Books.

A forewarning. I don't feel like I've explained it very well... Anyway:

What is ignition advance?
Igniting the mixture earlier, thus starting the combustion earlier. Advancing it is igniting the mixture while it is still being compressed, which the piston is already compressing the mixture, and now has to do that while the mixture is ignited (trying to push the piston down). This means the piston is working twice as hard, and the pressure inside the combustion chamber rises. Advancing the ignition means that the the peak pressure occurs earlier than before.

What are the effects?
Combustion starts earlier, and finishes earlier. It makes more power. The piston works harder. The mixture ignites while the piston is BTDC, which doesn’t help turn the crank, but there is a bigger push on the piston. Really high temperatures inside the engine.

What happens with too much advance?
You knacker your engine- your pistons can melt, high pressure can cause misfire or detonation (fuel exploding rather than igniting). Bearing wear out quicker.

How is this related to the fuel used?

When the engine is spinning faster, the fuel has less time to burn, so the advance needs to be started earlier and earlier as rpms increase. Is that what you mean?

Originally Posted by Sean_C

Matt-
Because there is such a long combustion chamber and low compression, the engine is much less efficient

I like the engine though, its very smooth

There is also a problem of "sealing" the chambers, and a problem with wear I think

Originally Posted by yorkie_chris

Why does long combustion chamber reduce efficiency then?

(p.s trying to get you to think about cause and effect rather than just "what happens when you do "x""...)

I'm not sure on this one Chris. All I can think of is that a long combustion chamber coupled with the fact that the fuel has to fully ignite to be an efficient engine is that with a long combustion chamber, the flame takes time to ignite the whole mixture. Which is why rotary engines have 2 spark plugs, to ignite the fuel/air mix quicker.
Care to enlighten me?

[yorkie_chris]

More wall area, the most efficient shape of combustion chamber in a perfect world is a sphere, hence the production of engines (2valve) with "hemi" heads, like a lot of V8's, and fords CVH engine. Produced by tilting the valves away from each other like this \ / (compound valve angle, hemisperical head ... CVH)

With wankel you've got lots of wall area, on which heat transfer can and does occur, reducing efficiency and power.

That's in addition to the problems with getting a good burn.

Grrr. That was obvious wasn't it.. Thanks YC.