The cylindrical device which forms the moving end of the cylinder. All power produced by the engine is transmitted as pressure applied to the working face of each piston.
Compresses air molecules before combustion.
Transfers the pressure of expanding gases to the crankshaft, forcing it to turn.
On a 2 stroke engine it acts as a valve by covering and uncovering cylinder intake and/or exhaust ports.
Inside each cylinder
Top of the Piston. Various shapes aid in creating air turbulence for combustion Confines each ring used to seal the space between the cylinder wall and piston Crown Ring Grooves Ring Lands Skirt Bosses Part of the piston above the top ring or between the ring grooves. Limit and support the piston rings in their grooves. Oil Drain Passages Permit oil to pass through the piston from the cylinder wall into the crankcase. Part of the piston between the the first ring groove above the piston pin hole, and the bottom of the piston. Reinforced openings in the piston body. Provide a means to connect the piston to the connecting rod.
Skirt is long enough to take up side thrust without scoring (scratching) the liner.
Trunk Type A two piece unit with a crown that can withstand the high heat and pressure of a turbocharged engine. The crown and skirt are held together by the piston pin. The downward load on the crown is directed onto the piston pin through a large bearing. Crosshead Type: Skirt is specifically designed to absorb side thrust. The skirt has less thermal distortion than the crown, and free of downward thrust. It guides the piston in the liner, takes up side thrust, and carries oil rings. The crown is subject to only a slight amount of side thrust.
Heat absorbed by the piston must be dissipated to prevent excessive metal temperature, and the carbonization of lubricating oils.
Piston Cooling Methods
Shaker: Piston Oil Cooling
A compartment in the piston head is supplied with oil from the drilled passage in the connecting rod by circulating through passages behind the rings.
The motion of the piston shakes the oil so it is spilled into channels or pipes which return it to the crankcase.
Oil from the connecting rod is sprayed through a nozzle against the underside of the piston.
Delivered from a jet at the bottom of the cylinder liner.
Seal the cylinder.
Prevent combustion pressure from entering the crankcase (blow-by).
Prevent oil from entering the combustion space.
Distribute oil to lubricate the cylinder walls
Transfer heat from the piston to the cylinder walls.
Rings fit in grooves around the piston crown and /or skirt.
Number and location will vary with type and size of piston
Ring gaps are staggered when installed to minimize blow-by.
Location General Classification of Rings
Principal function of the compression ring is to seal the cylinder and combustion space so gases cannot escape.
Made of gray cast iron
Some have inserts or facings for better seating (bronze or chrome)
Used for distribution, removal of excess oil, and control of oil film.
Note: Different manufacturers use a variety of terms in their Tech. Manuals to identify the oil rings, such as oil control, oil scraper, oil wiper, oil cutter, oil drain, oil regulating.
Furthest from the combustion chamber, Regulate the amount of oil passing between the skirt and cylinder wall.
Preventing excessive oil from entering the combustion chamber.
Oil Rings General Classification of Rings continued
The sharp edge on lower side, removes excess oil on the piston down stroke, the tapered upper side distributes the remaining oil on the piston upstroke.
If this ring is installed upside down, it will act as an oil pump rather than a scraper.
Oil Ring Expander
Basically a spring that is placed in the oil grooves of the piston prior to the oil rings being installed.
Usually hollow and made of alloy steel.
Machined, hardened, precision ground and lapped.
Some may be chrome plated to improve wearing properties.
Purpose: Connect the piston to the connecting rod.
Location: Ride on a carrier or boss inside the piston
Piston Pins/Wrist Pins Types
Secured to the piston at the bosses. Connecting rod oscillates on the pin.
Uses one bearing surface.
Secured in the middle of the connecting rod. Ends move freely in the bearing surfaces of the bosses.
Uses two bearing surfaces.
Full Floating Pins
Not secured at either the piston or connecting rod.
May be held in place with caps, plugs, snap rings or spring clips.
Pin rotates in both the rod and piston bosses.
Uses four bearing surfaces .
A bar or strut with a bearing at each end.
