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Electronically-controlle system for EFI engines Traditional LPG conversions of EFI engines involve fitting a simple carburettor to the air intake system & bypassing the fuel injectors during LPG running. This new system feeds the LPG to the fuel injectors & features an advanced electronic control unit. By JULIAN EDGAR Aftermarket LPG (liquid petroleum gas) systems for cars have been available for many years, with both Ford and Holden now also offering factory-fitted systems. However, the technology used in converting a petrol engine to run on LPG has been fairly basic. Until recently, there has been no use of mixture-monitoring feedback loops, although the design rules now state that the emissions level from a petrol engine must not be degraded by the conversion to gas. As a result, the oxygen sensor is used on current systems as a control input. Even systems that are factory-fitted to EFI vehicles use a traditional converter (which changes the LPG from a liquid to a gas) and a mixer (essentially a simple carburettor) which adds the gas to the intake airstream ahead of the throttle butterfly. In other words, no use is made of the fuel injectors or other me­chanical elements built into the EFI engines used in these vehi­cles. In addition, the electronic control methods used for LPG fuel metering tend to be far simpler than those employed for petrol injection systems. Now, however, a South Australian company has introduced new technology which is said to overcome many of these deficiencies. The company, Liquiphase Management Pty Ltd, has developed a new system which uses full electronic control to inject LPG in liquid form through the factory-fitted petrol injectors. Their system is currently only available for Falcons but development of a Holden V6 system is also under way, with others likely to follow. Improvements over traditional LPG systems are claimed in the areas of power, economy and starting. In fact, Liquiphase has chassis dynamometer sheets which show an improvement in The LPG tank mounts in the conventional boot location & differs only slightly from any other automotive LPG tank. 4  Silicon Chip ed LPG power over the same engine running on petrol! Given that there is almost universally a power decrease when running on LPG as com­pared to petrol, the latter point is quite intriguing. Unlike other LPG systems, the Liquiphase design uses an in-tank fuel pump, which can be seen in this cutaway view. Based on an EFI petrol pump usually used in Magnas, this operates at 250kPa above tank pressure. Mechanical layout Starting at the rear of the car, the Liquiphase system differs from a traditional LPG system by using a pump within the boot-mounted LPG cylinder. Normally fitted to the electronic fuel injection system used in Magnas, the roller-cell pump is sub­ merged within the liquid and increases the fuel pressure to 250kPa above the tank pressure (which varies depending upon temperature). The other major difference in the tank is the provision for a return line, as found in EFI petrol systems. Under the bonnet, the system looks quite unlike a conven­tional LPG system. Two new fuel rails are used, the top fuel rail supplying the injectors in the conventional manner but having more plumbing connections. The bottom fuel rail uses collars which fit around the base of the Ford injectors, with the fuel flowing through a slot which is cut into the bottom section of the injector for this purpose. The LPG (in liquid form) is then sprayed through the injector’s nozzle each time it opens. This method of “bottom feeding” the fuel injectors is necessary to prevent fuel vaporisation. If top-fed to the injec­tors in the conventional manner, the LPG can vaporise as it passes around the relatively warm solenoids. Any LPG which is surplus to the engine’s requirements flows out through the top of the injectors and into the upper fuel rail. This fuel Differences from other LPG tanks include the provision of a return fuel line & the use of a flanged fitting to allow the insertion & removal of the fuel pump. An in-line filter is used to prevent small particles blocking the injectors. Unlike a conventional EFI filter, this must operate at the very high pressures associated with a gas system. June 1995  5 Above: The additional equipment required by the Liquiphase LPG injec­tion system is positioned near to the stock injector location. The gas converter & mixer of a conventional LPG system are abs­ent. then circulates back to the LPG tank via two one-way check valves. Conversely, when the car is running on petrol, the fuel is supplied to the injectors via the conventional top feed points by the upper fuel rail. As a result, the upper fuel rail is not