Compression Systems

1. Compression Systems Small Engines

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

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

11. 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.

12. 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.

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

14. 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.

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

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.