4.4    INTERNAL COMBUSTION ENGINES.

The almost universal power source in the 5-500hp range, in areas having no mains electricity, is the internal combustion (i.c.) piston engine in either of its two main forms; the petrol (gasoline) fuelled spark ignition engine (s.i .) or the diesel fuelled compression ignition engine, (c.i.).

The main reasons for the widespread success of the i.c. piston engine are its high power/weight ratio, compact size and instant start-up capability, which led to its general adoption for powering motor vehicles in particular, and small  isolated  machinery  and  boats  generally.  Mass  production, and decreasing fuel prices (until 1973) made small engines based on designs used for motor vehicles both cheap to buy and inexpensive to run. Since the 1973 and 1979 oil crises the trend of declining petroleum prices has generally reversed, although during the mid-1980s we have again a period of declining oil prices which may even last a few years. But even though crude oil prices have again declined (in US dollar terms), petroleum fuels frequently remain in short supply in many developing countries due to inability to finance sufficient oil imports. Almost everywhere the most serious supply shortages are most prevalent in the rural areas, in other words, precisely where farmers are in need of power for irrigation pumping. It is this actual or potential fuel supply problem which makes the alternatives to petroleum fuelled engines, reviewed later in this chapter, worth considering at all; the great versatility of the i.c. engine makes it exceedingly difficult to improve on it from any other point of view.

4.4.1 Different Types of I.C. Engines

This is not the book to describe the technicalities of small engines in detail; there are many standard textbooks on this subject and manufacturers and their agents will normally provide detailed information in the form of sales literature. However, when comparing the many available engines, it soon becomes apparent that there is a variety of different types of engine available even at one particular power output.

i. Diesel versus petrol/kerosene

The two main categories of engine to choose from are c.i. or diesel engines and s.i. or petrol (gasoline), kerosene or 1.p.g. fuelled engines; (1.p.g. - liquified petroleum gas supplied in cylinders and usually primarily propane or butane). C.i. engines ignite their fuel by the heating effect when a charge of air is compressed suddenly enough; finely atomized fuel droplets are sprayed at very high pressure into the cylinder through the injector nozzle at the appropriate moment when the temperature is high enough to cause ignition. Spark ignition engines, on the other hand, work by mixing vapourized fuel with air, compressing the mixture and then igniting it at the correct moment by the electrical discharge of a spark plug set into the cylinder. Diesel engines therefore need to be heavier and more robust in construction to allow the high pressures needed to cause compression ignition to be sustained, and they also require a high pressure metering and injection pump to force the fuel in the right quantities at the right instant in time through the injector nozzle. The diesel injection pump and nozzles are built to a high level of precision and are therefore expensive components despite being mass-produced; they also depend on clean fuel and careful maintenance for reliable operation.

Therefore, petrol/gasoline and kerosene engines are inevitably lighter, more compact and usually cheaper than diesels. Although diesel engines are inherently more expensive to manufacture, they compensate for this by being more efficient, more reliable and more long-lasting (but more complicated to maintain in good running order). The main reasons for their better efficiency are firstly, the higher compression ratio allows a diesel to "breathe more deeply", to draw in more air per stroke in relation to the size of combustion space; secondly, fuel injection allows the diesel readily to run on a leaner fuel/air mixture than the equivalent s.i. engine. A spark ignition engine cannot be designed to run at such a high compression ratio, or the fuel/air mixture would ignite prematurely causing  "knocking"  or  "pinking". Another less well known advantage of the diesel is that diesel fuel is 18% "richer" in energy than gasoline per litre (mainly due to its higher density); Table 14 indicates the calorific value of the three main petroleum fuels. Since fuel is generally bought by the litre (or some equivalent volume measure such as gallons), rather than by its weight or by its energy value, you can buy 18% more energy per litre of diesel than with petrol (gasoline).

Table 14 COMPARISON OF DIFFERENT PETROLEUM-BASED ENGINE FUELS

Therefore the diesel is generally to be preferred as a power source, in terms of efficiency and reliable operation for long periods per day. For pumping applications, however, the choice of petrol or diesel relates largely to the scale of pumping required. Where a small, lightweight, portable system is needed which will only be used for one or two hours per day, and where simplicity of maintenance is important, and where "affordability" matters -i.e. the farmer has only the minimum capital to invest, then an i.e. gasoline or kerosene engine may be best and for that reason is frequently used.

It should be explained that the kerosene engine is similar to a petrol (gasoline) engine; indeed most kerosene engines need to be started and warmed up on petrol, because kerosene will not vapourize adequately in a cold engine. Many kerosene engines have a separate compartment in their fuel tank for a small supply of petrol and a tap to switch the fuel supply from petrol to kerosene once the engine is warm; it is also important to switch back to petrol a few moments before stopping the engine so that the carburettor float chamber is refilled with petrol ready for the next time the engine has to be started. Some farmers start kerosene engines simply by pouring petrol into the air intake, but this practice is not to be recommended as it can cause a fire. The advantage of a kerosene engine is that kerosene is normally available for agricultural purposes in an untaxed, subsidised or lightly taxed form and it also contains approximately 10% more energy per litre than petrol. The latter also usually carries a motor fuel tax in most countries as it is mainly used for private cars; therefore fuel costs for kerosene are generally much lower than for gasoline. Kerosene is also much less dangerous to store in quantity as it is much less easily ignited. The kerosene supply is also used for lighting and cooking fuel in many rural households and is therefore a more generally useful fuel.

