When talk turns to improving fuel vehicle economy, two technologies are likely to enter the discussion: hybrid petrol/electric drivelines like that used in the Toyota Prius and high pressure common rail diesels fitted to vehicles from makers like Audi, Peugeot, and Mercedes.

European manufacturers have long championed diesels, while Japanese company Toyota has an apparently unassailable lead in hybrids. Despite German automotive electronics powerhouse Bosch being a prime mover in the development of car electrics and despite Toyota building and selling diesel passenger cars, the obvious step of combining frugal diesel power with low emissions hybrid technology hasn’t yet occurred.

Or has it?

Coming in from left field is a completely new player – GM Allison. GM currently sells some ‘soft’ hybrids but it is their heavy vehicle transmission arm Allison that is the dark horse in hybrid technology development.

Not only has Allison developed a high efficiency, patented, two-mode hybrid transmission but it is a self-contained unit that can be bolted to a variety of different engines – including conventional diesels.

Rather than being controlled by the engine management system, the Allison EP system controls the engine via a standard communications interface, allowing it to work with a range of engines.

The Allison EP system is currently available in two configurations, both primarily suited to heavy vehicles that work in a stop/start environment such as urban buses and garbage trucks. However, GM and partners DaimlerChrysler and BMW will soon incorporate the technology in hybrid passenger cars, potentially providing some real competition for Toyota and Honda. So what do the heavy vehicle systems consist of and why has the technology implications for fuel-efficient passenger cars?