1 TITLE OF INVENTION Dual Fuel Engine Control TECHNICAL FIELD [0001] This invention concerns the electronic control of 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 suffer power reduction as the diesel fuel temperature rises and or the air temperature entering the engine rises even at modest ambient air temperatures. 2012-07-13,C:\Users\Sharlene\Documents\SPECIFICATIONS\gastech.divisional.wpd, 2 [0005] If the engine is to use diesel and hydrocarbon gas as a mix the ratio must change to optimise the combination. The engine therefore is mapped, taking measurements during dynamometer tests to determine the operating values across the rpm/load range of the engine. These parameters are data for the ECU which controls the operation of engine when running on a single fuel but the use of an additional fuel adds to the parameters and supplementary ECU becomes necessary. SUMMARY OF INVENTION [0006] The first apparatus aspect of the invention provides a dual fuel multi-cylinder internal combustion engine operable to accept diesel and hydrocarbon gas fuels, having an OEM electronic control unit for controlling the performance of the diesel injectors, the inlet manifold with multiple ports for admitting hydrocarbon gas, a primary heat exchanger for changing liquid hydrocarbon into gaseous fuel, a secondary heat exchanger for cooling the diesel fuel by heat exchange with the liquified hydrocarbon gas, multiple sensors for detecting the same engine parameters as the OEM electronic control unit including throttle opening, an additional sensor for hydrocarbon gas pressure, an additional sensor for hydrocarbon gas temperature, and a supplementary electronic control unit for varying the ratio of diesel to hydrocarbon gas in response to the throttle opening in dependence upon the above parameters, whereby when the engine idles the injectors admit gas to render the fuel mix richer in hydrocarbon gas than diesel. [0007] The parameters utilised by the supplementary electronic control unit may comprise air temperature coolant temperature, gas pressure, gas temperature, diesel temperature, lubricant pressure, lubricant temperature and diesel temperature. [0008] The flow of air and hydrocarbon gas in the manifold may be controlled by a motorised valve in turn controlled by the supplementary electronic control unit. [0009] The diesel injectors' performance may be controlled by the combined outputs of the OEM and supplementary electronic control units. 2012-07-13,C:\Users\Sharlene\Documents\SPECIFICATIONS\gastech.divisional wpd,2 3 [0010] The diesel heat exchanger for cooling the diesel may be in series with the heat exchanger which cools the air charge to the inlet manifold. [0011] The diesel heat exchanger is preferably downstream of the heat exchanger for cooling the air charge to the inlet manifold. [0012] Long individual intake pipes are fitted to ensure cylinder to cylinder air mass induced equally. This is important to ensure equal Air Fuel ratio in between cylinders remains equal. [0013] Fuel delivery is via sequentially timed port gas injection. This enables reduced gas emission loss during valve overlap period and superior transient Air Fuel Ratio control during acceleration. [0014] Operators of vehicles can correct the engine to accept diesel fuel by using a kit of parts. [0015] The second apparatus aspect of the invention provides a kit of parts for converting a hydrocarbon gas engine to a dual fuel engine consisting of a diesel fuel cooler with pipes to connect the cooler inlet to a source of diesel fuel and to connect the cooler output to diesel injectors, a manifold with at least one gas port for the admission of hydrocarbon gas, a motorised valve in the manifold for adjusting manifold flow, a sensor for hydrocarbon gas pressure, a sensor for hydrocarbon gas temperature, an electronic control unit for supplementary the OEM electronic unit in the gas engine to vary the proportion of diesel/hydrocarbon gas in the fuel mix in response to throttle opening. [0016] 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. [0017] Cooling the diesel fuel with its own heat exchanger and utilizing the LNG that has 2012-07-13,C:\Users\Sharene\Documents\SPECIFICATIONS\gastech divisional.wpd,3 4 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 shown that more stable smooth combustion takes place. [0018] These two installed heat exchangers allows the dual fuel diesel/LNG engine to maintain the rated engine power at high