Guy N�gre's first prototype compressed air engine was run successfully in 1993. Applying his tremendous knowledge and skill from his Formula One engine building career he has subsequently developed three evolutions of his engine, and the latest EV4 generation produces the greatest power output yet.
In principle the technology is very similar to the internal combustion system in that compressed air is used to drive a piston in a barrel. The secret of the engine lies in the way it efficiently converts the energy stored in the tanks of compressed air.
By way of explanation, it has long been known that to compress air to high pressures a staged process should be used, compressing air to first 50 bars, then to 150 bars then three hundred and so on. This technique, commonly employed by the air and gas liquefaction industries, uses a fraction of the energy used to compress the gas in one operation. The secret of the compressed air motor is simply to reverse the process - decompress the air in stages and in so doing efficiently release energy at each point in the chain.
To compensate for the cooling effect that takes place, a thermal exchanger heats the compressed air using the warmth of external air.
This process is repeated as many times as possible to extract the maximum energy efficiency from the compressed air.
For the somewhat technically minded, the following drawing illustrates the theoretical explanation for this process.
The Isotherm, the green line, represents the ideal transformation of the compressed air: in effect, the air temperature is the same coming in and going out of the cylinder, and power is maximized.
On the contrary, the worst transformation is the Adiabatic transformation, represented by the red line. The derived power is minimal, and the air leaves the system at a very low temperature indeed.
The blue line, or polytropic curve, represents the transformation that the MDI motor realises, and the individual stages outlined above can be seen. The transformation going through the first cylinder is represented by the polytropic line (somewhere between our ideal isotherm and the adiabatic curve). The following temperature rise brings the line closer to the isotherm, and allows the second and subsequent stages to produce more power.
In other words, if we realize an adiabatic transformation no heat is exchanged between the external air and the compressed air meaning that the power produced is minimal. On the contrary, following the isotherm means a maximum exchange and the power so produced is optimised.
In practical terms compressed air at 300 bars is stored in the carbon fibre tanks A. The air is released through the main line firstly to an alternator B where the first stage of decompression takes place. The now cold air passes through a heat exchanger C which adds thermal energy to the air and provides a convenient opportunity for air conditioning D. The warmed compressed air now passes to the motor E. where a two more stages of decompression and re-heating take place. The motor drives the rear axle G through the transmission F. Control of engine speed is through a conventional accelerator pedal H controlling a valve within the motor.
An energy recycler J is under test which uses engine braking K to recompress air during braking into a secondary storage facility, providing additional energy for re-start and acceleration. Conventional hydraulic braking L is supplied.
The vehicle can be refilled by using the onboard compressor M or by refilling the tank at an air station at N. Ultimately the engine generates 37 Kilowatts, notwithstanding the small size of this unit. The "exhaust" leaves the engine at about zero degrees Celsius, a result of the expansion and cooling action. The exhaust is totally pure and fit to breathe.
A compressed air driven engine offers enormous benefits to the car designer. Because of its small size and weight, and the removal of a host of devices and parts not required, the designer has free reign to maximise his materials and space to provide a simple, economic platform for the vehicle.
