Additional Engine Considerations
Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time. Valve overlap occurs when the piston nears TDC between the exhaust event and the intake event. Duration of valve overlap is between 10° - 20° of crankshaft rotation, depending on the engine design. The intake valve is opened during the exhaust event just before TDC, initiating the flow of the new charge into the combustion chamber.
As the exhaust gases are evacuated from the combustion chamber, a small but distinct low-pressure area is created on the surface of the piston head. By opening the intake valve earlier that TDC, the charge begins to fill this low-pressure area while exhaust gases exit. The low-pressure area on the head of the piston assists the fresh charge in filling the combustion chamber to its maximum capacity.
Valve overlap is designed into the engine and is most useful at higher speeds. At higher speeds, the extra amount of intake charge brought into the combustion chamber provides a substantial increase in available power. The amount of time that both valves are open in directly related to engine rpm. The higher the engine rpm, the shorter the amount of time that both valves are open. The angle of crankshaft rotation when both valves are open do so change, only the amount of time both valves are open varies. Thus at idle, the amount of time both valves are open is relatively long compared to that at top no-load speed.
Compression is required in a small gasoline engine to prepare the charge for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Compression system components direct, contain, compress the air-fuel mixture, and discharge exhaust gases. The compression ratio of most small gasoline engines ranges between 6:1 and 8.5:1. For example, with a compression ratio of 8:1, the charge is compressed into a space 1/8 the original volume of the air-fuel mixture before the compression event. Compression of the air-fuel mixture allows more energy to be released when the charge is ignited. 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 itself. During compression, heat is produced from the work applied by the piston. Heat is not introduced from an external source. The increase in temperature of the charge is directly proportional to the quantity of work applied to compress the charge.
Heating of the charge occurs in hundredths of a second. As the piston moves toward TDC, the volume of the combustion chamber is reduced. The charge in the combustion chamber is compressed. As the compression ratio increases, charge temperature increases proportionately from the work applied to the charge by the piston. There is little time for any heat energy to leave or to be transferred from the combustion chamber to the cylinder block.
The charge enters the combustion chamber in a gaseous state. In a gaseous state at the molecular level, atoms and molecules are as far apart from each other as possible, yet held together by cohesion. Cohesion is the molecular attraction by which atoms and molecules are united throughout the mass. As distance increases between the molecules, cohesion force is proportionately weaker. When the piston compresses the charge, an increase in cohesive force causes heat to be generated.
As the temperature of the charge is raised, gasoline molecules become more active. The increased activity from speed and molecular vibration and from being forced together by compression causes additional and increasing molecular collisions. This results in the addition of internal energy to the charge. As the process continues, the temperature of the charge increases hundreds of degrees.
When gasoline is heated, it also changes rapidly from a liquid to a vapor. In a vapor state, small droplets of gasoline liquid are suspended in the gaseous medium of the charge. The vapor is suspended, similar to the suspension of water in a rain cloud. The rate at which the gasoline becomes a vapor is a function of temperature, and the propensity of the gasoline to become a vapor. As the temperature increases, gasoline droplets release more vapor.
In addition to an increase in vapor, larger droplets tend to bread apart at higher temperatures. The smaller droplets increase the total surface area and expose more of the liquid gasoline to the air. This reduction in size and increase in surface area of the droplets provides additional volatile vapor for the combustion process.
The increase in the temperature of the charge reduces the energy needed to maintain the combustion process. The energy required to initiate combustion is provided by the spark jumping across the gap in the electrode of the spark plug. The energy required to compress the charge before combustion is typically 25% of the energy released during combustion, or a 1:4 ratio.
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. The break-in period is the period of operation time required for the running surfaces of piston rings and the surface of the cylinder bore to conform to one another after initial startup. In the past, the engine break-in period was very important to the overall life and durability of the engine. The break-in period required has changed over the years with improved piston ring materials and designs. The break-in period now is short in comparison with that of engines of the past. Aluminum cylinder bore engine piston rings break-in faster than those used on cast iron cylinder bores.
During the break-in period, the piston rings and cylinder bore wear rapidly to remove any rough edges on the piston ring running surface and cylinder wall. In general, there is no special engine operation procedure required during the break-in period. Break-in is accomplished by any speed above idle and may occur faster if the engine is operated at varying loads and speeds. However, break-in occurs at an acceptable rate if the engine is operated at slightly less than top no-load speed with or without a moderate load. Combustion pressures at this speed are sufficiently high to cause piston rings to conform to the cylinder wall. It is recommended that an engine not be operated continuously at full load during the initial hours of operation. This can lead to permanent deformation of the cylinder bore.
Once an engine has been broken in it is a good practice to change the oil and filter to remove any microscopic particles of debris which may have been dislodged during the initial engine operation.