For hybrid electric vehicles to be widely adopted by militaries, they must prove highly reliable with redundant power sources that can provide backup in the event of a power failure. New research by the French Army, using its Electrical Chain Components Evaluation (EECE) mobile laboratory, looked at ways to ensure a hybrid vehicle can keep moving if a power source fails.
In 2004, the French Army created the ECCE to test hybrid electric components of a heavy-duty vehicle in real-world conditions. The original test vehicle had two diesel internal combustion engines and a battery pack. In 2009, the diesel engines were replaced with a fuel cell system (FCS) and an ultracapacitor system (UCS).
The latest research evaluated what happens when one of those two power sources fails. Could the vehicle continue operating long enough to reach a maintenance center or complete the mission? For the study, the team modified the vehicle’s global energy management system (EMS) to allow degraded operation if an energy source fails.
“Our goal was to increase the global reliability of the electric hybrid power train,” said Professor Marie-Cecile Pera, deputy director of the FEMTO-ST Institute. “Confidence in the availability of the power is one of the hurdles to overcome for a wide marketing of this technology.”
The global EMS illustrated in Figure 2 shows its two outputs–the reference power of the FCS and the reference power of the UCS. If one of the power sources fails, the EMS only has one output during degraded operation.
The team first studied the effects of degraded operations in a simulated environment. After a UCS failure, they found the battery voltage was highly variable, and the state of charge was regulated by the FCS. Conversely, after a FCS failure, the simulation revealed the battery voltage was regulated by the UCS, but the state of charge could not be controlled.
“When the UCS fails, travel time is unchanged because the UCS stores some energy. The vehicle can operate for several hours,” said Prof. Pena. “When the FCS fails, battery life drops to about 20 minutes. The EMS avoids immediate shutdown and allows the vehicle to travel to a safer location.”
Both failures were tested and confirmed in live experiments. Based on the simulation and experimental results, the team concluded the EMS allowed operation of an HEV to continue after a failure occurs.
While the team does believe their study was a success, they also believe more work needs to be done to improve their solution’s effectiveness and the travel time post-failure.
One day this technology could be used in civilian vehicles too, Prof. Pena said. The EMS can be directly adapted to light trains, trucks and busses, making these vehicles more reliable and boosting confidence in hybrid electric transportation systems.
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