Speed and torque?

Question

What is actually more torque at low speeds and less torque at high speeds
why more torque is needed at the start
I am confused when it comes to torque and speed of ic engines can anyone please clarify my doubts

Answer 1

I'm a mechanical engineer, been in the engine business for 25 years. Few people have a good understanding of this. Let's give it a try.

Engines produce POWER by burning a quantity of fuel over a period of time. The heat energy in the fuel is measured in Joules, but burning takes time, so the amount of heat released over time is Joules per second, or Watts. Usually a lot of heat is released quickly, so kilowatts, kw, is more typical. Engine output power is also measured in kilowatts.

Engines produce power by combusting air and fuel. An IC engine is basically an air pump. Theoretically it can pump just so much air per revolution. Because of inertia effects in the intake and exhaust systems and flow losses as air velocity increases in the manifolds and ports, an engine will be most efficient at pumping air within a certain speed range, and less efficient at higher and lower speeds. As the speed increases, the flow losses will get so high that the engine's efficiency at taking in air really deteriorates. Fuel is much denser than air, so there is usually no problem giving the engine all the fuel it needs. The fuel must be metered in proportion to the air. If too much fuel is provided for the amount of air the engine processes, combustion efficiency will deteriorate. So, the fuel provides the energy and it must be metered in proportion to the air, and the engine "breathes" most efficiently over a certain speed range. Therefore, the engine will also be able to utilize fuel more effectively over a certain speed range.

Then, there is mechanical friction which increases dramatically as the engine speed increases.

An engine's power output, therefore, is a function of its breathing efficiency and its friction characteristics. The faster an engine turns, the more fuel and air it can bring in. As it speeds up, it becomes less efficient at bringing in the fuel and air, so the power doesn't increase as rapidly. Finally, as the engine speeds up even more, friction increases so much that it outweighs the increased fuel and air. The engine's power curve is simply the graphical representation of the combined breathing efficiency and friction effects.

Obviously, since fuel and air must be burned over some time, an engine cannot make power if it isn't turning. So at zero RPM, there is zero power.

The value for "torque" is derived mathematically from the power. (I'm switching to english units now, BTU's hp, lb-ft --sorry, its easier for me.) Torque (lb-ft)= Power (hp) * 5252 / RPM.

An engine's torque curve is simply its power curve "normalized" for RPM. It is usually easier to understand a torque curve because it doesn't rise as steeply as a power curve, because it has sort of taken out the speed effect.

An engine "makes" power directly. It does not "make" torque. If an engine is better at making power at lower speeds, it will also produce more torque at lower speeds. If it is a very high speed engine, it will usually not be as good at making power at lower speed, so it's lower speed torque will also be lower.

The equation for torque shows that for a given power, any torque value can be obtained if the speed is changed. That is what transmissions do. The transmission's output speed is higher or lower than the engine's speed, and the torque at the output is also changed. Remember that the engine is turning the same speed and producing the same power, but if the transmission output is slower, the torque will be higher. The power is unchanged.

It is power that we always need, and we can get whatever torque we want with gearing. Usually we want to go fast, so we need lots of power. If all we care about is moving something, even if it is very slow, we don't need a lot of power and we can use alot of gearing. Theoretically, we could lift the space shuttle with the tiny motor in my wristwatch if we gear it down enough. It might take years to move it an inch, but it could be done. On the other hand, if we want to launch the shuttle into orbit, we need to move it FAST, and that takes an enormous amount of POWER.

Acceleration takes power. The clutch in your car lets the engine produce some power even though the wheels aren't turning at all. You slip the clutch (wasting a lot of power), and the wheels begin to turn until they are going fast enough that they match the engine speed. Of course you are in first gear -- a lot of reduction -- so your engine can speed up to where it produces a lot of power for acceleration even though the wheels are still going fairly slow. Then when the engine runs out of power, you shift into second gear. Now there is more wind resistance on the car, so the engine's power won't make it accelerate quite as fast as when it was in first gear, but it still accelerates. Finally, you are in top gear, and the car is going about 120 mph. It won't accelerate any more because the wind resistance on the car just matches the power available from the engine.

In the end, it is all about POWER, and the ability to deliver the power at the speed the vehicle needs. A heavy duty truck engine might be rated at 350 hp and 1000 lb-ft max, but it needs more than 10 speeds in the gearbox to keep the engine in its power band as the truck slowly accelerates.

Answer 2

Torque times speed (angular velocity) equals power. For an engine with constant power, the torque will be inversely proportional to speed. If a gearbox is used to reduce (or increase) speed, it cannot add power, so torque must change inversely with respect to speed.

For an IC engine, it is not that more torque is needed to start, but that the engine cannot put out high torque and high speed at the same time. Moreover, IC engines do not increase their torque if you slow them down by fuel starvation (you are merely reducing their power), so the engine torque is increased by using a gearbox which also reduces the output speed.

More torque is needed at start because you are accelerating the car, and that requires more force than just keeping it running at constant speed. Same goes for going uphill. More torque is needed.

Answer 3

The link below is to a torque speed curve of a Porsche engine on Wikipedia. The torque tails off faster at low speed than it does at high speed. As you can see from the graph, max torque occurs at about 5000 rpm, while max hp occurs at about 6500 rpm

Answer 4

Yeah, you could do speed on the tongue, but what would you want to?