THE OXYGEN CAR

I’m very worried that a letter about a lost invention of a car that runs on pure oxygen should have been given pride of place in your May 2018 edition. I’m worried because I believe a lot of people look to PM to provide informed opinion about technology and “How the World Works”. You published the letter with a response that indicated that you thought there might be some complications with, for example, liquid oxygen being heavier than the equivalent volume of petrol, and drew a comparison with N O. It’s

2 as though you offered commentary on a perpetual motion machine that drew attention to the design of its bearings. Perpetual motion machines do not work. Nor does a car that “burns” only pure oxygen.

Somehow, the car is supposed to liquefy oxygen out of the atmosphere (which requires cooling air to -183˚C – no simple task), and then use that liquid oxygen to produce energy. How is the liquid oxygen going to be transformed into a motive force?

Oxygen by itself can do nothing. The only way to use oxygen to release any useful energy is to combine it with something. This could be in an internal combustion engine, using heat and expansion, or in a fuel cell to generate electricity, but, whatever the means, it involves oxygen combining with other substances. Most engines that employ oxygen use the oxygen in the air (free) and combine it with a fuel that the vehicle carries (for example petrol, diesel, hydrogen, or coal). This oxygen car would then have to provide the complementary fuel, making it just like other cars, except with the added and unnecessary complication of first liquefying the oxygen.

I suspect the concept is based on someone hearing that rockets such as the Saturn V carried liquid oxygen as its fuel, without understanding that it also carried kerosene. The oxygen was needed because (a) it couldn’t scavenge enough from the air at the prodigious rate needed while still in the lower atmosphere, and (b) once it got to very high altitudes, there is precious little atmosphere anyway; and it was liquid, because it had to be to cram enough oxygen into the spacecraft.

In sum, the invention never became the answer to cheap transport because it could never work, and having an article written about it in a Sunday paper is not an endorsement of its feasibility. Regrettably, having a letter about it not only published but also commended as the winning letter by PM, is such an endorsement, because PM is supposed to provide technically informed content. CHRIS CROZIER Hi Chris. The purpose in publishing the letter was to inspire debate and grow a community around our shared passion for truth.

Did I expect that letter to receive a response? To be honest, yes. It also got me thinking about the actual mechanics of hydrogen vehicles.

I did idiotically omit a crucial fact, though. The air-power I was referring to was an Indian prototype I came across years ago which used compressed air to drive pistons. Actually I got it confused in my own head while hurriedly finishing up the magazine on deadline day.

Thank you for calling me out on it and reminding me about the responsibility Popular Mechanics has to our readers. I can be a bit f lippant at times, to my own detriment. – Lindsey