Search Results

A quasi-dimensional computer simulation model is presented to simulate the thermodynamic and chemical processes occurring within a spark ignition engine during compression, combustion and expansion based upon the laws of thermodynamics and the theory of equilibrium. A two-zone combustion model, with a spherically expanding flame front originating from the spark location, is applied. The flame speed is calculated by the application of a turbulent entrainment propagation model. A simplified theory for the prediction of in-cylinder charge motion is proposed which calculates the mean turbulence intensity and scale at any time during the closed cycle. It is then used to describe both heat transfer and turbulent flame propagation. The model has been designed specifically for the two-stroke cycle engine and facilitates seven of the most common combustion chamber geometries. The fundamental theory is nevertheless applicable to any four-stroke cycle engine.

This paper describes an experimental comparison of loop and cross scavenged single-cylinder research engines. The cross scavenged engines have employed the QUB type deflector piston. The initial results show that the QUB cross scavenged engine exhibited inferior performance characteristics. Utilizing the QUB single cycle test rig, a study of the QUB cross scavenging system has shown that the bore-to-stroke ratio significantly influences the scavenging behaviour; reduction of the bore-to-stroke ratio from over-square values gave improved characteristics. On the basis of this finding, a new cross scavenged cylinder barrel was designed. In a subsequent series of dynamometer tests, improvements in power, fuel economy and emission characteristics were recorded for the new cylinder. These improved results approximate closely to those recorded for the loop scavenged engine and are considerably superior to those of the original cross scavenged cylinder.

The improvement in both performance and thermal efficiency of internal combustion engines at higher compression ratios is a well known phenomena. Indeed, a simple Otto Cycle analysis show a potential efficiency improvement of 13% by increasing the compression ratio from 9:1 to 15:1. However, the dilemma for engineers has always been in the realization of a practical operational mechanism. This paper describes the ECCLINK VCR mechanism which enables compression ratio to be altered within given limits on a running engine. A single cylinder 500 cm3 four-stroke research engine, incorporating the ECCLINK mechanism, has been built and tested. Results are presented at both full load and part load over a range of compression ratios, showing improvements in performance and fuel economy. Of particular interest is the fact that full load bsfc improvements equate to typical Otto cycle values.

This paper details a theoretical and experimental study of combustion phenomena within a two-stroke cycle, spark ignition engine. The theoretical part of the work involved the development of an improved quasi-dimensional combustion model. This model was incorporated into a computer program which was used to predict the thermodynamic and chemical changes occurring within a two-stroke engine during the closed cycle of the engine. The simulation uses a turbulent kinetic energy model to predict flame front velocity. Combustion chamber geometry is used to estimate entrained mass and mass fraction burned is calculated from a simple eddy-entrainment approach. The experimental work was undertaken to validate the combustion model. Two separate cylinder heads were designed with different combustion chambers and tested on a standard loop-scavenged engine over a range of operating conditions.

A computer simulation of a high performance two-stroke engine has been validated by a transient test method. The simulation included unsteady gas dynamics, together with detailed mechanical models for the reed valve, etc. The combustion model used Vibe coefficients derived directly from measured cylinder pressures. Cylinder scavenging characteristics which were measured on The Queen's University of Belfast (QUB) single-cycle rig were also incorporated in the simulation. Validation of the model involved use of an inertial dynamometer and data acquisition system which has been developed at QUB. This dynamometer incorporates a flywheel which is directly coupled to the engine. During a test, the engine is accelerated, and the torque and power are calculated from the flywheel speed characteristics. Further, at pre-determined engine speed intervals, the cylinder and exhaust pressures may be recorded. This testing system provided a convenient and rapid experimental method to acquire data.