chapter 6 intake, exhaust and in-cylinder flow

8/20/2019 Chapter 6 Intake, Exhaust and in-cylinder Flow

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Valve Flow

At WOT the most significant gas flow restriction in an IC engine is the flow

through the intake and exhaust valves

9.12

1 1≥

+=

−k k

v

o k

P

PFlow chokes when

)1(2

1

)1(2

1

1

2

1

2 −+

−+

+

=

+

=k

k

o

ov f

k

k

ovo f cr k T

P

R

k Ac

k c Acm ρ

, mass flow rate independent of Pv

2

112

1

2

−

−=

+k

k

o

vk

o

v

ovo f P

P

P

P

k c Acm ρ

Po = upstream stagnation pressure, Pv = valve static (for subsonic =Pcyl)

ρ o = stagnation density, co = √kRT o stagnation speed of sound,

Av = valve area, c f = flow coefficient

ooPm ρ ,,

Pv

Pcyl

The mass flow rate through the valve is given by:


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Minimum areas:low lift - Av = A1= π dl

high lift - Av= A2= π d 2 /4

)(Aareavalveactual

)(Aareafloweffective )(ctcoefficiendischarge

)(Aareavalveactual

)(Aareafloweffective )(ctcoefficienflow

1

2

v

f d

v

f f

=

=

Valve Flow

d

A2

A1

d

l

Low lift High lift


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Flow Coefficient Measurement

Set: Av

Measure: mi , T i , Pi

Calculate: c f

F l o w

c o e f f i c i e n t ( c

f )

Nondimensional valve lift (l/d )

2

112

1

2

−+

−

−=

k

k

o

vk

o

v

ovo

f P

P

P

P

k c A

mc

ρ


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6

Heads are often wedge-shaped or domed, this permits Av /A p up to 0.5.

Valve Sizing

Permits more than two valves

per cylinder

Limited to two valves per cylinder


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Double overhead cams per cylinder bank are used to accommodate

multiple valves, one cam for each pair of intake and exhaust valves

Valve Sizing


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Valve Opening and Closing

In thermo cycles it is assumed the valves open and close instantaneously

CA

V a l v e d i s p l a c e m e n t

( l )

Valve starts

to open

Valve completely

closed

Duration

BCTC

EVO

IVO

IVC

EVC

180o

In reality a cam is used to progressively open and close the valves, the

lobes are contoured so that the valve lands gently on the seat.


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Valve Overlap

In real engines in order to ensure that the valve is fully open during a stroke,

for high volumetric efficiency, the valves are open for longer than 180o.

TCBC BC CA

i

e

BCTC

IVO

IVC

EVO

EVC

180o

1

4

5

The exhaust valve opens before BC and closes after TC

At TC there is a period of time called valve overlap where both the intake

and exhaust valves are open.

The intake valve opens before TC and closes after BC.


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When the intake valve opens bTC the cylinder pressure is at roughly Pe

Part throttle (Pi < Pe): residual gas flows into the intake port. During intakestroke the residual gas is first returned to the cylinder then fresh gas is

introduced. Residual gas reduces part load performance.

Valve overlap

Pi

Throttled

Pi < Pe

Pe Pi

Supercharged

Pi > Pe

Pe

WOT (Pi = Pe): some fresh gas can flow out the exhaust valve scavenging

residual (increases power but reduces fuel efficiency and increases emissions)

Pi

WOT

Pi = Pe

Pe

Supercharged (Pi > Pe): fresh gas can flow out the exhaust valve


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Engine Operating Conditions

Conventional engines operate at low rpms, with idle and part load fuel

economy being most important.

Engine speed:

Idle - 1000 rpm

Economy - 2500 rpm

Performance - 4000 rpm

WOT bmepsfc

“ E n g i n e

l o a d ”

High performance engines operate at high rpms, with WOT torque (i.e.,

volumetric efficiency) being most important.


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@1000 rpm intake duration: 230o = 38.4 ms

@2500 rpm 230o = 15.4 ms

@5000 rpm 230o = 7.7 ms, 285o = 9.5 ms

Valve Timing

i

e

BCTC

EVO

IVO

IVC

EVC

180o

Conventional

i

e

BCTC

EVO

IVO

IVC

EVC

180o

Performance


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Valve Overlap

Overlap

15o

65o

At high engine speeds less time available for fresh gas intake so need more

crank angles to get high volumetric efficiency large valve overlap

Variable Valve Timing (VVT) used to obtain optimum performance over

a wide range of engine speeds and load

At low engine speed and part throttle want to minimized valve overlap

Variable valve l ift : high speed want high lift to increase air mass flow rate,

low speed want low lift to minimize overlap effects


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Honda Variable valve Timing and lif t Electronic Control (VTEC)

Intake valve pair has three cam lobes, two that operate the valves at

low-rpm, and a third that takes over at high rpm (4500 rpm).