Table 15 compares the general attributes of the three main i.e. engine options. It should be noted that diesels are sub-divided into two main categories; "low speed" and "high speed". The former run at speeds in the 450-1200 rpm range and tend to be much heavier and more expensive in relation to their power rating than the latter which typically run at speeds in the 1200-2500 rpm range. The slow speed diesel tends to have a much longer operational life and to be better suited to continuous operation, or long duty cycles, but its initial purchase cost is much higher.

Table 15 COMPARISON OF SMALL I.C. ENGINES

ii. Relationship between  size, speed and durability

A general characteristic of all i.c. engines is that the smaller and lighter they are for a given power output, the lower will be their initial purchase price (which correlates to some extent with the weight) and the shorter will be their useful life. This is because a high power/weight ratio is normally achieved by running an engine at high speed; the faster an engine runs, the more air/fuel mixture it can consume and the greater will be the energy delivered. However, a faster machine will wear out quicker simply because its moving and rubbing components travel further in a given number of hours of use. There is therefore a tradeoff between heavy, expensive and slow engines on the one hand, and cheap and fast ones of the same power rating. Therefore, small, lightweight engines are recommended for such duties where portability and low first cost are important. In most cases, especially if the engine is part of a fixed installation and to be used for lengthy duty cycles, it will generally be worth investing in a suitably heavy and slower machine in the interests of achieving better reliability and a longer operational life. In general light s.i. engines are restricted to duties requiring less than 500 hours running time per season.

iii.   Derating

If an engine is run continuously at its Rated Power, premature wear will occur. All engines therefore require to be derated from the manufacturer's rated power (which is the maximum power output the engine can achieve for short periods). Small engines are usually derated to about 70-80% of their rated power; eg. a 5kW rated engine will be necessary to produce a continuous 3.5-4.0kW.

The main reason for derating an engine is to prevent premature wear, but also the optimum efficiency for most engines is achieved at a speed corresponding to about 70-80% of its speed for maximum power. Therefore, derating an engine usually improves its specific fuel consumption (the fuel required per unit of output).

Further derating is necessary at high altitudes or at high ambient temperatures; recommendations to this effect are usually made by the manufacturer. Typically a further 10% derating is recommended for each 1 000m above sea level, plus 1% for each 5°C temperature rise above 16°C at the engine air intake. Therefore at 2 000m altitude and an ambient temperature of 26°C it would be necessary to derate an engine by say, 0.8 (generally) times 0.8 (for altitude) times 0.98 (for temperature) which totals .63 or 63% of rated power which would be the correct load to apply. Therefore a 2 O00W load would require an engine nominally rated at 2 000/0.63 or 3.2kW (4.3bhp) under those conditions.

Excessive derating is to be avoided, as (particularly with diesels) running at a fraction of the design power tends to cause coking of the cylinder. Also, the engine efficiency will of course be much poorer than normal under such conditions.

iv. Four-stroke or two-stroke

Both s.i. and c.i. engines can be designed to run so that ignition takes place either every other revolution, (four-stroke or four-cycle) or every revolution (two-stroke or two-cycle). The four-stroke s.i. engine tends to be more efficient as the "non-firing" revolution gives more time for inducing a fresh charge of fuel and also for effectively driving out the exhaust gases from the previous firing stroke (s.i. two-strokes tend to be less well scavenged of exhaust). Two-stroke diesels do not suffer an efficiency penalty in the same way, but are not generally available in the small size range of relevance for small scale irrigation. The two-stroke s.i. engine tends to be high revving and lightweight; it usually has fewer components than a four-stroke and therefore is cheaper to manufacture; typical applications are as moped engines. They are less suitable for irrigation pumping than four-strokes as they use more fuel and wear out more quickly. Most s.i. two-strokes use the downward movement of the piston into the crankcase to displace the air/fuel mixture into the cylinder; in which case it is not possible to lubricate the engine with a separate oil supply and the lubricant is mixed with the petrol, (two-stroke mix). This removes the need for oil changes, but is wasteful of lubricant, tends to cause a smoky exhaust, causes the need for more frequent "decokes" (de-carbonization of the cylinder head) and introduces a risk of damage caused by an inexperienced operator failing to mix sufficient lubricating oil with the fuel, or using the wrong type of oil. For these reasons, two-stroke s.i. engines are tending to be phased out and replaced by four strokes.

v. Air or water cooling

About one third of the heat produced when the fuel is burnt has to be dissipated through the walls of the cylinder and through the cylinder head; the two methods generally used for removing this heat and preventing the cylinder overheating are either by surrounding the cylinder with a water jacket which has water circulated through it and a separate radiator, or by having many cooling fins on the cylinder (to increase its effective surface area), and blowing air over the fins with a fan driven off the engine. A few small, low-powered, and old fashioned low-speed stationery engines have a water jacket with an open top and keep cool simply by boiling the water, which needs to be topped up from time to time, but most modern liquid cooled engines have their coolant circulated by a pump just like car engines.

