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
aftertreatment technologies must be reassessed from time to time.
Also, the effects of traffic induced CO2 emissions must be taken into account in the evolution of emissions reduction technologies. In order to meet the aim of decreasing anthropogenic CO2 emissions by 25% by the year 2005 compared to 1990, which was required by the German government during the
1st World Climate Conference in Berlin, a significant drop in traffic CO2 emissions needs to be effected.
The properly working standard closed-loop TWC
reduces emissions of HC, CO and NOx of a spark ignition engine by 80-90%. The main potential to further reduce emissions is to reduce the time needed for the catalyst to reach light-off temperature. Currently, about 60-80% of total HC emissions over the New European Driving Cycle (NEDC) and the FTP 75 cycle are emitted within the first 200 sec of the cold start phase. As a large part of inner-urban driving is done over small distances (less than 3 km), the reduction of cold start emissions can significantly reduce the overall traffic induced immission burden.
Legislation in the U.S., especially California,
already requires much lower emission standards than can be achieved with the conventional TWC technology. Yet for Europe more stringent emission regulations are also scheduled, including a more realistic test procedure (e.g. omitting the 40 sec idle period at the beginning of the driving cycle), and new requirements regarding evaporative emissions and on-board diagnostics.
The basis for the new U.S. regulations are the 1990
Clean Air Act Amendments (CAAA), with the objective to improve air quality, particularly in large cities where in summer high ozone levels and in winter high CO levels are encountered. Besides tighter tail-pipe emission standards, the CAAA require the establishment of improved inspection-maintenance programmes, a reformulated gasoline program, legislation covering standards for vehicle fleet operators and for 'clean fuels-clean vehicles', which could lead to the introduction of alternative fuels.
Exhaust emissions standards defined in the CAAA
are listed in Table 3. The current standard is Tier I which was phased in between 1994 and 1996, when certification up to 100,000 miles will also be required,
Fig. 2. Structure of the monolithic catalytic converter.
enforced by the possibility of an 'emission-recall'. Tier II standards will only come into effect if EPA decides that they are necessary, technically feasible and cost-effective.
More stringent emission standards are being intro
duced in California with the aim of progressively reducing vehicular emissions, to achieve the national air quality standards. In Table 3, the classes of low emissions vehicles are also defined. Only the intro duction of ZEV is mandatory, whereas for TLEV, LEV and ULEV implementation is up to the manufacturer, as long as his average NMOG fleet emissions are below the mandatory limits. Non methane organic gases (NMOG) are the sum of NMHC (up to l2C atoms) plus oxygenated com pounds (up to 5 C atoms) such as aldehydes/ketones and alcohols (for alcohol-fuelled vehicles). The inten tion is to evaluate the tropospheric ozone formation potential, therefore methane is excluded, and the measured NMOG emissions are corrected using reactivity adjustment factors (RAF) which are basic characteristics for a given vehicle/fuel combination. For gasoline and alcohol vehicles additional tests are required at 10°C,with the same CO and NOx standards, but less stringent NMOG and HCHO limits.
3. CATALYTIC CONVERTERS FOR STOICmOMETRIC SPARK IGNITION ENGINES
3.l. Catalyst Types
The catalytic converters used in spark-ignition engines consist of an active catalytic material in a specially designed metal casing, which directs the exhaust gas flow through the catalyst bed. The active
material employed for CO and He oxidation or NO
reduction (normally noble metals), must be distrib uted over a large surface area, so that the mass transfer characteristics between the gas phase and the active catalyst surface are sufficient to allow close to
100% conversion with high catalytic activity.f The most widespread type today employs a ceramic (or
metallic) honeycomb structure or monolith held in a metal can in the exhaust stream. The noble metals are impregnated into a highly porous alumina washcoat about 20-40 J.Lm thick that is applied to the passage way walls (Fig. 2). A typical monolith has square cross section passageways with inside dimensions of the order of 1 mm separated by thin (0.1-0.15 mm) porous walls. The number of channels per square inch varies between 400 and 600,9 although even higher cell densities of the order of 1200cpsi have been demonstrated for metallic substrates. 10 The washcoat,
5-15% of the monolith weight is mainly composed of
AI203 and has a surface area of 100-200m2/g.
The majority of the present day monolithic three way emission control catalysts for gasoline engines use a combination of platinum and rhodium as precious metal components. There has been a keen interest lately in replacing a part or all of the platinum content of an automotive catalyst by palladium, mainly as a means to reduce the precious metal cost of the catalyst. II Most of the oxidation catalysts used in the U.S.A. on model years 1975-
1980 vehicles, contained Pt and Pd in the ratio 5 Pt-2
Pd at a typical loading of 50-70 g/ft3. Also, a lot of the bead type three way catalysts used by General Motors on model year 1981-1992 vehicles, used Pt together with Pd and Rh. In the literature, precious metal loading ranges are reported for Pt 30-100 g/ ft3; Pd 0-120 g/ft3; and Rh 5-10 g/ft3. The ratio of Pt to Pd was in most cases higher than one. During the development of these catalysts it was recognized that each of the three precious metals Pt, Pd and Rh need a particular position in the catalyst to function properly. To this end, advanced impreg nation techniques were developed. A number of disadvantages have been reported when Pd is incorporated in Pt-Rh catalysts. For example it has been demonstrated that Pd can form alloys with Rh in which Pd is enriched at the alloy surface, thereby suppressing the full deployment of the excellent catalytic activity of Rh.12 To avoid this, advanced, costly impregnation technologies are necessary.
In addition, several fundamental studies have shown that the resistance of Pd against poisoning by for example lead and sulfur is substantially inferior to that of both Pt and Rh.It was shown that sulfur decreases, in particular, the ability ofPd to convert CO and HC in net reducing exha