BASIC PRINCIPLES OF 4- CYCLE & TWO CYCLE ENGINES BULLETIN 8793A

BRIGGS & STRATTON

PURPOSE Small internal combustion engine is an important power source for agricultural operations. In US alone, some 70,000,000 or more single-cylinder engines are in operation today.

SMALL ENGINE USES Mowers Chainsaw Post hole diggers Feed grinders Air compressors Brush cutters Rotary tillers sprayers Irrigation pumps Concrete equipment Generators Outboard boats Etc.

Internal vs external combustion engine Internal combustion engine obtained its power by burning fuel inside an engine. External combustion engine uses power from fuel burned outside the engine. Example: steam engine

EXTERNAL COMBUSTION ENGINE

ENGINES

Internal vs external combustion engine Both engines require air, fuel, and ignition to produce power in order to move a piston. –For external engine, steam is the moving force, whereas in, –Internal engine, an explosion occurs inside an enclosed area that exerts great pressure against a piston, forcing it to move.

Two types of Small Gasoline Engines 1. Four-stroke Cycle Engine 2. Two-stroke Cycle Engine

Four-stroke Gasoline Engine Crankshaft makes two complete revolutions for each cycle of events. Complete cycle is: 1.Intake stroke 2.Compression stroke 3.Power stroke 4.Exhaust stroke

Intake Stroke Piston moves downward Downward motion draws in a mixture of air/fuel into the cylinder. Intake valve is open Exhaust valve is closed

Compression Stroke Upward movement of the piston compresses the air/fuel mixture. Intake valve is closed. Exhaust valve is closed.

Power stroke The air/fuel mixture is ignited by an electrical spark creating the explosion to push the piston downward. Intake valve is closed Exhaust valve is closed

Exhaust stroke Piston moves upward, forcing burned gases out through the exhaust port. Exhaust valve is open. Intake valve is closed. Note: The exhaust valve closes at the end of the stroke, than the intake valve opens as the next cycle begins.

Two-stroke Cycle Engine To complete a cycle, the piston makes two strokes and the crankshaft turns one revolution. Stroke I – power stroke Stroke II – Compression stroke Exhaust & intake occurs between strokes

Stroke I Power Stroke –Piston moves downward from explosion –Reed valve in crankcase is closed. –Intake port is closed –Exhaust port is closed

Two-cycle Exhaust Piston continues to move downward under pressure. Exhaust port opens Intake port is still closed

Two-cycle Intake Piston continues to move downward. Intake port opens allowing fuel/oil mixture to enter into the cylinder

Stroke II - Compression Piston moves upward. Intake port is closed. Exhaust port is closed. Reed valve in crankcase opens allowing fuel/oil mixture to enter the crankcase. Fuel mixture is compressed in the cylinder.

Major Differences Four Stroke –Regular intake and exhaust valves –Oil is used as a lubricant only –Takes two revolutions of the crankshaft to complete a cycle. –Crankcase contains only the lubricant(oil) Two Stroke –Reed valve serves as a valve –Oil is mixed with gasoline to serve as a fuel and lubricant. –Takes only one revolution of the crankshaft to complete cycle –Crankcase contains oil and fuel mixture

Major Differences Four stroke –Four strokes of the piston complete one cycle. –Has valve lifters, valve springs, and camshaft. –Difficult to handle on very small equipment. –Starts easier. –Less power produced per unit of weight Two-stroke –Two strokes of the piston complete one cycle. –Has no valve lifters, valve springs, and camshaft. –Less difficult to handle on small equipment –Difficult to start. –More power produced per unit of weight

Major Differences Four stroke –Easy to lubricate –Fuel and oil are separate from each other. –Limitation – engine cannot be tilted due to oil in the sump. Two-Stroke –Lubricating oil is mixed with gasoline—less efficient in lubrication. –No limitation – engine can be tilted due to oil and fuel mixture.

Methods Used to Distinguish between a 4-cycle & a 2-cycle Check operator’s manual for crankcase capacity or the kind of oil needed. Check to see if engine has an oil sump (place to add oil or place to drain the oil) Check to see if engine has an exhaust muffler. (4-stroke is at the end of the cylinder – 2-stroke muffler is located in the center of the cylinder. Check compression – 4-stroke every other revolution, whereas, 2-stroke is on every revolution.

Primary Functions of Engine Operation There are 3 primary function for engine to operate. 1.Compression 2.Carburetion 3.ignition

Secondary Functions There are two basic secondary functions needed for engine operation. 1.Cooling 2.lubrication

What has to occur during compression? 1. No leakage can appear between the piston and cylinder walls. 2. First compression ring is fitted in a groove near the top of the piston so pressures can expand the ring on the cylinder wall. 3. Second compression ring is considered as a back-up, but also serves as a scraper ring. It scrapes the excess oil from the cylinder, preventing oil to accumulate in the cylinder

What has to occur during compression? 4. Perforated Oil ring, located below the compression rings, lubricates the cylinder wall on each stroke. Please note: All rings must meet engine specifications for engine performance.

How important are valves to an engine? Valves are one of the most important parts of engine. Primary function is the intake to allow fuel-air mixture and exhaust the burned residue. Engine operating at 3000rpm – two valves are opening and closing 1/50 of a second. Valves must seal well enough to withstand pressures up to 500 psi (pounds per square inch)

How important are valves to an engine? Exhaust valve are damaged by heat than any other part on the engine. Exhaust valve is subject to high temperatures as much as 2500 degrees on the head of a valve. Parts of a valve – head, margin, face, stem Please note: discard any valve that are burned, dished, necked

What is displacement? Displacement is the amount of air/fuel mixture to be compressed in a cylinder. Depends on the size of bore and the length of stroke.

