Fuel Economy and Mobility of Multi-Wheel Drive Vehicles: Modeling and Optimization Technology

A distinctive feature of unmanned and conventional terrain vehicles with four or more driving wheels consists of the fact that energy/fuel efficiency and mobility depend markedly not only on the total power applied to all the driving wheels, but also on the distribution of the total power among the wheels. As shown, under given terrain conditions, the same vehicle with a constant total power at all the driving wheels, but with different power distributions among the driving wheels, will demonstrate different fuel consumption, mobility and traction; the vehicle will accelerate differently and turn at different turn radii. This paper explains the nature of mechanical wheel power losses which depend on the power distribution among all the driving wheels and provides mathematical models for evaluating vehicle fuel economy and mobility. The paper also describes in detail analytical technology and computational results of the optimization of wheel power distributions among the driving wheels. The presented math models of a multi-body vehicle with any given number of the driving wheels and type of suspension are built on a novel inverse vehicle dynamics approach and include probabilistic terrain characteristics of rolling resistance and friction, microand macro-profiles of surfaces of motion. Computational results illustrate up to 15%-increase in energy efficiency of an 8x8 vehicle that is guaranteed by the optimal power distribution among the driving wheels. This technology can be applied for improving energy/fuel efficiency and mobility of tactical and combat military and unmanned robotic vehicles with mechanical and mechatronic driveline systems, vehicles with individually-driven wheels and vehicles with hybrid driveline systems. INTRODUCTION Improving mobility and fuel efficiency of highmobility terrain vehicles are mutually contradictory technical problems – to improve mobility usually requires extra fuel, and the optimization of fuel consumption can negatively impact mobility. Traditional vehicle dynamics, as the theory of vehicles in motion, has never developed analytical approaches to parallel optimization of mobility and energy efficiency. As a result, high-mobility vehicles demonstrate poor fuel economy (MPG) performance. For example, High Mobility Multipurpose Wheeled Vehicle (HMMWV) at 15,400 pounds gross vehicle weight currently gets 4-8 miles per gallon [1]. To improve mobility of terrain vehicles, multiwheel drive platforms, e.g. platforms with four or more driving wheels have been in use for decades. However, the simple addition of a drive axle can drastically increase fuel consumption and negatively impact the overall vehicle dynamics and performance. The problem here is that the performance of multi-wheel drive vehicles depend markedly not only the number of the drive axles, but also on the distribution of the engine power among the drive axles, and to the left and right wheels of each axle. When the power is differently distributed among the driving wheels, a given vehicle will demonstrate different fuel consumption, different terrain mobility and traction performance; the vehicle will accelerate differently and turn at different radii. Depending on wheel power split, the vehicle can run into either understeer or oversteer and then sometimes become unstable and skid in lateral direction, and finally move into rollover [2-5]. Wheel power distribution is largely determined by a vehicle’s driveline systems, which consists of a set of power dividing units (PDUs, see Fig. 1). Figure 1. A driveline system layout of an 8x8 vehicle There is no consensus among experts concerning the effect of driveline system parameters on the road and terrain of vehicle's fuel consumption. Many are of the opinion that, for example, the use of two drive axles instead of one Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 17 AUG 2009 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Fuel Economy and Mobility of Multi-Wheel Drive Vehicles: Modeling and Optimization Technology 5a. CONTRACT NUMBER