Test Bed 3: Highway Vehicles

Hydraulic Hybrid Passenger Vehicle

Leader

Prof. Perry Li (UMN)

Statement of Test Bed Goals

To investigate the potential, technological barriers and solution for hydraulic hybrid power-train technologies to significantly increase fuel economy of passenger vehicles, and to spur and integrate research from the rest of the center. Design metrics are: fuel economy of 70-100 mpg, 0-60mph acceleration times in 8.0s range.

Project Role in Support of Strategic Plan

This testbed directly supports the goal of the center to develop and to migrate fluid power technologies to passenger cars segment so as to enable major fuel savings and to reduce negative impact on the environment. The passenger car segment is targeted because of its large size. While hydraulic hybrid technologies have been demonstrated for large vehicles, the stringent weight and size constraint of passenger cars will require breakthrough in compact efficient fluid power concepts. TB3 serves to determine and focus research effort, and to integrate results from individual projects.

Fundamental Research Barriers and Methodologies for Addressing Them

Hydraulic hybrid passenger vehicles have great potential due to the intrinsic power density of hydraulic systems. Nevertheless, their development is constrained by several challenges requiring research.
These challenges are identified below, and related work in other center projects is identified where appropriate.

1. Lack of control strategies for managing power flow in complex hydromechanical transmissions: 1A.1
2. Low efficiency of hydraulic pumps and motors: 1B.1, 1B.2, 1D & 1G.1
3. Size and weight of available hydraulic components: 1E.1, 1E.3, & 2D
4. Limited energy storage density: 2C.1 & 2D
5. Conflicting requirements for high efficiency under city and highway driving conditions: TB3
6. Noise, vibration and harshness: 3B.1

Other hydraulic hybrid vehicle researches, commercial and academic, have focused mainly on the parallel and series architectures. Series architecture (e.g. EPA, INNA, Eaton-UPS) has the ability to fully decouple the engine operation from vehicle but requires the hydraulic components to be efficient. The parallel architecture cannot fully decouple the engine but power transfer efficiency is less dependent on hydraulic efficiency due to the use of efficient mechanical transmission for power transmission. Both can capture and reuse braking energy. The specific architecture that has been chosen for initial investigation for TB3 is a hydro-mechanical transmission (HTM) capable regeneration capability. It has the ability for full engine decoupling and the advantage of using the efficient mechanical transmission for some power transfer.

Achievements to Date

The first generation for the test bed is to utilize the open mechanical structure of a utility vehicle (a donated Polaris Ranger) as a platform to implement a hydraulic hybrid train-train with off-the-shelf components. This is to elucidate operational and component performance requirements. A novel hydro-mechanical hybrid architecture (Figure ) with regeneration and independent wheel torque control has been chosen as the architecture for detailed development and study. Mechanical design and manufacturing have been completed, and a downsized diesel engine, and hydraulic components have been installed. Sensing and control instrumentation (Matlab/xPC target system on a PC104 platform) have also been implemented. The design is flexible enough to accommodate various hydraulic hybrid architectures. During the past year, the vehicle was successively implemented, modeled and controlled under a) mechanical drive-by-wire mode; b) parallel mode, and finally, c) HMT with independent wheel-torque control mode. A 3-level hierarchical control architecture for the HMT mode has been specified. Two low level control modes have been designed (engine on, hydraulics only) based on a dynamic decomposition approach, and have been implemented and tested; a novel mid level control has been developed that translates high level energy management controller commands into desired set points and operating modes for the low level controller to track. The mid-level control significantly reduces the complexity of the high-level energy management controller. A Dynamic model of the system was also refined by developing a more comprehensive pump/motor model, and an engine model. An efficiency analysis of various hydraulic hybrid architecture based on power flow path and efficiencies of these paths is currently under way (Fig. 3)

Figure 1: Polaris Ranger utility vehicle retrofitted with a hydraulic hybrid drive train.

In support of the test bed, a pump/motor test stand has also been constructed and is nearly ready for operation.

Figure 2. Hydro-mechanical hybrid architecture.