parts & operation
Internal Combustion Engine – Fuel is burned ‘Inside’ of the engine to produce power. Power is a direct result of burning the fuel.
External Combustion Engine – Fuel is burned ‘Outside’ of the engine, Such as a steam engine. Power is produced indirectly from burning fuel.
External Combustion Engine – Fuel is burned ‘Outside’ of the engine, Such as a steam engine. Power is produced indirectly from burning fuel.
An Engine converts up / down motion into rotary motion (similar to a bicycle). Four actions are required for an engine to operate.
The four stages required for engine operation are: Intake, Compression, Power and Exhaust. One engine cycle is all four stages. A four stroke engine does 1 stage, per stroke of piston. (A Two stroke engine does 2 stages per stroke of the piston). A two stroke requires only 2 piston strokes to complete its cycle (1 Rotation) a Four stroke engine requires 2 rotations to complete the Cycle.
2 Stroke Cycle
Stroke 1) Intake / Compression
Stroke 2) Power / Exhaust
- Take fuel in 2) Compress Fuel 3)Ignite Fuel 4)Expel burned gasses
The four stages required for engine operation are: Intake, Compression, Power and Exhaust. One engine cycle is all four stages. A four stroke engine does 1 stage, per stroke of piston. (A Two stroke engine does 2 stages per stroke of the piston). A two stroke requires only 2 piston strokes to complete its cycle (1 Rotation) a Four stroke engine requires 2 rotations to complete the Cycle.
2 Stroke Cycle
Stroke 1) Intake / Compression
Stroke 2) Power / Exhaust
Basic Four stroke operation
A four stroke engine has 2 valves. An Intake Valve, and an Exhaust Valve. Both are mechanically lifted by valve lifters that rest on lobes of the camshaft. Fuel and air are mixed inside of the carburetor before entering the engine through the intake valve.
Intake Stage
Exhaust Valve is closed. Intake valve is open. Piston moves down creating a vacuum that draws fuel & air into the engine from the carburetor
Compression Stage
When piston reaches BDC, Both valves close. The combustion chamber is now sealed. Piston moves upward, compressing the fuel and air mixture into the combustion chamber.
Power Stage
Near, at, or slightly after TDC, the ignition system produces a spark igniting the compressed fuel mixture. With both valves closed, the piston is forced down by the rapidly expanding exhaust gases caused by the explosion.
Exhaust Stage
During the power stage, the exhaust valve slowly opens as the piston reaches BDC. At BDC The exhaust valve is open completely. The piston now moves upward moves the exhaust gases out of the engine through the exhaust valve.
Intake Stage
Exhaust Valve is closed. Intake valve is open. Piston moves down creating a vacuum that draws fuel & air into the engine from the carburetor
Compression Stage
When piston reaches BDC, Both valves close. The combustion chamber is now sealed. Piston moves upward, compressing the fuel and air mixture into the combustion chamber.
Power Stage
Near, at, or slightly after TDC, the ignition system produces a spark igniting the compressed fuel mixture. With both valves closed, the piston is forced down by the rapidly expanding exhaust gases caused by the explosion.
Exhaust Stage
During the power stage, the exhaust valve slowly opens as the piston reaches BDC. At BDC The exhaust valve is open completely. The piston now moves upward moves the exhaust gases out of the engine through the exhaust valve.
Basic Two Stroke Operation
Two stroke engines are great in small power equipment because they have a good power to weight ratio. Two stroke engines used to be very popular in the power sports industry as well. They are still desired by some for their light weight and power ratio. They are also simpler in design than a four stroke engine and have less moving parts.
A two stroke engine has no mechanical valves in the combustion chamber. Instead the cylinder has ‘ports’ that are covered and uncovered by the piston. In a way, the piston acts like a valve. A two stroke engine takes in fuel through the crank case, not directly into the cylinder. They fuel mixture enters the combustion chamber when the piston opens the intake port. The four stages of operation overlap in a 2-cycle engine. All four stages happen within 2 piston strokes (1 revolution). This means a two stroke has a power stroke on every rotation of the crank shaft.
