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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.

A mathematical model has been developed which predicts the tailpipe exhaust emissions of two-stroke cycle engines utilising an oxidising catalytic converter. This model is currently one-dimensional and has been developed as an aid to the design of engine/exhaust systems. The experimental rig employed has a two-fold function, its primary task was to aid in the validation of the model. Secondary to this it was used to simulate the gaseous properties of the exhaust gas at various positions in the exhaust system. The validation exercise is currently proceeding utilising metallic substrate technology with preliminary results indicating that the model is showing good correlation to measured 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.

Previously, an initial investigation examined the effect of the catalytic substrate on the gas dynamics of the blowdown pulse on the QUB single shot rig. This initial investigation measured the resulting waves from the catalytic converter in the exhaust pipe. In this early study the substrate was at ambient temperature but it is recognised that after light-off higher reaction temperatures will result from the exothermic nature of exhaust gas oxidation and reduction. Therefore substantially different results will occur. This paper details a series of experiments which investigate the influence of an operating catalyst on the unsteady gas dynamics in an exhaust system using the QUB single shot rig. In addition to measuring the effect of temperature on the gas dynamics previous work is reviewed with emphasis now on specifically measuring the features present rather than having to decipher superimposed pressure traces.

The continued pursuit in Europe for lower emissions from transport vehicles now identifies several new areas to be targeted as their total emissions become ever more significant when compared to the continued decrease in automotive emissions. One such transport area that now faces pressure in the reduction of exhaust emissions, is the scooter/ moped market. The new ECE R47 cycle that governs the operational mode of the vehicle specifies a typical driving cycle over which the total emissions are collected and analysed. This paper evaluates a carburetted 50 cc moped over such a cycle and from the results ascertains the possibility of catalysing the exhaust gas to achieve acceptable limits. An empirical catalyst model is used to predict exhaust gas and substrate bed temperatures with the view to prolonging durability of the catalyst support. Results are presented for operating strategies which offer better long-term durability.

An experimental transient catalyst test rig has been developed and used to investigate the location and intensity of the reactions in a two-stroke oxidation catalyst as the inlet temperature is increased from ambient through light-off at a rate which is similar to that found in an engine exhaust after a cold start. The catalyst samples used in this apparatus are relatively large by plug flow reactor standards, (50mm diameter x 70mm long), which allows significant radial and axial variations in temperature and activity to occur. The gas and temperature readings recorded during these tests show that, even though the front face of the catalyst is hotter than the rear due to the transient temperature ramp, the highest reaction rate occurs in the rear half of the catalyst during the early stages of light-off. As the light-off process progresses, the reaction zone moves in a radial direction and migrates towards the front face.

The after-treatment of exhaust gas using 3-way catalytic converters is now normal practice in automotive applications. For other applications, such as outboards, motorcycles and utility engines, new legislation is now in place in both Europe and North America. Further reduction of the permitted emission levels require the use of catalysts for two-stroke engine applications. However, current automotive catalyst systems are not suitable for durable operation in most two-stroke engines and new analytical tools are required to aid the development engineers in the implementation of revised designs and operating strategies. This paper reviews the range of modeling techniques which have been developed for automotive uses and presents new and modified models suitable for two-stroke engines. This requires particular emphasis to be placed on the oxidation reactions that predominate in the two-stroke engine exhaust.

The results of this paper will show the reader how to quantify a minimum light-off temperature to meet the required emissions standards with the use of a 3-way catalytic converter. The method can be applied to both motorcycle and larger automotive catalysts to help meet their respective emissions standards (Euro 5/Euro 7). The ability to predict a light-off temperature for any catalyst at the beginning of the project saves both time and resource. With an emphasis on how the shape of the light-off curve affects the cumulative tailpipe emissions and how shape of the light-off curves change with the ageing process. Changes in the light-off curves will be reviewed to understand how the chemical reactions and pore diffusion mechanisms within the catalyst deplete to negatively affect performance over its life time.

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.

This paper demonstrates the very useful technique of calculating air-to-fuel ratio, AFR, from exhaust gas emissions for a two-stroke cycle engine. Such methods are widely used for four-stroke engines where direct air flow measurement has now become redundant. Two modified methods are presented and compared with three standard methods, showing the accuracy to be quite good for a large set of test data from a standard two-stroke engine. A procedure for estimating AFR of the in-cylinder burning region, using trapping efficiencies, is presented for stratified charge engines, such as those with direct fuel injection. Accuracy of emissions measurement is assessed by calculating the total dry exhaust emissions, a method which could easily be automated for general test cell use. Finally, exhaust gas molecular weight and wet/dry ratio calculations are considered.