# What is the theory behind rpm and torque? [duplicate]

*I am sorry if this seems too basic for you guys.

From what I understood is that fuel and air enters the cylinder, combustion occurs, Piston expands generating work and torque.

The Rpm should match the Rpm of the tires or at least match it's ratio, the torque as well matches the torque on the tires or it's ratio, depending on which gear.

So why is it the if you come to complete stop your rpm will remain between 500 and 1500. (I am talking sedan 4 cylinder cars)

And how does acceleration occur?

I hope I have made my question clear.

• Welcome to the site! It turns out that your question about fundamentals is not too basic. In fact, we've discussed many of the related aspects in the linked question: mechanics.stackexchange.com/q/7922/57. You'll see one of my "Bob Cross Wall of Words" answers over there. If you'd like me to add to it, please let me know! Commented Dec 4, 2015 at 14:32
• @BobCross - Sure, I create that big long answer and you shoot me down because you've already done the same ... good catch :-) Commented Dec 4, 2015 at 14:57
• @Paulster2, take it over there! You have good content and the other question has room for new knowledge. Commented Dec 4, 2015 at 15:00
• @BobCross - I'd have to do an entire re-write to get it to work ... after my "wall-o-words" below, I'm not feeling up to it right now ... maybe later, lol. Commented Dec 4, 2015 at 15:21
• @BobCross sure please do :) Commented Dec 5, 2015 at 21:52

So why is it the if you come to complete stop your rpm will remain between 500 and 1500.

Here, I am assuming you are talking about an automatic transmission (AT). The major part of the AT which allows this to happen is the torque converter (TC). The heading of Wikipedia article sums this up very nicely:

In modern usage, a torque converter is generally a type of fluid coupling (but also being able to multiply torque) that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. The torque converter normally takes the place of a mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated from the power source. It is usually located between the engine's flexplate and the transmission.

The key characteristic of a torque converter is its ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. Some of these devices are also equipped with a temporary locking mechanism which rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.

This answer I wrote describes how a TC works, so may be of value here.

And how does acceleration occur?

Since you used the engine tag, I'll start at the front of the vehicle.

• Engine - The vehicle needs a power source to move the vehicle and to power the different accessories (alternator, power steering pump, brakes, etc) throughout the vehicle. When designing a vehicle, engineers design an engine which not only fits in the space provided, but also design it so it will produce torque/horsepower specific to its needs.

• Transmission - The transmission allows the vehicle to utilize the torque/horsepower of an engine to full advantage through torque advantage and through reducing engine speed at higher engine speeds. (NOTE: I'm including the torque converter/clutch as part of the transmission.)

• Differential - Allows for a change in direction of the rotation produced from the engine (through the transmission) as well as a means of gear reduction/torque multiplication.

• Tires - This is where the rubber meets the road ... literally. Tires also create a torque reduction with regards to their overall diameter. The taller the tire, the more torque it takes to turn the wheel/tire assembly. Again pulling from a Wikipedia article:

Torque, moment, or moment of force (see the terminology below) is the tendency of a force to rotate an object about an axis,[1] fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object ...

Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.

• Tires (cont.) - If looking at tires from the side, you were to draw a line between the center of the tire to the ground (radius of the tire), this then becomes your rod length. As this rod becomes longer (increase tire radius), it would become easier to turn the tire by moving it directly (rolling the tire directly by hand). Conversely, if you are twisting the rod from the center point, as the drive axles do, it becomes harder to turn the axle as the rod becomes longer. The differential's torque multiplication helps overcome the tire's torque reduction.

The engine produces the torque needed to motivate the vehicle. When sitting at a stop, the engine is happily purring along at idle. It can do this because either the TC or the clutch has allowed it to be decoupled from the rest of the drive train. As you prepare to pull away from the stop, you press on the gas pedal and the engine RPMs increase. If you have an automatic transmission, the torque converter allows slippage between the engine and transmission while still transmitting torque and multiplying it at the same time. The TC will allow the slippage until it reaches its stall speed. An excerpt from the Banks Power website:

Even under light loads, a vehicle with an automatic transmission will start moving as soon as you take your foot off the brake. The stall speed comes into play under all load conditions. When we talk about stall speed, we're referring to engine RPM. If the vehicle isn't moving by the time the impeller reaches the stall speed, either it will start to move, or the engine RPM will no longer increase. In other words, stall speed is the engine RPM at which the torque converter transfers the power of the engine to the transmission.

A vehicle's engine is designed with a certain torque/power band so that, along with the transmission, the engine will be allowed to operate within that torque band to allow the vehicle to move. If you try to apply too much load (taking off from a light with a standard transmission, for instance) without enough engine RPMs, you'll stall the engine (NOTE: Engine stall has nothing to do with TC stall) and the vehicle won't move. Slipping the clutch in this situation is analogous to how the torque converter allows a slippage. By doing so, the engine is allowed to get into its designed toque/power band and the vehicle can then start moving. Once the vehicle is moving and the engine operates above the TC's stall speed, the engine is then motivating the input of the transmission at the same general speed as it is running (there is a bit of slippage there until TC lockup ... but that's another story entirely).

• Thank you very much for your answer :) I was wondering if you recommend any books on the topic? Commented Dec 4, 2015 at 16:10

There is one of two mechanisms that sit between the engine and transmission.

The first is a clutch/manual transmission. The clutch connects and disconnects the engine from the transmission. The clutch is effectively a direct drive, so if you come to a stop without disconnecting the clutch then the engine will stall. Accelerating from a stop looks like this; The engine is idling at a stop with the clutch disconnected, the driver raises the rpm slightly and slowly engages the clutch, as the clutch engages it will slip for a second or two to get the car moving, when the clutch fully engages that one to one connection is made and full acceleration can be achieved.

The second is a torque converter/automatic transmission. The simplest definition of a torque converter is a fluid coupling. This coupling can tolerate up to between 1000 and 2000 rpm of slip. This slip allows sitting at a stop with the engine idling at 500 to 1500 rpm. When taking off from a stop the engine is idling. Hitting the gas causes the rpm to climb, there comes a point where the torque converter stalls which is the condition of reaching the limit of the slip, at this point the engine provides power to the wheels to get it moving.