7. Engine Breathing

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Engine Breathing and Advanced

Valvetrain Technologies

Dr. Rui Chen

Department of Aeronautical & Automotive Engineering

Loughborough University


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2

1. Volumetric Efficiency

Definition

2/V

andambientatcylinderpermeswept voluoccupyair toofmass

cyclepercylinderperinhaledairofmass

am bswept

actualair,

.

N

m

TpV

typically: 85115%VOL

Engine speedtypically 30004500rpm

Typical curve for a naturally aspirated engine at wide open throttle (WOT)

where,

= the mass of air inhaled per

cylinder per cycle

= the engine swept volume

= air density at ambient pressure

and temperature

= the engine speed in (rev/s)

am

sV

a

N


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3

Importance of Breathing: The maximum engine power output is directly proportional to the mass flow

rate of air into the engine since

ath m

.

AFR

LCVPower

Vasth V

AFR4

LCVTorque

bmenp typically:

8.0 13.0 bar

Torque

Engine speedtypically 30004500rpm

where,

= thermal efficiency

LCV = fuels calorific value

AFR = air to fuel ratio

= air mass flow rate

th

am


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4

Although high volumetric

efficiency is desirable, may

compromise to:

Produce swirl (masked/offset

valves)

Produce high in-cylinder

turbulence

Achieve good fuel distribution

and conditioning Exhibit good transient response

and idle speed stabilityFigure 2-2 Torque and volumetric efficiency vs. engine speed


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Quasi-static

Charge heating

Flow friction

Back flow

Choking

Tuning

Ram

Charge heatingCharge heating

Quasi-static effects

Flow friction

Backflow

Tuning

Choking

Ram effect

2. Effects on volumetric efficiency


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2.1 Quasi Static Effects

These include the following effects:

Residual gases

Inlet manifold density

Fuel partial pressure

Fuel vaporisation/latent heat


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2.1.1 Residual gas fraction

The residual gas fraction in the cylinder during compression is primarilya function of inlet and exhaust pressures, speed, compression ratio,valve timing and exhaust system dynamics.

Its magnitude affects engine volumetric efficiency.

The residual gas mass fraction is usually determined by measuring theCO2 concentration in a sample of gas extracted from the cylinder duringthe compression stroke.

e

cr

x

x

x2

2

CO

CO

~

~

where

the subscripts c and e denote compression and exhaust,

are mole fractions in the wet gas.2CO

~x


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2.1.2 Charge-mass law

Perfect gas law:

Assuming:

Charge mass into the cylinder:

Gas density in the cylinder:

Charge flow rate:

RTM

mPV

ei TT

e

ec

i

isei

RT

PVM

RT

PVMmmm

V

e

iiV

V

cs r

P

PRTr

Mr

VV

m

V

m

1

V

ei

Vi

sVs

r

PP

rRT

VrMNNVm

122/

NOTE: The charge-mass law ignores

Valve overlap

Inlet valve closure angle

Manifold dynamics

Port and manifold flow coefficients


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2.1.3 Fuel partial pressure

For most liquid fuels, this effect is small but for gaseous fuels the effecton volumetric efficiency is significant.

fam ppp 11

AFR

111

f

a

a

f

fa

a

m

a

M

M

p

p

pp

p

p

p

Manifold pressure:

therefore

For example:

For gasoline (C8H18),

For natural gas (CH4),

983.0114

96.28

15

11

1

m

a

p

p

983.0114

96.28

15

11

1

m

a

p

p

Hence, with gasoline, the reduction in is about 2% due to the presence of thefuel vapour while natural gas is over 10%.


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2.1.4 Fuel vaporisation

The enthalpy of the fuel vapour is higher than the liquid fuel, the difference

being the latent heat of vaporisation, vfh

Applying the steady state flowenergy equation,

vfflfpfapavfpfapa hmTcmcmTcmcmHHQ

1,,2,,

12

AFRAFR

1

AFR

11,,2,,

vf

lfpapvfpap

a

hTccTcc

m

Q

yields

Assuming , we get fplfpvfp ccc ,,,

AFRAFR

1,,12

vf

fpap

a

hccTT

m

Q

AFR

AFR

AFR

,

,

12

vf

afp

ap

vf

a h

m

Q

cc

h

m

Q

TT

i.e ,

Hence, with gasoline, would expect about 7% increase in air density due to fuelvaporising cooling effect which outweighs the loss due to fuel partial pressure.


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2.2 Charge heating effects

The inlet charge may absorb heat as it passes through the engines

intake system (i.e. warm inlet manifold).

This increase in temperature decreases charge density and hence lower

. The heating effect is speed dependent.


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2.3 Frictional Losses

Friction causes a pressure drop across each

component of the inlet and exhaust systems. This pressure drop is proportional to velocity squared

and results in the pressure in the cylinder being lessthan and greater than manifold pressures duringinduction and exhaust, respectively.

