Compression Systems Small Engines

Power Conversion Components The crankshaft is often considered the “backbone” of the engine. It converts the linear motion of the piston to rotary motion of the crankshaft. The crankshaft carries the entire power output. It rotates the flywheel, and drives other parts.

Power Conversion Components The main bearing journal are supported by the main bearings in the crankcase. Both the journals are the crankpin need to be machined to a high degree of smoothness. Counter weights or balances are added to the crankshaft to maintain balance and to reduce vibration.

Power Conversion Components The flywheel is mounted on the end of the crankshaft. The flywheel is may be part of the engines cooling system. Vanes designed to channel air across the engine block aid in carrying away heat produced by the engine. In most small engines the flywheel is part of the ignition system.

Power Conversion Components Permanent magnets are mounted in the outer or inner rim of the flywheel. This serve an important part of the magneto-ignition system.

Connecting Rods The connecting rod serves as the link between the piston and the crankshaft. Most connecting rods are made of aluminum or steel. Nuts and bolts used to secure a connecting rod cap to the connecting rod should always be torqued upon installation.

Bearings Bearing support the crankshaft and connecting rods. There are two classifications of bearings. Friction or a sleeve type Usually consists of a hard metal piece called a retainer and a soft metal bearing surface called a liner. Antifriction Ball, roller, or needle types. Usually used in two stroke engines. Most are either press of shrink-fitted into place.

Piston Most pistons are made of cast iron or aluminum. Cast iron pistons are high in strength but are heavy. Aluminum pistons are lighter in weight which means less stress on the rest of the rotating parts, better acceleration. Aluminum pistons run cooler due to the greater heat conductivity which allows for higher compression ratios.

Rings The main function of the rings is to provide a tight seal between the piston and the cylinder wall. Another function of the rings is to control lubrication of the cylinder walls. Piston rings reduce friction and the accompanying heat and wear caused by friction.

Rings Piston rings are found in grooves near the top of the piston.

Rings There are two classifications of rings: Compression Rings and Oil rings. The two upper rings are compression rings. Although the middle ring is a compression ring it is also referred to as a scraper ring and aids in oil control. This is done by scraping the cylinder walls to keep oil out of the combustion chamber.

Rings The lower ring or oil ring is perforated to permit lubrication of the cylinder wall. It spreads the correct amount of oil on the wall of the cylinder, scrapes off the excess, and returns it to the crankcase. Two clearances are important in piston rings. Piston Ring Gap This gap permits the ring to put pressure on the cylinder wall for a tight seal.

Rings Rings are separated from the piston lands by a slight gap called ring-groove clearance.

Valve System Camshaft One of the primary functions of the camshaft is to open and close the valves at the proper time. There are two cams for each cylinder, one is for intake and the other for the exhaust. The tappet of valve lifter rides on the cam. The valve is pushed open when the nose or high point of the cam moves under the tappet. The contour of the lobe determines the speed of valve opening, length of time open, and the speed of valve closing.

Valve System The camshaft is found only on four-stroke engines because two stroke engines do not have valves, instead they have ports.

Valve System Valves Properly sealed valves are important in maintaining efficient compression. The valve face must seal properly against the valve seat to produce an air tight cylinder. The most common valves used today are called poppet valves.

Valve System The important parts of a valve are the head, margin, face, and stem. The valve face makes contact with the seat and the stem slides along the valve guide. Temps around the head and margin of the exhaust valve may exceed 1200 F.

Valve System The cylinder and the cylinder head and the piston top are exposed to the same heat; however these parts are cooled by air from the flywheel fins and oil from the crankcase. A general rule valves should be discarded when the margin becomes less than one-half of its original thickness.

Valve System Valve inspection may reveal a number of valve failures: Burned, Dished, or Necked A Burned valve will have considerable carbon deposited on the valve. The side or margin may be cracked or melted.

Valve System A dished valve is caused by operating at too high a temperature with too strong a spring. Highly leaded fuels can also erode away the head. A necked valve is one that has the stem directly beneath the head eroded away by heat.

Valve System Valve sticking may be caused by gum or varnish forming on the valve stem and in the valve guide. The valve stem and guide should be checked for wear. Correct valve timing is very important for smooth, efficient operation. When an engine is operating at 3,000 rpms each valve opens and closes in about 1/50 of a second.

Valve Timing An engine will have optimum performance only if the valves open and close at the proper time. Air-fuel ignition must also take place that the proper time.

Lubrication Systems A constant supply of oil must be provided to moving parts of the engines to avoid engine failure. An injection oil system is common on the two stroke. All four stoke engines are lubricated from an oil reservoir in the crankcase.

Lubrication Systems Splash A system that is simple and widely used. It consists of a dipper attached to the connecting rod cap. The dipper strikes the oil on the lower end of the stroke and splashes oil on all parts within the crankcase.

Lubrication Systems Pump The positive displacement gear pump is the types of lubrication system often found on larger engines. The pump delivers oil through small drilled passages to all main, connecting rod, and camshaft bearings. Oil is also sprayed on other engine components in the crankcase.

Lubrication Systems The ejection pump is really a combination splash-pump system. A cam operated pump draws oil from the bottom of the crankcase. This oil is sprayed onto the connecting rod. Some of the oil is deflected onto other parts within the crankcase while some enters small holes to lubricate the connecting rod bearing.