What is the difference between torque and power

Automotive engineers have long and absolutely known the concepts and features of such purely physical quantities as engine power and torque. Questions arise from beginners, but motorists interested in technology.

Especially recently, when numerous popularizers and car reviewers, who themselves do not really understand the theoretical foundations, began to cite the amount of torque in the descriptions of motors, putting it forward as almost the most important indicator of the operational value of a car.

Without explaining the essence, and therefore misleading readers and viewers.

What is engine power

Power is the ability to do work per unit of time. In relation to an automobile engine, this concept characterizes the output of the motor as much as possible.

The car in motion resists the thrust of the engine, the losses go to aerodynamics, friction and a set of potential energy when moving uphill. The more energy that goes into this work every second, the higher the speed of the car will be, and hence its efficiency as a vehicle.

Power is measured in horsepower, which has developed historically or in kilowatts, this is accepted in physics. The ratio is simple - one horsepower is approximately 0,736 kilowatts.

Power types

Engine thrust is created by converting the energy of the burning mixture in the cylinders into mechanical work to rotate the crankshaft and the associated transmission. The key value is the pressure on the piston in the cylinder.

Depending on the method of calculation, the power may be different:

• indicator - is calculated through the average pressure per cycle and the area of ​​the piston bottom;
• effective - approximately the same, but the conditional pressure is corrected for losses in the cylinder;
• nominal, it is also the maximum - a parameter closer to the end user, indicating the motor's ability to fully return;
• specific or liter - demonstrates the perfection of the motor, its ability to give the maximum from a unit of working volume.

Since we are talking about work per unit of time, the return will depend on the speed of rotation of the crankshaft, with increasing speed it increases.

But only theoretically, since losses increase at high speeds, the conditions for filling the cylinders and the operation of the support mechanisms worsen. Therefore, there is a concept of revolutions of maximum power.

The engine can spin up more, but the return will decrease. Up to this point, each value of operating speed corresponds to its power level.

How to find out engine power

The value of the parameter is calculated during the development of the motor. Then tests, fine-tuning, optimization of modes are carried out. As a result, the rating data of the engine indicates its rated power. Practically referred to as the maximum, it is clearer to the consumer.

There are motor stands that can load the engine and determine its power at any speed. This can be done in the vehicle as well.

 

It is installed on a roller stand, the energy released into the load is accurately measured, losses in the transmission are taken into account, after which the computer gives a result related directly to the motor. This is useful in diagnosing the condition of the car, as well as in the process of tuning, that is, refining the engine to improve the selected characteristics.

Modern engine control systems store its mathematical model in memory, it is used to supply fuel, develop ignition timing and other operational adjustments.

According to the available data, the computer is quite capable of indirectly calculating the power, sometimes the data is even displayed on the driver's indicator displays.

What is torque

The torque is equal to the product of the force and the lever arm, which can be the engine flywheel, any transmission element or the drive wheel.

This value is exactly related to the power, which is proportional to the torque and rotational speed. It is she who is taken as the basis of the engine model during the operation of the control computer. The moment is also uniquely related to the pressure of the gases on the piston.

The main difference in the torque value is that it can easily be changed in the transmission. Downshifting in a box or changing the gear ratio of the drive axle gearbox, even a simple increase or decrease in the radius of rotation of the wheel proportionally change the moment, and hence the tractive effort applied to the car as a whole.

Therefore, it is meaningless to say that the car is accelerated by the engine torque. It is enough to turn on the gear lower - and it will increase by any amount.

External speed characteristic (ВСХ)

The relationship between power, torque and revolutions clearly demonstrates the graph of their correspondence. Revolutions are plotted along the horizontal axis, power and torque are plotted along two vertical ones.

In fact, there can be many VSHs, they are unique for each throttle opening. But they use one - when the accelerator pedal is fully depressed.

It can be seen from the VSH that the power increases with the increase in speed. Not surprising, since it is proportional to them at a constant torque, but it cannot be the same at all speeds.

The moment is small at the lowest, then increases, and decreases again as it approaches the maximum. And so much so that the power has a peak at those same nominal speeds.

The practical value is not so much the moment as its distribution over revolutions. It is desirable to make it uniform, in the form of a shelf, it is more convenient to control such a motor. This is what they strive for in civilian vehicles.

Which engine is better, with high torque or power

There are several types of engines:

• low-speed, with a "tractor" moment on the bottoms;
• high-speed sports with a pronounced peak of power and torque closer to the maximum;
• practical civilians, the torque shelf is leveled, you can move with a minimum of switching, while having a power reserve if you spin up the engine.

It all depends on the purpose of the engine and the preferences of the driver. Power is important for athletes, they are not too lazy to switch in order to have moment on the wheels for acceleration from any speed. But such engines need to be promoted, which gives extra noise and a decrease in resource.

Truck diesels and engines with modern turbocharging systems are tuned to work with high torque at low revs and low speed at maximum power. They are more durable and easier to manage.

Therefore, now it is the main trend in motor building. It is automatic transmissions and an even distribution of torque along the rpm curve that allow you not to think when choosing an engine, but to look only at its maximum output power.

The CVT or multi-speed automatic transmission will choose the optimal moment on the drive wheels themselves.