Mechanisms of HC Formation in SI Engines - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

Download Mechanisms of HC Formation in SI Engines - Engine Combustion - Lecture Notes and more Study notes Sustainability Management in PDF only on Docsity! Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture9/9_1.htm[6/15/2012 2:58:00 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 9:Mechanisms of HC Formation in SI Engines.... contd. Mechanisms of HC Formation in SI Engines.... contd. The Lecture Contains: HC from Lubricating Oil Film Combustion Chamber Deposits HC Mixture Quality and In-Cylinder Liquid Fuel HC from Misfired Combustion Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture9/9_2.htm[6/15/2012 2:58:00 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 9:Mechanisms of HC Formation in SI Engines.... contd. HC from Lubricating Oil Film Fuel hydrocarbons are absorbed in the oil film present on the cylinder walls during intake and compression strokes, which get desorbed back into the burned gases during combustion and expansion. On combustion, the partial pressure of fuel in the burned gases becomes nearly zero and the concentration gradient makes the fuel to be desorbed from oil and diffuse back into the burned gases. The desorbed fuel vapours from oil film are oxidized depending upon the temperature, pressure and composition of the burned gases. The maximum amount of fuel that can be dissolved per unit volume of oil is given by: (2.33) where, nfoisthe number of moles of fuel absorbed in oil, no is number of moles of oil per unit volume, Xfc mole fraction of fuel in the combustion chamber gases close to the oil film, P is instantaneous cylinder pressure, H is theHenry’s constant. Mole fraction of fuel vapours in oil, (2.34) As nfo << nfo, xfcan be approximated as nfo/ no. Henry’s constant is a measure of fugacity of the fuel components (solute) in liquid phase or inverse of its solubility. Henry’s constant increases with temperature and decreases exponentially with increase in molecular weight of the solute (in present case the fuel). At 400 K, Henry’s constant for n-hexane, iso- octane and ethyl-benzene is 200, 120 and 45 kPa, respectively. A larger fraction of the heavier fuel components would be absorbed in the oil as they have a smaller value of H. Taking average cylinder pressure under compression stroke as 0.5 MPa and typical oil film temperature equal to 400 K, the mole fraction of fuel vapour absorbed in oil film at equilibrium for the stoichiometric mixture of isooctane and air would be about 0.07 .The lubricant oil film thickness is a strong function of oil viscosity and hence the oil temperature. It also varies with engine speed. The oil film thickness on the cylinder wall varies between 1 and 10 µm. At temperature of 400 K, the diffusion time to reach equilibrium for fuel vapour absorption in oil film of 1 µm thick ness is about 10-3 seconds and for a 10 µm thick film it would equal to 10-1 seconds. For an engine speed of 3000 rpm, intake and compression strokes together would take 2x10-2 seconds. Thus, for oil films of 1 to 2 µm thickness state of equilibrium in fuel vapour absorption would be achieved under engine conditions. The absorption and desorption of fuel in the oil film and its contribution to the HC emissions involves several processes. Some of the absorbed hydrocarbon vapours in oil are carried to the crankcase where these are desorbed. Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture9/9_3.htm[6/15/2012 2:58:01 PM] Figure 2.13 Schematic of Port Fuel Injection and fuel vaporization process Some features of fuel induction into the cylinder of PFI engines are: The conventional PFI system produces the droplets of Sauter mean diameter (SMD) ranging from 130 µm to 300 µm. The droplets larger than 10 µm are unable to follow the air stream and they impinge on the combustion chamber walls producing a non-uniform fuel distribution in the cylinder. As the injection is made at the back of intake valve, liquid fuel film is formed at the port and on the back of the valve. The intake air strips this liquid fuel film and carries along into the cylinder. In the process, substantial amount of liquid fuel droplets enter the engine cylinder and is deposited on cylinder walls. Shearing of the liquid film from the back of intake valve and port by intake air produces larger droplets than by the injectors which impinge on the cylinder walls depositing liquid fuel film. Injection at a higher pressure although would produce finer droplets but the fuel jet velocity and droplet momentum are also higher, which increases the probability of the impingement of the fuel droplets on walls. During cold start as 8 to 15 times of the stoichiometric fuel requirement is injected for the first few cycles, more liquid fuel is deposited inside the cylinder. The liquid fuel deposition inside the cylinder decreases as the engine is warmed up. During cold starting and warm up, much of the injected fuel remains in the cylinder for several cycles. It vaporizes during and after combustion and thus, contributes to higher HC emissions. During cold start, with PFI up to 60% higher HC emissions could result compared to fully vaporized and premixed air and fuel mixture. At 90º C coolant temperature, the contribution of the liquid fuel deposited inside the cylinder to HC emissions is almost zero compared to 20 to 60 percent at 20º C. In the modern catalyst equipped vehicles, more than 90 percent of HC emissions under standard test driving cycle conditions result during the first minute of operation. due to use of over-rich mixtures Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture9/9_3.htm[6/15/2012 2:58:01 PM] during engine start-up and secondly the catalytic converter has not yet warmed up and is not functional. Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture9/9_4.htm[6/15/2012 2:58:01 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 9:Mechanisms of HC Formation in SI Engines.... contd. HC from Misfired Combustion Engine may misfire under engine idling and low load operation as the residual gas dilution is high. Presence of high residual gas content retards combustion and more fuel burns during expansion stroke. However, during expansion as the cylinder pressure falls the temperature of the unburned mixture ahead of the flame also decreases which may result in extinction of flame and consequently in partially misfired combustion. Such bulk flame quenching in spark-ignition engines leads to very high HC emissions. Use of excessively lean mixtures also decreases burning rates and increases incidence of bulk gas flame quenching. The propensity to partial misfiring increases under transient engine operation. Use of large amounts of exhaust gas recirculation (EGR) or high residual gas dilution though reduces NO formation but results in high HC emissions.