Study Of Diesel Engine Exhaust Heat Recovery With Pre-turbine Steam Injection And The Miller Cycle

The problems of energy and environment become more and more serious, and depending solely on improving the combustion process cannot fulfill people’s high requirement of thermal efficiency. Traditional waste heat recovery technologies mainly focus on transforming waste heat into mechanical or electrical energy for storage or output. Thus the heat-power or heat-electric conversion process is independent of the in-cylinder process of the internal combustion engine, such as the electrical generation, compounding turbine and Rankine cycle. If the recovered waste heat is used to improve the in-cylinder process, the torque output and the fuel economy can be improved directly. A waste heat recovery technology based on the pre-turbine steam injection and the Miller cycle is proposed to improve the in-cylinder cycle with the recovered exhaust heat. In this paper, a turbocharged diesel engine is selected and researched with thermodynamic, simulational and experimental methods, and the thermodynamic model of this new proposed waste heat recovery approach is built first, followed by investigations of parameter effects, matching strategies and experimental tests.Firstly, a thermodynamic cycle model of the turbocharged diesel engine is established with all the assumptions of the ideal cycle are relaxed, such as the working fluid properties, heat transfer loss, combustion loss, air exchange loss and so on. In order to depict effects of steam injection on the turbine flow characteristics, a part empirical model are deduced based on the radial turbine physical model of two nozzles in series, which has been demonstrated with high regression accuracy, less fitting coefficients, simple function structure and powerful extrapolating and interpolating capacities for the turbine map. With the deduced turbine model, the thermodynamic cycle model of the exhaust heat recovery system is established on the base of the original turbocharged diesel engine model.Investigations on the turbocharged diesel cycle show that there exists an optimum excess air coefficient under different operating conditions, which increases with the increase of the turbocharger efficiency and the decrease of the load. The pre-turbine steam injection process is studied with energy balance and exergy balance theories. The results show that the injected steam mass flow has the biggest influences on the turbine output followed by the injected steam temperature, and the injected steam pressure almost has no influence. The adoption of the Miller cycle always needs a high performance turbocharger and intercooler. Effects of pre-turbine steam injection and the Miller cycle on the engine performances are studied. The results show that the pre-turbine steam injection shows benefits under operating conditions with low excess air coefficient and high turbocharger efficiency, solely adopting the Miller cycle can improve the cycle performance with high excess air coefficient and low turbocharger efficiency, and the pre-turbine steam injection combined with the Miller cycle can further improve the engine performance when the excess air coefficient is located near the optimum value.Then the turbocharged diesel engine matching map is drawn to study the parameters matching strategies under different operating conditions based on air consuming characteristics of the diesel engine and air supplying characteristics of the turbocharger. The matching map shows that the pre-turbine steam injection should be combined with the Miller cycle at high speed operating conditions considering the compressor choking, the injected parameters at middle speed are limited by the maximum cylinder pressure, and effects of the pre-turbine steam injection on the intake pressure are not obviously due to the slopes of the air consuming and supplying characteristics of the turbocharged diesel engine. In order to avoid surging, the compressor with a high compress ratio range is always needed when adopting this waste heat recovery system. With the turbocharged diesel engine HC4132 matched at the maximum torque speed, the parameters matching strategies are validated in GT-Power software. For the test bench diesel engine HC4132 matched at the rated speed, the matching parameters of waste heat recovery system at different operating conditions are optimized with the theory of Design of Experiment, which can be used to guide the design of experiment components and schemes.Lastly, experimental components of this waste heat recovery system are designed and manufactured, followed by the construction of the waste heat recovery test bench. The diesel engine thermal balance experiment, heat exchanger performance experiment, pre-turbine steam injection experiment, Miller cycle experiment and pre-turbine steam injection combined with Miller cycle experiment are conducted in sequence under the full load operating conditions. The results show that although the cooling system bring much more waste heat than the intercooler, the exergy contained in those two heat sources are almost equal and much lower than that in the exhaust gas. The heat exchanger efficiency mainly depends on the steam mass flow ratio. For the high speed operating condition, the increased air mass flow caused by the pre-turbine steam injection increase the pumping loss and the friction loss, which make the special fuel consumption increase slightly. For the low speed operating condition, the increased air mass flow increases the high pressure cycle output, which makes the fuel consumption decrease obviously. With the Miller cam shaft, the pumping loss at high speed condition decreases, which benefit the fuel economy slightly. For the medium and low speed condition, decreased air mass flow with the Miller cam deteriorates the combustion process and the power output. At high speed condition, pre-turbine steam injection combined with the Miller cycle can keep the excess air coefficient at the original level and decrease the pumping loss at the same time, the fuel economy can be further improved.