Engine efficiency

[drmiller100]

Originally Posted by Frank Lee

Don't be fooled about "engine speed" by only looking at crankshaft rpms:

Most powerful diesel in the world: 98" stroke at 102 rpm = 1,666 ft/mn piston speed

Honda 50cc Super Cub at max fuel efficiency speed: 1.63" stroke at 4300 rpm = 1,168 ft/mn piston speed.

Hmmmm.

Yes one is a supercharged two-stroke diesel and one is a four stroke gasser. Different engine types seem to have different piston speed ranges for max fuel efficiency.



You can easily drop off the bottom end of the engine's efficiency zone by doing that.

FWIW, that is AVERAGE piston speed. If you look at the PEAK piston speeds, you might find some interesting results.

But to do that, you must do a bit of math and know the Rod length and stroke.

[Frank Lee]

I think as concerning fuel efficiency, we want to look at average piston speeds.

If we are getting into the nuances of intake and exhaust tuning, then peak piston speeds get more relevant.

[jakobnev]

Originally Posted by drmiller100

Apparently in some places Expansion Ratio is used instead of Compression Ratio.

For an Otto-cycle engine they are interchangeable, for Atkinson, ER is often significantly higher than CR.

Edit: Damn you Frank!

[dcb]

Originally Posted by Frank Lee

I think as concerning fuel efficiency, we want to look at average piston speeds.

Is there a more accurate predictor than piston speed? Assuming relatively constant VE/RPM. I don't quite understand why piston speed alone is a factor, but I could understand that there is an optimal rate of expansion for the combustion gasses, and that the combustion chamber should not overly retard, or outrun the expanding gasses, or mimize the discrepancy anyway. So that it would be a function of bore*stroke or something.

[Frank Lee]

^Yeah.

I think it has to do with how much combustion heat energy pushes on the piston vs dissipating into the cooling system.

Too slow rpm = more time for heat to go into coolant
Too fast rpm = less push on piston; more internal engine friction

[user removed]

The larger bore sizes have lower cylinder wall surface area as a percentage of displacement, so they will also have higher efficiency. Optimal piston speed is, as Frank already posted, the ideal point between the excess losses of the two extremes.

Other factors are the harmonics of induction and exhaust pulses and how they can reduce pressure waves which would affect in cylinder compression and exhaust scavenging. Also the amount of residual exhaust remaining in the combustion chamber which would tend to increase efficiency to a point, one reason why highly scavenged exhausts usually produce lower mileage.

Combining gas and diesel engines could yield best of both worlds

regards
Mech

[Ecky]

I've read that a long stroke relative to rod length increases stress on certain engine components. Does this mean that it increases friction?

Are we talking rod/stroke ratio, or absolute stroke length? Can you have a good rod/stroke ratio and still approach an "ideal" stroke length simply by building a bigger engine?

Also, why does increased stroke length increase efficiency anyway?

[drmiller100]

longer rod = more better. more efficient, more power, more everything, but slower maximum piston speed, which again, is better.

[drmiller100]

Originally Posted by Frank Lee

Wrong. Ex: Atkinson cycle - Wikipedia, the free encyclopedia

Sigh.

Expansion ratio - Wikipedia, the free encyclopedia

if we want to get really picky, the Otto by definition has different compression then what you are calling expansion because of exhaust valve timing differences then intake.

So Frank. How does this affect efficiency for mileage? (hint: variable cam timing).

[user removed]

Originally Posted by Ecky

I've read that a long stroke relative to rod length increases stress on certain engine components. Does this mean that it increases friction?

Are we talking rod/stroke ratio, or absolute stroke length? Can you have a good rod/stroke ratio and still approach an "ideal" stroke length simply by building a bigger engine?

Also, why does increased stroke length increase efficiency anyway?

Always a trade off in reciprocating engines. Longer rods and longer stroke generally give you more torque at lower RPM, but also increase reciprocating mass which will reduce efficiency.

I think the 225 Chrysler slant 6 had a 3.5 inch bore and a 4.5 inch stroke.

regards
Mech