Turbulent Combustion in Piston Engines part 1 - LTHX.S. Bai TC in piston engines Schematical diagram of Reciprocating internal combustion engine B = bore S = stroke r = connecting

X.S. Bai TC in piston engines

Lecture 10.

Turbulent Combustion in Piston Engines

part 1


Content

• Piston engines – introduction and definitions• Flow and turbulence in piston engines• Laminar and turbulent burning velocity• Combustion duration• Engine knock and compression ratio



Schematical diagram of Reciprocating internal combustion engine

B = boreS = stroker = connecting rode lengtha = crank offsets = piston positionθ = crank angleVc = clearance volumeVd = displacement volume

/ 2, 2pU aNθ π π= ± = ∓5 20 m/spU −∼

a

s

B

r

S

BDC

TDC

Vc

Vd

θ


Table 10.1 Examples of engine operating parameters

Very small Small Largeengines engines engines

Bore (cm) 2.00 9.42 50Stroke (cm) 2.04 9.89 161Displacement/cylinder (L) 0.0066 0.69 316Engine speed (N, rpm) 13000 5200 125Power/cyl (kW) 0.72 35 311Average piston speed (m/s) 8.84 17.1 6.71Power/displacement (kW/L) 109 50.7 0.98BMEP* (kPa) 503 1170 472Examples Model airplane Automobile large stationary

two-stroke four-stroke two-stroke

*BMEP = brake mean effective pressure: it represents cylinder pressure that contributes effectively to mechanical work

Formula 1 engine run at higher speed – 14000 rpm, 35 m/s


c dc

c

V VrV+

=

Compression ratio

Thermodynamic analysis of motored run

motor = motor run = no combustion


TDC,motor c BDC,motord c

TDC,motor BDC,motor

-1

TDC,motor BDC,motor d c

TDC,motor BDC,motor c

(Equation of state),

p V p(V +V ),

T T

p p (V +V )= ,

T T V

isentropic compression: entropy=con

iu

i

c

Ypv R T RT V mv

W

r

⎛ ⎞= = ⇒ =⎜ ⎟

⎝ ⎠

⇒ =

⎛ ⎞=⎜ ⎟⎜ ⎟

⎝ ⎠

∑

TDC,motor BDC,motorγ/(γ-1) γ/(γ-1)TDC,motor BDC,motor

stant p p1 0, T Tv vTds C dT pd C dT pdv

ρ= + = + = =

In-cylinder pressure and temperature due to isentropic compression




c dc

c

V VrV+

=

TDC,motor c BDC,motord c

TDC,motor BDC,motor

TDC,motor BDC,motorγ/(γ-1) γ/(γ-1)TDC,motor BDC,motor

p V p(V +V ),

T Tp pT T

=

=

TDC,motor

BDC,motor

TDC,motor 1

BDC,motor

p,

p

TT

c

c

r

r

γ

γ −

=

=

In-cylinder pressure and temperature due to abiabatic compression

Compression ratio



X.S. Bai TC in piston engines9

An example of a four stroke engine


Four stroke engine cycles

Intake stroke Compression stroke Combustion/power stroke

Exhaust stroke


Sequences of events in four stroke SI engine operating cycle


P-V graph

Otto cycle effciency (indicated thermal efficiency)

111cr

γ

η−

⎛ ⎞= − ⎜ ⎟

⎝ ⎠

SI engine p-v graph Local zoomed p-v graph


Works

• Indicated work: work generated in the combustion chamber (area in the P-V graph)

– Gross indicated work: area A– Pump work: area B– Net indicated work (without supercharger): area A – area B

• Friction work

• Brake work: actual work available at the crankshaft– Brake work = indicated work – friction and parasitic work

• Mechanical efficiency: brake work/indicated work (40-90%)– lower engine speed has higher mechanical efficiency (friction)– lower engine speed has lower thermal efficiency (heat loss)


Mean effective pressure (mep) and power

• Mean effective pressure = work / displacement volume– imep– fmep– bmep– pmep

• Power: rate of work of the engine– Power = work x engine speed / number of revolution per cycle– Power = (mep) x face area of all pistons x averaged piston

velocity

• Torque: force acting at a moment distance = brake work /2π– Torgue = (bmep) x dispacement distance / 2π / number of

revolution per cycle


• Combustion during injection• Diffusion flame• Yellow flame (=soot)• Local lambda ~1• Overall lambda ~1,5 (no three-way cat)

• Premixed flame (injection prior to combustion)

• Blue flame (no soot)• lambda ~1 (three-way cat OK)

• Premixed “Homogeneous combustion”• “zero” NOx and soot• High efficiency ~45%

Also at partload• Control• Supercharge/EGR• Noise• Cold start

Otto Diesel HCCI


SI engines


Most cars run with SI engine



Flow field


Flow field

0

0

0.5 ,( ) ' 'rms

Bu u u≡


Flow field near the piston


3-D turbulence eddies in an engine

-270 CAD

Intake stroke


-20 CAD

3-D turbulence eddies in an engine

Compression stroke


Turbulent flows inside the cylinder

Ensemble average based on multi-cyclesTime average based on individual cyclesTurbulence velocity


Turbulence large eddy velocity u’rmswith cycle-cycle variation (solid), without cycle-cycle variation (dashed)


Turbulence large eddy velocity u’rms at TDC

Mean piston speed m/s

Turbulencem/s 'u KN


Combustion process in SI engines


Turbulent flame propagation in SI engine

No swirl, central plug

No swirl, two plugs


Turbulent flame propagation in SI engine

No swirl, side plug

normal swirl, side plug

10 15 20 25crank angle, degrees from ignition

high swirl, side plug



laminar burning velocity

,0,0 0

uL L

u

T pS ST p

α β⎛ ⎞ ⎛ ⎞= ⎜ ⎟ ⎜ ⎟

⎝ ⎠⎝ ⎠

2.18 0.8( 1)0.16 0.22( 1)

αβ= − Φ −= − + Φ −

( 1) 0.648 0.012( 1) 0.648,0 ,0 ,0L L c L c L cS S r S r S rα γ βγ− + − Φ−= =

Empirical expression


Laminar flame thickness

1.7

0.7200 0

,0

0 ,0 ,0

,0 1.368,0 ,0

0

,

, ,

uc

u

L L L L

LL L L c

L

p TD D D rp T

D S D S

S D rS D

δ δ

δ δ δ

−

−

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠


Turbulent burning velocity and combustion duration

' 'T LS S u u KN+

'flameT

B B B BN N NS u KN K

θΔ ∼ ∼ ∼ ∼


Combustion duration in SI engine

Initial flamedevelopment

Turbulent flame propagation


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.


Emissions


Compression ratio in SI engines

• Engine knock at high compression ratio


Compression ratio in SI engines

• Engine knock at high compression ratio


Spark ignited engine(”Otto-engine”, ”gasoline engine”)

• Premixed flame (injection prior to combustion)• Blue flame (no soot)• lambda ~1 (three-way cat OK)

• Low efficiency, ~30% at full load, ~15% at part load, due to:

– Load control through air amount => throttlerequired => low part load efficiency

– Knock sensitivity requires low compression ratio=> low overall efficiency

Documents

Turbulent Combustion in Piston Engines part 1 - LTHX.S. Bai TC in piston engines Schematical diagram of Reciprocating internal combustion engine B = bore S = stroke r = connecting