The engine of the HyMotion. Hydrogen flows into the fuel cell, which grabs electrons to run an engine. Water vapor comes out of the tailpipe.

The liquid hydrogen tank takes up nearly all of the trunk in Volkswagen's HyMotion prototype hydrogen car. But better compression will shrink the tank size. The car is also based on a standard Volkswagen and not specially designed as a hydrogen vehicle.

BMW is working on a communication system that will let one car convey information about road conditions to those behind it. In this photo, the LCD screen on the dashboard flashes up a warning to a moving car that the road is slippery ahead. Minutes before, another car did a power slide through a wet patch on the same path.

With no hazards imminent, the screen provides data on the current status of the car. Motorola and GM are working on similar systems.

BMW's Connected Car. The cars are on display at the 12th World Congress on Intelligent Transport Systems taking place in San Francisco this week.