Intake / Compression Stage
As the piston moves upwards, it creates a vacuum drawing air/fuel into the crank case. The vacuum opens a reed valve inside the crank case. While the piston is moving upward, it is also compressing the fuel / air mixture that was previously drawn into the combustion chamber during the end of the exhaust stroke.
Power / Exhaust Stage
Near TDC, the spark plug will ignite the compressed mixture and the piston will be forced downwards. As the piston moved down, pressure is built up in the crank case, the reed valve is closed and pressure continues to built inside the crank case. The piston open the exhaust port first, and slightly after the intake port begins to open. When the intake port opens, pressurized air / fuel mixture rushes into the combustion chamber pushing out remaining exhaust gases.
A two stroke engine has no mechanical valves in the combustion chamber. Instead the cylinder has ‘ports’ that are covered and uncovered by the piston. In a way, the piston acts like a valve. A two stroke engine takes in fuel through the crank case, not directly into the cylinder. They fuel mixture enters the combustion chamber when the piston opens the intake port. The four stages of operation overlap in a 2-cycle engine. All four stages happen within 2 piston strokes (1 revolution). This means a two stroke has a power stroke on every rotation of the crank shaft.
Intake / Compression Stage
As the piston moves upwards, it creates a vacuum drawing air/fuel into the crank case. The vacuum opens a reed valve inside the crank case. While the piston is moving upward, it is also compressing the fuel / air mixture that was previously drawn into the combustion chamber during the end of the exhaust stroke.
Power / Exhaust Stage
Near TDC, the spark plug will ignite the compressed mixture and the piston will be forced downwards. As the piston moved down, pressure is built up in the crank case, the reed valve is closed and pressure continues to built inside the crank case. The piston open the exhaust port first, and slightly after the intake port begins to open. When the intake port opens, pressurized air / fuel mixture rushes into the combustion chamber pushing out remaining exhaust gases.
Power & Speed
The speed of the engine is controlled by the amount of air / fuel mixtures allowed to enter the combustion chamber. More mixture = more speed. Less mixture = less speed. The power output of the engine increases as the crankshaft speed increases. The amount of fuel mixture entering the combustion chamber is controlled by a valve in the carburetor (throttle).
Idle Speed – When an engine is operating at its lowest operating speed. Idle speed will range from around 500 - 1200RPM. Idle speed will vary from one engine to another. Full speed of an engine will range from 3000 – 7000RPM. Some engines will operate slower or faster as well.
Engine Components
Blower Housing – Usually a metal cover mounted on the top of an engine covering the flywheel. The blower housing serves many purposes and has many important functions. And the engine turns the flywheel acts as a fan, the blower housing directs the air flow to effectively cool the engine. On many engines the blower housing also consists of the starting system (recoil or pull start). This housing protects the flywheel and some components from exposure, and also protects the user from exposed parts.
Cylinder – The proper name for the cylinder is the ‘Bore’. Inside of the bore is called the cylinder wall. The cylinder is machined to a precise diameter to fit the piston. The inside of the bore is a very smooth surface so that the piston and rings can slicer very easily.
Cylinder Block – The most basic part of any engine. All components mount inside, or onto the cylinder block. The block is usually made from cast aluminum and is cast in a mold to form the shape. The exterior of the block may have fine to dissipate heat. Aluminum alloy fins dissipate heat very well. Note that liquid cooled engine may not have fins on the block. Most small engine have a single cylinder. Some engines have many cylinders. Multi cylinder engines are usually classified by the position of the cylinders. The three most common types are In line, V and opposed configurations.
Cylinder Head – Bolts onto the top of the cylinder with a head gasket to form the combustion chamber. The head has a threaded hole for the spark plug. The shape of the head will change from engine to engine depending on the valve configuration. Three common types
L =Side Valve Engine. Both valves are on one side of the engine. The valves open upward. Very common on small engines
T = Intake and exhaust valves are on opposite sides of the cylinder. Rare on small engines.
