GB2342390A

GB2342390A - Providing an oxygen-rich atmosphere in the combustion chamber of a gas-fuelled compression-ignition engine

Info

Publication number: GB2342390A
Application number: GB9821579A
Authority: GB
Inventors: John Mcneil
Original assignee: Finch International Ltd
Current assignee: Finch International Ltd
Priority date: 1998-10-02
Filing date: 1998-10-02
Publication date: 2000-04-12
Anticipated expiration: 2018-10-02
Also published as: GB2342390B; GB9821579D0

Abstract

Oxygen enriched air, eg from a membrane-type oxygen/nitrogen separator 16, is supplied to the intake of a compression-ignition engine to ensure spontaneous combustion of gaseous fuels without further engine modification. A catalyst 44, eg a platinum group metal catalyst on a ceramic substrate 40, may be coated on the piston crown 34 and cylinder head 48 to accelerate the rate, and reduce the temperature, of the reaction between the fuel and the oxygen. The concentration of oxygen in the intake air may be controlled by a valve 20 which selectively admits atmospheric air to the oxygen-enriched air from the separator 16. Valve 20, together with fuel gas supply valve 6, may be controlled according to sensed operating conditions. The invention is applicable to automotive engines or stationary engines for electrical power generation. In the latter case, the exhaust gas may be used to produce steam to drive a further electrical generating system.

Description

Combustion of gaseous hydrocarbon fuels in internal combustion engines.

The present invention relates to a method of combusting gaseous hydrocarbon fuels, in a compression ignition engine.

The fundamental design and method of operation of internal combustion engines has been established for many years. A compression ignition engine works by injecting liquid fuel, under high pressure, into air which has been compressed by a piston travelling up a cylinder. The fuel and air mixture is further compressed until it is hot enough to ignite the fuel.

This leads to a rapid increase in temperature and pressure inside the combustion chamber, forcing the piston back down the cylinder. A spark ignition engine operates in a similar manner except that a spark is used to initially ignite the liquid fuel. Internal combustion engines are very fuel specific and will only run efficiently with the particular petrochemical fuels designed for the engine in question. For example, compression ignition engines use diesel oil, whilst spark ignition engines run on petrol. The specifications and properties of these fuels are carefully controlled by the fuel manufacturers to ensure that they ignite and burn reliably inside the engine for which they have been designed.

It has been proposed to burn gaseous hydrocarbon fossil fuels, such as petroleum gas and natural gas, in compression ignition engines. Such fuels are advantageous in that they generally have a cleaner emission profile than common petrochemical liquid fuels.

However they can only be burned satisfactorily in specially modified compression ignition engines because these fuels require either a flame or spark to initiate combustion. The most common methods of achieving this are by the use of spark or glow plugs in either the combustion chamber or in a pre-ignition chamber, or alternatively a small amount of diesel oil is injected into the combustion chamber, or into a pre-chamber, to provide initial ignition prior to the injection of the primary gas fuel. Because of their relatively clean emission profiles, there is considerable interest at the present time in the use of gas fuels for both transit and static installations. If a more standard type of compression ignition engine could be used, rather than a specially modified design, the potential for using gas fuels would be greatly improved.

The present invention seeks to provide an improved method of combusting gaseous hydrocarbon fuels, in a compression ignition engine. It has surprisingly been found that this may be achieved simply by providing an oxygen enriched atmosphere in the combustion chamber, which allows the spontaneous ignition and subsequent efficient combustion of gaseous hydrocarbon fuels. The enriched oxygen atmosphere encourages the flameless ignition of gaseous fuels and a more complete combustion of the hydrocarbons.

Thus viewed from one aspect, the present invention provides a method of combusting gaseous hydrocarbon fuels in a compression ignition engine, wherein an enriched oxygen atmosphere is provided in the combustion chamber of the engine.

With such a method, the need to provide additional ignition means is obviated, leading to a much simplified engine design.

It has also been found that if a catalyst is further provided in the combustion chamber of the engine, this further encourages the flameless ignition of the gaseous fuels and a more complete combustion profile, while reducing the level of oxygen enrichment of the combustion atmosphere required, which may be desirable from a commercial point of view.

Thus from a particularly preferred aspect, the invention also provides a method of combusting gaseous hydrocarbon fuels in a compression ignition engine, wherein a catalyst and an enriched oxygen atmosphere are provided in the combustion chamber of the engine.

