Thermal investigations on a high-speed direct injection diesel engine

Abstract

Modern compression ignition engines offer higher thermal efficiency compared to gasoline engines, thus offering superior fuel consumption performance and lower CO₂ emissions, a major greenhouse gas. With future legislation pushing the automotive manufacturers for even lower fleet average CO₂ emissions the compression ignition engine can assist in achieving these goals, however further research is required to extend their efficiency. Understanding where the chemical energy of the fuel is transferred during the combustion process and from that identifying strategies that can assist in converting part of these energy flow terms into useful piston work can help enhance the engine’s efficiency. This work looked into the study of these energy flows by using a first law analysis approach and by developing the necessary instrumentation and methods that allow for the more accurate measurement of the various energy flows, subsequently increasing the accuracy of the first law analysis. Two thermal studies were carried out on a single cylinder diesel engine. The first study investigated the effect of different high-pressure EGR strategies on engine efficiency and emissions, in an attempt to reduce the negative effects of EGR application on soot emissions under two load/speed conditions. The second study compared the effects of different piston material on engine efficiency under two speed/load conditions. A baseline aluminium design, was compared against an alloy steel piston which, due to its lower thermal conductivity, was shown to provide lower heat transfer losses during combustion thus increasing efficiency. The results of the thermal studies showed that ~40% of fuel energy was transferred to the exhaust. Therefore, being able to accurately measure the exhaust temperature can offer significant insights to engine designers. The exhaust event is highly unsteady and the exhaust temperature is typically measured using a 3 mm sheathed thermocouple which, due to its thermal mass, cannot capture this transient event. Instead, a time-average measurement is only possible. This can result in an under prediction of the exhaust enthalpy since the measured temperature is lower than that of the flow field of interest due to measurement errors. A lumped capacitance model was developed in order to better understand the behaviour of thermocouple sensors under an unsteady flow environment. The sensors are subject to both dynamic errors, due to their thermal inertia, and conduction and radiation errors due to temperature gradients between the sensor and the surrounding environment. Understanding how different size thermocouples react under unsteady flow conditions has the potential to improve the measurement process and increase the accuracy of the measured exhaust temperature. A temperature reconstruction method was developed which can correct both the dynamic and conduction errors that are prevalent during the engine cycle, thus approaching the true exhaust gas temperature. This technique requires the use of thermocouples with different thermal masses, thus resulting in a different response under the same flow conditions. This reconstructed temperature then allowed the estimation of exhaust enthalpy on a mass-average basis improving the accuracy of the first law analysis.