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Diesels. The Basics

Diesels. The Basics

Here at Darkside we specialise in diesel vehicles. From a customer point of view, Diesel engines are torquey, strong and respond well to tuning, while maintaining fuel economy that tuned petrol cars can only dream of. So how and why do diesel engines perform in this way?



As most people know, a diesel engine runs on diesel fuel. Like petrol, it comes from crude oil, but it is a heavier fuel, and is less volatile (does not form a vapour as easily). Diesel fuel is injected directly into the cylinder, and the high temperatures and pressures cause it to auto-ignite. This means a diesel engine is what is known as a Compression Ignition engine. This differs to petrol engines, in which the mixture is ignited by a spark (Spark Ignition engine).


Obviously this means diesel engines don’t have spark plugs. Many diesel engines do have glow plugs, which many think are the diesel equivalent of a spark plug. This isn’t true - glow plugs are used to create extra heat in the cylinder to help the engine start from cold.


In a petrol engine, the air fuel ratio is extremely important - if the mixture is too lean, it will not burn. Traditionally petrol engines have to run close to an AFR of 14.7:1, and at low load this means that the intake air has to be throttled to achieve the correct mixture. In a diesel engine there is no limit to how lean the mixture can be, the fuel can still burn as it is injected, a Diesel engine can be anywhere from 15:1 AFR all the way up to 30:1 or more. This means that diesel cars do not need a throttle. It is this lack of a throttle that gives diesel some of its major advantages over petrol engines - it is more efficient at part throttle due to reduced pumping loss (the energy used to suck air past the throttle) and there is less turbo lag because the throttle is always open.

The Anti Shudder Valve (or ASV) is actually a throttle like part that is used on most Diesel engines to smooth out the shaking inherent with a high compression engine when the engine is turned off.

It is worth a mention that the most modern of engines do actually use this valve at very low load to increase the velocity inside the intake manifold to induce further swirl to improve efficiency and emissions, slightly contradictory to the above generalisation but certainly no reason why you should consider the below...



Diesels don't need them - dump valves/blow off valves are designed to vent the area between the turbo and the throttle. In a petrol engine, at high boost, when you snap the throttle closed the turbo is still spinning. This causes a buildup of pressure between the turbo and the throttle, and the air can send shockwaves back through the turbo compressor, potentially causing damage. A dump valve vents this pressure off. Diesels don't have a throttle, so this problem just doesn't exist. We dont fit them and neither should you.


Diesel engines are generally better on fuel than petrol's. This is due to two main reasons - firstly, the lack of throttle reducing the pumping loss as mentioned earlier. Traditionally this is a major reason petrol engines have had poor fuel economy at part load - as soon as the throttle is anything other than wide open, you create a restriction and the engine has to do work to overcome it. Diesels don’t have this restriction no matter how lightly you press the accelerator.

The second reason is due to the properties of the fuel itself. The amount of energy a fuel releases when it is burned is called the ‘Calorific Value’ and is measured based on weight. By weight, petrol actually has slightly more energy in it than diesel - but we don't purchase fuel by weight. We buy it based on volume volume. A litre of petrol weighs roughly 745g, but a litre of diesel weighs about 832g - this means that with diesel you get more for your money. A litre of diesel has about 15% more ‘energy’ in it than a litre of petrol. Obviously this means that you use less to drive the same distance!



The redline on most modern diesel cars is around 5000rpm, and even state of the art race engines don’t rev past 6000rpm. Many standard petrol engines rev to well over 7000rpm, with race engines capable of over 20,000rpm. The reason for the difference is in how the fuel is mixed and burned. In a petrol engine, the fuel and air is premixed to an ideal ratio. It is then compressed and ignited with a spark, after which it burns very quickly - so much so you can think of the burn as being pretty much instantaneous.

In a diesel engine, the fuel is injected and burns as it mixes with the air - known as ‘mixing controlled’ combustion. Because the mixing and burning are happening together, the burning is slower. It is the speed of this burn that stops the engine being able to rev higher - at high engine speeds there isn't enough time for the fuel to mix and burn effectively, so you just don’t make power. The stronger, thus heavier, Rotating and Reciprocating mass of the internal components is a small contributing factor but ultimately, the speed of the burn is the main constraint. You may see some crazy 300TD Mercedes or Peugeot D-Turbo revving to 7000rpm and beyond but these are indirectly injected engines (IDI) so the fuel and air are already well mixed in the Pre-Cup prior to entering into the main combustion chamber.



Diesel engines run high boost pressures as standard, anything from 1 bar up to 3 bars standard, while most petrol engines have usually half that. Compression ratios are also much higher from the factory, anything from 16:1 up to 19:1 compared to a turbo petrol engine at 9 or 10:1. This means that in general, the engine hardware needs to be very strong. It is very rare that you can double the power of a petrol engine reliably without changing internals like pistons and rods - but on TDI engines we regularly achieve double stock power with a turbocharger and injector upgrade. This makes diesels great for tuning, as not having to build the engine makes tuning much more affordable for the majority of people. It is also a bonus that highly tuned diesels maintain great fuel economy if tuned properly - many of us here have daily driven 300bhp/420ftlb+ 1.9 TDIs commuting as much as 100 miles a day for years and achieved 50mpg. Even highly tuned diesels can make very practical daily drivers, which cannot be said for the majority of highly tuned petrol engines.


Many comparisons have been made with petrol engines in this article - In recent years, the development of petrol engines has brought them closer to diesels in a few areas. Modern petrol engines are capable of running lean with the throttle open, improving fuel economy. They run high pressure fuel systems and inject fuel directly into the cylinder, just like diesels do, and if you saw a modern petrol and diesel engine in parts next to one another, you would realise that many of the components look far more similar than they did a few years ago. In stark contrast, most of the top end F1 teams were running Indirect injection petrol engines that had more in common with the 1990’s Diesels.

The Indirect Injection engines were phased out a long time ago due to their inefficiency in comparison to direct injection, the main issue is the heat management from the pre-combustion chamber and the CO2 emissions. Its now widely accepted that the IDI engines are able to produce less NOX while producing larger Soot particles which is the opposite of the high pressure, low compression Direct injection engines. With a modern spin on the design, this may mean the comeback of the IDI if the Diesel engine is to survive in the future!

Technologies like the e-boosting system found on the new 435bhp Audi SQ7 mean that diesels are as powerful and responsive as they have ever been. As fuel system technology improves, and injection pressures increase towards 3000bar (44,000psi), diesel engines will continue to impress with each new generation - it is hard to believe that Volkswagen now sell a 2.0 diesel engine making 240bhp from the factory, when it wasn’t all that long ago the Fabia VRS was showing the world that diesel could be fun with just 130bhp.

At Darkside we will be harnessing all of the latest technologies and continuing to offer the best possible products to make the most of these engines.

Autoengplus - Audi SQ7

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