Powertrain Configuration

The University of Michigan will be building a series hydraulic hybrid powertrain. The general architecture schematic is shown below. The propulsion for the vehicle will be provided by hydraulic components. The major hydraulic components of this system are the front pump/motor, the rear drive motor, the engine pump, the low pressure accumulator, the high pressure accumulator, the valves, and the hoses and fittings. The major non-hydraulic components are the engine, the batteries and other mechanical components providing the added traction of all wheel drive.

Pump and Motor

The front and rear drive motors, as well as the engine pump, will be bent-axis,variable-displacement pump/motors. The bent-axis pump/motors have higher efficiencies than other designs thus making them ideal for hybrid vehicles. The pump/motor displacements will be hydraulically actuated and controlled using a solenoid. At an axis angle of zero degrees, the pump/motor will rotate with zero displacement. As the axis angle increases, so does the displacement. The front drive pump/motor will be of a smaller displacement than the rear drive motor, which will be of the same size as the engine pump. The smaller front motor will be primarily used at high speeds as the torque requirements are lower. The lower displacement will allow for longer more efficient vehicle operation and fewer charge cycles. The rear motor will be used for needed power boosts for acceleration as well as providing the added traction of all wheel drive. Based on benchmarking existing hydraulic hybrid designs and simulation results, the front drive is a 60 cc/rev pump/motor, the rear a 100 cc/rev motor and the motor attached to the engine is a 100 cc/rev unit. Bosch- Rexroth and Eaton, two major hydraulic manufacturers, were considered for the project. There is no clearly superior pump/motor our application, thus either one will do. A decision will be made pending availability and cost.

Accumulators

Both composite and steel bladder-type accumulators were considered for the vehicle. It was decided that both the low and high pressure accumulators for the system will be carbon/e-glass fiber composite. The accumulators store energy using a nitrogen spring, compressed by oil. This is essentially a rubber bladder filled with nitrogen and foam. This allows them to be much stronger and lighter than steel piston accumulators. A high pressure steel accumulator, similar in size to the composite accumulator is approximately three times the weight of its composite counterpart. The steel accumulator weights 170 kg while the composite is approximately 56 kg. The accumulators will have integrated valves to ensure safety in operation and prevent the rubber bladders from herniating out of the tank. The sizing of the accumulators was based on spatial considerations. 15 gallon accumulators were chosen for both low and high pressure. The low pressure accumulator will have a service pressure of 200 psi, and with a safety factor of 5, a 1,000 psi burst pressure, and a net mass of 24 kg. The high pressure accumulator will have a service pressure of 5,000 psi a burst pressure of 15,000 psi, and a net mass of 56 kg. For safety issues, the system will also include check valves and relief valves. The check valves will ensure that flow will only go in the appropriate direction when necessary. The relief valves will prevent over pressuring of the system. Upon reaching a preset pressure, the relief valves will open and dump excess pressure back to the low pressure side of the system. A relief valves works using a pretension spring. Adjusting the force on the spring will adjust the maximum system pressure. A 5,000 psi relief valve will prevent over pressuring of the system.

Engine

The team will be using the GM 1.9 liter diesel engine. As part of their competition sponsorship GM is providing the engine. The engine utilizes common rail direct injection and is also turbocharged and intercooled. The diesel engine has a clear advantage over the comparative gasoline units in terms of both fuel economy and low-end torque. An Isuzu 1.7 liter engine to which the team already had access to was to be used initially. But once GM provided the option of using the GM engine, the team decided to go with it on account of the substantially more power and torque, lower emissions and good fuel economy of the GM unit. B20 (20%-Biodiesel; 80%-pure diesel) fuel will be used in the engine. The fuel is being supplied by B.P as part of the Challenge X sponsorship.

Engine Manufacturer	GM
Displacement (cc)	1910
No of cylinders	4
Max. power (Kw)	107@4000 rpm
Max.Torque (N-m)	318@2000 rpm
Valves per cylinder	4
Compression ratio	17.5
Fuel system	Common Rail Direct injection
Turbo charged intercooled air induction system

The Oil Conditioning System

The system must also contain an oil conditioning system, to both filter and cool the oil. The oil will be run through a radiator to cool it. There will also be an inline filter to clean the oil. The filtration and cooling system will be placed between the drive motors and the low pressure accumulator where flow losses are less detrimental. There will be a relief valve inline with the filter to prevent the backpressure from becoming too 4 high. There will also be a thermostatic valve in this line to allow oil at lower temperatures to bypass the cooling system.

Hydraulic Fluid

The hydraulic oil for the system is critical. The fluid must have a high viscosity index and a long service life. Based on research done at the EPA, the most effective fluid, in terms of cost and performance, is synthetic automatic transmission fluid. Its viscosity index is high, meaning that the viscosity changes very little with temperature fluctuations, and it has a long service life.