5 Principles of Engine Operation, Two- and Four-Stroke Engines

Learning Objectives Explain simple engine operation. Explain why gasoline is atomized in the small engine. Describe four-stroke engine operation and explain the purpose of each stroke. Explain the concept of valve timing. Compare the lubrication system in a four-cycle engine to the system in a two-stroke engine.

Learning Objectives Describe two-stroke engine operation and explain the principles of two-cycle operation. List the advantages and disadvantages of two-stroke and four-stroke engines.

Principles of Engine Operation Converts chemical energy into mechanical energy Gasoline engine is an internal combustion engine Gasoline must ignite easily and burn quickly Energy produced by burning gasoline must be controlled

Gasoline Must Be Atomized The more surface area exposed to air, the more vapor will be given off More vapor leads to faster burning Gasoline must be atomized Atomization Increased burning area Explosive release of heat energy

Elementary Engine

Two- and Four-Stroke Engines Engines identified by number of piston strokes required to complete one operating cycle Each stroke is either toward the rotating crankshaft or away from it Bottom dead center Top dead center Strokes identified by job they perform

Four-Stroke Engine 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

Intake Stroke 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 combustion is increased

Power Stroke Both valves closed Air-fuel mixture ignited Burning action forces piston downward

Exhaust Stroke Intake valve closed and exhaust valve open Rising piston pushes exhaust gases from engine

Exhaust Stroke Exhaust valve Allow a streamlined flow of exhaust gases Heat must be controlled

Four-Stroke Engines Valve Timing Lubrication 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 (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

Scavenge Loss Occurs when a significant portion of the air-fuel charge flows out through the open exhaust port Scavenge loss results in increased hydrocarbon emissions and reduced fuel economy

Stratified Scavenge Engine Two-cycle engine that eliminates scavenge loss Extra intake port and a specially shaped piston introduce fresh air into the transfer port ahead of the air fuel charge Fresh air flows into the cylinder before the air-fuel charge, pushing the spent exhaust gases from the cylinder and preventing the air-fuel charge from escaping through the exhaust port

Four-Stroke Engine vs. Two-Stroke Engine