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In Reply to: Engine Math...a little posted by pictureguy on April 19, 2021 at 16:20:41:
BTW, I got a little different numbers.
Displacement = pi * bore * stroke * number of cylinders
1 liter = 61.0237cc
If we assume "square" dimensions (bore = stroke), then a single cylinder of 1 liter displacement will have bore and stroke = sqrt(61.0237/pi) = 4.407 inches. For the 8 cylinder engine, it will be sqrt(61.0237/pi/8) = 1.558 inches.
So at the same RPM, the piston speed will be 2.83 times faster in the single cylinder engine (4.407/1.558). Or to put it another way, given the same piston speed, the eight cylinder engine can spin 2.83 times faster. So it can theoretically pump 2.83 times more air.
Piston speeds are mainly limited by metallurgy and the loads on the wrist pin, big end, and main bearings. The higher the piston speed, the faster the pistons have to accelerate, the greater the accelerating force.
The upper limit for a production sports car is a mean piston speed of about 25 m/s (984 in/s), achieved by the original Honda S2000 at 8900 RPM. Only race engines can move their pistons faster, and only by 5-10% if the engine has to run for hours. Top fuel dragster engines can go 20% higher.
Mean piston speed = 2 * stroke * RPM / 60
So if we assume a piston speed at redline of 984 in/s, our hypothetical single cylinder 1 liter race engine will have a redline of 6700 RPM. And our hypothetical eight cylinder 1 liter race engine will have a redline just shy of 19000 RPM.
Using a greater number of cylinders is one way to allow a higher redline for the same displacement. The other way is to make the cylinders over-square (bore larger than stroke).
But the valve train is a limiting factor. Coil springs might allow some valve float in our 1 liter 8 cylinder engine as it approaches it's high redline, so you probably couldn't make it oversquare and rev higher without pneumatic valves. Also, as you make the combustion chamber wider and flatter, the fuel/air mixture takes longer to burn. So the oversquare cylinders will require more ignition advance, and BMEP at high RPM will be lower.
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Follow Ups
- Piston speed is as much of a limit as BMEP - Dave_K 10:09:30 04/20/21 (1)
- RE: Piston speed is as much of a limit as BMEP - pictureguy 11:21:23 04/20/21 (0)
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