Each method of cooling has its pros and cons. Water cooled engines tend run slightly quieter and their engine temperature is more easily regulated through the use of a thermostat, than with air cooling.  However, with water cooled engines, internal corrosion can occur and also water can leak out, evaporate or freeze. This last problem can be prevented by the use of anti-freeze (ethylene glycol) mixed with the cooling water in winter; most anti-freezes also contain corrosion inhibitor and are therefore useful to add to the cooling water even in climates where freezing is not likely to occur. Loss of coolant generally causes severe engine damage if the engine is allowed to continue running in that condition; various safety devices are available either to warn of overheating (caused by loss of coolant or for any other reason) and in some cases to automatically cut off the fuel supply and stop the engine. Air cooled engines obviously cannot lose their coolant, but it is important to ensure that their cooling fins do not get clogged with dust or dirt and that any cooling fan (when fitted) is clean and functioning correctly.

vi. Cylinder arrangment

The smallest engines usually have a single cylinder, mounted vertically above the crankshaft, as this is convenient for access to the main engine components and also allows an oil sump to be conveniently located where oil can drain down to it from the cylinder. A large flywheel is needed to smooth the output from a single cylinder engine as excessive vibration can cause problems with parts resonating and fatiguing and nuts and bolts working loose. Single cylinder low speed diesels need particularly heavy flywheels because they have large heavy pistons and connecting rods which run at low speeds; traditional designs are "open flywheel" (Fig. 98) while the more modern style of high-speed diesel engine usually has an internal enclosed flywheel (Fig. 99).

With larger sizes of engine it becomes feasible to have two or more cylinders. Twin cylinder engines have a smoother power output because the cylinders fire alternately and partially balance each other. Multi-cylinder engines therefore run more smoothly and quietly than ones of the same power with fewer cylinders, but the more cylinders there are the more components are involved, so obviously a multi-cylinder engine will be more expensive and more complicated to overhaul and maintain.

Fig. 98 Open flywheel low-speed single-cylinder diesel engine

Fig. 99. Belt driven 3 cylinder Lister diesel engine coupled to a centrifugal pump via multiple 'V' belts

vii.   Special features and accessories

Most small pumping systems have a hand crank starter or a pull chord (recoil) starter (the former is more common with small diesels and the latter with small petrol engines). Diesels often include a decompression valve to aid starting, in which a cylinder valve can be partially opened to release the pressure when the piston comes up on the compression stroke, allowing the engine to be hand wound up to a certain speed, when the decompression valve is suddenly closed and the momentum of the flywheel(s) carries the machine on sufficiently to fire the engine and start it off. Larger engines often have an auxilliary electrical system and a battery with an electric starter; (spark ignition engines generally need a battery to run the ignition, although very small s.i. engines use a magneto which generates and times the spark from the rotation of the engine). Engines fitted with electrical systems or engines coupled to generators can have various electrical controls to warn of or to prevent damage from overheating, loss of lubricant, etc. Some of these options, although sophisticated, are not expensive and are therefore a sound investment.

A vital, and often neglected accessory is the air filter, especially in dusty climates. Usually there is a choice of paper element filter (as used on most cars) where the paper element needs to be regularly replaced when clogged with dust or torn or, alternatively, an oil bath filter. The latter is slightly more expensive, but much more effective and practical in an agricultural context, since at a pinch even old engine oil can be used to refill it.  A worn or malfunctioning air filter can greatly reduce the  useful life of an engine, a fact which is often not fully understood by farmers judging from the number of engines that can be found running without any air filter at all.

viii.   Installation

The smallest engines are supplied mounted on skids or in a small frame and therefore need no installation other than coupling their pump to the water conveyance system. But larger machines, and many diesels, need to be properly installed, either on a concrete pad or on a suitable trolley or chassis. Most manufacturers will provide a detailed specification, when necessary, for the foundations of any engine driven pumping system; this should be accurately adhered to.

The engine often needs to be installed in a small lockable building for security. It is essential, however, that any engine house is well ventilated and that the exhaust is properly discharged outside. This is not only to avoid the serious danger of poisoning the user with exhaust gases, but also to ensure the engine does not overheat. Similarly, engines with direct coupled centrifugal pumps sometimes need to be installed in a pit in order to lower them near enough to the water level to avoid an excessive suction lift. Considerable care is needed with such installations to ensure that neither exhaust nor oil fumes will fill the pit and poison anyone who enters it; carbon monoxide in i.c. engine exhaust emissions can, and frequently does, cause fatal accidents. Care is also needed to ensure that the water level will never rise to a level where it could submerge the engine.

4.4.2 Efficiency of Engine Powered Pumping Systems

This is a controversial subject where little reliable data on actual field performance exists in the literature. A few field tests have been carried out on "typical" irrigation pumping systems, and in some cases surprisingly poor efficiencies were achieved. For example, Jansen [32] reported on tests on three kerosene fuelled small pumping sets in the 2-3 bhp range in Sri Lanka; total system efficiencies in the range 0.75-3.5% were recorded although engine/pump efficiencies (without a pipeline) were in the range from 2.6-8.8%. Excessive pipeline friction losses obviously caused the very poor system performance. Unfortunately there is good reason to assume such losses are quite common.

Many of the reasons for poor performance can be corrected at little cost once it is recognized that a problem exists, but unfortunately, it is easy to run an inefficient pumping system without even realizing it, because the shortfall in output is simply made up by running the engine longer than would otherwise be necessary.

Fig. 100 indicates the principle components of a small engine pumping system and the range of efficiencies that can typically occur for each. Some explanation of these may give an insight into how such poor total system efficiency can sometimes occur, and by implication, what can be done to. improve it.

Firstly, fuel spillage and pilferage could perhaps result in, anything from 0-10% of the fuel purchased being lost; i.e. 90-100% of fuel purchased may be usefully consumed as indicated in the figure.  Spillage can occur not only when transferring fuel, but due to leaky storage or very frequently due to either a leaky fuel line on the engine or leaky joints especially on the high pressure lines of a diesel engine; well established fuel  stains on the ground ought to give warning that something is wrong with fuel management. As the value of fuel increases, pilferage becomes increasingly a problem. One standard 200 litre drum of fuel is worth typically US $50-100; a small fortune in many developing countries.

