Engine torque: what it is and how it affects the performance of your engine

When we see the publicity that the different brands make of their cars in the media, we can see that, at a technical level, they usually show a series of figures related to speed, consumption, acceleration... in short, some cold numbers that also a high percentage of drivers will never be able to match. However, there is a physical fact that all drivers do enjoy, that is rarely publicized and that not many years ago was given a certain importance: the torque.

Not so long ago, when cars had not yet undergone the current power escalation, the reprise of the car as the ability it had to gain speed. This popular affirmation, although when it comes to interpreting what reprís is is correct, to understand what torque is it falls a little short or rather inaccurate

What is torque?

Engine torque, also known as torque, is a physical magnitude that measures the moment of force to be applied to an axis that rotates on itself at a certain speed. Applied to the automotive world and explained in a way that we can all understand, it can be defined as the force required for the engine crankshaft to rotate and, therefore, be capable of transmitting said movement to the rest of the mechanical elements necessary to move the vehicle.

And this is where we observe the first difference between reality and custom; When we refer to engine torque to express the acceleration capacity of a vehicle, we are not really defining what engine torque is, we are only describing one of its applications. This is so because the torque of an engine measures the power required for the engine to turn a certain number of revolutions but does not take into account the additional power that must be applied to modify the angular speed of the shaft or crankshaft.

A bit of physics to explain torque

To explain to you what motor torque is, fleeing from physical principles, I will explain the function of the crankshaft and the forces that act on it.

A heat engine generates Energy in the cylinders. Specifically, it is in the combustion chambers where the fuel-air mixture explodes. It is the energy released by this explosion that generates a linear movement by pushing the piston in the opposite direction to that of the engine head. The pistons of the different cylinders are attached to the crankshaft by Rods and it is just in the union of these with the crankshaft where the linear movement is transformed into rotational motion.

It is worth mentioning at this point the exceptional construction of the rotary motors, in which the circular chambers of the "cylinders" directly surround a central axis that rotates on itself moved by the explosions produced in the chambers, so that in this case the rotational motion. In any case, the physical principles that act in regard to engine torque are the same.

Even without going into an excessive study, to simplify the idea of ​​energy transformation, it could be said that the rotating blocks generate torque instead of power. No faith can be made in this regard because neither the chambers nor the rotor of rotary engines are exactly circular and the ignition of the fuel occurs in a portion of the chamber, unlike conventional cylinder engines in which the fuel-air mixture occupies its entire volume.

Going back to the physical explanation, the force exerted by the piston on the crankshaft is not constant throughout the expansion process. This is because within each cylinder the maximum value of power is generated at the moment of fuel ignition. And with these moments of maximum power come moments of maximum torque.

The delay between the moment in which the maximum power is generated in the cylinder and the maximum applied to the crankshaft is not easily calculated. This is because the pistons do not make a purely linear movement but rather, because the crankshaft is not completely straight either, they make a movement that combines the linear effect of the piston with the circular effect of the connecting rod bearings.

However, these moments of maximum power and maximum torque are of great importance in terms of the perception of smoothness in the operation of the engine.

The more cylinders the vehicle has, the more times per minute that moment of maximum force will exist and more homogeneous will be the driver's perception of the smooth running of the engine.

This is due to the fact that in a 2-cylinder engine, there will be a single moment of maximum force every 360º of rotation of the crankshaft, in a three-cylinder engine it will happen every 240º, in one of six every 120º and so on. Of course, this must be interpreted as pure theory since today manufacturers strive to make their engines as smooth as possible in terms of their operation.

This factor also influences the fact that at idle an engine generates more vibrations and that they are also more noticeable: at 1.000 revolutions per minute there are half the moments of maximum force than at 2.000 revolutions. For example, starting from an average idle speed of 850 revolutions per minute, a three-cylinder engine will generate less than ten moments of force per second, while a six-cylinder block will generate almost twenty.

If we take into account that the "normal" human, faced with an intermittent force of continuous application, better recognizes intervals greater than a tenth of a second than those less than, here is the banal explanation by which the general public recognizes the vibrations of the motors of two or three cylinders: because the interval between the moments of maximum outside is greater than a tenth of a second.

What torque does your motor deliver?

In many publications on the motor world, the torque that a vehicle's engine "delivers" is usually measured. This statement, by definition, is not correct as long as we understand that the pair is a applied force and not one resultant force. However, also due to the physical principle of action-reaction, when a moment of force is applied to an axis that rotates on itself, another moment of force is automatically generated with the same intensity and direction but in the opposite direction to the original (Newton's third law).

