Hybrid-electric drive system developer ISE on Monday announced it raised $17.5 million in a fourth round of funding. 

The funding was led by Siemens Venture Capital, with Macquarie Clean Technology Fund, DTE Energy Ventures, and return investors RockPort Capital Partners and NGP Energy Technology Partners also participating.

ISE plans to use the funding toward sales and expansion of the company’s hybrid-electric drive systems.

ISE makes what's know as a "series"  hybrid system, one that leave the internal combustion engine not connected to the wheels and is used solely for electrical power generation. The vehicle is then controlled by electric motors rather than the combustion engine.

That compares to such "parallel" systems as that of Allison Transmission’s hybrid system, where the combustion engine is linked to the wheels, and the work is more evenly split between the engine and the electric drive.

The parallel hybrid system is currently the more widely used system, although more companies are starting to explore series hybrid systems. The Toyota Prius uses a combination of parallel and series.

The ailing General Motors, which owned Allison Transmission until it sold Allison off last year for $5.6 billion, will utilize a series hybrid system in the upcoming Chevy Volt plug-in hybrid.  

ISE previously raised $37.6 million.