TDI direct injection
In 1892, when Rudolf Diesel applied for a patent on his first engine, he was already considering direct fuel injection, but for many years this proved impossible to achieve, since it was too difficult to manufacture high-performance injection pumps.
The first diesel engines in passenger cars were introduced in 1936. They were naturally aspirated, with fuel injected at a pressure of just over 100 bar into a pre-combustion chamber. This was located ahead of the main combustion chamber in the cylinder head. The fuel vaporized and ignited there, after which the combustion process spread through an aperture into the main chamber. This principle had the advantages of relatively smooth, gradual mixture ignition at an acceptable acoustical level, and improved dynamics. The principal disadvantages were flow losses, heat dissipated through the cylinder wall and the slow speed of the combustion process. The turbocharged diesel that gained some ground in the 1970s suffered from the same shortcomings.
These handicaps were eliminated by the direct injection principle, which has evident benefits in terms of efficiency. It was first adopted in the 1950s for commercial vehicles. As injection pump design progressed and electronic system management became practicable, Audi produced a version suitable for use in passenger cars in the late 1980s.
The first Audi TDI, the 2.5-liter five-cylinder engine introduced in 1989, had an electronically controlled distributor-type injection pump operating at a pressure of up to 900 bar. The engine incorporated a large number of high-end features: swirl-action inlet ports to impart turbulence to the incoming air, four-hole injector nozzles for an accurate fuel spray pattern and two-spring nozzle holders that permitted a pre-injection phase which made the combustion process less violent and reduced the noise level. The charge-air intercooler lowered the temperature of the air compressed in the turbocharger, so that the combustion chambers were supplied with a higher proportion of oxygen. With an output of 88 kW (120 hp), and the notably high peak torque of 265 Nm (195.45 lb-ft), even this first TDI engine was an impressive source of power.