Optimum piston speed?

[Duffman]

Originally Posted by Frank Lee

From the Autospeed Brake Specific Fuel Consumption link:



Maybe this is why 1000-1200 ft/sec is better than lower speeds?

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

[tasdrouille]

At least 60% of the energy in an ICE is lost as heat. Thermal losses through the cylinder walls are significant at any rpm but more so at lower rpm.

[dremd]

A few notes on Large diesels operating more efficiently at lower speeds

1) CRAZY long strokes = Higher Mean piston speeds
2) Diesel must have a different burn rate than Gasoline
3) Diesel components are forced to be stronger due to stress, heavier= stronger = more reciprocating mass= less reciprocations to reduce energy loss in reciprocations


I'm no expert here, but it is what came to mind.

[}{ead$hot Zod]

IIRC when it comes to diesels it's based mostly on torque. They best way to figure that is to get it dyno tested. Since the torque curve will always change when you mod the motor this will have to done every time you make a mod. With this knowledge you will know exactly when to shift the tranny.

[Duffman]

Originally Posted by tasdrouille

At least 60% of the energy in an ICE is lost as heat. Thermal losses through the cylinder walls are significant at any rpm but more so at lower rpm.

True about the heat losses, I dont have the source handy but the majority of the heat lost in a ICE is pumped out of the engine through the exhaust and replaced with cool intake air. The actual losses to the engine coolant is small in comparison. Thats why air cooled engines can exist.

[jonr]

No, it's roughly 1/3 lost to engine heat and 1/3 out the exhaust.

Agreed - heat loss is the problem with low piston speeds. In general, you want high piston speeds, quick burn rate, high cylinder temps and pressures, long stroke, high compression, and insulating materials or coatings inside the combustion chamber (aluminum is not your friend).

Of course you also want the engine to stay in one piece.

[aerohead]

Originally Posted by Frank Lee

I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?

Boy Frank,that's a new one for me.Although many things are new to me.I was looking at BSFC maps last night and don't recall a term like that even on the charts.Rpm seems to matter.Road load is an issue.Volumetric efficiency.The oil ring,by itself,is supposed to be responsible for 10% of all engine friction.And friction is given as a square function of rpm and is borne out by the curves for internal friction.I'll look into it,but off the top of my head,I'm clueless.It's worth knowing,and it seems like there would be a context to if making such a general claim like that.Maybe there's a powerplant expert in the house?

[jonr]

You won't see piston speed on a map because the only way to change it independent of RPM is to redesign the engine.

You can build an efficient engine for 100 rpm or for 3000 rpm - and the 100 rpm one might have more friction losses.

[aerohead]

Originally Posted by Frank Lee

I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?

Sorry everybody,I took a couple days to dig through my rat's nest.Here's some stuff I culled out.The only statement citing an optimum piston speed range is from "Internal Combustion Engines",an old textbook,by Edward F. Obert.Piston speed wasn't even listed in the index,but after forraging,I found this citing:-------------------------- "...large and small diesel engines(designed to develop 125-200 bmep),attain their minimum specific fuel consumption in the range of 1,200-1,500 ft/min mean piston speeds."------------------------------------- A large-bore 100-rpm,700 hp/cylinder diesel engine for direct propeller drive was given with the lowest BSFC,@ 0.32 lb/hp-hr.------------------------------------ Friction seems to be the overriding factor for maximum BSFC.----------------------------------------- for spark-ignited gasoline engines,best BSFC occurs at point of maximum charge loading efficiency,which is basically where torque peaks,which is at about 80% load.At light load,or full load,efficiency suffers.--------------------------------------- BSFC maps showed minimums of about 0.45 lbs ( 275 grams ) per hp-hr.(0.746 kWh).-------------------------------- The Orbital Engine Co. 2-stroke is rated at 7-12% better BSFC,but they can't seem to clean up the emissions enough to use it.------------------------------------------- SAE said that an"adiabatic compound engine" operating at 1,300-1,500 degrees F,with no cooling system,and all waste energy into exhaust,could be harvested with compound turbos doing double duty,supercharging and doing shaft work with a BSFC 0.29 lb/hp-hr.------------------------------------ I guess that means a ceramic engine we've been hearing about for 20-years or so.

[bryn]

Originally Posted by aerohead

---------- for spark-ignited gasoline engines,best BSFC occurs at point of maximum charge loading efficiency,which is basically where torque peaks,which is at about 80% load.At light load,or full load,efficiency suffers.---------

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