Transcript Slide 1

[Translation: In the name of Allah, the Most Merciful, the Most Kind.]
1
THE UNIVERSITY OF LAHORE
REG NO:
BME: 01083004
SECTION:
B
DEPARTMENT: MECHANICAL
2
The Sparking Ignition engine.
The term spark-ignition engine is normally used to
refer to internal combustion engines where the fuel-air
mixture is ignited with a spark. The term contrasts with
3
compression-ignition engines, where the heat from
compression alone ignites the mixture. Spark-ignition
engines can be either two-stroke or four-stroke, and are
commonly referred to as "gasoline engines" in America
and "petrol engines" in Britain. However, these terms are
not preferred, since spark-ignition engines can (and
increasingly are) run on fuels other than gasoline, such
as
auto gas (LPG), methanol, ethanol, compressed natural
gas (CNG), hydrogen, and (in drag racing) nitromethane.
A four-stroke spark-ignition engine is an Otto cycle
engine.
4
Until recently, a major distinction between spark
ignition
and compression-ignition engines has been
where the fuel is mixed - spark-ignition engines mix
fuel
outside the cylinders and compression-ignition
engines
mix fuel inside the cylinders. However, both twostroke
and four-stroke spark-ignition engines are
increasingly
being designed with gasoline direct injection (GDi),
eliminating this distinction between the two
systems.
5
Pressure - Volume Diagrams
• Graph of pressure as a function of cylinder
volume above the piston
Bottom Dead
Center - volume
is maximum
Top Dead Center
- volume is
minimum
6
Cylinder Volume
• Change in volume as piston goes up and
down
Volume going
TDC
Volume going
7
Volume
Change in Volume
BDC
TDC
BDC
8
Bottom Dead Center
Start of compression stroke volume above piston filled with
fuel/air mixture
Pressure
P-V Diagram - Gas Engine
TDC
BDC
Volume
9
Piston travels up, fuel/air
compressed and
pressure rises
Pressure
P-V Diagram - Gas Engine
TDC
BDC
Volume
10
P-V Diagram - Gas Engine
End of compression stroke volume in cylinder at a minimum
Pressure
Top Dead Center
TDC
BDC
Volume
11
Then have an
instantaneous
introduction of heat,
which increases
pressure again
COMBUSTION
Pressure
P-V Diagram - Gas Engine
TDC
BDC
Volume
12
Pressure
P-V Diagram - Gas Engine
TDC
BDC
Volume
Pressure forces piston down,
creating torque on crank shaft
13
P-V Diagram - Gas Engine
Pressure
Piston reaches bottom
dead center again,
exhaust valve opens,
burned fuel/air expelled
TDC
BDC
Volume
14
Gas Engine Efficiency
• Remember, efficiency is work out divided
by heat in
• P-V diagram tells us both things
Pressure
Work out: area
between curves
Heat in:
temperature
change from
burning fuel
And, make a few assumptions,
can calculate efficiency
TDC
BDC
Volume
15
Gas Engine Efficiency
• Define compression ratio as:
Pressure
VBDC
r
VTDC
• Then efficiency can be shown
to be:
1
e 1
r
0.4

TDC
BDC
Volume
• Higher efficiency means higher
compression
ratio.

