Bond Graph Modeling of the Cold Start Capabilities of an Automotive Starter/Generator System

摘要

The use of a combined starter/generator integrated into the drive train of an automobile offers several possibilities for improvement of fuel economy. Among these are recovery of braking energy, start/stop operation in city traffic and the electrical drive of auxiliaries. The use of such a starter/generator system is made feasible by a switch from a 14 volts electrical system to a 42 volt system, but the sizing of the components is not a trivial problem. A difficulty arises because such a starter/generator must provide enough torque to start the engine at low temperatures. It cannot utilize a large gear ratio as does a conventional starter that decouples as soon as the engine starts. The starter/generator remains coupled to the engine even when the engine is running at high speed so a large gear ratio would lead to unacceptably high starter speeds. This study combines a dynamic electromechanical model of the starter, battery and power electronics with the nonlinear mechanics of the piston/crankshaft system and a thermofluid model of the compression and expansion processes to investigate the cold start problem. The example involves the start of an eight-cylinder engine at -25 degrees Celsius.