Preferably, an inside surface of the combustion chamber is selectively coated with a suitable catalyst, preferably a metal catalyst such as a platinum group based catalyst or a precious metal catalyst, or alloys or combinations of such metals. A typical composition consists of at least 90% by weight platinum group metal or metal alloy combined with up to 10% by weight of performance enhancing additives such as the product PAL 56 supplied by Surcotech International Limited.

For example, the catalyst may be provided as a coating on the top of the piston and/or the base of the cylinder head in the combustion chamber of the engine, so as to avoid interference with the operation of the piston within the cylinder. Of course the catalyst may alternatively be provided elsewhere within the combustion chamber for example an upper side wall of the cylinder.

In a particularly preferred embodiment of the invention the catalyst is deposited onto a ceramic substrate. Use of a ceramic substrate support may further improve the thermal efficiency of the engine, since the ceramic material acts as an insulating layer between the combustion chamber and the metal piston and cylinder head and reduces the amount of heat being conducted away by the metal components. More of the heat of compression and combustion is retained inside the combustion chamber to further encourage early and efficient ignition of the fuel.

Preferably the level of oxygen enrichment is over 0. 5% above normal (ie. an air composition 21.5% oxygen, 78.5% nitrogen) and most preferably the level is between 0.5% and 2% above normal (21.5%, 78.5% nitrogen and 23% oxygen, 77% nitrogen respectively).

However, it will be appreciated that an increase in oxygen concentration of less than 0.5% above normal, may achieve the desired ignition of the fuel, if the rest of the conditions are optimally controlled and a suitable catalyst is present in the combustion chamber of the engine.

The precise level of oxygen enrichment which will be required for a given combustion engine will be dependent on the quality and type of fuel used. For instance, to enable combustion of gas mixtures with a low calorific value and a low methane number, a level of oxygen enrichment more than 2% above normal may be required. It is a simple matter for a person skilled in the art to adjust the level of oxygen enrichment in accordance with the type of fuel and the engine operating conditions.

The level of oxygen enrichment may be controlled in dependence on an analysis of the engine operation conditions and the exhaust gas stream. Thus for example the engine operation conditions to be monitored may include, but are not limited to, the temperature and pressure inside the combustion chamber, the number of strokes per minute, the rate of fuel injection or carburation and the rate and concentration of the enriched-oxygen supply.

The analysis of the exhaust gas stream would include monitoring for example the temperature of the exhaust gas, the carbon monoxide concentration, the concentration of nitrogen oxides or detection of any unburned hydrocarbon residues in the exhaust gas stream.

If the amount of such pollutants in the exhaust stream increases, the concentration of oxygen in the air inlet stream may be increased accordingly.

Any suitable gaseous hydrocarbon fuel may be used, according to the availability of such fuels and other economic factors. Preferably however, the fuel is petroleum gas, natural gas, or a mixture of gases, for example a mixture of gases such as might be obtained from the gasification of organic or fossil matter. Thus the term"hydrocarbon fuel"is intended to encompass fuels such as petroleum gas and natural gas and also fuel mixtures, for example containing methane, hydrogen and carbon monoxide, as might be obtained from a gasification process.

In order to achieve efficient operation of the engine, the fuel injection or carburation is preferably controlled in dependence on an analysis of the engine operation conditions and the exhaust gas stream.

It is believed that the present invention will have application to both transit and static installations.

However, a particular application of the method of the present invention is in the commercial generation'of electricity. In addition, it is thought that engines designed to operate according to the method of the present invention will be particularly suitable for use as mobile units for power generation.

Accordingly, a preferred aspect of the present invention includes a method of generating power wherein an engine operating in accordance with the invention is coupled mechanically to an electrical power generating device. Furthermore, more efficient use of the power produced by the engines may be harnessed for other purposes. Thus for example the hot exhaust gas may be used to produce steam to drive a further electrical generator device or to provide local heating.

It will be appreciated that the invention also extends to a combustion system for operation in accordance with the invention.

Viewed from a further aspect therefore, the invention provides a combustion system comprising a compression ignition engine, means for supplying an enriched oxygen atmosphere to the combustion chamber of said engine, and a means to supply a gaseous hydrocarbon fuel to said combustion chamber. Such a system may have any combination of the preferred features of the method of the invention as described above, and particularly preferably, a catalyst is provided in said combustion chamber.