How to Calculate Motor Torque – Motor Load

The motor torque can be measured but its calculation is extremely complicated and almost impossible for mortals, so it is easier to leave it to professionals capable of handling modern machines and very complex computer programs, although at first glance we only see a roller bank.

As follows from its definition, in a combustion engine torque is a variable which depends on the power generated in the cylinder chambers and the number of revolutions at which the engine is turning at that particular moment, so its value could be calculated from the formula P = T · ω where P is the power expressed in watts or watts, T is the torque expressed in Newton meters and ω is the radial speed of rotation expressed in radians per second.

However, there are other factors that affect the theoretical values ​​that could be obtained from the direct application of the formula, such as the internal engine friction. These internal frictions mean that a part of the power obtained by the motor cannot be used externally but rather is "lost" in the same process of movement of the motor, normally in the form of heat. Remember that energy is neither created nor created nor destroyed, it only transforms.

There are also external factors that can affect the power generated by an engine, even in situations that could be internally comparable. For example, the same engine turning at a constant speed of 2.000 revolutions per minute will generate more power when driving on a flat road than going down a slope. Although the number of revolutions is constant, and therefore also the angular speed of the crankshaft, the different value of the power generated at each moment also translates into a different value of the torque applied to the crankshaft.

Many of you will wonder how this can be and the explanation is very simple. As we all know, the movement is generated thanks to the ignition of the stoichiometric mixture of fuel-air in the cylinder chambers and if less power is required the solution is to inject a mixture that is leaner in fuel and richer in air. This is also the reason why the computers in our cars mark a lower or even zero instantaneous consumption when we lower a port.

All these parameters that modify the operation and the theoretical results of a mechanism are called engine load, which can be defined as the amount of torque that a motor must produce to overcome the resistances that oppose its movement.

As we have seen, the engine load depends both on internal causes of the engine, such as the friction of its different moving parts, and on external agents such as the friction of the tires or the car's own aerodynamics. I have given these two examples totally external to the mechanics of the vehicle because in both cases they generate forces that are contrary and constantly variable to the movement of the vehicle, which also has repercussions on the motor load value will be a parameter too constantly variable.

Engine load also affects us while driving in a very clear way that all drivers appreciate. If we continue with the same example of a vehicle traveling at a constant speed and at a constant engine speed, why is it harder for the car to gain speed on an uphill section than on a downhill section? Well, due to the variation of the motor load.

Entering again in a theoretical world, when a car circulates at a constant speed on a flat road, it has two external forces that oppose its movement: aerodynamics and drag. When the vehicle begins to circulate on an ascending section, if we keep the speed constant, we can consider that the aerodynamic force contrary to the movement is maintained, but the friction is modified in the sense that it is a gravitational force and at the moment that the vehicle starts to rise, there will be a part of the friction that "pulls" the car backwards.

If we want to spin very finely, we can also bring into play kinetic energy and potential energy. The kinetic energy depends on the mass and speed of the vehicle and the potential energy on the mass and height. As the height increases, by the principle of conservation of energy, the kinetic energy will be transformed into potential energy.

In this case of uphill road, by adding the set of external forces that oppose the movement, we can say that the motor load increases and therefore, the amount of "usable" torque of the motor decreases, and several situations can be observed:

• If we want maintain constant rotation of the motor we must demand more power by pressing harder on the throttle to inject a richer mixture of fuel into the cylinder chambers.
• If the inclination of the road increases, the time may come when the vehicle begins to lose speed. This is due to the fact that the motor load (forces contrary to movement) is greater than the torque capable of being generated in the motor (positive forces to movement).

• by staying constant power and torque, and increasing engine load, less power will be available to increase vehicle speed because acceleration is proportional to applied force: less power means less acceleration power.

Engine torque and gearbox

However, physics is also capable of modifying the behavior of bodies subjected to different forces, and in the case of our car's engine crankshaft, it can be said that it is capable of send the torque it receives from the cylinders to other parts of the vehicle, such as the gearbox.

Torque comes from the engine to the gearbox in the form of rotational motion through the input shaft. This is why when a manufacturer talks about its catalog of changes, it always talks about torque limitations and not power. Inside the gearbox there is a transformation from torque to tangential force and back to torque. How?

Inside the gearbox there are a number of toothed wheels that transmit the movement to each other simply by the meshing of the teeth with each other. These toothed crowns, which refer to the number of gears that the transmission has, have a different size or “gear ratio”, that is why it can sometimes be read that a transmission has x speeds or x ratios; is the same.

