• Four strokes needed to complete operating cycle • Intake stroke • Compression stroke • Power stroke • Exhaust stroke • Two strokes occur during each crankshaft revolution • Two crankshaft revolutions complete one operating cycle Four-Stroke Engine Intake Stroke • Piston travels downward • Volume of space above piston increases • Creates partial vacuum • Intake valve open and exhaust valve closed • Atmospheric pressure forces air through carburetor, through intake valve port, and into cylinder • Intake valve must open and close at the correct time • Incoming air-fuel mixture cools valve during engine operation Compression Stroke
• Piston moves upward • Both valves closed • Mixture is compressed • Force of compression is increased • Pressure and temp of the gas increases Power Stroke • Both valves closed • Air-fuel mixture ignited • The pressure and temp of the mixture increases rapidly • Burning action forces piston downward and this power is transferred to the crank shaft. Exhaust Stroke • Intake valve closed and exhaust valve open • Rising piston pushes exhaust gases from engine. • The engine now gets ready to execute the next cycle of operation. Exhaust Stroke
• Exhaust valve • Allow a streamlined flow of exhaust gases • Heat must be controlled Four-Stroke Engines
• Valve Timing • Measured in degrees of crankshaft rotation • Varies with different engines • Valve overlap • Lubrication • Provided by oil in the crankcase • Splash and pump systems • Oil must be drained and replaced periodically Two-Stroke Engine
• Two strokes occur during each revolution of crankshaft • Advantages over four-stroke engine • Simpler in design • Smaller • Lighter • Adequate lubrication at extreme angles Two-Stroke Engine
• Cross-scavenged • Contoured piston head prevents air-fuel charge from going out exhaust port • Use reed valves or rotary valve • Loop-scavenged • Flat or slightly domed piston head • Transfer ports cause incoming air to swirl Two-Stroke Engine
(Kohler Co.) Two-Stroke Engine Operation
(Rupp Industries, Inc.) Intake into Crankcase
• Piston moving upward • Crankcase pressure drops • Intake port exposed • Intake air through carburetor pulls fuel and oil into crankcase Ignition-Power
• Piston moving upward • Compresses air-fuel charge • Spark ignites air-fuel mixture • Piston driven downward • Creates crankshaft motion Exhaust
• Piston moving downward • Exhaust port exposed • Exhaust gases expelled • Complete exhausting occurs when transfer ports are opened • New charge rushes in Fuel Transfer
• Piston moving downward • Air-fuel charge in crankcase compressed • Transfer port opened • Compressed charge rushes through port Four-Stoke Engine vs. Two-Stroke Engine
Actual and Ideal Cycles in Spark-Ignition Engines and Their P-v Diagram
V1
S.No Two-Stroke Engine Four-Stroke Engine In two-stroke engine thermodynamics In four-stroke engine thermodynamic cycle is completed in two strokes of the cycle is completed in four strokes of piston or in one revolution of the the piston or in two revolutions of 1 crankshaft. Thus there is one power the crankshaft. Thus, one power stroke for every revolution of the stroke for every two revolutions of crankshaft. the crankshaft.
Because of the above, turning moment Because of the above, turning 2 is more uniform and hence a lighter moment is not so uniform and hence a flywheel can be used. heavier flywheel is required.
Again, Because of one power stroke for Because of one power stroke for two every evolution, power produced for revolutions, power produced for same 3 same size of two-stroke engine is size of four-stroke engine is less or twice, or for the same power of the for the same power the engine is engine is lighter and more compact. heavier and bulkier. Because of one power stroke in one Because of one power stroke in two revolution greater cooling and revolutions lesser cooling and lubrication 4 lubrication requirements. Higher rate requirements. Lower rate of wear and tear of wear and tear in two-stroke in four-stroke engine. compared to four-stroke engine.
Two-stroke engines have no valves but Four-stroke engines have valves and valve only ports (some two-stroke engines 5 actuating mechanisms for opening and are fitted with convectional exhaust closing of the intake and exhaust valves. valve or reed valve).
Because of light weight and simplicity Because of comparatively higher weight and due to the absence of the valve 6 complicated valve mechanism, the initial actuating mechanism, initial cost of the cost of the four-stroke engine is more. two-stroke engine is less. Two-stroke engine is used where Four-stroke engine is used where low cost, compactness and light efficiency is important, viz., in cars, 7 weight are important, viz., in buses, tucks, tractors, industrial scooters, motorcycles, hand engines, airplane, power generation sprayers etc. etc.
Volumetric efficiency is lower due Volumetric efficiency is higher due to 8 to lesser time for mixture intake. more time for mixture intake.
Lesser thermal efficiency is lower; Higher thermal efficiency; part load 9 part load efficiency is poor. efficiency is better.