The purpose of this presentation is to promote common electronic presentation format within R&A for all R&A presentations. It is also in response to the directive issued by Nick Scheele on August 27, 2003.
INTRODUCTION TO SI ENGINE Air Fuel Mixture: In traditional SI engines, the fuel and air are mixed together in the intake system using a low pressure (circa 2 to 3 bar) fuel injection system (carburettors no longer used). Fuel injection system is normally multi-point port injection, which means that there is one fuel injector (sometimes two) in each inlet port. Multi-point injectors normally inject fuel onto the back of the closed inlet valve using sequential timing with the required amount o f fuel quantity being updated by the ECU every engine event. Air/fuel Ratio, AFR: The AFR has a very significant effect on the power output, thermal efficiency and exhaust emissions and has to be controlled precisely over the whole operating range. All modern engines use an electronic control unit (ECU) and various sensors and actuators to control the AFR. The air to fuel ratio by mass (AFR) is typically 14.3 to 14.7 for gasoline fuels.
Spark Ignition Combustion Homogeneous mixture of air, fuel and residual gas. Spark ignition shortly before TDC. Flame propagation. The combustion typically takes 50 degrees of crank angle The products of combustion: N2, CO2, Figure 1.19 Idealised SI engine flame propagation H2O vapour, O2, CO, H2, HCs, NOx. Cycle to cycle variation knock
THERMODYNAMIC GAS CYCLESOtto Cycle 1 – 2: isentropic compression 2 – 3: constant-volume heat addition p 3 3 – 4: isentropic expansion 4 – 1: constant-volume heat rejection Compression ratio V V r= 1 = 4 V V 2 2 3 4 Heat addition Qin=mcv(T3-T2) 1 Heat rejection Qout=mcv(T4-T1) V 5
Isentropic compression Perfect gas pV = mRT Isentropic process pVγ = constant Isentropic expansion p T 2 = rγ 2 = rγ −1 p T 1 1 γ γ −1 Cycle efficiency p4 = 1 T4 1 p3 r = T3 r Wout Qin − Qout 1 η = = = 1− Otto Q in Qin γ −1 r 6
Fuel & Air Gasoline Kerosene Diesel Heavy fuel Fuels C 85.5 86.3 86.3 86.1 • Gasoline: assume iso-octane (C8H18) H 14.4 13.6 12.8 11.8 • Diesel: assume deodecane (C12H26) S 0.1 0.1 <0.9 2.1 Air • Molar mass of air = 0.21 x 32 + 0.79 x 28 = 28.8 (kg/kgmol) O2 N2 Fuel and Air Mixture % by volume 21.0 79.0 • Stoichiometric air/fuel ratio • Rich mixture % by mass 23.3 76.7 • Weak (or lean) mixture Stoichiometric air/fuel ratio Fuel/air equivalent ratio φ: φ= Actual air/fuel ratio Air/fuel equivalent Actual air/fuel ratio 1 ratio λ: λ= = Stoichiometric air/fuel ratio φ 7
Gasoline Air C7 H13 + 10 O2 + 39 N2 The Combustion Process Energy!! (theoretical) 7 CO 2 + 6.5 H2 O + 39N 2 Carbon Water Nitrogen Dioxide (Steam)Gasoline : CnH1.87n
Automotive EmissionsFuel + Air → Combustion CO 2 + O 2 + N 2 + H 2 + ... + Products CO + HCs + PMs NOx + Pollutants 10
Todays Air Real FuelTheCombustionProcess Pollutants: Unburned(actual) Exhaust: • Nitrogen Hydrocarbons Carbon Monoxide • Water (steam) Oxides of • Carbon Dioxide Nitrogen • Pollutants Other elements or compounds
• In the engine -incomplete combustionHow -"wall quench" -high pressureEmission and temps -"Blowby" • Due toare evaporation ofFormed fuel -"breathing" -hot engine and fuel -displacement of vapors
TYPICAL ENIGNE OUTEMISSIONNOx : 100 to 1000 ppm or 10g/kg fuelCO : 1 to 2 percent or 200g/kg fuelHC : 1500 ppm (as C1) or 10g/kg fuel
MAJOR CAUSES OF HC EMISSIONS1. Evaporative losses from fuel tank, fuel linesand carburetor.2. Fuel composition.3. Air/Fuel (A/F) ratio deviation from stoichiometry. Fuel air mixture is too lean to burn. Lower temperature reduces evaporation. Fuel air mixture is too rich to burn resulting in-complete combustion.4. Incomplete combustion.5. Flame quenching at walls.6. Absorption and desorption in lubricating oilsand deposits.7. Crevices in combustion chamber and pistonrings.8. Short-circuiting of fresh charge.
The sequence of processes involved in the engine out HC emissions is:1. Storage2. In-cylinder post-flame oxidation3. Residual gas retention4. Exhaust oxidationHC Sources1. Quench Layers • Quenching contributes to only about 5-10% of total HC. However, bulk quenching or misfire due to operation under dilute or lean conditions can lead to high HC. • Quench layer thickness has been measured and found to be in the range of 0.05 to 0.4 mm (thinnest at high load) when using propane as fuel. • Diffusion of HC from the quench layer into the burned gas and subsequent oxidation occurs, especially with smooth clean combustion chamber walls.
2. Crevices • These are narrow volumes present around the surface of the combustion chamber, having high surface-to-volume ratio into which flame will not propagate. • They are present between the piston crown and cylinder liner, along the gasket joints between cylinder head and block, along the seats of the intake and exhaust valves, space around the plug center electrode and between spark plug threads. • During compression and combustion, these crevice volumes are filled with unburned charge. During expansion, a part of the UBHC-air mixture leaves the crevices and is oxidized by the hot burned gas mixture. • The final contribution of each crevice to the overall HC emissions depends on its volume and location relative to the spark plug and exhaust valve.
