Air Cars and Hydraulic Hybrids

A reader pointed me to a video about the MDI Air Car:

I think it’s fantastic to see exploration of new ideas like this. It has the obvious benefit of no local emissions, as well as freeing us to use renewable power sources to compress the air that powers the car. In addition, this technology is much less expensive than electric vehicles or hybrid electrics. However, when the air is compressed to put into the on board storage cylinders, the air gets hot (think about what happens when you use a bike pump). This heat is lost when the compressor and storage cylinders cool off, meaning that a lot of energy is lost in the process. This reduces the overall efficiency of the system. I have not done any calculations myself yet, and I don’t have any efficiency data, but I’m skeptical that an air car would compare in energy efficiency to an electric vehicle. There may be specific applications where an air-powered car makes great sense, particularly where purchase price is more important than overall efficiency and operating cost throughout the life of the vehicle. We need to strive for both freedom from fossil fuels and lower energy consumption. The air car addresses only the former objective.

Unfortunately the company overextends itself in the claims made in this ad. Early on they claim ZERO cost to fill up this car, conveniently neglecting the cost of compressing air. MDI acknowledges this cost only at the end of the ad. When they discuss an on-board compressor, they flaunt the term ‘perpetual motion‘, which is a huge red flag.  

Innovation is essential to our progress, and I commend exploration of new ideas. But we need to complement exploration with healthy debate that keeps us honest and helps drive us to seek the very best solutions. I’m concerned that wild claims may hurt the transition to clean energy and transportation. Let’s put our ideas out there, but let’s be up front about what we have actually accomplished and what the real potential is. We can’t afford to be deceptive; even the appearance of snake oil marketing could damage the movement.  

The Automotive Research Center, in conjunction with industry and government partners, has investigated a similar technology that does indicate some practical benefit: hydraulic hybrid powertrains. Unlike air, hydraulic fluid doesn’t change volume when compressed, so doesn’t heat up much when compressed. Hydraulic regenerative braking systems have been installed on heavy trucks (and even bikes). A hydraulic pump connected to drive wheels pumps fluid into a pressure accumulator while providing some braking force. When the driver is ready to move again the pressure accumulator feeds the hydraulic motor, which assists the conventional engine. This system has been shown to have significant energy benefit, at a low level of cost and complexity when compared to hybrid electric systems (although all energy still comes from fossil fuels). This may be a good short term patch for current vehicles, but we also need to be working on long-term solutions that don’t involve fossil fuels.

I would be especially interested in hearing from anyone who has experience with compressed air powertrains to learn more about both their benefits and problems. Has anyone found data on the overall efficiency (grid to wheels) of an air-fueled powertrain system so we can compare then quantitatively to EVs? Or, has anyone done any calculations to estimate the overall efficiency?