Converting diesel engines:
Diesel engines are designed for continuous operation (10000 or more operating hours).
Basically, they are well-suited for conversion to biogas utilization according to either of two
In the dual fuel approach the diesel engine remains extensively unmodified, except for the
addition of a gas/air mixing chamber on the air-intake manifold (the air filter can be used as a
mixing chamber). The injected diesel fuel still ignites itself, while the amount injected is
automatically reduced by the speed governor, depending on how much biogas is injected
into the mixing chamber. The biogas supply is controlled by hand. The maximum biogas
intake must be kept below the point at which the engine begins to stutter. If that happens, the
governor gets too much biogas and has turned down the diesel intake to an extent that
ignition is no longer steady. Normally, 15-20% diesel is sufficient. As much as 80% of the
diesel fuel can thus be replaced by biogas. Any lower share of biogas can also be used,
since the governor automatically compensates with more diesel.
As a rule, dual-fuel diesels perform just as well as comparable engines operating on pure
diesel. As in normal diesel operation, the speed is controlled by an accelerator lever, and
load control is normally effected by hand, i.e. by adjusting the biogas valve (keeping in mind
the maximum acceptable biogas intake level). In case of frequent power changes at steady
speed, the biogas intake should be somewhat reduced to let the governor decrease the
diesel intake without transgressing the minimum diesel intake. Thus, the speed is kept
constant, even in case of power fluctuations. Important: No diesel engine should be
subjected to air-side control.
While special T-pieces or mixing chambers with a volume of 50 to 100% of the engine
cylinder volume can serve as the diesel / biogas mixing chamber, a proper mixing chamber
offers the advantage of more thorough mixing.
To sum up, conversion according to the dual-fuel method is:
• a quick & easy do-it-yourself technique;
• able to accommodate an unsteady supply of biogas;
• well-suited for steady operation, since a single manual adjustment will suffice and
• requires a minimum share of diesel to ensure ignition
Conversion to Spark Ignition (Otto cycle) involves the following permanent modifications
at the engine:
• removing the fuel-injection pump and nozzle
• adding an ignition distributor and an ignition coil with power supply (battery or
• installing spark plugs in place of the injection nozzles
• adding a gas mixing valve or carburetor
• adding a throttle control device
• reducing the compression ratio (ratio of the maximum to the minimum volume of the
space enclosed by the piston) to E=11-12
• observing the fact that, as a rule, engines with a pre-combustion or swirl chamber are
not suitable for such conversion.
Converting a diesel engine to a biogas-fueled spark-ignition engine is expensive and
complicated so that only pre-converted engines of that type should be procured.
Converting spark-ignition engines:
Converting a spark-ignition engine for biogas fueling requires replacement of the gasoline
carburettor with a mixing valve (pressure-controlled venturi type or with throttle). The spark-
ignition principle is retained, but should be advanced as necessary to account for slower
combustion (approx. 5°-10° crankshaft angle) and to avoid overheating of the exhaust valve
while precluding loss of energy due to still-combustible exhaust gases. The engine speed
should be limited to 3000 rpm for the same reason. As in the case of diesel-engine