Abstract
Research and/or Engineering Questions/Objective
The compression ratio of the diesel engines is an important parameter regarding engine efficiency and emissions. The reduction of effective compression ratio by means of late intake valve closing (LIVC) shows potential to reduce NOx and smoke emission, which can improve the overall trade-off relationship between NOx and smoke emission. However, its effect on the engine efficiency deterioration requires further discussion in combination with other engine control parameters. Therefore, the objective of the study is to clarify the effects of LIVC strategies along with the exhaust gas recirculation (EGR) and intake boosting on the engine performance and the emissions.
Methodology
The LIVC strategies consisted of intake valve closing (IVC) retardation up to 88 crank angle degrees after bottom dead center (CAD ABDC), which is 60 CAD retarded from the original IVC timing. Two engine load with fuel quantities of 12.0 and 19.8 mg/stroke was selected for the evaluation. The effects of LIVC strategies in combination of conventional EGR was performed to examine each effect on the engine performance and emission. The operating conditions which required intake boosting caused by low volumetric efficiency was selected. The effects of intake boosting with LIVC strategies were further examined in these conditions.
Results
The LIVC strategy improved the conventional NOx-smoke trade-off relationship. NOx reduction was attributed to the lowered adiabatic temperature, while the smoke reduction was attributed to the prolonged ignition delay. The smoke emission was deteriorated with LIVC strategy when the engine load was increased due to insufficient amount of oxygen mass. The intake boosting was required to compensate for the repelled charge from the cylinder caused by the retarded IVC timing. In general, the LIVC strategy deteriorated the engine efficiency. However, the engine efficiencywas able to be recovered to original value with intake boosting.
Limitations of this study
The experiments were conducted only at two engine load conditions with fixed engine speed. The effects of the control parameters at different engine speeds should also be evaluated in the future.
What does the paper offer that is new in the field including in comparison to other work by the authors?
The effects of LIVC strategies were confirmed as it was stated in the previous literatures. In addition, the combustion phenomenon was visualized inside the combustion chamber to reveal the effects of asymmetric cam profile on the emission
Conclusions
The LIVC strategy effectively improved the conventional NOx-smoke trade-off relationship, under the premise of sufficient oxygen content inside the cylinder. Combination of EGR and LIVC showed promising reduction of both NOx and smoke emission. LIVC strategy reduces the volumetric efficiency, therefore the intake boosting is required for high load condition. The efficiency decreased as the IVC timing was retarded; however, the efficiency was increased again when the intake boosting was applied.
Key Words : diesel engine, compression ignition, late intake valve closing (LIVC), exhaust gas recirculation (EGR)