Various Methods of measuring Efficiency
Carnot Efficiency is a measurement of the utilisation of the thermal energy contained in fuel. Petrol engined vehicles typically run at 30% or so Carnot efficiency. The remaining 70% of the energy of the fuel is lost as rejected heat.
Electric vehicles can utilise 80% or more of the energy stored in the battery.
I have not calculated a figure for the Carnot efficiency of the FAE. It all depends on the time span that the compressed air is stored and the insulation quality of the storage tank and engine. If these losses can be minimised then the FAE can run at very high levels of Carnot efficiency.
It may be noted that Carnot efficiency does not take into account the kinetic energy recovered by regenerative braking systems.
Energy Density
of various fuels per. Wikipedia
Uranium 235 – 79,000,000 Mega Joules/Kilogram
Petrol 47.2 MJ/kg
Coal 24 MJ/Kg
Wood 16.2 MJ/Kg
Li-ion Battery 0.72 MJ/Kg
Compressed Air 0.5 MJ/Kg
Lead Acid Battery 0.1 MJ/Kg
Whilst this chart shows up the obvious comparative disadvantage of compressed air and battery power compared to fossil fuels; it also shows compressed air compares favourably to battery power as a means of energy storage. Whereas a 1000 Kg battery pack weighs the same fully charged as when half charged, a compressed air tank fully charged at 1000Kg may weigh only 600 Kg when half charged (assuming 100Kg weight of tank).
Carbon footprint / Greenhouse Gas emissions. From carbonindependent.org website, the CO2 emissions from petrol cars are;
3.15Kg/ Litre – for ongoing fuel usage and extraction costs
0.75Kg/Litre - for construction and maintenance of cars
Assuming a fuel consumption of 8Litres/100Km (or 12.5 km/Litre), the CO2 output of an average car is;
0.252Kg/Km – fuel use
0.060Kg/Km – construction and maintenance.
A compressed air car is assumed for now to have the same construction and maintenance costs as a petrol car; the ongoing fuel costs is dependent on the means by which air is compressed and on the efficiency of the compressor. A typical 240V workshop air compressor outputs 289 Litres of air compressed to 1.1 MPa and draws 2.25 KW. In one hour of operation this compressor will generate 17,340 Litres or 17.34 M3 of compressed air. My preferred means of powering the air compressor is by solar panels, wind generators or windmill (where the compressor is driven directly by the wind rotor) .
If we assume that a compressed air car is fuelled for a 100Km commuter run and uses an average of 26.8 KW during the run. The 26.8KW is my guess as to the average power demand of the car for this trip given normal traffic conditions. The amount of air consumed is 102.54 M3 per 100 km(per earlier calculations). The workshop compressor will need to run for 5.91 Hours to produce this air. (Regenerative braking energy recovery is ignored for now).
CO2 emissions from power generation are
Coal – 0.955 Kg/KW
Solar Panel- 0.106 Kg/KW
Wind Power – 0.021 Kg/KW
CO2 output from shop compressor for the 100 km trip is thus;
Coal Power 5.91 hours x .955 Kg/KW/hour x 2.25 KW/ Hour = 12.699 Kg/ Hour
Solar Power = 1.409 Kg/Hour
Wind Power = 0.279 Kg/Hour
Ongoing fuel use CO2 emissions per kilometre for a compressed air car is therefore;
Coal Power =0.1270 Kg/Km
Solar Power =0.0141 Kg/Km
Wind Power =0.0028 Kg/Km
To summarise, a compressed air car put to the same task of a commuter run of 100 km outputs only 50% of the CO2 of a petrol car when the air compressor is powered by a coal powered generator, and only 4.7% of the CO2 when powered by a wind powered generator. Accounting for regenerative braking energy recovery of the FAE will significantly improve that figure. Further efficiencies and reductions in greenhouse gas emissions are achievable by using a more efficient means of compressing air than the workshop compressor.
Compressed Air Car v Electric Cars
A compressed air car using the FAE engine will have similar performance and range to an electric car because of the comparable energy density. The FAE engine initially may (or may not) cost more to manufacture than an electric motor, but a compressed air tank will have a lower cost and a much longer life cycle than a battery pack.
At the moment, brush-less DC motors powered by Li-Ion battery packs are leading the field in zero emission vehicles. I hope to show that compressed air vehicles can be competitive and can ultimately be proven to have lower life-cycle greenhouse emissions than a battery powered vehicle.
Other Compressed Air Engine Designs
In brief outline, some of the other designs that I am aware of,
MDI – This firm started in 1991 and is a well funded company that is working exclusively on its compressed air vehicles. The MDI engine does not have regenerative braking and uses a external combustion process to make up for the lost heat energy.
Di Pietro Rotary Air Engine – This engine also does not have regenerative braking. The FAE engine cycle (which can be applied to rotary engines also) is, frankly, superior to the conventional pneumatic engine cycle of the Di Pietro engine.
Permo-Drive Technologies – is an Australian firm that has developed a hydraulic regenerative braking system primarily for use in heavy vehicles. In it's original form the system had a separate hydraulic pump and separate motor. Looking at their website now, it appears that they may have blended both pump and motor functions in the same body. The permo-drive system is not intended for, and is not capable of, being the primary means of powering a vehicle.
Cargine Engineering Camless Combustion Engine – This design, of which I am only recently aware (thanks to Gizmag.com), is a hybrid pneumatic/combustion engine. This design is very close to a FAE variant that I envisaged in my Patent Application. This company has clearly solved all the engineering challenges in building a prototype that can run at high RPM. As far as I can tell, the regenerative braking cycle is not optimised ,(as it is in the FAE), in that use is not made of vacuum during the down-stroke of the piston. Volvo, SAAB, Koeningsegg AB, and Lund University in Sweden are partners in this venture.
