Oxygenates, as a component of gasoline (MTBE or alcohol 10-17% or 3-4% O2 equivalent), leads to lower CO emissions and somewhat lower HC including benzene. NOx and acetaldehydeemissions can go up somewhat depending on the type of oxygenate,whereas evaporative emissionscan go up stronglydue to blockingof activecarbon canistersites by alcohols.
Regardingthe effectof dieselfuel on emissions,the following composition parameters have been, and are being, studied extensivelyfor their relationship with exhaust emissions:density, cetane number and index, aromatics content, sulfur content, boiling range (T50-T90). In some studies an attempt has been made to differentiate between mono- and polyaromatics. Of all the fuel parameters, cetane number appears to be the only consistentparameter; it clearlyaffectsthe quantity and quality of particulate emissionsin IDI and high speedDI engines.Rapeseed oil is discussed as a new agricultural product, that could replace other overproduced crops in Europe and elsewhere.In terms of regulated and unregulated emissions there appear to be major disadvantages for the straight oil. In esterified form (rapeseed oil methylester),small advantagesin regulatedemissions against increase of smell result. Like all vegetable oils, it offers an overall advantage in terms of CO2 emissions. However, if the overall greenhouse gas
emissionsfrom the full production cycleof esterified rapeseedoil are considered,advantagesover the diesel fuel cycleessentiallydisappear.
2. EMISSIONS LEGISLATION IN EUROPE AND THE u.S.
Figure 1 shows the evolution of U.S. emissions standards over the last 25 years. In the past, the exhaust emissions standards for passenger cars and other road vehicleswere strengthened stepwiseevery few years. This concept was based on the experience that the manufacturersof passengercars construct the vehiclesin such a way that the emissions are lower than the limit values by a safety margin. The thus attainable emissionslevelscould then be considered as a new basis of common available technology,for the negotiations of the next step. This approach has an advantage for the manufacturer, since only small changes are required every2 or 3 years. On the other hand, frequent modifications in the produc tion had to be done and expensesfor new certifica tions incurred. In the more recent EPA legislation,as well as in the European directive 94/12/EEC, it is required that the proposal for emissionstandards for passenger cars for the year 2000 should aim at a substantial pollutant reduction. For the following steps, the potential of the existingengineand exhaust
Table 3. Current and future emission standards for light-duty vehicles in the U.S. Implementation plan of the Clean Air Act
Amendments