Optimum piston speed?

[aerohead]

Originally Posted by bryn

so if 80% load at ~1700 rpm is 50 hp, that is only the most effecient engine speed if you need 50 hp. most of our cars might only need 15 hp to cruise down the road at 55mph.
if we change the gearing so that 15 hp is closer to 80% load, maybe down around 1000 rpm. the engine will still be more effecient even though piston speed is less than ideal.

a manual cvt and a scan guage setup would be really interesting to see

Yeah bryn,Hucho calls it gear-matching,and says that, say ,you streamline a car,you could lose up to 60% of the mpg potential if you don't redo the gearing.That really sucks! The engine wants to see the same "load" or it falls off it's "sweet-spot".

[metromizer]

As if this wasn't confusing enough...

Piston speed isn't constant, right? The piston stops at top and bottom dead centers, therefore accelerates and decelerate in between, or every 180 degrees of crankshat rotation. So using 'average piston speed' is kinda like using average speed while calculating fuel economy: Great if you only vary a few mph, but not if your commute invovles stop-n-go traffic.

The variable of rod ratio governs max piston speed for a given crankshaft rotational position, so wouldn't average speed be the same for every rod ratio if the same rpm is used?

Acceleration and deceleration rates, dwell time where the piston is at it's slowest, burn rate and bore size. Race engine builders ofter get to a place where the piston outruns the flamefront. Phew... so much going on I get dizzy. But wait, there's more, there's cylinder filling efficiency... what is optimal for the power cycle may hurt the intake or two other cycles to the point you have 1+1 steps forward, then 2 steps backward.

I think if you fix certain parameters (bore, stroke, port and head design) every engine has it's own 'sweet spot' where it is most fuel efficient, with fifteen (a number taken out of thin air) variables that influence it. I think you can make tweaks and changes that improve on FE, but question weather simply decreasing the final drive ratio until the engine no longer accelerates is the magical answer.

On the high performance side of things, I spent hours playing with "Engine Analyser" software varying only camshaft and exhaust headers selection. Then I put the thing on the dyno

I concluded the whole thing is one giant compromise.

[dremd]

Originally Posted by metromizer

As if this wasn't confusing enough...

Piston speed isn't constant, right?

I typically see it rated as Mean (average) Piston Speed. Also bear in mind that rod length alters the time that the piston is near/ at TDC/ BDC. It is actually an incredibly complicated science once you get down to the tid bits.

Originally Posted by metromizer

I concluded the whole thing is one giant compromise.

I second your conclusion.

[bryn]

Originally Posted by metromizer

I concluded the whole thing is one giant compromise.

so how about a wankel rotary? just to throw one more wrench into the mix. anyone play with one of those for fuel economy?

[jonr]

Generally not very efficient - they have too much combustion chamber surface area - so the hot gases cool off before you can get work out of them.

[bryn]

Originally Posted by jonr

Generally not very efficient - they have too much combustion chamber surface area - so the hot gases cool off before you can get work out of them.

that would make sense

not to get off topic but are there any other non fourstroke engines that people are working with? is there another thread somewhere that covers different engine types?

[MechEngVT]

Originally Posted by metromizer

Piston speed isn't constant, right? The piston stops at top and bottom dead centers, therefore accelerates and decelerate in between, or every 180 degrees of crankshat rotation. So using 'average piston speed' is kinda like using average speed while calculating fuel economy: Great if you only vary a few mph, but not if your commute invovles stop-n-go traffic.

The variable of rod ratio governs max piston speed for a given crankshaft rotational position, so wouldn't average speed be the same for every rod ratio if the same rpm is used?

Using Mean Piston Speed is the effective average speed of the piston as it travels through the given stroke over the given time. It doesn't matter how wide the engine's operating range is the only factor affecting mean piston speed is the physical engine design (considered fixed) and the actual engine speed. It's similar to using mean effective pressure to talk about combustion pressure...it's not REALLY how much pressure is in the cylinder since theoretically at TDC pressure is tremendously large and drops off very quickly as the piston descends on its power stroke, but there is an average effective pressure that would complete the same amount of work as the actual combustion does and that (the MEP) is a useful design tool.

