Turbocharged Direct Injection (acronym: TDI) is the technology used to describe and name modern turbocharged diesel engines produced by Volkswagen Group, and widely used in all marques of passenger cars and light commercial vehicles produced by the company (particularly those sold in Europe).

TDI is a registered trademark of Volkswagen AG.

TDI: Overview

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The 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 is coupled with a turbocharger to increase the amount of air going into the engine cylinders, and an intercooler to lower the temperature (and therefore increase the density) of the air from the turbo, thereby increasing the amount of fuel that can be injected and combusted. These, in combination, allow for greater engine performance (from a more complete combustion process compared to indirect injection), while also decreasing emissions and providing more torque than its petrol engined counterpart.

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

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

TDI: History

The first Volkswagen Group TDI engine was a 2.5L inline five-cylinder introduced in the Audi 100 in 1989. The TDI arrangement has been enhanced 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" engines. In 2000, the Pumpe Düse (PD, variously translated "pump nozzle", "unit injector", "pump injector") engine began to appear in Europe, eventually coming to North America a few years later.

The PD design was a reaction to the development of common rail fuel injection 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 not a match with the very latest common rail, and weren't able to control injection timing as precisely (a major factor in improving emissions). New engines appearing in 2009 model year Volkswagens are using the common-rail technique with piezoelectric injectors.

TDI: Motor racing

A racing version of the common rail TDI engine made an impact in 2006 when it was used in the 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 races. Fuel economy was a significant factor as the car didn't have to refuel as often as others in the race. The car used a special synthetic diesel from Shell as fuel.

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

Direct injection turbodiesel engines are frequent winners of various prizes in the International Engine of the Year Awards. In 1999 in particular 6 out of 12 categories were won by direct injection engines: 3 were Volkswagen, 2 were BMW and 1 Audi. Notably that year the Volkswagen 1.2L TDI beat the Toyota Prius to win "Best Fuel Economy" in its class.

TDI: Fuel

TDI engines, like most diesel engines, can run on 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. This is often overlooked because they do not drive on petrol. A 2007 Volkswagen Jetta 1.9L TDI with 5-speed manual, for example, achieves 5.2 L/100 km (54 mpg UK or 45 mpg US) on the European combined-cycle test while a DSG automatic reaches 5.9 L/100 km (48 mpg UK or 40 mpg US).

Newer TDI engines, with higher injection pressures, are less forgiving about poor-quality fuel than their 1980s ancestors. Volkswagen's warranty does not cover damage due to bad fuel (petrol or bio) and has in the past recommended that only mixtures up to 5% biodiesel (B5) be used. VW 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.

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 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.

TDI: Lubricants

For all Volkswagen Group diesel engines VW requires that lubricants must meet VW 505.00, 505.01, 506.00, 506.01, or 507.00 standards.

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A number of brands of motor oils are now "VW Approved" to the above standards; there do exist, however, a few off brands not approved by VW that print the VW number on the label and 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, therefore, has not carried out such tests (or if a submitted oil has failed their tests), VW 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 does not permit any independent testing facility to grant their own standards.

European models with "LongLife Servicing", "Extended Service Intervals", "ESI", or "WIV", must be run on official VW LongLife lubricants, namely 506.00 (for non-PD), 506.01 (for PD), or the very latest 507.00. Volkswagen of America, however, warranties require frequent oil changes even if ESI hardware is installed. Finally, the very latest TDIs with Diesel Particulate Filter (DPF) must ONLY use 507.00 oils.

TDI: Tuning

The Volkswagen TDI engines are known to enthusiasts as easily tunable engines. There is a large market of aftermarket components and software upgrades for the ECU's. The TDI engine is made of cast iron and heavily overbuilt so as to withstand the high compression and torque outputs associated with diesels. The main focus of TDI tuning is that the cost of modifications for the resulting power gain is in a very good ratio.

A remap on a TDI costing around £400 could increase engine power output by over 30%, to achieve this on an N/A petrol engine would involve heavy modification to internal components and great expense. Most Turbo petrol and diesel engines can be tuned similarly, but the Volkswagen TDI is particularly easy to tune while maintaining reliability and fuel economy.

Fuel Resistor modifications add a slight power increase by fooling the fuel temperature sensor to permanently preheat the fuel, these resistor modifications are cheap and can even be hand made for very little cost. Tuning boxes that connect to the OBD2 diagnostic port that can upload a "generic" performance software upgrade are available, these usually have the advantage of being removable for warranty purposes or for when extra power is not required. A Full custom remap can tune the car to the peak of the individual engines capabilities and can take advantage of upgraded components such as a better turbocharger or new injectors.

As an example, the PD 105 engine found in cars such as the Volkswagen Golf and Skoda Octavia can be remapped to over 130 bhp (97 kW) on standard components. By interchanging the injectors and turbo from a PD150 engine and applying a custom remap, it is possible to achieve over double the factory horsepower and a large torque gain. All power gains vary due to each engine and its ancillary components having different tolerances.