Viewed from yet a further aspect, the invention provides an electrical power generating system comprising a generator coupled to at least one engine, said engine being adapted to combust gaseous fuels by means of an enriched oxygen atmosphere. Once again, such a system may have any combination of the preferred features of the method of the invention as described above.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic illustration of a system embodying the invention; Figure 2 shows a cylinder of the engine of the system of Figure 1 with a piston in its compression stroke; Figure 3 shows the piston of Figure 2 in its top position; and Figure 4 shows the piston of Figure 2 in its return stroke.

With reference firstly to Figure 1, which shows a schematic illustration of a system embodying the invention, a fuel pump 2 pumps a gaseous fuel from a fuel container 4 via a control valve 6 to the fuel injector 8 of a compression ignition engine 10.

Air from the atmosphere is pumped into a gas separation module 12 by pump 14 and the filtered air is drawn out of the module 16 by a vacuum pump 18. The module 16 is of a known construction of the type which contains a plurality of hollow fibre membranes which can selectively separate oxygen and nitrogen to provide large volumes of oxygen enriched air. Oxygen enriched air leaves the module 16 via a control valve 20. The outlet of the control valve 20 is connected to the air intake manifold of the engine 10 and the oxygen enriched air is introduced to the combustion chamber 42 of the engine 10 via an air inlet valve 24. Nitrogen rich residual air 26 leaving the separation module 16 is vented to the outside atmosphere.

The control valve 20 controls the concentration of oxygen in the air supplied to the engine 1 by selectively admitting atmospheric air to the oxygen enriched air, and operates to provide optimum engine operating conditions, which are sensed by sensors 28 which analyse the exhaust gas stream 30. Similarly, the fuel injection is controlled by the control valve 6 in dependence on the engine operation conditions and an analysis of the exhaust gas stream by the sensors 28.

Turning now to the combustion process within the engine 10, Figure 2, this illustrates a cylinder 32 of the engine 10 with the piston 34 of the engine travelling up the cylinder 32 on its compression stroke.

Oxygen enriched air has already been introduced into the cylinder 32, via the air inlet valve 24, and is being compressed by the piston 34. In the drawing as shown, the piston 34 has reached the position in the cylinder 32 where injection of the gaseous fuel, by the fuel injector 8, has just commenced. The air inlet valve 24 and the exhaust gas valve 36 are in their closed positions.

The top surface 38 of the piston 34, inside the cylinder 32, has first been coated with a ceramic substrate 40 which provides an insulating layer between the combustion chamber 42 and the metal surface of the piston 34. This ceramic substrate 40 is then coated with a thin layer of a suitable catalyst 44 manufactured, for example, from platinum group metals.

A typical composition consists of at least 901 boy weight platinum group metal or metal alloy combined with up to 10% by weight of performance enhancing additives such as the product PAL 56 supplied by Surcotech International Limited. The bottom surface 46 of the cylinder head 48 of the engine 10 has also been coated with the ceramic substrate 40, to provide an insulating layer, and this is again coated with a thin layer of catalyst 44. The exposed surfaces of the air inlet valve 24 and the exhaust gas valve 36 have also been coated with both the ceramic substrate 40 and the catalyst 44. The tip of the fuel injector 8, which is manufactured from high quality stainless steel, is left uncoated.

Figure 3 shows the piston 34 at its top position after travelling up the cylinder 32. The catalystcoated top surface 38 of the piston 34 is in close contact with the catalyst-coated bottom surface 46 of the cylinder head 48, so that the combustion chamber 42 forms a restricted, enclosed space, which is almost completely coated on the inside with the catalyst 44 and contains a homogeneous mixture of the gaseous fuel and oxygen-enriched air. At this stage, gaseous fuel is still being injected by the fuel injector 8 into the combustion chamber 42 and the air inlet valve 24 and the exhaust valve 36 remain closed.

Figure 4 shows the position of the piston 34 shortly after it starts its downward power stroke. At this point, fuel injection has just ceased and the air inlet valve 24 and exhaust gas valve 36 both remain closed. The exhaust gas valve 36 will then open and the exhaust gases will be expelled from the cylinder 32 when the piston 34 returns back up the cylinder 32 on its exhaust stroke.

The sequence of the combustion process in the combustion chamber 42 is as follows: The oxygen enriched air, which has been introduced into the cylinder 32 via the air inlet valve 24, is compressed and heated by the piston 34 travelling up the cylinder 32. The injection of the gaseous fuel by the gas injector 8 starts when the piston 34 is at the approximate compression position shown in Figure 1.

