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A novel intra-cycle waste heat recovery (ICWHR) methodology, applied to an internal combustion engine is presented in this study. Through a split type thermodynamic cycle design, quasi-isothermal compression of the charge air and isobaric combustion of the air/fuel mixture can be performed separately in two chambers. Within such a design, the exhaust heat can be recovered to the intake air flow between the compression chamber and combustion chamber. Consequently, the recovered energy can be re-utilized in the combustor directly, and an intra-cycle waste heat recovery process can be achieved. To investigate the fundamental aspects of this new methodology, a comparative study between the conventional Rankine based WHR and the new ICWHR was undertaken. Both theoretical and numerical analysis were applied to evaluate the performance characteristics of these two technologies. The ICWHR cycle differs from the Rankine cycle in that an energy conversion subsystem is not necessary since the recovered energy is sent back to the combustion chamber directly, and then the system efficiency is improved significantly. Furthermore, the theoretical results indicate that the full cycle efficiency of ICWHR system is determined by the regeneration effectiveness, the compression ratio and the fuel equivalence ratio, then the limitations of Rankine cycle, such as working fluid selection and system parameter calibration can be avoided mechanically. Finally, through a one dimensional system model, analysis of optimal operation range, system efficiency and the heat transfer behaviours of ICWHR system are discussed in this paper and comparisons made with a Rankine cycle WHR system. [ABSTRACT FROM AUTHOR]

The split cycle engine is a new reciprocating internal combustion engine with a potential of a radical efficiency improvement. In this engine, the compression and combustion–expansion processes occur in different cylinders. In the compression cylinder, the charge air is compressed through a quasi-isothermal process by direct cooling of the air. The high pressure air is then heated in a recuperator using the waste heat of exhaust gas before induction to the combustion cylinder. The combustion process occurs during the expansion stroke, in a quasi-isobaric process. In this paper, a fundamental theoretical cycle analysis and one-dimensional engine simulation of the split cycle engine was undertaken. The results show that the thermal efficiency (η) is mainly decided by the CR (compression ratio) and ER (expansion ratio), the regeneration effectiveness (σ), and the temperature rising ratio (N). Based on the above analysis, a system optimization of the engine was conducted. The results showed that by increasing CR from 23 to 25, the combustion and recuperation processes could be improved. By increasing the expansion ratio to 26, the heat losses during the gas exchange stroke were further reduced. Furthermore, the coolant temperatures of the compression and expansion chambers can be controlled separately to reduce the wall heat transfer losses. Compared to a conventional engine, a 21% total efficiency improvement was achieved when the split cycle was applied. It was concluded that through the system optimization, a total thermal efficiency of 53% can be achieved on split cycle engine. [ABSTRACT FROM AUTHOR]

To achieve a step improvement in engine efficiency, a novel split cycle engine concept is proposed. The engine has separate compression and combustion cylinders and waste heat is recovered between the two. Quasi-isothermal compression of the charge air is realised in the compression cylinder while isobaric combustion of the air/fuel mixture is achieved in the combustion cylinder. Exhaust heat recovery between the compression and combustion chamber enables highly efficient recovery of waste heat within the cycle. Based on cycle analysis and a one-dimensional engine model, the fundamentals and the performance of the split thermodynamic cycle is estimated. Compared to conventional engines, the compression work can be significantly reduced through the injection of a controlled quantity of water in the compression cylinder, lowering the gas temperature during compression. Thermal energy can then be effectively recovered from the engine exhaust in a recuperator between the cooled compressor cylinder discharge air and the exhaust gas. The resulting hot high pressure air is then injected into a combustor cylinder and mixed with fuel, where near isobaric combustion leads to a low combustion temperature and reduced heat transferred from the cylinder wall. Detailed cycle simulation indicates a 32% efficiency improvement can be expected compared to the conventional diesel engines. [ABSTRACT FROM AUTHOR]