The finalists: Start-Stop Operation in Fuel Cell Vehicles

Project details

The start/stop function is very popular due to the fuel savings — and thus CO₂ reductions — it helps to achieve with internal combustion engines. Nevertheless, the good old Swabian virtues of frugality and economy weren’t the only things engineers Hans-Frieder Walz and Jörg Schütz had in mind when they decided to take a closer look at energy management in F-Cell vehicles. After less than a year of development work, the two finalists for the 2007 Daimler Research Award had scored a huge success, and the start/stop concept they subjected to endurance tests will now be installed in second-generation F-Cell cars of the B-Class.

The benefits the start/stop function offers in fuel cell drives differ from the advantages it brings to conventional combustion engines. In both cases, the initial objective is to save energy by shutting down the drive unit in situations where it isn’t needed. Additional advantages can be achieved, however, when innovative fuel cell drives are also equipped with a smart energy management system. The effect of such a system is immediately noticeable in fuel cell cars, where every noise is conspicuous. Here, the sound of the ancillary components can be clearly heard when the fuel cell drive is “idling,” so using the start/stop feature to shut off such equipment makes F-Cell cars even quieter.

“Because of the complex electrochemical makeup of fuel cell systems, the low or zero loads required of the fuel cell in idle mode set into motion mechanisms that negatively impact a system’s lifespan — due to the high voltage that the cells in the stacks continue to be exposed to,” says Schütz.

Walz points out another problem: “There are also difficulties involved when the system begins working again after a long idling phase, because the high air mass flow in the idle mode decreases fuel cell humidity to an extent that puts it in danger of drying out.” That’s why the two engineers combined their start/stop feature from the beginning not only with energy-management and acoustic benefits, but also with measures that reduce the strain on the drive system.

In the end, they tested two control concepts. The first was a “zero-electricity strategy,” which they wanted to use to ensure that the start/stop function itself would never place an undue burden on the drive system. This approach was in line with the iron-clad electrochemical axiom that electricity should never be allowed to flow into fuel cells.

The conditions for activating the stop function were defined as follows: the fuel cell system must be at operating temperature; the battery needs to be at least half charged; and the load requirement has to be less than two kilowatts. The fuel cell system then “awakes” as soon as the load requirement exceeds a predefined value, or if the battery charge falls below a set minimum.

All Walz and Schütz needed to implement this strategy was control software and a controller. They refitted an F-Cell vehicle accordingly, and subjected it to endurance testing on a roller test rig. They also drove the car along a test route of approximately 100 kilometers, between Nabern and Sindelfingen. In just six months, the vehicle had tallied more than 1,500 operating hours and had gone through around 280,000 start/stop cycles. The performance of the fuel cell stack was regularly monitored during all of this, and the engineers documented all signs of drive-unit aging. “With regard to all parameters, our test car performed at least as well, and in some cases better, than a reference vehicle without a start/stop function,” Walz reports.

Because of these positive results, and after extensive discussions between Schütz and Walz and fuel cell system specialists at NuCellSys, the two engineers turned to the use and testing of an alternative system control strategy, whereby the voltage in the stack would remain constant, but a minimal amount of current would be allowed to flow from the battery circuit back into the fuel cells. “The electrochemical specialists on the team encouraged us to do this and told us they thought the reverse current wouldn’t damage the stacks,” says Schütz.

This system control strategy is simpler than the first, as it only requires flashing the fuel cell vehicle with the corresponding control software for the start/stop function. It also offers another advantage in that it nearly eliminates the need in the “zero-electricity strategy” for interim engagement of the compressor during long stop phases. The compressor had to be run here for several seconds in order to supply the stack with fresh materials and ensure that voltage would not fall below a critical level.

Schütz and Walz have since equipped several vehicles with this start/stop system, and these cars are now being put through a series of endurance tests similar to the ones carried out for the first strategy.

The development of a start/stop feature for fuel cell vehicles inevitably led the two engineers in a direction involving much more than just that one function. “As can be seen from the basic conditions for a stop, this function activates not only when the vehicle comes to a standstill, but generally when the drive system requires only a low load,” Schütz explains. In fact, it’s also possible to shut down the stack while the vehicle is in motion — for example, when it’s rolling down a hill and the electric motor is recuperating energy and supplying it to the battery, or when a highly charged battery can power the vehicle on its own in stop-and-go traffic.

“That’s why we’re extending our lifespan studies to include lithium-ion batteries,” says Walz. “What we’re trying to do here is identify the most favorable parameters for our operating strategy, and to find ways to optimize energy management in the battery and drive unit.”

Their work here not only holds great significance for F-Cell vehicles, but also for any type of hybrid drive in which a battery and a drive unit jointly or alternately provide energy to propel the vehicle.

A convincing demonstration of the concept was provided last fall at the IAA International Motor Show in Frankfurt, where several F-Cell vehicles with start/stop function were used to shuttle guests around the exhibition center.