The gaseous fuel meets and mixes with the hot compressed oxygen-enriched air, and at the same time the air and gas mixture simultaneously contacts the catalyst 44 which is coated on the piston 34 and the cylinder head 48.

The catalyst 44 aids and accelerates the rate of chemical oxidation between the fuel and oxygen molecules, so that under the enriched oxygen conditions the gaseous fuel can ignite spontaneously without the need for flame or spark ignition. The catalyst 44 also encourages the initial reaction to take place at lower temperatures than would normally be expected in a conventional engine.

Increasing the concentration of oxygen molecules by using oxygen enriched air further assists the reaction process and ensures that the gaseous fuel oxidises quickly and as completely as possible. The ceramic substrate 8 ensures that more of the compression and reaction heat is retained in the cylinder 32, which again assists the thermal reactions.

By the time the piston 34 has reached its top position, as illustrated in Figure 2, the carbon and hydrogen molecules in the fuel have had a longer time to react with oxygen than in a normal compression ignition engine, and the reaction has been more controlled and more complete. This is due to the cumulative effect of early fuel ignition achieved by the method of the present invention, and sustained combustion which is aided by the increased availability of oxygen in the oxygen-enriched air. The peak temperature and pressure attained within the combustion chamber 42, at about this point in the combustion process, are both significantly lower than in a conventional compression ignition engine.

The pressure generated inside the combustion chamber 42, because of the combustion reactions, forces the piston 34 back down the cylinder 32 on its power stroke and the gas injection ceases, as shown in Figure 3. The excess oxygen and catalyst 44 continue to aid the reaction process, so that any gas remaining in the combustion chamber 42 is fully combusted. The catalyst 44 aids this continuing reaction process, even though the temperature starts to fall rapidly from its peak level once the piston 34 travels back down the cylinder 32.

The method according to the present invention provides a number of significant benefits relating to the efficient combustion of a gaseous hydrocarbon fuel in a compression ignition engine: More complete combustion of any pollutant byproducts which may otherwise be formed inside the combustion chamber during the thermal reaction processes is aided by the enriched oxygen atmosphere. More complete combustion is achieved by encouraging pollutant by-products to react both with each other and with oxygen to form relatively harmless gaseous products, such as nitrogen, water vapour and carbon dioxide.

Typical such undesirable by-products which can form during the combustion process include carbon monoxide and oxides of nitrogen. Some typical inter-reactions which can be assisted by the oxygen-enriched atmosphere and further assisted by a catalyst are shown below in simplified form: 2 NO + 2 CO = 2 CO2 + N2 2 NO2 + 4 CO = 4 CO2 + N2 2 CO + 02 = 2 CO2 In addition, the oxygen-enriched atmosphere encourages more complete combustion of the hydrocarbon fuel itself. The combustion process may be represented by the following reaction scheme in which the hydrocarbons are expressed as carbon and hydrogen molecules: C + 02 = C02 2 H2 + 02 = 2 H2O Any particulate matter, such as carbon-based particles, which may have formed during combustion will also tend to burn off better with the increased level of oxygen inside the combustion chamber.

As a result of the more complete combustion of any pollutant by-products and the hydrocarbon fuel, the exhaust gas stream will be relatively clean and contain a reduced level of pollutants when compared to the exhaust of a conventional naturally aspirated compression ignition engine.

The use of the oxygen-enriched atmosphere encourages the carbon and hydrogen in the gaseous hydrocarbon fuel to react quickly with oxygen molecules inside the engine cylinder and, thus under carefully controlled conditions, spontaneous ignition of the fuel may be achieved without the need for flame or spark ignition, or any other means to aid the initial ignition. In this way, the invention provides a method to allow conventional compression ignition engines to be able to burn gaseous hydrocarbon fuels which are not normally suitable for this type of engine without modification to the engine. The method of the invention may be further improved by incorporating a catalytic coating inside the combustion chamber, as mentioned above.

A particular problem associated with the use of a catalyst in an internal combustion engine is that the catalyst can become contaminated by combustion pollutants, such as carbon and sulphurous deposits which can form during the combustion process, or are caused by incomplete combustion. It has been found however, that by controlling the enriched oxygen atmosphere, even relatively small increases in oxygen concentration above normal levels, ensures that such deposits, particularly carbonaceous pollutants, are removed during the combustion process, thereby avoiding problems associated with catalyst poisoning.

