influence of design & operational parameters on volumetric efficiency of engine p m v subbarao...

Influence of Design & Operational Parameters on Volumetric Efficiency of

Engine

P M V SubbaraoProfessor

Mechanical Engineering Department

Clues to Improve Filling & Emptying Effectiveness….

Variables Effecting Volumetric Efficiency

• Overall volumetric efficiency is affected by following variables.

• Intake and exhaust manifold and port design.

• Intake and exhaust valve geometry: size, lift and timings.

• Fuel type, fuel/air ratio, fraction of fuel vaporized in the intake system, and fuel enthalpy of vaporization.

• In-cylinder mixture temperature, as influenced by heat transfer with cylinder wall.

• Ratio of exhaust to inlet manifold pressures.

• Compression ratio.

• Engine speed.

• The effects of many of above variables are quasi-steady in nature.

• Their impact is either independent of speed or adequately function of speed.

“ When you can measure what you are speaking about, and express it in numbers,

you know something about it; but when you cannot measure it in numbers, your knowledge

is of a meagre and unsatisfactory kind . . . ”.

•-- Lord Kelvin

Experimental Analysis of Valve Geometry

Pseudo Flow Velocity

• Pseudo Flow Velocity is a parameter to study the effect of manifold geometry, valve geometry and the rate change of cylinder volume on air flow into cylinder.

• PFV is proportional to the ratio of rate of change of cylinder volume to instantaneous minimum valve flow area.

d

dV

APFV

m

1

1

sin

cos

4

sin22

2

RA

aBCPFV

mv

d

ds

A

BCPFV

mv 4

2

Intake System Geometry Vs Volumetric Efficiency

Compression Ignition

Speed Vs Volumetric Efficiency

CI Engine

SI Engine

Dwell Piston Mechanism

The design geometry creates a pause or dwell in the piston’s movement at the TDC

and the BDC, while the crank continues to rotate for up to

20 degrees.

Instantaneous Displacement & Velocity of Dwell Piston

d

dSLN

dt

dSS p

60

22 sin

cossinsin

ael

aeaa

d

dS

22 sincos aelaS

Piston path for Dwell four stroke cycle

Comparison of standard and Dwell four stroke Cycles

SI Engine PV diagram due piston dwell in TDC and BDC

Comparison of standard and Dwell four strokecycles in PV diagram

Influence of Dwell on volumetric efficiency

Selection of Valve Timing for Better Flow

Selection of Maximum Valve Lift

Selection of Maximum Lift & Valve timings

Flow Kinematics through Valves

Valve Timing for Better Flow

Thermodynamic Parameters Vs Volumetric Efficiency • The volumetric efficiency is a function of

• Intake mixture pressure pi.

• Intake mixture Temperature Ti.

• Fuel/ air ratio (F/A).• Compression ratio r.

• Exhaust pressure, pe.

da

a

v V

m

0,

10, 11

1

Vr

r

AF

xm

c

c

a

rv

Where m is the mass in the cylinder at the end of intake stroke.

0,0,0,11 & aaaami TRpTmRVp

Effect of Fuel composition, Phase & Fuel/air Ratio

• In a SI engine, the presence of gaseous fuel and moisture in the intake system reduces the air partial pressure below the mixture pressure.

• For a mixture:

iegifiai pppp ,,,

1

, 1

eg

a

a

eg

f

a

a

f

i

ia

M

M

m

m

M

M

m

m

p

p

Fuel Chemistry Vs Volumetric Efficiency

Pa,I/pi

1.0

0.6

Equivalence Ratio0

1.5

H2

CH4

C8H18

Compression Ratio Vs Volumetric Efficiency