For inspiration, here's us running a 10kW Honda V Twin gen set recently:
Characteristics of the perfect wood gas engine / genset:
Spark ignition otto cycle engine so do not have to fuss with the dual fuel diesel scenario.
Slower RPM so the piston is not outrunning the slow flame speed of wood gas
Adjustable timing so can advance 10-15deg from typical gasoline timing.
High compression to take advantage of the detonation resistance of wood gas. Somewhere in the 13:1 - 15:1 range appears ideal on spark fire engine
Quality construction for long life. The lower the rpm, the longer the life expectancy usually.
Ubiquity of manufacture and distribution, so parts and knowledge are easily accessible.
The usual cheap DIY options:
Home Depot type 3600rpm generators are where most start, but quickly try to get away from. Yes, you can run one of these on woodgas, but there are many problems. First and foremost is the rpm of the engine is too high. Wood gas has a very slow flame front and 3600rpm is on the edge of the piston outrunning the flame speed. 2500-2700rpm is usually considered the tops for continuous woodgas operation. These cheapie gensets are claimed to be 500hr engines, but often keel over much before. And while still extant, most of them will rattle your brain enough to encourage you to find another option well before 500 hrs. All and all, better to have a slower rpm engine or reasonable quality and configuration.
Listers are heavy and of variable quality, though beautiful and a pleasure to have around. I have run a 6/1 (6 HP, 1 cylinder) on woodgas many times, but the speed control in dual fuel is sensitive and buffering the gas flow with a pony tank is advised. A 12/2 is a much better choice if you can get one.
China Diesels are of variable quality and again the dual fuel is sensitive. However, they are cheap. Conversion to spark or other ignition scheme is suggested.
The Redstone diesel looks interesting as a base, but still requires spark conversion and is heavy.
Dual fuel is not turning out to be very reasonable at the small scale. So you can either start with a spark engine and have lower than ideal compression ratios, or start with a direct injection diesel engine and convert to spark.
An outlier in the "convert a diesel realm" is the detroit diesel 2-71. these were common heavy industrial gensets, of near eternal life expectancy. Most interestingly for woodgas, they are two stroke, thus do not have intake valves to foul. They do, however, have a blower, and that has bearings that might be foulable. Not sure if the pressure side of the blower would encourage tarry gas into the bearing housing. Here's one dealer of these: http://affordablepower.com/Generators.htm
The best DIY options:
a common 4 cylinder small car gas engine, run at 1800rpm, into a 4 pole ST-style generator head. Common inline-6 and V8 engines will do well for larger sizes.
the common v-twin larger Honda engine, run at 1800rpm, into the same 4 pole generator head. It will run at half its usual speed, so you'll need 2x the size rating. Or, this amount of lowered rpm may be too much for the honda cam to still be in its reasonable power band. Thus a belt drive with a ratio that keeps the engine in the 2200-2700rpm range might be more appropriate. This is often how smaller engines are made into lower rpm gensets. This is the common mode in RV gensets, for instance.
Either of these scenarios can be set up with pulley drive or direct Lovejoy drive. Regular V pulleys are likely better for developed world. Lovejoy direct drive is likely better for places where they can be easily sourced. We've done both and are not strongly convinced of one method or the other. The V belt drive creates the option for ratio changes so the engine can be set to run in its desired zone, not just 1800 or 3600.
Here's how we currently combine all this into an integrated gasifier / genset:
Natural gas or LP gensets in the lower RPM variety are a reasonable match. You do not need the LP or natural gas mixer, as the woodgas requires a different set up, but the rest of the engine is well suited to your needs. There are often 2200 RPM LP generators for RV use. Then there are proper 1800 RPM LP and natural gas generators for industrial use, which will often have higher compression. Natural gas and LP are more detonation resistant than gasoline. See here for our inventory of lp / nat gas genset models and sellers of interest.
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