The foundation for the new five-cylinder diesel engine was the Group’s already successful 1.5-litre four-cylinder diesel engine, which produced 37 kW (50 hp). Following a logical development path, Audi engineers extended this design by one cylinder, creating a 2.0-litre five-cylinder engine with an output of 51 kW (70 hp).
The Audi five-cylinder diesel operated according to the swirl-chamber combustion principle. In this system, the incoming combustion air was set into a controlled rotary motion before entering the main combustion chamber. This promoted highly effective mixture formation through the homogeneous mixing of air and diesel fuel, resulting in a particularly smooth and progressive combustion process.
Optimized fuel utilization was achieved through carefully calibrated injection timing across the entire engine speed range and precise fuel metering by a rotary distributor injection pump. The exceptional fuel efficiency of a diesel engine is primarily due to its significantly higher compression ratio, which provides a substantially greater thermodynamic efficiency than that of a gasoline engine. In addition, a diesel engine does not suffer from the pumping losses caused by intake-air throttling during part-load operation.
With a compression ratio of 23:1, the Audi five-cylinder diesel operated at more than twice the compression ratio of a typical gasoline engine. This high level of compression generated the heat required to ignite the fuel-air mixture, eliminating the need for an electrical ignition system. This represented one of the fundamental differences between diesel and gasoline engines. Unlike a spark plug in a gasoline engine, the glow plug in the Audi five-cylinder diesel served solely as a cold-start aid. When the engine was cold, it preheated the swirl chamber sufficiently to ensure reliable initial combustion.
Fuel injectors were positioned in the upper section of the swirl chambers located ahead of the main combustion chamber. Their purpose was to ensure that the diesel fuel and incoming air mixed as uniformly as possible, thereby promoting the smoothest possible combustion process. The specially designed shape of the swirl chambers contributed significantly to the engine’s refinement and low fuel consumption.
The rotary distributor injection pump drew fuel from the tank, pressurized it, and distributed it to the individual injectors according to the engine’s firing order. In addition, the pump controlled engine speed and varied the injection timing to suit operating conditions.
In the Audi 100’s five-cylinder diesel engine, the camshaft was driven by a toothed timing belt, an unusual solution for a diesel engine at the time. This contributed significantly to the engine’s quiet operation. The injection pump was also driven from the camshaft via a second timing belt.
The coolant pump was integrated into the crankcase and simultaneously served as a tensioning device for the timing belt by which it was driven. Engine lubrication was provided by a reliable crescent-type gear oil pump, driven directly from the crankshaft.
A torsional vibration damper mounted on the crankshaft further enhanced the engine’s smoothness and refinement. At the same time, it served as a pulley for various auxiliary components, including the hydraulic power-steering pump and the air-conditioning compressor.
Because of the considerably higher combustion pressures encountered in a diesel engine, the connecting-rod bearings were designed to be substantially stronger than those used in a gasoline engine. Likewise, the crankshaft rotated in particularly robust main bearings specifically engineered to withstand the increased mechanical loads associated with diesel operation.