You’ve probably heard of engine torque. It’s that thing that has something to do with power. But what is it exactly? And how do you design an engine to have more of it?
Torque is rotational force. It’s what allows engines to overcome resistance, like towing a heavy load. It’s also a key factor in engine power, which largely dictates how fast something will go.
Think of torque like a linear force. If something is hard to push in a straight line, then you need more force to get it moving. It’s just like that with an engine, except we are dealing with a rotational force driving a crankshaft. This rotational force finds its way to the wheels of the car to provide movement.
To explain how we get more torque, let’s first dig a little deeper into it.
The torque equation
The fundamental equation for torque is:
τ = f x r
Where,
- τ is the torque
- f is the force applied to a lever.
- r is the perpendicular distance from the force to the pivot point.
According to this, there are two ways to increase torque; provide more force to the leaver, or increase the lever length.
It’s just like when you are trying to loosen a rusted-on nut. To get it going, you get out a longer wrench, and you push harder. Both those actions give you more torque.
But what if the force is not perpendicular to the lever? Then you need to use your trigonometry skills to break down the force into components and find how much is acting at right angle. Without going too deep into this, it basically means that torque will be less if you apply a force in a non-perpendicular fashion.
The units for torque are Newton-metres (N·m) in the metric system. For the imperial system it is foot-pound (ft-lb).
Applying the torque equation to engines
Now that we’ve learnt the equation for torque, how can we apply it to an engine?
With an engine, the lever is the crankshaft throw. And the force applied to the crankshaft comes from the combustion force on the piston.
The crankshaft throw is the distance between the centre of the crankshaft and the centre of a crankpin. The dynamics of the crankshaft are not as simple as a basic force and lever diagram, as there is a piston rod connecting it to the piston, and there are changing force angles. However, the same principles still apply, and increasing the crankshaft throw will increase torque. To do so, we also need to increase the engine stroke, which is the distance the piston travels up and down. In fact, the crankshaft throw is equal to half the stroke.
To increase the force of combustion, we can increase the engine bore which is the diameter of the piston. Why does this increase the force? To explain, consider the the formula, pressure = force / area, and rearrange it to, force = pressure x area. When combustion occurs, it exerts pressure on the piston. By looking at the formula, we can see that by increasing the area, we are also increasing the force, even if the pressure is constant.
Same engine displacement, same torque
Engine torque is a function of displacement. In other words, two engines of the same displacement but different stroke and bore sizes will have the same torque. At least in theory.
To illustrate, let’s look at two engines:
Engine 1
- Stroke: 11cm
- Bore: 9cm
- Displacement: 3.14 x (9 / 2)2 x 11 = 699cc
Engine 2
- Stroke: 8cm
- Bore: 10.55cm
- Displacement: 3.14 x (10.55 / 2)2 x 8 = 699cc
Now, assuming both these engines have the same cylinder pressure of 4000kPa at the time of combustion, we can calculate the torque they make.
Engine 1 torque
- Force = 4000 x 3.14 x (9 / 2)2 = 25.4 kN
- Crankshaft throw = 11 / 2 = 5.5cm
- Torque = 25.4 x 0.055 = 1.4 kN·m
Engine 2 torque
- Force = 4000 x 3.14 x (10.55 / 2)2 = 34.9 kN
- Crankshaft throw = 8 / 2 = 4cm
- Torque = 34.9 x 0.04 = 1.4 kN·m
As you can see, no matter how we change the bore and stroke of an engine, the theoretical torque will be the same if displacement is the same. This is assuming that the cylinder pressure made at combustion is the same.
In reality, large bore engines may make a little less torque because of their larger cylinder surface area at the time of combustion. This allows more heat to escape, which means less of the combustion energy turns into kinetic energy and torque.
Torque varies with engine RPM
What makes engines tricky is they show different levels of torque at different RPM.
In the above example, we assumed both engines had the same cylinder pressure at combustion. But in reality, the cylinder pressure, and therefore the force, varies across an RPM range. The quality of the intake, compression, and exhaust causes this variation.
Engine geometry plays a large part in where peak torque occurs. Some engine shapes allow for better air intake at low RPM and therefore we see peak torque at a low RPM. Other engine geometries show peak torque at a higher RPM.
The intake and exhaust valve tuning is another factor. The opening and closing of these valves is timed by a camshaft. Depending on the exact moment the valves open or close, greater piston force can be made at a low RPM or high RPM.
Undersquare engines and low end torque
Engines with a long stroke and short bore, which are called undersquare engines, typically show peak torque at low RPM.
Why is this the case? For a given RPM, engines with a long stroke have a higher piston speed because the piston needs to travel further for each revolution. With this comes high intake air speed as it rushes in after the downward moving piston. This provides a full intake and effective mixing of the air and fuel.
Tuning of the intake and exhaust valves can further optimise torque at low RPM. For example, the intake valves may be timed to close a little earlier to prevent leaking and compression losses at low RPM.
But as engine speed increases, torque drops off with undersquare engines because the small bore limits the size of the intake valves. As a result, less air volume can enter the engine and intake cannot keep up with demand if RPM gets too high.
On top of this, undersquare engines have a lower maximum RPM because of the higher piston speed. If RPM and piston speed get too high, engine damage can occur.
Harley Davidson engines are a classic undersquare example. Their low end growl produces high torque without the need to rev the engine high.
Oversquare engines with high end torque. And power!
Oversquare engines, on the other hand, have a bore that is larger than the stroke and typically show peak torque at high RPM.
As these have a shorter engine stroke, the piston speed is lower for a given RPM. The result is suboptimal intake air velocity at low RPM. It’s only when the engine speeds up that fuel and air mixing gets better and the combustion force gets stronger.
Although the piston speed is lower for these engines, they have the advantage of being able to fit larger intake and exhaust valves. This means the engine breathing can keep up with high RPM demand. They can also reach higher RPMs as the lower piston speed with respect to RPM means they are less likely to break.
On top of the natural advantages these engines see, valve timing can be tuned for high torque at high RPM. Here, the exhaust valves may open for longer to give the exhaust gases more time to exit while running at high RPM.
For these reasons, sports bikes typically use oversquare engines for maximum power!
Engine power and torque – how are they related?
So now we understand torque is the rotational force an engine can make. But what is power and how does it relate to torque?
Power is the rate at which work can be done. In other words, it’s a measure of what a force does with respect to time. For example, if a force pushes something really far in a short amount of time, then it has higher power! It’s the same for torque, except that we are looking at rotational force instead.
The metric equation for power in relation to torque (τ) is:
Power = τ x π x RPM / 30
For those of you who work with horsepower, the formula is:
Horsepower = τ x RPM / 5252
As you can see from this equation, having peak torque occur at high RPM means the engine will produce maximum power. Therefore, sports bikes and cars are designed and tuned in this way.
This formula also explains why family sedans are typically tuned to have peak torque at a low RPM. It allows the car to have a usable amount of power without revving hard, which reduces engine strain and fuel consumption.