Fig. 100 Principal components of a small engine pumping system and their efficiencies

Most basic thermodynamics textbooks claim that s.i. engines tend to be 25-30% efficient, while diesels are 30-40% efficient. Similarly, manufacturers' dynamometer tests, with optimally tuned engines, running on a test-bed, (often minus most of their accessories), tend to confirm this. However, such figures are optimistic in relation to field operations. The difference between theory and reality is greatest with the smallest  sizes of engines, which are inherently less efficient than larger once, and the text-book figures quoted above really only apply to well tuned engines of above 5-10kW power rating. The smallest tow-stroke and four-stroke s.i. engines also tend to vary in quality, engine to engine, and can easily be as poor and only 10% efficient at around 1kW power rating. Small diesels (the smallest are generally about 1.5 to 2kW) will probably be better than 20% efficient as engines, but components like the injection pump, cooling fan and water circulating pump (all parasitic energy consumers) can reduce the fuel-to-useful-shaft-power efficiency to around 15% (or less) for the smallest engines. This large drop is because the parasitic accessories take proportionately more power from very small engines. Obviously engines are also only in new condition for a small part of their, lives, and on average are worn and not well tuned, which also undermines their efficiency. Hence depending on size, type and quality plus their age and how well they are maintened, engines may in reality be at best 35% efficient and at worst under 10% efficient.

The next source of loss is any mechanical transmission from the engine to the pump. In some cases the engine is direct coupled to the pump, in which case the transmission losses are negligible, but if there is any substantial change of speed, such as a speed-reducer and gearbox to drive a reciprocating borehole pump, then the transmission can be as low as 60% efficient, particularly with small systems of less than 5kW, where geaz box losses will be relatively large in relation to the power flow.

As discussed in the previous section, the most common type of pump used for irrigation with a small engine will be a centrifugal pump, either direct coupled or belt driven, usually working on suction;  (eg. Figs. 72 and 99). If properly installed, so that the pump is operating close to its optimum head and speed, the pump efficiency can easily exceed 60% and possibility be as high as 80% with bigger pumps. However there is a lot of scope for failing to achieve these figures; bad impeller designs, worn impellers with much back-leakage, operating away from the design flow and head for a given speed will all have a detrimental effect and can easily singly, or in combination, pull the efficiency down to 25% or less. Given a reasonably well matched and well run system, pump efficiencies will therefore be in the 40-80% range, but they could easily be worse (but not better) than this range.

It is often not appreciated that the choice of delivery pipe can have a profound effect on system performance. Engines can deliver very high volumes at low head, so pipe friction can grossly increase the total head across the pump, particularly with long delivery lines at low heads. When this happens, it is possible for the total head to be several times the static head, which multiplies the fuel requirement proportionately. Fig. 4 allows this to be quantified; eg. even a small portable petrol engine pump will typically deliver over 360 l/min or 6 l/s through 5m head (600W hydraulic output). The friction loss for each 100m of delivery pipe with this flow and head will approximately as follows:    

from this, the pipe line efficiency for various lengths of pipe with the above diameters is as follows:

Quite clearly, when pumping at such low heads, it is easy to achieve total heads that are several times the static head, giving rise to pipeline efficiencies as poor as 20% when 100m of 2" pipe is used; (i.e. in that case the pumped head is five times the static head so that five times as much fuel is needed compared with a 100% efficient pipeline). The situation gets proportionately less serious at higher static heads, because it is the ratio of pipe friction head to static head that matters; for example, at 20m static head the above example would give the same friction head of 20m which although unacceptably high would at least imply 50% rather than 20% pipeline efficiency. It is a common mistake to use pipework which is too small in diameter as a supposed "economy" when larger pipe can often pay for itself in saved fuel within months rather than years. Also, some pumps which have a 2in discharge orifice may actually need a 3in pipeline if some distance is involved, yet uninformed users will usually use a pipe diameter to, match the pump orifice size and thereby create a major source of inefficiency. Incidentally, inch pipe sizes are quoted here simply because they are in fact more commonly used, even in countries where every thing else is dimensioned in metric units.

The performance curve in Fig. 101 indicates how centrifugal suction pumps can also suffer reduced performance as the suction head increases, mainly due to cavitation, particularly when the suction head is a large fraction of the total head. The figure shows how at 10m total head, 6m suction head causes a 20% drop in output compared with 3m suction head. This is without any reduction in power demand, so the former system is 20% less efficient than the latter, simply due to suction losses. There is therefore a potentially large fuel cost penalty in applying excessive suction lifts (apart from the usual priming problems that can occur).

Fig. 101 Effect of increasing the suction head on the output for a typical engine pump set

Pipe losses, whether due to friction or suction can therefore cause increased head and reduced flow and will typically have an efficiency in the 30-95% range, as indicated in Fig. 100.

The efficiency factors discussed so far apply to the hardware, while it is being run and water is being usefully applied to the field. Inevitably the system needs to run when water is not being usefully applied. For example, when starting up, any engine will often be run for a few minutes before the farmer can arrange the discharge to reach the correct part of the field, and water will be wasted. Similarly when rearranging the distribution system to deliver water to another part of the field, some wastage may occur. There is therefore a factor relating to the type of water distribution system and to the management skill of the farmer at applying the pumped water for as large a fraction of the time it is being pumped (or the engine is running) as possible. Even moderately bad management could cause 20% loss compared with ideal usage, and really bad management can be much worse, so this efficiency is taken as ranging from 80-100% for the purpose of Fig. 100.

