Combustion, efficiency, and inlet dynamics in a Recuperated Split Cycle Expander

Student thesis: Doctoral Thesis


The recuperated split cycle engine (RSCE) is a split cycle engine with quasi isothermal compression and exhaust recuperation. This novel concept allows the decoupling of efficiency and peak combustion temperature. In this work a methodology for evaluating the efficiency limits brought on by emissions limitations of thermal engine is demonstrated, before focussing on modelling and experimental work on the expander/combustor of a RSCE.

When evaluating the RSCE expander in isolation of the other systems, thermal and brake efficiency metrics can be skewed by the free Rankine work of the simulated recuperator. Metrics are proposed in this work which discount this effect to enable evaluation of the efficiency of the expander. A 0D model of the single cylinder combustion research engine (SCCRE), representing the RSCE expander, is presented. With studies and analysis of responses to key variables effecting performance, engine setup, and expander design analysed and discussed.

Experimental results and analysis produced from the SCCRE test bed demonstrated 35.6% efficiency. However, this was limited by low combustion efficiencies of 60- 70%. If combustion efficiency (CE) can be increased, 50.6% efficiency in the expander is demonstrated to achievable, before consideration of additional work reductions and heat recuperation from quasi-isothermal compression and exhaust recuperation in a full RSCE system.

High FSN (> 1) results combined with lowering CE at the higher pressures tested (>3.5MPa) demonstrated low combustion system optimisation at high inlet pressures and therefore high load conditions. With these factors in mind, the best BSNOx achieved from the parameter swings was 2.16g/kWh. With nitrogen dilution to 18% oxygen by volume, this drops to 0.58g/kWh. Emissions data from the expander cylinder suggested a lower plateauing of FSN and increasing NOx response to diesel rail pressure than that of a conventional ICE.

Hypotheses of mixing methods and conditions affected by high pressure air injection at that could be occurring in the SCCRE and RSCE are proposed, due to the unique inlet dynamics in a RSCE expander at intake valve opening. Schlieren optical data from a high pressure flow rig, replicating the SCCRE cylinder head, is presented which starts to investigate these hypotheses. The initial results confirm that significant shockwaves are created in cylinder by the intake valve and high pressure upstream conditions. This is unique to split cycle engines, has not been reported in the literature, and, as hypothesised, is likely contributing to responses seen.

To fully capitalise on the potential of the RSCE, a combustion, fuel, and air injection system needs to be designed and developed from the ground up with the understanding of unique operation and conditions. A well designed RSCE could in theory provide an on demand premix style of combustion with high combustion efficiency and low emissions. A few potential methods and concepts to achieve this are proposed and discussed.
Date of AwardJun 2023
Original languageEnglish
Awarding Institution
  • University of Brighton
SupervisorRobert Morgan (Supervisor), Morgan Heikal (Supervisor), Dr Konstantina Vogiatzaki (Supervisor), Andy Atkins (Supervisor), Konstantina Vogiatzaki (Supervisor) & Robert Morgan (Supervisor)

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