I = Both Valves are located at the top of the cylinder head and open downward. This style is used for Over Head Valve configurations (OHV)
L =Side Valve Engine. Both valves are on one side of the engine. The valves open upward. Very common on small engines
T = Intake and exhaust valves are on opposite sides of the cylinder. Rare on small engines.
I = Both Valves are located at the top of the cylinder head and open downward. This style is used for Over Head Valve configurations (OHV)
Head Gasket – A head gasket is placed between the cylinder and the cylinder head to ensure a proper seal. The head gasket is required to hold pressure inside of the combustion chamber and must be able to withstand high temperatures. In a liquid cooled engine, the head gasket must also keep water, or coolant out of the combustion chamber.
Crankcase – May be molded as part of, or bolted onto the bottom of the cylinder. The crankcase must enclose, and support the crank shaft. In some engines, the crank case also holds the engines lubricating oil and acts as a reservoir. It is sometimes called an oil sump. The crankcase must also be sealed (with the exception of vents) to prevent oil leaking out, and contamination coming in. In a two stroke engine the crankcase must seal to hold pressure.
Piston – The Piston is a can shaped cylinder made from cast steel or aluminum. The Piston fits inside of the cylinder. The top of the piston is called the head, or dome. It is mounted to the connecting rod via a wrist pin and fastened in place by retaining clips. The sides / bottom of the piston are called the piston skirt. The piston will have grooves on the upper portion for ‘rings’. Between the grooves are ‘lands’ A piston usually has between 1 and 3 rings. The top ring(s) are for compression. The bottom ring is sometimes an oil ring. An oil control ring will only be present on a 4-stroke engine. Many pistons have a pin inside of the ring groove to keep the ring from rotating.
Connecting Rod – The connecting rod connects the piston to the crank shaft. The connecting rod will have a bearing on each end. The piston is attached with a wrist pin or piston pin and secured via cir clips. The connecting rod can be a single, or two piece unit. On a two piece unit, the bottom part of the connecting rod is removable, and is called a connecting rod cap.
Crankshaft – This is the main rotating part of the engine located inside of the crank case. RPM (Revolutions Per Minute) is a reference to crankshaft speed. This part converts the up / down motion of the piston into a rotary motion and also drives all times functions with the piston (valves, spark…) A one piece crank shaft requires a two piece connecting rod. The connecting rod is clamped around the crank pin (or journal). The entire crank pin structure is called a throw. The crankshaft is equipped with heavy counter weights to balance it. A ‘built up crankshaft’ is a crankshaft that easily comes apart. This style crankshaft may use a one piece connecting rod. The crank pin goes through a hole in the crankshaft, through a roller bearing, and out the other side, fastening the connecting rod.
The crank shaft is always at 90 degrees to the cylinder. Engines come with three different operating positions, Horizontal, Vertical, and Multi-positional. In a horizontal shaft engine, the crank shaft goes from side to side. This style of engine is found in cars, some lawn tractors, garden tillers. A vertical shaft engine has the crank shaft going up and down and are most often seen is push lawn mowers, ice augers, outboard marine engines. Multi – Position engines can run in either orientation. These engines are used in applications where the engine must operate on steep angles, or even upside down. A chain saw is a good example of a multi position engine. 2-cycle engines often meet this requirement as they have no engine oil in the crank case.
The crank shaft is always at 90 degrees to the cylinder. Engines come with three different operating positions, Horizontal, Vertical, and Multi-positional. In a horizontal shaft engine, the crank shaft goes from side to side. This style of engine is found in cars, some lawn tractors, garden tillers. A vertical shaft engine has the crank shaft going up and down and are most often seen is push lawn mowers, ice augers, outboard marine engines. Multi – Position engines can run in either orientation. These engines are used in applications where the engine must operate on steep angles, or even upside down. A chain saw is a good example of a multi position engine. 2-cycle engines often meet this requirement as they have no engine oil in the crank case.
