Often people will try to calculate the flow in a wood gas system by doing some variation of multiplying the engine displacement by the operating rpm. I feel this is fundamentally wrong because it assumes the gas density through the engine is constant. Because the manifold vacuum changes we know it isn’t. Consider the following example:
Imagine your truck in your driveway and you rev the engine to 2000rpm. It will use a certain fuel flow rate. Now imagine your truck loaded with rocks in the bed, towing a trailer full of cement up an incline at 2000rpm in fourth gear. Will it use the same amount of fuel to turn 2000rpm? Of course not. The loaded case will require more energy input because you are lifting a load on top of overcoming internal friction(among other things).
The better way to consider the problem is to say, “How much wood gas gives the same energy input as my petrol consumption rate?”. That is what the spreadsheet below does. To my non-US readers, sorry about the US customary units. Feel free to change the input values and try different scenarios.
Please note: Friction is not considered. This spreadsheet WILL still underestimate the flow for generators. For a 389cc generator running at 3600rpm with no load roughly 10cfm is still needed to overcome friction on top of what is indicated.