Experimental Studies of Advanced Ignition Systems for Lean Burn Natural Gas Engines

Abstract: One of the greatest challenges for the transport sector today is reducing its environmental impact. This applies especially to the emission aspects, much due to increasingly stringent emissions legislations. Carbon dioxide (CO2) is considered an important greenhouse gas, but also unburned hydrocarbons (HC), nitric oxides (NOx) and soot pose environmental as well as health risks. Compared to diesel and petrol fuels, natural gas offers a competitive alternative regarding CO2 emissions as well as availability. High efficiencies and low emission levels can be achieved with lean-burn natural gas engines. However, lean mixtures present a few drawbacks: The flame propagation is slow compared to stoichiometric conditions, which increases the risk of misfire and reduces ignition stability. So, there is a need to improve ignition and enhance flame propagation in spark-ignited engines.

The focus of this work is to study and gain knowledge about what factors are important for reliable ignition of lean methane/air mixtures. This is done through experiments involving advanced, new ignition systems, commercially available as well as newly developed ones.

The first studies were performed with a conventional capacitive discharge ignition (CDI) system in a heavy-duty (HD) natural gas engine. The ignition system produces a pulse train, which can be modified according to the desired spark characteristics and current profile, e.g. multispark, increasing current level or low current and long duration. Sparks with different energy content were tested and how fast the energy was released was also studied. Long duration and quick energy release were found to be more important than high current for stable ignition of lean mixtures. The current level did not show a large importance for the ignition quality.