Why diesel engines produce more torque than petrol engines – TopAuto
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Wednesday / 8 December 2021
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Why diesel engines produce more torque than petrol engines

When you compare a diesel and petrol engine with identical displacements, the diesel version tends to produce more torque.

For example: The current BMW 3 Series range includes 2.0-litre turbo-diesel, and turbo-petrol options.

The diesel engine generates 140kW and 400Nm, while the petrol produces 135kW and 300Nm.

Below, we find out why this is the case.

Petrol vs Diesel

Petrol and diesel are both types of fuel derived from crude oil through fractional distillation at different temperatures.

The fuel types have different chemical compositions, though, with diesel generally having a ratio of 75% saturated hydrocarbons (primarily paraffins) and 25% aromatic hydrocarbons.

Petrol primarily consists of a mixture of paraffins, naphthenes, aromatics, and olefins – with the ratio of these elements varying on a sample-by-sample basis.

Petrol is more volatile and is generally less dense than diesel, due to this inconsistent ratio, as well as the individual additives that manufacturers put in their products.

It therefore contains less energy, leading to less efficient power delivery per volume of petrol when compared to diesel.

A petrol engine also works with a spark plug igniting the fuel in an engine cylinder, whereas diesel engines use heated air and pressure to ignite fuel and produce power.

Compression ratio and combustion speed

Another factor is the compression ratio of an engine.

This is the ratio between the total volume of the cylinder and the volume of the open chamber once a piston is fully extended inside the cylinder at its uppermost point – the top dead centre (TDC).

A petrol engine generally has a lower compression ratio due to the piston’s TDC being at a lower point to allow for a larger combustion chamber between the surface of the piston and top of the cylinder.

The combustion chamber is also larger than on diesel engines to avoid high temperatures from built-up pressure igniting the air/fuel mixture prematurely.

In petrol engines, a spark plug ignites the air/fuel mixture when the piston’s TDC is reached, with this explosion creating torque when pressure pushes the piston back down.

The explosion takes longer than in diesel engines, which leads to slower and weaker delivery of torque.

Diesel engines’ combustion chambers are smaller, due to the engine relying on the high heat of compressed air igniting the fuel when it is pumped into the cylinder as the piston hits the TDC.

While not exactly instant, this explosion happens much quicker and closer to the top of the cylinder, and due to the higher density of diesel, also delivers more instantaneous power – thus pushing the piston down at a faster, more powerful rate, as well as further.

This creates more torque.

Bore vs Stroke

Petrol and diesel engines also have different bores and strokes, which refers to the width and length of the cylinder.

The bore is the distance between the outermost edges of the chamber, whereas the stroke is the distance that the piston travels in the cylinder.

Petrol engines generally have a wider bore and shorter stroke than diesel engines and need to reach higher engine speeds (revolutions per minute) to match the equivalent output of a diesel motor.

As torque is the product of distance times force, both bore and stroke measurements have an effect on torque.

The difference in strokes generally has a higher impact than the differences in bore, however, as there’s usually a more significant deviation between the length of pistons than the width.

This means that the pistons in diesel engines travel further than petrol engines, thus producing more torque when the engines are operating at the same revolutions per minute (rpm).

While an engine using petrol and one using diesel might have the same maximum power output, the different bore and stroke will affect the levels of torque produced at various engine speeds.

Turbos

Additionally, diesel engines tend to be turbocharged and due to their higher operating temperatures, are generally made from stronger materials.

The engine also burns at a higher air to fuel ratio which makes turbocharging more efficient, as the turbo compresses more air while using less fuel when compared to an equivalent petrol engine.

The more air that is compressed, the higher the output will be.

Coupled with the stronger engine block and denser fuel type, meaning more energy per volume of fuel, a turbo is able to provide a much higher boost more efficiently in a diesel engine when compared to an equally-sized petrol engine running at the same rpm.


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