Techniques for CO2 Emission Reduction over a WLTC. A Numerical Comparison of Increased Compression Ratio, Cooled EGR and Water Injection

In this work, various techniques are numerically applied to a base engine - vehicle system to estimate their potential CO2 emission reduction. The reference thermal unit is a downsized turbocharged spark-ignition Variable Valve Actuation (VVA) engine, with a Compression Ratio (CR) of 10. In order to improve its fuel consumption, preserving the original full-load torque, various technologies are considered, including an increased CR, an external low-pressure cooled EGR, and a ported Water Injection (WI). The analyses are carried out by a 1D commercial software (GT-Power™), enhanced by refined user-models for the description of in-cylinder processes, namely turbulence, combustion, heat transfer and knock. The latter were validated with reference to the base engine architecture in previous activities. To minimize the Brake Specific Fuel Consumption (BSFC) all over the engine operating plane, the control parameters of the base and modified engines are calibrated based on PID controllers. The calibration procedure is also verified with a direct fuel consumption minimization carried out by an external optimizer. The calibration provides the optimal Spark Advance (SA), Air-to-Fuel (A/F) ratio, Waste-Gate (WG) opening, and VVA setting, complying with limitations on knock intensity, turbine inlet temperature, boost level, turbocharger speed and in-cylinder pressure peak. The performance and calibration maps are computed for various combinations of the above technologies, including a two-stage CR system, and are compared to the ones related to the base architecture. The results show that EGR offers some BSFC benefits at low load, mainly thanks to the pumping work reduction, while it is practically ineffective for knock mitigation at high load. On the contrary, WI has the potential to substantially increase the knock resistance, improving the fuel consumption at high load. No substantial advantages are, indeed, detected with WI under knock-free operation. Computed BSFC maps are then embedded in a vehicle model with the aim of estimating the CO2 emission of a segment A vehicle over a WLTC. The proposed results give a clear outlook of the above technique potentials and offer a guideline to assess the trade-off between engine complexity and improved CO2 emission.