An air hybrid is a vehicle with an ICE modified to also work as an air compressor and air motor. The engine is connected to two air reservoirs, normally the atmosphere and a high pressure tank. The main benefit of such a system is the possibility to make use of the kinetic energy of the vehicle otherwise lost when braking.
The main difference between the air hybrid developed in this paper and earlier air hybrid concepts is the introduction of a pressure tank that substitutes the atmosphere as supplier of low air pressure. By this modification, a very high torque can be achieved in compressor mode as well as in air motor mode.
A model of an air hybrid with two air tanks was created using the engine simulation code GT-Power. The results from the simulations were combined with a driving cycle to estimate the reduction in fuel consumption.
The system was designed to slow down a bus that stops from its maximum speed in urban traffic without using the friction brakes or any other braking system. When this criterion was applied to a 15000 kg bus, the required high pressure tank capacity was 140 litres and the low pressure tank capacity was 600 litres. The low pressure tank can be made very light and is ideally placed on the roof of the vehicle.
According to the simulations, 55 percent of the energy absorbed by the engine when braking can be regenerated and returned to the drivetrain. The total reduction in fuel consumption was estimated at 23 percent for urban driving.