Bearings – Bearings come in many styles and sizes and are used to support engine parts that rotate, or slide against each other, absorb heat, and reduce friction. Bearings are highly resistant to corrosion and scoring. The most common types of bearings are ball bearings, roller bearings, and needle bearings. Bearings are typically made of two rings called races. An inner race fits inside of the outer race. Very hard metal balls roll between the races. Some needle bearings do not have an inner race. A needle bearing is thinner and longer than a roller bearing.
Ball bearings are often used to support crankshafts, camshafts and transmissions. Needle bearings are found in connecting rods and sometimes transmissions.
Plain bearings are thin metal disks that are hard on the outside (steel) and softer on the inside (Aluminum, lead, zinc.) Plain Bearings can be a one or two piece design. A two piece design is called a “split sleeve bearing” and is sometimes used with a two piece connecting rod.
Bushings handle low speed load very well and are usually single piece construction often made of bronze and are often used as valve guides. Bushings may or may not require lubrication. Most bearings do require lubrication; many have small holes in the exterior so that lubricant can reach internal components.
Ball bearings are often used to support crankshafts, camshafts and transmissions. Needle bearings are found in connecting rods and sometimes transmissions.
Plain bearings are thin metal disks that are hard on the outside (steel) and softer on the inside (Aluminum, lead, zinc.) Plain Bearings can be a one or two piece design. A two piece design is called a “split sleeve bearing” and is sometimes used with a two piece connecting rod.
Bushings handle low speed load very well and are usually single piece construction often made of bronze and are often used as valve guides. Bushings may or may not require lubrication. Most bearings do require lubrication; many have small holes in the exterior so that lubricant can reach internal components.
Flywheel – A Heavy, usually cast metal disk attached to the crankshaft. The flywheel has a tapered hole that fits onto the tapered end of the crankshaft. On small engines, the flywheel is located underneath of the blower housing. The flywheel performs several important functions. It is an important component of the starting system in all engines and it acts as a weight carrying momentum between power strokes. In air cooled engines, the flywheel has fins on it that move air like a fan as it rotates, cooling the engine. On many engines, the flywheel has magnets on it, and is part of the ignition system.
Camshaft – A turning shaft that operates the intake and exhaust valves. This part is not found in a two stroke engine. Cam lobes are positions along the camshaft and there is one lobe per valve. As the Camshaft rotates, the lobes lift the valves. The camshaft is by the crankshaft via gears, chain, or a belt. Regardless of the drive style, the camshaft turns half the speed of the crankshaft (one revolution per two crankshaft revolutions.) In an OHV engine the camshaft typically operates a push rod. The pushrod moves a rocker arm that operates the valves. In an overhead camshaft engine (OHC) the camshaft is located in the head and operates rocker arms directly. OHC is usually driven by a timing chain.
Valves – Intake and exhaust valves are called poppet Valves and are made from a high grade of steel. The top portion of the valve is called the head of the valve. The flat section under the head is called the margin. The valve face is the sealing surface under the margin. The valve seat is where the valve seals, and is located on the cylinder block, or the cylinder head depending on valve configuration. The intake valve is larger than the exhaust valve to allow better air flow into the engine. The exhaust is forced out by the piston, so a larger port is not necessary. This also keeps the exhaust valve cooler. The longest part of the valve is called the stem. A valve guide is in place to prevent contact with cylinder and is made of a softer metal. It must also be lubricated. A Valve lifter or tappet rests on the cam lobes. A valve stem is placed directly on cam lobes.
The period of time a valve is open for is called duration and is measured in degrees of crankshaft rotation, not time. Vertical valve travel is called lift. At the beginning of the int6ake stages both valve will be open slightly to help expel all exhaust gas. This is called valve overlap.
The period of time a valve is open for is called duration and is measured in degrees of crankshaft rotation, not time. Vertical valve travel is called lift. At the beginning of the int6ake stages both valve will be open slightly to help expel all exhaust gas. This is called valve overlap.
