I understand how both types of engines work, but I don't understand the pros and cons of using one vs the other.

Why are wet heater engines so suitable for torpedoes, but seemingly no other applications?

  • $\begingroup$ Cost? Mass? Reliability? Fuel choice? $\endgroup$
    – Solar Mike
    Commented Jun 5, 2022 at 7:57

2 Answers 2


The wet-heater torpedo engine was an air-engine. A variation of a steam-engine. Hot gas was introduced in the piston cylinder, expanded, and drove the crank shaft. The Wet Engine was a development from the previous torpedo air-engines which were powered by compressed air, (the 'dry' engines).

Compressed air becomes cold on expansion, and as the storage pressure was increased, there was a problem with the supply freezing up.

[Storage supply pressure was limited by the capacity of the pressure vessel. Volume was limited by size and weight. To get more speed and range, more pressure was required. High pressure metallurgy technology developed in the early 20th century]

To add power and reduce freezing, the air supply of the air-engine was pre-heated. It came from the storage cylinder, through a reducing valve, into the pre-heater, then into the expansion cylinder. This is steam-engine technology, and by the early 20th century steam engine engineering and science had advance to a high level.

The early version of this technology also injected water into the pre-heater. The purpose was to prevent the pre-heater from burning out. This pre-heated technology was a great improvement on the previous torpedo engines. Injection of water into the pre-heater also changed the characteristic of the working fluid, but it wouldn't be fair to say it made it 'more efficient'. It was part of the design of the engine, and the engine was more effective because there was heavy fuel oil as well as compressed air, and the steam generation in the pre-heater was part of the overall design. The 'wet' description also comes from steam, where this would be known as 'wet steam' A Wet (or Burner-Cycle) torpedo would still run even if the heater didn't fire, because it was an air engine running on compressed air. It just wouldn't run nearly as far.

Historically, the Wet engine was followed by the Burner-Cycle torpedo engine, in which fuel was also injected into the cylinder, making it a two-stroke diesel engine. You may consider the Burner-cycle to be an internal-combustion variant of the steam engine, or you may consider it to be a diesel with a compressed-air pre-heater supercharger: most descriptions land somewhere between and just say it was a kind of variation of a sort of diesel.

Modern Diesel engines still use intake compressors which also preheat the intake air: they just don't use oil-injection pre-heaters to do so. Engine design is complex, but one reason would be because the extremely 'lean' pre-heater would produce a lot of NOx pollution.

[There were turbine Wet-Heater engines as well as reciprocating Wet-Heater engines. Same-as Same-as]


After some digging, a wet-heater just just appears to be a variant on the water injection internal combustion engine:


Thus, being a "wet-heater" does not imply a turbine nor does it preclude an engine from being reciprocating engine:

By the end of the Great War the standard British torpedo engine was a wet-heater four-cylinder radial made of bronze, with integral cylinder barrels and heads as in contemporary automobile practice.

At around the same time, the British were perfecting the burner-cycle reciprocating engine, which retained the classic four-cylinder radial layout. The bore/stroke ratio of these compact radial units bore little resemblance to the contemporary automobile long-stroke inline reciprocating engine, with its inherent disadvantages of piston friction and heavy, out of balance, reciprocating weights. The radial engine could thus continue to rival the American preference for the turbine engine.


As such, there are torpedoes that use reciprocating engines, both wet and dry, specifically radial engines:





The real question is why don't you see wet heaters on ships or submarines which run surrounded by water? It seems that while some torpedoes used freshwater, others used sea water so maybe they do. Or maybe freshwater is too valuable on long stays at sea and saltwater too harsh for non-single use engines.

According to the first link even freshwater is enough of a problem that oil is mixed in to protect engines from corrosion.

  • 1
    $\begingroup$ Yes, seawater injection is pretty common in marine engines. It is primarily used to lower temps and reduce thermal NOx. There are direct water injection systems and steam injection systems, but both require a good bit of kit for filtering the seawater. Water injection timing leads fuel injection. Efficiency is supposed to be a bit higher too, at least at the high-power/high temp end where it is mostly used. $\endgroup$
    – Phil Sweet
    Commented Jun 7, 2022 at 0:37
  • $\begingroup$ wartsila.com/encyclopedia/term/direct-water-injection-(dwi), fuel consumption vs water/fuel ratio - sds-max.com.ua/downloads/M_DIS.pdf $\endgroup$
    – Phil Sweet
    Commented Jun 7, 2022 at 0:41

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