Supercar makers prefer to build their vehicles to run on petrol, mostly due to the lower power-to-weight ratio of diesel engines.
This is evidenced by the fact that in 2021 there are very few high-performance vehicles running on diesel.
The closest thing we currently have to a diesel-powered “super” vehicle is the limited-edition Audi SQ7 and SQ8 SUVs that went on sale earlier this year.
While these diesel-driven SUVs feature a 4.0-litre, twin-turbocharged engine that puts out more torque than the RS Q8 with its identical petrol setup, they produce much less power and accelerate far slower than the petrol-powered flagship.
Trying to make this comparison with supercars is futile, as there are no new diesel-powered supercars on the market.
One of the last high-performance diesel vehicles from a premium brand was introduced in 2013 in the form of the Maserati Ghibli Diesel.
Development of this vehicle ceased in 2020 as a petrol-hybrid Ghibli replaced the diesel version in the line-up.
Even brands like Porsche have sworn off diesel in pursuit of improving its other propulsion technologies.
Why petrol?
The reason petrol engines are preferred to diesel for high-end cars is because they offer a superior power-to-weight ratio.
This means they produce more power per kilogram than diesel engines of identical size.
The compression ratio for a petrol engine generally ranges between 1:8-10, whereas a diesel engine is usually between 1:15-20.
“The compression ratio (CR) is defined as the ratio of the volume of the cylinder and its head space when the piston is at the bottom of its stroke to the volume of the head space when the piston is at the top of its travel,” according to ScienceDirect.
This smaller ratio results in petrol engines having to do less work to achieve the same level of power – in kilowatts (kW).
Furthermore, petrol engines tend to have shorter cylinder strokes and wider bores, which cause these motors to operate at higher revolutions-per-minute (rpm) when generating the same amount of torque – in Newton metres (Nm) – as a diesel engine.
With torque levels equal, a petrol engine is working faster than a diesel engine, thus producing more power at that moment due to the varying compression ratios.
As lightweight, cutting-edge supercars are more dependent on power than torque to achieve their impressive performance statistics, petrol engines are preferred as they are lighter and produce more power than an identical diesel motor running at the same engine speed.
The thermal efficiency of petrol motors is also higher than diesel due to their lower operating temperatures, allowing thinner and lighter materials to be used to build the engine.
Lower operating temperatures also have a positive effect on motor longevity, which is another welcome benefit for expensive supercars.
The low-rev torque advantage that diesel motors have over petrol motors is generally more useful for utilitarian purposes such as towing and transporting loads, which is why this powertrain is more often seen in larger and heavier vehicles.
As technologies improved, turbocharged petrol engines have started cutting down diesel’s torque advantage at low engine speeds, however, often providing the full torque band from under 2,000rpm.
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