lect10 engines part2 - lth | lunds tekniska … duration in si engine initial flame development...
TRANSCRIPT
X.S. Bai TC in piston engines
Content
• Spark-ignition engines• Auto-ignition of fuels, engine knock• Fuel injection and spray flame structures in Diesel engines• Fuel consumption and NOx emission in Diesel engines• HCCI engines
X.S. Bai TC in piston engines
Combustion stages
• Ignition stage: spark ignition needs to last a short time to initiate the flame propagation. The flame kernel initially is very small. The small kernel is not highly wrinkled by turbulence. The propagation speed of the small flame kernel is low.
• Turbulent flame propagation stage: As the flame kernel grows theflame surface area becomes also more wrinkled. The flame propagation speed is much higher than the laminar flame speed. The higher the turbulence level the faster the turbulent flame speed. This makes the combustion duration in terms of crank angle degrees roughly the same at different engine speed.
• Burning in the post flame zones: If the engine runs at very highspeed, the combustion intermediates such as CO may not fully burned at the flame front. Since the engine is at very high pressure and temperature, these intermediate can be further oxidized later before the exhaust gas is released.
X.S. Bai TC in piston engines
Combustion duration in SI engine
Initial flamedevelopment
Turbulent flame propagation
X.S. Bai TC in piston engines
Compression ratio in SI engines
• Engine knock at high compression ratio
X.S. Bai TC in piston engines
Compression ratio in SI engines
• Engine knock at high compression ratio
X.S. Bai TC in piston engines
Octane number, cetane number
• Octane number indicates the tendency of fuels to knock. The higher the octane number the more difficult the auto-ignition.
– n-Heptane (C7H16) has a octane number 0, – iso-octane (C8H18) has a octane number 100. – Gasoline has a octane number 93 – 97.
• cetane number denotes the ignition delay time (the start of the injection of diesel fuel to the onset of the auto-ignition). The cetane number ranks the fuels; the higher the cetane number the faster the auto-ignition.
– Isooctane has a cetane of 15 – diesel has a cetane number about 37 – 56. – Cetane (C16H34) has a cetane number 100.
X.S. Bai TC in piston engines
Compression ignited engine (”Diesel combustion”)
• Combustion during injection• Diffusion flame• Yellow flame (=soot)• Local lambda ~1• Overall lambda ~1,5 (no three-way cat) •Diffusion flame
Mixing controlled
X.S. Bai TC in piston engines
Compression ignited engine (”Diesel combustion”)
• Combustion during injection• Diffusion flame• Yellow flame (=soot)• Local lambda ~1• Overall lambda ~1,5 (no three-way cat)
• High efficiency, ~45%, high also at part load
– Load control through fuel amount– No throttling (smoke…)– Compression ignition => high
compression ratio
X.S. Bai TC in piston engines
n-heptane-air auto-ignition
n-heptaneC7H16
Heptyl-radicals (R)C7H15
Low temperature chemistry
RO2
KET First ignition
High temperature chemistry
Second ignition
e.g. C7H16 + O2 = C7H15 + HO2 C7H16 + HO2 = C7H15 + H2O
C7H14O3 = C7H13O2 + OH
C7H15 + O2 = C7H15O2
e.g. HO2 + O2 = H2O2 + O2 H2O2 = OH + OH
X.S. Bai TC in piston engines
Octane number, cetane number
• Octane number indicates the tendency of fuels to knock. The higher the octane number the more difficult the auto-ignition.
– n-Heptane (C7H16) has a octane number 0, – iso-octane (C8H18) has a octane number 100. – Gasoline has a octane number 93 – 97.
• cetane number denotes the ignition delay time (the start of the injection of diesel fuel to the onset of the auto-ignition). The cetane number ranks the fuels; the higher the cetane number the faster the auto-ignition.
– Isooctane has a cetane of 15 – diesel has a cetane number about 37 – 56. – Cetane (C16H34) has a cetane number 100.