Other general advantages of the invention include the more efficient engine operation conditions which may be achieved. For example, in the preferred embodiments, the concentrated oxygen atmosphere and catalyst combine to accelerate combustion and provide more controlled and complete oxidation reactions at lower temperatures than normal. Because of the longer and more efficient combustion, the peak cylinder temperature and pressure within the combustion chamber may be significantly reduced. This, coupled with the more effective fuel burn, may improve both the thermal efficiency and the fuel consumption of the engine whilst decreasing the mechanical strain on the engine. This allows the engine to be run, if necessary, at increased thermal densities and over a wider range of power outputs without imposing undue mechanical stress on the engine.

In addition, when a catalyst is employed, a ceramic substrate may be used as a support for the catalyst, and this helps to retain the heat of compression and reaction inside the combustion chamber, which again aids the combustion of the fuel and increases the thermal efficiency of the engine. At the same time, the ceramic layer shields the vulnerable metal parts of the engine from extreme temperatures and helps to protect them from thermal stress.

If a catalyst is not being used, a ceramic layer may nevertheless be incorporated into the combustion chamber to achieve the same advantageous effects.

Although the method of the present invention may be applied to transit applications, i. e. where the engine is used to propel vehicles, it is considered that it may also have greater potential in stationary applications, where engines may be used for example to produce power to generate electricity.

In such stationary applications, the compression ignition engine may be mechanically coupled to an electrical generating device and the hot exhaust gas may preferably be used to produce steam to drive a further electrical generating system. The invention enables a conventional compression ignition engine to efficiently utilise gaseous hydrocarbon fuels to generate electricity, thus providing an alternative to the use of either gas turbines or more complicated specially modified diesel engines.

It is possible to run an engine embodying the invention at higher than normal energy density to provide improved power output, without mechanically straining the engine. The concentration of pollutants in the exhaust gas stream may be maintained at much reduced levels, compared to diesel engines running conventional liquid fuels for power generation, reducing the need for post treatment of the exhaust gas. Both the gas fuel injection and the level of oxygen concentration may be controlled electronically, with the aid of sensors monitoring the engine operation and the exhaust gas stream to achieve optimum combustion.

The relatively small increase in oxygen concentration required to improve performance and allow spontaneous ignition of the gaseous fuel, particularly in the presence of a catalyst, means that more economical means of oxygen enrichment, such as hollow fibre gas separation membranes, may be used to produce the supply of oxygen enriched air required for efficient operation. However, any suitable means may be used to achieve the desired levels of oxygen enrichment according to the particular application, which will depend on the fuel to be used and other operating conditions, as well as a consideration of the economic factors involved. Where very large volumes of oxygen are needed, for example in high output stationary applications, alternative methods of oxygen concentration may be used to either supplement or replace the gas separation membranes. Alternatives include compressed or liquid oxygen units, cryogenic air separation and pressure swing adsorption systems.

The oxygen enriched air may conveniently be supplied to the air inlet valves on the engine via an adjustable valve which can, for example, vary the percentage oxygen in the air by mixing oxygen enriched air with atmospheric air. The adjustable valve may be linked to the sensors to control the degree of oxygen enrichment required, thus achieving optimum running conditions.

Oxygen could also be added, in desired concentrations, to the gaseous fuel itself which would reduce the level of oxygen enrichment required in the air supply to the engine. Thus in a further preferred embodiment of the invention, the oxygen enriched atmosphere is provided at least in part by supplying oxygen into the combustion chamber in admixture with the gaseous hydrocarbon fuel.

Although the system of the present invention is particularly suitable for gaseous fuels, it will be appreciated that it may also have other applications, for example in the combustion of other hydrocarbon fuels such as liquid fuels which burn with a clean profile, such liquid fuels being of animal, vegetable or mineral origin.

Although the invention has primarily been designed for use in conventional types of compression ignition engine, it could also be used inside diesel engines which have already been modified to burn gaseous fuels.

Modified engine types which could well benefit from the invention include those which use spark or glow plugs in either the main chamber or in a pre-ignition chamber.

The enriched oxygen combustion atmosphere would improve the thermal efficiency of these engines and reduce exhaust pollution levels.

Similarly, the system could have applications in other forms of internal combustion engines, such as spark ignition engines, which usually run on petrol fuel.

From the above it will be seen that the present invention, in particular, allows the combustion of gaseous hydrocarbon fuels, without the need for flame or spark ignition, in a conventional type of compression ignition engine, so that the power and heat produced can be harnessed efficiently to generate electricity.

