It is known to use liquid cryogenic fuel in engines and to pre-pressurise the cryogen before injection into the engine. The present invention is a development of the cryogenic engine system described in U.S. Pat. No. 6,983,598 B2. A cryogenic engine is an engine which is driven by the expansion of a cryogenic fuel.
In existing systems, such as that described in U.S. Pat. No. 6,983,598 B2, problems have arisen in delivering low quality (near fully liquid phase) cryogen to the engine due to boil off caused by heat transfer from the surroundings into the feed lines. For subcritical feed pressures, this manifests itself as a significant increase in specific volume of the fluid.
Other issues which have been identified in existing systems include:                a reduction in tank pressure as liquid cryogen is removed; the reduction in tank pressure reduces the feed pressure to the inlet of the cryogenic pumping or injection system, potentially reducing flow rates. This issue needs to be addressed to allow continuous running of the engine, otherwise tank pressure may reduce to a point where cavitation at the pump inlet prevents sufficient liquid flow being delivered to the engine for the desired power output. It is also possible that cavitation could damage pumping equipment;        reducing the pressure under which a cryogenic liquid is stored may also cause unwanted boiling of the liquid. Increased flow rates reduce the unwanted boiling of liquid nitrogen between the tank and the pump, and between the pump and the engine;        flow pulsations in the feed line caused by the cyclic operation of the cryogenic pump and injection valves. Pulsations caused by valve inlets may interact with those caused by the pump, may be amplified by cavitation, and ultimately may affect flow rates into the engine, causing unsteady engine operation;        difficulty in introducing cryogen and heat exchange fluid (HEF) during the short time frame prescribed by the engine operating speed, which can impact the efficiency of operation of the engine; and        the limited period of time in which heat exchange can take place between the cryogen and the HEF, which, again, can impact the efficiency of operation of the engine.        
US 2008/0271455 A1 relates to a heat exchange subsystem of a continuous flow cryogenic engine system intended to circumvent the problem experienced in conventional cryogenic engines of icing up from moisture found in ambient air, which can decrease the efficiency of the heat exchangers in their function. Heat is generated by burning hydrocarbon based fuel (e.g. propane). By mixing the heated air and combustion products with liquid cryogenic fuel inside a chamber, the subsystem delivers a continuous flow of high pressure gas at near ambient temperature which can be used in a connected expansion device to extract work. However, the gaseous oxygen required for combustion is provided by the vaporisation of the cryogen within the combustion chamber, but no consideration is given to the flash point of the combustible fuel which dictates whether the fuel will ignite. Moreover, the reduction in temperature and consequent contraction of the combustible gases due to vaporisation and heating of the cryogen mitigate the expansion of the cryogen and, therefore, the build-up of pressure.
Therefore, there is a need for an improved cryogenic engine system which overcomes these issues.