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I understand the concept of SFC, but after a few tanks of gas and use of P&G but no ridiculous improvements in fuel mileage, I was wondering what the hell was going on. Then it occured to me that the isopleths on the BSFC graph (that I was trying to emulate in my P&G technique) are of fuel flow rate per hp produced. Which I had read but didn't "compute" in my mind. I can't directly determine what the BMEP of my engine is at any given time, so I goto the SFC vs RPM chart with the load curves. I was able to find a dyno run for a 4 cylinder Camry (believe it or not), so to get an idea of fuel flow rate I plugged in the hp number for given RPM numbers for different load lines. Lo and behold that at less-than-ideal RPM and throttle settings, I can easily reduce the rate of fuel flow. In other words, at 2,000 RPM on the 50% load line I can use less fuel per hour than at 2,500 RPM at 100% load. Therein lies the point - to minimize fuel use. I can maximize efficiency by operating in certain load+RPM combinations, but in doing so can actually use more fuel in total. And the point is to use the least amount of fuel for the most amount of miles - this is why idling (which is less efficient) can improve you rmileage. I'm not sure I can hit crazy MPG numbers by worrying about throttle angle and RPM alone - the other variable is the HP created. This defines the fuel used. <sigh> I think I need a scangauge. |
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As for the leanburn vs P&G argument...
To me, the answer seems simple. Leanburn is obviously there to compensate for poor engine efficiency at light throttle by leaning the engine, which reduces power, which requires you to give it a bit more throttle to maintain power... which reduces pumping losses in addition to being a slightly more efficient burn. But, it also seems obvious that the engine produces more output energy per lb of fuel at high load (low rpm obviously) So, the answer is to use them according to driving conditions. If you can P&G, DO IT! If you're not in a position to do so, you may need to DWL for a while, and leanburn is better for that than non-leanburn. |
Honestly, thermal efficiency is by far the biggest player in all of this.
At part throttle there isn't much air in the chamber. That small amount of air doesn't have a lot of heat in it when you ignite it so more of the heat goes to the cylinder walls instead of pushing the piston back down, it burns slower because it isn't as dense making for timing advances that rob the engine of efficiency and at part throttle it's working against a vacuum, a partial parasitic loss only if there is enough vacuum in the chamber left to pull the piston up on the bottom portion of the compression stroke. At WOT you have way more air in the chamber. The larger amount of air has much more heat that would have to be absorbed into the engine, it does get absorbed but the ratio of heat created to amount absorbed plummets as engine speed and throttle increases. Since there is more heat in the chamber the pressure stays higher longer and can actually perform work. The air is very dense when you get close to the top of the compression stroke and results in a faster, better burn requiring less timing advance and increasing efficiency. At WOT you have MUCH less loss from pumping on the intake stroke but you end up with more losses on the compression stroke as has been mentioned, however, when you are compressing the gases you are building potential for the event happening in the power stroke. Not just throwing it away. |
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I think one of the interesting things about open-loop is that it's something my car generally doesn't do, as far as I can tell. I think a characteristic of wideband-sensor systems is that open loop occurs only when the engine is cold, or when there's some kind of a sensor failure. But I understand why avoiding WOT makes sense in your situation. Quote:
Coming out of open loop, I could see how your car's brain (ECU or ECM or whatever VW calls it) could choose to lag and hold open loop for a few extra seconds, for good reasons. But I would think that going into open loop would be immediate, when you floor it under certain conditions. After all, the idea is to give you good throttle response, and provide the power you're demanding. Also to prevent an overly lean mixture from hurting your motor. So it should happen without a lag, I think. Quote:
EOC is great, but it requires very specific driving conditions. |
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If you were correct that there is "more resistance at WOT," then it would be possible to maximize engine braking on a gas engine by using WOT (with the injectors off, of course). With current drive-by-wire systems, this could be easily accomplished. I wonder why no one has ever built an engine that works this way. Then again, maybe you know of one? Aside from all that, I have no idea what point you're trying to make by mentioning diesels and Jake brakes. Quote:
On the other hand, the energy that goes into fighting the throttle restriction is waste. We don't get it back. Quote:
If you monitored your lean burn, I imagine that you would notice things that would be helpful to the rest of us. Since you're not monitoring your lean burn, I have a hunch that there are quite a few moments when you think you're in lean burn, but you're not. It's very, very sensitive to throttle position; you can rapidly move from lean to rich with a very small increase in throttle angle. Quote:
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Somewhere I saw a photo of someone who rigged a hand throttle, for this purpose, essentially. You would need that kind of precise control to really be able keep the throttle in exactly the best position to maintain lean burn. Quote:
I think you're oversimplifying the relationship between lean-burn behavior and SIL behavior. Quote:
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Not much else, that I'm aware of. |
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