I understand that it drops off in the high end because air can’t get into the cylinder fast enough, but what limits are there at low RPMs? Why does an explosion at say 1000 RPM not produce the same torque as one occuring at 3000-4000? And is it the same reason in both gasoline and diesel?
In: Engineering
Engine seals aren’t that perfect. Gas can escape, given time.
Engines are also tuned for specific RPM’s. They’re most powerful/efficient at these RPM’s. This RPM is usually pretty low, since engines spend most of their time at low rev. However, at *even lower* revs, they go beyond the range they’re tuned for. By changing timings and such, this can be partially worked around, but things like exhaust geometry don’t really change easily.
There are probably other reasons that I don’t know about, too.
Each combustion event (the bit where the air-fuel mix burns) takes a finite amount of time. The burning of the fuel generates high pressure in the cylinder which, of course, is done to push down on the top of the piston to extract work.
Maximum mechanical advantage (and hence most torque) happens with the piston half way down its stroke, as the throw on the crank is at 90 degrees and thus has the most leverage to turn the crank.
If the crank is rotating slowly (low RPM, near to idle speed), the combustion event and its peak pressure on the piston crown happens before this point. Thus the pressure on the top of the piston from the expanding gasses doesn’t get maximum mechanical advantage to turn the crank.
The same is also true at very high RPM, when peak cylinder pressure happens ‘late’ in the travel of the piston, and again torque is lost.
Ignition timing (when the spark is fired) is adjusted on-the-fly by the engine computer to minimise this and centre the combustion event as well as possible on the piston movement, but you can’t stretch or compress the time the combustion takes.
There are undoubtedly more than a few reasons, however I’ll attempt to explain briefly the ones I know.
1.) Momentum of air coming from intake manifold. The higher the rpm, the higher the flow rate of air into the cylinder until a point. When the intake valve is opened, the faster the air is flowing, the more can get into the cylinder before the valve closes. This would then create a more powerful explosion and thus more torque. (Comparing between 1000-mid to high range rpm)
2.) Increased mixing – the quicker the air coming into the cylinder, the better the air and fuel would mix to create an essentially better explosion.
Edit: the top of some pistons are shaped in order to enhance mixing
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