Heat engines (which include anything with a piston, turbine, or rocket nozzle) start out with a big strike against them.
GM and GE invest hundreds of millions trying to get friction and other losses out of their V6’s and turbojets. They want to make their engines efficient at using the energy available in a gallon of gasoline or Jet-A.
Even if you don’t worry too much about how many miles per gallon you get driving around town, you can bet the airlines worry about how many miles per gallon (or is it gallons per mile?) a 757 gets going back and forth between Dallas and Boston.
But GM and GE are up against a wall. They always have been, and they always will be. That wall goes by the name of the Carnot Cycle. I won’t bring that up again, but keep this in mind: An engine designer has to work hard to squeeze out as much of the available energy from burning fuel as possible. Notice something? I’ve repeated that word, ‘available’.
When you burn fuel, chemical energy is transformed into heat energy. That’s why fire is hot. The job of the engine is to convert that heat energy into mechanical energy, to keep you cruising safely 35,000 feet over Detroit or down the Interstate. But only a certain fraction of heat energy can ever be converted into mechanical, and it depends on how hot the combustion is and how cold the exhaust is. The hotter and colder, the better, but there are always limits, and a lot of unusable energy is pumped out the tailpipe.
Now, electric cars try to make use of an intriguing fact. Electric motors are very, very good at converting electrical energy to mechanical. They’re not up against the same wall as heat engines. But the electricity has to come from somewhere, and since it’s not very practical to equip cars with arms to scrape the juice from overhead wires, like trolleys do, we equip them with batteries instead.
But batteries are heavy and expensive. They’re too expensive for use-once-throw-away, so they have to be charged and recharged. And where does that energy come from? Well, you know the answer. From big heat engines called ‘power plants’.
Suppose the power plant is a pretty good heat engine. The power still has to get to your garage, and it turns out that electric transmission lines just can’t be built big enough to carry nearly as much energy, pound for pound, dollar for dollar, as a pipeline or tanker car. They’re limited and not terribly efficient. So electric car makers spend hundreds of millions making marginally better batteries, but they’re stuck with the electric distibution system.
If batteries aren’t the answer for electric cars, is there another? George W. Bush was convinced hydrogen fuel cells were the answer. And it’s true that carrying liquid hydrogen in tankers gets around electric power lines, and fuel cells can be very efficient. But making hydrogen clean and pure enough for fuel cells is expensive, because it’s done with a very inefficient process called electrolysis. It uses gargantuan amounts of electricity from big heat engines. So fuel cell designers bypass electric power lines, but instead they’re stuck wasting a lot power while making the hydrogen.
So some people are looking at a very different approach, going back to a sort of piston engine. But instead of expanding a hot combustion product in a cylinder to do mechanical work, they’re expanding liquid nitrogen by quickly warming it to the surrounding air temperature.
Now, this is not necessarily a very attractive idea. It’s still a heat engine, but the ‘hot’ is the kind of temperature you and I are comfortable with, say 70°F, and the ‘cold’ is -320°F. You just can’t get the sort of temperature difference that is found with very hot combustion.
But never mind that, because liquid nitrogen is a byproduct of making liquid oxygen, and apparently we make so much the stuff is cheap.
Now, when something is very cheap, in a sense it is being wasted, if a more expensive use can be found.
So think of the liquid nitrogen engine as a machine to make use of wasted energy. Which might be something very efficient after all, in the grand scheme of things.
Still, one really big technical problem with the idea is that it’s hard to get liquid nitrogen to boil fast enough for use in an engine. Recently a British engineer figured out a way to squirt a little antifreeze into the cylinder, and that makes the cold stuff boil faster. Sounds pretty slick. Maybe someday we’ll see one on the road.
And here I was thinking that all they grow in Britain these days are curry and bankers.