Petrol or Diesel: Which is Better?

Petrol or Diesel: Which is Better?
Image from Pexels: Petrol

The distinctions between petrol and diesel engines are that diesel engines are lower revving and more fuel-efficient than petrol engines, but do you know exactly how and why they’re different? If not, let this article be your ultimate guide for petrol.

Understanding Their Similarities

Gasoline and diesel engines operate on the same four-stroke cycle. This starts with the intake stroke with a piston that descends, sucking air into the cylinder through open-air intake valves as fuel is injected.

It’s then accompanied by the compression stroke where the valves close and the piston goes back up the cylinder to squeeze the air and fuel mixture. The third step is the power stroke, where the mixture is ignited to force the piston back down the cylinder. The final exhaust stroke occurs where the ignited fuel and air mixture is pushed back out of the piston’s cylinder through the exhaust valves.

Knowing Their Differences

As you can see, petrol and diesel have few similarities, but a ton of contrasting qualities (hence, this article). For one, petrol and diesel engines differ in how they ignite the air and fuel. To understand the difference, we need to understand the self-ignition temperature.

What is Self-Ignition Temperature?

The self-ignition temperature is the level at which an air-fuel mixture will ignite without using a spark plug, purely due to compression. This compression allows the air and fuel mixture to ignite in a diesel engine without the need for a spark because the more significant compression, the higher the temperature. That’s why diesel engines have higher compression ratios than petrol engines that don’t compress the air and fuel mixture to the ignition point. 

If you don’t know what a compression ratio is, to put it simply, it’s the ratio of the maximum to the minimum volume within the cylinder. A ratio of ten to one means that when the piston is at its lowest point, there is ten times as much volume as when it’s at its highest point.

To give you an example of compression ratios, a 2015 VW Golf TSI has a compression ratio of 9.6 to one while a 2015 VW Golf TDI has a compression ratio of 16.2. Again, a diesel will have a higher compression ratio to ensure that the air and fuel mixture is compressed enough to the ignition point.  Simultaneously, a petrol engine uses less compression and is ignited by the spark plug, though no longer correct for all modern oil burners.

What is Throttle Body and Why is it Important for a Petrol Engine?

Another significant difference between a diesel versus a petrol engine is the diesel’s lack of throttle body. This means that when you press on the accelerator pedal, the fuel injectors supply more diesel to create more power. Petrol or gasoline engines, on the other hand, require a throttle body. As you press on the accelerator pedal, you open up the throttle and allow more air to flow into the engine. More air means the injectors send in more fuel, and more fuel means more power.

What are Their Differences When Engine Braking?

Petrol and diesel engines also differ in how they can engine brake—particularly when you lift off the throttle in gear to slow the car down in a petrol vehicle. Engine braking is achieved because as you lift off the throttle, the throttle body closes, creating a vacuum within the throttle body and the cylinders to slow the car down in a diesel engine vehicle with no throttle. Body engine braking occurs during the compression stroke, where the exhaust valve is open to allow pressurized air to escape. 

Why is Diesel More Efficient Than Petrol?

So why exactly are diesel engines more efficient than petrol engines? One of several reasons is down to the fuel itself. Diesel consists of more long-chain hydrocarbons, which means it’s got more energy than petrol.

It contains about 15% more energy by volume, roughly 36.9 megajoules per liter compared to petrol. 33.7 megajoules per liter diesel engines are also more efficient because of the thermal efficiency gains you get from igniting the air with high compression rather than a petrol engine’s lower compression ratio that requires a spark for ignition. 

The higher compression ratio, longer stroke, and typically higher turbo boost pressure over diesel also allow it to produce more torque at lower RPM than a petrol engine, meaning less fuel is needed to move a car. Lastly, diesel is more fuel-efficient because it can be turbocharged. After all, a turbo feeds compressed air into the cylinder, which helps the piston descend into the cylinder, saving energy. 


Now that you know petrol’s inner workings versus a diesel engine, which one are you a bigger fan of? Let us know in the comments down below. But if you’re still on the fence, we sure hope this article provides some sort of clarity.

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Arnold Alexander
Arnold Alexander
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Arnold knows how cars work, from the inside-out. He’s the team’s go-to guy when it comes to technicalities.
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