F2008-06-112
Stop-Start Micro Hybrid: An Estimation of Automatic Engine Stop Availability
It is widely accepted that hybrid electrical vehicles (HEVs) have achieved market acceptance in North America and Japan, while they still remain a niche market in Europe. The penetration by 2004 was less than 0.5%, with conventional diesel vehicles dominating the market [1]. The higher purchase price of full hybrid vehicles may be the deterrent to many customers from buying them unless payback can be guaranteed through reduced operating costs [2].
Micro hybrid vehicles provide an attractive compromise, offering some fuel economy improvement at limited increase in purchase price, with fuel consumption reduction reaching 3-5% in city driving. As such, micro hybrid vehicles are gaining increased attention, which may lead to their breakthrough in the European market [1]. Conversely, car manufacturers are under increasing pressure to reduce their fleet CO2 emissions and micro hybrid stop-start systems are viewed as a key part in achieving this aim.
One of the main features of a micro hybrid vehicle is the automatic stop and restart of the engine to avoid engine idling when the vehicle is at rest. However, in real world usage the availability of this feature is limited by the control logic to avoid stopping the engine when it may not be desirable, for reasons of safety, comfort or other factors.
In this research reported here, a methodology to analyse the frequency of automatic engine stop events in real world usage has been developed taking into account the situations where the automatic engine stop is unavailable (or inhibited). A statistical model has been constructed using a real world, real time, vehicle usage dataset measured for passenger cars and publicly available climate data combined by Monte Carlo analysis. This allows the prediction of the occurrence of inhibitors during real world customer usage. The real world usage dataset, measured by an OEM in Europe, provided information regarding the vehicle speed profile, gear usage, engine warm up periods, ambient conditions as well as generic information about all the measured trips. Using the above mentioned sources, inhibiting conditions have been analysed individually, using simplified models where necessary. The simultaneous occurrence of inhibitors has been investigated and accounted for in the results. The inhibitor models have been analysed to determine the sensitivity of the output (the expected number of automatic engine stops starts in the vehicle lifetime) to changes in the calibratable values which define the automatic engine stop control logic strategy.
Conclusions: The methodology developed in this study provides a useful tool which supports the calculation of durability requirements as well as the initial calibration for micro hybrid systems in vehicle applications. Parametric studies show the routes to further system optimization.
Session: Hybrid Powertrains II