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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

  • Intermittent power generation as in petrol and diesel engines

  • Continuous Power generation as in

  • (ii) Bore/Stroke Ratio

  • Passenger car engine if Bore/stroke = 1

  • Sport/racing cars and motorcycle engine if Bore/stroke > 1

  •  Long stroke engine if Bore/stroke < 1. These are not in use.

 

 (iii)   Compression Ratio, ε 

  •  ε = (Stroke volume+ Clearance Volume)/clearance volume

  • 9 to 12 for spark ignition engines (Petrol engines)

  • 16 to 20 for compression ignition engines (Diesel engines)

(iv)  Crank radius/Length of connecting rod = λ

  • 0.10 to 0.20 for large Marine engines

  • 0.25 to 0.35 for small and medium engines (commonly used engines)

    (v)   Number of crank revolutions for each power stroke

  • RPM/2 for four stroke engines

  •  RPM/1 for two stroke engines

(vi) On the basis of these exert torque or thrust

  • Engines which exert a torque are automobile gasoline engine, diesel engines and turbo-shafts.

  • Engines which produce thrust include rockets and turbofans

(vii)  Internal and external combustion

  • Internal combustion engines like petrol, diesel and CNG engines

  • External combustion engines like steam engines and steam turbines

(viii)  Number of cylinders

  • Single cylinder engines

  • Multi-cylinder engines (up to 16 cylinders engines)

(ix) Number of strokes

  • Two stroke engines

  • Four stroke engines

(x)  Engine mounting

  • Front mounted

  • Middle mounted

  • Rear mounted

  • Front mounted engines are very common

(xi)  Number of doors

  • Two doors

  • 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

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