A study of turbulent jet ignition combustion in an optical research engine with alternative fuels

Abstract

Turbulent Jet Ignition (TJI) is an advanced ignition process where ultra-lean mixtures can ignite in standard gasoline spark ignition engine. In this research, a TJI unit by Mahle Powertrain USA was adopted and studied in a bespoke single-cylinder engine with optical acess. The TJI device features a very small pre-chamber that is connected to the main chamber by multiple small orifices and can be separately fuelled by a direct fuel injector. The spark plug shifts from the main chamber to the pre-chamber to ignite the pre-chamber mixture. A new cylinder head was designed and manufactured to accommodate the TJI unit and optical windows on the top and sides of the cylinder head block. A new direct inejector (DI) fuel supply system was set up for direct fuel injection in the pre-chamber. A new engine control and a data system were commissioned and used for engine experiments and heat release analysis. High-speed combustion imaging and spectroscopic techniques were developed to study the ignition and combustion in the main chamber through high-speed cameras and spectrographic equipment. Thermodynamic studies on TJI combustion in a single-cylinder engine demonstrate the ability of TJI to extend the lean-burn limit of gasoline operation at different engine speeds and loads. Similar effects are also observed with engine operations fuelled with ethanol and wet-ethanol. TJI exerts the greatest effect in extending the lean-burn limit of ethanol fuel and leads to near-zero NOx emissions near the lean-burn limit. In addition, the TJI ethanol engine operation has higher thermal efficiency as well as lower HC and CO emissions than the gasoline operation. Spectroscopic results reveal that ethanol combustion produces higher chemiluminescent emissions than gasoline during the normal spark ignition combustion in the main chamber. The OH spectral peak at 310 nm is the highest throughout the ignition and combustion, followed by CH emission at 430 nm and HCO at 330 nm. Their intensities peak before the maximum heat release rates measured by the in-cylinder pressure. Emission spectra produced by the pre-chamber ignition are stronger than the normal spark ignition in the main chamber. The highest emission intensities are observed with the fuelled pre-chamber ignition even with leaner air–fuel mixture in the main chamber. As pre-chamber fuel is increased, the pre-chamber pressure rises faster to a higher peak value, producing greater pressure differential between the pre-chamber and main chamber and faster turbulent jets of partially burned products at higher temperature. The increase in the pre-chamber pressure causes the jets to travel deeper into the main chamber and enlarges the ignition sites. In addition, the ignition delay of the main chamber combustion is shortened due to the higher temperature of turbulent jets, as indicated by the stonger emission spectra. The turbulent ignition jets of ethanol are characterised with greater momentum than gasoline due to the faster combustion speed of ethanol and higher energy input. When the pre-chamber spark timing is advanced, the OH and CH emission intensities increase due to higher pressure and temperature in the pre-chamber, causing the pre-chamber products to travel deeper to ignite most of the main chamber charge. In comparison, the pre-chamber fuel injection timing has minimal effect. Finally, the spectroscopic investigation at different air–fuel ratios with fuelled pre-chamber ignition shows that the peaks of OH, CH and HCO drop towards the lean-burn limits for both fuels. The intensity of the emission spectra is dependent on the ignition type, fuel properties and air–fuel ratios.