In SBBlue's thread "Value of Engine Efficiency? (thread71-105903), SBBlue said:
"The smaller the engine, the better the engine efficiency, because the smaller engine operates at a higher rpm to produce the same amount of power as a larger engine."
In considering this, I think I don't at all know what I thought I knew.
I mean, a smaller displacement engine should get better fuel economy than a larger displacement engine for the same load, right? This seems intuitive. But wait – why???
Why can’t I run a bigger engine in bigger gears? Answer: I can. American cars do it. They run bigger, longer stroke engines, and run ‘em slower (and in bigger gears), and this doesn’t murder their fuel economy as much as it should, because, say I:
1) this bigger, slower engine has higher apparent compression ratio than the little one. The throttle is open wider per stroke -- more charge per rev, thus higher compression.
2) the big engine has fewer reciprocating loads and less friction than the little one. Slower revs mean lower recip loads, and fewer strokes per minute means less friction.
3) the big engine closer approximates constant volume combustion. Slower piston speed, so the piston recedes before the flame front more slowly than it does in the higher revving engine. Closer to constant volume combustion equals better fuel efficiency, because relatively higher peak temps and pressures, so more area under the expansion curve, if that's what it's called.
Hmmm. So maybe it’s not so intuitive. It’s pretty much common knowledge that running in gears as big as possible for a given load improves fuel economy for a given engine size and load. So how does this translate when you start shifting engine size around? My buddy gets 30 mpg in his Pontiac Bonneville with an engine bigger than 4 liters, vs. my getting only 40 mpg in a 1.7 liter engine with a significantly smaller car. He really has no right to get mileage that good with such an obscenely huge engine, but he does.
I’m suspecting there must be some kind of continuum. Hmmmmm. And this consideration points to another vexing one:
4) Does running an engine closer to its optimal torque and power (generally toward the higher end of revs) also mean it’s closer to its optimal sfc? How are these related?
I await enlightenment.
"The smaller the engine, the better the engine efficiency, because the smaller engine operates at a higher rpm to produce the same amount of power as a larger engine."
In considering this, I think I don't at all know what I thought I knew.
I mean, a smaller displacement engine should get better fuel economy than a larger displacement engine for the same load, right? This seems intuitive. But wait – why???
Why can’t I run a bigger engine in bigger gears? Answer: I can. American cars do it. They run bigger, longer stroke engines, and run ‘em slower (and in bigger gears), and this doesn’t murder their fuel economy as much as it should, because, say I:
1) this bigger, slower engine has higher apparent compression ratio than the little one. The throttle is open wider per stroke -- more charge per rev, thus higher compression.
2) the big engine has fewer reciprocating loads and less friction than the little one. Slower revs mean lower recip loads, and fewer strokes per minute means less friction.
3) the big engine closer approximates constant volume combustion. Slower piston speed, so the piston recedes before the flame front more slowly than it does in the higher revving engine. Closer to constant volume combustion equals better fuel efficiency, because relatively higher peak temps and pressures, so more area under the expansion curve, if that's what it's called.
Hmmm. So maybe it’s not so intuitive. It’s pretty much common knowledge that running in gears as big as possible for a given load improves fuel economy for a given engine size and load. So how does this translate when you start shifting engine size around? My buddy gets 30 mpg in his Pontiac Bonneville with an engine bigger than 4 liters, vs. my getting only 40 mpg in a 1.7 liter engine with a significantly smaller car. He really has no right to get mileage that good with such an obscenely huge engine, but he does.
I’m suspecting there must be some kind of continuum. Hmmmmm. And this consideration points to another vexing one:
4) Does running an engine closer to its optimal torque and power (generally toward the higher end of revs) also mean it’s closer to its optimal sfc? How are these related?
I await enlightenment.
