Automobile Emissions Essay Research Paper ABSTRACTPollution from — страница 5

  • Просмотров 531
  • Скачиваний 9
  • Размер файла 22

However, with current engine technology, in order to achieve nitrogen emissions consistent with US legislation, the engine must operate in a very lean region where, as shown in Figure 3.3, hydrocarbon emissions that increase to levels which may exceed current American standards. In these situations an oxidation catalyst is incorporated into the exhaust system to control hydrocarbon emissions. Figure 3-3 The Effect of Air/Fuel Ratio on Engine Operation A feature of the ECE15 European test cycle was its low average speed as it is intended to be representative of city driving. The emissions that result are therefore typical of low speed, low acceleration conditions. A more representative cycle incorporating higher speeds and accelerations has been introduced so as to assess

emissions under other conditions including urban and highway driving. In order to develop and maintain a higher speed more power is required from the engine which, in the case of the lean burn system, means decreasing the air/fuel ratio. This in turn increases nitrogen oxide emissions to levels where current engine technology is likely to exceed standards (See Figure 3.3). It is therefore desirable that catalysts used on lean burn engines should in addition to having a hydrocarbon oxidation capability also have a nitrogen oxide reduction capability when fuel enrichment occurs for increased engine power. The effect on the reduction of hydrocarbons and nitrogen oxide emissions which can be achieved on a lean burn engine using a catalyst with oxidation and reduction capabilities is

shown in Table 3.4 for a Volkswagen Jetta Series 1, powered by a 1.4 litter Ricardo High Ratio Compact Chamber lean burn engine. ECE 15 Cold Start Cycle g/test Hydrocarbons Carbon Monoxide Nitrogen Oxides Without Catalyst 11.7 15.9 5.9 With Catalyst 1.7 12.4 4.2 Table 3-4 Lean Burn Engine Emissions 3.2.4 Diesel Exhaust Emission Control Although Diesel engines emit relatively low concentrations of carbon monoxide and hydrocarbons and have a better fuel economy compared to gasoline powered vehicles, particulate emissions are of concern. Along with the carbon particulates which are produced during the combustion process are a range of aromatic hydrocarbons, which was one of the main reasons that the EPA established standards to limit particulate emissions.8 The carbon and the

associated organics produced during combustion may be collected on a filter and removed by oxidation so that the filter regenerates and is effective for the life of the vehicle. As the particulates are not oxidized at a significant rate below 600?C which occurs in the exhaust system only when the engine is running at or near full power, catalysts are introduced into the filter which reduces the oxidation temperature to approximately 300?C. Table 3.5 compares emissions from an exhaust system with a catalyst to that of a system without.9 g/mile HC CO NOX Particulate Without catalyst 0.24 1.01 0.90 0.23 With catalyst 0.05 0.16 0.79 0.11 Table 3-5 Catalytic Control of Diesel Exhaust Emissions 3.2.5 Catalytic Combustion Nitrogen oxide emissions result mainly from the reaction between

oxygen and nitrogen at temperatures arising from the combustion of fuel whether it is initiated by spark, as in the gasoline engine, or compression as in the diesel engine. Leanburn operation of a gasoline engine, as described earlier, offers a partial solution to the problem but is limited by hydrocarbon emissions as the non-flammability limit for spark ignition is approached. While the diesel engine does not have these advantages it is limited by high particulate emissions. A solution to this problem is to use a catalyst to ignite the air/fuel mixture thus overcoming the constraining factors of the gasoline and diesel engines. Having removed this constraint, the engine is able to operate at a compression ratio of 12 to 1. Combustion efficiency and mechanical energy is thus

optimized which results in a maximized fuel economy.10 The principle of the catalytic engine is that during the engine operating cycle, the fuel is injected into the combustion chamber just before the start of combustion is required. This fuel is then mixed with the air already in the cylinder and then passed through the catalyst, where heat release occurs. Since the charge is passed through a catalyst, oxidation can occur at low temperatures and very lean mixtures. This results in complete fuel oxidation which enables the engine to run unthrottled and therefore lean, which provides good fuel economy. The formation of nitrogen oxides and carbon monoxide in the combustion chamber is also strongly dependent on the air/fuel ratio and lean operation results in reduced emissions of