Asish K Sarangi

The effects of split injections on high exhaust gas recirculation low-temperature diesel engine combustion

Diesel engine emissions of oxides of nitrogen and smoke can be reduced simultaneously through the use of high levels of exhaust gas recirculation to achieve low-temperature combustion. However, single fuel injection per cycle diesel low-temperature combustion is also characterized by high fuel consumption and high total unburned hydrocarbons and carbon monoxide emissions. This work focuses on investigating the potential of a split (50/50) main fuel-injection strategy to reduce smoke, total unburned hydrocarbons and carbon monoxide emissions at exhaust gas recirculation levels lower than those required to achieve single-injection diesel low-temperature combustion at a medium-load, medium-speed operating condition. Experiments were performed on a 0.51 l single-cylinder high-speed direct-injection diesel engine running at 1500 r/min at an operating condition corresponding to a gross indicated mean effective pressure of 500 kPa. At this load, exhaust gas recirculation levels of 62% are needed to realize near-zero nitrogen oxide and smoke emissions, but this leads to an unacceptable reduction in thermal efficiency as well as high total unburned hydrocarbons and carbon monoxide emissions. This work compares the effects of split fuel injections at an exhaust gas recirculation level of 52% by volume to those from single injections at exhaust gas recirculation levels of 52% and 62%. The results demonstrate that the combined effects of exhaust gas recirculation rate and split injections can achieve near-zero nitrogen oxide with good thermal efficiency and total unburned hydrocarbons and carbon monoxide emissions much lower than at 62% exhaust gas recirculation. Single injection at this point results in excessive smoke, which can be reduced by over 75% through the split-injection strategy. These results are particularly relevant as they demonstrate very low nitrogen oxide emissions from an engine operation with acceptable thermal efficiency and at practical exhaust gas recirculation levels.

Effects of engine operating parameters on diesel low-temperature combustion with split fuel injection

This paper shows that a split-fuel-injection strategy can achieve robust, near-zero smoke and nitrogen oxide emissions at reduced exhaust gas recirculation levels under low-temperature combustion conditions. The overall objective of the work was to investigate the sensitivity (in terms of the engine emissions and the fuel economy) of a 50:50 (by mass) split-injection strategy to variations in the key engine operating parameters. Experiments were performed at operating conditions corresponding to a gross indicated mean effective pressure of 500 kPa at an engine speed of 1500 r/min in a 0.51 l single-cylinder high-speed direct-injection diesel engine. The paper presents the effects of different relative fuel injection timings at a variable intake oxygen mass fraction (10.5% and 12%) at a constant intake pressure (120 kPa, absolute) on the smoke, total hydrocarbon and carbon monoxide emissions with the split-main-injection strategy. The effects of a variable fuel injection pressure (90 MPa and 110 MPa) on diesel low-temperature combustion with split injection are also reported, as are the effect of an increased intake pressure (150 kPa, absolute). The combined effects of the operating parameters and the fuel injection timing on the smoke, nitrogen oxide, total hydrocarbon and carbon monoxide emissions and the gross indicated specific fuel consumption are described. For selected operating conditions, the cycle-resolved spray and combustion processes are visualized together with the flame temperature measurement using two-colour optical pyrometry to understand the combustion phenomena occurring in the split-injection strategy. The results of the optical studies show that different low-temperature combustion operating conditions producing similarly low levels of ‘engine-out’ smoke emissions have substantially different histories of soot formation and soot oxidation. An increase in the intake oxygen mass fraction reduced the total hydrocarbon emissions and the gross indicated specific fuel consumption at a given intake pressure, while a higher intake pressure reduced them further. Although significant soot formation took place from the second injection event, the majority of the soot was subsequently oxidized because of a slightly higher flame temperature and slightly higher oxygen concentration than in single-injection high-exhaust-gas-recirculation low-temperature combustion. A higher injection pressure did not have any significant effect on the emissions and the gross indicated specific fuel consumption.

Load transient between conventional diesel operation and low-temperature combustion

The operation of diesel low-temperature combustion engines is currently limited to low-load and medium-load conditions. Mode transitions between diesel low-temperature combustion and conventional diesel operation and between conventional diesel operation and diesel low-temperature combustion are therefore necessary to meet typical legislated driving-cycle load requirements, e.g. those of the New European Driving Cycle. Owing to the markedly different response timescales of the engine’s turbocharger, exhaust gas recirculation and fuelling systems, these combustion mode transitions are typically characterised by increased pollutant emissions. In the present paper, the transition from conventional diesel operation to diesel low-temperature combustion in a decreasing-load transient is considered. The results of an experimental study on a 0.51 l single-cylinder high-speed diesel engine are reported in a series of steady-state ‘pseudo-transient’ operating conditions, each pseudo-transient test point being representative of an individual cycle condition from within a mode transition as predicted by the combination of real-world transient test data (for fuelling and load) and one-dimensional transient simulations (for intake manifold pressure and exhaust gas recirculation rate). These test conditions are then established on the engine using independently controllable exhaust gas recirculation and boost systems. The results show for the first time that the intermediate cycle conditions encountered during combustion mode change driven by the load transient pose a significant operating challenge, particularly with respect to control of carbon monoxide, total hydrocarbon and smoke emissions. A split-fuel-injection strategy is found to be effective in mitigating the negative effects of the mode change on smoke emissions without significantly increasing oxides of nitrogen or decreasing fuel economy; however, unburned hydrocarbon emissions are increased. Additional experimental testing was also conducted at selected intermediate cycles to understand the sensitivity of key fuel injection parameters with the split-injection strategy on engine performance and emissions.

Managing the Transition Between Low Temperature Combustion and Conventional Diesel Combustion During a Load Change

High-EGR diesel low temperature combustion breaks the traditional diesel NOx-PM trade-off, thereby facilitating ultra-low NOx emissions with simultaneously low smoke emissions. High-EGR LTC is currently limited to low and medium load and speed conditions. Therefore, in order to implement a high-EGR diesel LTC strategy in a passenger vehicle, a transition to conventional diesel operation is required when either a high load or high speed is demanded. This transition must be carefully managed to ensure smooth operation and to avoid excessive pollutant emissions—a task that is complicated by the markedly different response time-scales of the engine’s turbocharger, EGR, and fuelling systems.This paper presents the results of a combination of numerical simulation and steady-state engine experiments that describe the performance and emissions of an automotive-sized 2 litre turbocharged diesel engine during a rapid transition from high-EGR LTC to conventional diesel operation. The effects of load change at constant engine speed during the Extra-Urban Drive Cycle (EUDC) part of the New European Drive Cycle (NEDC) are first evaluated using a one-dimensional engine simulation (Ricardo WAVE). The inputs to the model are; the initial and final fuelling quantities, the duration of the transient events, and the response of the engine’s control systems. The WAVE model outputs the intake manifold pressure and EGR level for each cycle during the transition.The predicted intake pressure, EGR rate and the corresponding known injected fuel mass for each individual cycle are used to define a set of ‘pseudo-transient’ test conditions—matching the conditions encountered at discrete points within the modelled transient—for subsequent steady-state engine testing on a 0.51 litre AVL single cylinder diesel engine. These test conditions are established on the engine using independently controllable EGR and boost systems and the corresponding emissions (NOx, smoke, CO, and THC) and performance data (GISFC) were recorded. The experimental emissions and performance data are subsequently presented on a cycle-by-cycle basis. The results of this study provide significant insight into the combustion conditions that might be encountered during mode switching and their deleterious effect on emissions and performance. Strategies to mitigate these effects are examined.Copyright