Effect of CI Engine Design - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

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Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_1.htm[6/15/2012 3:01:41 PM] Module 3: Influence of Engine Design and Operating Parameters on Emissions Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions Effect of CI Engine Design and Operating Variables on Emissions The Lecture Contains: Engine Design Variables Operating Variables Compression Ratio Direct Injection (DI) versus Indirect Injection (IDI) Engines Combustion Chamber Design Fuel Injection Timing and Injection Pressure Engine load and Speed Exhaust Gas Recirculation Fuel Quality Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_2.htm[6/15/2012 3:01:41 PM] Module 3: Influence of Engine Design and Operating Parameters on Emissions Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions CI Engine Design and Operating Variables and Emissions Important diesel engine variables that influence emissions are: Engine Design Variables Compression ratio Combustion chamber type Combustion chamber design Injection system: injection pressure and timing, nozzle holes, nozzle sac volume Operating Variables: EGR Engine speed Engine load Fuel quality Compression Ratio In the diesel engines the minimum compression ratio that can be used is governed by the ease of engine cold starting ability. For the high speed direct injection engines CR of around 16 to 17.5:1 is used. The turbocharged heavy duty engines employ CR in the range of 13 to 14:1. Cold starting requirements prevents further reduction in the compression ratio. Use of higher compression ratio results in a shorter ignition delay period. A shorter delay would result in less ‘overmixing' of fuel and air and hence, lower HC emissions. Further, the higher combustion temperatures obtained at a higher compression ratios tend to increase oxidation of the unburned HC. At a low compression ratio, a longer delay increases the fraction of fuel burned during the premixed phase resulting in higher peak pressures and temperatures which cause an increase in NO x formation. On the other hand, increase in compression ratio due to higher combustion temperatures would tend to increase formation of NOx. If the ignition delay is too long the combustion may begin in the expansion stroke reducing combustion pressure and temperature. Too long an ignition delay leads to lower NOx emissions along with poor fuel efficiency. Use of a low compression ratio results in too long a delay during engine warm up under cold conditions, and it causes high emissions of unburned fuel which due to its appearance is called ‘white smoke'. A high compression ratio leading to high combustion temperatures would increase soot formation while on the other hand it increases soot oxidation. For obtaining low particulate and NOx emissions simultaneously, an optimum compression ratio is to be used. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_4.htm[6/15/2012 3:01:41 PM] Figure 3.5 PM – NO x trade-off for two valve and four-valve passenger car DI diesel engines. A centrally placed combustion bowl has lower swirl requirements and results in more equal fuel distribution and availability of equal air to each spray for mixing. In four-valve engines, symmetrical air motion in the piston bowl and equal fuel distribution between different sprays lead to optimum mixture formation and combustion with very low smoke levels. Use of lower air swirl in 4-valve engines compared to 2-valve engines for the same PM and NOx emission levels, results in reduction of fuel -air ‘overmixing' during premixed phase of combustion. As ‘overmixing' of fuel is an important source of HC emissions, lower HC emissions in 4-valve engines are obtained. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_5.htm[6/15/2012 3:01:42 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions Fuel Injection Timing and Injection Pressure Effect of fuel injection timing on NOx emissions from CI engines is quite similar to that of spark timing in SI engines. Typical effect of injection timing on NOx , HC, smoke emissions and fuel consumption is shown on Fig 3.6. With retarded injection timing, as expected the NOx emissions decrease sharply. On the other hand an increase in smoke results with retarded injection timing If the injection timing is retarded too much, HC emissions in naturally aspirated engines also may increase sharply. An increase in injection pressure results in higher NOx and HC, but yields lower smoke and PM emissions. Effect of injection pressure on PM emissions is shown on Fig. 3.7. With high injection pressures of 1500 bars and higher, small nozzle orifices of about 0.15 mm dia. are used. The high injection pressure and small nozzle holes give very good fuel atomization with very small fuel droplets which vaporize rapidly resulting in low soot emissions Figure 3.6 Effect of injection timing on emissions and fuel economy for a heavy-duty diesel engine. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_5.htm[6/15/2012 3:01:42 PM] Figure 3.7 Effect of Injection pressure on PM emissions for aheavy duty diesel engine. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_7.htm[6/15/2012 3:01:42 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions Exhaust Gas Recirculation The role of EGR is to act as inert diluents and heat sink that reduces the oxygen concentration during combustion and lowers the combustion temperatures. The flame temperatures are reduced as a result of EGR. The NOx formation being an exponential function of temperature, even a small reduction in flame temperature has a large effect on NOx formation. Increase in heat capacity of charge caused by EGR has generally been thought to result in reduction of NOx emissions from SI engines. However, in the diesel engines, EGR can affect NO x reduction in three possible ways. These effects are: Dilution effect: It is the reduction in inlet charge oxygen concentration Thermal effect: It is the increase in inlet charge heat capacity, and Chemical effect: Modification in combustion process as a result of dissociation of CO2 and water vapour The dilution effect (reduction in inlet oxygen) is the dominant effect in case of diesel engines. The chemical and thermal effects are relatively small. Figure 3.9 Typical effect of effect of EGR on NOx, HC and fuel economy for a turbocharged, intercooled passenger car DI diesel engine Typical effect of EGR on NOx , HC and CO emissions for a turbocharged passenger car DI diesel engine is shown on Fig 3.9 . At around 10% EGR, 50% reduction in NOx is obtained with little change in CO and HC. As the EGR rate is increased beyond 15 %, NOx decreases further, but CO, smoke and HC are increased. The excess air declines with increase in EGR causing sharp increase in smoke and loss in fuel economy. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_7.htm[6/15/2012 3:01:42 PM] The effect of EGR at the same rate (% of intake air) on diesel NOx is lower compared to SI engine as the exhaust gas in diesel engine contains smaller amounts of tri-atomic gas CO2 .. Exhaust gas recirculation has been used on diesel passenger cars since mid-1990s to reduce NO x emissions. This is being applied now on more and more diesel engines as the emission standards are being tightened. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture15/15_8.htm[6/15/2012 3:01:42 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 15:Effect of CI Engine Design and Operating Variables on Emissions Fuel Quality For petroleum fuels many of the properties such as hydrocarbon composition, natural cetane number, volatility, viscosity and density are interdependent. As the fuel density decreases the fuel contains more of paraffinic hydrocarbons, which results generally in higher cetane number and fuel volatility and lower viscosity. So, the effect of change in one fuel quality parameter on emissions may be some times the result of several interactions. A high fuel cetane number improves cold starting and results in faster warm-up thus reducing cold engine HC emissions. As an increase in the cetane number reduces ignition delay it results in lowering of HC and NO x emissions. On the other hand, with higher fuel volatility a larger lean flame out ‘overmixing' region may result and due to faster fuel evaporation the fraction of fuel burned during premixed combustion is also higher. Therefore, an increase in NO x as well as HC may be observed with more volatile diesel fuels. The fuel sulphur increases sulphates in PM emissions increasing the particulate mass Summary The effect of various design and operating variables on DI diesel engine performance and emissions are summarized in Table 3.3. Table 3.3 Emission Trends with Engine Design and Operating Variables for Diesel Engines Parameter BSFC BSNOx BSHC PM Engine design variables Compression ratio Stroke/Bore ratio Crevice volume Swirl Valve number Fuel injection variables Injection pressure Retarded injection timing Sac volume Number of holes (size smaller) Pilot injection Eccentricity of Injector Operating parameters