Heat Engines of the Future

Heat engines have come a long way from the slow, awkward steam engines of the 18th and 19th centuries. Smaller parts, better fuel, and superior materials have produced engines powerful enough to propel cars and airplanes to breathtaking speeds. Although engines of the future will continue to be restricted by the laws of thermodynamics—no engine will ever be able to convert 100 percent of its heat input into work—there is still room for improvement.

Fighter pilots and other people who need to get from one place to another in a hurry never seem to be satisfied—they

Heat Engines 97

always want engines with more power and speed. The problem, though, is temperature.

The F-15, a fighter jet of the United States Air Force, can reach

a speed of 1,660 miles per hour (2,656 km/hr.), and the SR-71 can fly at close to 2,000 miles per hour (3,200 km/hr.). Flying at even greater speeds than this presents several difficulties. One difficulty is that friction with the air causes the exposed surfaces of the plane to overheat. Another difficulty is the jet engine’s compressor; at

The SR-71, an air force plane used in the 1970s and 1980s for reconnaissance missions, was the world’s fastest and highest-flying airplane during its years of service. (United States Air Force/Tech. Sgt. Michael Haggerty)

98 Time and Thermodynamics

high speed, the temperature of the compressor becomes so hot that it begins to melt.

Afterburners add extra power but also create a disadvantage in terms of military combat. The addition of burning fuel in the exhaust of an airplane with an afterburner will increase the temperature, which is undesirable because it creates a “heat signature” that can be easily detected by a heat-seeking missile (a missile designed to locate its target by sensing the high temperatures of the enemy’s airplanes). The afterburner is an excellent target for a heat-seeking missile, and the result could be disastrous for a pilot. For this reason, the United States Air Force wants its future jets to be able to produce a lot of power without having to turn on an afterburner. The F/A-22 Raptor, the newest air force fighter, has this ability, and its two Pratt & Whitney engines produce more thrust than any previous fighter engine. The F/A-22 can cruise at a speed of one-and-a-half times the speed of sound, without engaging the afterburner.

Problems associated with the compressor’s high temperature can be partially solved by keeping it cool with a circulating fluid, creating convection currents. Another way is to find new metal alloys to withstand even higher temperatures without melting. Yet

The F/A-22 Raptor (also known as the F-22) is the newest jet fighter in the air force. (United States Air Force/Tech. Sgt. Ben Bloker)

Heat Engines 99

another way is to get rid of the compressor entirely. But high pres- sures are mandatory in order for the engine to do a lot of work, as mentioned earlier, and if the compressor is gone, what will compress the air? The answer is simple—the air itself.

When a jet airplane is traveling at an exceptionally high speed, the compressor gets so hot that it cannot function, but at these high speeds, the air rushes into the engine with a great deal of force anyway. A careful arrangement of the engine inlet can funnel the incoming air and use the air’s own momentum to compress it. In this way, a high-enough pressure occurs without a machine compressor. An engine that uses this idea is called a ramjet—the air rams itself into the engine, achieving high pressure.

Ramjets obviously must be going at high speeds before they become usable. The minimum required speed is about 300 miles per hour (484 km/hr.), and most ramjets only become efficient at about twice that speed. The ramjet engine is an extremely simple heat engine, consisting of a long tube in which fuel is burned in the compressed air. Ramjets work quite well at supersonic speeds, although the air is slowed down as it goes through the engine. An even more powerful engine at supersonic speeds is the scramjet (supersonic combustion ramjet), which is a ramjet engine that allows air to travel through the engine at supersonic speed—a lot of compression is available with this kind of speed!

Ramjets and scramjets have been built, but they are still being developed and are not quite ready for everyday use. The National Aeronautics and Space Administration (NASA) tested a scramjet in an unmanned experimental aircraft called X-43A in November 2004. A larger plane carried the attached X-43A aloft; when the operator remotely launched the X-43A, it used a rocket to gain enough speed for the scramjet to function. The scramjet’s fuel was hydrogen. Eventually, the aircraft reached a record speed (for this type of engine) of 7,000 miles per hour (11,200 km/hr.). Although the X-43A flew at high altitudes where the air is thin, the speed was so fast that the plane experienced a great deal of friction as it moved through the air. This friction generated so much heat that the craft required circulating water as a coolant to keep from melting.

100 Time and Thermodynamics

This drawing shows the X-43A Hypersonic Experimental Vehicle in flight. (NASA)

Heat engine development will not stop with scramjets. Speed is not the only important aspect of engines—efficiency is also important. The laws of thermodynamics place strict limits on the ability of heat engines to convert heat into work, and all heat engines must obey these laws no matter what fuel they burn or how they burn it. But even modern heat engines such as the internal combustion engines in automobiles fall well short of Carnot’s ideal engine because of heat losses from conduction, convection, and radiation. If heat engines of the future are to wring out all possible work for a given input of energy, they must be made from strong insulators and heat-resistant materials, designed with thermody- namic principles in mind.

Heat Engines of the Future