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Old 12-20-2020, 09:38 PM   #10 (permalink)
swineone
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So I went out for another drive today, paying attention to ignition advance -- PID 0x0E (hex)/14 (dec).

So indeed, at lower boost, ignition advance is higher (more degrees before TDC), while at higher boost, ignition advance is lower (fewer degrees before TDC). Either way, while pulsing, it's always positive (before TDC). I assume none of this is news.

So you claim ignition is retarded (pushed closer to TDC) to avoid knock, and that makes sense: if you ignite too early, there is the risk of the flame front meeting the piston head while it's still compressing rather than expanding.

What I'm trying to understand is how that "dumps part of the useful energy that could have been extracted from combustion, out the exhaust". I imagine early ignition, when the flame front meets a compressing piston, besides being destructive (through knocking), would also waste useful energy by pushing the piston head in the opposite direction of its movement, therefore doing negative work. But if the flame front meets the piston head when it's already expanding, then isn't that doing positive work as expected? Where exactly is useful energy being dumped out the exhaust?

Sorry if it's a stupid question, but I really want to understand this. It would help me understand why my car could have a BSFC sweet spot at part load, and why it would be better to pulse at lower boost pressures rather than higher ones.
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