How the Rotating Detonation Engine Works

We tend to think of the pressures created inside an engine as being more controlled rather than disorganized "detonation" -- we want it to be predictable, not chaotic. But if an engine can control a typical cycle with enough precision to produce steady power, it can also control detonations. And the burst of pressure that a detonation produces can result in a more efficient engine, if it's harnessed correctly. Typically, an engine mixes air with fuel to prepare it for detonation, which is when the fuel releases its energy. The efficiency of the fuel's energy release, though, is highly dependent on the type of engine.

This is where the rotating detonation engine differs from more common types of gas turbine engines. The air and fuel are mixed (as usual) before they're injected into a long, circular combustion chamber, in what "Physics Today" has described as "a sequential, circular manner." The first detonation sets off a cycle in which the pressure from ignition continues around the chamber, lighting each injection in sequence. The pressure from each ignition keeps the cycle moving. The pressure then forces the exhaust gas out of the combustion chamber through an exhaust nozzle, which is actually the thrust generated by the engine, and moves on to power whatever type of vehicle the engine is installed in (in this case, typically, a ship or an airplane).

The rotating detonation engine is actually a variation of another engine design: the pulse detonation wave engine. Though pulse detonation engines provide energy savings over many other types of engines, they still have their own inefficiencies. One reason is the combustion chamber which must be purged after each pulse. The rotating detonation engine is an improvement over the pulsing design, because the detonation wave constantly cycles around the chamber, eliminating the need to waste time and energy by purging.

Because detonations create extreme pressures, a detonation engine can be designed without the additional compressor that's usually required. Not only are compressors typically complex, but their operation generally sucks up a lot of energy, too. However, adding a compressor to a detonation engine actually makes it even more efficient. This compatibility makes it easier to retrofit gas-turbine engine vehicles to be used with detonation engine technology [source: Green Car Congress].

The United States Navy is getting all the credit for its recent investment in the technology (and perking up the news cycle), but the rotating detonation engine has actually been in the works for a few decades, at this point. The patent for the engine was filed in 1982 and granted in 1988 to a Rockville, Md.-based inventor named Shmuel Eidelman. (The patent is actually called the "rotary detonation engine," rather than "rotating" -- it's unclear when the moniker changed.)

Shmuel Eidelman has been a busy man. He has been awarded 14 patents since 1982, focusing on aeronautics, propulsion and chemicals through his work with scientific corporations and military organizations [source: PatentBuddy]. So when the patent was filed, it seemed like the Navy had pushed the gas-turbine as far as it could go, and it was time to start thinking fresh.

The U.S. Navy was initially interested in pulse detonation engines (as described earlier), and invested in research to develop these fuel-saving systems. The Navy's researchers say that maximizing the potential of this type of engine relies on understanding its complex physics [source: U.S. Naval Research Laboratory]. The rotating detonation engine is still a gas-turbine engine, like the engines currently powering the Navy's fleet of ships and aircraft, but tweaking and refining the cycle unlocks a lot of additional power. Navy researchers believe that rotating detonation engines have the potential to reduce fuel consumption in new equipment by 25 percent, which would be an annual savings of about $300 to $400 million [source: Quick]. Another benefit of the system is that it could be configured to power up an electric motor, which would, in theory, allow military fleets to start transitioning to (potentially) cleaner, cheaper and more efficient electric drivetrains.

Rotating detonation engines aren't ready to go just yet, so simulations are the best predictor of their efficiency -- yet they're still promising enough that the Navy is pushing forward with development. There's no publicly-known ETA for completion or implementation, but the rotating detonation engine is likely to arrive -- someday, anyway -- at a naval base near you.