The discussion revolves around the specification of engine power and torque, particularly why maximum power and torque are reported at certain engine speeds. Participants explore the relationship between engine speed, torque, and the factors influencing these specifications, including design limitations and airflow dynamics.
Participants express various viewpoints on the factors affecting engine performance, particularly regarding airflow and runner size. There is no consensus on the optimal design parameters, and the discussion reflects multiple competing perspectives on how these factors interact.
Some claims about airflow dynamics and engine design are made without fully resolving the complexities involved, such as the balance between runner size and air velocity, as well as the impact of camshaft selection on engine performance.
Filip Larsen said:Welcome to PF!
Are you asking in general or for a specific engine?
Anyway, most engines which produces power by turning an axle are usually characterized by a relationship between angular speed and torque. If you search for "engine speed torque diagram" you should be able to find some useful explanations, like for instance [1].
If such explanations do not help you understand then try explain what it is you don't understand. The best would be if you could point to some calculation (from a book or web) and say which part of it that eludes you.
[1] http://lancet.mit.edu/motors/motors3.html
jack action said:The engine gets its torque from the combustion of fuel and air. Power is the product of torque and rpm. In theory, the torque should be constant no matter the rpm of the engine, if the quantity of air and fuel is the same. But the air has to go trough the intake and exhaust runners to get in and out the cylinder, which size is greatly dependent on the cylinder bore. The cross-sectional area of these runners will determine the maximum quantity of air that can get in the cylinder. At a certain rpm, the runners will create such a restriction that there won't be enough air to fill the cylinder to give the same torque as the one from lower rpm.
The runner's CC amount does have influence on cylinder filling, but, depending on your application your air velocities are more important than the runner area to a point, sure the higher the rpm range you want to operate at or the higher horsepower goal, you need larger runners. Too big is bad because of 'sluggish' flow happening, finding the right runner size is much needed. Camshaft events immensely dictate the cylinder filling action in correlation to the velocity of flow throughout the induction manifold and intake & exhaust ports. You could have great flowing heads, but, have a terrible running engine simply from a bad camshaft choice or slow air flow velocities in general.The cross-sectional area of these runners will determine the maximum quantity of air that can get in the cylinder.