1. Overview

The TDI engine uses direct injection,^[1][2] where a fuel injector sprays atomised fuel directly into the main combustion chamber of each cylinder,^[1][2] 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^[1][2] to increase the amount of air which is able to enter the engine cylinders,^[2] 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.^[1][3] These, in combination, allow for greater engine efficiency, and therefore greater power outputs^[2] (from a more complete combustion process compared to indirect injection), while also decreasing emissions and providing more torque^[2] than the non-turbo and non-direct injection petrol engined counterpart from VAG.

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.

Because these engines are relatively low displacement and quite compact, they have a low surface area. The resulting reduced surface area 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,^[4] because of 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 3 L beat the Toyota Prius to win "Best Fuel Economy" in its class. The TDI engine has won "Green Car of the Year" award in the years 2009 (Volkswagen Jetta 2.0-litre common-rail TDI clean diesel) and 2010 (Audi A3 TDI clean diesel) beating other various electric cars.

2. History

The first passenger car to be powered by direct injection was the 1986 Fiat Croma 2.0 TD i.d. (The Turbo Diesel iniezione diretta pump was developed by Bosch in accordance with Fiat's engineering specifications).

Rover introduced its MDI turbocharged direct injection diesel developed with Perkins, (also known as the Perkins Prima) in 1988 in the Rover Montego. It was also sold in marine form by Volvo. It used a Bosch VE injection pump. The engine had been launched in naturally aspirated form for commercial vehicles in 1986.

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 variant is still used today in Volkswagen Marine applications. The TDI arrangement has been enhanced through various stages of development – 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^[1] 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, were not able to control injection timing as precisely (a major factor in improving emissions). Some current PD TDI engines now use piezoelectric unit injectors, allowing far greater control of injection timing and fuel delivery.

From the 2009 model year onwards, TDI engines using the common rail (CR) technique, again with piezoelectric injectors, are now used in various Volkswagen Group models.^[1] The CR engines are available in many sizes, including 1.2, 1.6, 2.0, 2.7, 3.0, 4.2 and 6.0 litres, with outputs from 55 to 368 kW (75 to 500 PS) from these engines.

2.1. Emissions Testing Falsification

On 18 September 2015 the US EPA and California Air Resources Board served notice to VW that approximately 480,000 VW and Audi automobiles equipped with 2.0 TDI engines sold in the US between 2009 and 2015 had an emissions compliance defeat device installed.^[5][6] The defeat device, in the form of specially crafted engine management unit firmware, detects emissions testing conditions, and in such conditions will cause the vehicle to comply with emissions regulations by properly activating all emissions controls. However, under normal driving conditions, the emissions controls are suppressed, allowing the engine to produce more torque and get better fuel economy, at the expense of emitting up to 40 times more nitrogen oxides than allowed by law.^[7] Such NOx emission levels are not in compliance with US regulations. VW has since admitted to these allegations, and said that the illegal software was in use in its diesel cars worldwide, affecting some 11 million vehicles.^[8]

Audi R10 TDI LMP race car. https://handwiki.org/wiki/index.php?curid=1987487

SEAT León Mk2 TDI race car. https://handwiki.org/wiki/index.php?curid=1958586

3. Motor Racing

A motor racing version of the common rail TDI engine made an impact in March 2006 when it was used in the Le Mans Prototype (LMP) Audi R10 TDI, and made its debut win in the 12 Hours of Sebring race.^[9][10][11] This victory was followed three months later by another one in the 24 Hours of Le Mans race, becoming the first diesel-powered car to win these prestigious endurance races.^[12][13][14] Fuel economy was a significant factor, as the car did not have to refuel as often as petrol engined race cars in the race.^[15] The car was fueled with a special synthetic V-Power diesel from Shell.^[16] The Audi R10, R15 and R18 TDIs have won at Le Mans seven times in eight years, from 2006 to 2011, with only the 2009 race being won by Peugeot's 908 HDi FAP, which is also a diesel powered car.

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,^[17][18] 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).^[19][20]

In 2008, SEAT – with the León Mk2 TDI at Donington Park in England – became the first manufacturer to win a round of the British Touring Car Championship (BTCC) in a diesel-powered car. Jason Plato won race 1 of the weekend and Darren Turner won race 3.^[21]

4. Fuel

The fuels required for TDI engines include diesel fuel (also known as petrodiesel), or B5, B20, or B99 biodiesel, depending on emissions equipment, location dependent.

A 2007 Volkswagen Jetta Mk5 with a 1.9 TDI engine and a five-speed manual transmission achieves 5.2 litres per 100 kilometres (54 mpg_‑imp; 45 mpg_‑US) on the European combined-cycle test (a US EPA test of the same vehicle would achieve around 34 MPG), while a six-speed direct-shift gearbox (DSG) automatic version reaches 5.9 litres per 100 kilometres (48 mpg_‑imp; 40 mpg_‑US).^[22]

Newer TDI engines, with higher injection pressures, are less forgiving of 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.^[23]

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-sulphur petroleum-only diesels are known to cause some seals to shrink,^[24] and may cause fuel pump failures in TDI engines used in 2006 to 2009 models. TDI engines from 2009 on and before 2006 are designed to use ULSD exclusively; biodiesel blends are reported to prevent that failure.