Emissions Variability in Gaseous Fuel Direct Injection Compression Ignition Combustion 2005-01-0917
Measurements of ignition characteristics and emissions were made in a shock tube facility operating at engine-relevant conditions. Methane and methane/ethane fuels were injected down the centerline of the shock tube using an electronically controlled prototype gaseous fuel injector developed by Westport Innovations. Air was preheated and compressed using a reflected shock technique that produced run times of 4-5 ms.
Particulate matter (PM) emissions were found to be highly intermittent. In only 6 out of 97 experiments was PM detected above background levels. In all of these 6 sooting experiments ignition kernels were located relatively close to the injector tip and ignition occurred prior to the end of fuel injection. Using the large orifice injector tip with pure methane fuel, PM was detected in 4 out of 28 experiments; using the small orifice with pure methane fuel, no PM was detected in any of 50 experiments. When 10% ethane was added to the fuel, PM was detected in 2 out of 20 experiments.
Orifice size was found to significantly influence emissions of nitrogen oxides (NOx). Emissions were reduced by over an order of magnitude when the orifice size was reduced by a factor of 4. Run-to-run variability in NOx emissions was relatively high at nominally constant operating conditions. NOx variability was associated with variability in ignition timing. At fixed operating conditions, NOx emissions were found to increase significantly with advancing ignition timing. Over the range of conditions studied, NOx emissions were found to be relatively insensitive to injection pressure ratio and injection duration.