Air hybrid vehicles could halve fuel consumption


February 7, 2011

The air hybrid engine used in the Lund University study

The air hybrid engine used in the Lund University study

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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.

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag. All articles by Darren Quick

UPS trucks have been using this tech for some time so its not some pipe dream.


This direction is already being heavily pursued by the Scuderi group (see in combination with their intruiging split-cycle engine design. This tech has huge potential to improve IC engine efficiencies. And they\'re well on track to commercialization with several major manufacturers. Good stuff!


Basically it\'s the same as the famous american Scuderi Engine. See youtube

Akemai Olivia

I\'ve thought this before myself - compressing air and then releasing it is much more efficient that converting the electricity etc. I differ a little however in that I think of it more as an energy return system where the same special brakes that compress the air while decelerating also return the power to the wheels when acceleration is required. Getting the combined brake/air motor right would be the key.

Some Aussie company patented something a bit like this that the US army brought to use on their trucks etc.

John Hogan

\"while hydraulic systems store the energy as pressurized fluid.\" When you say pressurized fluid - are you implying that fluids cab be compressed? Air can be but fluids cannot. That is why water is used to hydrostatically test gas cylinders and air is used to force the water. The water can expand the metal and test it\'s ductility. Brake fluid works like a steel cable in liquid form because it can not be compressed.


\'hydraulic systems store the energy as pressurized fluid.\' WRONG. Pressurized fluid in hydraulic systems cannot STORE energy, they only CONVEY it, the very principle of hydraulics is the use of an incompressible fluid in order to transmit power from the source of pressure to where effort is needed. What allows a hydraulic hybrid to work is use of an accumulator whereby pressurized fluid COMPRESSES GAS in a piston. These and other concepts are explained in my report on Atlas Motor Works ducted blade rotary engine here; There is finally an alternative to pistons!


Don; Pressurized is not the same as compressed. All fluids can be pressurized, those that are gasses will compress proportionately, PV = nKT - liquids will compress very little. A hydraulic ram with a load on it will hold pressurized, but not compressed, hydraulic fluid. Well, just a little.


I think they are very much overrating the mileage gain of the system. Most regenerative braking contributes about 10% to overall mileage. And that\'s with systems that get 90% efficient energy recovery. The main ingredients to the Prius\' high mileage are low drag, and Atkinson cycle ICE. The Atkinson cycle engine gets about 50% better mileage than a comparable Otto cycle engine. The loss in torque from an Atkinson cycle is offset by the electric drive. So here we have a system that only get UP TO 48% recapture, and it\'s supposed to magically do 4 times better than a Prius? I really doubt it. Bench tests are one thing, real world tends to throw that stuff out the window.


To answer the comment by donwine about compressing fluids; The fluid itself is not compressed, but the pressurized fluid pushes against a piston which has on its other side a compressible innert gas such as nitrogen. This device is called an accumulator and has been used extensively on airplane controls and crane and other construction equipment controls and pressure equalizers or moderators. The pressures are much greater than those of a compressed air system. The accumulators I was familiar with operated with pressures greater than 1500 psi. and could go up around 10,000 psi I\'m not sure if this system would be of any value for propelling a vehicle because the volume of fluid involved is usually not as great as the volume of air as it uncompresses. Regards



Aldo Greco

Lithium batteries are presently very expensive and must be replaced periodically at great cost. The proposal of an compressed air/ICE hybrid seems good.

Adrian Akau

There are mechanical systems other than flywheels. Some designs use essentially giant elastic bands or springs that store energy by stretching or compressing.


The EPA has been pushing hydraulic hybrids in conjunction with industry partners. The results are impressive, much more so than pneumatic hybrid tech. 70% of braking energy gets recaptured.


The gains are higher than presented here if you improve the efficiency of the compression, the efficiency of the expander, raise the temperature of the stored air and fold back in all the thermal losses normally rejected in an IC setup. SI engines are less of a candidate than diesel, and external combustions systems have even higher gains from recapturing thermal losses. Using the IC cylinders for compressors and expanders cause immense losses due to thermal losses because the IC cylinders are cooled to maintain lubrication while the air expanders and compressors have a stratified thermal requirement for top efficiency. Our project in 1969 for Stanley Dashew (invented the credit card) for diesel powered systems showed almost as high efficiency as the Still engine without the complexity.

Facebook User

What ever happened to the idea of using a spring to store energy temporarily for reuse upon acceleration?

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