Experimental investigation of the influence of ignition system parameters on combustion behavior in large lean burn spark ignited gas engines

To achieve high efficiency and low pollutant emissions with large spark ignited gas engines, operation at high excess air ratios and high turbulence levels in the combustion chamber is necessary. Under these severe boundary conditions, it is a great challenge to reliably ignite the air-fuel mixture and keep cycle-to-cycle variation of the combustion process within reasonable limits. This paper investigates the influence of ignition system parameters on combustion behavior by analyzing the overall ignition and combustion process in detail: the ignition system, the electric arc, the early phase of combustion and finally the whole combustion process.

Measurements were carried out on a large bore spark ignited single-cylinder research engine and on a specially designed arc test rig. In the research engine, ion current probes were applied around the spark plug in the spark plug sleeve so that a detailed analysis of the early phase of combustion was possible in order to assess the influence of ignition system parameters on combustion. In addition, crank-angle resolved in-cylinder pressure measurements provided information about the overall combustion process. The arc test rig enables the optical investigation of the influence of ignition system parameters on electric arc behavior under non-combustible atmospheres and at thermodynamic conditions and flow conditions similar to those in the research engine at spark timing. For consistency, the same modulated capacitive discharge ignition system and spark plug were installed on both test rigs. The combination of the two test setups facilitates in-depth understanding of the phenomena observed during ignition and combustion.

Near the lean engine operating limit, increasing the nominal spark current duration to a certain level yielded faster combustion progress and lower cycle-to-cycle variation of the combustion process. The investigation of the early phase of combustion by means of the ion current measurements revealed the same trend: Increasing the nominal spark current duration leads to an increase in the flame front velocity during the early phase of combustion. The results from the arc test rig indicated that an increase in nominal spark current duration leads to an increase in the maximum of the cycle-averaged electric arc length. In total, the arc test rig results suggest that besides the increase in overall dissipated secondary energy, the increase in electric arc length is the second important factor in achieving the development of a stable flame kernel and a rapid combustion process in the engine when the nominal spark current duration is varied from 100 to 500 µs at very lean engine operation.