Air Engine

Compressed Air Engine
by Motor Development International, Luxembourg (MDI)



Mono Energy – Mode 1 (available on all MDI vehicles)The pre-compressed air in the tanks is transferred to a chamber (called “active”) which provi

des “work” before being expanded in the cylinders to perform the stroke phase. There are no polluting emissions generated and the use of renewable energy in the air filling stations allows the production of a completely clean energy loop (compression – expansion).

Dual Energy Engine – Mode 2

The engine is the same as the Mode 1 engine, however a burner is added between the compressed air storage tank and the engine. The burner provides a continuous low temperature combustion increasing the volume of the intake air and therefore increases the freedom and independence of the system. The performance of the engine in terms of torque and power remain identical to Mode 1.

This external continuous combustion at a controlled temperature at around 600°C does not produce nitrogen oxides or sulphur dioxides and eliminates any discharge of unburnt hydrocarbons. Although using a fossil fuel, it is never the less a clean engine. Unlike the internal combustion engine, it is totally insensitive to the load (throttle position) and provides unparalleled consumption efficiency.

For example, on an AirPod, only 0.3l/100km (or 6.8g of CO2 per km) are needed to heat up the air coming from the tanks and double the range of the vehicle, and 0.5 l/100 km (or 11.4g of CO2 per km) are used to triple the range, up to nearly 450kms.

The operation in Mode 1 is always possible on the products equipped with dual energy Mode 2 engines. Mode 2 is used on all MDI vehicles.

Dual Energy Engine – Mode 3

Compared to previous modes, the dual-energy engine – Mode 3 has an augmented cylinder for the same purposes. In fact, when the energy source is the fuel for the external combustor, part of the power is used for motor output (as in Modes 1 and 2), and the rest is used to compress the air necessary for operation. Being a real external combustion engine with active chambers, it augments the dual-energy engine Mode 2 in terms of emissions, i.e. no oxides of nitrogen and no unburnt hydrocarbons. It offers extremely low consumption with a range of fuel options .

Fitted on an AirCity, only 2.5 litres of fuel are required to travel 100 kms at 100kms/h (with only 57g of CO2 per Km). The range is no longer linked to the volume of compressed air stored and can reach close to 2,000 kms with a petrol tank equivalent to those of the current cars. Insensitive to the load (throttle position), the consumption obtained, thanks to the dual-energy Mode 3, are less than 1l/100km in urban cycle (for only 22.8g of CO2 per km).

The operations in Mode 1 and Mode 2 are always possible on the products equipped with the dual energy Mode 3. This clean engine will be offered on most of AVL’s vehicles (except the AirPod).


The MDI engineers and technicians have created an efficient an efficient thermodynamic cycle. They are constantly working to improve the performance of their engines by working on cycles which are as close as possible to the isothermal expansion ideal. This is done at a constant temperature. It’s also the one that produces the most work for a given quantity of air and therefore corresponds to the best expansion performance.

In the active chamber engines developed by MDI, the filling of a volume with air to expand already creates work potential. This is one of the ways to move closer to the isothermal expansion and therefore to increase the yield. The lack of expansion valve and multi staged expansions (allowing a recovery of thermal ambient energy), are other elements which, associated with the active chamber, allow MDI to obtain exceptional yields between 68 and 75%.

Dual energy mode efficiency When using the dual energy mode, one must not only consider the phases of compression and expansion of the compressed air, but also the efficiency of the combustion (being continuous and external to the engine). The increase in the temperature difference between the cold source (air tank at ambient temperature) and the hot source of the system (combustion) leads to an increase in the overall efficiency which in “Mode 2” is between 50 and 67% for a combustion not exceeding 600°C.







Bigger Engine 72 kW


MDI has two production engines that cover its multiple applications:
Bigger engine 72 kW @ 3,000 rpm, 230 Nm, 1,000 c.c.
Small Engine – 7 kW @ 1,500 rpm, 45 Nm, 430 c.c.






Small Engine 7 kW


The engines can be grouped together (‘banked’) so, by way of example for electricity production, if 8 engines were banked with each engine providing 7 kW of electricity the total electricity provided would be 56 kW. Similarly 8 of the larger engines banked together would provide over 500 kW of electricity.