When all the worst efficiencies suggested in Fig. 100 are compounded, they yield a theoretical worst total efficiency of 0.5% (which Jansen [32] and others have confirmed) while the best factors in Fig. 100 compounded together give 27%. The "best" figure is only even theoretically feasible however for a larger diesel (over 5kW), driving a pump at 10-20m (or higher) head, which is rather higher than usual for most irrigation applications. Most smaller engine pumped irrigation systems therefore in practice probably achieve 5 to 15% total efficiency, with larger diesels operating at higher heads being towards the top end of this range and small kerosene or petrol engines at low heads being at (or below) the bottom. The operator should probably be satisfied if a small diesel system achieves 10-15% efficiency and a small petrol or kerosene fuelled system achieves 5-10%. It may be very worthwhile for any reader to investigate the actual efficiency of any engine pumping system they may be responsible for, (by comparing fuel consumption against hydraulic energy output) so that if it is below par steps may be taken to find the causes and to correct them.

4.5    EXTERNAL COMBUSTION ENGINES

The difference between internal and external combustion engines, as their names suggest, is that the former burn their fuel within the power cylinder, but the latter use their fuel to heat a gas or a vapour through the walls of an external chamber, and the heated gas or vapour is then transferred to the power cylinder. External combustion engines therefore require a heat exchanger, or boiler to take in heat, and as their fuels are burnt externally under steady conditions, they can in principle use any fuel that can burn, including agricultural residues or waste materials

There are two main families of external combustion engines; steam engines which rely on expanding steam (or occasionally some other vapour) to drive a mechanism; or Stirling engines which use hot air (or some other hot gas). The use of both technologies reached their zeniths around 1900 and have declined almost to extinction since. However a brief description is worthwhile, since:

1. they were successfully and widely used in the past for pumping water;
2. they both have the merit of being well suited to the use of low cost fuels such as coal, peat and biomass;
3. attempts to update and revive them are taking place.

and therefore they may re-appear as viable options in the longer term future.

The primary disadvantage of e.c. engines is that a large area of heat exchanger is necessary to transmit heat into the working cylinder(s) and also to reject heat at the end of the cycle. As a result, e.c. engines are generally bulky and expensive to construct compared with i.c. engines. Also, since they are no longer generally manufactured they do not enjoy the economies of mass-production available to i.e. engines. They also will not start so quickly or conveniently as an i.c. engine; because it takes time to light the fire and heat the machine to its working temperature.

Due to their relatively poor power/weight ratio and also the worse energy/weight ratio of solid fuels, the kinds of applications where steam or Stirling engines are most likely to be acceptable are for static applications such as as irrigation water pumping in areas where petroleum fuels are not readily available but low cost solid fuels are. On the positive side, e.c. engines have the advantage of having the potential to be much longer-lasting than i.c. engines (100 year old steam railway locomotives are relatively easy to keep in working order, but it is rare for i.c. engines to be used more than 20 years or so. E.c. engines are also significantly quieter and free of vibrations than i.c. engines. The level of skill needed for maintenance may also be lower, although the amount of time spent will be higher, particularly due to the need for cleaning out the furnace.

Modern engineering techniques promise that any future steam or Stirling engines could benefit from features not available over 60 years ago when they were last in general use. Products incorporating these new developments are not yet on the market, but R&D is in hand in various countries on a limited scale; however it will probably be some years before a new generation of multi-fuel Stirling or steam powered pumps become generally available.

4.5.1 Steam Engines

Only a limited number of small steam engines are available commercially at present; most are for general use or for powering small pleasure boats. A serious attempt to develop a 2kW steam engine for use in remote areas was made by the engine designers, Ricardos, in the UK during the 1950s (see Fig. 157). That development was possibly premature and failed, but there is currently a revival of interest in developing power sources that can run on biomass-based fuels (as discussed more fully in Section 4.10). However, small steam engines have always suffered from their need to meet quite stringent safety requirements to avoid accidents due to boiler explosions, and most countries have regulations requiring the certification of steam engine boilers, which is a serious, but necessary, inhibiting factor.

The principle of the steam engine is illustrated in Fig. 102. Fuel is burnt in a furnace and the hot gases usually pass through tubes surrounded by water (fire tube boilers). Steam is generated under pressure; typically 5 to 10 atmospheres (or 5-10bar). A safety valve is provided to release steam when the pressure becomes too high so as to avoid the risk of an explosion. High pressure steam is admitted to a power cylinder through a valve, where it expands against a moving piston to do work while its pressure drops. The inlet valve closes at a certain point, but the steam usually continues expanding until it is close to atmospheric pressure, when the exhaust valve opens to allow the piston to push the cooled and expanded steam out to make way for a new intake of high pressure steam. The valves are linked to the drive mechanism so as to open or close automatically at the correct moment. The period of opening of the inlet valve can be adjusted by the operator to vary the speed and power of the engine.

Fig. 102 Schematic arrangement of a condensing steam engine

In the simplest types of engine the steam is exhausted to the atmosphere. This however is wasteful of energy, because by cooling and condensing the exhausted steam the pressure can be reduced to a semi-vacuum and this allows more energy to be extracted from a given throughput of steam and thereby significantly improves the efficiency. When a condenser is not used, such as with steam railway locomotives, the jet of exhaust steam is utilised to create a good draught for the furnace by drawing the hot gases up the necessarily short smoke stack. Condensing steam engines, on the other hand, either need a high stack to create a draught by natural convection, or they need fans or blowers.

