Scavenging and auto ignition

Scavenging -: In an I.C. engine , scavenging is the process of replacing the exhaust gases in the cylinder with the fresh air or fuel mixture for the next cycle. 

                   In other words,  scavenging is the process of removing the brunt gases from the engine cylinder.  Efficient scavenging is necessary for good combustion of fuel in the engine cylinder. The passage of scavenge air will also assist cooling of the cylinder,  pistpn and the valves. 

There are three types of scavenging process-:

1. Loop scavenging 

2. Uniflow scavenging 

3. Cross scavenging 

1.Loop scavenging -: In loop scavenging process , the incoming air passes over the piston crown then rises toward the cylinder head.  The exhaust gases are forced before the air passing down and out of exhaust ports located above the inlet port .

                      Fig -: loop scavenging 

2.Cross scavenging -: In this type of scavenging process,  the incoming air is directed upwards and pushing the exhaust gases before it from the engine cylinder. The exhaust gases then travel down and out of the exhaust ports. 

                   Fig -: cross scavenging 

3.  Uniflow scavenging -: In the uniflow scavenging process, the incoming air enters at the lower end of the cylinder and leave at the top of the cylinder. During this travel incoming air push the brunt gases outside of the cylinder.  The outlet at the top of the cylinder may be ports or a large valve. 

Auto ignition -: It is the phenomenon of fuel catching fire without external source of ignition energy . As the name suggests, the fuel particles auto-ignite. So when the spark is introduced ,the flame front propagates,  this flame boundary further increases the temperature of fuel molecules which is already at a high temperature. 

                  Auto ignition occurs the mixture of the gases or vapour ignites spontaneously with no external ignition source and after reaching a certain temperature , the Auto ignition temperature.  The auto ignition temperature is not an intrinsic property of gases or vapours but is the lowest temperature in the system whene the rate of heat evolved from the gases or vapour increases beyond the rate of heat loss to the surrounding , resulting in ignition .

                 Auto-ignition is accelerated by the normal heptane and resisted by iso-octane.