The latest designs of diesel engines recently unveiled in Europe provide significant improvements in virtually all of the characteristics of interest. Most of the development in diesel technology is centered in Europe. Diesel penetration in the Japanese market is low, and Japanese automakers are focusing primarily on lean-bum gasoline engine concepts.
Diesel penetration is occurring, however, in the Japanese sports utility vehicle market. Until 1991, diesel powered passenger cars and light trucks sold in the United States were all of the IDI type, where fuel is sprayed into a prechamber, partially mixed and combusted with air before further mixing and combustion occurs in the main combustion chamber.
Benefits Diesel Engines
The prechamber 91 design results in smoother combustion with less noise and lower NOX emissions. However, heat transfer from the prechamber and pressure losses from the partially combusted gases as they flow through the small passages connecting the prechamber to the main combustion chamber result in reduced efficiency. In fact, the peak efficiency of an IDI diesel is comparable to, or only slightly better than, that of a spark ignition engine; most of its efficiency advantage occurs at light loads.
Direct injection (DI) systems avoid the heat and flow losses from the prechamber by injecting the fuel directly into the combustion chamber. The fuel injection system must be quite sophisticated, as it must be capable of injecting very little fuel during the ignition delay period, while providing highly atomized fuel and providing intensive mixing during primary combustion.
Advancements in fuel injection technology and diesel combustion chamber design has led to the recent introduction of passenger car DI diesels by Volkswagen in their Audi and VW model lines. Turbocharging has also been found to be particularly effective in combination with diesel engines. Many new diesel engines, including the Volkswagen DI diesel engines, are turbocharged and some feature intercoolers, which provide a cooler, denser charge to the engine.
As a result, the specific power of diesel engines with turbocharging now exceeds the specific power output of naturally aspirated, two-valve per cylinder gasoline engines and approaches that of four-valve per cylinder gasoline engines. Turbocharging and intercooling are quite costly, however, and turbocharged engines still have some low-speed drivability deficiencies.