Chapter 4 Compression System Compression • Valves • Valve Guides • Valve Seats • Pistons • Cylinder Bore • Crankcase Breather System • Compression Release System • Valve Resurfacing System • Valve Resurfacing Service Procedures

Competencies Describe the compression process in a small engine. List and describe compression problems. Describe important characteristics of valves. Describe valve head design and valve dynamics. Explain the function of valve guides. Explain the function of valve seats.

Competencies Explain piston design. Describe the function and types of piston rings. Explain piston ring dynamics. List and describe common piston ring materials. Describe cylinder block and bore design, and compression release systems.

Compression Process an adiabatic process heat is derived from the process work applied by the piston during compression produces heat as compression continues, temperature of the charge increases as temperature increases, additional volatile vapor is produced Compression is required in a small gasoline engine to prepare the charge for ignition. Compression of the charge in an internal combustion engine is an adiabatic process. An adiabatic process is a process in which heat is derived from the process, as in the compression stroke when heat is produced from the work applied by the piston. As compression continues, the temperature of the charge increases hundreds of degrees. As vaporized gasoline is heated, the reduction in size and increase in surface area of the droplets provides additional volatile vapor for the combustion process.

Compression Problems from inadequate or excessive compression compression leaks detonation Pre-ignition excessive carbon build up Compression problems occur from inadequate or excessive compression. Compression leaks cause an exponential decrease in performance, efficiency, and available power and a possible increase in exhaust gas temperature. Detonation is an undesirable engine condition in which there is spontaneous combustion of a significant portion of the charge before the spark-induced flame front reaches it. Figure 4-3. Engine damage from detonation includes piston failures (melting or breakage), piston skirt damage, connecting rod breakage, and occasional camshaft failures. Preignition reduces combustion efficiency and engine performance. Figure 4-4. The ability to properly seal the combustion chamber for proper combustion greatly affects the overall performance of the engine.

Valves one-piece valves projection-tip welded valves made from single piece of metal projection-tip welded valves small piece of metal welded to stem two-piece-stem welded valves head welded onto stem hardfacing Heat treated coating added on valve face for better wear valve stem surface treatments Valves commonly used in small engines are classified as one-piece, projection-tip welded, or two-piece-stem welded valves. Figure 4-5. One-piece valves are commonly used on engines that drive lighter loads or have occasional use, or where economy is the main consideration. A projection-tip welded valve is a valve that is constructed from austenitic steel that has approximately .09² of hardened steel welded on the end of the valve stem. A two-piece-stem welded valve is a valve that is constructed from an austenitic steel valve head welded to a hardened valve stem. Some valves and valve seats used in heavy service applications or premium engines require hardfacing to extend valve life. Valve stem surface treatments allow a reduction in valve stem to valve guide clearance and reduced friction-related wear.

Valve Head Design and Valve Dynamics 45° exhaust valve face angle 30° intake valve face angle valve spring retainer For optimum performance and durability, a 45° exhaust valve face angle is the industry standard. A 30° intake valve face angle allows a better flow into the combustion chamber and provides increased wear resistance. A 45° valve angle is expected to have a true 45° angle, but it is commonly manufactured with an interference angle so that the surfaces will seat better after break-in. Some rotation of a valve with a keyhole valve spring retainer is caused by the valve spring itself. Figure 4-7. A two-piece automotive-style valve spring retainer has minimal pressure applied at the valve stem groove, allowing the valve stem to rotate freely. Valve rotation should not be used on engines that use LP gas or propane because where the inherent lubricating qualities of gasoline are not present, the valve interface wears prematurely if significant valve rotation is allowed to occur.

Valve Guides foundation of the valve system provide consistent paths for valve stems valve guide length to valve stem diameter ratio is typically 7:1 no standard clearance specifications aluminum, brass, or sintered iron The valve guide is the foundation of the valve system and provides a consistent path for the valve stem to ensure maximum engine power, valve performance, and overall valve train durability. The valve guide length to valve stem diameter ratio is typically 7:1, which provides a sufficient running surface area for an oil film layer to provide lubrication for the sliding valve stem. There are no standard specifications for clearance between the valve stem and the valve guide wall for all engines. The most common type of valve guide is an aluminum valve guide, which is integrally machined into an aluminum cylinder block. Figure 4-10. Brass valve guides are commonly used on many types of small engine blocks and offer greater resistance to wear and lower coefficient of thermal expansion than aluminum valve guides. Sintered iron valve guides have microscopic pockets that provide an additional reservoir of oil to help maintain the proper amount of lubrication.

