1. S. I. Engines
The only adoption for a spark ignition engine is a gas (not gasoline!) carburetor to work at the supply pressure (just like an LPG conversion, but an evaporator would not be needed as the storage pressure is low). It is also a good idea to scrub the H2S (as it causes corrosion) and to
derate the engine (unless you want to replace it each year if operating
Modification of S.I. Engine -
S.I. engines can run completely on biogas, however, the engines are required to be started on petrol at the beginning, conversion of S.I. engine for the entry of biogas, throttling of intake air & advancing the ignition timing. Biogas can be admitted to S.I. engine through the intake manifold & air flow control valve can be provided on the air cleaner pipe connecting air cleaner & carburetor for throttling the intake air as shown in fig.
2. C.I.Engine:-
Diesel engines also need a gas carburetor and scrubbing, but require at least 10% diesel via the injectors for ignition (and cooling). The initial starting of diesel engine is done on pure diesel
Modification of C.I. Engine:–
C.I. engine can operate on dual fuel & the necessary engine modification include provision for the entry of biogas with intake air, provision of carburetor & system to reduce diesel supply, advanced injection timing. The entry of biogas and mixing of gas with intake air can be achieved by providing the mixing chamber below the air cleaner which facilitate through mixing of biogas with air before entering into the cylinder. The arrangement is shown in fig. is largely used in stationary engine commercially available in India. The capacity of mixing chamber may be kept equal to the engine displacement volume. The pilot injection of cycle is required to be advanced for smooth and efficient running of engine on dual fuel. The admittance of biogas into the engine at the initial stage increases engine speed and therefore a suitable system reduces the diesel supply by actuating the control rack needs to be incorporated.
There is a wide range of thoughts on what treatments should these biogases be subjected to before being used as fuel. Most operators simply remove the water present in the biogas, leaving it to the engine manufacturers to design engines which will cope with the impurities inevitably included in the biogas (significant maintenance costs); other Operators are seriously evaluating maintenance costs against initial investments in biogas clean up technologies such as has been developed by Acrion Technologies (although Acrion's technologies are mainly aimed at biogas contaminant removal and separation into methane and carbon dioxide as feed stocks for a variety of commercial applications).

To use the biogas as a fuel in SI engine there are some practical difficulties. It is not possible to compress the methane, separated from biogas by available method, because the gas could be liquefied through chilling below -161 0C.
This process is adapted by installing the units required when there use of methane separated from biogas as a fuel. Since gas can not be compressed it requires large space for storage.

1. In purification method, by reducing CO2 and moisture along H2S impurities in biogas, the engine performance is improved.
2. Effect of spark timing :-
Biogas is slow burning fuel. Hence in order to get optimum engine performance, spark timing does not advance, and then combustion continues in major part of the expansion stroke. This reduces effective work done. By advancing, spark timing power is improved on low speed at partial throttle condition as well as high speed at full throttle condition.

The exhaust emission contains three specific substances which contribute the air pollution, hydrocarbon, carbon monoxide &oxides of nitrogen. Hydrocarbons are the unburned fuel vapour coming out with the exhaust due to incomplete combustion. Hydrocarbon also occurring in crankcase by fuel evaporation. The emission of hydrocarbon is closely related to many design &operating factors like induction system, combustion chamber design, air fuel ratio, speed, load. Lean mixture lower hydrocarbon emission.
Carbon monoxide occurs only in engine exhaust. It is the product of incomplete combustion due to insufficient amount of air in air- fuel mixture. Some amount of CO is always present in the exhaust even at lean mixture. When the throttle is closed to reduce air supply at the time of starting the vehicle, maximum amount of CO is produced.
Oxides of nitrogen are the combination of nitric oxide & nitrogen oxide &availability of oxygen are the two main reasons for the formation of oxides of nitrogen. The spark advance means lower peak combustion temperature. It causes high NO concentration in the exhaust. With biogas, co emission levels are low than that of gasoline.