TYPES OF ENGINES-SPARK AND COMPRESSION ENGINES
TYPES OF ENGINES-SPARK AND
COMPRESSION ENGINES
There are many types of engines. Each
is for a specific purpose.
Petrol engine use a spark ignition cycle.
Diesel engine use a compression ignition
Cycle. Spark ignition is for light duty
vehicles. Compression ignition is for
heavy duty vehicles.
Definition of an engine
An engine is a device which does two functions.
(i)Produces motion (produces power) as in a scooter/bike, car, bus, train and aircraft
(ii) Generate electricity as in generators and vehicles
ENGINE TYPES
There are many considerations for the various types of engines
(I) Power generation
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Intermittent power generation as in petrol and diesel engines
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Continuous Power generation as in
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(ii) Bore/Stroke Ratio
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Passenger car engine if Bore/stroke = 1
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Sport/racing cars and motorcycle engine if Bore/stroke > 1
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Long stroke engine if Bore/stroke < 1. These are not in use.
(iii) Compression Ratio, ε
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ε = (Stroke volume+ Clearance Volume)/clearance volume
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9 to 12 for spark ignition engines (Petrol engines)
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16 to 20 for compression ignition engines (Diesel engines)
(iv) Crank radius/Length of connecting rod = λ
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0.10 to 0.20 for large Marine engines
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0.25 to 0.35 for small and medium engines (commonly used engines)
(v) Number of crank revolutions for each power stroke
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RPM/2 for four stroke engines
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RPM/1 for two stroke engines
(vi) On the basis of these exert torque or thrust
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Engines which exert a torque are automobile gasoline engine, diesel engines and turbo-shafts.
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Engines which produce thrust include rockets and turbofans
(vii) Internal and external combustion
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Internal combustion engines like petrol, diesel and CNG engines
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External combustion engines like steam engines and steam turbines
(viii) Number of cylinders
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Single cylinder engines
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Multi-cylinder engines (up to 16 cylinders engines)
(ix) Number of strokes
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Two stroke engines
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Four stroke engines
(x) Engine mounting
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Front mounted
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Middle mounted
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Rear mounted
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Front mounted engines are very common
(xi) Number of doors
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Two doors
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Four doors
-
Convertible
SPARK AND COMPRESSION IGNITION ENGINES
COMPARISON OF SPARK IGNITION AND COMPRESSION IGNITION ENGINES
Item/Description |
SI ENGINE |
CI ENGINE |
Thermodynamic cycle used |
Otto Cycle |
Diesel Cycle |
Heat addition |
At constant volume |
At constant pressure |
Fuel used |
Petrol/Gasoline |
Diesel |
Ignition temperature |
Self Ignition temperature is high and needs a spark plug |
Lower ignition temperature, achieved by compression |
Carburetor/fuel pump |
Requires Carburetor and spark plug. Battery supplies the required voltage. |
Requires injector and fuel pump. |
Fuel air mixture |
Fuel-air mixture enters the engine during the suction stroke |
First compresses air alone. Then injected fuel directly into the combustion chamber |
Fuel regulation |
Controls fuel air mixture as per load on the engine |
Controls the fuel as per load. It does not control the flow of air as per load |
Fuel ignition |
Spark plug ignites the fuel |
Fuel ignites due to high temperature of compressed air |
Compression ratio |
7 to 10 |
15 to 20 |
Weight of vehicle |
Light weight vehicle. Low pressure ratio require thin parts |
Heavy weight vehicle. High pressures require thick parts |
Speed achieved |
High speed as Light weight vehicles |
Low speed as Heavy weight vehicles |
Thermal efficiency |
Low. Low compression ratio produces less power. |
High. High compression ratio produce more power. |
SI ENGINES |
CI ENGINES |
|
1. |
Air and fuel are premixed. |
Air alone is compressed first and then fuel is admitted. |
2. |
Mixture is compressed and then ignites with the help of a spark plug. |
Fuel ignites because of high temperature of compressed air |
3. |
These are more useful in small power engines. In these, power/weight and power/volume is high. It is in light compact vehicles. |
These are more useful in large power engines. In these, high power/weight and power/volume is not important. High thermal efficiency is more important. |
4. |
In these engines, premixed charge is throttled in spite of losses during throttling since full power is not always required. |
In these engines, no throttling is done. Power produced depends on control of fuel flow in the fixed mass of air. |
5. |
If the equivalence ratio (φ) (fuel air ratio) falls below 0.7, the mixture may misfire and may stop altogether. |
In these engines equivalence ratio is changing from zero in pure air to 1 for complete combustion. The fuel will ignite when required equivalence ratio is achieved. |
6. |
Burning of fuel is slow only for lean mixtures since here there is no delay period. |
In these engines, there is always a delay period. Because the fuel will mix first with compressed air to the required equivalence ratio. Then it ignites. |
7. |
Knocking chances are more. Mixture may explode. It may cause severe damage to the engine because of intense pressure waves. |
Knocking chances are in case of large delay period. |
8. |
Petrol engines have lower efficiency at all loads because of throttling losses. |
Diesel engines have higher efficiency at part load. Throttling losses are not there. High compression ratio increases efficiency at all loads. |
9. |
There is no such situation in petrol engines |
Under heavy load condition, it is possible that no air left for the fuel admitted. It gives heavy smoke because of incomplete burning under more power requirements |