1 TITLE OF INVENTION Dual Fuel Engine with Fuel Cooling TECHNICAL FIELD [0001] This invention concerns a cooling system for dual fuel internal combustion engines. BACKGROUND [0002] The invention uses a hydrocarbon gas, namely liquefied natural gas [LNG] or compressed natural gas [CNG]. The hydrocarbon gas is injected into the inlet manifold, and delivered to the engine, in which it is controlled by an engine control module [ECM]. Thus the operator benefits from the economy of LNG or CNG fuel. Turbo-charging is standard for such engines. Compression of the inflowing air increases its temperature and reduces its density. It is not unusual for the inlet air temperature to reach in excess of 70'C. This heat level is recorded even with the intervention of an air-to-air intercooler because intercoolers are cooled by the passage of air moving through them and, when the vehicle is in a queue or is stationery, the intercooler effectiveness falls. [0003] In our co-pending Application No. 2010352515 we describe cooling modifications to these types of engines in which use is made of the phase change in the fuel from liquid to gas to cool the turbo-charged air. [0004] When large capacity engines operate continually at large throttle openings the cost of fuel is an important economic factor to the operator and the availability of fuel is a concern particularly in remote areas. It is an advantage if an engine can deliver adequate power from a choice of fuels, for example diesel and hydrocarbon gas. Many large diesel engines stiffer power reduction as the diesel fuel temperature rises and or the air temperature entering the engine rises even at modest ambient air temperatures. 2014-10-27,C:\Users\Sharlene\Documents\SPECIFICAT LON'Skgaste;liionl2.apdj 2 SUMMARY OF INVENTION [0005] The first apparatus aspect of the invention provides a cooling system for supplementing the air to air intercooler of a turbocharged engine using dual fuel comprising a cooler for turbocharged air, a cooling circuit including the cooler and a reserve of liquid coolant, a first heat exchanger in the circuit, a liquid fuel inlet to the heat exchanger, a gas fuel outlet from the heat exchanger to the engine, a second heat exchanger in series with the first and a diesel fuel inlet and outlet in the second heat exchanger, whereby the cooling capacity of the phase change of the cryogenic liquid fuel to gas is available to cool both the turbocharged air and the diesel fuel. [0006] Preferably the cooling system circuit is a closed loop providing coolant to air interchange and a header tank. [0007] The second apparatus aspect of the invention provides a kit for modifying an engine using gas fuel and an intercooler, comprising a liquid cooled intercooler for input air to the engine, a first heat exchanger coupled to the intercooler in which cold liquified gas fuel absorbs heat from the intercooler as the fuel changes from liquid to gaseous state, a diesel fuel cooler cooled by the gaseous fuel proceeding to the engine, unions for connecting the first exchanger to a fuel tank to accept liquid fuel and for connecting the first exchanger to the engine manifold and means for connecting the diesel fuel cooler to the diesel tank. [0008] A primary heat exchanger is fitted to transform the liquid methane into a gaseous state utilizing the heat from the warm air exiting from the originally fitted intercooler. This lowers the inlet air 20-30'C that supplies air to the cylinders and thereby reducing the combustion process vulnerability to ping or knock. The lower air temperature reduces the end of compression temperature and gives a smooth combustion process. [0009] Cooling the diesel fuel with its own heat exchanger and utilizing the LNG that has passed through the primary exchanger ensures that the all the LNG has changed its state to gaseous and that the methane gas temperature is above 10'C. This lowers the diesel temperature by about 20-40'C and reduces engine ping or knock in the combustion process. Tests have 2014-10-27,C:\Users\Shalene\Documents\SPECIFICATIONSgastechkdiisiomu]2.wpd,2 3 shown that more stable smooth combustion takes place. [0010] These two installed heat exchangers allows the dual fuel diesel/LNG engine to maintain the rated engine power at high ambient temperatures. Advantageous Effects of Invention [00111 1. Being able to utilise two different fuels eases availability problems especially in remote areas. [0012] 2. Overall fuel economy is improved. BRIEF DESCRIPTION OF DRAWINGS [0013] Embodiments of the invention are now described with reference to the accompanying drawings in which: [0014] Figure 1 is a coolant flow diagram. [0015] Figure 2 is a diagram of the circuit modification to Figure 1 showing the position of the diesel cooler. DESCRIPTION OF EMBODIMENTS [0016J Referring to Figures 1 of 2 of the drawings, a 141 truck engine (not shown) with a conventional turbo- charger delivers air at a pressure through an air-to-air intercooler 2. A duct 4 directs the compressed air at up to 70-75 0 C through a water-cooled intercooler 6 then to the engine. The water-cooled cooler 6 is tubular (500mm x 200mm) and delivers cooler air at 20 25*C lower than the incoming charge air before it reaches the engine intake manifold. [0017] The water-fuelled cooler 6 has a cold coolant inlet 8 and a warm coolant outlet 10 which 2014-10-27,C\Users\Sharke\Documents\PECFICATIONSIgastech divisicnal2.wpd.3 4 returns coolant to a header tank 12. The contents of the header tank create extra volume for the system to prime the coolant pump 14, which feeds the heat exchanger 16. This compartment is 900mm x 200mm and houses a multi-passage heat exchanger element which receives LNG at -160'C from an insulated fuel tank (not shown). Contact with the warm coolant of the exchanger 16 changes the liquid to the gaseous form, at which point the liberated gas leaves the exchanger through the outlet 18, passes through a gas filter 20 and reaches the regulator 22 before entering the injector fuel manifold. [0018] The continuous flow of liquid fuel to the exchanger provides a steady heat sink capable of cooling the coolant arriving from the intercooler by about 20'C with a consequent density increase in air for the engine. The advantage of increased inlet-air cooling is evident when the truck travels up a sustained incline: with such extra cooling the truck engine's power is sustained. With charge-air temperatures normally at 25-45 0 C, but which can reach up to 70 75 0 C at maximum load, it falls to between 45 0 C and 55 0 C with the addition of the heat exchanger 16. [0019] Referring now to Figure 2, an air to air intercooler 2 supplies pre-cooled air to water cooled heat exchanger 6. Exchanger 6 further cools the pressurised air blown into the inlet manifold (not shown) which is cooled by exchange with the LNG heat exchanger 16. The LNG conduit 24 leaves exchanger 16 and enters diesel oil cooler 26 and transfers fuel to filter 20. The cooled diesel passes through conduit 28 to be directed to the injector pumps beside the engine. [0020] It is to be understood that the word "comprising" as used throughout the specification is to be interpreted in its inclusive form, ie. use of the word "comprising" does not exclude the addition of other elements. [0021] It is to be understood that various modifications of and/or additions to the invention can be made without departing from the basic nature of the invention. These modifications and/or additions are therefore considered to fall within the scope of the invention. 2014-10- 27,C:\Users\Sharlene\Documents\SPECIFICATIONSigasteckdivisiona12.wpd,4