An Experimental Study of Combustion and Fluid Flow in Diesel Engines

Combustion zone (flame) propagation and flow velocities are measured using optical measurement techniques in a single–cylinder direct injected diesel engine. Combustion and, hence, flame propagation is detected by optical fibers and flow velocities have been measured by a new single–color, 2 dimensional Laser Doppler Velocimeter (LDV). Flame propagation and flow velocities could not be measured simultaneously. Consequently, pressure traces which were recorded during each measurement served as a criterion for selecting similar combustion cycles from flame and flow velocity measurements.The operational speed of the engine was 1100 rpm and the power cylinder was configured with a combustion bowl in the piston. For the purpose of the tests, the intake swirl number was either 1.8 or 4.2, the latter corresponding to the application of a shrouded inlet valve. It has been shown that variation of several significant engine parameters influences both combustion and fluid flow history. Retarding the start of injection produced an observable change in ignition delay. Higher swirl levels also decreased the ignition delay and enhanced the flame propagation. A similar result was obtained by reducing squish height. Soot emissions tended to increase if the flame location at the end of injection was in the vicinity of the injection nozzle and fuel was being injected directly into the flame. The tangential and radial flow velocities were measured at several locations in a plane located at the center of the combustion bowl with the piston at TDC. A solid body rotation takes place within the combustion bowl during the power stroke. With higher swirl levels and, hence, higher tangential velocities, radial velocities are much lower. A comparison between the motored engine results and those from the fired engine revealed that fuel injection and, possibly, the presence of combustion leads to higher turbulence and lower values for mean flow velocity. Cycle–resolved analysis indicates that the flow history tends to vary significantly from cycle to cycle.By analyzing pressure traces, a number (dependent upon the cycle–to–cycle variations) of single cycles from flow and flame measurements were selected. A correlation of these measurements provided qualitative insight into the combustion process and allowed determination of a quantitative estimate of the combustion rate.