Lect10 engines part2 - Lunds tekniska högskola

X.S. Bai TC in piston engines

Lecture 10.

Turbulent Combustion in Piston Enginespart 2


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


Spark ignition 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.


Combustion duration in SI engine

Initial flamedevelopment

Turbulent flame propagation


Emissions


Compression ratio in SI engines

• Engine knock at high compression ratio


Compression ratio in SI engines

• Engine knock at high compression ratio


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.


Compression ignition 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



• 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


Hydrogen/air auto-ignition

Initial temperature

Ignition delay time


Hydrogen/air auto-ignition

Stoichiometry H2/O2 auto-ignition


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


n-heptane-air auto-igntion


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.


The main component of gasoline and diesel

Primary reference fuel

PRFx = x%n-heptane +(100-x)%iso-octane


Oil refinery

DieselC10 – C15


[From Johan Dec]


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


Engine head and fuel injector

From Joseph C Oefelein, Sandia National Lab


Fuel injectors


Fuel injection


Fuel injection and flames


Jet model

air

air


Flame liftoff structures

Liftoff height


Liftoff height


Liftoff height


Liftoff height


• 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


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


NTC and cool flame, n-heptane flame


Emissions


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


Challenge for the diesel engine

Nitrous oxides

Fuel

con

sum

ptio

n &

CO

2

NOx= NO + NO2


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


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


Homogeneous charge compression ignition (HCCI) 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


Experimental setup


Local structure and burning rate

CAD 3 CAD 3.5

CAD 4 CAD 4.5 CAD 5


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





Operating characteristics -Timing/Emissions

Documents

Lect10 engines part2 - Lunds tekniska högskola