Operating Variables on Emissions - 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/lecture14/14_1.htm[6/15/2012 3:01:19 PM] Module 3: Influence of Engine Design and Operating Parameters on Emissions Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions Effect of SI Engine Design and Operating Variables on Emissions The Lecture Contains: SI Engine Variables and Emissions Compression Ratio Ignition Timing Air –Fuel Ratio Residual Gas and EGR Engine Speed Cold Start and Warm-up Phase Coolant Temperature Summary Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2.htm[6/15/2012 3:01:19 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions SI Engine Variables and Emissions Any engine variable that affects oxygen availability during combustion would influence CO emissions. The factors which influence flame quenching, quench layer thickness and post flame oxidation control engine out HC emissions. The burned gas temperature-time history and oxygen concentration control NO formation and emission. Hence the engine variables that influence burned gas temperature and oxygen concentration would affect the NO emissions. Principal design and operating variables affecting engine emissions are: Design Variables : Compression Ratio Combustion chamber surface to volume ratio Ignition timing Valve timings and valve overlap Air motion, swirl tumble etc Charge stratification Operating Variables : Air-fuel Ratio Charge dilution and exhaust gas recirculation (EGR) Speed Load Coolant temperature Transient engine operation: acceleration, deceleration etc. The effect of some variables discussed below is typical in nature and variations in the trends with specific engine design change are observed. Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_2_2.htm[6/15/2012 3:01:20 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions Ignition Timing The effect of ignition timing on NO and HC emissions is shown on Fig 3.2 When ignition occurs earlier in the cycle more heat is released before and around the top dead center. Thus, with advanced ignition timings higher peak cylinder pressures and temperatures result. As has been discussed lecture 5 with increase in combution temperatures NO formation increases. Hence , higher NO emissions are obtained as the ignition timing is advanced. As the ignition timing is retarded more burning takes place during expansion stroke resulting in lower peak combustion pressures and a lower of mass of charge is pushed into crevice volume. Also, at the retarded ignition timings exhaust gas temperature increases as the engine thermal efficiency is reduced. In the hotter exhaust gas with the retarded ignition timing higher oxidation rates of the HC and CO in the exhaust system are obtained.. Due to these reasons, lower HC emissions are obtained with retarded ignition timings. The disadvantage of the retarded ignition timing is lower engine efficiency, lower power and a poorer fuel economy. When the emission control legislation was introduced for the first time around 1970 in the USA and Europe, ignition timing versus speed and manifold vacuum curves were among the first engine parameters that were modified for control of NOx emissions due to ease of their adjustment. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_3.htm[6/15/2012 3:01:20 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions Contd... Fig 3.2(a) Effect of spark timing on NOx emissions. Fig Effect of ignition timing on HC Objecti ves_template |[8-2(b) _|lemissions. | q@[Previous Next [|> file://C\/...%20and%20Settings/iitkrana] /My%20Documents/Google%20Talk%20R eceived%20Files/engine_combustion/lecture] 4/14_3.htm[6/15/2012 3: Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_5.htm[6/15/2012 3:01:20 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions Engine Speed Volumetric efficiency of the engine changes with speed, it being highest in the mid-speed range. At high engine speeds the volumetric efficiency generally decreases resulting in high residual gas dilution. Although heat transfer rates increase with increase in engine speed as a result of higher turbulence, but total amount of heat transfer is lower due to shorter cycle time. This gives higher gas temperatures at higher speeds. However, at high speeds a shorter time is available for NO formation kinetics. The net result is a moderate effect of speed on NO although this is specific to the engine design and operating conditions. Increase in exhaust gas temperatures at higher speeds enhances post flame oxidation of unburned hydrocarbons. A reduction of 20 to 50 percent in HC emissions has been observed with increase in speed from 1000 to 2000 rpm. Cold Start and Warm-up Phase Engine cold start and warm-up phase contribute significantly to unburned hydrocarbons. One of the main sources of HC emissions during cold start and engine warm-up period is very rich fuel-air ratio needed for ignition and combustion for several seconds after engine start. During cold start, the engine has to be over-fuelled 5 to10 times the stoichiometric amount of gasoline. To obtain robust ignition on the first cycle on cold start, a fuel vapour- air equivalence ratio above lean threshold limit (f = 0.7-0.9) is required. This threshold is independent of the engine coolant temperature. The fuel-air equivalence ratio supplied to the engine during cold start is in the range, f = 4 to 7. For the first few engine cycles, a large fraction of inducted fuel is stored as liquid film in the intake port and cylinder as only the most volatile fractions evaporate when the engine is cold. The liquid fuel films do not participate in combustion and is emitted as unburned fuel emissions. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture14/14_6.htm[6/15/2012 3:01:20 PM] Module 3:Influence of Engine Design and Operating Parameters on Emissions Lecture 14:Effect of SI Engine Design and Operating Variables on Emissions Coolant Temperature As the coolant temperature is increased, the contribution of piston ring zone crevice becomes lower due to decrease in gas density within this crevice. Secondly, the top piston-land side clearance is also reduced due to higher thermal expansion of the piston. A thinner oil film and reduced fuel vapour solubility would result in reduced absorption of fuel vapours in engine oil. Increased postflame oxidation at high temperatures also contributes to reduction in HC emissions. Increase in coolant temperatures has been observed to reduce HC emissions by about 0.4 to 1.0 % per K increase in temperature. An increase in the coolant temperature from 20 to 90º C, roughly results in 25% lower HC emissions and hence, the need of a rapid engine warm up is obvious. For reduction of the cold start and warm up HC emissions, an important area of development is to improve the fuel injection and delivery to the cylinder with minimum wall wetting. Over-fuelling during cold start and warm-up is to be kept at a minimum, while still forming the combustible charge. Summary The effects of some of the important engine design and operating variables on emissions from SI engines are summarized below Table 3.1. Table 3.1 Effect of SI Engine Design and Operating Variables on Exhaust Emissions (Source: IC Engines - Combustion and Emissions by Pundir, 2010) Variable Increased HC CO NOX Fuel-air equivalence ratio Compression ratio ↑ - ↑ Surface/volume ratio ↑ - ↓ Bore/stroke ratio ↑ - ↓ Ignition timing advance ↑ - ↓ Port fuel injection ↑ ↓ ↓ Engine speed ↑ - ↑↓ Engine load ↑ - ↑ Coolant temperatures ↓ - ↑ Combustion chamber deposits ↓ - ↑ EGR ↓ - ↓ Intake swirl and turbulence ↓ - ↑ ↑= increase; ↓ = decrease; ↑↓= uncertain; - = no effect Objecti ves_template [Previous Next [|> file://C\/...%20and%20Settings/iitkrana] /My%20Documents/Google%20Talk% 20Received%20Files/engine_combustion/lecture] 4/14_6.htm[6/15/2012 3:01:20 PM]