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Petrol vs Diesel Engines

An in-depth Comparison

In this video we’re doing a detailed comparison of petrol, or spark ignition and diesel, or compression ignition engines.

The video covers pretty much all the key points, starting with the differences in the way combustion occurs as well as explaining the diesel combustion process in detail. After this we’re explaining why diesel engines don’t rev as high and after that the topics are compression ratios, power and torque, efficiency, emissions and fun factor.

In essence diesel and petrol engines share a very similar anatomy, and that makes sense because they’re both internal combustion engines, meaning they do their combustion internally. What’s different is HOW they do their combustion.

Petrol engines rely on a spark to ignite the air fuel mixture, while diesels completely forgo spark plugs, ignition coils and the like in favor an ignition started simply be the heat of the compressed air inside the engine. And this results in one of the few differences in anatomy between petrol and diesel engines. Most modern diesel engines actually don’t have a combustion chamber, instead the smallest volume of their cylinder is achieved by void in the piston. Therefore, diesel engines have a smaller smallest volume of the cylinder achieving a higher compression ratio. A higher compression ratio means more power and more efficiency.

Because the air fuel mixture combusts in a smaller area more energy is transferred onto the piston resulting in the harnessing of more of the energy stored in the fuel. But there’s a price to be paid for the higher efficiency generated by compression ignition, and that’s cylinder pressure spikes generated during the diesel’s combustion process.

As you may know, most diesel engines rev to anywhere between 4500 to 5500 rpm. On the other hand petrol engines usually rev to anywhere between 6000 to 9000 rpm.

Now there’s a couple of reasons for this. The first one is that the cylinder pressure spikes require heavy internals. The other one is that most diesels are under-square by design, but the final reason is key, and that is that diesels don’t have a wide range of ignition timing control.

They can’t control ignition timing to compensate for increased piston speeds, because they can start combustion only when the air is hot enough, and the air is always hottest when the piston is near top dead center. On the other hand a petrol engine with an ECU, a spark plug and an ignition coil or coil on plug can fire the spark almost at any point in the engine’s compression stroke.

We all know that diesels are also more efficient engines. Again, the reason is a higher compression ratio and the ability to squeeze more power from the fuel. The other reason is that diesels simply use less fuel. They have a stratified or heterogeneous air fuel mixture, meaning that only one part of the air is mixing with the fuel, allowing diesels to run extremely lean. On the other hand petrol engines compress a homogeneous air fuel mixture meaning that they have to worry about knock occurring which limits their compression ratio.

Diesels introduce fuel much later into the cylinder, at the end of the compression stroke, before that they compress only air, meaning that knock isn’t an issue. Diesel fuel itself is also more energy dense. Because it’s composed of hydrocarbons with longer chains it contains approximately 15% energy for the same volume.

When it comes to emissions and pollution, diesel and petrol emissions have been traditionally presented as a trade off between environment harming C02 and health harming nitrogen oxides and soot particles. But the picture isn’t really black and white and many independent road tests have demonstrated that the CO2 gap between modern petrol and diesel engines is extremely low. On top of this EURO 6 or equivalent standard diesel engines are very clean, and rely on diesel particulate filters and diesel exhaust fluid injection (selective catalytic reduction) to trap close to 99% of the soot particles and nitrogen oxides.

Problems arise when diesel cars hit the used car markets. Because of their more expensive emissions equipment maintenance becomes a problem for used car owners often resulting in more overall pollution from the diesels.

Fun is a subjective thing and fact is that both diesel and petrol engines can be extremely fun. Petrol engines have a wide power band and often a better soundtrack leading to more smiles per gallon on a twisty road. But diesel give you incredible torque sensations and these can be extremely addictive.

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