Efficiency
01-30-2012, 12:08 PM
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Efficiency
My question is: If a motor makes it's peak HP at say... 4000 rpm, is the motor running at it's most efficient at 4000rpm? My inner physics/science nerd knows that 100% efficiency isn't possible, there will always be losses, but does the fact that the motor is producing the most power at that point also mean that the motor is operating at peak efficiency? Is the motor working much harder/consuming more fuel at 2000rpm compared to it's peak HP rpm? Am I way off in thinking the two are related?
What say you, experts?
What say you, experts?
02-01-2012, 02:44 PM
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From the standpoint of amount of work done per unit of fuel, generally no. You will often have your torque peak near the peak efficiency RPM.
02-01-2012, 10:57 PM
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Lol, I am definatly no expert, but I can spout out a bunch of random semi-related thoughts that come to my head and maybee you could make some sense of some of it lol.
Do you mean fuel efficiency? The answer would be differnt on a dyno or the road, drag plays a huge role on fuel efficiency if on the road.
Every port injected engine has a throttle plate and this causes quite a conundrome. With a port injected engine, you have a butterfly valve throttle plate, this causes a pumping loss 100% of the time, if can't get out of the way even WOT. So some of your power made is being used to pull in that air through the narrow throttle. So the more shut it is, the higher the pumping loss is, so you are going to want it WOT in that regard.
The problem with WOT is you are generally accelerating when you do this, it takes much more energy to accelerate.
This is also very diffuclt to answer based on engine size and bore/stroke differences, many different engines will have different rpms.
Depeding on gearing too, if you have very small gearing, you can be at 1500rpm and require WOT to stay the same speed. But at lower rpm, the deflagration is too slow for the piston speed and it is almost like detonation, it will cause high cylinder pressures, but it is too quick, none of that can be translated to the crnak.
The higher the rpm, the higher the friction, and more heat (energy expended) so generally higer rpm isn't necessarily as good.
Now if you are talking about a diesel, I think they are generally more efficient at lower rpms and they don't have a pumping loss.
All of that said, I think the avg car is between 2000-2500rpm and/or between 50-70mph.
Do you mean fuel efficiency? The answer would be differnt on a dyno or the road, drag plays a huge role on fuel efficiency if on the road.
Every port injected engine has a throttle plate and this causes quite a conundrome. With a port injected engine, you have a butterfly valve throttle plate, this causes a pumping loss 100% of the time, if can't get out of the way even WOT. So some of your power made is being used to pull in that air through the narrow throttle. So the more shut it is, the higher the pumping loss is, so you are going to want it WOT in that regard.
The problem with WOT is you are generally accelerating when you do this, it takes much more energy to accelerate.
This is also very diffuclt to answer based on engine size and bore/stroke differences, many different engines will have different rpms.
Depeding on gearing too, if you have very small gearing, you can be at 1500rpm and require WOT to stay the same speed. But at lower rpm, the deflagration is too slow for the piston speed and it is almost like detonation, it will cause high cylinder pressures, but it is too quick, none of that can be translated to the crnak.
The higher the rpm, the higher the friction, and more heat (energy expended) so generally higer rpm isn't necessarily as good.
Now if you are talking about a diesel, I think they are generally more efficient at lower rpms and they don't have a pumping loss.
All of that said, I think the avg car is between 2000-2500rpm and/or between 50-70mph.