TDI (Diesel)

Fuel needs oxygen to burn and the engine has to be supplied with huge quantities of air to get enough. You can solve this problem with a bigger engine - or you can solve it with a turbocharger - as in the TDI. Driven by the exhaust gases, it squeezes air more tightly into the cylinders.

After being drawn through the turbocharger the air is then cooled by passing it through an intercooler (cool air takes up less space than hot air), before entering the combustion chamber where diesel is injected directly into the cylinders at very high pressure through a nozzle. It's this intensive mixing of highly atomised fuel with the cooled compressed air that leads to better, more efficient combustion.

Your driving experience is quiet and refined because effective sound insulation keeps noise to a minimum, while hydraulic engine mounts ensure smooth, low-vibration running.

The turbocharger

To boost power output and torque, we fit our TDI engines with exhaust turbochargers featuring variable turbine geometry. They compress the air required for fuel burning, letting the engine draw in more air while its displacement and revs stay the same.

A turbocharger is powered by the energy in the exhaust gas. It has two turbines. The turbine wheel in the exhaust stream drives a second in the intake stream that compresses the intake air. Before it is fed into the combustion chamber it is cooled by a charge air cooler (intercooler). Because cool air is denser than hot air, more oxygen can be fed into the cylinder to burn the fuel, enhancing power and efficiency.

Overcoming turbo lag

The main disadvantage of a turbocharger is that it needs a certain gas pressure to work, only available when engine revs are high enough. To avoid 'turbo lag' a delay in available power - and be very efficient at lower engine revs, the turbocharger needs to be able to control the exhaust pressure.

A variable turbine geometry (VTG) turbocharger does this with a system of mechanical guide vanes. It alters the cross-section of the exhaust flow inlet on the powertrain side. If the gas pressure falls at slower engine revs, the control system adjusts the guide vanes to narrow the cross-section. This speeds up the exhaust flow and increases the pressure. And as the exhaust gas pressure rises with the engine revs, the control system makes the inlet cross-section larger by altering the position of the guide vanes.

How injection works

The pressure at which the diesel is injected into the cylinder is the key factor in diesel direct injection. The fuel has to mix swiftly with the compressed air in the cylinder. The higher the pressure, the more finely the diesel is atomised for an intensive mixing of the fuel and air particles. This, in turn, leads to better and more efficient fuel burning. The energy from the fuel is used more effectively and emissions are reduced.

We use various injection stages within one power stroke - referred to as multiple injection. Depending on the engine design, revs and load, modern diesel engines use a pilot or double pilot injection, a main injection and a post injection. Pilot injection achieves smooth combustion, ensuring that the extremely high pressures necessary for combustion to take place are reached more gradually. This significantly reduces combustion noise and cuts emissions. Post injection helps the combustion process further, achieving even lower exhaust emissions.

Common rail - third-generation diesel direct injection

The common rail system stores the injection pressure in a high-pressure fuel reservoir referred to as the 'common rail' as it supplies all the injectors. In this system the generation of pressure and the fuel injection processes are separate.

Lines connect all the cylinder injectors to the common rail in parallel, ensuring they all have an uninterrupted supply of constant pressure.

The advantage of common rail is that fuel can be delivered at higher pressure, giving better mixing with air for a more efficient and cleaner combustion. This gives higher performance combined with improved fuel consumption.

The ever-higher injection pressures that make diesel engines cleaner and more efficient than before place big demands on the common rail system. Our latest generation of diesel engines reach injection pressures as high as 1,800 bar. For this reason we make the rail ourselves, and we are the first car maker to do so.

The diesel particulate filter

Our advanced diesel engines are much cleaner than older engines. One important factor is their diesel particulate filters (DPF), which are very effective in cutting emissions, trapping even the finest soot particles that are produced as the engine burns diesel fuel.

The result is a cleaner environment as less pollution enters the atmosphere. The latest generation of filters operate without additives. This makes them maintenance-free for a long time: an initial inspection is not usually carried out until after 90,000 miles. Their lifespan is dependent on factors such as fuel quality, driving style, use and oil consumption. The filter uses a catalytic coating containing precious metals. Passive regeneration converts the particulate matter contained in the catalytic converter into CO2. This process takes place at temperatures between 350 and 500°C and can run continuously, particularly if you drive your car mainly on long runs.

The latest generation of filters operate without additives. This makes them maintenance-free for an exceptionally long time: an initial inspection is usually carried out only after 150,000 km. The filter's lifespan is dependent on factors such as fuel quality, driving style, use and oil consumption.