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As its name indicates the spark Ignition (SI) engine, uses spark to ignite the fuel. And in Compression Ignition (CI) engine, the air is compressed within the cylinder and the heat of this compression air is used to ignite the fuel. Since spark and compressed air is used to ignite the fuel in these engines, so they are called as spark ignition (SI) engine and compression ignition (CI) engine.
Let's See the major difference between SI engine and CI engine :-

S.noAspectsS.I EnginesC.I Engines
Otto Cyle 
 Diesel Cycle... For slow speed engines
 Fuel Used 
 Air Flow Ratio
10:1 to 20:1 
 18:1 to 100:1.
 Comparison Ratio
Up to 11; Average value 7 to 9; 
Upper limit of compression ratio fixed by
anti-knock quality of fuel. 
 12 to 24 ; Average value 15 to 18 ; Upper limit of compression ratio is limited by thermal and mechanical stresses.
 Spark Ignition
 Compression Ignition.
 Fuel Supply
By Carburetor...Cheap method 
 By injection.....explosive method.
 Operating Pressure
(i)Comparison Pressure 
(ii)Maximum Pressure

(i)7 bar to 15 bar 
(ii)45 bar to 60 bar  
 (i)30 bar to 50 bar (ii) 60 bar to 120 bar
 Operating Speed
High Speed; 2000 to 6000 rpm 
 Low Speed: 400 rpm.Medium Speed:400 to 1200 rpm.High speed: 1200 to 3500 rpm.
 Control Power
Quantity Governing... by throttle 
 Quality Governing... by rack.
 Calorific Value
44 MJ/KG 
 42 MJ/KG
 Cost Of Running
 Maintenance Cost
 Minor Maintenance is required 
 Major Overhaul required but less frequently.
 Limited by detonation. Used only in Air Craft Engines.
 Limited by blower power and mechanical and  thermal stresses.widely used.
 Two Stroke Operation
Less Suitable, fuel loss in scavenging.But small two stroke engines are used in mopeds, scooters and motorcycle due to their simplicity and low cost. 
 No fuels loss in scavenging. More suitable. 
 High Powers
Mopeds,Scooters,Motorcycle,Simple Engine passenger cars,Air Crafts etc.. 
 Buses,Trucks,Locomotives,Tractors,Earth Moving machinery and Stationary Generation Plants 

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