25Principles of Engine Operation, Two- and Four-Stroke Engines
3Learning 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.
4Learning ObjectivesDescribe two-stroke engine operation and explain the principles of two-cycle operation.List the advantages and disadvantages of two-stroke and four-stroke engines.
5Principles of Engine Operation Converts chemical energy into mechanical energyGasoline engine is an internal combustion engineGasoline must ignite easily and burn quicklyEnergy produced by burning gasoline must be controlled
6Gasoline Must Be Atomized The more surface area exposed to air, the more vapor will be given offMore vapor leads to faster burningGasoline must be atomizedAtomizationIncreased burning areaExplosive release of heat energy
8Two- and Four-Stroke Engines Engines identified by number of piston strokes required to complete one operating cycleEach stroke is either toward the rotating crankshaft or away from itBottom dead centerTop dead centerStrokes identified by job they perform
9Four-Stroke Engine Four strokes needed to complete operating cycle Intake strokeCompression strokePower strokeExhaust strokeTwo strokes occur during each crankshaft revolutionTwo crankshaft revolutions complete one operating cycle
11Intake Stroke Piston travels downward Volume of space above piston increasesCreates partial vacuumIntake valve open and exhaust valve closed
12Intake StrokeAtmospheric pressure forces air through carburetor, through intake valve port, and into cylinderIntake valve must open and close at the correct timeIncoming air-fuel mixture cools valve during engine operation
13Compression Stroke Piston moves upward Both valves closed Mixture is compressedForce of combustion is increased
15Exhaust Stroke Intake valve closed and exhaust valve open Rising piston pushes exhaust gases from engine
16Exhaust Stroke Exhaust valve Allow a streamlined flow of exhaust gases Heat must be controlled
17Four-Stroke Engines Valve Timing Lubrication Measured in degrees of crankshaft rotationVaries with different enginesValve overlapLubricationProvided by oil in the crankcaseSplash and pump systemsOil must be drained and replaced periodically
18Two-Stroke EngineTwo strokes occur during each revolution of crankshaftAdvantages over four-stroke engineSimpler in designSmallerLighterAdequate lubrication at extreme angles
19Two-Stroke Engine Cross-scavenged Loop-scavenged Contoured piston head prevents air-fuel charge from going out exhaust portUse reed valves or rotary valveLoop-scavengedFlat or slightly domed piston headTransfer ports cause incoming air to swirl
27Why is a gasoline engine considered an internal combustion engine Why is a gasoline engine considered an internal combustion engine? Because gasoline is combined with air and burned inside the engine.
28What is atomization and why is gasoline atomized What is atomization and why is gasoline atomized? Atomization involves breaking gasoline up into tiny droplets and mixing it with air. Gasoline is atomized to produce the rapid burning required in an engine.
29What are the four-strokes in a four-stroke cycle What are the four-strokes in a four-stroke cycle? Intake stroke Power stroke Compression stroke Exhaust stroke
30What is valve overlap? Valve overlap is a condition in which both valves are open at the same time.
31Why do two-stroke engines receive adequate lubrication even when operated at extreme angles? Because it receives its lubrication as fuel mixed with oil is passed through the engine.
32How many crankshaft revolutions are required to complete a two-stroke cycle? One
33What advantages does a two-stroke engine have over a four-stroke engine? A two-stroke engine: …is simpler in design than a four-stroke engine …is smaller and lighter than a four-stroke engine of equivalent horsepower …will receive adequate lubrication even when operated at extreme angles
34Glossary Atomization Bottom dead center Compression ratio Process of breaking gasoline into tiny droplets and mixing it with air to produce the rapid burning required in an engine.Bottom dead centerThe lowest point of piston travel. Abbreviation is BDC.Compression ratioVolume of combustion chamber at end of compression stroke as compared to volume of cylinder and chamber with piston on bottom center.
35Glossary Compression stroke Cross-scavenged Exhaust stroke Occurs as the piston moves upward in the cylinder with both valves closed. During this stroke, the air-fuel mixture is compressed into a smaller space.Cross-scavengedAn engine with a special contour on the piston head, which acts as a baffle to deflect the air-fuel charge upward in the cylinder.Exhaust strokePiston stroke during which the exhaust valve opens and the rising piston pushes the exhaust gases from the cylinder.
36Glossary Four-stroke engine Intake stroke Also known as Otto cycle. A combustion cycle that consists of an intake, a compression, a power; and an exhaust stroke. Also called four-stroke engine.Intake strokePiston stroke in which the piston travels downward in the cylinder with the intake valve open. This increases the volume above the piston and creates a partial vacuum that draws the air-fuel mixture into the cylinder.
37Glossary Internal combustion Loop-scavenged engine Power stroke The burning of a fuel within an enclosed space.Loop-scavenged engineEngine in which the fuel transfer ports are shaped and located so that the incoming air-fuel mixture swirls.Power strokePiston stroke that occurs when both valves are in the closed position and the force of combustion drives the piston downward.
38Glossary Stroke Top dead center Two-stroke engine Movement of a piston in the cylinder from one end of its travel to the other.Top dead centerThe point at which the piston is at its upper most position on the compression stroke.Two-stroke engineAn engine design that produces one power stroke for each revolution of the crankshaft.
39Glossary Valve overlap An interval expressed in degrees where both valves of an engine cylinder are open at the same time.