The most commonly used form of regenerative braking is where a vehicle’s electric motor is used as an electric generator to capture the vehicle’s kinetic energy, which is otherwise lost as heat when braking. The generator converts the kinetic energy into electricity that is then fed back into the vehicle’s battery pack where it is stored for later use. New research suggests that pneumatic or air hybrids that instead store the energy as compressed air would be much cheaper to produce than the current crop of EVs and battery-electric hybrids and could halve the fuel consumption of ICE powered vehicles.

Although converting kinetic energy to electricity is the system found in the majority of EV’s and hybrids on our roads, such as the Toyota Prius, there are also systems that allow the kinetic energy to be stored in other forms. Mechanical systems use a flywheel to store it as rotational energy, while hydraulic systems store the energy as pressurized fluid.

Pneumatic systems that store the energy as compressed air are yet another option and it is pneumatic or air hybrid systems that Sasa Trajkovic believes can be used to produce more fuel efficient vehicles. Trajkovic, a doctoral student in Combustion Engines at Lund University in Sweden, did his doctoral thesis on pneumatic hybrid vehicles and he says that compressed air could be used to provide extra power to the engine when starting up and save fuel by avoiding idle operation when the car is at standstill.

For the study the researchers converted heavy duty Scania engines converted to operate as pneumatic hybrid engines. During pneumatic hybrid operation the engine can be used as a 2-stroke compressor for generation of compressed air during vehicle deceleration (compressor mode) and during vehicle acceleration the engine can be operated as an air-motor driven by the previously stored pressurized air (air-motor mode).

The compressed air is stored in a pressure tank connected to one of the inlet ports. One of the engine inlet valves has been modified to work as a tank valve in order to control the pressurized air flow to and from the pressure tank.

The air hybrid engine, which would work with gasoline, natural gas and diesel fuel-powered engines, doesn’t require any expensive materials such as those used in battery packs, so they would be cheaper to manufacture. They would also take up much less space than an electric hybrid engine.

Trajkovic calculated that 48 percent of the brake energy, which is compressed and saved in a small air tank connected to the engine, could be reused later. This matches the degree of reuse of today’s electric hybrids and, like current electric hybrids, Trajkovic says the technology would be even more attractive for slow and jerky driving, like that found on a bus in urban traffic. His simulations showed that buses in cities could reduce their fuel consumption by 60 per cent.

Although pneumatic motors have been around for over a century and there have been some (so far unsuccessful) attempts to bring a compressed air car to the market, the idea of air hybrids has only been around for a couple of decades. With most of the previous research into the technology theoretical, the Lund researchers say theirs is the first time anyone has done experiments on an actual air hybrid engine.

The Lund University researchers studied a single cylinder air hybrid engine but hope to conduct further research into a complete, multi-cylinder engine with the aim of bringing the concept another step closer to a real air hybrid vehicle.