This work investigates the implications of natural gas composition on the combustion in a heavy-duty natural gas engine and on the associated pollutant emissions. In this engine system, natural gas is injected into the combustion chamber shortly before the end of the compression stroke; a diesel pilot that precedes the natural gas injection provides the ignition source. The effects of adding ethane, propane, hydrogen, and nitrogen to the fuel are reported here. The results indicate that these additives had no significant effect on the engine's power or fuel consumption. Emissions of unburned fuel are reduced for all additives through either enhanced ignition or combustion processes. Black carbon particulate matter emissions are increased by ethane and propane, but are virtually eliminated by including nitrogen or hydrogen in the fuel.
The ignition delay time of two stoichiometric methane/hydrogen/air mixtures has been measured in a shock tube facility at pressures from 16 to 40 atm and temperatures from 1000 to 1300 K. Overall, the observed reduction in ignition delay with some methane replaced by hydrogen is relatively small given the large concentration of hydrogen involved in the current study. With a high hydrogen mole fraction (35% of the total fuel), a reduction of the ignitionpromoting effect was observed with reduced temperature. A detailed chemical kinetic mechanism was used to simulate ignitions of test mixtures behind reflected shocks. An analysis of the mechanism indicates that at higher temperatures, the rapid decomposition of hydrogen molecules leads to a quick formation of H radical pools, which promote the chain branching through H + O 2 O + OH. At lower temperatures, the branching efficiency of hydrogen is low; a weak effect of hydrogen on methane ignition could be result from the reaction between H 2 and methylperoxy CH 3 O 2 , which contributes extra H radicals to the reaction system. The effects of hydrogen also decrease with increasing pressure; this is related to the negative pressure dependence of hydrogen at the second ignition limit. C 2006 Wiley Periodicals, Inc. Int J Chem Kinet
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