Investigation into the effect of the Thermal Management system of a Diesel engine on the rate of heat transfer through the combustion chamber

Student thesis: Doctoral ThesisPhD


Modern diesel engines are being continuously developed in order to improve their specific output thus reducing the fuel consumption. This is in response to both increasingly stringent regulations and to the demands of the customer evolving. With this in mind a more detailed understanding of some of the fundamental processes within the engine are required. A prime example of one of these processes is heat transfer. In the region of 17-35% of fuel energy will pass to the coolant, therefore the rate of heat transfer has a considerable effect on the design and function of the engine. This thesis describes a method to calculate heat transfer through a combustion chamber wall of a modern production engine and uses the data to improve the understanding of the impact of active thermal management systems on the rate of heat transfer and how this can be incorporated into empirical modelling, also improving fundamental understanding and evaluating established correlations focusing on the warm-up period.Development of prototype engines over extended test schedules can prove expensive, therefore in order to reduce the necessary test and development period of a new engine it is useful to be able to predict the heat transfer within the combustion chamber using modelling data. A large number of correlations have been developed over the years, however with the rate of development of the diesel engine; some of these correlations have been left behind. A number of steady state operating conditions were employed to approximate the New European Drive Cycle. By approximating the drive cycle performance it was possible to further the understanding of the effect of changing the engine operating conditions on the temperature distribution in key areas of the engine, including the crankshaft bearings, camshaft bearings and combustion chamber walls. The main focus of this thesis is the cylinder wall temperatures and the rate of heat transfer through the cylinder wall from the combustion gases. It was found that there was a temperature rise of 7°C between the oil in the main oil gallery and that used to lubricate the journal bearings; however the oil in the camshaft bearings was found to decrease in temperature along the inlet camshaft but increase along the exhaust camshaft. In addition the temperature and heat transfer profiles were significantly different between the inlet and exhaust sides of the engine. Existing correlations were found to in general over-predict the gas side convective heat transfer coefficients at high power conditions. The accuracy of these correlations could be improved by 77.6% by modifying the correlation coefficients; however the introduction of a cylinder wall temperature component, in addition to modifying the correlation coefficients, led to an improvement in the correlation by a further 17.4%.Prototype hardware and a Design of Experiments (DoE) based test programme were used to investigate the impact of actively changing the external oil and coolant circuits during a New European Drive Cycle on the engine warm-up. It was found that the driving force for improved warm-up when throttling the engine coolant flow was the increase in the gas side wall temperature, and the reduction in the local coolant temperature. The coolant local to the combustion chamber wall was cooler as it remained isolated from the majority of the external cooling system. In addition, the low engine coolant flow retained the heat energy which had been transferred through the cylinder wall, within the engine structure. This unique approach of combining cylinder wall temperature measurements and active thermal management systems allowed for the fundamental heat transfer paths to be explained.The insight obtained during the steady state experiments and the transient tests was then used to evaluate the potential to calculate the convective heat transfer during different stages of the engine warm-up. A strong correlation, a R-squared of between 0.77 and 0.83, was found between the rate of energy transfer to coolant across the engine and the gas side convective heat transfer coefficient during early stages of the drive cycle, however this decayed as the engine warmed up. A correlation was also found between the convective heat transfer coefficient and the temperature used by the ECU for engine control, in this case measured in the cylinder head. The modified convective heat transfer correlation developed in this thesis allows for the gas side convective heat transfer coefficient to be estimated at different stages of the engine warm-up based on the average cylinder wall temperature. This was not possible with existing correlations and could significantly improve thermal modelling during engine warm-up.The findings of this thesis are relevant to engine designers, combustion simulation engineers and thermal management teams as 1-D modelling will continue to play a significant role in the development process of internal combustion engines for the foreseeable future. This is primarily due to the cost implications of high performance computers for 3-D modelling; therefore the development of these models remains vital.
Date of Award14 Nov 2014
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorChris Brace (Supervisor) & Sam Akehurst (Supervisor)

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