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To increase the operational range of the vehicle equipped with direct injection gas supply and to eliminate the problems associated with the cryogenic pumps, a composite high pressure vessel is proposed for use as a thermocontrolled tank in vehicles. The test results showed that the liquefied natural gas can be brought from atmospheric pressure to the high pressure required for direct injection in a reasonable time period. Based on these results a mathematical model for the heat transfer to the natural gas was developed and simulated. Three different aspects of gas storage, namely, the initial operational temperature, the gas to vessel mass ratio and the gas weight to vessel volume ratio have been investigated and formulated as objectives for optimization. Due to the presence of different conflicting objectives, the problem was formulated as a nonlinear multi objective optimization problem and then solved.

A new concept of a semicryogenic tank for storage and supply of natural gas into the cylinders of an internal combustion engine is presented. It allows the gas to be pressurized before injection and uses the heat entering the tank as the means to evaporate the natural gas introduced in a liquid phase to the tank. To limit the heat transfer to the tank and the gas pressure increase, the semicryogenic tank has to be well insulated. However, it has to be also preheated to accelerate the gas pressure increase, as required. To maintain the right pressure and temperature of natural gas an electronic control of the semicryogenic tank is needed. The gas preheating system uses the waste heat of engine exhaust gases and, therefore, contributes to the increase of the thermal efficiency of the engine through the injection of hot natural gas.