X.S. Bai TC in piston engines
The main component of gasoline and diesel
Primary reference fuel
PRFx = x%n-heptane +(100-x)%iso-octane
X.S. Bai TC in piston engines
[From Johan Dec]
Compression ignited engine (”Diesel combustion”)
Diffusion flame in Scania 14-liters engine.Foto: Anders Larsson, Scania
FlameSpray
Conceptual model of diffusion part of Diesel flame (from Dec):
Airentrainment
Flame lift-off Diffusion flamestabilization
X.S. Bai TC in piston engines
Engine head and fuel injector
From Joseph C Oefelein, Sandia National Lab
X.S. Bai TC in piston engines
• Flame lift-off caused by intense turbulence in the early stage of the flame, chemistry is decreased
• At high enough turbulence intensity chemistry is quenched. blow-off.
Characteristic behaviour of stationary flame whenincreasing fuel flow (from Glassman):
Conceptual model of diffusion part of Diesel flame (from Dec):
Airentrainment
Flame lift-off Diffusion flamestabilization
Flame length
X.S. Bai TC in piston engines
Combustion in diesel enginesp
A
B
C
D
Normal engine run
Mortored run
θTDC
A: start of injection of fuel (SOI); B: start of combustion (SOC); C: end of fuel injection (EOI); D: end of combustion (EOC); AB: ignition delay; AC: duration of fuel injection; BD: duration of combustion
X.S. Bai TC in piston engines
Emissions legislations
LevelLevel From From yearyear NOxNOx (g/kWh)(g/kWh) PM PM (g/kWh)(g/kWh)
ECE R49ECE R49 19821982 1818 --ECE R49ECE R49--20%20% 19901990 14,414,4 --EuroIEuroI 19931993 88 0.250.25EuroIIEuroII 19961996 77 0.150.15EuroIIIEuroIII 20012001 55 0.100.10EuroIVEuroIV 20052005 3.53.5 0.020.02EuroVEuroV 20082008 2.02.0 0.020.02USA EPA07USA EPA07 20102010 0,270,27(NOx+HC)(NOx+HC) 0,0130,013
X.S. Bai TC in piston engines
Challenge for the diesel engine
Nitrous oxides
Fuel
con
sum
ptio
n &
CO
2
NOx= NO + NO2
X.S. Bai TC in piston engines
0
2
4
6
8
10
12
-5 -4 -3 -2 -1 0 1 2 3 4 5
Injection timing
NO
x (g
/kW
h)
500
520
540
560
580
600
620
640
660
CO
2 [g
/kW
h]
4.6
4.7
4.8
4.9
5
5.1
5.2
5.3
40 45 50 55 60
T inlet (after cooler)
NOx
(g/k
Wh)
Require appropriate EGR-cooling
012345678
0 5 10 15 20 25 30
EGR rate %
NOx
(g/k
Wh)
Intake air temperature Injection timing
Exhaust Gas Recirculation
X.S. Bai TC in piston engines
Base engine
Spray angleIntercooling
Increasedinj.pressure
fuel4 valves
Turbo compoundEGR
After treatment
New engineconcepts
Fuel
con
sum
ptio
n
NOx-emissions
Ways to decrease emissions and fuel consumption
X.S. Bai TC in piston engines
Homogeneous Charge Compression Ignition(HCCI)
Premixed“homogeneous combustion”
+ “Zero” NOx and soot+ High efficiency ~45%
also at partload
- Controll- Charging/EGR- MIT- Noise- Cold start
X.S. Bai TC in piston engines
Area burning rate
CAD 2 CAD 2.5 CAD 3 CAD 3.5
CAD 4 CAD 4.5 CAD 5 CAD 5.5
Global burning rateHCCI: 3.5 m2/sSI: 0.65 m2/s
Expansion speedHCCI: 70-90 m/sSI: 5-15 m/s