First introduced in N.A. 1991 Honda NSX model.

Camshaft

Follower

Rocker


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VTEC Intake Valve Operation

High rpm lobe has longer duration and higher lift raises max to 8000 rpm

giving higher peak power (good for racing) no benefit below 4500 rpm

During low-rpm operation, the two rocker arms riding the low-rpm lobes open

the intake valves.

During high-rpm operation a pin locks the three rocker arms and the valves

are opened by the larger center cam lobe.


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Honda DOHC 3VTEC


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Latest VTEC systems

i-VTEC (2001): VTEC + continuously variable camshaft phasing for

benefit even at lower speeds

Stage 1 (low speed): left valve left rocker arm driven by the low-lift left

cam. Right valve right rocker arm driven by the medium-lift right cam

Stage 2 (medium speed): left and right valve right rocker arm driven by

the medium-lift right cam

Stage 3 (high speed): left and right valve middle rocker arm driven by the

high-lift right cam

med-lift med-lift high-liftlow-lift

high-cammed-cam

no pin pin 1 pin 2


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VVT - Cam Phasing

Shifts the phase angles of the camshaft, does not change the valve “open

duration”.

Most systems provide inlet, two-stage discrete phasing (0o and 30o), others

provide continuous phasing (0o - 30o)

At low speed, 0o phasing is used so as to minimize valve overlap to minimize

residual gas backup into intake (good idle performance)

At high speeds, max phasing so as to increase valve overlap high-speed

exhaust gas inertia pulls in fresh gas purging residual gas out of cylinder

(improves volumetric efficiency)


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BMW Double VANOS and Valvetronic

Cap moves towards or away from the cam based on engine speed and gas

pedal position by varying hydraulic pressure in the two chambers

Double VANOS system provides continuous phasing for both the intake

(max range 40o) and exhaust valves (max range 25o)

Valvetronic also permits continuously variable intake valve lift, from ~0 to

10 mm, on the intake camshaft. This eliminates the need for a throttle valve

reducing pumping losses (10% improvement in power and fuel economy).


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Delphi cam phasing system

HPLP


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Toyota’s VVTL

VVTL uses cam phasing and two cam profiles for duration

At low rpm: long duration cam not engaged, short duration cam runs on

roller follower to reduce friction At high rpm: long duration cam engaged by sliding pin and locking

follower height also increases the lift (for Honda VTEC, both the

duration and lift are implemented by the cam lobes)


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Solenoid Activated Valves

Needs a large alternator to supply high current, also gently seating the

valve is difficult, needs sophisticated electronics


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Intake and Exhaust Processes in 4-Stroke Cycle

po

Lv, exh Lv, int

P = cylinder pressureLv = valve disp lacement

P o

P, Lv

P

TC BC

Exhaust

Intake

P o

3

2

4i

e

1

BCTC

WOT

Part thrott le

1st crank shaft rev: 1 - 3 – 22nd crank shaft rev: 4

EVO

1


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The valve spring normally keeps the top of the valve stem in contact with

the cam lobe

At very high engine speeds, and thus high camshaft speeds, it is difficult to

maintain contact between the cam lobe and the top of the valve stem as a

result the valves stay open longer than desired and slam into valve seat.

Valve “ Float”

Spring


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Muffler

Air cleaner

Intake and Exhaust System for Single Cylinder Engine

Cylinder

P

P o , T o

P o


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Intake and Exhaust Manifold

The intake manifold is a system designed to deliver air to the engine from

a plenum to multiple cylinders through pipes called runners.

Exhaust manifold used to duct the exhaust gases from each cylinder to

a point of expulsion such as the tail pipe.

Velocity magnitude (m/s)


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Manifold Pressure

3000 rpm

6000 rpm


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Supercharger and Turbocharger

These devices are used to increase the power of an IC engine by raising

the intake pressure and thus allowing more fuel to be burned per cycle.

Compressor

Patm

Pin t > Patm

Win

Superchargers are compressors that are mechanically driven by the engine

crankshaft and thus represents a parasitic load.

