Why do engines have torque and horsepower curves?


I don’t understand how there could be a difference in power and torque if you are burning the max amount of fuel possible 100 percent of the time.

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You are not burning the maximum amount of fuel possible. Generally speaking, you are burning the ideal ratio of air to fuel (~14.7:1) all the time. The throttle can open and close, which varies the air intake.

The volumetric efficiency (V.E.), which is how much of the cylinder gets filled during each cycle varies. Therefore, the torque and HP vary along with it. Generally speaking, the V.E. is the highest at maximum torque RPM and is lower on each side of that point.

HP is just a math problem that relates torque to power and is the rate at which torque is transmitted.

About 180 years ago someone determined that a horse could lift 550 lbs. at a rate of 1 foot per second. This (550 lb ft/sec) became the standard for 1 HP. A horse can actually produce about 15 HP.

HP = TQ x RPM / 5252

The HP and TQ curves are for wide open throttle (WOT) performance.

I assume you know the concept of how a combustion engine works. The pistons suck inn an air and fuel mix which it compresses and then ignites it which generates a huge pressure pushing the piston back down. The issue is that the combustion takes place over a certain amount of time. After the combustion is over the gasses cool down and you start loosing pressure. So if the engine rotates slowly then it will only generate high torque while the piston is at the top. But if the engine rotates faster then it will make torque all the time the piston goes down as the gasses still expand. You even get a flame front increasing the pressure at the piston compared to the engine head generating even more force.

However if the engine goes too fast then the combustion is not done when the piston gets all the way down. So you are pushing fuel out the exhaust pipe instead of using it in the engine. Even worse the engine loses energy when it compresses the air and this energy is just pushed out the exhaust and not used. Adding to this all the friction in the bearings, the water pump, the oil pump, etc. and the engine end up using more power then it can generate when going too fast.

A internal combustion engine, can not go from stand still to max RPM. So during the time it takes for the engine to spin faster and faster it creates more and more power(typically). The engine has to overcome its own momentum not to mention the momentum of the vehicle it’s attached to.

More bangs per second means more power. But too many bangs and the bangs don’t have enough time to fully bang. 😀

Anyway that result ends up giving you a curve that ramps up till it hits a peak then starts falling.

You also have little peaks and bumps when things like intake and exhaust resonance line up nicely allowing air to flow in and exhaust out of the engine faster.

Then you also have a bit of nonlinearity because another limiting factor is how much air can flow into the engine. Amount of fuel is easy to increase but air has to fit through the intake and be pulled in by vacuum created by the pistons.

I am not sure if it has been mentioned, but its also highly related to how and when the cameshaft opens the valves of the engine. As engine speed changes the valve events also are required to change to maintain optimal aspiration.

This is why you have systens like vtec and other variable valve systems. Some do this by having a second set of cam followers which engage when a certain oil pressure is reached. This is how vtec works.

The amout of valve lift, duration and overlap change how and engine breathes and thus change when you get your optimal power. This is why changing your camshaft to a more “aggressive” one will result in more horsepower at full throttle, but will cost you low end power and part throttle drivability because the engine valves will be opened at the wrong time and the engine cannot breath efficently.

The perfect camshaft would change continously to have the optimal lift and duration across all engine rpms, but this is increidbly complicated and difficult to do. So we are forced to design the camshaft with the maximum amount of compromise to have a smooth power curve. The vast majority of an engines powe and torque curve has to do with how and when the valves are opened and closed. They often call the camshaft the brain of the engine.