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:

i. they were successfully and widely used in the past for pumping water;

ii. they both have the merit of being well suited to the use of low cost fuels such as

coal, peat and biomass;

iii. 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.

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 . 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. 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.

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, (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.

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 directaction

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.

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.

Some insight into the mechanics of a ' small Stirling engine can be gained ,

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 re generator 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.