The Audi TDI – Tech Workshop 2014
The 4.2 TDI
The 4.2 TDI has more torque than any other engine in the Audi lineup. The latest version in the Audi A8 delivers 850 Nm (626.9 lb-ft) between 2,000 and 2,750 revolutions per minute. Its peak output of 283 kW (385 hp) is available at just 3,750 rpm.
The tremendous power provides for a sporty and composed driving experience. Mated with an eight-speed tiptronic and quattro drivetrain, the V8 diesel accelerates the luxury sedan from 0 to 100 km/h (62.1 mph) in 4.7 seconds on its way to a governed top speed of 250 km/h (155.3 mph).
The top-of-the-line TDI from Audi displaces 4,134 cc (bore x stroke 83.0 mm x 95.5 mm (3.3 in x 3.8 in). Its vermicular cast iron crankcase is a major reason for the engine’s comparatively low total weight of approximately 250 kilograms (551.2 lb). Four chain-driven camshafts actuate a total of 32 valves via roller cam followers.
The developers updated the 4.2 TDI for use in the Audi A8. The aluminum pistons got reinforced bowl edges, which, like the new exhaust valves, further improve temperature stability. The common rail system develops up to 2,000 bar of pressure, and piezo inline injectors with newly developed, eight-hole nozzles spray the diesel fuel into the combustion chambers.
Improvements were also made to the two VTG turbochargers, primarily with respect to the mounting of the compressor wheels. Each of the two turbochargers, which produce up to 1.7 bar of relative boost pressure, feeds air to one cylinder bank via a downstream intercooler. With the variable turbine geometry, power begins to develop just slightly above idle speed.
Thanks to this and the highly refined running behavior, the diesel engine can be operated at engine speeds of just 800 rpm if the Audi drive select system is set to efficiency mode. The low engine speed significantly reduces actual fuel consumption. In the ECE cycle, the Audi A8 4.2 TDI clean diesel consumes just 7.4 liters of fuel per 100 kilometers (31.8 US mpg), corresponding to 194 grams CO2/km (312.2 g/mi).
The 4.2 TDI clean diesel in the Audi A8 meets the requirements of the Euro 6 emissions standard. Two heated oxidation catalysts are located close to the engine; the two particulate filters with an SCR coating are located in the rear. A metering pump injects the AdBlue additive that breaks down the nitrogen oxides. The solution is supplied from two tanks having a combined volume of 27 liters (7.1 US gal).