2. MINIMISING ENGINE BULK
Volumetric division of an AFV:-
a. Power plant - 38%
b. Crew & storage - 48%
c. Ammunition - 6%
d. Swept volume of gun - 8%
CI Engines for their low sfc ( high mileage ) & robustness have been
adopted for high mileage applications; but intrinsically has poorest power
to bulk ratio in its simpler form at least. It does not offer the required
compactness for an AFV.
Military engines must develop more kW from each m3 of bulk than is
commercially available. ( As a guide, commercial vehicle engines achieve
about 180 kW/m3 whereas military CI engines are in the range of 600-
1000 kW/m3; underlining their special status).
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CV systems: Prof (Col) GC Mishra
3. MINIMISING ENGINE BULK
Increasing the air swallowing capacity of the engine without commensurate
Bulk increase; can be made possibly by :-
1. Increasing swept volume, Vs, within the existing bulk.
Ratio of bulk/swept volume:-
a. 100/1 for 4 stroke truck engines;
b. 50/1 or less for military vehicles
Lower ratios require removal of power pack for maintenance/repair.
2. Inducing more times per second.
Engine may be run faster or, operate on 2- stroke.
Maximum engine speed is limited in a CI Engine due to smoky
combustion coupled with little gain.
2- stroke engines at the highest power ratings compare unfavorably
with 4- stroke engines.
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CV systems: Prof (Col) GC Mishra
4. 4
3. Filling the Cylinder More Completely on each Cycle.
Use of many small cylinders accented towards high speed performance;
racing car approach; resulting into narrow operating speed range.
Not suited to AFVs requiring a wide range of road speed & having
inadequate number of gears.
4. Supercharging .
A naturally aspirated engine has a limited supply of air for combustion.
The air has only atmospheric pressure pushing it into the cylinders.
Supercharging provides pressurized air, which allows for more air to be
packed into a cylinder for each firing.
This provides more power and much better combustion efficiency.
Thus, more power is realized from a given engine size, fuel economies
improve, and emissions are reduced.
CV systems: Prof (Col) GC Mishra
5. SUPERCHARGING
Definition.
Supercharging can be defined as the introduction of air ( or, air/fuel mixture)
into an engine cylinder at a density greater than ambient.
This is accomplished by means of:-
1. An engine-driven compressor- called Supercharger or, mechanical
supercharger.
2. A compressor driven by a turbine powered by exhaust gas energy;
called turbocharger.
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CV systems: Prof (Col) GC Mishra
6. 6
Aim of Supercharging.
1. To increase the power output for a given weight & bulk
of the engine.
Important for aircraft, marine, auto engines where weight
& space are at a premium.
2. To compensate for loss of power (due to lowering of
density) due to altitude.
a. Mainly for aircraft engines which loose power at an
approx rate of 1% per 100m altitude ( after 900 to
1200m onwards).
b. Also relevant for other engines used at high altitude.
CV systems: Prof (Col) GC Mishra
8. NA Engine;
W.D = Area 1-2-3-4-5-1
Supercharged engine;
W.D = Area 1’-2’-3’-4’-5’-1’
Thus supercharged engine gives a
greater power output & a much higher
maximum pressure.
To withstand higher maximum
pressure & stresses involved, the
engine has to be suitably designed to
be supercharged.
COMPARISION OF SUPERCHARGED & NATURALLY ASPIRATED(NA)
AIR STANDARD DUAL COMBUSTION CYCLES WITH SAME CR
8
The supercharged engine starts at a higher pressure ( and density ) point P1’.
Extra fuel can be burnt between 2’- 4’ because more air is available.
CV systems: Prof (Col) GC Mishra
9. COMPARISION OF SUPERCHARGED & NA AIR STANDARD DUAL
COMBUSTION CYCLES WITH SAME MAXIMUM PRESSURE
BUT DIFFERENT COMPRESSION RATIOs (CR)
By reducing the CR, the
clearance Volume ( Vc1) Is
increased & maximum pressure
is reduced.
If the CR is suitably chosen ,
the maximum pressure of a
supercharged engine can
equal that of NA engine.
The power output of a
supercharged engine will be
greater than a NA engine
with same maximum pressure.
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CV systems: Prof (Col) GC Mishra
10. Typical arrangement of an engine fitted
with a mechanically driven supercharger
A mechanically driven
supercharger takes drive from
the engine crankshaft;
consuming some useful power.
However the indicated power
output rises much more to
compensate for the above
power loss and any additional
friction losses.
10
Mechanically driven supercharger has been used in the past,
particularly on piston aircraft engines & large automotive engines (
Tank T-72 ). Not in common use today.
CV systems: Prof (Col) GC Mishra
12. EFFECT OF SUPERCHARGING
ON THE PERFORMANCE OF THE ENGINE
12
1. Power output is increased due to:-
a. Increase in mass flow rate of the air
b. Better scavenging and reduced residual gases.
c. Reduced exhaust gases.
2. Better mechanical efficiency.
3. Fuel Consumption
a. Low for CI engines
b. High for SI engines
CV systems: Prof (Col) GC Mishra
13. 13
Comparison between the performances of a 7.37-litre
spark ignition engine with and without a centrifugal
supercharger driven at ten times engine speed
CV systems: Prof (Col) GC Mishra
15. 15
Vane-type blowers
Eccentric rotor with spring loaded vanes for
continuous contact with outer casing.
a. Pulsating and noisy.
b. Efficiency very low.
c. Not useful for high speeds.
