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Internal Combustion Engines
Published in Mehrdad Ehsani, Yimin Gao, Ali Emadi, and Fuel Cell Vehicles, 2017
Mehrdad Ehsani, Yimin Gao, Ali Emadi
Proper fuel/air (or air/fuel) ratio in the fuel/air mixture is a crucial factor that affects the performance, efficiency, and emission characteristics of an engine, as shown in Figure 3.8. The mean effective pressure peaks at slightly rich stoichiometry (between φ = 1 and φ = 1.2). However, the fuel conversion efficiency decreases as the mixture is enriched above the stoichiometry (φ > 1) because part of the fuel is left after the combustion. When φ decreases, the fuel conversion efficiency increases because there exists sufficient oxygen in the cylinder to oxidize the fuel and the energy of the fuel is almost completely released as thermal energy. In SI engines, too lean mixtures will cause misfire. φ = 0.8 would be the bottom limit. With lean mixtures, the combustion produces lower temperature in the cylinder and thus results in low mean effective pressure.
Engine emission control
Published in M.J. Nunney, Light and Heavy Vehicle Technology, 2007
If in the interests of maximum power the air-fuel ratio is quite low and there is an excess of fuel, the quality of combustion will be poor and the emissions of both hydrocarbons and carbon monoxide will be high. These can be reduced by moving the air-fuel ratio to a region that is just rich of stoichiometry, which in fact better enables maximum power to be attained, but also incurs a penalty of increased emissions of nitrogen oxides. Should the primary aim be best economy and the air-fuel ratio moved across the dividing line to a region just lean of stoichiometry, then combustion temperatures will be raised to a maximum and produce the highest emissions of nitrogen oxides. These can be much reduced and the emissions of carbon monoxide kept to a minimum by further weakening the mixture until it comes into the ‘lean-burn’ region, which has been defined as one where the air-fuel ratios lie between 18:1 and 21:1, but the gains mentioned are bought at the expense of increased emissions of hydrocarbons and the possibility of rough running, if there is insufficient fuel for the mixture to burn properly.
Reciprocating Engines
Published in Neil Petchers, Combined Heating, Cooling & Power Handbook: Technologies & Applications, 2020
Depending on the air-fuel ratio used, engines may be classified as lean-burn, stoichiometric, or rich-burn. The ratio of oxygen (or air) to fuel that produces perfect combustion is referred to as the stoichiometric air-fuel ratio. If more oxygen (excess air) is used in the combustion process cycle, the engine is referred to as a lean-burn engine. If fuel intake is equivalent to or greater than the stoichiometric quantity, the engine is referred to as a rich-burn engine. Diesel engines always operate with lean air-fuel ratios and under part-load, operate with extremely lean air-fuel ratios. Spark-ignited engines may be designed to operate over a wide range of air-fuel ratios, from rich to very lean.
Combustion and emission characteristics of light duty diesel engine fueled with transesterified algae biodiesel by K2CO3/ZnO heterogeneous base catalyst
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Jayashri Narayanan Nair, Yeditha Veera Venkata SatyanarayanaMurthy, Syed Javed
Figure 8 shows the variation of relative air – fuel ratio (λ) at different loads for ABD5, ABD10, ABD15, ABD20& Diesel fuel. Figure 8 illustrates that increase in engine load decreases the air-fuel ratio for all blends. The air-fuel ratio for all the blends and diesel fuel is greater than unity. The air-fuel ratio influences the combustion phenomenon and completeness of the combustion mostly in the lean fuel zone. Air-fuel ratio of biodiesel blend is higher compared to that of neat diesel. This trend is also reflected in bsfc graphs in Figure 2a. Similar results are also reported by Sinha and Agarwal (2005) for experiments conducted on rice bran methyl ester at variable engine speed. The air-fuel ratio for ABD15 is less than that of ABD20 in 3/4th load and full load conditions.
Use of alternative fuels in compression ignition engines: a review
Published in Biofuels, 2019
The use of biofuels as an alternative fuel improves all performance and emission characteristics except NOx. Due to the availability of oxygen in the biodiesel, NOx emissions increase compared to diesel operation. To meet stringent emission regulations it is essential to reduce NOx. EGR is one of the most efficient ways whereby NOx emissions can be controlled. Exhaust gases are made up of carbon dioxide and nitrogen which, in combination, have a greater specific heat in comparison to atmospheric air. The carbon dioxide and water vapour found in the engine exhaust replaces the fresh air entering the combustion chamber, with the help of the recirculated exhaust gas. Therefore, there is less oxygen in the intake air available for combustion, which decreases the valuable air–fuel ratio, markedly affecting exhaust emissions. The temperature of the flame also decreases because of the exhaust gases combined with the fresh intake air. Hence, there is a significant reduction in NOx emissions due to less oxygen available in the intake air and the decreased flame temperature.
Performance and emissions of gasoline blended with fusel oil that a potential using as an octane enhancer
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2019
Ahmed N. Abdalla, Omar I. Awad, Hai Tao, Thamir K. Ibrahim, Rizalman Mamat, Ali Thaeer Hammid
The air–fuel ratio is the mass ratio between air and fuel in the mixture provided to the engine. The stoichiometric air–fuel ratio illustrates the minimum amount of air wanted to complete the fuel burning. The ratio between the stoichiometric air–fuel ratio to the actual air–fuel ratio is called the lambda (ƛ). The lambda a significant impact on the engine performance, combustion, and formation of emissions. A typical lambda has a direct impact of on the brake power and brake specific fuel consumption (BSFC) of an SI engine. Highest brake power is achieved at the rich mixture (ƛ < 1) because of the higher rate of heat release. While the least BSFC is achieved at the lean mixture (ƛ > 1) due to the better combustion of fuel molecules that results in higher thermal efficiency. The general trends of formation of NOx, HC, and CO2 emissions also affected by lambda. At the lean mixture (ƛ > 1) side, the NOx and HC emission increased, while CO continues to decrease slightly (Ivanič et al. 2005).