Methods to Reduce End Gas Temperatures for Lower Knocking Sensitivity – A Simulation Study

Downsizing of gasoline engines by forced induction faces limitations of the engine speed range of maximum torque by irregular combustion, such as knock and/or pre-ignition. Reducing the end gas temperature can slow-down precursor reactions, thereby reducing the likelihood of undesired auto-ignition of mixture, and hence enable a further increase of mean effective pressure or compression ratio. While charge dilution e.g. by cooled EGR is one possible path toward this target, this paper focuses on methods that can be combined with EGR but also enable knock and fuel economy benefits it applied without. At the given example of a small gasoline engine, different measures will be analyzed and compared by engine process simulation:
• Miller Cycle with early and with late intake valve closure
• Turbo-cooling and
• Controlled compression ratio using variable connecting rod length
These technologies have effects on turbocharger matching and knock control. The different measures will be discussed and compared with regards to fuel economy and full load characteristics.