3. Lubricant Oil Layer • The presence of lubricating oil in the fuel or on the walls of the combustion chamber is known to result in an increase in exhaust HC levels. • The exhaust HC was primarily unreacted fuel and not oil or oil-derived compounds. • It has been proposed that fuel vapor absorption into and desorption from oil layers on the walls of the combustion chamber could explain the presence of HC in the exhaust.4. Deposits • Deposit buildup on the combustion chamber walls (which occurs in vehicles over several thousand kilometers) is known to increase UBHC emissions. • Deposit buildup rates depend on fuel and operating conditions. • Olefinic and aromatic compounds tend to have faster buildup than do paraffinic compounds.
5. Liquid Fuel and Mixture Preparation – Cold Start • The largest contribution (>90%) to HC emissions from the SI engine during a standard test occurs during the first minute of operation.This is due to the following reasons: • The catalytic converter is not yet warmed up • A substantially larger amount of fuel is injected than the stoichiometric proportion in order to guarantee prompt vaporization and starting6.Poor Combustion Quality Flame extinction in the bulk gas before the flame front reaches the wall is a source of HC emissions under certain engine operating conditions.
HYDRO-CARBON COMPOSITION OF SPARK-IGNITION ENIGNE EXHAUST (BY CLASS) Carbon, Percent of total HC Paraffins Olefins Acetylee AromaticsWithoutcatalyst 33 27 8 32Withcatalyst 57 15 2 26
HOW CO EMISSIONS ARE FORMED?Carbon monoxide is formed due to in-homogenity offuel distribution with rich A/F mixture. This is anintermediate product in the combustion of hydrocarbonfuels.CO is formed when-• Oxygen is not available in adequate quantity.• Cycle temperatures are low.• Primarily dependent on the Air/Fuel Ratio.• Levels of exhaust manifold CO are lower than themaximum values measured within the combustion chamber• The processes which govern CO exhaust levels arekinetically controlled• The rate of re-conversion from CO to CO2 is slower thanthe rate of cooling.• This explains why CO is formed even with stoichiometricand lean mixtures.
HOW NOx EMISSIONS ARE FORMED?• There is a temperature distribution across thechamber due to passage of flame.• Mixture that burns early is compressed to highertemperatures after combustion, as the cylinderpressure continues to rise.• Mixture that burns later is compressed primarily asunburned mixture and ends up after combustion at alower burned gas temperature.
THE MAJOR CAUSES OF NOx EMISSIONS• Higher Combustion Temperature.• Higher oxygen content.• Ample Resident / reaction timeNO = Nitric Oxide (Predominant), NO2 = Nitrogen DioxideExtended Zeldovich mechanism: O + N2 = NO + N N + O2 = NO + O N + OH = NO + H Zeldovich was the first to suggest the importance of first two reactions and Lavoice added 3rd reaction to the mechanism.
Effect ofAir-FuelRatio onEmission(Typical) Fig. 1-1 Spark ignition engine emissions for different fuel/air equivalent ratios 26
Effect Air-Fuel Ratio on Engine PerformanceFig. 1-8 Response of specific fuel consumption and power output to 27 changes in air/fuel ratio
CRITICAL FACTORS & ENGINE VARIABLES IN HC EMISSION MECHANISMS (a) Crevices (1) Crevice volumes (2) Crevice Location (relative to spark Plug) (3) Load (4) Crevice wall temperature (5) Mixture composition1) Formation of (b) Oil layers (1) Oil consumption HC (2) Wall temperature (3) Speed (c) Incomplete combustion (1) Burn rate and variability (2) Mixture composition† (3) Load (4) Spark timing‡ (d) Combustion chamber walls (1) Deposits (2) Wall roughness
CRITICAL FACTORS & ENGINE VARIABLES IN HC EMISSION MECHANISMS (a) Mixing rate with bulk gas (1) Speed (2) Swirl ratio (3) Combustion chamber shape (b) Bulk gas temperature during expansion and2) In-cylinder exhaust mixing and (1) Speed (2) Spark timing‡ oxidation (3) Mixture composition† (4) Compression ratio (5) Heat losses to walls (C) Bulk gas oxygen concentration (1) Equivalence ratio (D) Wall temperature (1) Important if HC source near wall (2) For crevice: importance depends on geometry
CRITICAL FACTORS & ENGINE VARIABLES IN HC EMISSION MECHANISMS (a) Residual fraction (1) Load (2) Exhaust Pressure (3) Valve overlap3) Fraction HC (4) Compression ratio Flowing out (5) Speed of cylinder (b) In Cylinder flow during exhaust stroke (1) Valve overlap (2) Exhaust valve size and location (3) Combustion chamber shape (4) Compression ratio (5) Speed
CRITICAL FACTORS & ENGINE VARIABLES IN HC EMISSION MECHANISMS (a) Exhaust gas temperature (1) Speeds (2) Spark timing (3) Mixture composition (4) Compression ratio (5) Secondary air flow4) Oxidation in (6) Heat losses in cylinder and exhaust (b) Oxygen Concentration exhaust (1) A/F ratio system (2) Secondary air flow and addition point (c) Residence time (1) Speed (2) Load (3) Volume of critical exhaust system component (d) Exhaust reactors: (1) Oxidation catalyst (2) Three-way catalyst