6 Engine Components

Learning Objectives Identify the basic components of a small engine and describe the function of each component. Describe engine block variations. Describe the construction and operation of the crankshaft. Explain piston design considerations and differentiate between types of piston rings. Describe connecting rod and bearing variations. Identify common valve train configurations.

Engine Block Keeps engine parts in alignment Cast iron or aluminum alloy Consists of two sections: Cylinder block Crankcase

Cylinder Block Contains cylinder bore Cylinder may be bored directly in block Aluminum cylinder blocks cast around steel sleeve Cylinder head bolts to block or is cast as integral part Cooling fins on block and head of air-cooled engines

Crankcase Contains crankshaft Must withstand rotational forces of the crankshaft May contain oil for lubrication May be cast with engine block

Crankshaft Major rotating part of engine Converts reciprocating motion into rotary motion Transmits engine torque Drives camshaft Supports flywheel

Crankshaft Cast or drop forged steel Tapered end receives flywheel Flywheel keyed to crankshaft Crankshaft throw Offset portion of shaft Counterweights offset unbalance

Crankshaft Main Bearings Supports crankshaft in block Three types: Sleeve or bushing Roller bearing Ball bearing

Crankshaft Main Bearings

Crankshaft Main Bearings

Crankcase Seals Prevent leakage Shell makes fixed contact with crankcase Knife edge rubs crankshaft Coil spring keeps sealing lip in contact with shaft

Crankcase Seal

Pistons Straight line driving member Subjected to heat of combustion Adequate clearance Seal between combustion chamber and crankcase Piston rings exert tension on cylinder wall

Piston Construction Steel or aluminum Grooves accept piston rings Aluminum most popular Grooves accept piston rings Lands are full-diameter ridges between grooves Holes in bottom groove allow oil to flow back to sump Hole for piston pin (wrist pin) Pin boss surrounds the piston pin hole Piston skirt keeps piston from tipping

Piston Construction

Cam Ground Piston Oval shape Thrust surfaces fit more closely Helps prevent piston slap

Piston Head Runs hotter and expands more than skirt Smaller in diameter than skirt Shape depends on application Four-stroke engines Flat Domed Wedge Two-stroke Flat with loop-scavenged engine Raised baffle or deflector with cross-scavenged engine

Piston Rings Allow piston to compress fuel charge Prevent burning gases from leaking Ride on cylinder walls Separated by thin film or oil Rub sides of piston grooves Grooves hold rings squarely to bore

Piston Rings Four-stroke pistons use three rings Two upper rings are compression rings Lower ring is oil control ring

Compression Rings Provide strong seal Keeps compressed air-fuel mixture and burning gases above piston Prevents passage between piston and cylinder wall Twists in groove during intake stroke In tipped position during compression and exhaust strokes Flat against cylinder wall on power stroke

Compression Rings

Oil Control Rings Remove surplus oil from cylinder walls Light scraping action Slotted or perforated Oil passes through holes in ring and groove Oil flows to inside of piston and into crankcase

Piston Ring Construction Cast iron or steel Plated with long-wearing material Outside diameter slightly larger than cylinder bore Creates ring tension Ring gap Opening between ends of ring Allows rings to expand and contract

Piston Ring Movement Free to move inward or outward in piston grooves Floating rings Rotate around grooves Installed with end gaps staggered Pinned rings Held in position by pin in ring groove Pin prevents rotation of ring around grooves

Piston Pins Secure piston to connecting rod Case-hardened steel Solid or hollow Full-floating pin Retained by snap rings Press-fit pin Fits tightly in connecting rod

Connecting Rod Attaches piston to crankshaft Upper end accepts piston pin Lower end contains bearings and fits around crankshaft journal

Friction Bearings Used when lower end of connecting rod is split Types Rod metal Bearing bronze Removable precision inserts Locating tab Fits into slot in rod

Antifriction Bearings Rollers or balls reduce friction Elements held together by roller cage or can be free Hardened and ground to exact size Must fit accurately and have clearance for expansion

Intake and Exhaust Ports Provide means for admitting air-fuel mixture and exhausting waste products Two-stroke engines Porting of cylinder walls Alternately covered and exposed by piston Four-stroke engines Poppet valves open and close port opening

