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Why can’t I buy an automobile with a jet engine in it? —Steve Hunt

I hear you, brother. I understand energy efficiency and all that stuff. But in Despicable Me when you see Gru tooling around in his jet-powered SUV (or maybe it’s rocket-powered; the details of propulsion aren’t entirely clear), every red blood cell in your body screams: I want one of those.

Cars today overwhelmingly feature four-stroke gasoline or diesel internal combustion engines, with a few oddball vehicles powered by two-stroke engines, steam, or other methodology. To hear some fringe enthusiasts talk, the only reason the four-stroke engine has become so popular is due to a cabal of engineers named Otto in the pay of the piston-ring industry. The sane, however, tend to agree that four-stroke engines provide an excellent balance of power output, drivability, energy efficiency, tolerance of widely varying environmental conditions, and suitability to mass production. Plus, at least in the old days, all you needed to fix one was a well-stocked tool chest and some beer.

In contrast, strapping a jet engine to a car is something Wile E. Coyote would do. I’m not saying that like it’s bad. The jet-powered Thrust SSC broke the sound barrier during a 1997 run in the Black Rock Desert in Nevada, setting the current land vehicle speed record at 763 miles per hour. With its twin Rolls-Royce jet engines, the vehicle looks like a DC-9, except with a long, pointy car in the middle instead of a plane. However, and here’s why you don’t see jet cars in auto showrooms, you don’t get miles per gallon in this thing. You get 211 feet.

A few intrepid souls have successfully installed jet engines in street cars. One example I found on the Internet was a Volkswagen Beetle using a 1,350-horsepower GE helicopter engine converted to jet use. The flames shooting out the back end look impressive in the pictures, but the owner concedes he’s never really torqued out in the thing. More daring were the builders of a twin-jet Toyota MR2, who claim they got it up to 187 MPH during a run on the salt flats.

Plenty of rocket cars have been built as well, mostly for land speed record attempts. The most extreme example surely is the Bloodhound SSC, under development by the same team that came up with the Thrust SSC. This baby will use a fighter-jet engine and a rocket to reach 1,000 MPH—assuming it doesn’t first encounter a canyon wall.

There’s a simpler way to put a jet engine in your car: install a gas turbine. To give you a little background, a simple jet engine takes in air, compresses it, and sends it to a combustor where it’s used to burn fuel (typically jet fuel, diesel, or kerosene). The hot exhaust gas turns a turbine that powers the compressor, and exits the rear of the jet engine at high velocity to provide thrust.

For land vehicles, you can design a jet engine so most of the energy goes into turning the turbine, which then powers a driveshaft. Gas-turbine engines have definite advantages: fewer moving parts, a high power/weight ratio, and, believe it or not, smooth, quiet operation.

Gas-turbine cars enjoyed something of a vogue in the years after the Second World War, with inventors promising vehicles capable of 40 to 50 miles per gallon on the highway. Chrysler used its experience making aircraft engines during the war to introduce in 1963 the most popular gas-turbine car ever produced: a sedan featuring a 130-horsepower engine. At a time when people were just getting used to the idea of interstates, this was a car meant for high-speed cruising.

Only 55 Chrysler Turbines were made. They were reportedly a big hit with the 203 lucky families chosen to test-drive them from among a huge pool of volunteers. Noted car collector Jay Leno owns one and in his column for Popular Mechanics has praised its quiet engine and smooth acceleration.

Unfortunately, acceleration was also slow, one reason gas-turbine cars never made it to the mass market. In addition they were expensive, required diesel fuel rather than gasoline, and had high emissions and poor city fuel economy. They offered a good mix of power and fuel economy under heavy load, though, and so found a niche in military vehicles such as the Army’s M1 Abrams tank.

In recent times there’s been some interest in using gas turbines in hybrid vehicles, with an electric motor for city driving and a turbine that charges the batteries and kicks in for full power on the highway. Sadly for Grumobile fans, should such a vehicle ever make it to market, it’ll look like pretty much any other car—no giant spinning turbofan, no flaming plume of exhaust.

Have something you need to get straight? Take it up with Cecil at straightdope.com.