Allows the use of a 4 cylinder instead of 6 cylinder engines cost effective

and weight reduction


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Positive Displacement Compressors

Positive displacement compressors: piston, Roots, and screw

Pressurization occurs in the manifold when the air flow rate supplied

is larger than that ingested by the cylinders.

P1 P2

Most common is the Roots compressor – pushes air forward without

pressurizing it internally.

Produces constant flow rate independent of boost pressure (P2)


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Performance of Posit ive Displacement Compressors

s/co = rotor tip Mach#

~ pump speed

ηc = compressor efficiency = ratio of isentropic work and actual work

c

Screw

Roots

Extra energy goes to heat up air leading to a reduction in density


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Dynamic Compressors

Dynamic compressor has a rotating element that adds tangential

velocity to the flow which is converted to pressure in a diffuser.

Produces a constant boost pressure independent of the mass flow rate

Most common is the radial (or centrifugal) type


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Mass flow rate (Pounds of air per minute)

To the left of surge line the flow is

unstable (boundary layer separation

and flow reversal)

To the right of 65% line the compressor

becomes very inefficient:

a) air is heated excessively

b) takes excess power from the crank

shaft


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Turbochargers couple a compressor with a turbine driven by the exhaust

gas. The compressor pressure is proportional to the engine speed

Compressor also raises the gas temperature, so after-coolers are used

after the compressor to drop the temperature and thus increase the air

density.

Aftercooler


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The peak pressure in the exhaust system is only slightly greater than

atmospheric – small ∆P across turbine

INTAKE

AIR

EXHAUST

FLOW

Takes time for turbine to spool up to speed, so when the throttle is opened

suddenly there is a delay in achieving peak power - turbo lag

In order to produce enough power to run compressor the turbine speed

must be very fast (100k-200k rev/min) – long term reliability an issue


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Waste gate valve used to bypass exhaust gas flow from the turbine

Turbine Compressor

WASTE GATE

Proportional

valve

Engine

Exhaust

Patm

AIR

Patm

It is used as a full-load boost limiter and in new engines used to control

the boost level by controlling the amount of bypass using proportional

control to improve drivability

WASTE GATE


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Turbo Lag Reduction: Twin Turbo

Two turbochargers:

• Smaller turbo for low rpm low load and a larger one for high load

• Smaller turbo gets up to speed faster so reduction in turbo lag

2006 Volkswagen Golf GT 1.4 L GDI uses twin turbo:

0-2400 rpm roots blower

>3500 rpm turbocharger

Supercharger/turbo:

• Supercharger used at low speed to eliminate turbo lag

• At higher rpm turbo charger used exclusively to eliminate parasitic load


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BMW 2.0L I4 turbo diesel surpasses 100 hp/L (75 kW/L)

2008 BMW 4.4L V8 valley mounted twin turbo


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382006 Porche 911 Variable Turbine Geometry uses temperature-resistant materials

Turbo Lag Reduction: Variable Geometry Turbo (VGT)

Variable guide vanes direct the flow of exhaust gas from the engine in

exactly the direction required on to the turbine wheel of the turbocharger.

Guide vane

VGT used on diesel engines with exhaust temps (700-800 C) not

normally used in SI engine due to high exhaust temp (950 C)

Good response and high torque at low engine speeds as well as superioroutput and high performance at high engine speeds


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Low engine rpm (low exhaust flow velocity):

Vanes are partially closed accelerating the exhaust

gas flow. The exhaust flow hits the turbine blades at

right angle. Both make the turbine spin faster

High engine rpm (high exhaust flow velocity):

The vanes are fully opened to take advantage of the

high exhaust flow. This also releases the exhaustpressure in the turbocharger, saving the need for

waste gate.


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Variable Geometry Turbo

Holset VGT


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Volumetric Efficiency

Recall volumetric efficiency is defined as:

===

d

ad

o

o

cyl

cyl

d oa

ad cyla

d oa

av

V

V

T

P

T

P

V

V

V

m ,

,

,,

, ρ

ρ

ρ η

Volumetric efficiency is affected by :

i) Fuel evaporation

ii) Mixture temperature

iii) Pressure drop in the intake systemiv) Gasdynamic effects

v) Valve timing

or engine speed

Note: mean piston speed proportional

to air flow velocity

pU S N ⋅= )2/(


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Factors affecting v

Fuel evaporation:

• In naturally aspirated engines (no supercharging) the volumetric efficiency

will always be less than 100% because fuel is added and the fuel vapour

will displace incoming air.