Reed Valves and Rotary Valves Control fuel flow directly into crankcase of two-stroke engine Reed valve Opens during compression stroke and closes before the power stroke Rotary valve Attached to end of crankshaft Air-fuel charge enters crankcase when holes in valve and wear plate align

Reed Valve Action

Rotary Valve Action (Evinrude Motors)

Poppet Valves Installed in valve ports of four-stroke engines Control flow of air-fuel mixture into the cylinder and exhaust gases out of the cylinder Angled valve face closes tightly against seat Valve guide aligns valve and ensures accurate raising and lowering

Poppet Valves

Valve Spring Holds valve firmly against seat Connect to valve stem with retainer and keeper Allows valve to be opened and closes it when pressure is released from valve stem

Camshaft Used in four-stroke engines Shaft has lobe for each valve Opens valves the right amount at the right time Holds valve open for a specific period and allows them to close at correct instant Shaft has lobe for each valve Lobe lifts valve from seat when shaft rotates Rotates at half crankshaft speed Camshaft gear meshes with and is driven by crankshaft gear

Valve Lifter Located between cam lobe and valve stem Rises and falls as camshaft rotates, opening and closing valves May provide means to adjust clearance

Valve Train Transforms crankshaft rotation into opening and closing of valves Valve train configurations Valve-in-block Overhead valve Overhead cam

Valve-in-Block Camshaft located in crankcase Valves located in cylinder block, directly above camshaft lobes Lifters act directly on valve stems

Overhead Valve (OHV) Camshaft installed in crankcase Valves installed in cylinder head Pushrods transfer motion from valve lifters to rocker arms Rocker arm pushes down on valve stem

Overhead Cam (OHC) Both camshaft and valves in cylinder head Camshaft positioned directly above valves or offset Rocker arms used if camshaft is offset Camshaft driven by chain or belt

Starter Assembly Rewind starter assembly Electric starters Mounted above flywheel When rope is pulled, pawls engage flywheel clutch and assembly turns crankshaft When rope is released, pawls retract and assembly disengages Electric starters

Automatic Compression Release Makes hand cranking easier Lifts exhaust valve during cranking Releases part of compression pressure

Flywheel Fastened to one end of crankshaft Keeps crankshaft spinning during nonpower strokes Metal fins on flywheel force air over cylinder to cool engine Contains magnets that produce electrical current for ignition system

The two sections of the engine block are the cylinder block and the _____. crankcase

The _____ converts the reciprocating motion of the piston into rotary motion crankshaft

The shape of a cam-ground piston allows the _____ surfaces to fit more closely. thrust

The _____ rings keep the air-fuel mixture and burning gases above the piston. compression

Name the three types of friction bearings used in the big end of the connecting rod. Rod metal, bearing bronze, and removable precision inserts

_____ bearings use rollers or balls to reduce friction between moving parts. Antifriction

In a(n) _____ valve train, the valves are in the cylinder block and the camshaft is in the crankcase. valve-in-block

In an overhead valve arrangement, _____ transfer motion from the valve lifters to the rocker arms. pushrods

In an overhead cam valve train, both the camshaft and the valve assemblies are installed in the _____. cylinder head

Glossary Antifriction bearing Automatic compression release Camshaft A bearing constructed with balls or rollers between journal and bearing surfaces to provide rolling instead of sliding friction. Automatic compression release Mechanism on the camshaft designed to lift the exhaust valve slightly during cranking to release part of the compression pressure. Camshaft Shaft containing lobes or cams that operate engine valves.

Glossary Compression rings Connecting rod Cooling fins Piston rings designed to provide a strong seal, keeping the compressed air-fuel mixture and the burning gases above the piston by preventing passage between the piston and the cylinder wall. Connecting rod Link that attaches the piston to the crankshaft. Cooling fins Thin metal projections on the cylinder block that increase the surface area around the outside of the cylinder on air-cooled engines.

Glossary Crankcase Crankcase seals Crankshaft The housing for the crankshaft and other related internal parts. Crankcase seals Seals that prevent leakage of oil from the areas where the crankshaft and crankcase come together. Crankshaft The major rotating part of the engine. It converts the reciprocating (back-and-forth) motion of the piston into rotary (circular) motion.

