In this video we do a detailed comparison between petrol, or spark ignition, engines and diesel, or compression ignition, engines. The video covers pretty much all the key points, starting with the differences in the way combustion occurs and explaining the diesel combustion process in detail. After this, we explain why diesel engines don't rev as high, and then we touch on the topics of compression ratios, power and torque, efficiency, emissions and the fun factor. At their core, petrol and diesel engines share very similar anatomy, and that makes sense because they are 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 fuel-air mixture, while diesels forgo spark plugs, ignition coils and the like entirely, preferring to have ignition initiated simply by the heat of 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 do not have a combustion chamber, but instead their smaller cylinder volume is achieved by the vacuum in the piston. Therefore, diesel engines have a smaller cylinder volume achieving a higher compression ratio. A higher compression ratio means more power and more efficiency. Because the air-fuel mixture is burned in a smaller area, more energy is transferred to the piston, resulting in more of the energy stored in the fuel being harnessed. But there is a price to pay for the increased efficiency generated by compression ignition, and that is the pressure peaks in the cylinders generated during the diesel combustion process. As you may know, most diesel engines rev between 4,500 and 5,500 rpm. On the other hand, gasoline engines typically rev between 6,000 and 9,000 rpm. There are a couple of reasons for this. The first is that cylinder pressure peaks require heavy internal parts. The other is that most diesel engines are off-square in design, but the last reason is key, and that is that diesel engines don't have a wide range of ignition timing control. We all know that diesels are also more efficient engines. Again, the reason is a higher compression ratio and the ability to extract 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 only some of the air mixes with the fuel, allowing diesel engines to run extremely lean. On the other hand, gasoline engines compress a homogeneous air-fuel mixture, meaning they have to worry about detonation occurring which limits their compression ratio. Diesels introduce the fuel much later into the cylinder, at the end of the compression stroke, before that they only compress air, meaning detonation is not an issue. Diesel fuel itself is also more energy dense. Being made up of hydrocarbons with longer chains, it contains about 15% energy for the same volume. When it comes to emissions and pollution, diesel and petrol emissions have traditionally been presented as a balance between environmentally damaging C02 and health-damaging nitrogen oxides and soot particles. But the picture is not really black and white and many independent road tests have shown that the CO2 difference between modern petrol and diesel engines is extremely low. Furthermore, 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 about 99% of soot particles and nitrogen oxides. Problems arise when diesel cars hit the used car market. Because emissions equipment is more expensive to maintain, it becomes a problem for used car owners, often resulting in higher overall pollution from diesel engines. 00:00 Spark vs Compression 03:12 Injection Timing 04:05 Diesel Combustion Process 05:55 Why Diesels Don't Rev High 10:02 Compression 12:35 Detonation 14:49 Horsepower and Torque 16:16 Efficiency 18:42 Power Modulation 20:45 Economy 24:38 Fun Factor