Most of these involve the coupling of an engine, an electric motor and sometimes batteries, in various different ways to allow excess energy that would otherwise have been lost, to be stored in the vehicle. Energy is created by activities such as braking and descending hills, or by running the engine at its most efficient while the stored electrical energy takes up the variations in required power. This can improve the vehicle efficiency and fuel consumption dramatically. Many manufacturers are running development work on such systems. So far hybrids are more expensive and heavier than normal engines but offer the chance to downsize to obtain the same power. This type of system is also likely to drive the need for more efficient turbochargers.