Cherian A. Idicheria

Ignition, soot formation, and end-of-combustion transients in diesel combustion under high-EGR conditions

The ignition, soot formation, and end of combustion transients of n-heptane and #2 diesel jets were investigated in an optically accessible constant-volume combustion vessel under high exhaust-gas recirculation (EGR) environments. A wide range of EGR levels were simulated by systematically decreasing the ambient oxygen concentration from 21 to 8 per cent, while holding other experimental conditions constant. Characteristics of the effect of EGR on the ignition transient include: development of a cool flame early after injection for all EGR levels, an increase in the premixed-burn (high-temperature combustion) ignition delay inversely proportional to ambient oxygen concentration, ([O2]−1), and lower apparent heat-release rates during the premixed-burn with increasing EGR. The timing of soot formation is strongly dependent upon EGR, and the time between ignition and the first soot formation increases with decreasing ambient oxygen concentration. Soot-forming fuel jets are shown to become soot-free at high-EGR conditions by reducing the injection duration to be less than the soot formation time, but longer than the ignition delay time (negative ignition dwell). While past studies show success in reducing soot formation when the injection duration is less than the ignition delay (positive ignition dwell), this result shows that high EGR can suppress soot formation even with negative ignition dwell, thereby permitting higher-load operation by using longer injection durations. At the end of injection, increasing EGR presents difficulties in completing combustion because of the lower ambient oxygen concentration. Despite eventually reaching the same pressure rise (i.e., combustion efficiency) more time is required for the higher EGR conditions to mix fuel with sufficient oxygen to complete combustion.

A numerical study of high-pressure non-equilibrium streamers for combustion ignition application

We present a computational simulation study of non-equilibrium streamer discharges in a coaxial electrode and a corona geometry for automotive combustion ignition applications. The streamers propagate in combustible fuel-air mixtures at high pressures representative of internal combustion engine conditions. The study was performed using a self-consistent, two-temperature plasma model with finite-rate plasma chemical kinetics. Positive high voltage pulses of order tens of kV and duration of tens of nanoseconds were applied to the powered inner cylindrical electrode which resulted in the formation and propagation of a cathode-directed streamer. The resulting spatial and temporal production of active radical species such as O, H, and singlet delta oxygen is quantified and compared for lean and stoichiometric fuel-air mixtures. For the coaxial electrode geometry, the discharge is characterized by a primary streamer that bridges the inter-electrode gap and a secondary streamer that develops in the wake of the ...

Design of an optimum combustion chamber across multiple speed/load points for a heavy-duty diesel engine: analytical design and experimental validation

Analytical tools such as computational fluid dynamics (CFD) and micro-genetic optimization algorithm (GA) have been customized and applied at General Motors Research Laboratories (GMR) for designing a combustion chamber for GM heavy-duty diesel engines, capable of meeting present and future emission targets. In the combined analytical and experimental study described in this paper, the analytical design of the piston bowl shape spanned four key steady-state load points and the performance was validated by testing the analytical design in a single-cylinder engine (SCE). Computations were made to assess the fuel economy performance of the first analytical design. Based on the assessment, a second analytical design was performed with a different set of load points. Computations clearly revealed the progress of performance from the first to the second design. The two optimized piston bowl designs were tested in the SCE in order to validate the predicted progression of performance from the baseline bowl shape and the effect of the choice of the load points for analytical optimization. A multidimensional computer code (KIVA-3V) with a relatively simpler combustion model – the characteristic time-scale combustion (CTC) model – was used along with the GA optimization tool. Experimental results confirmed the predicted emission improvements and performance progression of the analytically designed piston bowls compared with the baseline. Detailed flow field analysis is presented for a selected load point to elucidate the physics behind the sensitivity of observed emission behaviour to variations in injector tip protrusion.

Potential of Advanced Corona Ignition System (ACIS) for Future Engine Applications

In this study, the advanced corona ignition system (ACIS) was evaluated. First, the system was evaluated in a direct-injected, single cylinder optical engine and high-speed imaging of enhanced combustion luminosity was used to visualize the ignition, flame kernel formation and flame propagation. The imaging results show that the ACIS promotes simultaneous ignition of the mixture at multiple locations in the combustion chamber as opposed to ignition being limited to the spark gap channel. Therefore, ignition delay is always shorter with the ACIS than with a traditional spark plug, inductive discharge system. The ACIS is able to support stable combustion (COV of IMEP < 3 %) in a leaner homogeneous mixture than the spark plug based system. Similarly, the ACIS has shorter ignition delay and faster burn rate for stoichiometric, rich and simulated EGR diluted mixtures. Second, the ACIS system was evaluated in a 2.0L, direct-injected, turbocharged, engine. Comparison of combustion results showed that significant reduction in 0–10 burn durations can be achieved over the production ignition system, enabling higher internal dilution limits. The ACIS was able to match the performance of the production ignition system for the idle spark sweep test. The test results showed that the close proximity of the piston to the ACIS electrode can transition the streamer discharge to arcing mode, which hampers optimal performance. Overall, ACIS has the potential to perform as well or better than the production ignition system and for best combustion performance the piston design must be optimized.

Ignition Systems Challenges for Next Generation Internal Combustion Engines

To achieve maximum fuel economy and minimum emissions potential for a diverse range of global application, future internal combustion engine automotive powertrains will need to synergistically integrate key building block technologies. Chief among these technologies are downsized boosting, closed loop engine controls, dilute and lean combustion and electrification. Improvements to conventional ignition systems and development of alternate ignition systems to enable lean-burn combustion strategies is a significant challenge.

Comparison of Experimental and Numerical Modeling of Reforming HCCI Combustion

Homogeneous charge compression ignition (HCCI) engines have high potential to provide better fuel economy with low emissions than conventional spark ignition (SI) engines. In an HCCI engine, combustion phasing strongly depends on the initial temperature and composition. Negative valve overlap (NVO) with reforming has been investigated as combustion phasing control strategy. However, the reforming process is not yet fully understood and further research is necessary to fully utilize the NVO reforming strategy. In this research, optically measured reforming process was modeled by 3D CFD simulation and the results were compared to understand the reforming process better. The optical measurement was carried out with sodium additive to enhance the combustion luminosity. Numerical simulation was carried out with state-of-art spray model with chemical kinetics for ignition and combustion. The chemical reaction mechanism was optimized for modeling the reforming process. It was found that the luminosity from the optical measurement correlates well with the chemical reaction source terms from the simulation.Copyright