solely a “petrol rail” and, in fact, there are times when the flows of fuel actually mix. This occurs during the change-over from LPG to petrol, for example. Such a change-over needs to be provided if the system Two ECU-controlled solenoids, two mechanical one-way valves & two fuel rails are used in the Liquiphase LPG system. Fuel rail pressure & temperature sensors are also fitted to provide inputs to the new ECU. 6  Silicon Chip The stock Falcon injectors are modified by having a slot cut into one side. This provides the LPG supply point for the injec­tors & prevents the fuel vaporisation that would otherwise occur if the injectors were “top-fed”. BOTTOMFEED INJECTORS PETROL NON-RETURN VALVE LPG SOLENOID VALVE PETROL SOLENOID VALVE FUEL RAIL FEED PIPE LPG NON-RETURN VALVE LPG SUPPLY LINE PETROL PRESSURE REGULATOR PRESSURE SENSOR FUEL RAIL RETURN PIPE LPG FILTER PETROL SUPPLY LINE NON-RETURN VALVE LPG RETURN PIPE LPG TANK PETROL RETURN PIPE PETROL TANK FUEL PUMP Fig.1: basic layout of the Liquiphase LPG injection system. Unlike other LPG conversion systems, it feeds the LPG to the car’s existing fuel injectors & features an advanced electronic control unit (ECU) which mates with the existing ECU. This ensures optimum performance when running on LPG. is to be acceptable in the marketplace. In addition, the system must be engineered so that the car is easy to start and yet comply with the design rules. These rules state that LPG cannot be allowed to circulate unless the engine is being started or is running. This precludes the use of an automatic circulation system when the engine is stopped. As a result, the LPG can vaporise in the fuel rail because of underbonnet heat-soak. In a worst-case scenario, it can take up to 60 seconds for the vaporised LPG to be displaced by liquid LPG and this would obviously lead to poor starting performance. To overcome this problem, the Liquiphase-injected engine is run on petrol provided by a “third party” seventh injector during a fuel changeover or when the car is being hot-started. This seventh injector is positioned prior to the inlet plenum chamber and supplies enough fuel for the engine to be driven at up to about 75% throttle opening. During a change from gas to petrol, for example, the con­ ventional six injectors are initially shut off and the engine is run on petrol from the seventh. The pressure is then reduced in the fuel rails until it drops below 250kPa, whereupon petrol flows into the top rail through a one-way valve, flushing out any remaining gas vapour in the process. When this process is com­ plete, the multi-point injection system takes over and the extra injector is switched off. Electronic control Cars to which the system is currently being fitted use the Ford EEC-IV engine After the slot has been cut into its side, the injector is flushed & tested on this rig to ensure that no particles of metal remain. June 1995  7 The system uses two new fuel rails. Shown here is the stock Ford rail (top), the new top feed rail (centre), & the bottom feed rail (bottom). The collars on the bottom fuel rail surround the modified injectors, with the fuel flowing to the injectors via the slots. This close-up shows the arrangement of the two new fuel rails & the modified Ford injectors. As can be seen, both fuels are injected just behind the intake valves in a multi-point arrange­ment. management system. This sophisticated manage­ment system relies on a number of inputs, including throttle position, air and coolant temperature, manifold absolute pressure (MAP), ignition pulses, road speed and exhaust gas oxygen cont­ent. When the Liquiphase cars are running on petrol, the Ford EEC-IV system is used in the conventional manner. In other words, the cars run in exactly the same manner as unmodified vehicles when petrol is used. Two different approaches have been used to control the fuel injec8  Silicon Chip tors and the ignition timing when running on LPG. The first system used a piggyback approach, where the output signals of the EEC-IV ECU were modified by another electronic control unit before being applied to the fuel injectors. In general, the injector opening times for LPG are shorter than for petrol. This is because of the much higher operating pressures of the gas system, which ensures that suffi­cient fuel flow occurs in a shorter time. At the same time, the energy value of LPG is lower than that of petrol. This close-up view shows one of the collars which surround the modified fuel injectors. The fuel injectors are “bottom-fed” when running on LPG to prevent fuel vaporisation. This means that a greater amount of LPG must be injected but, even so, the injec­tor opening times must still be reduced. The other major factor which the piggyback ECU