A novel split cycle internal combustion engine with integral waste heat recovery

Abstract

h i g h l i g h t sA novel engine thermodynamic cycle is proposed. Theoretical analysis is applied to identify the key parameters of the thermodynamic cycle.The key stages of the split cycle are analysed via one-dimensional modelling work. The effecting mechanism of the split cycle efficiency is analysed. a b s t r a c tTo 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.

The Recuperated Split Cycle - Experimental Combustion Data from a Single Cylinder Test Rig

“…The heat release values derived using WAVE were transferred as an input to the AMEsim cycle model. Details of the modelling approach were reported previously by Dong [8,9].…”

“…Advanced, low heat rejection combustion systems such as Reactivity Controlled Compression Ignition (RCCI) [3] and alternative engine cycles, such as the double expansion cycle [4] and opposed piston cycles [5] have been suggested, but the authors review of the literature suggests efficiencies of 55% are the maximum that can be expected from these concepts. The recuperated split cycle engine was first proposed by Coney [6] and further developed by Jackson [7] and Dong [8,9] as a radical departure from the conventional Otto and Diesel cycles. In the split cycle engine, the combustion and compression processes are separated into different chambers with intracycle waste heat recovery.…”

A review of split-cycle engines

“…This added flexibility allows utilisation of the features listed below to possibly increase performance compared to a four-stroke [7]. The potential benefits of these effects compared to alternatives are shown in 1.…”

“…Sealing issues with the compressor cylinder also caused water leaks into the crankcase when the engine speed was increased to 600 rpm, imposing speed limitations. A more recent analytical study published in 2015 [7] gave the IsoEngine a slightly lower brake thermal efficiency prediction of 53%.…”

“…The design, as described in the first paper [5], can be seen as the split-cycle in its most basic form, and is not too distant from Webb's 1913 design [9]. This is because it uses none of the unique split-cycle benefits listed earlier by Dong [7]. A small volume ratio, expansion cylinder to compression cylinder, of 0.88 was used.…”