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Old 01-27-2010, 12:50 PM   #25 (permalink)
jfitzpat
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Quote:
Originally Posted by tpwalsh View Post
Sorry about the confusion, normally "going lean" is refering to lamba's over 1, or over 14.7:1.
Actually, that is the myth, once you go past 14.7, temps plummet.

Quote:
Originally Posted by tpwalsh View Post
One instance of this is the 09+ Subaru STi's. They use a fuel cutout rev limiter. In the autocross world where I come from this is a very bad thing. When the fuel gets cut out, the mixture goes from "safe" in the 12.5:1 range, to lean, nearing 14.7:1, which promotes detonation. Normally this isn't a big deal in roadracing, or on the street, since when you hit the limiter, you shift. In autocrossing we've been known to ride the limiter for up to a second, or a second and a half, since shifting would actually make the car go slower. During this time, you're you high boost and load levels but leaner than the safe range. This has caused detonation which has destroyed multiple multiple engines in under 10,000 miles.
Look at your instance. The problem isn't that the mixture is leaner than lambda 1.0, but *just rich* of stoich ('nearing 14.7'), where CHTs and pressure soar.

Remember, stoich is the peak thermal reaction. The most heat that can be released from a quantity of fuel. As you go leaner, there is less fuel and the flame front alters, so temps drop. As you go just richer, there is still plenty of fuel, but you get a slower front and longer burn, so pressures soar.

As you continue to enrichen, the flame front continues to slow, but it also changes compositionally. Simply put, we get more exhaust gas pressure, but released more slowly. So, the peak pressures are lower, and more of the released energy goes into work (pushing the piston), then into heating the cyl head.

You see the myth at it's worst in general aviation. Literally, you manage mixture with a red knob and an EGT gauge (or, in some cases, engine roughness!). A lot of times, 'economy cruise' will be peak EGT, but people think that 'lean is bad', so they peak, then just go a bit richer. But this is the worst place to run the engine for CHT's and detonation. Better to go 25 degrees lean of peak (if your engine can do so without running rough) and get great economy, or go 100 degrees richer, and get terrible economy and best power, but either way, CHTs are a lot lower.

You can actually get away with running in the 'hot box' on a normally aspirated aircraft engine, because the air outside is already getting thinner. But, if you want to maximize engine life and reliability, it is not the place to run.

-jjf

P.S. Virtually any MY '96 and newer vehicle can be datalogged for basic ECU params.
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