2 Stroke Engines
2 Stroke EnginesScavenging 2 Stroke Evolution Timeline Dugald
Clerk ( 1881)Separate Cylinder for Charging
Joseph DayPiston As Pump Crankcase Scavenged (3 Port Engine)
Alfred Angas Scott (1908)Practical 2Stroke engine used in Motor
Cycle
Why Study when Its BannedNo Engine Type Produced Great Interest
Very Simple to Design and ManufactureOptimizationOne form of engine
produces high power output even in low speeds used extensively in
ships Design is Not Limited
Air Capacity in Two Stroke4 stroke retains almost all the air
entering during intakeIn 2 strokeSince exhaust ports are open most
time some air escapes through itPower depends on air retained and
not on air enteredBasic DefinitionsDelivery RatioTrapping
EfficiencyScavenging EfficiencyDelivery RatioTrapping
EfficiencyScavenging EfficiencyTheoretical Scavenging
ProcessPerfect ScavengingPerfect MixingComplete Short
CircuitingPerfect ScavengeThe fresh air-fuel mixture remains
separated from burnt mixture Inlet charge completely pushes out the
outlet chargeNo MixingNo Loss of inlet chargePerfect MixingIncoming
charge mixes completely and instantaneously with cylinder
contentsThe out substance is the same as the cylinder contents This
models results approximates to many actual scavenging processUsed
as a basis of comparisonShort CircuitingDead LossCharge goes
directly out of the exhaust without replacing the residual burnt
products
Classification of Scavenging SystemsBased on Engine TypeCrank
case Scavenged EngineSeparately Scavenged EngineScavenging
ArrangementsBased on Air FlowCross Flow Loop FlowUniflowCross
FlowOriginal methodTransfer Port & Exhaust Ports are opposite
to each otherPiston DeflectorGood Low speed ScavengeGood Low speed
Power Characteristics
Cross FlowFor High Speeds Detonation occursHot Edged Piston Fuel
EconomyShort CircuitingUnburned hydrocarbons
Cross FlowManufacturing AdvantagesPacking Space is lessCylinder
to Cylinder spacing is less; compact
Loop ScavengingInvented by SchnurleTransfer Port and Exhaust
Ports are placed on same sideAir Flow is in a loopFlat top
PistonHot Running Piston is EliminatedCompact Combustion
ChamberRapid and Efficient Combustion
Loop Scavenging
M.A.N TypeSchnuerle TypeCurtis typeUni FlowRegarded Most
Efficient Fresh Charge Enters At one End and Exhaust goes out at
some other endSwirling Motion is ImpartedSwirl Produces Good
Combustion in Engines
Uni FlowVery efficient for Low speed Marine EnginesProduce good
thermal Efficiency for large engines ( eg: 1000 mm bore, 2500 mm
Stroke)Not suited for automobiles Suits better to CI than SI
Separately Scavenged EnginesUses Separate Compressor to supply
AirImproved Emission Characteristics Thermodynamically Efficient if
TurboCharger arrangement is usedRecently,Petrol Injection Technique
used with it showed lesser unburnt and smoke emissionsIdeal For
Diesel Engines but Only Draw back is the visible smoke
Standards
Separately Scavenged with Turbocharger and Fuel InjectionDesign
is Not Limited
