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Audi Introduces High-End Diesel V8
30 May 2005
Following on the heels of DaimlerChrysler’s release of its most powerful diesel passenger car, the E420 CDI, (earlier post) Audi has upped the ante with the new diesel Audi A8 4.2 TDI quattro.
The new 240 kW (326 hp) engine deivers 650 Nm (479 lb-ft) or torque, with fuel consumption of 9.4 liters/100km (25.0 mpg).
This is an expensive, limited-market luxury car. The basic price of the A8 4.2 TDI quattro in Germany is €81,000 (US$101,000). But the developments and enhancements of the engine and exhuast system on this car (as on the Mercedes E420 CDI) highlight some of the possible technology paths and manufacturing enhancements that could be applied to smaller engines to increase fuel efficiency and reduce emissions.
| Power Diesels: Audi vs. MercedesBenz | ||
|---|---|---|
| Audi A8 4.2 TDI | Mercedes E420 CDI | |
| Displacement | 4,134 cc | 3,996 cc |
| Power | 240 kW (326 hp) | 231 kW (314 hp) |
| Torque | 650 Nm (479 lb-ft) | 730 Nm (538 lb-ft) |
| Power/liter | 58 kW/l | 57.81 kW/l |
| BMEP | 286.3 psi | 333.1 psi |
| Acceleration 0–100 km/h | 5.9 sec | 6.1 sec |
| Fuel consumption | 9.4 l/100km | 9.3 l/100km |
| Mileage | 25.0 mpg US | 25.3 mpg US |
| Emissions | Euro 4 | Euro 4 |
Audi engineers made a number of structural changes to the engine design to achieve the enhanced power. The new engine is made of vermicular graphite cast iron (GGV), reducing the engine weight by 15 kg from its predecessor, the 4.0 TDI. The 4.2 TDI is this one of the lightest diesel V8s ever. Cylinder spacing in the new engine block is 90 millimeters, giving the engine a compact overall lenth of 520 mm.
The surfaces of the cylinder bores are treated using UV laser exposure. This special honing process ensures significantly reduced oil consumption in the TDI engine, and, as a result, a significant reduction in particulate formation during combustion.
The cylinder head is made of aluminium, and two camshafts are used to operate the valves in each. They are driven via a maintenance-free chain drive, which also drives the oil, water and power-assisted steering pumps.
Roller cam followers with hydraulic valve clearance compensation operate the 32 valves (four per cylinder) on the new V8. This low-friction drive technique plays an important role in reducing both the fuel consumption and the emissions of the 4.2-litre engine.
Two turbochargers with variable turbine geometry deliver high torque even at low engine speeds, in conjunction with high power output. Variable turbine geometry optimizes the flow of exhaust gas to the turbine rotor depending on load and engine speed. By means of air mass metering, the engine management ensures that both turbochargers run constantly at the same speed and therefore achieve the same delivery rate. Maximum boost pressure is 2.5 bar; the turbochargers rotate at up to 226,000 revolutions per minute.
The variable turbine geometry is supported by electrically operated actuators on the new 4.2 TDI. These actuators adjust the vanes faster and more precisely: particularly at low engine speeds, this means a more rapid build-up of boost pressure, thus achieving an excellent response.
Each of the two turbochargers serves one bank of cylinders. The two intake manifolds are interconnected by means of a “feedthrough” system, to equalise the pressure between the right and left cylinder banks. This feature boosts cylinder charging and consequently enhances the engine’s output.
There are continuously variable swirl flaps integrated into the intake tract. These permit the air movement to be adjusted to suit the engine speed and load at the time. High tumble at low engine load results in optimised combustion, just as low tumble does at high engine load. In practice this means that a closed swirl duct at low load increases swirl, while open ducts at high load boost cylinder charging.
Audi has upped the pressue of its common-rail injection system to 1,600 bar (250 more than previous). The higher injection pressure results in even finer atomisation of the fuel, better mixture preparation and hence more efficient combustion. The consequence is not merely extra power and torque, but also reduced fuel consumption and emissions.
Piezo injectors enable considerably smaller and more precisely metered injection quantities. At the same time, piezo technology permits a higher injector needle speed. With a value of 1.3 metres per second, the piezo system responds twice as fast as other common rail injectors.
The number of injection processes per combustion cycle can thus be varied almost at will. In the case of the 4.2 V8, the Audi TDI development team decided on up to four injections. In the lower engine speed range there is a double injection in addition to the main injection, and a single pre-injection in the mid-range.
The fuel is injected into the combustion chamber via eight-hole jets. They too play a major part in making the combustion process ultra-efficient, and thus improving the quality of emissions, by distributing the fuel homogeneously throughout the combustion chamber.
The 4.2 TDI has a twin-pipe exhaust system with particulate filters as standard. In each exhaust pipe the cleaning of the exhaust gases is undertaken by an oxidizing catalytic converter close to the engine and a catalytically coated particulate filter.
The particulate filter, made of silicon carbide with thin-wall technology, is located in the under-floor area. The combination of thin-wall technology and catalytic coating enables controlled regeneration from temperatures of approx. 580 ºC, with simultaneously low exhaust back pressure. This is reflected in only marginally increased fuel consumption and enhanced regeneration efficiency.
The A8 4.2 can cover distances of up to 2,000 kilometres before the filter reaches its maximum charge level. The charge status of the filter is continuously checked by means of monitoring of the exhaust back pressure and parallel simulation calculation for both exhaust pipes separately.
When regeneration becomes necessary, the engine management system generates the required increase in temperature by means of measures taken within the engine and exothermic conversion of unburned hydrocarbons in the oxidising catalytic converter. Up to 5 injections are also possible with the common-rail injection system used at Audi.
May 30, 2005 in Diesel | Permalink | Comments (0) | TrackBack (0)
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