Low-Pressure Exhaust Gas Recirculation (LP-EGR) has been shown to be an effective means of improving fuel economy and suppressing knock in downsized, boosted, spark ignition engines. LP-EGR is particularly beneficial at low-speed, high-load conditions, but can lead to combustion instability at lower loads. The transport delays inherent in LP-EGR systems slow the reduction of intake manifold EGR concentrations during tip-out events, which may lead to excessive EGR concentrations at low load.
This paper explores leveraging Variable Valve Timing (VVT) as a means of improving the rate of reduction of intake manifold EGR concentration prior to tip-out. At higher boost levels, high valve overlap may result in intake manifold gas passing directly to the exhaust manifold. This short-circuiting behaviour could potentially improve EGR evacuation rates. However, introducing short-circuit flow may lead to lean exhaust flow through the catalyst, and/or necessitate rich in-cylinder conditions that could counteract the fuel economy benefits of increasing high load LP-EGR rates. Therefore, this paper seeks to quantify the improvement in EGR evacuation rate and duration of short circuiting that may be achieved while at boosted conditions with high valve overlap, in preparation for a tip out.
To conduct this investigation, a controller is first proposed, capable of regulating torque at boosted operating conditions with high valve overlap and external EGR. This controller extends a published control architecture, by accounting for both short-circuit flow and external EGR.
The developed controller is then applied to a GT-Power model to regulate torque during constant torque LP-EGR evacuations, where high valve overlap is shown to improve evacuation times by 15-25%.