It will be appreciated that the embodiment described above is merely exemplary and that various changes may be made thereto without departing from the scope of the invention. For example, while a fuel injection system has been described, it may also be possible to supply the fuel through a carburettor.

Claims

Claims 1. A method of combusting gaseous hydrocarbon fuels in a compression ignition engine, wherein an enriched oxygen atmosphere is provided in the combustion chamber of the engine.
2. A method as claimed in claim 1, wherein a catalyst is provided in the combustion chamber of said engine.
3. A method as claimed in claim 2 wherein said catalyst is supported on a ceramic substrate.
4. A method as claimed in claim 2 or claim 3 wherein said catalyst is a platinum group metal catalyst, or an alloy or combination of platinum group metal catalysts.
5. A method as claimed in any one of claims 2 to 4 wherein said catalyst is coated on the top of the piston and/or the base of the cylinder head in said combustion chamber.
6. A method as claimed in any preceding claim wherein the atmosphere is enriched with oxygen to above 0. Soi above normal (21.5% oxygen, 78.5% nitrogen).
7. A method as claimed in any preceding claim wherein the atmosphere is enriched with oxygen to between 0.5% and 2% above normal (21.5% oxygen, 78.5% nitrogen to 23% oxygen, 77% nitrogen).
8. A method as claimed in any preceding claim, wherein said oxygen enriched atmosphere is provided at least in part by supplying oxygen into the combustion chamber in admixture with the gaseous hydrocarbon fuel.
9. A method as claimed in any preceding claim, wherein the level of oxygen enrichment is controlled in dependence on an analysis of the engine operation conditions and/or the exhaust gas stream.
10. A method as claimed in any preceding claim, wherein the fuel is petroleum gas, natural gas, or a mixture of gases obtained from the gasification of organic or fossil matter.
11. A method as claimed in any preceding claim, wherein the fuel injection is controlled in dependence on an analysis of the engine operation conditions and the exhaust gas stream.
12. A method as claimed in any preceding claim, wherein the engine is coupled mechanically to an electrical power generating device.
13. A method as claimed in claim 12, wherein the hot exhaust gas is used to produce steam to drive a further electrical generator device.
14. A combustion system comprising a compression ignition engine, means for supplying an enriched oxygen atmosphere to the combustion chamber of said engine, and a means to supply a gaseous hydrocarbon fuel to said combustion chamber.
15. A system as claimed in claim 14 further comprising a catalyst provided in said combustion chamber.
16. A system as claimed in claim 15 wherein said catalyst is supported on a ceramic substrate.
17. A system as claimed in claim 15 or claim 16 wherein said catalyst is a platinum group metal catalyst, or an alloy or combination of platinum group metal catalysts.
18. A system as claimed in any one of claims 15 to 17 wherein said catalyst is coated on the top of the piston and the base of the cylinder head in said combustion chamber.
19. A system as claimed in any one of claims 15 to 18 wherein the atmosphere is enriched with oxygen to between 0.5% and 2% above normal (21.5% oxygen, 78.5% nitrogen to 230 oxygen, 77% nitrogen).
20. A system as claimed in any one of claims 15 to 19 further comprising means for supplying oxygen into the combustion chamber in admixture with the gaseous hydrocarbon fuel.
21. A system as claimed in any one of claims 15 to 20, wherein the level of oxygen enrichment is controlled in dependence on an analysis of the engine operation conditions and/or the exhaust gas stream.
22. A system as claimed in any one of claims 15 to 21, wherein the fuel is petroleum gas, natural gas, or a mixture of gases obtained from the gasification of organic or fossil matter.
23. A system as claimed in any one of claims 15 to 22, wherein the fuel injection is controlled in dependence on an analysis of the engine operation and the exhaust gas stream.
24. A system as claimed in any one of claims 15 to 23, wherein the engine is coupled mechanically to an electrical power generating device.
25. A system as claimed in any one of claims 15 to 24, wherein the hot exhaust gas is used to produce steam to drive a further electrical generator device.
26. A system as claimed in any one of claims 15 to 25, comprising a means for analysing the engine operation conditions and the exhaust gas produced and a means for controlling the concentration of oxygen in the air supply in dependence on the analysis.
27. A system as claimed in any one of claims 15 to 26, comprising a means for analysing the engine operation conditions and the exhaust gas and a means for controlling the fuel injection in dependence on the analysis.
28. An electrical power generating system comprising a generator coupled to at least one engine, said engine being adapted to combust gaseous fuels by means of an enriched oxygen atmosphere.