29 September 2010 ~ 4 Comments

Dealer view: Volkswagen’s TDI
engines explained

In the second of our series of dealer view articles, Victoria Stubbs from Volkswagen retailer group Vindis explains the technology behind the company’s TDI engines, as used in the new Polo SE and SEL 1.6 TDIs

If you’re comparing engine options on Volkswagen models, you probably asked yourself at some point ‘What is a TDI engine?’ Well, TDI stands for Turbocharged Direct Injection. TDI engines are Volkswagen’s advanced diesel engines, and are more powerful (with faster acceleration and smooth performance), more economical, produce less CO2, and will require less maintenance and servicing. To find out why, read on.

You need oxygen to burn fuel in an engine, and as only 21% of our atmosphere is oxygen, huge amounts of air are needed for an effective engine. The TDI’s turbocharger squeezes air tightly into the engine’s cylinders (where the combustion takes place) and then cools it so it shrinks further. The injection system then adds fuel (also under high pressure) into the compressed air. This compressed mix burns better but uses less fuel.

The knock-on effect of this more efficient combustion technique is an engine that is powerful even at low revs, and burns less fuel whether at low or high speeds.

More about turbochargers
A turbocharger has two turbines; the first is powered by the exhaust gas and drives the second, which sucks in the air with the oxygen needed for combustion. The air is compressed and then cooled to compress it still further before it is mixed with the fuel and burnt.

A traditional problem with turbochargers is that if the engine is running at low revs then there will be less exhaust gas and therfore less power to drive the turbines. VW uses a nifty feature called Variable Turbine Geometry to overcome this problem. Vanes are used to create a smaller area of airflow through the exhaust turbine, increasing the speed of flow and making the turbine work as if the engine were running at a higher speed.

More about fuel injection
The key factor here is the fuel pressure. The more pressure you can put the fuel under, the finer the spray will be that gets injected into the cylinder, and the quicker and more thoroughly it will mix with it the compressed air. This means you get more power – and less emissions – from the same amount of fuel.

Volkswagen use Piezo crystal injectors, which are lighter and twice as fast as the solenoid valves traditionally used. This speed increase means the injector valve can adjust the rate of fuel injection five times faster. This means smoother, quieter and more efficient combustion.

The latest weapon in the war to increase fuel pressure is the ‘Common Rail’ direct injection system. This separates the pressure generation and the fuel injection processes, meaning each can be more efficient. The common rail is the name given to the high-pressure fuel reservoir to which all the injectors are connected. Each injector receives an uninterrupted supply of fuel at the same high pressure from the common rail.

Finally, as well as burning less fuel, the TDI engines also produce less emissions, thanks to VW’s diesel particulate filters. These not only trap even the smallest particles of soot produced by burning fuel, but need no additives to run, meaning they need little or no maintenance. In fact VW recommend the first inspection at 150,000km.

Victoria Stubbs, Vindis
Vindis are retailers for all Volkswagen models including used Volkswagen Passat

Read the PoloDriver first drive of the 74bhp Polo SE 1.6 TDI and the rest of the fifth-generation Polo range here

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