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 Cross-scavenged Loop-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

Why 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.

What 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.

What 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

What is valve overlap? Valve overlap is a condition in which both valves are open at the same time.

Why 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.

How many crankshaft revolutions are required to complete a two-stroke cycle? One

What 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

Glossary 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 center The lowest point of piston travel. Abbreviation is BDC. Compression ratio Volume of combustion chamber at end of compression stroke as compared to volume of cylinder and chamber with piston on bottom center.

Glossary 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-scavenged An 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 stroke Piston stroke during which the exhaust valve opens and the rising piston pushes the exhaust gases from the cylinder.

Glossary 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 stroke Piston 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.

Glossary Internal combustion Loop-scavenged engine Power stroke The burning of a fuel within an enclosed space. Loop-scavenged engine Engine in which the fuel transfer ports are shaped and located so that the incoming air-fuel mixture swirls. Power stroke Piston stroke that occurs when both valves are in the closed position and the force of combustion drives the piston downward.

Glossary 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 center The point at which the piston is at its upper most position on the compression stroke. Two-stroke engine An engine design that produces one power stroke for each revolution of the crankshaft.

Glossary Valve overlap An interval expressed in degrees where both valves of an engine cylinder are open at the same time.