Compression Is an adiabatic process. This means heat is derived from the process. As we squeeze the air fuel mixture it gets hot. Normal compression ratio in gasoline, small engines is some where form 6:1 to 8.5:1. Cohesion :Is the molecular attraction between gasoline molecules that helps to create heat. One fourth of the energy produced by the combustion of the air fuel mixture is required to compress the air fuel mix.

Compression problems LeaksWorn rings, bad valves, bad head gaskets, worn cylinders may let the compression escape lowering the power produced. Maverick air Air that enters the air fuel mixture that does not go through the carburetor. This leans out the air fuel mix so it burns hotter. Excessive compression Detonation: The charge ignites before the spark flame reaches the area. Deposits which increase compression. Preignition: A hot spot causes a flame pattern to start while the charge is entering the cylinder.

Compression system components Valve, guides, seats, pistons, rings, cylinder, heads, Valves: Austenitic: heat resistant metal. A combination of Cobalt Tungsten, and chrome. One piece: One solid piece, economical to make, hard to get wear surface and hardness correct. Projection tip: Tip of hard steel welded to stem for wear. May also have a 3 piece austenitic head welded on stem also. Uses a friction welding process. Hard surfacing: Spray on material that hardens areas of the valve. (stellite, cobolite)

Valves continued: Exhaust valves: Because they run hotter they are most likely made out of heat resistant metal. Also the most common angle on the face is 45 degrees for crushing carbon deposits. Intake valves: They run cooler so heat resistance is not as critical. 45 or 30 degree angle on face. 45 seals better, 30 is better for flow of air fuel mix into cylinder. Interference angle: It is a one degree difference between the valve seat and the valve face. This helps the valves seal better after the engine has been overhauled.

Valves continued: Rotation: Valve rotation is a good thing. It scrapes the carbon off between the valve face and seat. Too much rotation will cause excess surface wear. Slotted valve spring retainers offer the least amount of valve rotation. On some applications a special valve rotator is installed Engines that operate using LP gas have less lubrication in the valve area because the fuel is “dry”. This may cause problems with the intake valve seat and valve guides.

Valve guides Length of guide:A normal ratio is 7:1 (guide is 7 times longer than the diameter of the valve stem. This will change with different guide material used. Side loading:Twisting or pushing force that applies pressure to one side of the valve stem causing it to wear the guide more on one side. A bent valve spring, bent stem, or worn lifter may cause this condition. Materials used: Aluminum, brass, sintered iron, and cast iron are all common types of guide material. The harder the material the longer they will last but the harder it will be on the stem.

Valve seats: Integral: The valve seat is part of the block, or head. This is usually done in cast iron material. Insert: These are usually used in blocks that are made of softer material. The inserts are usually made of cast iron or cast steel. They are press fit, usually 2 to 3 thousands of an inch larger than the hole they fit into so they do not move.

Pistons Cam ground: Pistons that are not round when cold. When heated they become round. Tapered:The head of the piston is slightly narrower when cold, because it heat up and expands more than the rest of the piston. Offset pins: In V type engines the piston pins may be offset up to 1/16th of an inch from the centerline of the piston. This is to insure proper clearance between the pistons on the same throw of the crank. Piston windows: Holes in the piston behind the oil ring that returns the oil to the crankcase in 4 cycle engines. Most modern pistons are aluminum alloy

Pistons Ring lands: Parallel surfaces between rings that support the rings. Ring groove depth: The grooves for the rings must be deep enough for the rings to fit in flush with the piston. Carbon can build up in the groove and not let new rings go deep enough. Piston operating environment: Must be able to change direction 120 times per second at 3,600 RPM. Must be able to withstand 500 PSI Must be able to withstand 3,000 degrees F for short periods of time.

Piston rings: Radial pressure: Rings are made slightly larger than the cylinder so the ring pushes out. Ring design also allows the pressure of the compressed air fuel mixture to push the rings out. Top ring: Called the compression ring. It may have a Tapered face for sealing, or a barrel for lubrication purposes. It will have more ring clearance (distance between ring and ring land) than the other rings. Second ring: Called the wiper ring. It is tapered to wipe excess oil off the cylinder wall so the top ring can do its job. Third ring: Called the oil ring. Can be a one piece ring with holes in it or a three piece build up ring.

Piston rings: Rotation: Rings will rotate around the piston when the engine is operating. Make sure when you place the piston into the cylinder that all the end gaps of the rings are not lined up. This will cause blow by on startup. End gap: Is the space between the ends of the ring when it is placed into the cylinder. It shows the amount of wear in the cylinder or the ring. Ring coating material: The base for almost rings is gray cast iron. Other materials are added to improve the ring wear. Chrome: Added to ring to improve wear. It has a 1:1 wear ratio with cast iron. Phosphating and ferroxiding:Improves the ring by making it softer for fast break in. Nitriding: Used on cast iron bores it makes the ring tough and hard.

Piston installation: Place rings on the piston in the right position with the correct side facing up. Place ring compressor over the piston and rings with the dimples toward the top of the cylinder. Tighten the ring compressor. Lubricate the ring compressor Place piston and compressor into cylinder with the rod down. Gently tap piston down into the cylinder.

Cylinder bore New cylinders and reconditioned cylinders will have a crosshatch design of scratches that hold the oil to the cylinder wall. Aluminum cylinders may not the rings provide enough scraping to hold the oil. Compression release: Some engines will have a system to “bump” open one of the valves at low RPM. This allows the engine to turn over faster when starting. After a higher RPM is reached it is ineffective. Valve resurfacing: In small engines valves are normally NOT reground for service. This removes the special coating that they may have and removes the margin.