The speed and range of later models were improved by increasing the pressure of the stored air. In 1906 Whitehead built torpedoes that could cover nearly at an average speed of . At higher pressures the adiabatic cooling, experienced by the air as it expanded in the engine caused icing problems. This drawback was remedied by heating the air with seawater before it was fed to the engine, which increased engine performance further because the air expanded even more after heating. This was the principle used by the Brotherhood engine. Heated torpedoes Passing the air through an engine led to the idea of injecting a liquid fuel, like kerosene, into the air and igniting it. In this manner, the air is heated more and expands even further, and the burned propellant adds more gas to drive the engine. Construction of such heated torpedoes started circa 1904 by Whitehead's company. Wet-heater A further improvement was the use of water to cool the combustion chamber of the fuel-burning torpedo. This not only solved heating problems so more fuel could be burned but also allowed additional power to be generated by feeding the resulting steam into the engine together with the combustion products. Torpedoes with such a propulsion system became known as wet heaters, while heated torpedoes without steam generation were retrospectively called dry heaters. A simpler system was introduced by the British Royal Gun factory in 1908. Most torpedoes used in World War I and World War II were wet-heaters. Compressed oxygen The amount of fuel that can be burned by a torpedo engine (i.e. wet engine) is limited by the amount of oxygen it can carry. Since compressed air contains only about 21% oxygen, engineers in Japan developed the Type 93 (nicknamed "Long Lance" postwar) for destroyers and cruisers in the 1930s. It used pure compressed oxygen instead of compressed air and had performance unmatched by any contemporary torpedo in service, through the end of World War II. However, oxygen systems posed a danger to any ship that came under attack while still carrying such torpedoes; Japan lost several cruisers partly due to catastrophic secondary explosions of Type 93s. During the war, Germany experimented with hydrogen peroxide for the same purpose. Oxygen enriched air The British approached the problem of providing additional oxygen for the torpedo engine by the use of oxygen-enriched air, up to 57% instead of the 21% of