Variable Twin Turbo technology takes the BMW four-cylinder diesel engines to unmatched levels of performance. It dual-stage design enables the engines to achieve performance figures that previously only larger engines could reach. Emissions and fuel consumption, however, are impressively low. The 4 cylinder engine featured rear-mounted chain drive system designed to satisfy even future pedestrian impact severity reduction requirements. The auxiliary units are positioned on the left-hand side of the engine. By having the auxiliary units all on one side, space is created on the right-hand side of the engine for locating exhaust treatment components close to the engine. There is also sufficient space for the Top model turbocharger assembly. The introduction of BMW's dual-stage turbocharging system, Variable Twin Turbo, represents another quantum leap in diesel engineering. This technology delivers a performance boost of 25 per cent, and enables instant responsiveness even at low engine speeds - as well as enormous reserves of power at higher revs. The heart of the Variable Twin Turbo system comprises two turbochargers of different sizes. In contrast to the bi-turbo system, the two turbochargers do not operate parallel, but are activated at different points. Precisely coordinated, they run individually or together, increasing the pressure of the intake air to the optimum level. The greater pressure of the intake air enables a more effective combustion of the diesel fuel in the cylinder. At lower engine speeds, the intake air passes through the inactive larger turbocharger into the active smaller turbocharger, which increases the air pressure. At medium engine speeds, the larger turbocharger becomes active, forcing air under pressure into the smaller unit, which increases the pressure still further. At higher engines speeds, the larger turbocharger alone forces air into the cylinders. Despite these impressive dynamics, the fuel consumption of the Variable Twin Turbo diesel engine is exemplary low. It has also been possible to minimize the height of the cylinder head. The inlet ports now run parallel and the intake system with integral swirl flaps is an ultra-compact design. The 3rd-generation common-rail system with an injection pressure of up to 1,800 bar, a new high-pressure pump and piezo-electric fuel injectors complete the fuel-system. So fuel injection is even more precise and finely measured. Thus reducing consumption as well as exhaust emission levels, while boosting engine power.