An Investigation of Premixed and Lean Combustion in Engines

Spark ignited internal combustion engines are expected to continue to be the mainstay for the passenger cars and light duty trucks for the next few decades. It is understood that to conform to the stringent fuel efficiency legislations as well as meet the regulated exhaust emission limits, combustion technology must evolve significantly. It is imperative to develop a deeper understanding of the fundamental engine processes such as air intake, fuel-air interaction, and ignition so that avenues for incremental improvements may be explored. With this broad objective, the present study focuses on spark ignition engines in which premixed and lean (air in excess) charge of fuel and air can be burned efficiently. Studies have indicated that under these conditions, it is possible to simultaneously reduce the oxides of nitrogen (NOx), while keeping the carbon monoxide (CO) and unburned hydrocarbons (UHCs) at low levels. The in-cylinder turbulence plays a major role in the fuel-air mixture preparation. When this mixture ignites, the combustion may propagate through what is known as a premixed turbulent flame. Turbulence is beneficial since it enhances the mass burning rate. This is particularly critical in lean burn engines in which it is difficult to complete the combustion within the extremely short time scales typical of modern engines. Excess turbulence however, may lead to flame quenching. In order to investigate the conditions leading up to and the propagation of the turbulent flame itself, analytical and empirical studies are performed. Tests are conducted on a constant volume combustion chamber with optical access to provide insight into the combustion characteristics of lean mixtures subject to turbulence. Fundamental studies on premixed flame propagation are performed with a variety of fuels at different equivalence ratios with different fuels. Impacts of engine operating conditions such as air-fuel ratio, exhaust gas recirculation, engine load, fuels, and ignition strategies on the flame initiation and development are investigated in detail on a research engine test setup. Chemical simulation and computational fluid dynamics (CFD) tools are used to supplement the understanding of the results. Finally, an attempt is made to comprehensively understand the combined effects of in-cylinder flow and fuel reactivity on premixed and lean combustion

An Investigation of a Diesel Liquid Injector in a Simulated Exhaust Flow

Certain exhaust after-treatment devices used in modern diesel engines need an injector to spray a liquid, including a fuel, into the exhaust stream. For optimum performance, it is desired that the liquid must atomize and vaporize before it enters the device. The spray of an after-treatment injector was simulated in a computational fluid dynamics (CFD) suite, and showed that evaporation increased with increase in the gas flow rate and gas temperature. The results of the calculations were used to design an experimental setup to study a diesel after-treatment injector. Water was injected into air flowing with a speed of 1.4 m/s and at temperature of 423 K. High speed imaging and phase Doppler anemometry (PDA) were used to identify regions of high particle count. In these regions, diameter decreased with increasing vertical and horizontal distance from the injector. The vertical velocity of the particles was found to increase marginally with increasing vertical distance from the injector tip

An investigation of near-spark-plug flow field and its effect on spark behavior

In the recent decades, the emission and fuel efficiency regulations put forth by the emission regulation agencies have become increasingly stringent and this trend is expected to continue in future. The advanced spark ignition (SI) engines can operate under lean conditions to improve efficiency and reduce emissions. Under such lean conditions, the ignition and complete combustion of the charge mixture is a challenge because of the reduced charge reactivity. Enhancement of the in-cylinder charge motion and turbulence to increase the flame velocity, and consequently reduce the combustion duration is one possible way to improve lean combustion. The role of air motion in better air-fuel mixing and increasing the flame velocity, by enhancing turbulence has been researched extensively. However, during the ignition process, the charge motion can influence the initial spark discharge, resulting flame kernel formation, and flame propagation. Therefore, a combined empirical and simulation study is undertaken to investigate the flow field around the spark gap. The flow field generated by a steady flow of air across the spark gap of a conventional J-type spark plug under ambient conditions is studied using optical particle image velocimetry (PIV) measurements and computational fluid dynamics (CFD) simulations. The flow characteristics are compared to the high-speed direct imaging, and voltage and current measurement results of the spark channel in an effort to correlate the spark behavior to the local flow velocity. The flow field near the spark gap in an SI engine under motoring conditions is simulated, and the results are compared to the empirical current and voltage measurements taken during engine operation. The results show that the turbulence is generated in the wake of the spark plug as expected and flow velocity in the spark gap is higher than the free stream velocity. Optical and electrical measurements show the spark stretching and restrikes increase, and discharge duration decreases with increase in flow velocity. Similar behavior is observed during engine operation as well