CHAPTER 4

COMPRESSION Compression is required to prepare the charge for ignition. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Compression ratio for most small engine is 6:1 to 8.5:1. Which means the charge is compressed into a space 1/8 the original volume.

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. Not introduced from external source. Heating of the charge occurs in hundredths of a second. As piston moves toward TDC, the volume of the combustion chamber is reduced.

CHANGES IN THE CHARGE 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. As distance increased, cohesion force is weaker, when the piston compresses the charge, the increase in cohesive force causes heat to be generated. As the temperature of the charge is raised, gasoline molecules become more active.

CHANGES IN THE CHARGE When gasoline is heated, it also changes rapidly from a liquid to a vapor. In addition to an increase in vapor, larger droplets tend to break apart at higher temperatures, therefore creating an increase in total surface area and expose more of the liquid gasoline to the air. The energy required to compress the charge before combustion is typically 25% of the energy released during combustion.

COMPRESSION PROBLEMS Compression problems occur from inadequate or excessive compression. Inadequate compression is caused by leaks in the system. Excessive compression can cause engine performance problems which can lead to detonation and preignition.

DETONATION Engine condition in which there is spontaneous combustion of a significant portion of the charge before the spark-induced flame front reaches it. It causes the knocking and pinging noise in an engine. Major factors of detonation is the compression ratio.

PREIGNITION Occurs when a small portion of a combustion chamber component or a particle in the combustion chamber becomes excessively heated and ignites the charge as it enters the combustion chamber. It also causes knocking and pinging.