Valve Seats mate with valve face used for exhaust valves seals combustion chamber removes heat away from valves used for exhaust valves The valve seat mates with the valve face to seal the combustion chamber and remove a significant amount of heat away from the valves. Valve seat inserts are commonly used for exhaust valves because of their durability and resistance to heat. Figure 4-12.

Piston Design made from aluminum alloy cast and machined at the factory into a cam ground shape designed with a taper or with a barrel-shaped piston skirt piston windows machined channel near the piston pin The piston acts as the movable end of the combustion chamber and must withstand pressure fluctuations, thermal stress, and mechanical load. Most pistons are made from die- or gravity-cast aluminum alloy to reduce the overall mass and the force necessary to initiate and maintain acceleration of the piston. Some pistons are cast and machined at the factory into a cam ground (elliptical shape). Some pistons are designed with a taper or with a barrel-shaped piston skirt. Piston windows are a series of small holes machined into the oil ring groove surface of the piston to allow excess oil collected by the oil ring to drain into the oil reservoir. Figure 4-15. Another common method used to return oil to the oil reservoir is through a machined channel near the piston pin. The clearance between the piston ring and ring lands allows the required piston ring movement to seal the combustion chamber and to route combustion gases.

Piston Rings seal the combustion chamber, transferring heat to cylinder wall control oil consumption taper-faced or barrel-faced wiper ring oil ring By applying inherent and applied pressure, piston rings seal the combustion chamber, transferring heat to the cylinder wall and controlling oil consumption. A piston ring must provide a predictable and positive radial fit between the cylinder wall and the running surface of the piston ring for an efficient seal. A taper-faced compression ring is a piston ring that has approximately a 1° taper angle on the running surface. A barrel-faced compression ring is a piston ring that has a curved running surface to provide consistent lubrication of the piston ring and cylinder wall. A wiper ring provides a consistent thickness of oil film to lubricate the running surface of the compression ring. An oil ring has holes or slots cut into the radial center of the ring to allow the flow of excess oil back to the oil reservoir.

Piston Ring Dynamics combination of inertia, radial pressure, and conformability ensure required seal piston ring twist alignment of the piston ring end gaps A combination of inertia, radial pressure, and conformability of piston rings ensures the required seal for combustion efficiency. Piston ring twist causes piston rings to provide a seal against the ring land during non-combustion strokes of the piston. In four-stroke cycle small engines, all piston rings rotate during operation. Rotation is caused by vibration in the engine and is critical to engine durability. Alignment of the piston ring end gaps under normal conditions does not cause an immediate decrease in performance or an increase in oil consumption.

Piston Ring Materials Chromium plating break-in is accomplished at any speed above idle piston ring expander prevents improper stressing or bending of the piston ring Chromium plating is a piston ring surface treatment that adds a layer of chromium to increase hardness and durability. Chromium-plated piston rings have cracks and fissures that retain lubricating oil and provide a place for debris to accumulate. During the break-in period of a piston ring, the piston ring and cylinder bore wear at an accelerated rate and conform to a mutual shape and size. Break-in is accomplished at any speed above idle and may occur faster if the engine is operated at varying loads and speeds. A piston ring expander prevents improper stressing or bending of the piston ring during installation and a piston ring compressor is used to install the piston and piston rings into the cylinder bore.

Cylinder Block and Bore Design the number and size of cooling fins on a cylinder block are determined by the thermal characteristics of the engine compression release system The number and size of cooling fins on a cylinder block are determined by the thermal characteristics of the engine. Figure 4-23. For easier starting, some engines use a compression release system. Figure 4-24 and Figure 4-25.

Chapter 4 Review What types of valves are commonly used in small engines? Valves commonly used in small engines include one-piece, projection-tip welded, or two-piece-stem welded valves. What is the function of a piston ring? A piston ring seals the combustion chamber. Valves commonly used in small engines are classified as one-piece, projection-tip welded, or two-piece-stem welded valves. By applying inherent and applied pressure, piston rings seal the combustion chamber, transferring heat to the cylinder wall and controlling oil consumption.