Let's start with the fact that an internal combustion engine is a device in which the chemical energy of fuel burning in the working area is converted into mechanical work.
Schematically it looks like this:
Ignition of fuel in the cylinder (6) leads to movement of the piston (7), which, in turn, leads to rotation of the crankshaft.
That is, expansion and compression cycles in the cylinders drive the crank mechanism, which, in turn, converts the reciprocating motion of the piston into rotational motion of the crankshaft:
Engine RPM
The widely used term "engine RPM" refers to the number of revolutions of the crankshaft per unit of time (per minute).
Both power and torque are not constant values; they have a complex dependence on engine RPM. This dependence for each engine is expressed by graphs similar to the following:
Engine manufacturers strive to ensure that the engine develops maximum torque in as wide an RPM range as possible ("the torque shelf is wider"), and maximum power is achieved at RPM as close as possible to this shelf.
Engine power
Power is the ratio of work done over a certain period of time to that period of time. In rotational motion, power is defined as the product of torque and angular velocity of rotation.
Engine power has recently been increasingly indicated in kW, whereas previously it was traditionally indicated in horsepower.
As can be seen in the graph above, maximum power and maximum torque are achieved at different crankshaft RPMs. Maximum power for gasoline engines is usually achieved at 5-6 thousand RPM, for diesel engines - at 3-4 thousand RPM.
Power graph for a diesel engine:
In practical terms - power affects the speed characteristics of the car: the higher the power, the greater the speed the car can develop.
Torque
Torque (moment of force) is the product of force and the lever arm. In the case of a crank mechanism, this force is the force transmitted through the connecting rod, and the lever is the crank of the crankshaft. The unit of measurement is Newton-meter.
In other words, torque characterizes the force with which the crankshaft will rotate, and how successfully it will overcome resistance to rotation.
In practice, high engine torque will be especially noticeable during acceleration and when driving off-road: at speed, the car accelerates more easily, and off-road, the engine withstands loads and does not stall.
More examples
For a greater practical understanding of the importance of torque, we will give several examples on a hypothetical engine.
Even without taking into account maximum power, some conclusions can be drawn from the graph reflecting torque. Let's divide the number of crankshaft revolutions into three parts - these will be low RPM, medium, and high.
The graph on the left shows a variant of an engine that has high torque at low RPM (which is equivalent to high torque at low speeds) - with such an engine it is good to drive off-road - it will "pull out" of any mud. The graph on the right shows an engine that has high torque at medium RPM (medium speeds) - this engine is designed for use in the city - it allows fairly brisk acceleration from traffic light to traffic light.
The next graph characterizes an engine that provides good acceleration even at high speeds - with such an engine it is comfortable on the highway. The graphs are completed by a universal engine - with a wide shelf - such an engine will pull out of a swamp, allow good acceleration in the city, and on the highway.
For example, the 4.7-liter gasoline engine of the Toyota Land Cruiser 200 develops maximum power of 288 hp at 5400 RPM, and maximum torque of 445 Nm at 3400 RPM. And the diesel 4.5-liter engine installed on the same car develops maximum power of 286 hp at 3600 RPM, and maximum torque of 650 Nm with a "shelf" of 1600-2800 RPM.
The 1.6-liter engine of the Mitsubishi Lancer X develops maximum power of 117 hp at 6100 RPM, and maximum torque of 154 Nm is achieved at 4000 RPM.
The 2.0-liter engine of the Honda S2000 provides maximum power of 240 hp at 8300 RPM, and maximum torque of 208 Nm at 7500 RPM, being an example of "sportiness".
Summary
So, as we have already seen, the relationship between power, torque, and engine RPM is quite complex. Summarizing, we can say the following:
- torque is responsible for the ability to accelerate and overcome obstacles,
- power is responsible for the maximum speed of the car,
- and engine RPM complicates everything, since each RPM value corresponds to its own value of power and torque.
And overall it looks like this:
- high torque at low RPM gives the car traction for off-road driving (diesel engines can boast such a distribution of forces). In this case, power may become a secondary parameter - let's recall, at least, the T25 tractor with its 25 hp;
- high torque (or better - "torque shelf") at medium and high RPM makes it possible to accelerate sharply in city traffic or on the highway;
- high engine power provides high maximum speed;
- low torque (even with high power) will not allow realizing the engine's potential: having the ability to accelerate to high speed, the car will reach this speed incredibly slowly.