Used in refrigerators
Roots-type blower
1. It has twin-lobe rotors, but some have three
or four lobes.
2. Helical lobes; gradual uniform loading due to
gradual cutting of air.
3. Efficiency 63-70%.
4. Most commonly used on superchargers.
Types of Compressors
CV systems: Prof (Col) GC Mishra
16. 16
CENTRIFUGAL COMPRESSOR
SCREW-TYPE COMPRESSORS
1. Air is trapped between helical
rotors and the casing.
2. Efficient but complex.
1. Simple, small and cheap.
2. Good efficiency: 90-95%
3. Very pressure ratios can be obtained in 1-stage.
CV systems: Prof (Col) GC Mishra
18. 18
1. Increase in the valve overlap period. To allow complete scavenging of the
clearance volume.
2. Increase in clearance volume. By decreasing the compression ratio.
3. Supply of more fuel. Injection system of a diesel engine must be modified
for the same; requiring greater nozzle area.
4. Exhaust Valve. In case of turbocharged engines, the EV opens a bit
earlier to supply more energy to the turbocharger. Also, the exhaust
manifold will need to be insulated to reduce heat losses.
MODIFICATION OF AN ENGINE FOR SUPERCHARGING
CV systems: Prof (Col) GC Mishra
19. Power consumed by supercharger
19
1
.
.
.
.
0
1
0
k
c
p
c
c
p
c
P
P
T
C
m
T
C
m
c
c
Power consumed by the
compressor of the supercharger
0 1
CV systems: Prof (Col) GC Mishra
21. Mechanical superchargers: Drawback
Supercharging is intrinsically less
efficient than turbo charging.
This is because the compressor
absorbs some of the useful engine
power increase; particularly at high
speed, light load conditions;
It may absorb nearly as much power
as at full load.
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CV systems: Prof (Col) GC Mishra
22. 22
Major advantages of turbo-charging over supercharging
1. It does not require a mechanical drive from the engine,
which reduces the cost and bulk of the installation.
2. Turbocharging increases overall thermal efficiency by about
3 to 5%; up to 10%.
3. The turbocharger also reduces exhaust emissions and
exhaust noise.
CV systems: Prof (Col) GC Mishra
23. 23
Working of Turbocharger
A centrifugal compressor pulls air through
a rotating wheel at its center, accelerating
the air to a high velocity, which flows
radially outward through a shell-shaped
housing. The air velocity is slowed after
leaving the wheel, which converts velocity
energy into pressure.
This type of compressor is a high speed
device running at 80,000 to 130,000 rpm.
Since the turbocharger uses a majority of
the energy of the exhaust gases, the
sound of the exhaust is muted to a great
extent and the engine runs more silent.
Of the fuel energy available for an engine, about 40% is wasted in the exhaust.
A turbocharger uses some of this waste energy to drive a turbine. The turbine in turn
runs a compressor which is mounted on a common shaft inside a common housing.
CV systems: Prof (Col) GC Mishra
30. PULSE CHARGING
• The actual exhaust
pressure pulses are
made to run the turbine.
• Gases pulses are led
through narrow exhaust
pipes by shortest
possible route to the
turbine; recovering large
proportion of energy.
• Separate exhaust pipes
are used so that exhaust
process of various
cylinders do not interfere
with each other.
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CV systems: Prof (Col) GC Mishra
31. PULSE CHARGING
ADVANTAGES
1. Recovery of exhaust blow down energy is quite high except, in case of
highly supercharged engines with one/two cylinders per turbine inlet.
2. Smaller space required due to short & small diameter pipes.
DISADVANTAGES
1. Complicated inlet & exhaust pipe arrangements needed for large number
of cylinders.
2. Poor turbine efficiency due to intermittent gas supply.
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CV systems: Prof (Col) GC Mishra
32. CONSTANT PRESSURE TURBOCHARGING
• The exhaust from all the cylinders are made to discharge into a
common manifold at pressure higher than atmospheric pressure.
• Gases from all the cylinders expand in the exhaust valves to an
approximately constant pressure in the common manifold and pass
from here to the turbine.
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CV systems: Prof (Col) GC Mishra
33. CONSTANT PRESSURE CHARGING
Advantages.
1. Because of constant pressure & temperature, the turbine runs on a specific
pressure ratio and hence better efficiency
2. Simple design with simple exhaust pipes.
3. The engine speed is not limited by the pressure waves in the exhaust
pipes.
Disadvantages.
1. Response is poor to sudden load changes- only a small increase in energy
in the constant pressure pipes is available to meet the increased
requirement of acceleration at high loads.
2. Scavenging is poor due to the requirement of higher pressure drop during
scavenging.
34. PULSE CONVERTER
1 2 4 6
3 5
CONSTANT
PRESSURE
TURBINE
VENTURI JUNCTION
BOX ( MAINTAINS
CONSTANT
PRESSURE
VENTURI JUNCTION
PULSE CONVERTER
INCREASES SPEED OF
GASES FOR BETTER
SCAVENGING
- Combines advantages of both; pulse and pressure charging.
- Turbocharger turbine is a constant pressure machine and requires steady
flow Conditions for maximum efficiency ( Pulse charging provides partial
admission Operation ).
- However at low level of available exhaust energy especially at part load,
pulse charging required for efficient utilization of energy and better
scavenging.
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CV systems: Prof (Col) GC Mishra
