Quote:
Originally Posted by mpgmike
That makes my point perfectly! It isn't enough to simply burn the fuel, it really matters when it burns in the combustion cycle, and how quickly. Burning out the exhaust manifold contributes absolutely nothing towards making power at the wheels. However, the emissions analyzer will show that it got burned.
Engineers use the term Critical Crank Angle (CCA) to denote the optimal phase of crank angle where highest peak cylinder pressures do the most good; usually 17-18 degrees ATDC. With a slow burning charge, it takes more ignition timing advance to build pressures by CCA (which can have disastrous side effects). Faster flame propagation and oxidation rates allow more conservative ignition timing while still delivering the goods in time. Furthermore, a faster burn rate is more likely to oxidize more fuel within the combustion cycle than a slow burn. In other words, a faster burn is able to release more of the chemical energy within the "magic window" when the engine can convert it to usable kinetic energy.
So your observation that it is not an issue of unburned fuel is correct; it's an issue of when-burned fuel.
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What evidence do you have that any significant amount of fuel is still burning after it exits the exhaust manifold? It's under considerably higher pressure and temperature prior to this.
I'm quite familiar with CCA - I write ignition tables as a hobby. 17-18 degrees ATDC is for more conventional engine geometries - most engines made in the last 20 years have offset crankshafts, which alters that considerably. Add to that, modern combustion chambers are
much faster than older designs. I believe the Insight's stock 3 cylinder would actually go into the negatives at times with ignition timing. When leaning the charge out to a 25:1 AFR, the additional ignition timing at part throttle was typically around 20-25 degrees extra advance due to the lower flame speed, but it was still only in the high 20's or maybe even 30 degrees, whereas many other older engines would be in the 40's, up to 50-60 degrees of ignition advance at low loads.
This is an engine that achieves near 40% thermal efficiency over a pretty broad powerband. Toyota's new "Dynamic Force" branded engines achieve similar ~40% thermal efficiency numbers, and these exist in just about all of their vehicles now, including the Tacoma, Corolla, etc. If you read through Toyota's publications, they explicitly state that while combustion is effectively "complete" even in older designs, where the low hanging fruit is, is in speeding it up, so more of the energy is released when the crank angle is favorable, and the pressure wave isn't chasing the piston down the bore.
Again, I'm calling misinformation here.