The differences between petrol and diesel engines. We all know 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, then let this video be your ultimate guide.

Petrol aka gasoline and diesel engines operate on the same four-stroke cycle. This starts with the intake stroke with a piston descends, sucking air into the cylinder through open air intake valves as fuel is injected. This is followed by the compression stroke where the valves closed, and the piston goes back up the cylinder to compress the air & fuel mixture.

The third step is the power stroke, where the mixture is ignited to force the piston back down the cylinder. And then the final exhaust stroke occurs where the ignited fuel and air mixture is pushed back out of the cylinder by the piston through the exhaust valves.

Where petrol and diesel engines differ is how they ignite the air and fuel. To understand the difference we need to understand the self ignition temperature. This is the temperature at which an air fuel mixture will ignite without the use of a spark plug, purely due to compression. It's this compression that allows the air & fuel mixture to ignite in a diesel engine without the need of a spark, because the greater compression, the higher the temperature.

That's why diesel engines have higher compression ratios compared with petrol engines that don't compress the air & fuel mixture to the point of ignition. If you don't know what a compression ratio is, let me quickly explain. Simply put, it's the ratio of the maximum to minimum volume in the cylinder.

So, a ratio of 10 to 1 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 nine point six to one while a 2015 VW Golf TDI has a compression ratio of sixteen point two to one.

Again, a diesel will have a higher compression ratio to ensure that the air & fuel mixture is compressed enough to the point of ignition. While a petrol engine uses less compression and is ignited by the spark plug. Though no longer true for all modern oil burners, another significant difference where the diesel versus petrol engine is that Diesel's lack a throttle body.

This means that when you press on the accelerator pedal, the fuel injectors simply 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.

Where petrol and diesel engines differ again is how they're able to engine brake where you lift off the throttle in gear to slow the car down. In a petrol vehicle, engine braking is achieved because as you lift up the throttle the throttle body closes, creating a vacuum between 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. And the reason engine braking with diesel engines is so now it's because you're hearing that compressed air.

So why exactly are diesel engines more efficient than petrol engines? One of the many reasons is down to the fuel itself. Diesel is comprised of more long-chain hydrocarbons which simply means it's got more energy than petrol. In fact, it contains about 15% more energy by volume, roughly 36 point 9 mega joules per liter compared to petrol's 33.7 mega joules 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 engines lower compression ratio that requires a spark for ignition.

The higher compression ratio, longer stroke, and typically higher turbo boost pressure of a diesel also allows it to produce more torque at lower rpm than a petrol engine. Meaning that less fuel is needed to move a car. Another reason why diesels are more fuel efficient is their tendency to be turbocharged and that's because a turbo feeds compressed air into the cylinder which helps the piston descend into the cylinder therefore saving energy.