D. Yap

Effect of hydrogen addition on natural gas HCCI combustion

Natural gas has a high auto-ignition temperature, requiring high compression ratios and/or intake charge heating to achieve HCCI (homogeneous charge compression ignition) engine operation. Previous work by the authors has shown that hydrogen addition improves combustion stability in various difficult combustion conditions. It is shown here that hydrogen, together with residual gas trapping, helps also in lowering the intake temperature required for HCCI. It has been argued in literature that the addition of hydrogen advances the start of combustion in the cylinder. This would translate into the lowering of the minimum intake temperature required for auto-ignition to occur during the compression stroke. The experimental results of this work show that, with hydrogen replacing part of the fuel, a decrease in intake air temperature requirement is observed for a range of engine loads, with larger reductions in temperature noted at lower loads. It is also shown that the low NOx emissions and high rates of heat release, typical for HCCI, are retained with hydrogen-assisted operation, especially at low engine loads. A practical possibility of producing the necessary hydrogen in a fuel reformer fitted in the exhaust gas recirculation system is illustrated for one engine condition.

Research on expansion of operating windows of controlled homogeneous auto-ignition engines

Abstract A major effort has been made to surmount the current obstacles to expanding the operating window of homogeneous charge compression ignition (HCCI) engines. The research involves extensive experimental studies on single-cylinder and multi-cylinder engines and the work is devoted to the development of on-board fuel-reforming technology and to the application of supercharging combined with trapping of residual gases in the cylinder. Fuel reforming yields significant quantities of hydrogen and is used to extend the lower load boundary while supercharging and internal exhaust gas recirculation (EGR) trapping are used to increase the upper load limit of HCCI engines. The present paper highlights the main findings of the research to date; in particular it reveals that using a combination of technical elements for effective control of auto-ignition in a typical passenger car engine configuration is possible and promising.

Effect of inlet valve timing on boosted gasoline HCCI with residual gas trapping

With boosted HCCI operation on gasoline using residual gas trapping, the amount of residuals was found to be of importance in determining the boundaries of stable combustion at various boost pressures. This paper represents a development of this approach by concentrating on the effects of inlet valve events on the parameters of boosted HCCI combustion with residual gas trapping. It was found that an optimum inlet valve timing could be found in order to minimize NOx emissions. When the valve timing is significantly advanced or retarded away from this optimum, NOx emissions increase due to the richer air / fuel ratios required for stable combustion. These richer conditions are necessary as a result of either the trapped residual gases becoming cooled in early backflow or because of lowering of the effective compression ratio. The paper also examines the feasibility of using inlet valve timing as a method of controlling the combustion phasing for boosted HCCI with residual gas trapping.

AN EXPERIMENTAL STUDY OF BIOETHANOL HCCI

ABSTRACT Bioethanol has been successfully used in conventional spark ignition (SI) internal combustion engines. Homogeneous charge compression ignition (HCCI) combustion, a novel combustion method, has shown the potential for low nitric oxides (NOx) emissions with no particulate matter formation. This paper explores two different approaches to achieve HCCI with bioethanol; namely, trapping of internal residual gas and intake temperature heating with a high compression ratio. For naturally aspirated HCCI operation with residual gas trapping on a spark ignition engine, although the NOx emissions were low, the load range was unacceptably small. When inlet manifold pressurisation was employed, a substantial increase in the upper load boundary could be achieved without any substantial increase in NOx emissions. With forced induction, the feasibility of using boost control as the main method of load control for higher engine loads during HCCI operation has been explored with possible methods of utilizing boost control. One possible strategy is a map based strategy where fuelling rates are correlated versus boost pressure and trapped residual amounts. A proof of concept using this strategy showed that transient operation from a low load to a much higher load, using boost control might be possible without engine misfire.