Ideal Computer Analysis of a Novel Engine Concept

Paper #:
  • 960080

  • 1996-02-01
Craven, K. and Clark, N., "Ideal Computer Analysis of a Novel Engine Concept," SAE Technical Paper 960080, 1996,
A novel engine concept, currently under study, addresses many of the problems commonly associated with conventional internal combustion engines. In its simplest form the novel engine consists of a single crankshaft operating both a piston compressor and a piston expander which are connected by a continuous flame combustion chamber. One might regard this as a Brayton piston engine which is similar to a previous engine investigated by Warren. Also, due to the use of piston cylinders as the compression and expansion devices, this engine varies little mechanically from current engine technology thus allowing for easy implementation. The main improvement from conventional engine design is that the expansion cylinder can have a larger displacement than that of the compression cylinder. This allows more power to be extracted by lowering the loss due to blowdown and this will increase the thermal efficiency.The air-standard efficiency of the novel engine falls between that of the Diesel and Brayton Cycles. This has been shown to be the case both numerically and algebraically. A dimensionless ideal computer model has been developed and the displacement ratio (expansion cylinder displacement to compression cylinder displacement) has been extensively explored. The computer program was used to study the effect of step size on the model and it was found that a step size of 0.1 degrees crank angle achieved the best numerical results when compared to the algebraic solution. Implementing this step size and analyzing several different configurations, the results of the ideal computer model matched closely the algebraic model. The computer model was then used to identify trends that can be expected from the novel engine. In a working engine, the optimum displacement ratio would be influenced by friction losses which will decrease the efficiency at higher values and may affect the trends seen in the ideal computer analysis.The computer program was written to follow an ideal steady-state situation where all of the working gas which entered the combustion chamber then entered the power cylinder. This forced the ideal computer model to assimilate a steady-state of operation by allowing the intake valve of the expansion cylinder to remain open until such time as the appropriate amount of working gas had entered. For an actual engine, the intake valve closing time will not vary and therefore, a non-steady flow could occur. There will exist, however, one (and only one) pressure in the combustion chamber which would cause the flow to be steady from cycle to cycle. Several values of expansion cylinder intake valve closing time were investigated and it was found that 40 degrees after top-dead-center of the expansion cylinder was an optimum for the ideal case.Future research will address transient operation and will include real world losses due to friction, blowby, and heat transfer.
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