SI engine combustion

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Owen Chase
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1 SI engine combustion 1 SI engine combustion: How to burn things? Reactants Products Premixed Homogeneous reaction Not limited by transport process Fast/slow reactions compared with other time scale of interest Premixed flame Examples: gas grill, SI engine combustion Detonation Pressure wave driven reaction Non-premixed Diffusion flame Examples: candle, diesel engine combustion 2 1

2 SI ENGINE COMBUSTION Premixed flame Laminar flame speed Turbulent enhancement of combustion Wrinkled laminar flame 3 LAMINAR FLAME SPEEDS For inert diluent Fig Laminar burning velocity of several fuels as function of equivalence ratio, at 1 atm and 3 K. Fig Effect of burned gas mole fraction in unburned mixture on laminar burning velocity. Fuel: gasoline. (Note that actual burned gas from non-stoichiometric combustion would render the charge different from the metered. 4 2

3 Schematic of SI engine flame propagation Heat transfer Work transfer Fig. 9-4 Schematic of flame propagation in SI engine: unburned gas (U) to left of flame, burned gas to right. A denotes adiabatic burned-gas core, BL denotes thermal boundary layer in burned gas. 5 Typical pressure and mass fraction burned (x b ) curves P(bar); x b *1; d(x b )/d (% per CA-deg) SI engine;15 rpm,.38 bar intake pressure P mass fraction burned x b x b *1 dx b /d Useful conversions: 1 rpm: 6 o CA/ms 12 rpm: 2 Hz (For 4 stroke engine 1 1 cycle/s 1 ms/cycle) crank angle (deg) 6 3

4 SI engine part-load operation P (bar) rpm; X 1 MAP=38kPa; =1; b 3 3 o BTC P Xb Temperature (K) T u T b cumulative flow (g) m intake m exhaust Crank Angle, degree 7 T(K) 4 2 m/s (%/deg) and bar Pressure 1 Mass burn rate SI engine part-load operation Burned gas Laminar flame speed 2*R/B Unburned gas V b /V Laminar expansion velocity Mass fraction burned 8 4

5 Combustion produced pressure rise u Flame b Flame u b m time t m time t + t 1. Pressure is uniform, changing with time 2. For mass m: h b = h u (because dm is allowed to expand against prevailing pressure) 3. T rise is a function of fuel heating value and mixture composition e.g. at = 1, T u ~ 7 K, T b ~ 28 K 4. Hence burned gas expands: b ~ ¼ u ; V b ~ 4 V u 9 Combustion produced pressure rise 5. Since total volume is constrained. The pressure must rise by p, and all the gas in the cylinder is compressed. 6. Both the unburned gas ahead of flame and burned gas behind the flame move away from the flame front 7. Both the unburned gas and burned gas temperatures rise due to the compression by the newly burned gas 8. Unburned gas state: since heat transfer is relatively small, the temperature is related to pressure by isentropic relationship T -1)/ u /T u, = (p/p ) ( u u 9. Burned gas state: Later burned gas, lower T b Early burned gas, higher T b u Flame 1 5

6 Thermodynamic state of charge Fig. 9-5 Cylinder pressure, mass fraction burned, and gas temperatures as function of crank angle during combustion. 11 Burn duration Burn duration as CA-deg. : measure of burn progress in cycle For modern fast-burn engines under medium speed, part load condition: -1% ~ 15 o -5% ~ 25 o -9% ~ 35 o As engine speed increases, burn duration as CA-deg. : Increases because there is less time per CA-deg. Decreases because combustion is faster due to higher turbulence Net effect: increases approximately as rpm.2 6

7 Optimum Combustion Phasing Heat release schedule has to phase correctly with piston motion for optimal work extraction In SI engines, combustion phasing controlled by spark Spark too late heat release occurs far into expansion and work cannot be fully extracted Spark too early Effectively lowers compression ratio increased heat transfer losses Also likely to cause knock Optimal: Maximum Brake Torque (MBT) timing MBT spark timing depends on speed, load, EGR,, temperature, charge motion, Torque curve relatively flat: roughly 5 to 7 o CA retard from MBT results in 1% loss in torque Spark timing effects Fig. 9-3 (a) Cylinder pressure versus crank angle for overadvanced spark timing (5 o BTDC), MBT timing (3 o BTDC), and retarded timing (1 o BTDC). (b) Effect of spark advance on brake torque at constant speed and A/F, at WOT 7

8 Control of spark timing Borderline knock spk adv WOT Fig Fig Obtaining combustion information from engine cylinder pressure data 1. Cylinder pressure affected by: a) Cylinder volume change b) Fuel chemical energy release by combustion c) Heat transfer to chamber walls d) Crevice effects e) Gas leakage 2. Obtaining accurate combustion rate information requires a) Accurate pressure data (and crank angle indexing) b) Models for phenomena a,c,d,e, above c) Model for thermodynamic properties of cylinder contents 3. Available methods a) Empirical methods (e.g. Rassweiler and Withrow SAE 8131) b) Single-zone heat release or burn-rate model c) Two-zone (burned/unburned) combustion model 8

9 Typical piezoelectric pressure transducer spec. 6.2mm Kistler 6125 Source unknown. All rights reserved. This content is excluded from our Creative Sensitivity of NIMEP to crank angle phase error SI engine;15 rpm,.38 bar intake pressure Percent error in NIMEP Crank angle phase error (deg)

10 Cylinder pressure Fig. 9-1 (a) Pressure-volume diagram; (b) log p-log(v/v max ) plot; 15 rpm, MBT timing, IMEP = 5.1 bar, =.8, r c = 8.7, propane fuel. 19 p Pressure, kpa p f Burned mass analysis Rassweiler and Winthrow (SAE 8131) p Ignition End of combustion slope =n Advantage: simple Need only p( ), p, p f and n x b always between and 1 During combustion V = V u V b Unburned gas volume, back tracked to spark () 1/n V u, V u (p/p ) Burned gas volume, forward tracked to end of combustion (f) 1/n V b,f V b (p/p ) f Mass fraction bunred Vu, Vb,f x b 1 V Vf Hence, after some algebra 1/n 1/n p V p V x Fraction of maximum volume b 1/n 1/n p f V f p V (There are two procedures described in the paper; this is one of them) 2 Society of Automotive Engineers. All rights reserved. This content is excluded from our Creative 1

11 Heat release analysis 1 zone model Fig Open system boundary for heatrelease analysis Fuel chemical energy release Energy balance: dq ch /dt = du s /dt Sensible energy change + pdv/dt Work transfer + dq ht /dt Heat loss to walls + h dm cr /dt Flow into crevice - h inj dm f /dt Injected enthalpy Net heat released 21 Results of heat-release analysis P intake Fig Results of heat-release analysis showing the combustion inefficiency and the corrections due to heat transfer and crevice effect

12 Flow and Combustion Process in Spark-Ignition Engine A Color Schieren Movie taken in a Special Visualization Engine Square piston engine Visualization by color-schlieren method Captures density gradients Note: Flame propagation process Outgasing from crevices 23 Square piston flow visualization engine Bore 82.6 mm Stroke mm Compression ratio 5.8 Operating condition Speed 14 rpm.9 Fuel propane Intake pressure.5 bar Spark timing MBT 24 Source unknown. All rights reserved. This content is excluded from our Creative 12

13 Flame Propagation (Fig 9-14) 14 rpm.5 bar inlet pressure 13

14 MIT OpenCourseWare Internal Combustion Engines Spring 217 For information about citing these materials or our Terms of Use, visit:

Engine Heat Transfer. Engine Heat Transfer

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