SI engine combustion 

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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

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SI ENGINE COMBUSTION • Premixed flame – Laminar flame speed • Turbulent enhancement of combustion – Wrinkled laminar flame

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LAMINAR FLAME SPEEDS

For inert diluent

Fig. 9-25 Laminar burning velocity of several fuels as function of equivalence ratio, at 1 atm and 300 K.

Fig. 9-26 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

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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

P(bar); xb*10; d(xb)/d (% per CA-deg)

Typical pressure and mass fraction burned (xb) curves 25 SI engine;1500 rpm, 0.38 bar intake pressure Useful conversions:

20

P

1000 rpm: 6oCA/ms

mass fraction burned xb xb *10

15 10

dxb/d

1200 rpm: 20 Hz (For 4 stroke engine 10 cycle/s 100 ms/cycle)

5 0 0

200

400

crank angle (deg)

600 6

3

SI engine part-load operation 40 30

1500 rpm; MAP=38kPa; =1; ign @ 30oBTC

0.5

20

P

10 0 0

100

200

300

400

500

600

700

500

600

700

0

3000 2000

Tb

1000 0 0

cumulative flow (g)

Temperature (K)

1

Xb

Xb

P (bar)

50

Tu 100

200

300

400

0.6 0.4 0.2

mexhaust

mintake

0

0

100

200

300 400 500 Crank Angle, degree

600

700 7

SI engine part-load operation T(K)

4000

Burned gas

2000

(%/deg) and bar

m/s

0 0 2 1

0 0 20

Unburned gas

0.2

0.4

0.5 0 0

0.8

1

Laminar expansion velocity Laminar flame speed

0.2

0.4

0.6

0.8

1

0.8

1

0.8

1

Pressure

10 0 0 1

0.6

Mass burn rate

0.2

0.4

0.6

2*R/B Vb/V

0.2

0.4 0.6 Mass fraction burned

8

4

Combustion produced pressure rise Flame u

Flame u

b

b m

m time t

time t + t

1.

Pressure is uniform, changing with time

2.

For mass m: hb = hu (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, Tu ~ 700 K, Tb ~ 2800 K



Hence burned gas expands: b ~ ¼ u ; Vb ~ 4 Vu

4.

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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 

9.

Tu/Tu,0 = (p/p0)(u-1)/u

Burned gas state:

Later burned gas, lower Tb

Early burned gas, higher Tb

u Flame 10

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Thermodynamic state of charge

Fig. 9-5 Cylinder pressure, mass fraction burned, and gas temperatures as function of crank angle during combustion.

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Burn duration • Burn duration as CA-deg. : measure of burn progress in cycle • For modern fast-burn engines under medium speed, part load condition: – 0-10% ~ 15o – 0-50% ~ 25o – 0-90% ~ 35o • 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 rpm0.2

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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 7oCA 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 (50o BTDC), MBT timing (30o BTDC), and retarded timing (10o BTDC). (b) Effect of spark advance on brake torque at constant speed and A/F, at WOT

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Control of spark timing Borderline knock spk adv

WOT

Fig. 15-17

Fig. 15-3

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 800131) b) Single-zone heat release or burn-rate model c) Two-zone (burned/unburned) combustion model

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Typical piezoelectric pressure transducer spec.

6.2mm

Kistler 6125

Sensitivity of NIMEP to crank angle phase error SI engine;1500 rpm, 0.38 bar intake pressure

Percent error in NIMEP 15 10 5 0 -3

-2

-1

-5

0

1

2

3

Crank angle phase error (deg)

-10 -15

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Cylinder pressure

Fig. 9-10 (a) Pressure-volume diagram; (b) log p-log(V/Vmax) plot; 1500 rpm, MBT timing, IMEP = 5.1 bar,  = 0.8, rc = 8.7, propane fuel. 19

Burned mass analysis – Rassweiler and Winthrow (SAE 800131)

• Advantage: simple Need only p(), p0, pf and n xb always between 0 and 1

During combustion V = Vu  Vb

Unburned gas volume, back tracked to spark (0) Vu,0  Vu (p / p )1/n 0 End of combustion pf

p

Pressure, kPa

p0

|slope|=n

Burned gas volume, forward tracked to end of combustion (f) Vb,f  Vb (p / p f )1/n

Ignition

Mass fraction bunred V V xb  1  u,0  b,f V0 Vf Hence, after some algebra

Fraction of maximum volume

xb 

p1/n V  p01/n V0

pf 1/n Vf  p01/n V0

(There are two procedures described in the paper; this is one of them)

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Heat release analysis

Fig. 9-11 Open system boundary for heatrelease analysis

1 zone model

Energy balance: Fuel chemical energy release

dQch/dt

= dUs/dt

Sensible energy change

+ pdV/dt

Work transfer

+ dQht/dt

Heat loss to walls

+ h’ dmcr/dt

Flow into crevice

- hinj dmf/dt

Injected enthalpy

Net heat released

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Results of heat-release analysis

Pintake

Fig. 9-12 Results of heat-release analysis showing the combustion inefficiency and the corrections due to heat transfer and crevice effect. 22

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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 Stroke Compression ratio

82.6 mm 114.3 mm 5.8

Operating condition Speed  Fuel Intake pressure Spark timing

1400 rpm 0.9 propane 0.5 bar MBT

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Flame Propagation (Fig 9-14)

1400 rpm 0.5 bar inlet pressure

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