BMW Technology - Diesel Engine Development

Efforts to improve the basic properties of the diesel engine with BMW in the forefront were so successful that, particularly from the 1980s onwards, it came to be considered as a serious alternative to other types of engine for all classes of passenger car. With improved dynamics and acoustics, the decision was then soon taken to implement series production of BMW dieselfuelled cars. This led to the foundation of the BMW engine factory in Steyr, Austria, where BMW's diesel engines have been developed and built ever since 1980.

The turbodiesel engine was presented for the first time in the 524td -- delivering 85 kW/ 115 hp -- and was promptly dubbed the "sports diesel" thanks to its outstanding performance, typical of the BMW brand. In 1987 BMW beca, me the first car manufacturer in the world to introduce a fully electronic diesel fuel-injection control. Operating faster and more precisely than a mechanical control, the electronic system regulates the "tuning" of exhaust and consumption parameters, engine noise and smooth running performance. A completely new development followed in 1991, a 2.5-litre turbo-diesel with charge-air cooler; delivering 105 kW/143 hp, it was the most powerful diesel in its class anywhere in the world.

2000: the most powerful direct-injection diesel car in the world - from BMW

The turbocharged, direct-injection diesel engines from BMW are shining examples of today's state of the art and the four-cylinder engine of the 320d, the in-line six-cylinder versions of the 330d, 525d, 530d and 730d, and the world's first and ultra-powerful V8 diesel engine of the 740d.

Diesel engines with direct fuel injection ("DI engines") into the cylinder have a higher thermal efficiency and thus consume less fuel. A high injection pressure at the nozzle is essential for an immediate and effective "swirling" of the fuel, leading to an optimum mix formation and complete combustion. This in turn provides high torque and power output, with low emissions.

Very few systems achieve these high injection pressures. BMW employs two different systems: the distributor pump (up to 1,750 bar) and common rail (up to 1,600 bar). In the first instance, the distributor pump separately supplies each valve via its own delivery pipe; this system proved ideal for the four-cylinder engine concept of the BMW 320d. It reaches its limits, however, in the context of six or eight-cylinder engines. With more cylinders and their longer feed lines, this system is unable to achieve the necessary coordination of the timing of each fuel injection.

In order to avoid compromised performance, the engines of the 330d, 525d, 530d, 730d and 740d are supplied with fuel by the common rail system. This employs a high-pressure pump to generate the optimum pressure for each phase in the operating cycle, the pump acting on a common supply line (the "common rail") for all injection valves. The common rail system is also markedly quieter than the distributor-pump injection system, and thus the ideal companion to the smooth, vibration-free ride offered by an in-line sixcylinder or V8 engine.

A common means of increasing torque and effective performance in diesel engines, which has been in use for a considerable time, is pressure charging by means of an exhaustdriven turbocharger. This compresses the intake air prior to combustion and ensures a denser charge within the cylinder. To increase the volume of compressed air delivered by the turbocharger still further, it also flows through a charge-air cooler between the turbine and the engine. Here, the compressed and thereby heated air is cooled and increased in density, in turn increasing the charge within the cylinder. The consequence is a further improvement in torque.

In the past, advances in turbocharger design had always been compromised by the conflicting requirements of improving throttle response and torque at low revs on the one hand, and raising power output on the other. The turbo-chargers used in the BMW DI engines solve this problem by using a variable turbine geometry and high-pressure charging of up 2. bar.The system adjusts the guide 1 vanes which direct the exhaust flow into the turbine. Increasing the rate of flow of the exhaust gas causes the turbine and, concomitantly, the compressor rotor, to rotate at a higher speed. More air is taken in, leading to higher boost pressure.

The system introduced under the designation VNT (variable nozzle turbine) has the advantage that sufficient exhaust counterpressure is always generated to provide the delivery pressure required in any given situation: greater torque at slow speeds ? previously only attainable with a small charger and greater performance through lower exhaust counterpressure at high revs. For the driver, the VNT technology has tangible benefits. The engine provides more powerful torque at both low and high revs.

The fast charge cycle of the turbocharger and four-valve technology also improves the throttle response. Whereas conventional diesel engines tend to respond sluggishly to accelerator pressure in comparison with petrol engines, the BMW diesel immediately turns a touch of the accelerator pedal into a rise in the level of torque.

The world's first series-manufactured eightcylinder diesel engine with common rail direct injection and another first electronically controlled twin-turbocharging opens up a new dimension of powerful, comfortable and yet economical motoring. The injection delay is five times shorter when controlled by the electrical actuator rather than being induced by underpressure. The result is that both chargers are now far more precisely attunable than before to the parameters of speed and revs, which brings about a marked improvement in throttle response.

Perfectly smooth engine tuning and optimum fuel economy in all ranges is ensured by the electronic synchronisation now possible for the first time between the chargers and the exhaust-gas recirculation system, since the combustion chambers of the V8 cylinder banks are supplied at all times with exactly the same pressure of air. Use of this technology also makes it possible to reduce the previously normal safety margin providing protection against excess pressure. Resultantly, higher performance is achieved from the same capa-city, with a lower engine weight.

Future prospects - new emission laws may well require new fuels

In spite of its peerless economy and low consumption of energy and resources, the diesel engine faces enormous challenges. The reason for this is legislation on exhaust emissions, in particular restrictions on the levels of oxides of nitrogen (NOx) and soot particles released into the atmosphere. Oxides of nitrogen are generated during combustion upward of a certain temperature, depending on the proportion of oxygen in the fuel-air mixture.

Soot is a characteristic of the compression-ignition engine: homogeneous combustion is not a property of the diesel. Soot results from an insufficiency of oxygen in certain "zones" of the combustion chamber. The aim of reducing both pollutants therefore brings about a conflict fewer oxides of nitrogen mean more particles of soot, and vice-versa.

A standard oxidation catalytic converter reduces emissions of hydrocarbons by up to 50 per cent, and as a side-effect also the emission of particles by between 10 and 30 per cent. This lies well within the limits prescribed by current regulations on emissions, and all diesels in the current BMW range have emission levels well below EU3 stipulations. In order to meet the tolerance levels soon to be enforced as EU4, several measures must be implemented in tandem.

In principle, the NOx and soot particle emissions of a diesel engine with a common rail direct-injection system can be reduced by increasing rail pressure. But this alone is not enough; larger vehicles in particular will also need the so-called "de-NOx" cat as well as a particle filter. Both are still at the technical development stage, and will ultimately increase the price of new cars.

Not only that: in order to operate a de-NOx cat, a fuel is required containing less than 10 ppm of sulphur. This is due to the fact that the real function of such catalytic converters is to capture and store the oxides of nitrogen generated during particularly lean - i.e. economical combustion. During "richer" combustion, such as when accelerating, they are released once again, and subsequently converted in the oxidation converter.

The storage medium of a de-NOx cat, however, also collects sulphur even before capturing oxides of nitrogen. This reduces the effect of the converter, or even prevents it from working at all. Moreover, sulphur binds with soot particles, prevents their combustion, and thus increases their pollutant effect. These technical reasons alone illustrate why simultaneous introduction of low-sulphur diesel fuel of the required quality is an elementary prerequisite of compliance with the planned emission laws.