BMW's V8 direct injection diesel engine: CAD image of crankshaft, valve gear, and common rail.
CAD front view of BMW's V8 direct-injection engine.
BMW's V8 direct injection diesel engine.
Audi V8 direct-injection diesel engine.
It may seem a conflict in terms to install a diesel engine in a large luxury car, but in Europe it is now established as a clear trend. Last year, BMW presented its bi-turbo 3.9-L V8 prototype diesel engine 7-series. Now, it has entered production as the 740d, the latest iteration of the 7-series. BMW's large car line began in 1939 with the 335 and continued in the 1950s and early 1960s with the six- and eight- cylinder 501 and 502, a design known as the "baroque angel." BMW used an aluminium V8. After a gap of a few years, its successor was the 2500 - 2800 range, which clearly signaled the design cues of modern BMWs.
Mercedes-Benz is also developing a V8 diesel for its luxury cars, and so is Audi. But it is BMW that lays claim to producing the world's first production luxury performance car with a direct-injection V8 diesel engine. The basic figures are impressive: 180 kW (245 hp) output at 4000 rpm with maximum torque of 560 N***149;m (413 lb***149;ft) delivered between 1750 rpm and 2500 rpm. It is said to be the world's most powerful diesel engine in a production passenger car. The engine is marginally more powerful than originally projected, said BMW's Dr. Helmut Kratochwill, due to gains made during fine-tuning of the fuel injection and intake and exhaust systems. Performance figures include a 0-100 km/h (62 mph) time of 8.4 seconds and a top speed of 242 km/h (150 mph). EU standard composite fuel consumption is 9.9 litres/100 km (24 mpg). The V8 engine and its associated technology is based closely on BMW's direct-injection, four- and six-cylinder diesel units which share the main cylinder dimensions.
BMW lists the new engine's notable features as: adjustment of the turbochargers by electric motors; two intercoolers; four-valve combustion chambers with the injection nozzle located in the middle; fully controlled, cooled exhaust gas recirculation; and twin-chamber exhaust system with one advance and one main catalyst on each chamber. It also has a crankcase made of high-strength grey cast-iron incorporating vermicular graphite. BMW believes this is a first for a production car. The material is said to be up to 20% lighter than conventional grey cast-iron, also having acoustic benefits.
Another first is the use of a cracking method, patented by BMW, for manufacture of the crankshaft bearing supports: "The 100% positive engagement obtained in this way avoids even the slightest micro-movements on the bearing surface, enabling our V8 to achieve a very high level of peak combustion chamber pressure despite the substantial transverse forces with engines of this kind," said Kratochwill The V8 has two fuel-injection rails, with the injection pipes from the rails to the injector kept very short. Maximum injection pressure within the system is 135 MPa (19,600 psi), generated by a newly developed high-pressure pump, according to Kratochwill. The pump can cope with an injection pressure up to 160 MPa (23,200 psi), making it suitable for the next generation of common rail technology.
Kratochwill said the cylinder charge cycle also uses new techniques: "Virtually the entire system, extending from the intake snorkel via the air filter, air-mass meter, turbocharger, intercooler, advance and main catalyst, all the way to the final muffler catalyst, is built throughout in two-chamber design. In conjunction with the four-valve combustion chambers, this provides an ideal basis for maximum power and torque combined with minimum fuel consumption and emissions."
A particularly interesting aspect of the big diesel V8 involves its turbocharger technology. The turbocharger turbine blades of the company's in-line direct-injection diesel engines are adjusted pneumatically, but the V8 uses an electrically operated system. Kartochwill explained the logic behind the system: "The big advantage is an adjustment rate 10 times faster than before, plus much more precise control and management. This allows both turbochargers to respond far more accurately and quickly to the turbocharger pressure requirements defined by engine management, which in turn means far better and smoother behavior."
The exhaust gas recirculation (EGR) system, as with all other components in the charge cycle, consists of a separate unit for each row of cylinders. An exhaust gas cooler integrated in each of the two reflow pipes serves to keep the temperature of the recycled exhaust gas to a minimum, thus allowing a higher reflow rate and serving to further reduce the formation of nitric oxides. The V8 engine exceeds EU2 emissions limits and "comes close" to EU3, according to BMW.
New diesel engine technology often raises the question of particle filters. BMW says it feels that particle filter technology and the regeneration process have "not yet achieved the requisite level of development." It prefers a different approach and is developing technologies able to improve the combustion process and reduce untreated emissions to a minimum. Introduction of low- sulfur fuel is an important element in this work, which would allow the application of new emission management systems.
A problem manifested by thermally efficient diesel engines is the supply of sufficient heat to the passenger compartment. This is solved by the use of an ancillary heater, integrated into the heating circuit. As for servicing, engine oil changes will be needed for the V8 on average every 22,000 km (13,670 mi).
When this AEI editor drove the prototype 740d, its overall performance was impressive, and it also had a sporty engine note, which gave it added "character," although BMW engineers at the time felt this might be inappropriate on a production car at the top of the luxury sector. However, interestingly not all of this sound has been squelched and when driven hard, the car retains its aural sportiness. In fact, BMW will even offer an M-sports package to further enhance this aspect of the car.
Performance delivery has an impressive linearity, with the electrically adjustable turbine geometry of the two turbochargers dealing very effectively with turbo-lag. It had been thought that the new V8 might be installed in the 5-series BMW, but the company says it has no plans for such a development. However, the engine is likely to be used by Land Rover for the next-generation Range Rover.
Although Audi's V8 engine is not yet fitted to a production car, it unveiled the 3.3-L direct-injection common-rail unit at the 20th International Vienna Engines Symposium. Audi has also opted for electronically controlled variable turbine geometry (VTG) turbochargers. Power output is 265 kW (355 hp) at 4000 rpm with 480 N***149;m (354 lb***149;ft) of torque available from 1800 to 3000 rpm. The engine uses four-valve technology and cooled, charged-air EGR. Significantly, Audi says it meets the EU3 exhaust emissions limits.
Like BMW, Audi also uses vermicular graphite cast-iron for the cylinder block. Instead of having individual main bearing shells, Audi opted for what it describes as "an extra-strong main bearing frame," which incorporates the five bearing shells, and is supported at each side and bolted from beneath at several points. Audi states that it uses six-jet injection nozzles. Compression ratio is 18.5:1. This is relatively low for a direct injection engine and produces a favorable relationship between high performance and low emissions, while preserving good starting behavior. The air intake system operates with two hot-film air-mass sensors and its responsiveness allows the guide vanes of the two VTG turbochargers to be adjusted so that both banks of cylinders operate on the same mass of air, the charge air cooled in an air/water cooler, which produces significantly lower pressure loss than an air/air cooler. The charge-air cooler is located in the V of the two cylinder banks, which contributes to the power unit's overall length of 717 mm (28.2 in). Recirculated exhaust gas is water-cooled, which helps keep NOx levels down. Although the level of HC and CO generated is marginally higher, the location of the catalytic converters close to the engine compensates for this.