Spark Breakdown Voltage Sampling During Early Stage Compression
This thesis proposes a novel methodology to enable cycle by cycle control of a two-stroke cycle type engine. These engines are well known for offering high specific power density solutions, however, this advantage cannot be fully exploited without new technologies enabling significantly reduced emissions and improved fuel economy. If this could be provided, working with direct fuel injection, new highly efficient, low emission, power units could result. One of the main reasons why this has not previously been achieved has been the inability to accurately measure and quantify the amount of combustible charge available for metering of the Air/Fuel ratio. This is due to the highly dynamic gas conditions in the engine which cause significant cyclic variations of scavenging and trapping efficiencies. Existing combustion control methods are unable to accurately compensate for these conditions because fuel quantity is determined using the results of previous combustion events which do not reflect the actual gases available for each combustion. This thesis proposes a different approach, whereby accurate fuel quantities could be determined cyclically from in-cylinder measurements ahead of each combustion event. The intention being, for optimal fuel quantities and ignition initiation timings to be calculated and provided for each cycle. This technology would significantly improve the ability to achieve an optimal combustion of each individual combustion event. The principle of measurement uses and extends proven existing extensive scientific knowledge of the relationships between the value of Spark Break-Down Voltage (SBDV) to gas density and speciation. The methodology presented, applied pulses of voltage to the spark plug, which is normally used only to initiate ignition, to also function as a non-intrusive in-cylinder sensor. Experimental results were obtained using three items of equipment purposely designed and manufactured for the present work. These consisted of a) A new high frequency spark breakdown voltage electronic circuit. b) A static volume sparking chamber. c). A motored test engine into which exhaust gas was supplied from an auxiliary engine via an air mixing system. The novel use of an auxiliary engine enabled a wide range of mass fractions to be subjected to cyclic compression events for evaluation independent of test engine conditions.
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