Knocking combustion places a major limit on the performance and efficiency of spark ignition engines. Spontaneous ignition of the unburned air-fuel mixture ahead of the flame front leads to a rapid release of energy, which produces pressure waves that cause the engine structure to vibrate at its natural frequencies and produce an audible ‘pinging’ sound. In extreme cases of knock, increased temperatures and pressures in the cylinder can cause severe engine damage.
Damage is thought to be caused by thermal strain effects that are directly related to the heat flux. Since it will be the maximum values that are potentially the most damaging, then the heat flux needs to be measured on a cycle-by-cycle basis. Previous work has correlated heat flux with the pressure fluctuations on an average basis, but the work here shows a correlation on a cycle-by-cycle basis.
The in-cylinder pressure and surface temperature were measured using a pressure transducer and eroding-type thermocouple. These sensors were installed side-by-side at the surface of the cylinder in order to investigate the relationship between knock and heat flux on a cycle-by-cycle basis. A finite difference method was implemented to solve the one-dimensional unsteady heat conduction equation and calculate the temperature distribution away from the surface of the combustion chamber, and thus the instantaneous heat flux. The knock intensity was varied by controlling the fuel quality, compression ratio, ignition timing and air-fuel ratio. The measured heat flux was compared with pressure-based knock indices and it was found that as knock intensity increases, the correlation with peak heat flux became stronger. Results from different fuels and engine operating conditions did not collapse onto the same trend line. At higher speeds and higher compression ratios the heat flux was higher at a given level of knock intensity. The co-location of the pressure transducer and heat flux sensor meant that a correlation was found on a cycle-by-cycle basis.