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