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.
The control of transient emissions from turbocharged diesel engines remains an important objective to manufacturers, since newly produced engines must meet the stringent criteria concerning exhaust emissions levels as dictated by the legislated Transient Cycles. In the present work, experimental tests are conducted on a medium-duty, turbocharged and after-cooled diesel engine in order to investigate the behavior and formation mechanism of nitric oxide (NO), smoke and combustion noise emissions under various transient operating schedules including acceleration, load change and starting. To this aim, a fully instrumented test bed was set up in order to record and research key engine and turbocharger variables during the transient events. The main parameters measured are nitric oxide concentration and smoke opacity (both using ultra-fast response analyzers) as well as combustion noise. Various other variables were monitored, such as in-cylinder pressure, engine speed, fuel pump rack position, boost pressure and turbocharger speed. The main focus of the experimental investigation was devoted to engine acceleration tests representative of automotive and truck applications, commencing from various engine speeds and loads. The experimental test pattern also included load increases and (cold and hot) startings. Analytical diagrams are provided to explain the behavior of exhaust emissions development in conjunction with turbocharger and governor/fuel pump response. Turbocharger lag was found to be the main cause for the emissions peak values observed during all transient events. During starting, the lack of air and its mismatch with fueling caused excessive black smoke, identified by the extremely high values of exhaust gas opacity.
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