2015 Toyota Prius Spyshot

The Toyota Prius is undeniably the king of the hybrid market in the United States, with a 39.4 percent market share in 2013. With the next-generation Prius likely to go on sale in 2015, Toyota is trying to build an even more efficient hybrid to keep its control of the market.

Keeping cost down will be one of the major concerns of the new Prius. The next generation will ride on the new, modular Toyota New Global Architecture platform. The lighter underpinnings will improve efficiency and will reduce production costs by allowing for more shared components among vehicles. Toyota will not reveal how many vehicles will use the new platform. But even with the cheaper platform, price will remain a concern. Toyota is still deciding whether all versions of the next Prius will use lithium-ion batteries or whether some models will stick with the heavier nickel-metal hydride batteries to keep cost down.

Of course, the reason most people buy the Prius is because of its great fuel efficiency. Toyota will aim for at least an 8 percent improvement in fuel economy in the next Prius, which would increase it to 58 miles per gallon city and 52 mpg highway.

To maximize efficiency, the next Prius will receive a new, more thermally efficient engine. Toyota will not go into many details about it yet, but engine will have thermal efficiency above 40 percent, compared to 38.5 percent now. That will mean more of the car's energy won't be lost to excess heat.

While the Prius remains the hybrid market leader, competitors keep emerging and eroding its lead. If it wants to remain on top, the next Prius has to show why it is still the king.