Valve Spring – Each valve has a spring. The valve spring serves two purposes. It keeps valves closed and ensures a tight seal as well as returning valves to the proper position after they have been opened. The Valve spring also keeps pressure on the camshaft to prevent floating. Floating reduces performance as valves do not close completely. Floating may also cause damage to the engine.
Muffler – Part of the exhaust system in place to reduce noise. It is filter like, and made of metal. The muffler may bolt or thread onto the engine. On some equipment it may be inline on the exhaust pipe.
Starter – Many different starter options are available. The main job of the starting system is to turn the engine fast enough to start the stages of operation. The engine must be turned over fast enough to create adequate compression, draw fuel into the cylinder, and create a spark from the ignition system. Many push mowers, weed eaters, or chain saws use a pull start system also called a rope rewind starter. Larger equipment such as lawn tractors, cars, many ATVs, and marine engines use an electric start system. Some power sports equipment will use a kick starter.
Rope Rewind Starter – This is one of the most common, and most reliable starting systems. This system does not require the use of a battery or electric motor to start the engine and is even used on equipment that has electric start for back up in emergency situations. A handle is attached to the rope for ease of use. When the rope is pulled pawls engage the crankshaft and the engine begins turning. As the rope is pulled, the recoil spring is wound tight. When you let go of the handle, the recoil spring pulls the rope back into the starter housing and the pawls are disengaged. This occurs whether or not the engine is started.
Electric Start – In an electric start system, a battery is used to power a small electric motor that turns the crankshaft. Most electric start systems operate on 12 Volts. Usually on an electric start engine, the outside of the flywheel will have teeth on it. The electric motor will have a moveable gear with teeth on it that engage the flywheel. When the electric motor begins to turn, the gear is pushed forward to make contact with the flywheel, when the power is removed from the electric motor the gear retracts so it does not make contact with the flywheel while the engine is running. The start will always operate at a gear reduction. This means that the starter motor is always turning faster than the engine it is starting. This gear reduction helps the motor have torque it needs to turn the engine.
Carburetor – The carburetor mixes fuel with air at the appropriate ratio for combustion. A ratio of approximately 15:1 (air to fuel) is desired. A ratio that has more fuel than the desired ratio is considered ‘rich.’ Any time the mixture has more air, or not enough fuel it is considered to be ‘lean’. Running a mixture that is to rich or lean will affect engine performance or cause the engine to stop running. Both lean and rich mixtures have affects that may cause permanent damage to the engine if operated for extended periods of time. A Gravity fed carburetor is located underneath the fuel tank and no external fuel pump is in place. Engines that locate the carburetor above the fuel tank, such as an outboard engine, will often have fuel pumps built into the carburetor. If the fuel is located far away from the carburetor, an electric, or mechanically driven fuel pump may be installed.
Float & Needle Valve – The float and needle valve are located inside of the carburetor bowl. As the bowl fills up with fuel, the float will rise. The float operates a needle valve. When the bowl is full the float ‘floats’ and pushes the needle valve closed, stopping the flow of fuel into the carburetor. As the engine uses fuel, and float will drop and allow more fuel into the carburetor. The function of these parts is very important. If fuel were allowed to flow freely, the carburetor will become ‘flooded’. If the flow were allowed to continue endlessly, the engine would fill up with fuel, causing a hydro lock, and fuel will begin coming out of the air intake.
Speed Governor – All engines are required to wok under varying load conditions. Small engines have the largest variations, and most frequent changes of work load. The governor varies the throttle position to keep the engine operating at a certain RPM as the load changes. If the engine slows down, it opens the throttle to increase engine speed. As the engine reaches the desired RPM, the governor begins to close the throttle to eliminate over-revving. Without a governor, if the throttle was in a fixed position, the engine would run to fast when no load is applied, or the engine would lack power or even stall under heavy load. Small engines usually come with one of two governor styles, an Air Vane Governor, or a Mechanical Governor.
