Search Results

Heat transfer study is particularly important in a modern engine exhaust system incorporated with a catalytic converter. Heat loss along the exhaust system, which includes exhaust ports, manifold, tailpipe and catalytic converter, causes prolonged lightoff time of catalysts at engine cold-start. This implicitly means that more noxious gas emissions in the cold-start phase will be produced at the engine tailpipe, which contributes to an overall increase in exhaust toxic emissions, in particular for urban driving where heavy traffic is frequently encountered. Modeling of exhaust heat transfer is thus necessary, as it is a powerful and cost effective tool for estimating the lightoff time and conversion efficiency of the catalysts. Furthermore, the concentrations of noxious gases in the exhaust tailpipe can be calculated.

Theoretical thermodynamic analysis reveals that, when a fixed amount of heat energy is added into an Otto cycle the thermal efficiency of that cycle can be substantially improved by increasing the expansion ratio while keeping the compression ratio unchanged to achieve a greater net work output. As such, to maximise the cycle work output, the exhaust gas is allowed to expand to atmospheric pressure within the power machinery itself. With this approach, the pressure versus specific volume diagram of this modified cycle at exhaust valve opening is thus minimised and hence waste heat recovery/utilisation such as the implementation of turbocharging system can be eliminated. This paper presents the development and design considerations of a double helical screw internal combustion engine.

Ever stringent emission legislation has called for more advanced measures for petrol engine emission control. The main chanllenge is to largely cut off the engine cold-start emissions. The problem is that the catalytic converter does not operate effectively at temperature below the range of 250 - 300 °C at engine cold start. This paper presents an alternative approach, which needs no assistance from the external heat source such as electrically heating or burner/afterburner-assisting, etc, for rapid light-off an automotive catalyst using high value of ignition retard control. Proper control strategies permit a catalyst to reach lightoff within 50 seconds from cold-start. Emission data of unburnt hydrocarbons (in terms of hexane-equivalence) and carbon monoxide at both the upstream and downstream of a catalytic converter are measured. Results show that significant reduction in emissions can be achieved with proper ignition control strategies.

This paper presents three different methods for obtaining equivalence ratio/air-fuel ratio from the internal combustion engines. Emphasis is placed on reviewing several important air-fuel ratio models based upon the analysis of gaseous emission constituents in the exhaust of the engines as alternate to that obtained conventionally from fuel and air flow rate consumptions of the engines. A thorough analysis of all the factors that may affect the accuracy of fuel-air equivalence ratio is conducted which include the ambient conditions such as temperature and humidity, the assumed water gas reaction constant and the inclusion of oxides of nitrogen constituent in the model. A comprehensive model based upon chemical reaction of air-fuel mixture is proposed with flexible inputs of fuel type and the above factors in addition to the volume concentration inputs of carbon monoxide, carbon dioxide, oxygen and unburnt hydrocarbons.

This paper claims that, instead of relying on direct fuel and air flow rate measurements, exhaust gaseous emissions analysis should be considered as the standard method for the determination of air-fuel ratio, in particular for petrol-powered engines. Various sensitivity parameters are defined and the maximum possible relative divergences of the fuel-air ratio are calculated to support the above claim. It is proved that to obtain the air-fuel ratio with the same accuracy for a petrol engine, the accuracy of the instruments used for fuel and air flow rate measurements should be at least 10 times more accurate than the emission analysers. Estimations of hydrogen and oxygen concentration based on other measurable emission concentrations in the exhaust gas stream are also conducted and validated satisfactorily with the experimental data.

A reacting multi-fluid model, based on the Favre-averaged separate transport equations for reacting gas-liquid “multi-phase” flow, is presented. New density-weighted (Favre-averaged) transport equations for multi-phase mixture fraction f͂ and its variance g are derived. The new multi-fluid transport equations for f͂ and g are equally applicable to spray flames as well as liquid metal fuel combustors. The fuel spray is discretized into a number of size groups; each group is considered as a separate “fluid” or “phase”. A pdf approach, to the reaction process, is adopted. An evaporation variable e is introduced, which is a measure of a nonequilibrium phase state, defining a two-variable pdf as a function of f and e. The instantaneous thermo-chemical properties are computed from a nonequilibrium model. The predicted results, using the present density-weighted multi-fluid model, for an airblast kerosene spray flame are compared with corresponding experimental data.