Exhaust thermal management using intake valve closing timing modulation (2013)

In recent times, NOx and PM emission regulations for both light and heavy duty diesel engines have become very strict. Most engines meet these regulations using after-treatment systems. However, a major drawback of these after-treatment systems is that they are efficient in reducing emissions only when their catalyst temperature is within a certain range (usually around 250°C to 450°C). Thus the exhaust gases from the engine need to be at least 250°C to attain satisfactory after-treatment performance. This is not a problem when the engine is operating at higher loads. However, at lower loads this is a major problem as the exhaust temperatures are typically much below 250°C. This makes exhaust thermal management very important in diesel engines. The primary objective of this study is to explore intake valve timing modulation as a method for exhaust thermal management. The goal is to increase engine turbine out temperatures to ensure NOx and PM after-treatment systems operate efficiently. The first step in this work was to generate fuel consumption-NOx emissions trade-off curves for nominal IVC timings using a central composite design of experiments (DOE) and second order regression model based approach. This served as a baseline to compare fuel efficiency and NOx emissions for off-nominal valve timings. The effect of intake valve closing timing was studied on exhaust gas temperatures, fuel consumption, in-cylinder combustion and emissions by sweeping IVC timing, keeping other parameters constant at a given load-speed operating condition. A significant increase in turbine out temperature accompanied by a decrease in fuel consumption and NOx emissions was observed along the IVC sweep. Along the IVC sweep at 1200 RPM and 100 ft-lbf load, turbine out temperature went up from 200°C to 250°C accompanied by an increase in brake thermal efficiency from 31% to 32.5% and a reduction in brake specific NOx from 6.7 g/hp-hr to 3.7 g/hp-hr. Combustion was found to shift from a diffusion mode to a more premixed mode. Increases in exhaust gas temperatures due to IVC modulation were due to a drop in air flow through the engine. This drop in air flow was due to a reduction in the volumetric efficiency. Regression models to understand the effect of air-to-fuel ratio on turbine out temperature and IVC timing on volumetric efficiency were developed. The increase in fuel efficiency was analyzed by breaking down the brake thermal efficiency into three components: closed cycle efficiency, open cycle efficiency and mechanical efficiency. It was found that the increase was due to a drop in the pumping penalty. Reduction in NOx was attributed to the combined effect of a drop in in-cylinder temperature due to reduction in piston induced compression and a shift to a more premixed combustion mode. The effect of both early and late intake valve closing was found to be identical. Areas of the engine operating map where IVC modulation would be most effective to keep the after-treatment system and to get the after-treatment system hot were identified. This study is a first step towards exploring intake valve modulation as a method for exhaust thermal management. Intake valve closing modulation seems to be a promising strategy for exhaust thermal management.