Motors have a speed and torque at whic they are most efficinet. Hybrids allow a motor to operat at it's most efficinet range. Any extra acceleration or hill climbing, draws on the energy stored during braking or going down hills.
Parallel hybrid has both motors able to operate at the same time. Thus in most hybrid cars, both petrol and electric motors contribute at the same time thus increasing acceleration or hill climbing.
Series hybrid is used in diesel locomotives or large mine dump trucks. The diesel generates electricity that power the electric motors. They are usually cheaper to build and maintain.
It is unusual for a large machine to have battery storage. The exceptions are submarines, and shunting locomotives where the extra battery weight helps grip the rails. They are usually cheap as they are built on an old worn out loco.
This is a 3000 hp hub motor as used on the big dump trucks, with a 32:1 reduction gearbox attached.
Different storage devices
The most common storage is a battery.
However good results are being produced by fly wheels and compressed air (hydraulic).
Possibly cheaper and more efficient than a battery, Energy can be stored in compressed gas. The method being tested by several car companies is to pump hydraulic liquid into a tank with nitrogen in it, thereby compressing the nitrogen. The gas then powers the drive chain. Nitrogen is used because it won't start a fire in an accident.
Parallel hydraulic hybrids give a 40 percent increase in fuel economy.
Using Hydraulics in Urban Delivery Trucks
Energy can be stored in a flywheel. Porsche uses this in racing cars. More this website
Hybrid diesel trucks
The eHighway’s catenary system uses diesel hybrid trucks fitted out with software that senses when an overhead electrical line is available and automatically connects or disconnects as needed (although even when the trucks are in hybrid diesel mode they will consume 30 per cent less fuel). As SustainableBusiness.com (via Matter Network) puts it, “when trucks detect and attach to overhead wires they automatically go into electric mode, and when they detach they automatically switch back to diesel. Pretty cool.” Pretty cool indeed… and pretty dear too: Siemens estimates the system will cost $5 million to $7 million per mile to build. But cost aside, Siemens says that as the technology becomes more widely adopted, every truck equipped with an electric drive system will be able to use the eHighway: diesel electric, pure battery, fuel cell range extended or natural gas. Based on a pilot project in Germany (LA Times says the eHighway concept has been getting a test run on an old air strip near Berlin for the past year), trucks can make the switch at speeds of up to 90 kilometres/hour.
Plug-in hybrid conversion
Protean has developed a replacement wheel hub with and electric motor/generator. It can replace the normal wheels in a standard car converting it to an electric vehicle. It is being manufactured in China in 2013.