16
(sparking ignition engine)
Advantages for Spark Ignition Engines:
- Increase efficiency at part load operating conditions by
operating at a higher effective compression ratio
- Successfully avoid spark knock at full load conditions,
even during rapid transients
17
18
Following is the explanation
of
.
the Four-Stroke and Two-Stroke
SI Engines and their P-V
Diagrams.
19
Four Stroke SI Engine
Stroke 1: Fuel-air mixture introduced into cylinder
through intake valve
Stroke 2: Fuel-air mixture compressed
Stroke 3: Combustion (~constant volume) occurs
and product gases expand doing work
Stroke 4: Product gases pushed out of the cylinder
through the exhaust valve
FUEL
A
Ignition
I
R
Fuel/Air
Mixture
Intake
Stroke
Compression
Stroke
Combustion
Products
Power
Stroke
Exhaust
Stroke
Spark plug for SI engine
Fuel injector for CI engine
Cylinder
Components
Valves
Top
Center
(TC)
Stroke
Bottom
Center
(BC)
TC
0o
Crank shaft
q
270o
90o
180o
BC
Clearance
volume
Cylinder
wall
Piston
Four-Stroke SI Engine
Exhaust gas
residual
IVO - intake valve open, IVC – intake valve close
EVO – exhaust valve open, EVC – exhaust valve close
Xb – burned gas mole fraction
Two Stroke SI Engine
The two-stroke engine is simpler mechanically than
the four-stroke engine. The two-stroke engine delivers
one power stroke every two strokes instead of one every
four; thus it develops more power with the same
displacement, or can be lighter and yet deliver the same
power. For this reason it is used in lawn mowers, chain
saws, small automobiles, motorcycles, and outboard
marine engines.
23
24
Two Stroke SI Engine
Exhaust
port
Fuel-air-oil
mixture
compressed
Check
valve
Expansion
Exhaust
Intake (“Scavenging”)
Crank
shaft
Fuel-air-oil
mixture
Compression
Ignition
Two-Stroke SI Engine
scavenging
Exhaust area
Intake area
EPO – exhaust port open
EPC – exhaust port closed
IPO – intake port open
IPC – intake port closed
Achievements:
• Three prototype pistons were manufactured: two
pistons for a spark ignition engine and one piston
for a compression ignition engine. All three were
tested experimentally in single cylinder engines.
Models of the piston operation were also developed
to investigate the piston concept at different
operating conditions.
• Spark Ignition Engine Test Result Summary
The spark ignition PRP prototypes demonstrated the
utility of the PRP concept when applied to a spark
ignition engine. At low loads, the piston behaved
like the elevated compression ratio, yielding a brake
specific fuel consumption (BSFC) improvement of
3.5-10% during low load conditions. At high load
conditions, the PRP limited the peak cylinder
pressure to that of the baseline (lower compression
ratio) piston.
27
Cylinder Arrangement
Single-cylinder engine gives one power stroke per crank revolution
(2 stroke) or two revolutions (4 stroke). The torque pulses are widely
spaced, and engine vibration and smoothness are significant problems.
Used in small engine applications where engine size is more important
Multi-cylinder engines spread out the displacement volume amongst
multiple smaller cylinders. Increased frequency of power strokes
produces smoother torque characteristics. Engine balance (inertia forces
associated with accelerating and decelerating piston) better than single
cylinder.
Most common cylinder arrangements:
- In-line 4-cylinder
- In-line 6-cylinder
- V-6 and V-8
Power Regulation (Throttling)
An IC engine is basically an air engine, the more air you get into the
cylinder, the more fuel you can burn, the more power you get out.
The initial pressure in the cylinder is roughly equal to the pressure
in the intake manifold.
Pressure in the intake manifold is varied by opening and closing the
throttle plate to change the pressure drop. Maximum air flow (and
power) achieved at wide-open-throttle (WOT). Minimum air flow
at idle
Fuel
Patm
Pint < Patm
Idle
Intake manifold
WOT
Basic Carburetor Design
Air Flow
Venturi
Fuel
Throttle
Mixture to manifold
Fuel Injection System
Air intake
manifold
Throttle
Fuel tank
During start-up the components are cold so fuel evaporation is very slow, as a result
additional fuel is added through a second injecting valve
Superchargers and Turbochargers increase the power of an IC engine
by raising the intake pressure and thus allowing more fuel to be burned
per cycle. Knock or autoignition phenomenon limit precompression.
Superchargers are compressors that are mechanically driven by the
engine crankshaft and thus represent a parasitic load.
Pint > Patm
Patm
Compressor
Turbochargers couple a compressor with a turbine driven by the exhaust
gas. The compressor pressure is proportional to the engine speed
The peak pressure in the exhaust system is only slightly greater than
atmospheric – small DP across turbine.
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.
It takes time for turbine to get up to speed so when the throttle is opened
suddenly there is a delay in achieving peak power - Turbo lag.
Waste gate valve controls the exhaust gas flow rate to the turbine.
It is controlled by the intake manifold pressure
EXHAUST
FLOW
INTAKE
AIR