Steam pumps can easily include a condenser, since the pumped water can serve to cool the condenser. According to Mead [13], (and others) the typical gain in overall efficiency from using a condenser can exceed 30% extra output per unit of fuel used. Condensed steam collects as water at the bottom of the condenser and is then pumped at sufficient pressure to inject it back into the boiler by a small water feed pump, which is normally driven off the engine. A further important advantage of a condensing steam engine is that recirculating the same water reduces the problems of scaling and corrosion that commonly occur when a continuous throughput of fresh water is used. A clean and mineral-free water supply is normally necessary for non-condensing steam engines to prolong the life of the boiler.

The most basic steam engine is about 5% efficient (steam energy to mechanical shaft energy - the furnace and boiler efficiency of probably between 30 and 60% needs to be compounded with this to give an overall efficiency as a prime-mover in the 1.5 to 3% range). More sophisticated engines are around 10% efficient, while the very best reach 15%. When the boiler and furnace efficiencies (30-60%) plus the pump (40-80%) and pipework (40-90%) are compounded, we obtain system efficiencies for steam piston engine powered pumps in the 0.5 to 4.5% range, which is worse, but not a lot worse than for small s.i. internal combustion engines pumping systems, but allows the use of non-petroleum fuels and offers greater durability.

4.5.2 Stirling Engines

This type of engine was originally developed by the Rev. Robert Stirling in 1816. Tens of thousands of small Stirling engines were used in the late nineteenth and early twentieth century, mainly in the USA but also in Europe. They were applied to all manner of small scale power purposes, including water pumping. In North America they particularly saw service on the "new frontier"; which at that time suffered all the problems of a developing country in terms of lack of energy resources, etc.

Rural electrification and the rise of the small petrol engine during and after the 1920s overtook the Stirling engine, but their inherent multi-fuel capability, robustness and durability make them an attractive concept for re-development for use in remote areas in the future and certain projects are being initiated to this end. Various types of direct-action Stirling-piston water pumps have been developed since the 1970s by Beale and Sunpower Inc. in the USA, and some limited development of new engines, for example by IT Power in the UK with finance from GTZ of West Germany is continuing.

Fig. 103 Rider-Ericsson hot air pumping engine (Stirling cycle) circa 1900

Stirling engines use pressure changes caused by alternately heating and cooling an enclosed mass of air (or other gas). The Stirling engine has the potential to be more efficient than the steam engine, and also it avoids the boiler explosion and scaling hazards of steam engines. An important attribute is that the Stirling engine is almost unique as a heat engine in that it can be made to work quite well at fractional horsepower sizes where both i.c. engines and steam engines are relatively inefficient. This of course makes it of potential interest for small scale irrigation, although at present it is not a commercially available option.

To explain the Stirling cycle rigorously is a complex task. But in simple terms, a displacer is used to move the enclosed supply of air from a hot chamber to a cold chamber via a regenerator. When most of the air is in the hot end of the enclosed system, the internal pressure will be high and the gas is allowed to expand against a power piston, and conversely, when the displacer moves the air to the cool end, the pressure drops and the power piston returns. The gas moves from the hot end to the cold end through a regenerator which has a high thermal capacity combined with a lot of surface area, so that the hot air being drawn from the power cylinder cools progressively on its way through the regenerator, giving up its heat in the process; then when cool air travels back to the power cylinder ready for the next power stroke the heat is returned from the regenerator matrix to preheat the air prior to reaching the power cylinder. The regenerator is vital to achieving good efficiency from a Stirling engine. It often consists of a mass of metal gauze through which air can readily pass, [33], [34 ].

Some insight into the mechanics of a ' small Stirling engine can be gained from Fig. 103, which shows a 1900 vintage Rider-Ericsson engine. The displacer cylinder projects at its lower end into a small furnace. When the displacer descends it pushes all the air through the regenerator into the water cooled volume near the power cylinder and the pressure in the system drops, then as the displacer rises and pulls air back into the hot space, the pressure rises and is used to push the power piston upwards on the working stroke. The displacer is driven off the drive shaft and runs 90° out.of phase with the power piston. An idea of the potential value of engines such as this can be gained from records of their performance; for example, the half horsepower Rider-Ericsson engine could raise 2.7m³/hr of water through 20m; it ran at about 140 rpm (only) and consumed about 2kg of coke fuel per hour. All that was needed to keep it going was for the fire to be occasionally stoked, rather like a domestic stove, and for a drop of oil to be dispensed onto the plain bearings every hour or so.

4.6    ELECTRICAL POWER

If a connection is available to a reliable mains electricity supply, nothing else is either as convenient or more cost-effective for powering an irrigation pump. Unfortunately, the majority of farmers in developing countries do not have mains electricity close at hand, and even those that do often find that the supply is unreliable. Electricity supply problems tend to be particularly prevalent during the irrigation season, because irrigation pumping tends to be practised simultaneously by all farmers in a particular district and can therefore easily overload an inadequate rural network and cause "brownouts" (voltage reductions) or even "blackouts" (complete power cuts). Therefore there is a major inhibition for many electricity utilities in encouraging any further use of electricity for irrigation pumping in developing countries where the electrical supply network is already under strain.

The real cost of extending the grid is very high, typically in the order of $5 000-10 000 per kilometer,of spur. Although connections in many countries have in the past been subsidised, whatever the pricing policy of the utility, someone has to pay for it and the tendency today is to withdraw subsidies.  Therefore, although an electric motor considered in  isolation  is an extremely inexpensive and convenient prime-mover, it is only useful when conbnnected to a lot of capital-intensive infrastructure which needs to carry a substantial electrical load in order to be self-financing from revenue.

