DETROIT - Here's how you know that automakers are feverishly pursuing ways to improve fuel efficiency by harnessing engine heat: They won't talk about it.

The pressure to improve mileage has been stepping up for years, and the straightforward gains - cutting aerodynamic drag, lowering rolling resistance - have been long since mined. So researchers are increasingly probing small improvements once thought too troublesome to bother with.

The competition is fierce and the secrecy intense.

An example: After an exchange of emails requesting answers to questions about thermodynamic research, a Honda spokesman, Brad Nelson, shut off further discussion with a reply that "several of these things touch on confidential engineering work at the company, so our team prefers not to offer comments at this time."

It may be, industry experts hint, that thermodynamic advances will lengthen the life of internal combustion engines.

Thermodynamics involves the physics of heat and energy, and automakers are applying this science to a quest for efficiency, developing devices to use heat that would otherwise be wasted to improve fuel economy and reduce emissions.

A number of automakers, including BMW, are exploring ways to use heat energy to generate electricity. The principle is simple: Voltage can be generated between two thermoelectric semiconductors held at different temperatures. NASA has long relied on this technology, known as the Seebeck effect, to power space probes.

BMW has said that it developed a thermoelectric device positioned in the exhaust system that provides 200 watts of power. The latest generation of the device uses more advanced materials to generate 600 watts. The automaker's goal is 1,000 watts of thermoelectric power.

Electricity produced by a thermoelectric generator can be used in place of an alternator, reducing workload on the engine, or it can be used to charge batteries for a hybrid system.

Thermoelectric devices are just one of many ways auto engineers are trying to use the heat energy rather than waste it.

Waste is a key word, because much of the energy produced when fuel is burned is lost to the atmosphere as heat.

How much is dispersed through thermal losses? The Energy Department puts the number at 58 to 62 percent for a typical internal combustion engine. This energy is transferred to the atmosphere primarily through the tailpipe and radiator. Much of that heat transmission is intentional. If the engine isn't cooled, gasoline ignites prematurely, lubricants are rendered ineffective, and moving parts are destroyed. But while the engine must be cooled, some extracted heat can be put to good use.

Turbocharging is the age-old way to use heat energy for increased power and efficiency. The earliest turbochargers were a compromise for automobile engines because of their poor low-speed response. But turbocharging has come a long way and modern systems use twin turbos that operate in sequence to reduce lag.

Exhaust-driven turbines can also drive a generator that provides electricity for accessories, batteries and powertrain motors, and turbocompound engines use turbine power directly, coupling it to the vehicle's drivetrain to help turn the drive wheels.

Ford was willing to discuss its work in thermodynamics. In a telephone interview, Zafar Shaikh, manager of powertrain cooling and thermal energy recovery, said that while he was not at liberty to discuss proprietary projects, he could say that Ford's research efforts are many and include turbocompounding, electricity generation, heat storage devices and Rankine cycle thermodynamics that use engine heat to produce steam that can power an electric generator.

A paper by two Ford engineers, Quazi Hussain and David Brigham, presented at the 2011 Directions in Engine-Efficiency and Emissions Research Conference described a closed-loop Rankine cycle device: A pump delivers fluid to an exhaust-heated vaporizer and the resulting steam drives a turbine, which turns an electric generator. The vapor is returned to liquid form in a condenser, and the cycle is repeated.

Shaikh said the challenge is fitting the Rankine device into an automobile. "We're close in hardware," he said, "but packaging remains a challenge."

BMW is developing a system that is similar in principle. Jürgen Ringler, BMW's team leader for thermal energy converters, described a heat exchanger that recovers heat from the engine exhaust.

"This energy is used to heat a fluid which is under high pressure - this heated fluid then turns into steam, which powers an expansion turbine that generates electrical energy from the recovered heat," he said in a statement.

The automaker says that when complete, the system is expected to reduce fuel consumption by up to 10 percent on long trips.

Toyota hybrids use a heat storage device based on a three-liter stainless-steel thermos that stores coolant after the engine is shut off. When the car is restarted, the warm coolant is cycled back to the cylinder head. This allows for a leaner fuel mixture on startup, improving efficiency and reducing emissions. A warmer startup also means the passenger cabin can be heated rapidly on cold mornings.

FiatChrysler uses engine waste heat to preheat the transmission fluid of Ram trucks with the eight-speed automatic transmission. In a telephone interview, James Adams, chief transmission engineer, said that starting cold, a typical automatic transmission requires 35 to 40 minutes to reach optimum operating temperature, but by preheating fluid with engine coolant, which warms more rapidly, the transmission reaches optimum temperature in about 10 minutes. The technology, according to Adams, is worth "a couple percent in fuel economy on the overall cycle."