2.3.1 Flow friction:

For the intake system, quasi-steady analysis assuming

incompressible flow gives

2

2

v

p

paA

AVkp

where,= piston velocity (instantaneous

= piston area

= valve flow area (instantaneous)

= a constant

pV

pA

vA

k

Notes:

High flow areas are

advantages

Dependence of engine

speed


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2.3.2 Flow through ports

At low lifts, flow area is given by the curtain area,

At high value lifts, area given by project area

Flow separation occurs at high lifts reducing the discharge coefficient which is

defined in terms of an effective area

Mass flow rate through a poppet valve can be calculated by assuming 1-D nozzle

flow

VVCV

LDA ,

22,4

VVhV dDA

where, = port diameter

= valve lift

VD

VL

where, = valve stem diameterVd

V

eD

A

AC

12

1

2

6 i

c

i

c

i

ie

p

p

p

p

RTN

pA

d

dm


14/3014

Different flow regimes

insufficient valve lift can severely reduce the volumetric efficiency, there is little to be

gained from excessive valve lift.

port area is important for achieving high at high speeds and this area is

proportional to valve diameter. The 4-valve gives approximately 25% more flow area

than 2-valve design. The highest breathing efficiency should be obtained with 5-valve

(3-inlet and 2-exhaust) design.

Mass flow rate through a poppet valve can be calculated by assuming 1-D nozzle flow

VOL


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2.4 Choking Effects

The flow will be choked if the criticalpressure ratio is exceeded, which is given

by

The choked flow rate is given as

At high engine speeds during induction,

the flow can become choked. The inlet

Mach Index or Gulp Factor, , can be used

to assess the performance of inlet valves.

1

12

i

c

pp

1

1

1

2

6

i

ie

RTN

pA

d

dm

aDV

pp

CaA

AVZ

,

FFF


16/3016

2.5 Volumetric Efficiency Correction

To include for some of these effects, factors can be added, theseusually being empirically derived

where,

= inlet valve closure factor

= valve overlap factor

= pressure factor

= uncorrected volumetric efficiency

The Ford ESA engine simulation program uses the inlet mach index in

conjunction with flow coefficients and a manifold tuning factor.

pvoive FFFZVOLVOL

pvoive FFFZVOLVOL

iveFvo

F

pF

ZVOL


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Ram pipe supercharging

The intake work of the

piston is converted into

kinetic energy of the

column of gasupstream of the intake

valve, and

this kinetic energy, in

turn, is converted intofresh charge

compression work.

3. Dynamic Supercharging


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Define

Note: This parameter is to make test results independent ofengine speed. Experimental results show that for any enginespeed, engine peak performance corresponds to FrequencyRatioclose to 3, 4, and 5.

If Frequency Ratio equals to 4, then

frequencyopeningvalvepulsespressureoffrequencyratioFrequency

4L

a

a

4L

1pulsespressureofFrequency

120

NopeningvalveofFrequency

N

7.5a

16N

120aLopt

Tuned-intake tube charging


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Long inlet pipe:

Short inlet pipe:


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4. Variable inlet systems


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http://www.km77.com/marcas/mercedes/2005/clase-s/gama/gra/82.asphttp://www.km77.com/marcas/mercedes/2005/clase-s/gama/gra/84.asp


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Figure 1.14 Valve timing

5. Valve Timing


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Figure 1.14 Valve timing

Valve timings

Inlet valve opening (IVO):

Around 10-25oBTDC. Engine performance

is fairly insensitive to the IVO.

Inlet valve closing (IVC):

Around 40oABDC.

At low speeds, late IVC reduces volumetric efficiency. At high speeds, early IVC reduces volumetric efficiency.

Exhaust valve opening (EVO):

About 40oBTDC. Ensure all burned gas have sufficient time to escape.

Slight penalty in the power stroke, represents about 12% of power stroke.

Exhaust valve closing (EVC):

Usually 10-60oATDC. It appears not affect the level of residuals. Overlap

Large overlap, part load and idling operation suffers since the reduced inductionmanifold pressure causes back-flow of the exhaust.

Full load economy is poor since some unburned mixture pass straight throughthe engine when both valves are open at TDC.


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igure 4-2 Influence of inlet valve closure angle on the volumetric efficiency

There are compromises in valve timing:

high speed versus low speed performance, and

full load versus part load performance.

Variable valve timing can be used to

reduce the throttling losses in SI

engines.

Variable valve timing has significant

effect on effective compression ratio improve engine torque and

derivability

improve engine idle quality and

reduce idle speed

improve engine fuel economy and

emission

eliminate engine throttle andthrottling losses with gasoline

engines

directly control internal EGR for NOx

emission reduction

faster catalyst light-off

improve turbocharger performance

6. Variable Valve Timing (VVT)

Introduction:


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Reduce pumping losses caused by throttling early IVC (inlet valve closed partway through induction strokk)

late IVC (inlet valve closed partway through compression stroke)


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Multi-profile VVT systems:


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Variable Timing and Lift - BMW Valvetronic :


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Variable Phasing :


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Continuously variable VVT systems

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7. Engine Breathing

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