Investigations on the Potential of a Variable Miller Cycle for SI Knock Control

A promising combustion technology for DISI downsizing engines is the Miller cycle. It is based on an early intake valve closing for the separation of effective and geometric compression ratio. Therefore IAV has prepared a turbocharged DISI test engine with a high geometric compression ratio. This engine is equipped with the Schaeffler “UniAir” variable valve train in order to investigate a variable Miller cycle valve timing in the turbocharged map area. The goal is to investigate whether and how a rapidly variable Miller cycle can influence the knocking behavior. Therefore its potential for a SI knock control can be evaluated. The investigated parameters in a steady-state engine dyno mode were the intake valve closing timing, the intake camshaft phasing and the ignition timing. A variable intake valve closing Miller cycle strategy, a variable intake camshaft phasing Miller cycle strategy and a state-of-the- art ignition timing strategy have been investigated. The experimental study investigated the middle turbocharged load range and the low-to-middle speed range in two stages. The first stage considered the final engine operation after the different anti-knock interventions and appropriate trim measures. The second stage represented the instantaneous anti-knock intervention. It was possible to evaluate the impact of each solitary anti-knock intervention measure. The Miller cycle strategies offer interesting results for cyclic variations of combustion, fuel economy and exhaust gas temperature. These results are promising as a slow adaptive strategy. Due to the highly dynamic behavior there must be a combination of VVT and ignition based knock controls for practical applications.