Optimization of a CNG Driven SI Engine Within a Parallel Hybrid Power Train by Using EGR and an Oversized Turbocharger with Active-WG Control 2010-01-0820

The hybrid power train technology offers various prospects to optimize the engine efficiency in order to minimize the CO₂ emissions of an internal-combustion-engine-powered vehicle. Today different types of hybrid architectures like parallel, serial, power split or through-the-road concepts are commonly known. To achieve lowest fuel consumption the following hybrid electric vehicle drive modes can be used: Start/Stop, pure electric/thermal driving, recuperation of brake energy and the hybrid mode.

The high complexity of the interaction between those power sources requires an extensive investigation to determine the optimal configuration of a natural-gas-powered SI engine within a parallel hybrid power train. Therefore, a turbocharged 1.0-liter 3-cylinder CNG engine was analyzed on the test bench. Using an optimized combustion strategy, the engine was operated at stoichiometric and lean air/fuel ratio applying both high- and low-pressure EGR. The range of possibilities given by hybrid power train architectures allows a phlegmatic operation strategy for the internal-combustion engine, where the load demand is subject to slow transients. By applying an oversized turbocharger (wider turbine-neck cross-section) with Active-WG control (open wastegate at part-load operation) the engine efficiency was optimized over the whole operation range.

Secondary simulations of the longitudinal vehicle dynamic behavior based on the measured steady-state performance maps from the engine test bench were carried out. To manage the different power sources a modified version of the common "Equivalent Consumption Minimization Strategy" was used. This "non-forward" looking operation strategy was developed to attain the initial state of battery charge at the end of the driving cycle. Consequently no compensation calculation had to be performed. Furthermore this strategy was designed to achieve lowest fuel consumption over the whole driving cycle. Hence, fuel consumption and CO₂ emissions for legal driving cycles were determined.