Producer gas in power generation
Practical Action
Mixing valve
Efficient engine operation requires adequate adjustment of the proportion of air and gas, and
the quantity of the resulting mixture reaching the engine. In its simplest form the valve
consists of a Y-piece. The gas enters through one branch, the air from an air cleaner through
the second branch, and the resulting mixture flows through the stem to the existing engine
induction system between the carburettor and the induction manifold. The admission of air is
controlled by a butterfly valve in the air branch.
Some design refinements
While the preference would be to use purpose designed gas engines it is possible to use
producer gas as a fuel for either spark ignition (SI) and compression ignition (CI) engines.
However, as producer gas cannot easily be liquefied it has to be introduced by means of a
gas carburettor and is present during the compression stroke for both types of engine. In the
SI engine the gas is ignited (as for petrol) by a high voltage spark but in the CI engine
ignition is achieved by injecting a small amount of diesel into the cylinder at the end of the
compression stroke. Modern gas engines are available which are similar in design to CI
engines (for strength and durability) but use a spark plug or glow plug to provide ignition for
the gas
Spark ignition engines are cheap, if based on mass-produced automotive engines, and are
simple to operate but are sensitive to changes in gas quality. Little modification is required to
an automotive engine other than changing the carburettor for one suitable for use with gas
and hardening the valves and valve seats. These engines can be operated on 100% producer
gas achieving efficiencies of around 25% at full load but this can fall off rapidly when
operating at part loads.
Consideration also needs to be given to the ignition timing of the engine. Typically there is a
greater ignition delay with producer gas compared with petrol. Therefore the ignition point
(the crank angle at which the spark occurs) of a SI engine is usually advanced when a
gaseous fuel is used to ensure optimum performance.
The CI engine is much more complex to operate and will not work on biogas or producer gas
alone. If biogas were admitted in anything like the stoichiometric ratio, it could
spontaneously ignite during the compression stroke. The engine would then ‘knock’, run
unevenly, and overheat very quickly. Therefore the gas can only be admitted to the engine at
quite high air/fuel ratios grater than 30:1 (by mass). The exact ratio will be dependant on the
design of the engine and its compression ratio.
Most large, medium and low speed engines normally operate as diesel engines with an
air/fuel ratio of around 30:1 and so it is possible to operate them on 90% gas and 10%
diesel especially if they have relatively low compression rations of around 14:1. Smaller, high
speed CI engines (auto derivative) are normally rated with relatively low air/fuel ratios of 20:1
and compression ratios of between 16 and 18:1. These engines require much more diesel
fuel injected at maximum load some times as much as 60%. Compression ignition engines
can operate on all ratios of producer gas/diesel oil, which can be desirable when producer gas
production is subject to fluctuations.
Compression ignition engines are more expensive but are less sensitive to changes in gas
quality and have better efficiencies (30 and 35%), which can be maintained through out
regardless of load.
Spark and compression ignition engines will require modifications to the valves, valve seats
and carburettor before they can be used with producer gas. The dry nature of producer gas
means that it does not have lubricating properties and therefore valve wear is more likely to
occur. The materials used for the valves and valve seats will need to be modified (i.e.
hardened) or changed to improve wear resistance.
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