It sounds like pie in the sky, admittedly, and most industry insiders were justifiably sceptical when Peugeot first mooted the idea of running hybrid cars on a combination of petrol and compressed air.
But really, all hybrids do is to recapture some of the energy that's thrown away in the form of heat, friction and chemical by-products when liquid fuel is converted into physical propulsion - and compressed air is as efficient a method of storing energy as an electrical battery.
And, in this case the compressed air is just a medium; the actual hybrid power source is a small hydraulic motor, as used on most fork-lifts - there's nothing new about that.
What's new is the car - it's the Peugeot 2008 urban crossover, developed by and for three continents, and due to debut at the Geneva motor show early in March. Peugeot says it's the natural home for a drivetrain that combines petrol and compressed air - and it's difficult to argue with that.
HOW IT WORKS
The Hybrid Air system combines two energies for optimal efficiency, with compressed air assisting, or even replacing the petrol engine during the phases which consume the most energy, i.e. starting and acceleration.
It uses an 'energy tank' containing pressurised air in the central tunnel, a low pressure tank at the rear suspension cross member acting as an expansion bottle and a hydraulic unit consisting of a motor and a pump on the transmission, under the bonnet.
The output of the two power sources is managed by an electrically controlled gearbox that replaces the mechanical transmission and even offers automated gear changes.
The petrol engine is Peugeot's latest three-cylinder VTi, kept as light and compact as possible by integration of components, with internal friction reduced by diamond carbon coating, thermo-management and split-cooling.
All of which means that the Hybrid Air system is compatible with existing platforms because the passenger compartment, chassis layout, and the position and volume of the fuel tank remain unchanged.
THREE MODES: Air, Petrol and Combined
The transmission controls the two energy sources to achieve the highest efficiency in any given situation; the process is seamless and the driver doesn't even feel it happening.
In the Air mode, the energy contained in the compressed-air tank drives the vehicle. As it expands into the low-pressure tank, it pressurises a corresponding volume of oil, which drives the hydraulic motor coupled to the drivetrain, so the car moves without using any fuel or emitting any CO2 - and that's great in pollution-sensitive urban areas.
In Petrol mode, only the 1.2-litre VTi three-cylinder petrol engine drives the car. It's an all-new engine, 21kg lighter than the previous generation, with internal friction reduced by 30 percent and 'intelligent' thermal management to reach its optimum operating temperature more quickly.
This mode is particularly suited to steady speeds on the open road.
In the Combined mode, the petrol engine and hydraulic motor share the load, in proportions adjusted according to the situation for optimum fuel economy.
As long as there's air in the high-pressure tank, that's used to drive the hydraulic motor, which is useful for quick bursts, as in take-offs and accelerating for overtaking manoeuvres.
Once the tank runs out of puff, however, the hydraulic pump cuts in to power the hydraulic motor (exactly as in a conventional forklift) so that full acceleration is always available on demand - unlike the Honda CR-Z sports hybrid, which loses the edge off its performance when the battery goes flat!
The energy tank is filled in two ways: under braking and on the overrun, the car sheds speed by using its kinetic energy to drive the compressor or, when there's no 'free' energy available, the petrol engine can drive the compressor.
Either way, maximum pressure is reached very quickly - in just 10 seconds.
TARGETING TWO LITRES PER 100KM
The adaptability of the Hybrid Air system means it can be used in a variety of vehicles technology and, because it uses proven components it's durable enough to be used in a third-world situation.
Peugeot says it can deliver zero-emissions driving about 80 percent of the time in heavy urban traffic, with fuel savings of around 45 percent and smooth, seamless powerflow due to the auto transmission - without losing any of the versatility, occupant space or even fuel-tank capacity of the equivalent conventional model.
In fact, in a homologation cycle test a prototype Hybrid Air vehicle recorded fuel consumption of 2.9 litres per 100km, with CO2 emissions of only 69g/km.