Currently, energy shortage and environment pollution are becoming increasingly serious. Energy saving has become an important topic requiring urgent attention. Engines are primary power source which have been widely used in vehicles, agricultural machineries, industrial machineries and stationary power units. In an industrial country, about 60–70% of fossil fuel is consumed by engines. However, over 50% of the total fuel combustion heat in an engine is wasted by the exhaust gas and engine coolant under most operating conditions, resulting in energy waste and emission problems [1], [2] and [3]. Hence, engine waste heat recovery is important. Using thermodynamic cycle generating power is a suitable method for doing so.
Generally speaking, the waste heat recovery of exhaust gas should be considered first due to its high temperature property and large amount of heat. The maximum exhaust temperature of a gasoline engine can reach about 800 °C. Due to such a high-grade heat, the traditional waste heat recovery system, Organic Rankine Cycle (ORC) system, is restricted because most organic fluids have a relatively low thermal decomposition temperature. It is infeasible to transfer heat from a high temperature exhaust gas to organic fluids directly. To avoid the decomposition of the working medium at high temperature, some researchers have added a thermal oil circuit between exhaust gas and working fluids [4] and [5]. But this method requires complicated modification engineering on the existing plant and would result in additional energy and exergy loss. Hence, certain research focuses on high-temperature ORCs with high-decomposition temperature organic fluids (e.g. Alkanes [6] and Siloxanes [7]), or with a dual-loop configuration [8] and [9]. Unfortunately, high-temperature organic fluids are mostly flammable, which may cause serious safety issues. Hence it is difficult for the experiment and the application of a high-temperature ORC. In a word, it is still important to find environment-friendly, high-efficiency, feasible and safe working fluids to match the high temperature exhaust gas.
For the past few years, choosing carbon dioxide (CO2) as the working fluid of transcritical Rankine cycle system has attracted increasing levels of attention. The characteristics of CO2 can overcome the shortcomings of organic fluids mentioned above. First of all, CO2 has no limitation of thermal decomposition temperature. The heat transfer process between CO2 and exhaust gas can be realized directly, and a better temperature match is obtained by the transcritical process in the evaporator, which reduces both energy and exergy loss. Secondly, CO2 is environmental friendly, non-flammable and non-corrosive. Hence, CO2-based systems feature less safety issues even in the leakage event of the working fluid. Furthermore, CO2 is low-cost and readily available. These advantages make its experiment and application convenient. Echogen Power Systems (EPS) company [10] and [11] in the USA has done preliminary tests of a 250 kW gas waste heat recovery system using CO2 as the working fluid, which indicates the possibility of recovering the waste heat of engine exhaust gas. In the automotive field, CO2 has been applied in automobile air conditioning which also indicates the miniaturization possibility of a CO2-based system used for waste heat recovery of vehicle engine [12] and [13].
The CO2-based transcritical Rankine cycle (CTRC) used for engine waste heat recovery is available but rare in the open literatures. Only some simple thermodynamic analysis of the CTRC was conducted for engine waste heat recovery [14] and [15]. Further analysis of the CTRC used for engine waste heat recovery is required, such as component size analysis, which is not included in previous research but of key importance for system assessment, especially for waste heat recovery of automotive engine. Other research involving the CTRC put more emphasis on the field of low temperature waste heat recovery and usually in comparison with the ORC [16], [17], [18], [19], [20] and [21]. Their results show that the CTRC system is superior in heat transfer performance and performs good economic performance compared with the traditional ORC, but the corresponding net power output and thermal efficiency are lower. The solution to improve the net power output and thermal efficiency generally focuses on adding regenerator to the CTRC system, but results show that the regenerator improved the thermal efficiency but had little effect on the net power output [22] and [23].
In contrast to the traditional scheme mentioned above, this paper proposes an improved CTRC system used for engine waste heat recovery, which improves not only the thermal efficiency but also the net power output. This improved CTRC system (PR-CTRC) contains a preheater and a regenerator simultaneously, which uses the engine coolant as a preheat source.
Except for routine thermodynamic analysis, the analy