The diesel engine is known as the most fuel efficient combustion engine. Its acceptance for use in passengers cars, however, varies geographically. Today, the diesel car plays an important role in Europe; in France, for instance, it is achieving a remarkable market share of about 42 percent, while in the US its market penetration can be neglected. Many questions are expressed concerning the future of diesel powered cars. The question affecting market acceptance is as follows: can the significantly better fuel efficiency of a diesel car outweigh perceived detrimental characteristics? Such unfavorable properties are thought to be low specific power, objectionable noise, higher exhaust emissions (including smoke), and higher vehicle price. These features are closely influenced by the state of passenger car diesel engine technology. This technology state and its potential must be evaluated with respect to current and future demands, for instance, tighter exhaust emission regulations. In addition, the commercial value and consumer acceptance of high fuel economy must be evaluated. It is clear that the ultimate result of weighing the pros and cons will depend not only on technological factors, but also on political factors such as fuel taxation. Regarding the state of technology, the diesel car is very promising. First, by employing a direct injection combustion system, the fuel efficiency can be improved by about 15 percent over current swirl chamber engines. Furthermore, the specific power (hp/ltr) can be increased by efficient supercharging to achieve values of today’s gasoline engines. By tuning the combustion system, low noise engine design features and incorporation of careful noise reduction measures on the vehicle, the noise behavior of a spark ignited vehicle can be reached. Exhaust emissions can currently be reduced to a level to satisfy today’s European and US Tier 1 emission limits. However, significant development effort remains. More stringent emission levels (California US, Tier 2 ULEV, and Stage 3 in Europe) require further advancements in diesel combustion. The strong development potential of 4-value engines and new unique injection systems is evident. In addition, there are promising developments with lean NOx catalysts and regenerative particulate filters. These technologies offer the potential to meet the very stringent future emission standards. It is anticipated that the sophisticated technology needed to meet these standards will make the future diesel car more expensive compared to a gasoline fueled vehicle. This raises the issue of what price will the consumer pay for the higher fuel economy of a diesel car. In light of the worldwide rapid increase in passenger car population and of the dwindling oil reserves and their global distribution, the fuel efficient diesel engine will play an important role in the future of passenger cars.

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