Compression Ignition Engine (C.I.) as compared to Sparkling Ignition Engine (S.I.)
6.3.1 Advantages of Compression Ignition Engine (C.I.)
i.
Low fuel usage especially low weight and low velocity handling.
ii.
Low fuel consumption.
iii.
Electric ignition is not needed.
iv.
Reliable because every cylinder has their own injection system. Therefore, no risk to all cut cylinders when there is failure ignition, for example, in S.I. engine.
v.
Well distribution of mixture because every cylinder has a nozzle injection.
vi.
Better attribution of torsion during large range of handling velocity.
vii.
Less maintenance, only need to be serviced after a longer period.
viii.
Less heat period.
ix.
of a higher compression H i g h e r t h e r m a l e f f i c i e n c y as a consequence of ratio (16-20 vs 9-12) needed for the self ignition of the mixture
x.
H i g h e r e f fi f i c i e n c y a t p a r t l o a d c o n d i t i o n (city driving) because of the
different load control with much inferior pumping loss for aspirating air into the cylinder: load control directly by varying the fuel delivery, while in the Otto engine by varying the air through an intake throttle xi.
L e s s e n e r g y spent to produce Diesel fuel
6.3.2 Disadvantages of Compressed Ignition Engine.
i.
Large fix (weight ratio to power brake) because of larger ratio from maximum pressure to minimum pressure. Therefore, bigger and strong cylinder is needed.
ii.
Starting cost which is very expensive.
iii.
Engine produces loud noise.
iv.
Polluted exhaust gas.
v.
H i g h e r w e i g h t for same power delivery, because of higher thermal and
mechanical stresses due to higher temperatures and pressures , almost double vs Otto engine, at the end of compression and combustion phases vi.
L o w e r m a x i m u m en g i n e s p e e d because a slower combustion process
and higher weight of the rotating an oscillating masses vii.
Engine roughness
that generates higher structural and airborne
vibration/noise.
6.3.3 Differences between petrol engine and diesel engine.
i)
Economic fuel
Because of high pressure ratio, C.I engine has higher heat competence than S.I. engine; higher heat competence means the usage of fix fuel is very low.
With reference to both test engines which have the same
loading cylinder, automotive engine usually works from ¼ and ½ weight load, whereby the difference of using fuel is very obvious. ii) Commercial vehicle engine Although the price of petrol and diesel is the same, C.I engine will show a higher economical price of fuel especially in this country where there is a big difference between the price of petrol and diesel. There is also a higher cost of maintenance, road tax and miscellaneous cost.
iii)
Pressure limit
C.I. engine can work contently with pressure ratio up to 23:1 and increase heat competence. Pressure ratio for ‘exploded equation’ is 8:1 for petrol
and 10:1 for high octane fuel.
iv)
Work pressure
As for the same power output, maximum pressure of C.I.engine. is higher than S.I.engine.
v)
Weight engine
Because of the effective maximum pressure is very high, C.I.engine. Is very heavy for power output.
Working
principles
of
spark
ignition
compression ignition engines Working Priciple of 2 Stroke Spark Ignition Engine
Intake The fuel/air mixture is first drawn into the crankcase by the vacuum that is created during the upward stroke of the piston. The illustrated engine features a poppet intake valve; however, many engines use a rotary value incorporated into the crankshaft.
Crankcase compression During the downward stroke, the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.
and
Transfer/Exhaust Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well.
Compression The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston).
Power At the top of the stroke, the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. (At the same time, another crankcase compression stroke is happening beneath the piston.)
2 Stroke Spark Ignition Figure
2 Stroke Spark Ignition Animations
4 Stroke Spark Ignition Engine A four -stroke engine, also known as four -cycle, is an internal combustion engine in which the piston completes four separate strokes—intake, compression, power, and exhaust— during two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.
Working Priciple of 4 Stroke Spark Ignition Engine
COMPRESSION stroke: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air, or fuel -air mixture into the combustion chamber of the cylinder head. POWER stroke: this is the start of the second revolution of the engine. While the piston is close to Top Dead Center, the compressed air–fuel mixture in a gasoline engine is ignited, usually by a spark plug, or fuel is injected into the diesel engine, which ignites due to the heat generated in the air during the compression stroke. The resulting massive pressure from the combustion of the compressed fuel -air mixture forces the piston back down toward bottom dead center.
EXHAUST stroke: during the exhaust stroke, the piston once again returns to top dead center while the exhaust valve is open. This action evacuates the burnt products of combustion from the cylinder by expelling the spent fuel -air mixture out through the exhaust valve(s).
st
1 Stroke = INTAKE stroke = 180
of
°
crankshaft revolution
Top Dead Centre (TDC)
nd
2 Stroke = INTAKE stroke = 360
4 Stroke = INTAKE stroke = 720 of crankshaft revolution
crankshaft revolution
°
Bottom Dead Centre (BDC)
r
3 Stroke = INTAKE stroke = 540 crankshaft revolution
of
°
of
°
4 Stroke Compression Ignition Engine In compression ignition (CI) engines, burning of fuel occurs due to compression of the fuel to very high pressures. At very high pressures the fuel, i.e. diesel, starts burning automatically without the need of any external flame. The cycle of operation of the CI engine is completed in four -strokes: suction, compression, expansion, and exhaust
A four stroke CI engine consists of the following four strokes. 1. Suction or Intake stroke 2. Compression Stroke 3. Expansion or power stroke 4. Exhaust stroke
1. Suction Stroke: This stroke starts when the piston is at the top dead centre. When it moves downwards it will create suction and only air enters the cylinder. The inlet valve is open at this time and exhaust valve is closed. When the piston reaches at the bottom dead centre the inlet valve closes and the suction stroke ends. It all takes place in 180º of the crankshaft rotation. 2. Compression stroke: In this stroke the piston starts moving upward. During this stroke both the inlet and exhaust valves are closed. The air is compressed by the upward movement of the piston. At the end of the compression stroke the fuel is injected into the combustion chamber. An injector is provided to inject the fuel. At the end of compression stroke the temperature is sufficient to ignite the fuel and the combustion of fuel-air mixture takes place. 3. Expansion or Power Stroke: Due to the high pressure of the burnt gases the piston moves towards bottom dead centre. Both the inlet and exhaust valve remains closed during the stroke.
