Turbocharged Direct Injection

Turbocharged Direct Injection or TDI is a design of turbodiesel engines, which feature turbocharging and cylinder-direct fuel injection, developed and produced by Volkswagen Group. These TDI engines are widely used in all mainstream Volkswagen Group marques of passenger cars and light commercial vehicles produced by the company (particularly those sold in Europe). They are also used in marine engine - Volkswagen Marine, and Volkswagen Industrial Motor applications.

TDI is a registered trademark of Volkswagen Aktiengesellschaft.

Overview
The TDI engine uses direct injection, where a fuel injector sprays atomised fuel directly into the main combustion chamber of each cylinder, rather than the pre-combustion chamber prevalent in older diesels which used indirect injection. The engine also uses forced induction by way of a turbocharger to increase the amount of air which is able to enter the engine cylinders, and most TDI engines also feature an intercooler to lower the temperature (and therefore increase the density) of the 'charged', or compressed air from the turbo, thereby increasing the amount of fuel that can be injected and combusted. These, in combination, allow for greater engine efficiency, and therefore greater power outputs (from a more complete combustion process compared to indirect injection), while also decreasing emissions and providing more torque than its non-turbo and non-direct injection petrol engined counterpart.

Similar technology has been used by other automotive companies, but "TDI" specifically refers to these Volkswagen Group engines. Naturally-aspirated direct injection diesel engines (those without a turbocharger) made by Volkswagen Group use the Suction Diesel Injection (SDI) label.

The reduced material volume of the direct injection diesel engine reduces heat losses, and thereby increases engine efficiency, at the expense of slightly increased combustion noise. A direct injection engine is also easier to start when cold, due to more efficient placing and usage of glowplugs.

Direct injection turbodiesel engines are frequent winners of various prizes in the International Engine of the Year Awards. In 1999 in particular, six out of twelve categories were won by direct injection engines: three were Volkswagen, two were BMW, and one Audi. Notably that year, the Volkswagen Group 1.2 TDI 3L beat the Toyota Prius to win "Best Fuel Economy" in its class.

History
The first Volkswagen Group TDI engine was the Audi-developed 2.5 litre R5 TDI – an inline five-cylinder engine (R5), introduced in the Audi 100 in 1989 – and this exact variant is still in use today, in Volkswagen Marine applications. The TDI arrangement has been enhanced through various stages of evolution – by improving the efficiency of the turbocharger, increasing the pressure at which fuel can be injected, and more precisely timing when the injection of fuel takes place. There have been a few major 'generations', starting with what are known as "VE", and "VP" (German: VerteilerPumpe) engines, which use a distributor-type injection pump. In 2000, the Pumpe Düse (PD, variously translated "pump nozzle", "unit injector", "pump injector") TDI engine began to appear in Europe, eventually coming to North America a few years later.

The Pumpe Düse design was a reaction to the development of high-pressure common rail fuel injection systems by competitors - an attempt by Volkswagen Group to create an in-house technology of comparable performance that would not require any royalties to be paid. While Pumpe Düse engines had a significantly higher injection pressure than older engines, they are slightly less refined when compared to the very latest common rail, and with the original solenoid-operated unit injectors, weren't able to control injection timing as precisely (a major factor in improving emissions). Some current PD TDI engines now utilise piezoelectric unit injectors, allowing far greater control of injection timing and fuel delivery. Furthermore, appearing in 2009 model year, Audi and Volkswagen TDI engines are starting to use the common rail (CR) technique, again with piezoelectric injectors, and CR TDI engines are starting to appear in models from other marques of the Group.

Motor racing
A motor racing version of the common rail TDI engine made an impact in 2006 when it was used in the Le Mans Prototype (LMP) Audi R10 TDI, which won the 12 Hours of Sebring and 24 Hours of Le Mans, becoming the first diesel-powered car to win either of those endurance races. Fuel economy was a significant factor, as the car did not have to refuel as often as petrol engined race cars in the race. The car was fueled with a special synthetic V-Power diesel from Shell.

In 2007, SEAT – with the León Mk2 TDI at the Motorsport Arena Oschersleben in Germany – became the first manufacturer to win a round of the World Touring Car Championship (WTCC) series in a diesel car, only a month after announcing it will enter the FIA World Touring Car Championship with the León TDI. SEAT's success with the León TDI was continued, and resulted in winning consecutively 2008 World Touring Car Championship and 2009 World Touring Car Championship both titles (for drivers' as well as for manufacturers').

Fuel
TDI engines operate on diesel fuel (also known as petrodiesel), or B5, B20, or B99 biodiesel subject to manufacturers' prior approval.

In fuel efficiency, and clean emissions when run on biodiesel, or when converted vegetable oil (which should NOT be used on the later PD engines without prior conversion, since irreparable damage will result), TDI engines are among the best on the market. A 2007 Volkswagen Jetta Mk5 with a 1.9 TDI engine and a five-speed manual transmission, for example, achieves 5.2 litres per 100 kilometres (54 mpg-imp; 45 mpg-US) on the European combined-cycle test, while a six-speed Direct-Shift Gearbox (DSG) automatic version reaches 5.9 litres per 100 kilometres (48 mpg-imp; 40 mpg-US).

Newer TDI engines, with higher injection pressures, are less forgiving about poor-quality fuel than their 1980s ancestors. Volkswagen Group's warranty does not cover damage due to bad fuel (diesel or bio), and has in the past recommended that only mixtures up to 5% biodiesel (B5) be used. Volkswagen Group has recently permitted mixes up to B20, and has recommended B5 be used in place of 100% petroleum-based diesel because of biodiesel's improved lubricating properties.

In North America, No. 2 diesel fuel is recommended, since it has a higher cetane number than No. 1 fuel, and has lower viscosity (better ability to flow) than heavier fuel oils. Some owners in North America, where cetane levels are generally poor (as low as 40), use additives, or premium diesel, to get cetane numbers closer to the standard levels found in the European market (at least 51) where the engine is designed. Improved cetane reduces emissions while improving performance, and may increase fuel economy.

New ultra low-sulfur petroleum-only diesel recipes cause seals to shrink and can cause fuel pump failures in TDI engines; biodiesel blends are reported to prevent that failure.

Lubricants
For all Volkswagen Group Diesel engines, Volkswagen AG requires that motor oil lubricants must only meet VW505.00, 505.01, 506.00, 506.01, or 507.00 officially approved standards. 'Generic' oil standards such as the American Petroleum Institute 'API', or European Automobile Manufacturers Association 'ACEA' are not recognised, nor approved by VW - so oils bearing these generic approvals must not be used, except when the oil has also been granted one of the specific VW approvals.

Many formulations of motor oils from most brands are now "VW Approved" to the above standards. However, there does exist a few unapproved "off brands" that print the VW number on the label, effectively 'claiming' to meet the said standard, but hide in small type the word "recommended". Volkswagen AG only carries out in-house testing on motor oils to grant the necessary approvals; if Volkswagen AG, therefore, has not carried out such tests (or if a submitted oil has failed their tests), Volkswagen AG will not grant approval. Volkswagen Germany and Volkswagen of America freely publish up-to-date lists of currently approved oils on their respective websites, along with the technical resource, erWin. Volkswagen AG does not permit any independent testing facility to grant their own VW standards.