• The earlier the fuel is added in the intake system the lower the volumetric

efficiency because more of the fuel evaporates before entering the cylinder.

• In Diesels and GDIs the fuel is added directly into the cylinder after the

intake stroke so get higher volumetric efficiency.


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Factors affecting v

Heat transfer:

• All intake systems are hotter than ambient air, e.g., injection system and

throttle bodies are purposely heated to enhance fuel evaporation.

am

Pcyl

f m

• Therefore, the density of the air entering the cylinder is lower than

ambient air density.

• Greatest problem at lower engine speeds more time for air to be heated.

• Use cold air intake


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Factors affecting v

Fluid friction:

• The air flows through a duct fitted with an air filter, throttle and intake valve

• Air moving through any flow passage or past a flow restriction undergoes a

pressure drop

• The pressure at the cylinder is thus lower than atmospheric pressure

• Greatest problem at higher engine speeds when the air flow velocity is high


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Muffler

WOT

Part throttle

P o = atmospheric pressure

P air = pressure losses in air c leaner

Pu = intake losses upstream of thrott le

P thr = loss across throttle

Pvalve = loss across intake valve

Air cleaner

Pressure losses over the length of the intake system

Cylinder

P

P o

P air

Pu

Pvalve P throttle

Extreme case of flow restriction is when the flow chokes at the intake valve

as engine speed increases flow velocity remains the same have less fill time.


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Factors affecting v

1i

e

4

BCTC

Residual gas:

Residual gas takes up cylinder volume that would otherwise contain air

k

e PPr

f /1

4 )/(1

=

Recall the residual fraction given by

As (Pe /P4 ~ Pe /Pi) increases, or r decreases the fraction of cylinder volumeoccupied by residual gas increases and thus volumetric efficiency

decreases.


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Opening intake valve before TC (valve overlap):

• The longer the valve overlap, more exhaust gases rush into the

intake port.

Factors affecting v

IO

EC

ICi

e

EO

BCTC

• Greatest problem at idle (part throttle and lower engine speeds) low

intake pressure and more time for exhaust gases to back up.

Factors affecting


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Factors affecting v

Closing the intake valve after BC (backflow):

• When piston reaches BC still have ∆P across theintake valve, mixture continues to flow into cylinder, close the intake valve after BC.

po

Lv, exh Lv, int

P, Lv

P

• As the piston changes direction the mixture is compressed, when the

pressure equals the intake manifold pressure the flow into the cylinder stops.

• Best time to close the intake valve is when the manifold and cylinder

pressures are equal, close the valve too early and don’t get full charge, too

late and air flows back into the intake port.

• At high engine speeds larger ∆P across intake valve because of higherflow velocity, so ideally want to close valve later after BC (60o aBC).

• At low engine speeds smaller ∆P across the intake valve so ideally wantto close the intake valve earlier after BC (40o aBC).

Factors affecting


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RAM Effect:

• As the intake valve closes at higher engine speeds, the inertia of the air in

the intake system increases the pressure in the intake port,

allowing more air to be injected

Factors affecting v

sPuP =+ 2 ρ

• This effect becomes progressively more important at higher engine speeds.

• To take advantage of ram effect close intake valve after BC.

Pcyl

F t ff ti


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Intake tuning:

• When the intake valve opens the air suddenly rushes into the cylinder and

an expansion wave propagates back to the intake manifold at the local speed

of sound relative to the flow velocity.

Factors affecting v

• For fixed runner length the intake is tuned for one engine speed.

• If the timing is appropriate the compression wave arrives at the inlet at the

end of the intake process raising the pressure above the nominal inlet

pressure allowing more air to be injected.

N

c L

N t

c

Lt

valvewave ∝⇒

≈=

2

3

2

2 π

• When the expansion wave reaches the manifold it reflects back towards to

intake valve as a compression wave. The time it takes for the round trip

depends on the length of the runner ( L) and the flow velocity.


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Since L~1/N : high engine speed use short runners,

low engine speeds use long runners

Audi V6

Similarly the exhaust system can be tuned to get a lower pressure at

the exhaust valve increasing the exhaust flow velocity.

Adjustable runner length


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Factors affecting v as a function of engine speeds

Fuel vapour pressure

I C li d Fl id fl


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In-Cylinder Fluid flow

Three parameters are used to characterize large-scale in-cylinder fluid

motion: swirl, squish, and tumble.

