S. Sreedhara

The Simultaneous Reduction of NOx and PM Using Ultra-Cooled EGR and Retarded Injection Timing in a Diesel Engine

Now-a-days, diesel engines are more popular over gasoline engines due to their undisputed benefit of fuel economy and higher torque output. However, meeting the stringent emission norms without affecting fuel economy is a major challenge with diesel engines. In this paper, optimization of various parameters like injection parameters, compression ratio (CR) and amount of ultra-cooled exhaust gas recirculation (EGR) has been done for a variable CR (VCR) engine, to achieve low-temperature combustion (LTC) mode by which simultaneous reduction of NOx and soot may be obtained. Taguchi analysis was employed to carry out minimum number of experimental runs and still get the essence of large number of test cases. Effect of those parameters on engine performance and exhaust emissions has also been studied with the help of signal-to-noise (SN) ratio analysis. Results of SN ratio analysis indicate that injection timing, CR and ultra-cooled EGR are the dominant factors while the effect of injection pressure, in range of study, is not much significant for chosen response variables. Flatter and wider HRR traces, hence simultaneous reduction in NOx and PM emissions has been observed for runs having optimized input parameters. NOx and soot found to be reduced by 98% and 60%, respectively, with an increase in brake thermal efficiency (BTE) by 5%. Higher values of HC and CO emissions were found for runs with optimized parameters, but these can be easily reduced by using a catalytic converter, a simple device.

Numerical investigation of late injection strategy to achieve premixed charge compression ignition mode of operation

Premixed charge compression ignition mode of operation simultaneously reduces oxides of nitrogen (NOx) and particulate matter with improved fuel economy. Late injection is one such strategy to achieve premixed charge compression ignition mode along with the control over the ignition timing. Late injection along with charge dilution retards combustion and prepares homogeneous mixture by providing sufficient ignition delay. The in-cylinder charge distribution dictates mixture formation which affects the performance and emissions characteristics. Experimental results fail to provide an insight of the in-cylinder processes involved. Hence, numerical simulations have been performed to get in-cylinder distributions of scalars, which help in understanding the combustion process. The work consists of late in-cylinder injection of Diesel wherein the heat release rate trace shows only a premixed mode with lower soot and NOx. The absence of diffusion combustion confirms that the engine operates in a premixed charge compression ignition mode. In this work, effects of various injection parameters and swirl ratio on engine in-cylinder processes, performance and exhaust emissions have been studied using a three-dimensional model with detailed chemistry by using Converge, a computational fluid dynamics tool. Increasing swirl ratio improved air–fuel mixing causing lower soot emission up to a certain limit beyond which further increase in swirl ratio led to higher soot emission due to increased heat loss. Delayed start of injection resulted in delayed combustion leading to lower soot and NOx emission but partial oxidation resulted in higher carbon monoxide (CO) emissions. Nozzle tilt angle beyond 70° led to the formation of fuel-rich mixture at the center of combustion chamber resulting in higher soot and CO emissions. A narrow spray cone angle improved air–fuel mixing leading to lower soot but produced higher NOx. Medium swirl with late injection timing and wide nozzle tilt angle along with narrower spray cone angle resulted in lower emissions without deteriorating engine performance.

Experimental and numerical investigations of effect of split injection strategies and dwell between injections on combustion and emissions characteristics of a diesel engine

In this work, effects of various split injection strategies and dwell between injections on diesel engine combustion and emissions characteristics have been studied experimentally and numerically using Converge computational fluid dynamics tool. The electronic fuel injection unit, which is capable of injecting fuel up to four injections per cycle, was attached to the engine to achieve various split injection strategies. Results of the study showed that a significant reduction in nitrogen oxides with an acceptable change in particulate matter, hydrocarbons, carbon monoxide and brake thermal efficiency has been achieved with 2-shot:50(−27)-50 split injection strategy with a dwell of 8 crank angle as compared to the single injection strategy. Hence, split injection technique may be considered as an alternative to exhaust gas recirculation technique. Results also showed a fall in premixed heat release peak with an increase in diffusion combustion phase, when the dwell period was increased for all split injection strategies. As a consequence, reduction in nitrogen oxides and increase in particulate matter, hydrocarbons and carbon monoxide were observed with increased dwell between injections. Results of numerical study showed that contour area with higher temperature was observed to be lesser for 2-shot:50(−27)-50 split injection as compared to that for single injection, resulting in reduced nitrogen oxides.