I don't think rod/stroke ratio affects maximum piston speed. It will have a *huge* affect on maximum piston acceleration and therefore wristpin loading (which will limit maximum RPM), but generally pistons accelerate from 0 at BDC to max speed halfway up the cylinder and decelerate to 0 at TDC. When piston speed is maximum the rod is perpendicular to the crankshaft throw and the piston's maximum linear speed at that engine rotational RPM is independent of connecting rod length. The higher the rod/stroke ratio the closer the piston's actual velocity approaches a sinusoidal variation, and lower ratios cause less dwell (i.e. sharper acceleration into and out of) the top/bottom centers. So succinctly, the average piston speed for a given engine RPM is independent of rod length, just as the formula quoted earlier indicates.

[metromizer]

Originally Posted by MechEngVT

Using Mean Piston Speed is the effective average speed of the piston as it travels through the given stroke over the given time. It doesn't matter how wide the engine's operating range is the only factor affecting mean piston speed is the physical engine design (considered fixed) and the actual engine speed....

I don't think rod/stroke ratio affects maximum piston speed. It will have a *huge* affect on maximum piston acceleration and therefore wristpin loading (which will limit maximum RPM), but generally pistons accelerate from 0 at BDC to max speed halfway up the cylinder and decelerate to 0 at TDC. When piston speed is maximum the rod is perpendicular to the crankshaft throw and the piston's maximum linear speed at that engine rotational RPM is independent of connecting rod length. The higher the rod/stroke ratio the closer the piston's actual velocity approaches a sinusoidal variation, and lower ratios cause less dwell (i.e. sharper acceleration into and out of) the top/bottom centers..

So succinctly, the average piston speed for a given engine RPM is independent of rod length, just as the formula quoted earlier indicates.

I think we agreed/I understand, independant of average piston speed for a given rpm.

But, I should have said piston speed (average, max, and profile) is stroke dependant, right? Also, the other key is piston speed has a profile that is rod ratio dependant. For a given rpm, the bigger the engine's stoke, the greater the piston speed. For a longer stroked engine for instance, the piston has a longer distance to travel over the same time, so piston travel in feet per minute (speed) must be greater.

In your expanding pressure wave example you used, that wave has a pressure profile. With dwell, max piston speed, the piston must have a speed profile we could graph over 180 crankshaft degrees. Short rod race engines are known for yanking the pin out of the piston boss from extreme acceleration rates, for example. Short rod race engines are also known for 'outrunning' the expanding pressure wave at very high rpms. Extreme acceleration rates are the result of high bore-to-rod angles present in a high rod ratio engine. If I think about a graph, I think about the (piston speed) magnatude is stroke dependant, and the speed profile is rod ratio dependant. The shape of the curve from TDC to where the rod is perpendicular to crankshaft throw, is rod ratio dependant.

Why do you care?

Some choices must be better for for extracting the maximum motion energy from that expanding pressure wave.

If what I've said is true, and average piston speed changes with stroke, to answer the question "what rpm do I want to acheive XX average piston speed" one needs more information... what's the stroke?

There may very well be a magic average piston speed, but not a magic rpm without considering the engine's stroke.

I would argue there must be an optimal piston speed profile (which include speed, decel and accel rates) to take the most advantage of a given pressure wave profile, but I also think there is a lot more to the puzzle. I also belive for every piston speed profile, the pressure wave profile changes.

You should want to find the best engine specific RPM for best FE, with so many variables I think you need an engine dyno to tell you that.

[jonr]

Some engine designs have used cams instead of crankshafts so they can better control the piston movement/speed profile.

Better combustion chamber shape at TDC is also an attribute of long stroke engines - a flat pancake chamber at ignition vs something a little closer to a sphere.

[aerohead]

This engine stuff is really a can of worms.I think the spirit of what Hucho is eluding to is that given whatever engine we have,it's going to have it's "sweet-spot." If we streamline the car,we move the engine out of this area of performance,and without gear-matching,we stand to lose the full benefit of the streamlining.If I don't alter anything under the hood,and switch to a lower numerical gear ratio,in theory,I should be able to restore the engine to it's plateau of efficiency.I think that's all he's saying.I will lose on acceleration,and everybody is in agreement,that it would be better to have another gear,twelve gears to be exact.Some guys at Bonneville are running series transmissions to get all kinds of gear splits,as their cam grinds and induction provide very narrow power bands,requiring very small rpm drops between gears.One day I may go this direction as I fear I may never see the full benefit of the wind-cheating.