Torque vs RPM

On a dynamometer chart you always see the torque and power produced by an engine plotted versus RPM. That shows you how the engine performance depends on speed. The engine speed is related to the vehicle speed but not directly because of the gear ratio in the transmission. Have you ever wondered why the torque curve has a peak? What causes the torque to be lower at low and high RPMs?

First of all, why is there a peak torque? Peak torque occurs when the engine receives the maximum amount of fuel air mixture in the cylinders to burn. The torque is produced by the pressure of the explosion on the pistons. This pressure is transmitted to the wheel rims as rotational force against the road. More cylinder pressure gives more torque, and to get more cylinder pressure you need to burn more fuel. So peak torque is when the cylinders are getting the most fuel with air sufficient to burn it all.

AS the RPM goes above the point of peak torque, torque decreases mainly due to difficulty getting air in and out of the cylinders. Basically the engine can't breathe fast enough to keep up with the pistons. The air will only move around so fast. The maximum external static pressure to drive air into the cylinders for example is one atmosphere. If the pistons begin to move up and down faster than the air can move into the spaces they leave behind then the air fuel charge amount will start to drop. As the exhaust gases are forced into the exhaust manifold faster than they can flow out through it, the back pressure in the manifold rises and rises. This rising back pressure robs output from the engine as it must now divert some effort to fighting it. Also mechanical efficiency drops at high RPM due to generally increased frictional resistance at high relative speeds between moving parts.

Now what happens at lower RPM? First of all, the pistons are now moving slowly. This gives time for heat to flow. During the compression stroke and power stroke at slow piston speeds, there is time for some of the heat in the gas mixture in the cylinder to escape through the cylinder wall. This loss of heat leads to a drop in temperature and pressure. This pressure loss directly reduces torque. Remember that the torque is being generated by the pressure on the pistons. There will always be an imperfect seal around the piston rings and the valves. At low RPM there is time for the pressure to bleed off through leaks. In addition to these factors the timing of the ignition and valve open/closing for normal engines is optimized for midrange RPM. Engines with variable timing systems do not have to suffer low RPM torque losses due to this factor, but the majority of today's engines do not have such systems.

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