Numerical Flow Simulation of a Natural Gas Engine Equipped with an Unscavanged Auto-Ignition Prechamber

A three-dimensional Reynolds-Averaged Navier-Stokes (RANS) simulation was carried out on an unscavanged auto-ignition prechamber-cylinder configuration in order to understand better the ignition conditions inside the engine. The sensitivity to different boundary conditions like initial gas and wall temperature was tested and flow simulations coupled with a 55 species chemical mechanism were carried out in order to determine the starting location of auto-ignition. It was shown that the initial gas temperature has the greatest influence on the ignition timing and that there still is a high risk of premature ignition inside the main chamber with the current prechamber design. ∗ Corresponding author: [email protected] EPFL / LENI-ISE-STI, Bat. ME A 2, Station 9, 1015 Lausanne, Switzerland; http://leni.epfl.ch Proceedings of the European Combustion Meeting 2007 Introduction This work was undertaken as part of on-going research at the Industrial Energy Systems Laboratory (LENI) on stationary natural gas (co-generation) engines equipped with ignition prechambers. Successful operation of engines with spark-ignited prechambers has been demonstrated for both natural gas and biogas [1-3]. A major advantage of this system is that the stringent Swiss emission regulations [4] may be met without catalytic post-treatment of the exhaust gases, while still achieving good thermal efficiencies. This is of particular importance in the case of biogas, where use of a catalyst would require costly gas pre-treatment. A new approach has been pursued at LENI in order to trigger homogeneous auto-ignition inside the prechamber, similar to HCCI. Ignition inside the prechamber is promoted by resistive heating of the upper prechamber part. This contrasts with previous works [5 8] where prechamber ignition was achieved by direct injection into the prechamber, or with a spark or glow plug ignition system, all requiring relatively large prechambers. Potential advantages of the new system include greater service intervals since there is no need to replace spark plugs. Lower emissions of carbon monoxide and unburned hydrocarbons are anticipated because, compared to a spark ignited prechamber, the auto-ignition prechamber design gives a faster transfer of the flame from the prechamber into the main chamber. Furthermore, homogeneous ignition avoids the initial expulsion of cold, un-reacted gas from the prechamber, which reduces the necessary volume of the prechamber for equivalent ignition performance. The heated auto-ignition prechamber design has been installed on an experimental single-cylinder engine at LENI. Its specifications are given in Table 1 and a schematic drawing of the prechamber setup is shown in Fig. 1. Table 1 Engine operation parameters