retarded spark makes heat

[fillibuster]

I'd sure like to hear a good explanation of why a retarded spark can cause overheating of an engine. Sounds like "everybody knows that", but darned few folks really know why and how. Heard a fair reasoning last night, but it was limited in "eloquence".
If you think you have a handle on it, please give it a try.

[c.o.]

It won't likely be good or even close to eloquent Phil but isn't it just a matter of advanced explosions in the combustion chamber should be favored over a delayed fire in the jug? Burned oil, friction and glowing valves.... scary stuff...

[Jack K]

I'll offer an explanation I saw on the antique engine forum several years ago. I saved that discussion to a file that was lost in a computer crash but I thought I had printed it,,, and I found it.

"When the engine fires normally, the fire burns completely (or nearly so) within the combustion chamber. The heat is kept within the combustion chamber, and as the pressure drops during the power stroke, the superheated gasses cool. As the power stroke occurs, the cylinder absorbs the heat, and when the exhaust valve opens, the burned cool mixture is blown out of the cylinder and combustion chamber.

When timing is retarded, peak pressure and heat is delayed, and is not confined to the combustion chamber. Delayed ignition causes the peak heat and pressure to occur in the cylinder body itself. The flame actually burns the lubricating oil off the cylinder walls, causing more friction. As the combusting mixture is not under high compression, it is less dense, again causing further delay in the burn. When the exhaust valve opens, extremely hot, still burning gasses are blown out the exhaust port past the valve. This flame heats the valve red hot and superheats the head and manifold."

I think the short answer is when firing is retarded you have a blaze in the cylinder rather than an explosion in the combustion chamber.

[Shaky Jake]

In very general terms, anything that makes an engine less efficient is probably going to make it run hotter. It takes a certain amount of power to push a bike down the road at a certain speed. An inefficient engine has to burn more fuel to make that amount of power than an efficient engine does. The extra heat from that extra fuel has to be rejected through the cooling fins or radiator, whichever the case may be, and the exhaust. That extra heat means higher temperatures. When the spark timing is optimized, the engine is more efficient, it burns less fuel, and it runs cooler. That's my simple minded way of looking at it. There may be exceptions.




Kevin


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[fillibuster]

Thanks Jack! I really appreciate your answer, and it makes sense.
I've also heard roundabout explanations of what happened to the energy produced? Well, ideally, most of it will force the piston down, leaving a small balance expended in heat and noise. With a late spark the energy is too late to force the piston efficiently, leaving a much larger balance of energy in heat to be absorbed by the cylinder.
.... but I like the way you said it better.