Belt-Driven Starter-Generator for Future 42-Volt Systems

Belt-Driven Starter-Generator for Future 42-Volt Systems

ABSTRACT

This paper explores issues related with the design and implementation of belt-driven starter-generators for future 42-V systems. Belt-driven starter-generators can offer many advantages including smooth restarts, high efficiency, and convenient packaging. Future vehicle systems require these characteristics to enable fuel economy functions like “engine off at idle” and “x-by- wire.” Belt-driven starter-generators are often easier to package in contrast with flywheel-mounted systems, which require powertrain modifications and in many cases a longer package. A prototype system based on a belt-driven induction machine mounted on a small, European engine is described in the paper. Test results for both cranking and generation are shown and analyzed. Efficient, high-power generation was confirmed and high-speed (beyond 400 engine rpm) cranking was demonstrated down to the targeted -20°C.The paper also provides comparisons with various engine-starting, machine thermal, and electrical model results.

INTRODUCTION

This study is part of on-going, industry-wide studies of 42-V power systems and components [1-7]. The proposed higher-voltage architectures constitute an opportunity to revisit the idea of combining the starter and generator functions into a single drive. The single unit could be designed to generate at higher power levels than the current Lundell alternators can, and
would be capable of providing smooth starts thus making it possible to turn the engine off at idle. A few years ago, perhaps because of the influence of hybrid vehicle research, much of the focus of starter-generator studies was on so-called flywheel systems, where the starter- generator is placed directly on the crankshaft in the general location of the present flywheel [8-12]. Such systems could provide engine torque smoothing capabilities and could also be scaled up in output power level to provide some hybrid propulsion capabilities.

A possible alternative to the flywheel design consists of driving the starter-generator through mechanical means, belt, chain or gear, an option often referred to as “side- mounted” design. Such a system would allow for a more traditional underhood layout, thus being more evolutionary, yet provide most of the expected benefits from starter-generators in terms of efficient, high power generation and smooth starting. It would, however, be more limited probably in terms of torque-smoothing ability and certainly in terms of propulsion capability.

A prototype belt-driven, induction-machine based starter- generator system was therefore studied, built and tested, see Fig. 1. The general design target was the ability to start a European-style small engine down to -20°C, to generate 3 to 4 kW at most speeds, and at least 1 kW at 6,000 engine r/min. In addition, cranking up to speeds of at least 400 r/min was required at room temperature for smooth restart. The system had to be packageable in the corresponding compact vehicle. The scope of the study was the mechanical drive, the machine and the inverter that interfaces with a single, 42 V bus. A design description, mathematical models and test results are included.

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