In any case, this different size of the ring gears is what varies the input and output torque also by the physical principle of conservation of energy: When two wheels turn in mesh (theoretically) they conserve energy, so the product of the torque times the angular velocity must be kept constant.

Explaining the basic principle that affects torque, the lower speeds have larger sprockets than those of the higher gears and its physical logic is very easy to understand with an example because it is something that all drivers perceive and know. take advantage, so we continue with the same car circulating at 2.000 revolutions per minute, generating constant power and torque.

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circulating in first gear, the input input shaft is torqueing the gearbox with a given angular velocity but is in gear. larger ring gear which will rotate at a lower speed than the input shaft. Since the power remains constant in the gear, As the angular speed of rotation decreases, the torque increases..

If, on the other hand, we circulate in the highest gear, with the ring gear even smaller than that of the primary input shaft, just the opposite will happen: the ring gear of the highest gear will rotate faster and therefore the output torque will decrease. .

This variation in torque in the face of a theoretical constancy of both the effectiveness of the block and the engine load is responsible for the different behavior that can be observed in the car when gaining speed. Because everyone knows that driving at a constant speed, it is easier to increase the speed of the engine in a low gear than in a long one, even though the power and torque generated in the engine are the same.

The reason is that in a higher gear less torque reaches the drive wheels. The reason is that at the same rpm, the tires will spin faster the higher the gear. That is why sometimes we can climb a fairly steep ramp in first gear at 1.500 revolutions per minute and other times, driving in 5th or 6th, the slightest slope makes us reduce a gear so as not to lose speed even if we drive at a higher regime of revolutions.

Logically, we are once again in a theoretical world because, in practice, as the speed increases, the aerodynamic force that tends to slow down the car also increases, the energy losses for example, due to the greater heating of the tires... In short, a series of external agents that generate forces contrary to movement and that it is simply worth that they sound a bit familiar to you to better understand the engine torque.

Torque in electric motors

As in rotary engines, electric motors generate directly rotational motion and, therefore, torque instead of power understood as such. This is because the operating principle of an electric motor is based on a basic principle of magnetism whereby charges of the same sign repel each other and charges of the opposite sign attract each other.

La constructive basis of an electric motor, explained roughly, for being a magnetized cylinder traversed by a rotor that rotates on itself thanks to the constant changes in load of the outer cylinder. The most basic example would be that of the compass: if it is not touched it points to the magnetic north of the earth, but if we bring a magnet closer and make it rotate in circular movements around the compass, its needle will rotate on itself at the speed with which that we are moving the magnet.

There is a basic difference when it comes to the quality of the pair obtained: es almost challenge. While in a heat engine the torque figure can vary depending on the number of revolutions at which the block rotates, in an electric motor the torque is almost constant. This is due to the basic operating principle of these engine types and the technology applied today.

As I have mentioned, the rotation of the rotor of an electric motor is due to the continuous stator bias which becomes a small magnetic field able to turn the rotor by the alternation of forces of attraction and forces of repulsion and it is at this point where current technical advances allow the gravitational forces generated in the rotor to have an almost constant maximum torque.

Electric motor torque vs. thermal motor torque

I have commented that the pair is almost constant for a very specific detail and that explains in a certain way the limitations of electric cars on motorways or dual carriageways but also their advantages in urban traffic. Unlike a heat engine, electric motors generate motor torque from the beginning of the rotation and they keep it constant until the maximum power level is reached, at which point the torque figure drops. To cite an example, the BMW i3 offers maximum power X and a maximum torque of 250Nm, but let's see how it is distributed:

• The electric motor of the BMW i3 offers a constant torque of 250Nm from almost 0 engine revolutions to approximately 4.500 engine revolutions per minute.
• In this interval from 0 to 4.500 revolutions per minute the power increases from 0 to 170 horsepower (127kw).
• Starting at 4.500 revolutions per minute, both torque and power begin to decrease.
• At 8.000 revolutions per minute the engine of the BMW i3 offers approximately 150 horsepower and a torque of 125Nm.

What reading can be made of these figures? Well, in the case of the BMW i3 engine, it can be said that it is equipped with a very cheerful engine up to 4.500 rpm, which makes this car very quick on acceleration at low speed. In fact, it reaches 100 km/h starting from a standstill in just 7 seconds, which allows it to challenge itself face to face with the BMW 120i.

However, from 4.500 revolutions Both power and torque begin to decrease and negatively affect both acceleration capacity and consumption, which may double compared to the approved figures. This is also why many electric cars have a “ECO” mode which limits its top speed to 90 or 100km/h, just when a car like the BMW 120i could obtain, by keeping the speed constant, very low consumption.