These types of governors are not used in automobiles, or power sports equipment. In a car, the governor is electronic, controlling either spark, or fuel (in a fuel injected engine.) In a car, the governor is most often used to limit the travel speed of the vehicle, not the engine RPM. Power sports equipment will usually only have a ‘rev limiter’ that only kicks in if the engine gets too close to a pre-determined speed that may damage the engine.
These types of governors are not used in automobiles, or power sports equipment. In a car, the governor is electronic, controlling either spark, or fuel (in a fuel injected engine.) In a car, the governor is most often used to limit the travel speed of the vehicle, not the engine RPM. Power sports equipment will usually only have a ‘rev limiter’ that only kicks in if the engine gets too close to a pre-determined speed that may damage the engine.
Air Vane Governor – An Air Vane Governor is mounted near the fly wheel. A spring keeps some tension on the governor to keep the throttle open. As the RPM of the engine increases, air flow from the fins on the flywheel begins to push the governor closed. As the engine slows down, air flow is reduced and the spring returns the governor to the open position. When the engine is off, the governor holds the throttle in the wide open position.
Mechanical Governor – This governor is located inside the engine and operates from centrifugal force. As the engine increases in speed, weights are pulled outward and the position of the governor changes to close the throttle. As the speed decreases, the weights are returned to their natural position and the throttle opens.
Lubrication Systems – Moving parts inside the engine require lubrication. Engine oil must lubricate the internal components. Most four stroke engines have an oil sump in the crank case. In small engines, you may find splash lubrication systems, or pressurized lubrication systems. A splash lubrication system may use an oil slinger, or dipper to distribute oil around the engine. A pressurized lubrication system uses an oil pump of some kind.
Two stroke engines to not have a sump of oil in the crank case. Instead Oil is mixed with the gas. Because the fuel mixture is drawn through the crank case, this lubricates most internal components. These systems are called ‘pre-mix’. Oil and gas are usually mixed at ratios between 40:1 and 100:1 in pre-mix applications. Some 2 stroke engines have an oil injection system. In this system a mechanically driven pump pumps oil to the carburetor or directly to an oiling point in the engine. Once it reaches the carburetor it is mixed with fuel and drawn into the engine. Some oil injection systems offer variable rate oiling. This increases performance by reducing the ratio to 80:1 at an idle, and increasing it to 40:1 at wide open throttle. This increases low speed performance and increases high speed protection. The most current 2 stroke engines that feature oil injection have direct oiling ports. In these engines very little oil is mixed with the fuel, instead the oil is injected directly where lubrication is required at various locations inside the engine.
Two stroke engines to not have a sump of oil in the crank case. Instead Oil is mixed with the gas. Because the fuel mixture is drawn through the crank case, this lubricates most internal components. These systems are called ‘pre-mix’. Oil and gas are usually mixed at ratios between 40:1 and 100:1 in pre-mix applications. Some 2 stroke engines have an oil injection system. In this system a mechanically driven pump pumps oil to the carburetor or directly to an oiling point in the engine. Once it reaches the carburetor it is mixed with fuel and drawn into the engine. Some oil injection systems offer variable rate oiling. This increases performance by reducing the ratio to 80:1 at an idle, and increasing it to 40:1 at wide open throttle. This increases low speed performance and increases high speed protection. The most current 2 stroke engines that feature oil injection have direct oiling ports. In these engines very little oil is mixed with the fuel, instead the oil is injected directly where lubrication is required at various locations inside the engine.
Transmission – A Transmission is used to convert the output speed of an engine. Transmissions are rarely seen in equipment such as push mowers, chain saws, and weed eaters, but are more common in lawn tractors, snow throwers, and garden tillers. Most often, a transmission is used to create a gear reduction. A gear reduction takes the fast moving crankshaft speed, and slows it down to a more suitable speed. (The drive wheels on a lawn tractor). A gear reduction also increases the torque output at the transmissions. If you have an engine operating at 4000RPM and running a 2:1 gear reduction the output of the transmission will be half of the engine speed (2000RPM).
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