Wood Gas Engine Selection

If you're considering building your own wood gas generator for running a car or electricity generation here's how to select an appropriate internal combustion engine for the job.

Most internal combustion engines can be made to run on wood gas. Both diesel engines and gas/petrol engines are suitable but some are better suited than others…

Running on wood gas, engines are de-rated compared to their original fuel. With diesels on wood gas you can expect around 85% of original horsepower and for gas/petrol engines it may be as low as 60%. Diesel engines will require a small supply of diesel to be maintained – generally the amount that is used at idle. Petrol/gas engines can be started and run on wood gas alone.

Fuelled by wood gas, engines best suited for self-sufficiency projects would be a conventionally carbureted, petrol/gas engine built prior to computerized engine management and overly complicated emission control plumbing. The engines are cheap and being older, the internal tolerances are not as exacting so they will handle a little more tar should a malfunction occur within the gasifier or the gas cleaning train. Low-tech is the real key to self-sufficiency!

Wood gas is a lower-energy fuel than petrol/gasoline and tends to burn a little slower in the combustion chamber - hence the de-rating. To overcome the de-rating it is wise to select the “right” configuration for engines. The ideal compression ratio range for wood gas engines is between 11:1 and 14:1. Tests have shown that wood gas is fine at a compression ratio as high as 16:1.

To get the best from wood gas engines seek high compression ratios.

Because wood gas burns a little slower there a 2 things to do:

1.. Select an engine that is designed to produce more power (higher torque) at low r.p.m. Generally, “under square” motors are the most suitable. “Under square” engines have less bore than stroke and by nature are not particularly high revving motors. 2500 rpm is regarded as the rpm ceiling for wood gas engines although in practice they do rev higher. The point here is that high piston speeds on the power stroke can’t be matched by the wood gas flame front speed!

2.. Advance the spark a few degrees. Most wood gas powered cars have been modified so that ignition can be advanced from inside the cab using a mechanism that rotates the distributor – the best setting is often found by ear and throttle response!