BMW's Twin Turbo Diesel

While twin turbos have been used on passenger car petrol motors for some time, there's never been a sequential twin turbo diesel for use in cars. Until now.

The new BMW engine is a 3-litre in-line six cylinder. In twin turbo form it develops 200kW at 4400 rpm and a staggering 560Nm at 2000 rpm. No less than 530Nm is available from 1500 rpm.

The twin overhead cam, iron block and alloy head design uses four valves per cylinder and weighs only 228kg. BMW state the twin turbo approach has resulted in a power increase of 20 per cent when compared with existing state of the art diesel engines.

As is typical with petrol car sequential turbos, two different sized turbos are used. The smaller operates at low engine speeds, and the larger at higher speeds.

Three different operating conditions occur:

1. At low engine speeds the intake air flows through the large turbocharger and is compressed in the smaller turbocharger. This supplies substantial amounts of air to the power unit, starting without any appreciable delay at idle speed. The 3.0 litre inline six-cylinder diesel develops 530 Nm of torque at as early as 1,500 rpm.

2. With increasing engine speed, the larger turbocharger becomes more important - initially as a pre-compressor. The intake air is additionally highly compressed inside the small turbocharger, the engine reaching its maximum torque of 560 Nm at 2,000 rpm. By means of a turbine control valve, the flow of exhaust air is variably distributed to both turbochargers, regulating their interaction.

3. At high engine speeds work is done primarily by the large turbocharger, the power unit reaching a maximum output of 200 kW/272 bhp at 4,400 rpm.

With a specific power output of 66.7 kW/l, the new 3.0 litre diesel even exceeds the rating achieved by the M5 sports saloon (58.8 kW/ltr), production of which ceased at the end of June 2003.

The basic construction of the BMW high-performance diesel is modelled on the diesel engine already used in Sports Activity Vehicles X5 and X3, or the large 7 Series. While the 2,993 cc capacity remained unchanged, the crankcase construction was reinforced predominantly within the area of the crankshaft bearing.

Second-generation Common Rail serves as an injection system, with the flow characteristic of the fuel-injection nozzles adapted to cope with the new level of output. The 3.0 litre diesel with Variable Twin Turbo has in certain driving situations lower specific fuel consumption than the basic power uni. Injection timing and combustion process (compression ratio 16.5:1) were adapted to the new parameters.

The Variable Twin Turbo offers the widest useful engine speed range in its class. In addition to the increase in output, the useful engine speed range was widened by + 400 rpm. Nominal output of 200 kW/272 bhp is at 4,000 rpm and maximum engine speed is now 5,000 rpm, which is unusually high for a diesel.

Acid test in the Dakar Rally At the 2003 Dakar Rally, specialists at the BMW Diesel Competence Centre in Steyr (Austria) used the new technology with two variable turbochargers in an X5 entered by the independent X-Raid team. During this first appearance, the X5 gained victory in the diesel category. To conclude the final stage of testing, the BMW X5 was once again entered for the 2004 Dakar Rally. On 17 January, two BMW X5s, powered by the innovative 3.0 high-performance, near-production diesel, were the first to cross the finishing line in Dakar.

When viewed from the side, the upper eye of the engine connecting rod is shaped like a trapeze. This new technology is used on the internal combustion engine to create an additional supporting surface for the gudgeon pin within the lower area of the connecting rod eye for high piston pressures. At the same time, weight is saved in the upper (narrower) part of the connecting rod eye.

Like all other BMW connecting rods, those installed in the new inline six engine have also undergone the cracking process. This method entails breaking the lower connecting rod eye in two. The two parts are then bolted together again when assembled onto the crankshaft. Due to the uneven surfaces resulting from the cracking process, the link between the two parts of the connecting rod has far greater stability than the even surfaces obtained on conventionally sawn connecting rods.

Thanks to lightweight camshafts, the new R6D has a weight advantage of 1.2 kg (-25%). The basic component of the new camshaft is a steel tube serving as a support for the cams prefabricated from high tensile refined steel. The cams are then joined and finally polished down to an accuracy of 1/1000 mm.

Engine electronics required to control a turbocharged power unit are significantly more complex. A system comprising two superchargers, turbine control, bypass and wastegate has to be controlled, not just a turbocharger with variable turbine geometry. The system elements have to be coordinated with each other and with the respective operating condition of the engine. The Bosch DDE 6.0 control system provides the required computing power and sufficient storage capacity to deal with processes adequately.

The exhaust system was specifically designed to cope with the high rate of air flow. Pipe diameter and silencer were developed independently and the sound was adapted to the sporty character of this high-performance diesel.

The new BMW high-performance diesel engine with Variable Twin Turbo debuts in the 535d Saloon and 535d Touring. The saloon sprints from 0 to 62 mph in 6.5 seconds and top speed is limited to 250 km/h (155 mph), using only 8.0 litres of diesel fuel over a distance of 100 km in the EU combined cycle (all figures apply to the automatic transmission supplied as standard).

Full Engine Specifications

Configuration	-	Inline-six
Maximum output	kW/bhp	200/272 at 4,400 rpm
Maximum torque	Nm	560 at 2,000 rpm
Combustion process	-	Direct-injection Common Rail diesel engine with multi-stage diesel turbocharging
Capacity, effective	cc	2,992.6
Compression ratio	-	16.5
Bore/stroke	mm	84/90
Max. combustion chamber pressure	bar	180
Minimum specific consumption	g/kWh	208
Crankcase material	-	Grey-cast iron (GG25) with cast-in liners
Cylinder block height	mm	285
Distance between cylinders	mm	91
Top land	mm	7
Main bearing diameter	mm	60
Conrod bearing diameter	mm	45
Conrod length	mm	137, crack technology, trapezoidal conrods
Cylinder head material	-	Aluminium
Camshafts	-	2 chain-driven camshafts (assembled camshafts) running in 7 bearings
Valve drive		Roller-type rocker arms; hydraulic valve play compensation
Valves per cylinder		4
Valve diameter intake/outlet	mm	25.9/25.9
Valve shaft diameter intake/outlet	mm	6.0/6.0
max. valve lift intake	mm	7.5
max. valve lift outlet	mm	7.5
Intake system	-	Twin turbo technology with 2 turbochargers, infinitely variable control, intercooling
Engine weight according to BMW standard	kg	228
Engine management/mixture preparation/ignition	-	Bosch DDE6.0/Common Rail 1600 bar system pressure with multiple injection
Fuel		Diesel
Certified emission standard	-	EU4
Exhaust system	-	Primary catalytic converter, isolated exhaust routing, diesel particulate filter (additive-free)