changed was the warm-up outputs of the EEC-IV unit. Because of the very low boiling point of LPG (ie, -43°C), it will vaporise even at very low temperatures. As a result, the normal cold-start injector pulse width extension required for petrol operation was found to be unnecessary for LPG and so this function was eliminated. However, the piggyback system did have some problems, due mainly to the fact that the ignition timing remained the same for both petrol and LPG. In practice, this gave some problems with driveability. LPG has different burning characteristics to petrol and therefore needs different ignition timing to give the best performance. Programmable ECU As a result, Liquiphase decided to use a fully-programmable aftermarket ECU to drive the LPG system and Injec were commis­sioned to do the development work. This new ECU uses all of the inputs fed to the original unit, picking these up via an inter­connect­ing panel which fits between the car’s standard wiring harness and the EEC-IV ECU (which is retained). Both the ignition timing and the fuel injector pulse widths are calculated on the basis of look-up maps, which use a light and full load axis every 500rpm of engine speed. This system is said to be able to interpolate accurately within this framework, giving a “very large” number of different outputs. In addition, the new ECU produces 48V injector “pull-on” pulses so that the injectors open in the same time as for operation with petrol, this despite the fact that the LPG pressure can be up to 10 times higher. Following this initial 48V pulse, the injectors are held on using just 12V. Because the pressure of the gas system varies with tempera­ture, the system changes the fuel injector pulse widths depending on the pressure being sensed in the fuel rail. Along with a temperature sensor in the rail and another in the tank, these are the only additional inputs to the new ECU over those provided by the factory-fitted EFI sensors. Shown here, from top to bottom, are the petrol solenoid valve, the petrol supply line, the LPG return pipe, the fuel rail pres­sure sensor, & the LPG solenoid & LPG supply line. On the road The Liquiphase organisation had available a Falcon sedan for testing. While the system looks highly-developed, both elec­tronically and mechanically, it was apparent after driving the vehicle that some further work still needs to be carried out. When running on LPG, the car drove well, with normal re­sponsiveness and other behaviour. The same goes for petrol opera­tion. On the debit side, the fuel changeover was clumsy, with the change from gas to petrol being somewhat protracted. While undergoing this change, Liquiphase recommends that the car not be driven but instead be fast-idled by the side of the road while the seventh injector supplies the fuel. However, on the advice of a technical officer who was pres­ent, we drove the car gently during the changeover period. It took several minutes for the car to switch to petrol and more than very gentle throttle action resulted in engine misfires. In one case, the engine had successfully changed from gas to petrol only to then go back to seventh-injector (low power) running. Software glitches were blamed for this behaviour. The performance testing was also interesting. The denser charge caused by the heat lost through the latent heat of evapo­ration of the fuel resulted in improved power torque while run­ ning on LPG. This was shown in the supplied dynamometer charts. Hand-timed 0-100km/h runs in the The LPG electronic control unit (ECU) was developed by Injec. It uses the sensor inputs of the existing engine management system & has unique fuel injector pulse width & ignition timing maps to give optimal performance when the vehicle is running on LPG. automatic Falcon indicated an average time of 10.0 seconds on petrol, while on gas the time was reduced to 9.7 seconds. However, on rolling 60-90km/h splits, the car was slower on LPG with a time of 3.6 seconds versus 3.5 seconds for petrol. From this, it would appear that further fine tuning of the ignition and fuel maps is required to maximise the performance on LPG. Conclusion By adopting a sophisticated electronic and mechanical ap­ p roach, the Liquiphase LPG injection system appears to have the potential to revolutionise LPG installations in EFI cars. The system is currently being fitted at a cost of $2500, which is claimed to be only about $500 more expensive than a conventional system. At this stage, it appears that just a little more development should result in an excellent system. For further information on the Liquiphase LPG system, contact Liqui­ phase Management Pty Ltd, 20/2 Gray St, Kilkenny, SA 5009. Phone (08) 345 SC 3500; fax (08) 347 3240. June 1995  9