CHAPTER 5 Principles of Engine Operation Two and

CHAPTER 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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Learning Objectives • Describe two-stroke engine operation and explain the principles of two-cycle operation. • List the advantages and disadvantages of twostroke and four-stroke engines. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Elementary Engine © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Four-Stroke Engine © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Intake Stroke • Piston travels downward • Volume of space above piston increases – Creates partial vacuum • Intake valve open and exhaust valve closed © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Compression Stroke • Piston moves upward • Both valves closed • Mixture is compressed – Force of combustion is increased © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Power Stroke • Both valves closed • Air-fuel mixture ignited • Burning action forces piston downward © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Exhaust Stroke • Intake valve closed and exhaust valve open • Rising piston pushes exhaust gases from engine © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Exhaust Stroke • Exhaust valve – Allow a streamlined flow of exhaust gases – Heat must be controlled © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Two-Stroke Engine (Kohler Co. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Two-Stroke Engine Operation (Rupp Industries, Inc. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Intake into Crankcase • Piston moving upward • Crankcase pressure drops • Intake port exposed • Intake air through carburetor pulls fuel and oil into crankcase © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Ignition-Power • Piston moving upward • Compresses air-fuel charge • Spark ignites air-fuel mixture • Piston driven downward – Creates crankshaft motion © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Exhaust • Piston moving downward • Exhaust port exposed • Exhaust gases expelled • Complete exhausting occurs when transfer ports are opened – New charge rushes in © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Fuel Transfer • Piston moving downward • Air-fuel charge in crankcase compressed • Transfer port opened – Compressed charge rushes through port © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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 © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Four-Stroke Engine vs. Two-Stroke Engine © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.