Optimization of the air handling system of a multi-cylinder light duty engine running on reactivity controlled compression ignition – A simulation study

Bharath, A. N. Optimization of the Air Handling System of a Multi-Cylinder Light Duty Engine Running on Reactivity Controlled Compression Ignition - A Simulation Study. University of Wisconsin-Madison, 2016.

Low Temperature Combustion (LTC) Strategies such as Reactivity Controlled Compression Ignition (RCCI) are highly sensitive to intake conditions, which are influenced by the gas exchange process. Because the gas exchange process is dependent on air handling system characteristics, optimizing the air handling system for improved RCCI engine performance is necessary. Major objectives were: 1. Improve combustion efficiency while mitigating unburnt hydrocarbon (UHC) and carbon monoxide (CO) emissions at low load, 2. Determine system parameters and configurations for high loads and 3. Examine variable valve actuation (VVA) and manifold redesign to maximize fuel efficiency. Zero-dimensional, one-dimensional and multi-dimensional models were used in this simulation study. Early Exhaust Valve Opening (EEVO) using fully flexible variable valvetrains and cam-phasers, and cylinder deactivation were evaluated for their impact on aftertreatment efficiency and fuel economy at low load. For near-idle conditions, cylinder deactivation in which only one cylinder was fired gave the best fuel economy and catalyst efficiency. For the second objective of performing high load system simulation, a low pressure (LP) EGR circuit was incorporated into the engine model. High Pressure EGR could not be used for high loads as the pre-turbine pressure was insufficient to drive EGR flow. Moreover, insufficient exhaust energy would be available to the turbine, resulting in lower boost pressures.

For the final objective, the stock exhaust manifold was redesigned using the Divided Exhaust Period (DEP) concept by splitting it into two manifolds, one connected to each exhaust valve. By using VVA to separately actuate the valves, overlap between the valves was varied, changing the exhaust distribution between the two manifolds, and thereby regulating boost pressure. With DEP, due to lower overall backpressures, pumping penalty decreased, but the pumping benefit was negated by parasitic losses from the supercharger which had to compensate for the boost deficit. Replacing the fixed geometry turbocharger with a variable geometry turbocharger (VGT) improved the Brake Specific Fuel Consumption (BSFC) over the base engine by 1%, while bypassing the turbine at low load gave elevated exhaust gas temperatures for thermal management.