Nissan will limit the introduction of mild-hybrid stop start-type technology to its bigger, more expensive vehicles because the technology adds too much cost to smaller cars.

“We will focus more on the 4×4-type of vehicle, where it can be quite good,” said Mitsuhiko Yamashita, global head of R&D.

In the UK this is likely to be selected models of the X-trail and Qashqai line-up, which together contribute about one-third of the company’s UK sales.

This will help Nissan get its corporate average CO2 figure down from last year’s 167g/km towards the EU target of 130g/km by 2012.

Yamashita believes the 5% to 10% saving possible with stop-start and intelligent alternators – usually bundled together under the ‘mild hybrid’ banner – won’t make a sufficient dent in the company’s CO2 output to justify the cost.

“It’s just not too positive on that aspect,” he said.

Nissan‘s view aligns with that of several other volume car-makers, such as Ford and the VW Group, which is making mild-hybrid technology available on a limited number of models, usually badged as a green sub-brand.

BMW on the other hand is going all-out to fit all its models with mild-hybrid equipment, including for the first time its new flagship 7-series, launched this autumn. “That may be okay for some companies with bigger cars,” said Yamahita.

Nissan‘s current and next-generation of Euro5 and 6 diesels are also unlikely to get mild-hybrid equipment.

These engines will effectively have to emit exhaust pollutants at petrol-engined levels and will be equipped with high-precision injectors and pumps and extra exhaust after-treatment. “These are expensive engines and the mild hybrid makes them even more expensive,” says Yamashita.

Yamashita is much keener to bring forward critical green breakthroughs like electric cars and hybrids. “Beyond the next three-to-five years we need a giant technology,” he said.

Nissan is forging ahead with plans to bring electric cars equipped with high-storage lithium ion batteries to the UK in 2012.

Part of this program also includes a medium-sized car with a range-extender petrol engine like the Chevy Volt. The go-ahead is expected around 2009/10, before the Volt goes into production.

Nissan is also working on its own hybrid system to replace the one offered in the US-only Altima, technology bought off-the-shelf from Toyota.

This hybrid will also use high-capacity lithium ion batteries, which will allow the car to run much more frequently as an electric car. “It will operate much differently to the Toyota system,” added Yamashita.