A further problem for developing countries in considering the mains electricity option is the high foreign exchange component in the investment; this is typically from 50-80%, according to Fluitman, [35] (quoting a World Bank source). Electricity generation in rural areas of developing countries tends to be by petroleum-fuelled plant (usually diesel generators) so this also is a burden on the economy. In fact a large fraction of many developing countries' oil imports goes to electrical power generation. The attractiveness of rural electrification as an investment for development is therefore being questioned much more now then it used to be; (eg. [35]). However it is not proposed here to deal with policy implications or macro-economic effects of the widespread use of electricity for irrigation pumping, other then to point out that it cannot be seen as a universally applicable solution to the world's irrigation pumping needs, because most countries will not be able to afford to extend a grid to all their rural areas in the forseeable future. Even where such an option can be afforded, it is still necessary to question whether it is the most cost-effective solution for irrigation pumping bearing in mind the high infrastructural costs.

4.6.1 Sources and Types of Electricity

Batteries produce a steady flow of electricity known as "direct current" or DC. Photovoltaic (solar) cells also produce DC. Electrical generators to produce DC are sometimes known as "dynamos"; they require commutators consisting of rotating brass segments with fixed carbon brushes. Alternators are almost universally used today for the generation of electricity from shaft power. Alternators are simpler and less expensive than DC generators, but they produce a voltage which reverses completely several times per revolution. This type of electrical output, which is almost universally used for mains supplies, is known as "alternating current" or AC.

AC mains voltage normally fluctuates from full positive to full negative and back 50 times per second (50Hz or 50 cycles/sec) or in some cases at 60Hz. The current fluctuates similarly. Sometimes the current and voltage can be "out of step", i.e. their peaks do not coincide. This discrepency (or phase difference) is quantified by the "power factor"; the output of an AC system is the product of the amps, volts and the power factor. When the amps and volts are in perfect phase with each other, the power factor is numerically 1. When the power factor is less than one (it frequently is 0.9 and sometimes less) then the power available is reduced proportionately for a given system rating. The rating of AC equipment is therefore generally given not in watts or kilowatts (kW), but in volt-amps or kilovolt-amps (kVA). The actual power in kW will therefore be the kVA rating multiplied by the power factor.

Another important principle to be aware of is that it is considerably more economic to transmit electricity any distance at high voltages rather than low. A smaller cross-section of conductor is needed for a given transmission efficiency. This is analagous to water transmission, where higher pressures and smaller flow rates allow smaller pipes to be used for equal hydraulic power. However, electricity is potentially lethal at AC voltages much above 240V and at DC voltages much above 100V (it can of  course kill at considerably lower voltages depending on the circumstances and state of health of the victim) and insulation becomes more difficult. Therefore, for safety reasons, 240V AC or about 110V DC are usually the maximum voltages used at the end-users' supplies and for electrical appliances.

The reason AC is generally used for mains applications rather than DC are that it has a number of important advantages:

1. AC generators and motors are much simpler, less expensive and less troublesome, since they do not require commutators;
2. AC voltages can be changed efficiently and with a high degree of reliability, using transformers, but it is a technically much more difficult problem to change DC voltages; therefore AC can easily be transmitted efficiently at high voltages and then transformed to low, safer voltages close to the point of use;
3. as a result of the advantages of AC, it has become the internationally used standard for mains supplies and virtually all mass-produced electrical applicances are designed for AC use.

It is sometimes necessary to convert AC to DC or vice-versa, for example to charge batteries (which are DC) from the AC mains or to run an AC appliance designed for the mains from a DC source such as a battery or a solar photovoltaic array. AC can quite readily be converted to DC by using a rectifier; these (like transformers) are solid-state devices which require no maintenance and are relatively efficient. A battery charger usually consists of a combination of a transformer (to step mains voltage down to battery voltage) and a rectifier to convert the low voltage AC to DC. Converting DC to AC is more difficult; traditionally an inefficient electro-mechanical device called a rotary converter was used; this is a DC motor direct coupled to an AC alternator. The modern alternative is an electronic, solid state device called an inverter. Inverters are relatively inexpensive for low power applications (such as powering small fluorescent lights from low voltage batteries), but they become expensive for such higher powered applications such as electric motors for pumping. The quality and price of inverters also varies a lot; if a good quality AC output is essential (and high efficiency of conversion) a more complicated and expensive device is needed. Cheap inverters often produce a crude AC output and are relatively inefficient; they can also seriously interfere with radio and TV reception in the vicinity.

4.6.2    AC Mains Power

Mains electricity is generally supplied as alternating current (AC) either at 220 to 240V and 50Hz frequency or at 110V and 60Hz frequency for low power connections, (including domestic ones) of up to about 10kW. The 220-240V 50Hz standard is normal in Europe while the 110V 60Hz standard is in use in the USA; either might be used in other parts of the world, although 220-240V is more common, especially in Asia and Africa.

When AC is supplied through two wires, it is known as single-phase. The two wires are not "positive" and "negative" but are "live" and "neutral"; there should always also be a third wire included for safety - the "earth" or "ground".  The latter is normally connected to the casing of any appliance or motor so that if any internal fault causes the casing to come into contact with the live supply, the leakage current will flow to earth (ground) and trip out the system or blow a fuse. Therefore if an electric pump keeps tripping or blowing fuses it is as well to have it checked to see if there is a short-circuit.

