wet heater

A wet heater is a closed-cycle combustion engine designed to run on a rich fuel-air mixture inside a sealed chamber, with water deliberately injected into the combustion zone. The engine burns a hydrocarbon fuel (typically kerosene or similar) in oxygen-rich air or pure oxygen; as it burns, the operator or automatic valve injects seawater or fresh water into the hot combustion gases. This water instantly flashes to steam, which expands dramatically and drives the engine's piston or turbine. The exhaust leaves as warm, moist air.

Wet heaters powered most military and some commercial torpedo engines from the 1890s through the mid-20th century. The addition of steam gave a conventional internal combustion engine roughly 50% more power output from the same displacement, because steam expansion adds a second, lower-pressure phase to the power stroke. This made for compact, lightweight propulsion in confined spaces: a submarine's torpedo tube or, later, a diver's rebreather harness. The thermal efficiency was poor and fuel consumption high, but power density was what mattered.

The mechanism works because water's latent heat of vaporization is enormous. Cold injected water absorbs heat from combustion products (typically running 1800, 2200 °C), boils instantly, and occupies roughly 1600 times its original volume as steam at atmospheric pressure. The pressure inside the chamber rises sharply, driving the piston harder than combustion gases alone would. The engine also runs cooler overall: the water injection cools the cylinder walls and head, reducing thermal stress and allowing higher compression ratios or richer fuel mixtures without detonation.

Trade hazards and the wet heater's decline

Wet heaters were notoriously difficult to control. Fuel-oxygen ratio had to stay within a narrow band, or the engine misfired, exploded, or corroded internally from unburned fuel or excessive heat. Corrosion and salt scale from seawater injection fouled valves and pistons. Timing of water injection had to be precise: too early and it quenched the flame; too late and it added no useful expansion. Military torpedoes sometimes used calcium carbide, which reacted with water to produce acetylene gas for extra power, but at greater risk of detonation.

After World War II, wet heaters gave way to closed-cycle diesel and thermal engines with external heat exchangers, which were more reliable and efficient. Small-scale wet heaters persisted in specialist applications such as experimental submarine propulsion and deep-diving rebreathers into the 1970s, but modern closed-circuit breathing equipment uses chemical oxygen generators (like lithium peroxide candles) or dense oxygen cylinders instead. The term survives mainly in historical and military engineering literature, and in the terminology of torpedo engineers who trained before 1960.