Powertrain Modes of Operation

The placement of both energy sources and sinks on both sides of the DC-DC power converter yields a powertrain energy balance equation that is unique in current fuel cell architectures. The possible current pathways within the vehicle are shown in Figure 29.

Figure 29: Current flow within the powertrain

From the energy flow diagram, the powertrain high voltage energy balance is developed. As a convention, a production of electrical energy is considered positive current, and

consumption is negative current.

Equation 6

Fuel Cell

Bi-Directional DC-DC

Rear Motor / Inverter High Voltage Battery

Front Motor / Inverter

High Side Hub Low Side Hub

R

eg

ener

atio

n

Front Motor Regeneration Fuel Cell Charging

Rear Motor Power Supplement Front Motor Power Supplement

Pr o p u lsi o n R eg ener atio n Pr o p u lsi o n Pr o p u lsi o n R eg ener atio n P_ro pu lsi o n

This expression simply demonstrates that all power produced at one point in the powertrain must be consumed at another point. Additionally, the DC-DC power converter is responsible for moving power between the high and low sides of the high voltage system, and this power transfer is expressed in terms of the difference in power generated / consumed on each side of the high voltage bus. For convention, the DC-DC transferred power is positive when boosting from the low side to the high side, and negative when bucking from the high side to the low side.

( ) Equation 7 The powertrain goes through various distinct modes of operation that are determined by where and how power must flow through the high voltage bus. The vehicle supervisory control software defines when each of these modes occurs and applies the appropriate power and torque control algorithms.

5.2.1 High Voltage Bus Pre-Charge

The only source of permanently energized on-board high voltage energy is the battery pack. The battery therefore must pre-charge the entire high voltage system before any high voltage components can operate. Thus, the DC-DC must be capable of being energized on the low side and subsequently energizing the high side from zero volts. Once a sufficient voltage is established on the high side, the start-up sequence for the fuel cell system can begin.

5.2.2 Fuel Cell Stack Start

The fuel cell system draws high voltage power from the battery pack to operate the balance of plant components (e.g. cathode air compressor) which enables the fuel cell stack to get to open circuit voltage in preparation to close its contactors and provide power to the vehicle. Up to 10 kW of power is required to start the fuel cell stack, which translates to 40 A at 250 V nominal battery voltage.

Additionally, the DC-DC converter must be able to “follow” the stack voltage during the start-up sequence. The fuel cell system requires that the high side bus voltage be within 25 V of the stack-side voltage before contactors are allowed to close. The final voltage of the stack

before contactors attempt to close is variable and depends on temperature, humidity, and other factors. Therefore the DC-DC must have the ability to control the high side voltage to a specific value (the stack-side voltage) as the high side component loads vary. Otherwise, failed starts may occur.

5.2.3 Propulsion – Excess High-Side Power

During normal propulsion there is a power balance which exists on each side of the high voltage system. An imbalance on either side is compensated for by transferring current through the DC-DC power converter to the other side. Every propulsion system component (with the exception of the fuel cell stack) can act as a source or a sink at any time. The system power balance is shown in terms of DC-DC power transfer below. All terms are signed positive when acting as a source and negative when acting as a sink. The DC-DC power term is positive when current is transferred to the low side.

For normal propulsion the control system is concerned with the power balance on the high side of the high voltage system. This is due to a combination of factors, including the slower fuel cell electrical inertia (compared to the battery) and the fact that the battery can act as a source or sink as needed without any battery-level control (it is treated as an automatic energy buffer). If, during normal propulsion the fuel cell power output exceeds the load of the front motor and the high voltage auxiliary loads, the DC-DC must be able to transfer the excess current to the low side (bucking mode), to be absorbed by the battery. This could be due to fuel cell inertia being too slow to respond to a reduction in the front motor load (heavy tip-out), which is shown in Figure 52 of section 7.1.

5.2.4 Propulsion – Excess Low-Side Power

If the fuel cell power output is insufficient to meet the loads of the high voltage auxiliaries and the front motor, current must be transferred to the high side (boost mode) from the battery. This typically occurs during heavy acceleration where the fuel cell cannot increase power output quickly enough to meet the motor demand. It may also occur during charge depleting operation, where the hybrid control strategy prefers a low fuel cell power output.

5.2.5 Regenerative Braking

During regenerative braking, the fuel cell is commanded to zero power output, which

maximizes the regenerative braking power that can be transferred through the DC-DC power converter. The fuel cell will internally generate the power required to satisfy all non-motor high side loads (auxiliaries). The DC-DC converter is commanded to maintain fuel cell open- circuit voltage on the high side of the high voltage bus to accomplish this. Thus, maximum current generated by the front motor is transferred to the low side.