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 joined with steady speed, the
biogas fraction should be reduced somewhat to let the governer decrease the diesel intake without
transgressing the minimum amount. Thus, the speed is kept constant, even in case of power
cycling. Important: No diesel engine should be subjected to air-side control.
While special T-pieces or mixing chambers with 0.5 to 1.0 times the engine displacement can serve
as the diesel/biogas mixing chamber, at which a true mixing chamber offers the advantage of more
Conversion according to the dual-fuel method is evaluated as follows
- a quick & easy do-it-yourself technique
- will accommodate an unsteady supply of biogas
- well-suited for steady operation, since a single manual adjustment will suffice
- requires a minimum share of diesel to ensure ignition.
Conversion to spark ignition (Otto cycle)
involves the following permanent alterations to the engine:
- removing the fuel-injection pump and nozzle
- adding an ignition distributor and an ignition coil with power supply (battery or dynamo)
- 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 to E = 11-12
- observing the fact that, as a rule, engines with a precombustion or swirl chamber are not
suitable for such conversion.
Converting a diesel engine to a biogas-fueled spark-ignition engine is very expensive and
complicated - so much so, that only preconverted engines of that type should be procured.
Converting spark-ignition engines
Converting a spark-ignition engine for biogas fueling requires replacement of the gasoline
carburetor 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
r.p.m. for the same reason. As in the case of diesel-engine conversion, a simple mixing chamber
should normally suffice for continuous operation at a steady speed. In addition, however, the mixing
chamber should be equipped with a hand-operated air-side control valve for use in adjusting the
air/fuel ratio (opt. d = 1.1).