General Motors Co. reveals it has added a “mountain mode” to the Chevrolet Volt extended-range electric vehicle, a driver-selectable option aimed at optimizing its performance under steep grade conditions.

Larry Nitz, GM executive director-hybrid and electrical powertrain engineering, offers a few details of the mode during a conference call with journalists from a Volt validation trip in California.

Mountain mode allows the Volt to reach into its battery while under range-extended operation for extra power to boost the performance of its 1.4L naturally aspirated 4-cyl. engine.

The Volt is designed to travel 40 miles (64 km) under all-electric power before the engine engages as a generator to provide power to the wheels for a full range of some 300 miles (483 km).

“When you have a substantially downsized engine as we do – it is half-sized in terms of the power-delivery capability of the electric side of the vehicle – you need to be able to reach into the battery and pull that extra performance,” Nitz says.

Selecting mountain mode places more of the battery’s capacity in reserve than “normal mode” would do. As a result, the engine revs slightly higher while under range-extended operation to generate the extra power.

Nitz admits drivers would have to plan ahead if they wanted optimum performance in mountainous conditions, but notes, “The whole nature of the Volt is as an interactive vehicle.”

“Mountain mode is very nice,” he adds. “We’re excited, and we like to have that feature for customers.”

Nitz says the Volt development team tested mountain mode this week, running the cars from sea level in Los Angeles up to 6,500 ft. (1,981 m) at Big Bear. The cars performed well, although he stops short of offering a fuel-economy number.

Volt engineers have previously said three factors will affect the Volt’s much touted all-electric range: technique, terrain and temperature.

In other words, the more aggressive the driver and the more demanding the terrain, the shorter the range.

A driver’s desired comfort level also plays a big role. “It takes more energy to heat the cabin than it does to move the car down the road,” Nitz says. “It’s simple physics.”

The Volt also will feature a “sport mode,” which changes the accelerator map to enhance torque response. Inside the car, selectable “eco” and “comfort” modes will regulate cabin conditions.

In addition to proving out the Volt’s various driving modes, Nitz tells Ward’s engineers are working to perfect the car’s diagnostics systems ahead of its sales launch later this year.

“This is a complicated vehicle with a lot of computers that talk to each other, some of them regulated and some of them our own,” he says.

“Having a car that drives well can sometimes be the easy part. We have to have a window to make sure we are correctly finding faults and not incorrectly finding faults when everything is fine.”

Nitz estimates 99% of the diagnostics testing is complete; a package that will have to be turned over to federal regulators for certification by mid-June.