Swirl is used to:i) promote rapid combustion in SI engines

ii) rapidly mix fuel and air in gasoline direct injection engines

iii) rapidly mix fuel and air in CI engines

The swirl is generated during air induction into the cylinder by either:i) tangentially directing the flow into the cylinder, or

ii) pre-swirling the incoming flow by the use of helical ports.

Swirl is the rotational flow about the cylinder axis.


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Helical port

Tangential inject ionSwirl motion

Contoured valve

Cylinder Swirl and i ts Generation


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Swirl Theory

Swirl can be simply modelled as solid body rotation, i.e., cylinder of gas

rotating at angular velocity, ω.

Tangential flow velocity is v = ω r

where N is the engine speed (revolutions per second)

ω is the air solid-body angular velocity (rad/s)

N Rs

π

ω

2=

The swirl ratio, Rs, is defined as the ratio of the gas angular velocity and

the crank shaft angular velocity, i.e.,

Most production engines have Rs in the range of 0.5 -1.0

S i l Th


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During the cycle some swirl decays due to friction, but most of it persists

through the compression, combustion and expansion processes.

Swir l Theory

8 cylinderafor

2 MB

I rdm I I ∫ ===Γ ω

where M is the total gas mass

B is the cylinder bore

The angular momentum, Γ, and moment of inertia, I , of a rotating

volume of gas is:

Neglecting friction, angular momentum I ω is conserved, I decreases ω increases


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Many engines have a wedge shape cylinder head cavity or a bowl in the

piston where the gas ends up at TC.

Engine Swirl

During the compression process as the piston approaches TC more of the

air enters the cavity and the air cylinder moment of inertia decreases and

the angular velocity (and thus the swirl) increases.

S i h d T bl


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Squish and Tumble

Squish is the radial flow occurring at the end of the compression stroke in

which the compressed gases flow into the piston or cylinder head cavity.

As the piston reaches TC the squish motion generates a secondary flow

called tumble, where rotation occurs about a circumferential axis near the

outer edge of the cavity.

Intake Flow


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The intake process governs many important aspects of the flow within the

cylinder. The gas issues from the valve opening as a conical jet with radial

and axial velocities that are about ten times the mean piston velocity.

Large vortices become unstable

and eventually break down into

turbulent motion

Shear layers

The jet separates from the valve producing shear layers with large velocity

gradients which generate turbulence.

The jet is deflected by the cylinder wall down towards the piston and up

towards the cylinder head producing recirculation zones.

Additional turbulence is generated by the velocity gradient at the wall

in the boundary layer.

Turbulent Flow


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Turbulent Flow

Turbulent flow is characterized by its transient and random nature that is

superimposed on a steady mean flow.

Steady flow

Turbulent flows are always dissipative, viscous shear stresses result in an

increase in the internal energy at the expense of its kinetic energy.

So energy is required to generate turbulence, if no energy is supplied

turbulence decays.

The source of energy for turbulent velocity fluctuations is shear in the mean

flow, e.g., jets and boundary layers.

Statistical Approach to Turbulence


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Statistical Approach to Turbulence

The fluid velocity measured at a point in a specific direction:

componentgfluctuatintheis

itymean veloc 1

where

)(')(

2

1

u'

U(t)dt Δt

U

t uU t U

t

t ∫=

+=

t1

U x(t1 )

U x mean velocity (steady)

t

U x(t)

Reynolds decomposition for statistically steady flow:

u‘(t 2)

t 2

It is common practice to define the turbulent fluctuation intensity, ut , in

terms of the root-mean-square of the fluctuations:

( )∫∆

=== 21

222 )('1

' where''t t rmst

dt t ut

uuuu

Turbulence Measurements in Engines


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In engines the flow is statistically periodic (the flow pattern changes with

crank angle) not steady.

Cycle i

CA

The following shows the velocity measurement at a point in the cylinder

over time for a two-stroke engine (cycle has 360 CA)

+ Instantaneous

Individual cycle mean

Measurement

point

BC TC BC TC BC

BC

TC

),(')(),( iuU iU θ θ θ +=

The instantaneous velocity measured at a specific crank angle θ in a

particular cycle i is:



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∑=

n

i EA iU nU ),(

1

)( θ θ

where n is the number of cycles averaged.

Flows that are statistically periodic are treated using ensemble average:

g

There are both cycle-by-cycle variations in the mean flow at any point in the

cycle as well as turbulent fluctuations about that specific cycle’s mean flow.

