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Old 01-05-2011, 11:17 PM   #22 (permalink)
t vago
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Quote:
Originally Posted by IamIan View Post
Just wanted to add one more bit I didn't notice mentioned previously.

Sense Leaner Air to Fuel Ratios burn slower ... some engines that make use of significant Lean Burn modes ( like the Gen-1 Insight ) ... also tend to lower the ICEs operating RPMs during high Lean Burn operations ... lower RPMs = less friction.

Attached is part of one EPA test describing both the BSFC already discussed and the friction benefits as well.
Book alert: The reason why lean burn is not done at higher RPMs is because combustion of the air fuel mixture is more complete than at richer AFRs.

At stoich or richer, during a combustion event, there's a small layer of air-fuel mix, hugging the combustion chamber and piston surfaces, that remains unburnt. This serves as an thermally insulating layer against the high temperatures associated with combustion, and is considered a good thing for this reason. Now, disrupt this layer, and the surfaces will now be directly exposed to those high temperatures (which are normally much higher than the melting point of aluminum alloys used for pistons and cylinder heads).

If the surfaces are not allowed sufficient time to cool off (say, when the heat energy moves from the metal surface into the meat of the metal engine, and then into the coolant and oil), the surface temperatures rise to the point where air-fuel mix will spontaneously ignite once it reaches those hot surfaces. This is called pre-ignition. If this pre-ignition occurs while the cylinder is in its compression stage, the air-fuel mix will combust and raise the temperature of the now-ignited charge just sucked in, and the temperature will then be further raised by the compression itself (basic thermodynamics). This will make the surfaces that much hotter, and if this is left unchecked, will quickly destroy an engine.

Unfortunately, lean burn tends to strip away this small insulating layer of unburnt air-fuel mixture simply because of the fact that the combustion process tends to be more complete. Usually, though, this in itself is not enough to cause problem, because at lower RPMs, the high surface temperatures can usually propagate through the metal surfaces into the coolant. However, sharp edges and carbon deposits may tend to become much hotter anyway, and this will lead to pre-ignition.

Now, there's no real way to sense for pre-ignition, but we can sense for a related bad thing called detonation. This is where air-fuel mixture actually explodes instead of burning normally. The explosion causes the flame front to move faster than the speed of sound, and the shock waves created by this flame front impact on the combustion chamber surfaces, disrupting that small insulating layer previously mentioned. Detonation can be heard as a pinging sound, or the sound as though rocks were being thrown against the engine. The engine computer can detect detonation, and enrich fueling and retard spark advance to compensate.

For lean burn engines, detonation would typically occur as a result of pre-ignition, and at lower RPMs, the engine computer can react in time to prevent engine damage. Combustion chamber surface temperatures as a whole won't become high enough to melt.

However, at higher RPMs, even the engine computer can't sense detonation in time to prevent engine damage. Once a lean-burn system detonates at high RPMs, it's too late. You just melted something.

That's why lean burn is not recommended at high RPMs.
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