Optimum piston speed?

[Frank Lee]

FYI I went up TWO sizes and the suspension remains stock. It ain't no monster mudder.

[George Tyler]

Originally Posted by Duffman

My old Thermodynamics professor use to say heat transfer is a slow process. I dont think its why.

think of a model airplain engine, if you turn it over fairly slowly while holding the prop, you feel a pulse as it fires, but that pulse is done in about 1/5th of a second, all the heat has been absorbed by the head , piston and cylinder in that time.

[George Tyler]

remember that those efficiency figues will be with wide open throttle, I don't think any of us would be running at 1000 RPM and wide open throttle for long, and this will change everything.

[Duffman]

Originally Posted by Frank Lee

VE is nice and all, but I was after the bsfc peak... the reason for an optimal piston speed has been mentioned earlier in the thread (thanks guys ) but I resurrected it just cuz I found this:

Quote:
Sfc measures how efficiently an engine is converting fuel into work. The graph of an engine's sfc plotted against engine RPM will be "U" shaped. At high speeds the sfc increases due to increased friction, while at lower speeds the sfc also increases due to higher heat losses from the combusted gas to the cylinder and piston wall because of the increased time the hot gases remain in the cylinder. This is also the reason why air cooled engines are usually less fuel efficient than water cooled engines.

Wow its been a long time since I posted in this thread. Frank I think your explanation of the U shape is oversimplified at least with respect to the lower RPM. The curve is U shaped in part because the torque curves in general are inverse U shaped. Its been over a decade since I learned the term "mechanical efficiency" wrt to engines but... The maximum torque is like the engines maximum potential, when the engine is operating below that it still carries the frictional losses associated with that RPM but its out put is less so it is mechanically less efficient.

[Frank Lee]

That's not my explanation but I've bought into it. They are saying why it's an inverse u.

WRT friction, I would think internal friction has more variables than just plain ol' rpm; I suspect as loading gets higher on the piston it slams harder onto the cylinder on the thrust side thus having more friction, plus the higher chamber pressures are going to drive the rings onto the cylinder walls harder.

[Duffman]

I agree with your last comment on friction too but bearing friction is small compared to ring friction and valvetrain friction is independent of engine load. Until recently though it was typical that the bsfc curve tended to correlate well with the torque curve. Newer engines with VVT have changed that somewhat.

[gone-ot]

Originally Posted by Duffman

...Until recently though it was typical that the bsfc curve tended to correlate well with the torque curve. Newer engines with VVT have changed that somewhat.

...agreed; VVT changes (pun intended) how the engine both BREATHES and EXHAUSTS in respect to both load and speed.

[George Tyler]

Originally Posted by Duffman

Wow its been a long time since I posted in this thread. Frank I think your explanation of the U shape is oversimplified at least with respect to the lower RPM. The curve is U shaped in part because the torque curves in general are inverse U shaped. Its been over a decade since I learned the term "mechanical efficiency" wrt to engines but... The maximum torque is like the engines maximum potential, when the engine is operating below that it still carries the frictional losses associated with that RPM but its out put is less so it is mechanically less efficient.

yes, true, and your consumption does not relate to this as you only open the throttle to get the torue you need, so that chops the top off the U.

[Duffman]

Yeah but if you made torque curves at 75%, 50% and 25% throttles the shapes would be similar to WOT, Obviously though less throttle would taper off faster at high RPM.

Talking about BSFC curves becomes pointless at low throttle openings, if your throttle is nearly closed then you are geared wrong and barking up the wrong tree to begin with.

[Frank Lee]

I wonder how much throttle opening those 30mpg Vettes are running?