Increasing Exhaust Temperature of an Idling Light-Duty Diesel Engine through Post-Injection, Throttling, and Cylinder Deactivation

Paper #:
  • 2018-01-0223

Published:
  • 2018-04-03
Abstract:
Manufacturers of light-duty Diesel engines are currently facing unprecedented scrutiny due to alleged emissions cheating and concerns of federal and local governments regarding the effect of Diesel emissions on urban air quality. Light duty vehicles must meet emissions standards established as standardized drive cycles using chassis dynamometers, while heavy-duty Diesel engines must comply with emissions regulations established by standardized torque and engine speed versus time test cycles on an engine dynamometer. It is recognized that these standardized tests are approximations for actual real-world driving conditions. Because actual driving practices can deviate significantly from these cycles, actual Diesel engine emissions can be significantly greater than those measured during the standard cycles, leading to high levels of pollutants being emitted. Modern Diesel engines rely on catalytic emissions control devices and Diesel particulate filters, some of which are catalyzed, to comply with emissions of unburned hydrocarbons, CO, NOx, and particulate matter. These catalytic emissions devices in the exhaust require heating to within a specific temperature range to be active. While there is some variability due to many factors, many of these catalytic devices must be heated to temperatures exceeding 200oC to function effectively. Many vehicles, particularly in urban settings, are operated under idle conditions for extended periods. Under these conditions, Diesel exhaust temperatures are often too low to maintain active catalyst temperatures, which can lead to high emissions. While Diesel particulate filters are effective at low temperatures, the DOC and SCR may not maintain a high enough temperature to control emissions of unburned hydrocarbons, CO, and NOx. This study investigated three methods of increasing Diesel exhaust temperature under idle conditions: post injection of fuel after TDC, intake throttling, and cylinder deactivation. In the first method, the baseline idle injection strategy was modified by adding a post-injection of fuel injected after TDC during the expansion stroke. The additional fuel and the reduced effective expansion ratio, post-combustion, leads to higher exhaust temperature. The second method was the combined use of this technique along with throttling of the intake air to reduce engine air/fuel ratio (lambda). The baseline injection strategy was adapted from a 2014 Chevrolet Cruze having an engine similar to the light-duty GM engine used for this study. For this particular study, EGR was not used in order to simplify the parameter space and examine the above mentioned effects separate from EGR. The effect of EGR will be considered in a future study. The engine was mounted on a motoring engine dynamometer but the dynamometer was not active for the study. A National Instruments LabView based engine control algorithm was developed to maintain the desired idle speed using a feedback loop to vary the duration of the main injection event. The engine operating parameters considered included two idle speeds of 800 and 1100 rpm, with the engine fully warmed up. Two rail pressures of 500 and 900 bar were studied with the injection strategy being the primary variable. The degree of throttling used was determined by emissions and the ability of the engine to maintain a stable idle. The parameters measured included exhaust temperature, exhaust concentrations of NOx, HC, CO, and CO2, as well as, IMEP and COV of IMEP. For the baseline idle conditions, manifold-out exhaust temperature was approximately 100oC. It was found that under idle conditions the post-injected fuel had to be injected within 30- 35 degrees after TDC for the fuel to combust completely enough to contribute to both IMEP and to higher exhaust temperatures. Without throttling the contribution of the post-injection to increased exhaust temperature was relatively modest, about 20oC. With heavy throttling it was possible to significantly increase idle exhaust temperature by more than 60oC. The addition of post-injection allowed further temperature increases, on the order of 20-30oC, yielding manifold-out exhaust temperatures as high as 200oC. There was a significant engine-out HC emissions penalty associated with this, however, with HC concentrations roughly doubling over the baseline idle condition. For conditions for which heavy throttling was used, it was interesting to note that the highest exhaust temperatures were found for a post-injection timing of approximately 25o aTDC which also corresponded to a minimum in engine-out NOx emissions. The reason for the NOx behavior is not clear.
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