Piston displacement formula Piston displacement = (bore)² X X stroke 4 Example of a 2” bore and 1-3/4” stroke 2 x 2 X X 1.75 = 5.5 cubic inch 4

Piston displacement formula Most small engines vary from 5:1 to 6:1 compression ratios. Generally, engines with high compression ratio is more efficient than one with a low compression ratio.

Carburetion The major functions of the carburetor are: 1.Atomize the fuel (reduce to a fine spray) 2.Mix air with the fuel 3.Vaporize the fuel 4.Deliver the mixture to the combustion chamber.

Best Operating Fuel Mixture 15 to 17 parts of air by weight to 1 part of gasoline.

What is the venturi? The venturi is the narrow part of the pipe in the carburetor. Accelerated air moves through the narrow part causing fuel to be drawn through the nozzle and atomized as a result of low pressure.

Types of fuel systems Two types of fuel systems. 1.Gravity 2.suction

Gravity Fuel system Fuel tank located above the carburetor. Fuel flows by gravitation to a carburetor bowl. A small air vent must be open on the tank cap, allowing air to move into the tank as fuel flows out.

Suction Fuel System Fuel is drawn from the tank through a tube that is located below the carburetor. Suction created by the venturi pulls gasoline from the fuel tank and a foot valve keeps it from flowing back into the tank.

Types of Governors 1. Air-vane governor – hand throttle increases or decreases the tension on the governor spring. The spring moves the flap which is attached to a hinge in the air current created by the fins of the flywheel. 2. Centrifugal governor controls the speed by a set of weights which moves outward for high speed and moves inward for low speed. This action causes the throttle to open and close.

Air Cleaners An engine running 3600 rpm, it is estimated that it takes in 390 cubic feet of air per hour. Regularly scheduled maintenance is needed for engine performance. Three type of air cleaners 1.Oil bath 2.Oil saturated polyurethane 3.Dry-type air filter element

Ignition Ignition system furnishes a spark at the proper time and strong enough to ignite the fuel-air mixture in the combustion chamber. Engine performs the best when the spark occurs just before the piston reaches TDC on the compression stroke.

Types of Ignition System 1. Battery 2. Magneto 3. Solid state

Battery System Battery system must include the following parts: –Battery –Ignition coil –Spark plug –Battery cables –Timer –Condenser –Cam –Ignition wires –Ignition switch –Breaker points –Starters –lights

Magneto Systems Use on small engines where no electrical load is necessary, except to ignite the fuel. Most economical to operate. Low maintenance costs.

Magneto Systems Most commonly used magnetos –1. flywheel –2. external

Magneto Systems Flywheel magnetos –Found on small engines –Magnets are imbedded in the flywheel –Magnets is made of a special alloy called “alnico”. –Poles of the armature are separated from the magneto by a small air gap.

Magneto Systems Armature coil –Primary windings (few turns of large wire) –Secondary windings (many turns of fine wire)

Magneto Systems Primary windings –Connected to the breaker points – other end connected to the frame of the engine. –Current flows through the primary coil as the moving magnets in the flywheel reach the poles. –Breaker points open-magnetic field collapses- sending about 100 volts to the coil.

Magneto Systems Secondary winding –Collapsing magnetic field cuts through the secondary coil increasing the current to several thousand volts.

SMALL ENGINE SPECIALITY TOOLS STARTER CLUTCH WRENCH

SMALL ENGINE SPECIALITY TOOLS FLYWHEEL PULLER

SMALL ENGINE SPECIALITY TOOLS FLYWHEEL WRENCH

SMALL ENGINE SPECIALITY TOOLS FLYWHEEL HOLDER

SMALL ENGINE SPECIALITY TOOLS TELESCOPIC GAUGES

SMALL ENGINE SPECIALITY TOOLS FEELER GAUGE

SMALL ENGINE SPECIALITY TOOLS SPARK PLUG GAP GAUGE

SMALL ENGINE SPECIALITY TOOLS COMPRESSION TESTER

SMALL ENGINE SPECIALITY TOOLS VALVE SPRING COMPRESSOR

SMALL ENGINE SPECIALITY TOOLS RING COMPRESSOR

SMALL ENGINE SPECIALITY TOOLS PISTON RING EXPANDER

SMALL ENGINE SPECIALITY TOOLS TORQUE WRENCH

SMALL ENGINE SPECIALITY TOOLS CYLINDER HONE

SMALL ENGINE SPECIALITY TOOLS GEAR PULLER

SMALL ENGINE SPECIALITY TOOLS VALVE GRINDING COMPOUND

SMALL ENGINE SPECIALITY TOOLS RING COMPRESSOR/WRENCH

SMALL ENGINE SPECIALITY TOOLS SNAP RING PLIERS

SMALL ENGINE SPECIALITY TOOLS VALVE SPRING COMPRESSOR

SMALL ENGINE SPECIALITY TOOLS BLADE BALANCER

SMALL ENGINE SPECIALITY TOOLS VALVE SEATING TOOL

SMALL ENGINE SPECIALITY TOOLS IGNITION TESTER

SMALL ENGINE SPECIALITY TOOLS PISTON GROOVE CLEANER