Usually forged from alloy steel.
I or H section to give it greater strength for its weight.
Upper end is attached to the piston by the piston pin.
Lower end is split so it can be fastened around the crankshaft.
Transmits reciprocating motion of the piston to the rotary motion of the crankshaft.
Transmits force of combustion on the piston to the crankshaft.
Transmits force to the piston from the crankshaft during the compression stroke.
In the bore of the cylinder, between the piston and the crankshaft.
Purpose Types of Connecting Rods
Conventional design : Most common type used.
Fork and Blade : Fork rod straddles the blade rod.
Hinged strap type : Variation of the fork and blade.
Forged steel with design determining the firing order for a given direction of rotation.
Converts the reciprocating motion of the piston and its connecting rod into rotary motion.
Supported by bearings below the block.
Block and main bearing caps are precision (line) bored.
Purpose Crank Journals (Main Bearing Journals) Induction hardened for durability and wear. Connecting rod journals Offset from the crankshaft centerline Orbit the centerline as shaft rotates. Machined to a highly polished finish. Mounted to the block by main bearing caps. Crank Throw 3 parts- 2 webs and a pin Provide attachment points for the connecting rods.
Construction Cont. Counter Balance Weights Heavy metal sections opposite the throws to offset the weight of the throws and connecting rods. Fine balancing is achieved by drilling the counterweight. Oil passages- Main bearings receive oil from the main oil galleries. Passages are drilled between main and connecting rod journals Rod journals receive oil from the drilled passages Flywheel Hub – allows a flywheel To be bolted to the crankshaft. Flywheel
a heavy wheel or disc for opposing or moderating speed fluctuations.
Purpose – helps the engine run smoothly by absorbing some energy of the power stroke and releasing it during the other strokes.
Some engines utilize the flywheel to install a starting ring gear, turning ring gear, or overspeed safety mechanism.
Location- firmly bolted to the hub of the crankshaft.
Multi-cylinder speed variations become less as the number of cylinders increases.
Cylinders are smaller and impulses more frequent.
Ends of the connecting rods, crank webs and crankpins have considerable weight and therefore have the same inertial effect as the flywheel.
Some engines the rotor of the generator serves as the flywheel.
Single cylinder engines require large flywheels to keep speed variations within limits. Vibration Dampers (Harmonic Balancer)
Purpose- operates to reduce the torsional (twisting) stresses on the crankshaft caused by the power strokes and loads on the engine.
Location- Free end of the crankshaft.
Two Basic Types
Usually incorporated with a fan pulley
Consists of a rubber ring, bonded to a heavy metal ring on one side, and a stamped metal disc on the other.
The rubber allows some flexing between the heavy ring and crankshaft to absorb vibration from the engine.
Consists of a heavy metal disc suspended in fluid inside a sealed drum.
Any movement of the internal mass is resisted by the fluid friction. This tends to dampen excessive torsional vibrations in the crankshaft.
Intake and exhaust valves are “Poppet Type”
Purpose- on 4 cycle engines the valves open and close to allow clean air to enter the cylinder and allow the exhaust of spent gases.
Some 4 cycle engines use 2 intake and 2 exhaust valves per cylinder.
2 cycle engines may have intake ports and exhaust valves or may have both intake and exhaust ports.
2 cycle engines using exhaust valves generally employ 2 or 4 valves per cylinder.
Location- Valves are located in the cylinder head assemblies.
Intake valves- constructed of carbon steel or low alloy steel.
Directly cooled by the air flowing past them
Exhaust valves- usually made of silicon-chromium steel or steel alloys.
High content of nickel and chromium in the steel or alloy for corrosion resistance.
Some exhaust valves use sodium as a cooling agent. At operating temperatures the sodium liquefies and splashes up and down in the hollow valve stem, transferring heat from the stem to the engine cooling system.
Valve Construction Sodium valves are effective but not commonly used.
Sodium can be highly explosive when it comes in contact with the atmosphere.
Seating edge of the face may be 30,45 or 60 degrees.
Seating angles are required to provide a positive seal.