InstantaneousIndividual cycle

mean

Ensemble average

U EA

u’

U ˆ

CA



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g

The difference between the mean velocity in a particular cycle and the

ensemble average is defined as the cycle-by-cycle variation in mean velocity:

)(),(),(ˆ θ θ θ EAU iU iU −=

If the cycle-by-cycle variations are small then the cycle mean is equal to

the ensemble average.

Thus, the instantaneous velocity can be split into three components:

),('),(ˆ)(),( iuiU U iU EA θ θ θ θ ++=

21

1

2 ),('1

)(

∑==

n

it iu

nu θ θ

The turbulent intensity is determined by ensemble averaging:



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g

At the end of compression when the piston is at TC, the turbulence

fluctuating intensity is about one-half the mean piston speed:

The two data sets shown with redlines are for individual cycle

turbulence intensity.

The rest of the points are for

ensemble averaged, which means

they include cycle-by-cycle

variations in the mean velocity,

making it larger by up to 2 times.

Pt U u21=

Turbulence Length-Scales


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Turbulent flow is comprised of unsteady eddies (vortices) with a multitude

of length-scales and time-scales (turnover time).

Most of the turbulent KE is contained in the large eddies that breakdown

into smaller size eddies via inviscid mechanisms.

The largest eddies in the flow are limited in size by the enclosure with

characteristic length-scale of L (e.g., large eddy associated with swirl).

The integral scale l represents the largest turbulent eddy, determined

by the fluctuating velocity frequency.

The turbulent KE cascades from the larger structures to the smaller

structures where it is converted to thermal energy via viscous effects.

What scale eddy is required to dissipate energy?

Length Scales of Turbulence


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eunit volum perforceviscous

eunit volum perforceinertia

'

''

Re

3

3

22

→==

L

Lu L

Lu Lu

µ

ρ

µ

ρ

Length-Scales of Turbulence

Reynolds number (Re) of an eddy with circulation velocity u' and size L is:

There is one more length-scale between the integral and Kolmogorov scales

known as the Taylor microscale which represents the distance over whichviscous effects can be felt, or the mean spacing between dissipative eddies.

Viscous forces are only important in the smallest scale where the Re →1

The eddy size at which the flow KE is dissipated by viscous effects is

known as the Kolmogorov scale, and the eddy dimension is η.

Length-Scales of Turbulence


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The scales are: Integral (l), Taylor micro (l), Kolmogorov (η)

l

λ

η

l

η

λ

Gas flow throughintake valve

The Length-Scales of Turbulence


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Dimensional analysis leads to the following relationships between the scales:

( ) 4341

2121

1

Re

Re15

−−

−

=

=

=

t

t

C l

C l

LC l

η

λ

η

λ

where C1, Cλ, and Cη are numbers unique to the flow.

If the integral scale can be determined, so can all the other scales.υ

lut t =Re

The turbulent Reynolds number is based on the integral scale and the

turbulent fluctuation intensity

As the engine speed increases the Re increases, so the smaller scales

of turbulence decrease in size.

T St k E i I C li d Fl


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Two-Stroke Engine In-Cylinder Flow

Most common two-stroke engines are crankcase-scavenged

PROD

AIR

AIR

Another class of two-stroke engine uses a separate compressor to deliver

air into the cylinder to scavenge the combustion products, fuel is

injected directly into the cylinder.

S f


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Scavanging Performance

Delivery ratio, Dr

If the cylinder volume is completely filled with air the delivery ratio is

given by: 11>

−==

⋅⋅

=r

r

V

V

V

V D

d

bc

d a

bcar

ρ

ρ

densityambientvolumedisplaced cycle perairdelivered of mass

×=r D

air delivered of mass

retained airdelivered of mass=ΓTrapping efficiency, Γ

chargecylindertrapped of mass

retained airdelivered of mass=seScavenging effic iency, es

Scavenging Models


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A. Perfect scavanging – no mixing, air displaces the products out the exhaust

if extra air is delivered ( Dr > r /(r-1) ) it is not retained

Scavenging Models

S c a v e n g i n g

e f f i c i e

n c y

T r a p p i n g

e f f i c i e n

c y

B. Short c ircuiting – the air initially displaces all the products within the path

of the short circuit and then flows into and out of the cylinder

C. Perfect mixing – the air that enters the cylinder mixes instantaneouslywith the products, so immediately the gas leaving includes

both air and products

chapter 6 intake, exhaust and in-cylinder flow

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