By the way, there is another very striking and interesting advantage of cars equipped with electric motors: they show less sensitive to sporty driving or city traffic and the increase in energy consumption is not as pronounced as it would be in a vehicle with an equivalent thermal engine. That's because by offering such a high and relatively constant torque, the motor can be said to have easier to increase the speed of rotation of the motor or that demands less increase in torque to increase its speed of rotation.

Petrol torque vs. diesel torque vs. supercharging torque

In this section it is not advisable to go too long because the differences between the torque obtained from a block powered by gasoline and another powered by diesel are due to the particular construction characteristics of each other and the released energy by the ignition of their respective fuels.

If we attend to a classic reading of these figures, understanding as such a comparison between atmospheric blocks fed by injection or what would more or less be a jump to the 80 years, the diesel fueled blocks offered more torque and at a lower rpm compared to the gasoline blocks, but in today's eyes, its power levels could even be ridiculous.

In this regard we can remember the beginning of the article where I explained that the theoretical power of the vehicle is proportional to the torque and the angular speed of rotation. An atmospheric gasoline vehicle has a actual margin of use approximately between 1.000 and 5.500 revolutions per minute and an atmospheric diesel between 1.000 and 4.000 revolutions per minute. In the real world, the practical margin of use It ranges between 2.000 and 4.000 revolutions per minute for gasoline engines and between 1.500 and 3.000 revolutions for diesel fueled mechanics.

If we leave one of the variables constant, for example the turn at 2.000 revolutions per minute, we will obtain less power in the diesel engine but at the same time it will offer us more torque. What is this about? Well, it's simple, the engine torque is caused by the linear movement of the pistons according to the ignition of the fuel in the cylinder chambers and the power that is generated depending on whether gasoline or diesel is burned is different. However, the mechanical explanation is valid for both cases.

Electronics and supercharging

To this day, what I have just explained to you remains for the memory of the most nostalgic. In fact, many of you will have noticed that sometimes a manufacturer offers vehicles with different torque and power figures extracted from the same engine block. Or even a vehicle that have a “ECO” mode capable of modifying these figures by simply pressing a button, as is the case, for example, with the Fiat Panda Cross TwinAir: in normal mode it offers 90cv and 145Nm and in “ECO” mode it stays at 78cv and 100Nm.

This is due to Technical advances and above all electronics applied to the automotive world. Today we are no longer surprised to hear about the phase variator for vehicles with multi-valve heads, diesel and gasoline engines with the same compression ratio or even variable compression engines, but if there is something that has represented a giant step in regard to the figures of torque and power of a vehicle is the supercharging.

Although its mechanical explanation can become very complicated, the basics of overfeeding is very simple: increase the pressure inside the cylinder chambers to increase the force generated in the ignition of the fuel, which makes the pistons descend with more force and, therefore, more torque reaches the crankshaft.

As expected, its mechanical implementation is somewhat more complicated and requires much study of its correct location inside the hood of a car, new inlet and outlet manifolds, specific reinforcements in the pistons, connecting rods, crankshaft... but the basic principle is to increase the pressure inside the cylinder chamber and this is what matters to relate it to the torque of an engine.

Supercharging can be driven directly by the rotation of the engine or by the pressure of the exhaust gases. Nowadays, electronics has also reached supercharging and the new Audi SQ7 TDI has premiered the first electric turbo on the market and the results could not be more spectacular: X constant between 3.750 and 5.000 revolutions per minute and 900Nm constant between 1.000 and 3.250 revolutions per minute.

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The torque yesterday and today

Until not many years ago, only the most knowledgeable knew that a car with square cylinders (diameter = stroke) was the most balanced to drive, that if the stroke was less than the diameter it would be a powerful car but with a modest torque figure and that if the stroke was greater than the diameter it would be just the opposite, quieter and with more torque.

Nowadays most of the motors belong to modular families, which allows manufacturers to offer blocks with more or fewer cylinders and gasoline or diesel with relative ease and minimal changes, variations in torque and power are given by the use and combination of different technical and electronic applications that the manufacturer wants to use.

Despite all this that I have explained in this article, reality surpasses theory in all aspects. In the current market we can find six-cylinder engines with the power of one of eight, three-cylinder engines as smooth or more than other four-cylinder engines of similar capacity or even diesel engines with the same compression ratio as gasoline ones and that is Today everything is possible.

La Fundamental reason of this article was to explain in an understandable way what is the engine torque or torque, that you be able to recognize how it affects daily driving and that you realize that the power of a car, if it is not related to the engine torque, It is not a very indicative value of its behavior. I hope I have succeeded.