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The effects of turbulence and combustion chamber geometry on combustion in a spark ignition engine

University of British Columbia

An experimental program has been undertaken to investigate the effects of turbulence on combustion in a spark ignition engine and to examine the effects of a new combustion chamber design. The experiments were conducted in a rapid intake and compression machine using the compression stroke alone in order to eliminate the effects of intake stroke generated turbulence. The effects of turbulence scale and intensity on combustion was investigated using perforated plates to generate turbulence in the combustion chamber. The turbulence generated with three different perforated plate hole sizes was measured with a hot-wire anemometer (HWA) at several locations and orientations so that the turbulence could be characterized. Combustion experiments were run for various ignition timings with a premixed stoichiometric methane/air mixture. Pressure measurements and a mass fraction burned (MFB) analysis program were used to calculate the MFB curves and the combustion durations. The turbulence generated by the perforated plates was separated into two stages. The first stage (pre-relaxation stage) was characterized by large anisotropics with a turbulence scale characteristic of the perforated plate hole size. The second stage (relaxed stage) was characterized by homogeneous isotropic turbulence with a turbulence scale characteristic of the chamber height. Although the scales were not measured they were implied by the decay rates and the turbulence intensity. The main combustion duration (5-90% MFB) was found to decrease with increased turbulence intensity and the flame initiation period (FIP, 0-5% MFB) was found to decrease with smaller scales even when counteracted by a lower turbulence intensity. This suggests that the generation of small scale turbulence could be used to decrease the FIP in a spark ignition engine. The compression stroke turbulence generating characteristics of a standard bowl-in-piston design and several new "forced squish-jet" designs were examined using HWA measurements. The "forced squish-jet" designs used a ridge at the top of the piston bowl and a step in the head to force the squish motion through jet slots located in the ridge. Combustion durations and MFB curves were determined from combustion experiments for all the designs as with the perforated plates. The flow field generated by the bowl-in-piston design resulted in the turbulence intensity at the centre of the bowl being generated at around top dead centre (TDC). With the "forced squish-jet" designs, although the main part of the motion was directed between the ridge and the step in the head and not through the jet slots, the turbulence intensity at the centre of the bowl was generated around 10 degrees before TDC. The jet slots main influence was to reduce the effectiveness of the ridge in generating turbulence. The combustion with the different combustion chamber designs was significantly affected by the generation of turbulence. The fuel/air mixture burned at a slow (laminar) rate until the pistons generated turbulence upon which the combustion rate dramatically increased. The timing of the increase in the combustion rate was determined by the timing of the turbulence generation with the different designs.

Text

2010-08-30

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http://hdl.handle.net/2429/27938

Mechanical Engineering

University of British Columbia

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