Why Do Diesel Engines Have More Torque?

Diesel engines were initially designed to produce enough power to move heavy loads. You’ll typically find them in light trucks, agricultural applications, construction equipment, and other types of automobiles. Diesel engines produce more torque to cause a rotation.

Where they fall short in horsepower, they compensate with torque. Gasoline engines are the opposite. Where they fall in torque, they compensate with horsepower. This is why they are a favorite among performance enthusiasts.

Here are 5 reasons why diesel engines have more torque.

1.   Higher compression ratio

Compression ratio (CR) is defined as the ratio of the volume of an engine’s cylinder when the piston is at top dead center to the volume of the same cylinder when the piston is at bottom dead center.

Top dead center (TDC) is the point at which the piston is at its highest point in the cylinder. The opposite is referred to as bottom dead center (BDC). The CR for petrol engines averages 8-10 while that of diesel engines averages 15-20.

This means that the piston moves up and down a larger internal volume in a diesel engine compared to a gasoline engine. This is due to the fact that diesel engines do not use spark plugs to ignite fuel. They use compression ignition.

The air is compressed inside the cylinder to produce the necessary high temperature before the fuel is introduced. The fuel is injected into charged compressed air which ignites it. This method is used because diesel is less volatile and has less self-ignition tendency.

So, a higher compression ratio provides enough capacity to ignite the fuel. The diesel piston runs all the way from the very top of the cylinder to the bottom, creating more compression. The stroke length is, therefore, longer. This results in more torque output.

Petrol engines have pistons that stop a bit lower at the top of the cylinder. They use spark plugs because petrol is more volatile and burns easily by a little spark. With this, less compression ratio is needed to produce torque.

2.   Speed of combustion

Compression ignition burns fuel faster than spark ignition. This means that diesel engines have better thermal efficiency. The compression builds a considerable amount of pressure and heat inside the cylinder before fuel is delivered.

A mist of metered fuel is sent into the hot compressed air where it bursts into a controlled explosion. This turns the rotating mass within the engine. The auto-ignition means that the engine is not limited by a premised fuel and air combustion knock which is common in gas engines.

The engine creates power immediately just as the piston is traveling back down the cylinder. The piston in gasoline engines doesn’t travel as quickly. The diesel moves much faster, hence, resulting in more torque.

3.   High calorific value

Calorific value refers to the amount of heat released during combustion. Diesel has a calorific value of 45.5 megajoules per kilogram (MJ/kg) compared to that of petrol at 45.8 MJ/kg. While the difference here may not be significant, diesel is denser than petrol.

It contains roughly 15% more energy per volume. Therefore, it produces more energy during combustion. The difference in energy density results in about 20% more efficiency. When you account for everything else, this can go to as high as 40%.

4.   Turbochargers

A petrol engine has a higher peak power-to-weight ratio when compared to a diesel engine. Power-to-weight ratio is the amount of power a car produces relative to its weight. The higher it is, the better.

Due to their large compression ratio, diesel engines are heavier and tend to have a lower peak power-to-weight ratio. To fill the gap, manufacturers add turbos that bring the peak ratio closer to that of a gasoline engine.

The turbo blows more air into the combustion chamber. This increases the air mass and causes more fuel to be sent to the engine for combustion. More air means more compression inside the engine.

More compression means higher pressure inside the cylinder, hence, an increase in torque. Most diesel engines now include turbos as they require a lot of air intake. The turbos improve efficiency and are designed to handle higher boost pressure.

5.   Stroke length

Stroke length refers to how far the piston travels inside the cylinder. As mentioned earlier, the piston has a longer stroke length in diesel engines compared to gasoline engines.

This is because it moves to the very top of the cylinder. Torque is equal to force multiplied by distance. In this case, the longer distance means we get more torque.

On That Note

Diesel engines produce more torque than petrol engines. This makes them ideal for towing applications and carrying heavy loads. However, petrol engines tend to produce more horsepower.

This is because petrol engines have a shorter stroke length that allows for more revs and more power output. The short distance means faster rotation, hence, higher speeds. This is why petrol engines are preferred for sports cars.

In summary, if you wanted to load a bunch of concrete blocks onto a trunk one by one and as quickly as possible, you’d need a petrol engine. If you were to load them all together but slowly, you’d need a diesel engine.

Frequently Asked Questions

Do diesel engines have more torque than electric motors?

Diesel engines generate torque through the combustion of fuel and air. Electric cars use an electric motor. An electric current moves through a magnetic field to create the force needed to rotate the armature that gets the car moving.

Why do diesels sound different?

Diesel engines are noisier than gas-powered engines because they work under high pressure. The diesel molecules tend to be bigger than those of petrol. So to ignite it, the engines lack spark plugs and use compression ignition to ignite fuel instead. They also have oil pipes, valves, and metal caps that make noise.

Why do heavy cars use diesel engines?

Diesel engines produce higher torque. This is required in heavy cars and machinery as it provides the strength to move them. Trucks and heavy machinery need a significant amount of force to run. This is only found in diesel engines.

Why do diesels have lower rpm?

They have a larger stroke length which requires the piston to travel longer inside the cylinder. This provides a high compression ratio necessary for compression combustion. The longer travel distance results in low RPMs in diesel engines.