Japan - Nissan Motor Co said it aims to nearly double the mileage on its first hybrid car developed in-house compared with the gasoline engine version, while keeping costs down with a simple, single-motor system.

Japan's No.3 automaker is due to launch a gasoline-electric Infiniti M sedan, called Fuga in Japan, late this year, lagging rivals Toyota Motor and Honda Motor by more than a decade in offering a proprietary hybrid model.

While admitting to the late start, Nissan said its one-motor, two-clutch system would achieve far better fuel economy, at a much lower technical cost compared with hybrid leader Toyota's complex, two-motor series parallel system.

Typically, carmakers say the fuel economy improvement on their cars using a 'strong' or 'full' hybrid system is roughly 30 percent, while for 'mild' hybrids (like Honda's), it's 15 percent, said Koichi Hayasaki, chief engineer of Nissan's rear-wheel-drive hybrid system.

We're aiming for an improvement of 60 to 90 percent, he said, adding that the hybrid Infiniti M would have mileage comparable to that of a compact car.

Nissan's hybrid system has a structure similar to that of Volkswagen AG's, mounting an extra clutch that separates the electric motor from the engine to allow for driving using only electric power when the battery is charged.

But Hayasaki said Nissan's unique system enables it to better control the motor by using a lithium-ion battery instead of nickel-metal hydride, which is slower in capturing and discharging energy. Using a lithium-ion battery, he said, eliminates the need for a torque converter, which the Volkswagen group uses on its Touareg SUV, Porsche Cayenne and other hybrids.

It was a technical hurdle that most hybrid engineers in the

industry believed could not be cleared, Hayasaki said, adding the system took about six years to develop.

Together with more accurate electronic controls, the system allows the engine to idle more frequently and therefore use less fuel. Hayasaki said millions of miles of testing had shown that the engine was stopped about half the time in city driving.

By having fewer components, Hayasaki said, Nissan's hybrid system was roughly 30 kg (66 lb) lighter than Toyota's series parallel system, which mounts two electric motors and two inverters, which make up the bulk of a hybrid system's cost.

As part of its drive to reduce carbon dioxide emissions from its cars, Nissan is planning to also launch fuel-efficient 3- and 4-cylinder gasoline engines and stop-and-start technology this business year.

The new March, due for launch in Japan this month, will be the first to use the stop-start feature, which automatically shuts down the engine when the car comes to a brief stop. The March will have class-leading fuel economy of 26 km/liter, Nissan said.

Nissan, held 43 percent by France's Renault SA, is due to mass-market the zero-emission Leaf electric car starting in December. Its sole hybrid model now is the Altima sedan, which uses Toyota's hybrid system.

(Editing by Michael Watson)