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Old 10-09-2010, 07:06 AM   #65
It is what it is
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Joined: Aug 2010
Location: Wilmington, DE
Oddometer: 205
Originally Posted by johnjen
Yeah I learned about this on carbs. FI is different. Mostly because the particle size is much smaller with FI.

Has nothing to do with particle size. But, mixture balance between cylinders is crucial (more on that later). The boxer is a glorified aircraft engine. The concept is very important in piston aircraft engine operation, as the fuel consumption vs power is a major concern (Avgas is around $5 per gallon and a typical trip will burn 50-100 gallons). Imagine going from 30 gallons per hour (gph) to 18 gph but only reducing speed by 5 knots. That's the effect of operating the fuel/air mixture on the rich vs lean side of peak EGT.

Here is a graph showing various values of EGT, CHT (cylinder head temp), ICP (internal cylinder pressure) and BSFC (brake specific fuel consumption - in other words engine efficiency).

(source: John Deakin - Pelican's Perch article)

Note the three vertical dotted lines. The leftmost dotted line would be on the rich side of peak EGT. HP peaks on this side of peak EGT so this is where the engine should operate for max HP and torque (acceleration). However, fuel consumption sucks.

Moving to the right in mixture control (leaner mixture), the temps start to increase up to the middle dotted line (peak EGT) and so does fuel efficiency. Emissions are reduced. However, note that HP starts to decrease. The high cylinder pressure, combined with the higher temps, are not good for the engine, so this area is often referred to as the "red box", meaning you don't want to stay here for long periods of time.

As the mixture is further leaned past peak EGT, a funny thing happens, temps start to decrease. So does HP but fuel efficiency continues to improve. So, past peak EGT, leaning REDUCES temps. The most efficient engine operation is where BSFC is minimized (not in the graph it is inverted for easier viewing).

Here's the rub. All of this applies to each cylinder of the engine INDIVIDUALLY. Each cylinder doesn't care what the others are doing as the fuel/air charge is specific to that cylinder. Why is this important? If one cylinder is running significantly richer than the other, it's possible to have one operating at min BSFC (low EGT) while the other is at max EGT. That's bad. So, air/fuel mixture balance between cylinders is important.
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