Glossary Crankshaft throw Cylinder block Engine block The offset portion of the crankshaft measured from the centerline of the main bearing bore to the centerline of the connecting rod journal. Cylinder block The portion of the engine block that contains the cylinder bore. Engine block Casting of iron or an aluminum alloy designed to keep all engine parts in alignment.

Glossary Floating rings Flywheel Piston rings installed with the ring end gaps staggered to avoid gap alignment and possible oil flow through the series of gaps to the combustion chamber. Flywheel A heavy wheel fastened to one end of the crankshaft to improve the running quality of the engine. The inertia of the flywheel keeps the crankshaft spinning and smoothes engine operation.

Glossary Friction bearings Lands Oil control rings Bearing that use smooth, sliding surfaces to reduce friction between moving parts. Lands The full-diameter ridges between the piston grooves. Oil control rings Piston rings designed to remove surplus oil from the cylinder walls.

Glossary Overhead cam (OHC) design Overhead valve (OHV) arrangment Valve train arrangement in which both the camshaft and the valve assemblies are located in the cylinder head. Overhead valve (OHV) arrangment Valve train arrangement in which the camshaft is installed in the crankcase and the valves are installed in the cylinder head. Pin boss The section of the piston surrounding the piston pin hole. It is thick and often reinforced with cast-in webs.

Glossary Pinned rings Piston Piston rings that are held in position by a short pin that fits into the piston ring groove. The pin prevents rotation of the ring around the groove. Piston A part that moves up and down in the cylinder in order to draw air-fuel mixture into the cylinder during the intake stroke, compress the mixture during the compression stroke, transfer force to the crankshaft during the combustion stroke, and force exhaust out of the cylinder during the exhaust stroke.

Glossary Piston rings Piston skirt Poppet valve Circular rings that fit the piston grooves and exert tensions on the cylinder wall. Piston rings prevent blowby of exhaust gases into the crankcase and leakage of oil into the combustion chamber. Piston skirt The part of the piston below the rings and the bosses. Poppet valve A valve structure consisting of a circular head with an elongated stem attached in the center.

Glossary Porting Pushrods System that consists of two holes (ports) in the cylinder wall of some two-stroke engines. One port admits the air-fuel mixture and the other port allows exhaust gases to escape. Pushrods A connecting link in an operating mechanism, such as the rod between the valve lifter and rocker arm on an overhead valve engine.

Glossary Reed valve Rewind starter assembly Ring tension A flat, springy valve covering the ports between the carburetor and the crank chamber in a two-cycle engine. Rewind starter assembly Mechanism mounted above the flywheel on many small engines that spins, or cranks, the engine during starting. Ring tension Force that the piston ring exerts on the cylinder wall.

Glossary Rocker arms Rotary valves Slap Device used in overhead valve system to transfer the upward motion of the pushrod to a downward force on the valve. Rotary valves A valve construction in which ported holes move in and out of register with each other to allow fluids or gases to enter and exit. Slap Side-to-side movement of the piston in the cylinder caused by too much clearance.

Glossary Snap rings Sump Thrust surfaces Fasteners that are compressed and placed in grooves in the piston pin bosses. They prevent the pin from rubbing on the cylinder surface. Sump The part of the block in a small four-stroke engine that holds and collects the lubricating oil. Thrust surfaces The sides of the piston forced against the cylinder wall. These surfaces are at right angles (90°) to the centerline of the crankshaft and piston pin.

Glossary Valve guide Valve lifter Hole machined in the engine block to align the valve and ensure accurate raising and lowering in relation to the seat. Valve lifter A rod or plunger that transfers motion from the cam and the other valve train components.

Glossary Valve spring Valve train A spring attached to a valve to return it to the seat after it has been released from the lifting or opening operation. Valve train All the components that work together to transform the rotation of the crankshaft into the opening and closing of the valves.

Glossary Valve-in-block arrangement Wrist pin Valve train arrangement in which the camshaft is located in the crankcase and the valves are located in the cylinder block, directly above the camshaft lobes. Wrist pin The journal for the bearing in the small end of an engine connecting rod. The piston pin also passes through the piston walls, holding the piston to the connecting rod.