Mains power is normally generated as "three-phase", in which the alternator transmits three "single phase" AC outputs down three wires. Each phase is shifted by one third of a revolution of the alternator, so the voltage peaks in the three conductors do not coincide, but are evenly spaced out. The three phases, if equally balanced, will cancel each other out if fed through three equal loads, but in practice they are not usually perfectly balanced so there is normally a fourth return conductor called the neutral. A single phase AC supply is simply a connection to one of the three "lives" of a three-phase source with a return to its neutral. For this reason it is important in many cases not to confuse the live and neutral; also it is the live which should be protected by fuses or contact breakers.

At higher power levels, usually above 5kW, and always above about 25kW, it is normal to use three-phase AC. This is supplied mostly at 415V line to line (Europe) or 190 or 440V (USA).

4.6.3    Electric Motors

An electric motor seems almost the ideal prime mover for a water pump. Power is supplied "at the flick of a switch", and water  is produced at a constant rate until the motor is turned off.  Electric motors have relatively long service lives and generally need little or no servicing.

The cheapest and simplest type of electric motor is the squirrel cage induction motor which is almost universally used for mains electric power applications; see Fig. 104 (a) and Fig. 105. Here there are no electrical connections to the rotating "squirrel cage", so there are no brushes or slip rings to wear or need adjustment. Motors of this kind are available in either three-phase or single-phase versions. They run at a fixed speed depending on the frequency of the power supply and the number of poles in their stator windings. The most general type (which is usually the cheapest) runs at a nominal 1 500rpm at 50Hz (1 800 at 60Hz), but other speeds are available. It is normal to direct-couple a motor to a centrifugal pump where possible (eg. Fig. 105). Non-standard speed motors may be used where this does not suit the pump, or alternatively a belt speed reduction arrangement may be used, such as in Fig. 106.

A problem with induction motors is that they normally need over three times as much current to start as they do once running at rated speed and power. This means that the peak current that can be supplied must be significantly higher than that needed for operation, which often causes not just technical but also financial problems, as some electricity tariffs are determined by the maximum current rating on a circuit. Recently electronic starting devices have become available which limit the starting current while the motor runs up to speed and which in some cases also improve the overall efficiency of an electric motor.

Fig. 104 The four main types of electric motors

Fig. 105 Direct coupled electric motor and centrifugal pump

Fig. 106 Electric motor powered, belt driven piston pump (Climax) (note air chambers provided to prevent water hammer)

Induction motors are typically 75% efficient for a 300W (0.5 hp) size and may be around 85% efficient at 10kW size (subject to having a unity power factor). They are not generally made in sizes significantly smaller than 100-200W.

For very small scale applications, the so-called "universal motor" is most commonly used. The universal motor (Fig. 104 (b)) is the "classic" electric motor with a brushed commutator and wound armature. Fixed field coils produce the magnetic flux to run the motor. Motors of this kind can use either an AC or a DC supply and they are typically used for very small-scale power applications (such as in power tools, and domestic appliances, for example). They are more efficient than would be possible with a very small induction motor and their starting current is smaller in relation to their running current. They suffer however from needing periodic replacement of brushes when used intensively, as for pumping duties.

There are small-scale electrical power applications independent of a mains supply, which use a DC source such as a photovoltaic array, or batteries charged from a wind-generator. In these applications a permanent magnet DC motor is the most efficient option (Fig. 104 (c)). In these, permanent magnets replace the field coils; this offers higher efficiency, particularly at part-load, when field windings would absorb a significant proportion of the power being drawn. Permanent magnet DC motors can be 75-85% efficient even at such low power ratings as 100-200W, needed for the smallest solar pumping systems. Most permanent magnet motors have brushed/commutated armatures exactly like a universal motor, which in the pumping context is a major drawback particularly for submersible sealed in motors. However brushless permanent magnet motors have recently become available (Fig. 104 (d)). Here the magnets are fixed to the rotor and the stator windings are fed a commutated AC current at variable frequency to suit the speed of rotation; this is done by sending a signal from a rotor position-sensor which measures the speed and position of the shaft and controls electronic circuitry which performs the commutation function on a DC supply. Motors of this kind are mechanically on a par with an induction motor, and can be sealed for life in a submersible pump if required, but they are still produced in limited numbers and involve a sophisticated electronic commutator which makes them relatively expensive at the time of writing. With the increasing use of solar pumps they are likely to become more widely used and their price may fall.

Submersible pump motors, whether AC induction motors or DC brushless permanent magnet motors, are commonly filled with (clean and corrosion inhibited) water as this equalises the pressure on the seals and makes it easier to prevent ingress of well water than if the motor contained only air at atmospheric pressure. Filling motors with water is obviously only possible with brushless motors, or short-circuits would occur. Another advantage of water filled motors is that they are better-protected from overheating.

4.6.4 Electrical Safety

AC electrical voltages over about 110V and DC over 80V are potentially lethal, especially if the contact is enhanced through the presence of water. Therefore, electricity and water need to be combined with caution, and anyone using electricity for irrigation pumping should ensure that all necessary protection equipment is provided; i.e. effective trips or fuses, plus suitable armoured cables, earthed and splashproof enclosures, etc. Also all major components, the motor, pump and supporting structure should be properly earthed (or grounded) with all earth connections electrically bonded together. It is vital that electrical installations should either be completed by trained electricians or if the farmer carries it out, he should have it inspected and checked by a properly qualified person before ever attempting to use it; (in some countries this is in any case a legal requirement). It is also prudent to have some prior knowledge what action to take for treating electric shock; most electrical utilities can provide posters or notices giving details of precautions with recommendations on treatment should such an unfortunate event occur.