ambient temperatures. [0019] If any fault occurs in the control imposed by the supplementary ECU, the operator reverts to control by the OEM electronic unit. Advantageous Effects of Invention [0020] 1. Being able to utilise two different fuels eases availability problems especially in remote areas. [0021] 2. Overall fuel economy is improved. BRIEF DESCRIPTION OF DRAWINGS [0022] Embodiments of the invention are now described with reference to the accompanying drawings in which: [0023] Figure 1 is a flow diagram. [0024] Figure 2 is a diagram of the circuit modification to Figure 1 showing the position of the diesel cooler. [0025] Figure 3 is a diagram of an engine electronic control unit as supplied with a multi cylinder diesel engine and a supplementary electronic control unit. 2012-07-13,C:\Users\Sharlene\Documents\SPECIFICATIONS\gastech.divisional.wpd,4 5 [0026] Figure 4 is a table showing fuel ratios at 75% throttle. [0027] Figure 5 is a table showing fuel mix ratios at 25% throttle. DESCRIPTION OF EMBODIMENTS [0028] 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'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. [0029] The water-fuelled cooler 6 has a cold coolant inlet 8 and a warm coolant outlet 10 which 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. [0030] 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'C, but which can reach up to 70 75'C at maximum load, it falls to between 45'C and 55'C with the addition of the heat exchanger 16. [0031] 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 2012-07-13,C:\Users\Sharlene\Documents\SPECIFICATIONS\gastech.divisional wpd,5 6 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. [0032] Referring now to Figure 3, the ECU 30 supplied with the engine by the manufacturer as original equipment is in a housing fixed to engine exterior and connected by electrical leads to engine components. In a six cylinder engine there is a lead 32 from the ECU 30 to each injector to vary the injector pulse. A lead 34 connects to the accelerator pedal sensor 36. The ECU 30 collects input from Sync sensor 38, reference sensor 40, diesel temperature 42, oil temperature 44, oil pressure 46, coolant temperature 48, air temperature 50 and map sensor 52. These inputs allow the unit 30 to change the pulse period of the injectors to respond to the throttle opening. [0033] An intake manifold drive by wire throttle is fitted and commanded from the supplementary ECU 54 to maintain a maximum lean A/F ratio. The throttle controls the amount of air entering the engine. If the Air Fuel ratio is too lean (no throttle control), then the natural gas (methane) that has started to bum after the pilot diesel injection, will bum too slowly and incompletely and therefore green house emission will rise as well as fuel consumption. The substitution ratio of methane versus diesel falls and the cost benefit is significantly reduced. By having this throttle control the substitution of methane can be utilised in almost all load conditions even at idle. [0034] The supplementary ECU 54 requires inputs from the same sensors referred to above and in addition there is input from hydrocarbon gas pressure sensor 56 and gas temperature sensor 58. The ECU 54 has leads 60 to the six diesel injectors, leads 62 to six gas injectors 64 in the manifold and a lead 66 to the servo meter (not shown) which rotates the flap valve 68 in the manifold. The supplementary ECU 54 is connected to the injectors by sharing the lead used by the ECU 30 and in a variant a changeover switch allows the operator to cancel the sharing. All parameters are controlled via the supplementary ECU 54 and the ECU 30 is only monitoring the engine parameters but not controlling. [0035] If any problems were to arise the original ECU 30 would take over control of the engine 2012-07-13,C:\Users\Sharlene\Documents\SPECIFICATIONS\gastech.divisional.wpd,6 7 and run under full diesel mode, and the supplementary ECU 54 switched off. [0036] Referring now to the Table in Figure 4, at 75% throttle and 1500rpm the fuel mix is 20% diesel, 80% gas. [0037] Referring now to the Table in Figure 5, at 25% throttle and 1500rpm the mix is 3% diesel and 97% gas. As vehicles idle for a substantial portion of their journey, this saves on emissions. [0038] 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. [0039] 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. 2012-07-13,C:\Users\Sharlene\Documents\SPECIFICATIONS\gastech.divisional.wpd,7