Over half the heat a valve must dissipate, leaves through the valve face.
Face and seat surfaces may be hardened with a cobalt-tungsten alloy to resist thermal damage.
Fillet (neck): Constructed with the head and neck of a material which resists heat Hardened Tip: Reduce wear from rocker arms. Valve Springs
Made of highly tempered round steel wire,wound in a spiral.
Serve to close the valves
Provide sufficient force to overcome inertia of the valve assemblies caused by the rapid motion of the valve being lifted.
Springs surround the valve stems
Rotate the valve, preventing carbon buildup and hot spots that could damage the valve and seating surfaces.
May be installed above or below the valve spring, according to design requirements.
Valve rotators: not found on all engines. Valve Guides
Provide a guide and bearing for the valve stems.
Also aid in conducting heat from the stem to the water jacket which surrounds the guide.
Pressed into the cylinder head
Lock the valve spring retainers to the valve spring.
On top of the valve spring
Used to control the opening and closing of valves and fuel pumps/injectors
Camshaft: A long slender shaft with a number of projections called cam lobes.
The timing desired determines the shape of the lobes.
Lobes are elliptical so the valves are opened and closed gradually to avoid excessive inertial forces.
May be located low near the crankshaft, on the cylinder block or in/on the cylinder head.
Valve Actuating Mechanism Purpose
4 stroke engine- normally operates the intake and exhaust valves, fuel nozzles (spray nozzles), fuel injector pumps, fuel injectors or air starting valves.
2 stroke engine- operates the exhaust valves, fuel nozzles, fuel injector pumps, fuel injectors, or air starting valves
One end of the camshaft has a machined surface to which the camshaft drive gear is attached
Camshafts time the events of the intake, compression, injection, power and exhaust to the crankshaft.
Therefore the connecting drive must be positive, i.e. gear, chain or cogged belt drive.
In 4 stroke engines, a cycle of events occurs in 2 crankshaft revolutions, so the cam rotates at ½ the crankshaft speed.
Cam must rotate once for each cycle of events.
In 2 stroke engines a cycle of events occurs in one crankshaft rotation, so the cam and crankshaft rotate at the same speed.
Some V-engines use 2 camshafts to actuate the intake and exhaust valves. Other timing functions are divided between the two camshafts
Change the rotating motion of the camshaft to reciprocating motion to open the valves.
Hinged Roller type
Cam followers ride the cam and are raised by a section or the cam as the camshaft rotates.
transmit the motion of the cam and lifter to the rocker on the cylinder head.
Between the cam followers and the rocker arms.
Part of the valve operating mechanism which opens and closes the intake and or exhaust valves.
To actuate the valves through the use of push rod or cam followers and the camshaft.
Changes direction of motion only
On some engines a rocker arm operates the unit fuel injectors and fuel injection pumps.
Each rocker arm actuates either one or two valves.
Mounted on the head of an engine on a shaft, which acts as a pivot.
Make up a very important group of parts.
Support rotating shafts and other moving parts
Transmits load from one part to another.
Reduce friction between moving surfaces.
Dissipate heat produced by friction
Bearings 2 general groups
Bearings for rotary motion
Journal bearings (support radial loads)
Thrust bearings (support axial loads)
Combination (support radial and axial loads)
Bearings for reciprocating motion
Slipper type bearings
Same as precision type except babbitt is carried on over the edge of the shell, and is machined to give it a surface to absorb the axial thrust.
All sliding contact bearings will support radial loads or combinations of radial and axial loads.
Requires no fitting to the shaft.
May be split inserts which form a bushing when put together
May be of solid construction.
Classification by construction Thrust type
Identifies the group of parts which take power from the crankshaft and transmits that power to various engine components and accessories.
The drive mechanism does not change type of motion but it may change direction of motion.
Types of Drives
Consists of various helical gears arranged at one or both ends of the engine.
Most common type of drive mechanism.
Consists of a chain and sprockets.
Used on Fairbanks-Morse opposed piston diesel engines.
Modified to ensure positive engagement between the crankshaft and driven components.