Studies on the application of plasma-assisted ignition for lean combustion

Abstract

Non-thermal or non-equilibrium plasma is an emerging field in combustion science. Stable ignition of lean fuel-air mixtures continues to remain a challenge. The present work explores the potential of non-thermal plasma for igniting lean mixtures using fundamental studies in a constant-volume chamber and applied studies in an internal combustion engine. The first part of the study focused on evaluating non-thermal plasma as an effective combustion enhancement tool by initially conducting experiments in a constant-volume combustion Chamber (CVCC) with optical access. A high-voltage Nanosecond Pulse Discharge (NPD) generator that can operate at a maximum frequency of 3.5 kHz is used to generate non-thermal plasma. The improvement in combustion is confirmed by comparing the Flame Development Time (FDT), lean limit and flame kernel size between the conventional and NPD plasma ignition systems. The configuration of the sparkplug for NPD plasma ignition is decided based on the experiments carried out in the CVCC. With suitable modifications to the spark plugs, the FDT is observed to reduce significantly. With the NPD system, a modified, conventional J shaped spark plug (MJ) and a modified surface plug (MS) performed better compared to their conventional counterparts. With MJ and MS spark plugs, a 12% and 15% decrease in FDT is achieved, respectively, even with a single pulse of NPD as compared to a conventional ignition system. Similarly, a two-fold increase in flame kernel size is observed with a single pulse of NPD plasma ignition. The OH chemiluminescence measurements also confirm the improvement in combustion shown in NPD plasma. In the second part of the study, Optical Emission Spectroscopy (OES) is used to calculate the post-discharge gas temperature. The plasma system is characterized based on the reduced electric field (E/N) and electron temperature calculated using the BOLSIG+ software. Zero dimensional plasma simulations are carried out using the Chemical Workbench and CHEMKIN software to simulate NPD plasma combustion's discharge and ignition phases. The electron impact reactions in the discharge phase are observed to generate excited nitrogen and oxygen molecules, which in turn produce chain-carrying radicals such as O, H and OH. Among these radicals, the O radical generated by NPD plasma affects combustion the most, as observed in the sensitivity analysis of ignition delay and quantitative reaction path diagrams (QRPD). These calculations are conducted to establish the initial chemical kinetics in the flame kernel, which are then used to initiate three-dimensional simulations of the flame propagation. The simulations successfully capture the lower FDT with NPD plasma ignition, as observed in the CVCC. In the third and final part of the study, the efficacy of the plasma-assisted ignition is assessed in an internal combustion (IC) engine. Experiments are carried out in a single-cylinder Port Fuel Injection (PFI) based four-stroke spark ignition (SI) engine. The results are very encouraging as it is found that the engine could run leaner with the NPD plasma system, reducing HC and CO emissions by 25.7% and 62%, respectively, in cold start tests using gasoline fuel. Steady-state engine experiments are also carried out at two load points corresponding to 2500 rpm and 4000 rpm engine speeds. The lean operating condition of the engine is represented by the lambda (λ) value, which is defined as the ratio of the actual air fuel ratio to the stoichiometric air-fuel ratio. Thus, as lambda increases beyond 1, the mixture becomes leaner, resulting in lower fuel consumption and higher efficiency. With NPD plasma assisted ignition, the stable operating limit of the engine could be extended from λ = 1.2 to λ = 1.4 at an engine speed of 2500 rpm and from λ = 1.25 to λ = 1.3 at 4000 rpm. The engine torque and thermal efficiency have improved in the extended lean operation zone with NPD ignition. Similarly, the NPD plasma ignition system at these lean operating points shows a considerable decrease in fuel consumption, ignition delay, combustion duration, HC, and CO emissions. A similar enhancement of lean limit is noticed with a methane-operated engine. At an engine speed of 3000 rpm and 8% throttle, the lean limit of operation increased from λ = 1.1 to λ = 1.3 with a two-pulse NPD plasma-assisted ignition. The NPD plasma-assisted ignition could achieve the same power output as the conventional system with 9.4% less fuel consumption. Overall, the above results show that using an NPD plasma ignition system has great potential to achieve higher efficiency and lower emissions under lean conditions in IC Engines, particularly those operating on clean fuels such as natural gas and hydrogen