Quote:
Originally Posted by tpwalsh
This is an interesting statement. I come from the performance world, and especially in turbo motors, "going lean" is a recipe for disaster. Is it only under high load and RPM conditions that going lean is a problem, but in light loading situations when economy is the target, it's not as much of an issue?
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Actually, this was the confusion that I was trying to explain. "Going lean" in relationship to what?
For economy reasons, you would generally want to be leaner than a stock street car runs normally. They run typically at stoich (peak EGT), not for economy, but for emissions.
Best economy is leaner. There is a myth that leaner than EGT burns up engines, this is unquestionably false (this is what I'm referring to in the original post). The problem is that emissions would go up and cat efficiency would plummet.
In PERFORMANCE, we aren't running closed loop at stoich (peak EGT). When you stomp on the gas, the ECU on even a street car will go open loop and add fuel.
Stock tunes are too rich for peak performance. They pour on fuel to avoid heat and detonation. So, for max performance, you need to lean out the stock tune. The danger is when you (or more specifically, any cyl) get leaner than best power, but still richer than stoich. This is when all the ingredients for detonation, heat, fuel, and pressure, are at their highest.
This has led some performance folks to believe that you 'can't be too rich', but this is not true either. The basis of the myth is that they think the extra fuel is acting as a coolant. But this makes no sense in the physical world as we understand it. If you look at the *amounts* of fuel we are talking about, and the latent heats involved, this sort of 'cooling' by going rich would be like tinkling on a raging forest fire.
The reason that going rich helps with cooling is that it changes the speed of the reaction. That is, the flame front actually travels more slowly through the mix. This moves the point of peak pressure, helping less energy be transferred to the cyl heads as heat, and more put out the exhaust as heat. In the normal triangle, this leaves work (basically the energy goes into useful work, heat out the exhaust, or heat in the cyl heads). As we move from peak EGT and get richer, we initially get more work. Somewhere around 125-150 degrees rich of peak is typically best power.
This is because we are looking for the best combination of thermal release, exhaust gasses, and mechanical advantage pushing down.
But, as we continue to get rich, we start to lose power, for no real benefit in heat or detonation avoidance. Much the way a stock tune errors too rich for best performance to begin with! So, for performance reasons, you can be too rich.
My other point was that from an eco point of view, it is too rich as well. Ideally, we want to run as tightly to stoich as we can, as much as we can. This is primarily from our driving habits. But, when we can't - say, hilly terrain, we want the engine to use the excess fuel as efficiently as possible. In other words, performance folks want rich operation power efficient for max speed, etc. Eco folks want rich operation power efficient for *least demand of it*, it is better to be able to ease off the pedal and keep going up the hill than keep it down.
I hope that is clearer.
-jjf