less torque at high rpm's

[Ris91]

When we compare two engines say one producing less torque and high rpm and other producing high torque but lower rpms (as that of a bike and car)
then it's easy to understand that the latter would have greater stroke lenght or bore so more force but less rpm............................
but in case of a single engine, if we see torque vs rpm curves then we see that torque first increases and then decreses or in short it doesn't increase with increase in rpm at all rpms,,,,,,,,,,,"how is it possible?" greater torque means greater rpm bcoz more the twisting force(torque) fatser will be the speed of crankshaft and more should be engine rpm but this is not so!!! can anyone tell why???????
I thought that torque should always vary linearly with rpm however this is false.............why????????????
Please note that i am talking in a single engine here now..
if two engines then it;s ok there can be high torque at low rpm but in a single how can torque vary inversely with rpm..........
plz help me................................................

[curtis73]

Torque is a function of how much pressure is being exerted on the crankshaft by the piston. The pressures in the cylinder are obviously created by the burning fuel in the cylinder. The engine will make peak torque at the RPM where its most efficiently ingesting air and fuel. This is mostly a function of cam and head flow. Cams have a duration that suits a certain RPM. More duration means they are designed to make torque at a higher RPM.

So, lets say you're looking at a dyno chart from an engine where the torque peaks at 3000 RPM. At speeds below 3000, the cam is not as efficient at trapping the maximum amount of intake air and torque suffers. Above 3000 RPMs, you are passing where the cam is at its most efficient and torque suffers.

Everything about an engine is designed to work best at one speed. At every other speed from idle to redline, it is a compromise. That is why variable valve timing cams are better at making a flatter torque curve. They alter cam timing so that it can act like a small cam at low RPMs and a bigger cam at higher RPMs.

[jdmccright]

To throw in a bit more, as Curtis said the variable cams adjust to flatten the torque curve because even though the engine is spinning faster, the actual burning rate of the fuel/air mixture is the same. So to eke out as much push as possible from that pressure front, the ignition/cam timing is advanced enough to cause the burn to happen at the best moment in the cylinder's stroke.

To take it one step further, as the engine rpms increase, the difference between the cylinder's velocity and the advancing flame front decreases, reducing the pressure applied to the cylinder by the burning mixture. Its like the difference between acclerating a one-speed bicycle from rest and trying to accelerate at top speed...your feet and legs can only go so fast.

[MagicRat]

Originally Posted by Ris91

ves then we see that torque first increases and then decreses or in short it doesn't increase with increase in rpm at all rpms,,,,,,,,,,,"how is it possible?" greater torque means greater rpm bcoz more the twisting force(torque) fatser will be the speed of crankshaft and more should be engine rpm but this is not so!!! can anyone tell why???????
.

I think the original poster is simply confused about the definition of torque.

Torque is a measure of force. It exists completely separately from RPM

So, 100 ft/lbs of torque is not dependent on RPM. Whether the crankshaft is turning at 2000 rpm or 4000, the 100 ft/lbs torque is the same.

Horsepower is a function of work, that is, force applied over a distance.
If torque remains constant over an RPM range, then horsepower will increase.
Therefore, 100 ft/lbs of torque at 4000 RPM produces twice the horsepower as an engine turning at 2000 rpm.


So..... if a torque curve is flat, or declines somewhat as RPM increases, the horsepower curve will increase, because the amount of work being done is increasing, (due to increasing RPM), even though the twisting force is actually decreasing.