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An evaluation of partially stratified charge ignition in a direct injection natural gas engine

University of British Columbia

The challenge of reducing tailpipe emissions, while retaining performance, continues to motivate new engine technologies. To this end, natural gas shows promise as a clean burning alternative fuel. However, an efficiency penalty of conventional spark ignited engines persists when these are fuelled by natural gas. This penalty is due to pumping losses, which arise as the intake mixture is throttled to maintain stoichiometry over the engine's operating range. Direct injection (DI), used to create a globally lean stratified charge, provides a load control method which is decoupled from throttling. Previous work has shown this concept to be viable, but plagued by poor fuel usage and high emissions of unburned hydrocarbons. Partially Stratified Charge (PSC) technology is a novel high-energy ignition method. It involves a standard spark plug, modified such that a small portion of natural gas (<5% of the total fuel charge) is injected directly into the vicinity of the electrodes. The intention is to surround these with an easily ignitable mixture, thus ensuring a strong flame kernel that propagates through a marginal bulk charge. PSC has already been shown to improve combustion stability in ultra-lean, homogeneous mixtures. This study was conducted to determine if PSC can be combined with direct injection to improve combustion stability in a stratified charge engine. A single cylinder research engine was modified to include a natural gas direct injector, as well as PSC ignition. Experiments were conducted over a wide range of fuel injection timings, as well as different angles between the fuel jet and ignition source. Engine performance was poor at stratified charge operating conditions and results indicate that PSC did not reliably improve combustion. Experimental evidence suggests that the PSC system was not igniting the pilot charge as expected. Very high concentration gradients at the boundaries of the PSC and DI plumes may be the cause of the negative result. These make it difficult to reliably ensure that the spark plug discharges into the narrow and transient regions of combustible mixture. A secondary investigation was conducted in which a very dilute premixed fuel component was added to the intake air. This assured reliable ignition of the PSC pilot charge; and in this configuration, stratified charge combustion at late direct injection timings produced stable engine operation.

Text

2011-03-21

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http://hdl.handle.net/2429/32614

Mechanical Engineering

University of British Columbia

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