Detonation Engine

December 5, 2012

I worked on a variety of gas turbine engine components during my career in industrial research. The principle behind the gas turbine engine is so simple that it lends itself to many applications, from motors for medium-sized electric generators, to engines for tanks, battleships, and very large aircraft. Adding complexity to the basic design allows creation of high performance engines for military aircraft.

Schematic diagram of a gas turbine engine. Once the engine is started, the compressors in the cold section compress the intake air, which is mixed with fuel in the hot section combustor. The hot gases drive the turbine section. As can be imagined, there's a lot of science and engineering that goes into turbine engine research. (Illustration by Jeff Dahl, via Wikimedia Commons).

The gas turbine engine works well in its applications, but there's always room for improvement, especially where fuel efficiency is desired. Gas-turbine engines are used to provide propulsion and electricity for ships, and it takes a lot of fuel to propel a battleship across an ocean. Fortunately, just lurking around the technology corner, is the more fuel efficient detonation engine.

The fundamental difference between a detonation engine and a gas turbine engine is compression. The gas turbine engine has a huge array of compressor blades and stators to handle this task. A detonation engine, as its name implies, uses a detonation wave to compress the fuel-air mixture. Detonation engines don't supply power in a continuous fashion, like gas turbine engines. To get a quasi-continuous power output, you need to renew the detonation again and again.

PV curves for the gas turbine engine Brayton cycle and the detonation cycle, showing the enhanced efficiency obtainable. In this comparison, the operating pressure ratio (OPR) is 10 for the Brayton cycle, and 2 for the detonation cycle. (Fig. 1 of ref. 2).[2]

Detonations are nearly a constant volume process, so they generate high pressures that can do work without any compression. Elimination of the compressor section, and its array of moving parts, adds reliability. This advantage, however, comes at a price. Air and fuel inlet valves need to actuate at high rates to get a continuous series of detonations. There's also a potential noise problem, since you're trading the mostly white noise of a turbine engine for a continuous tone of the detonation engine.

Scientists from the Laboratory for Computational Physics and Fluid Dynamics of the United States Naval Research Laboratory are using computational techniques in the study of a variant of the detonation engine, the rotating detonation engine (RDE).[1] Their motivation is simply stated. The US Navy presently fields about 430 gas turbine engines on 129 ships, and these engines require about two billion dollars of fuel each year. RDEs could save about 300-400 million dollars per year in fuel cost.[1]

The rotating detonation engine addresses the problems of a basic detonation engine by causing the detonation to propagate azimuthally around an annular combustion chamber, as shown in the computer model, below. The naval trend is towards an all-electric ship propulsion system, so the rotating detonation engine would drive a hefty electrical generator.

A computer model of a rotating detonation engine. The detonation, as shown in red, propagates azimuthally around an annular combustion chamber. See refs. 2-3 for an overview of detonation wave engine technology.[2-3] (U.S. Naval Research Laboratory image).[1]

References:

1. Donna McKinney, "Navy Researchers Look to Rotating Detonation Engines to Power the Future," Naval Research Laboratory Press Release, November 2, 2012.
2. D.A. Schwer and K. Kailasanath, "Rotating Detonation-Wave Engines," NRL Review, U.S. Naval Research Lab (2011).
3. F. Falempin, "Continuous Detonation Wave Engine," NATO Report RTO-EN-AVT-150, March 26, 2008.