Hello Preston,
A piston and cylinder device, like an engine, converts pressure into useable work. A faster burning fuel, like gasoline, contains more energy on a volume basis, partly just because it is a liquid and woodgas is gas. However, a lot more of the energy in gasoline is unusable because it comes in the form a high pressure spike that the engine doesn’t know how to take advantage of. The two graphs may give you a rough idea of why this is true.
Now think about things that might cause the shape of the graph for gasoline to change. Have you ever noticed that engines seem to run better, and have more power on cold, wet days? This is because very small drops of water are going in with the air. These small drops absorb a lot of energy from the burning fuel and slow it down. Racers use this sometimes. They inject water to get more power.
Look at other fuels, like Methanol, Ethanol, Butanol, Propane, CNG…
You ask a good question. Keep studying the basic processes. The ‘devil is in the deails’, thats true, but the answers are there too.
Rindert