ISE auxiliary power units (APUs) are the primary source of onboard electrical power in the ThunderVolt® drive system. The standard APU subsystem for ISE diesel-hybrid bus applications consists of a conventional diesel or gasoline fueled engine, coupled to a Siemens 150 kW generator through a Stiebel gearbox. The generator is controlled by a Siemens inverter identical to the main drive motor inverters with the exception of its software. The combined weight of the generator and gearbox is approximately 215 kg. The APU is sized to enable a 40-foot bus to perform efficiently in local stop-and-go driving at grades up to 17%. For heavier vehicles such as refuse trucks, or applications requiring extended high speed driving, either a high-energy battery pack or a larger engine and a second generator can be added.

Fuel Cells

ISE offers many variants of this APU subsystem, which can be tailored to the performance and emissions requirements of each bus operator. One of ISE's unique APU offerings is an ultra-low emission gasoline hybrid APU, which uses a Ford 6.8 liter V-10 gasoline engine in place of the Cummins diesel engine. In October 2003, ISE's gasoline hybrid drive system was certified by the California Air Resources Board for use in transit buses, to an emissions rating 0.4 grams of NOx per brake horsepower hour - making this ISE Fdrive system the first hybrid drive system to be certified by the ARB for use in  buses or heavy-duty trucks. ISE also offers the same basic APU configuration using engines fueled by compressed natural gas (CNG) and hydrogen. ISE has integrated CNG-based hybrid drive systems into three different truck models to date.

Another unique aspect of the ThunderVolt-ELFA® drive system is that the engine-based APU subsystem can also be replaced with electric generators based on fuel cells. ISE has successfully integrated 30-foot transit buses using fuel cells, and is developing 40-foot fuel cell buses using the ThunderVolt-ELFA® drive system that are scheduled to enter service in 2005. Hybrid-electric buses integrated using the ThunderVolt-ELFA® drive system can be easily upgraded to fuel cell buses as fuel cell technology advances.