Combined Effects of Valve Strategies, Compression Ratio, Water Injection and Cooled EGR on the Fuel Consumption of a Small Turbocharged VVA Spark-Ignition Engine

In this work, various techniques are numerically investigated to assess and quantify their relative effectiveness in reducing the Brake Specific Fuel Consumption (BSFC) of a downsized turbocharged spark-ignition Variable Valve Actuation (VVA) engine. The analyzed solutions include the Variable Compression Ratio (VCR), the port Water Injection (WI), and the external cooled Exhaust Gas Recirculation (EGR). The numerical analysis is developed in a 1D modeling framework. The engine is schematized in GT-Power™ environment, employing refined sub-models of the in-cylinder processes, such as the turbulence, combustion, knock, and heat transfer. The combustion and knock models have been extensively validated in previous papers, at different speed/load points and intake valve strategies, including operations with a relevant internal EGR rate and with liquid WI. The 1D model is coupled to an automatic optimizer, to explore the potential BSFC benefits arising from the adoption of the above-listed solutions. The base engine architecture, only including the VVA device, is preliminarily optimized to define reference BSFC levels. Then, single and combined solutions are analyzed to outline the maximum achievable BSFC gains. Operating conditions typical of a Worldwide harmonized Light vehicles Test Procedure (WLTP) driving cycle are considered. More than proposing an advanced, very complex, engine architecture, the aim of the activity is to clearly outline isolated and mutual effects of each technique at various operating points. In this way, some guidelines are offered to engine developers to select the preferred solution and to have information on the expected improvements. The optimization outcomes show that the WI proves a higher effectiveness at medium-high load, mainly thanks to its knock suppression capability. Cooled EGR is preferable at low load, to reduce the pumping work. If coupled to the WI, a high Compression Ratio (CR) is always beneficial. The combination of the above techniques provides BSFC reductions of 6.9%, 5.2%, and 9.0% at low, medium, and high loads, respectively.