A typical LPG system consists of the following parts. Firstly, there's tank (usually boot mounted) that contains LPG under very high pressure. In fact, the pressure is so high that the fuel is in liquid form. It flows forward through a filter/lock-off valve, a valve that only opens when there is an engine rpm signal sensed. This is for safety, so that LPG cannot flow from the tank when the engine is stopped.
The LPG changes from a liquid to a gas in the converter, which is heated by connections to the engine cooling system. Converters are classified as 'simple' or 'complex'. A simple converter has no provision for external adjustment - all it does is change the state of the LPG. A complex converter has adjustment screws - for changing idle mixtures, for example.
Following the converter the LPG is in gaseous form, so it can be added to the intake system of the engine. Of course, the addition of LPG must be in the right proportions to the mass of air being breathed, otherwise the air/fuel ratio will not be correct for the operating conditions of the engine.
The device adding the LPG to the intake air is called a mixer. Mixers vary in complexity, depending on which converter they are working with. The simplest mixer is called an Amos Ring, and it comprises just a venturi (flow restrictor) and a gas pipe connection. The pressure drop caused by the venturi encourages the gas to flow through the tube. This simple mixer needs to be used in conjunction with a complex converter, so that the correct adjustment of the air/fuel ratio can occur.
More complex mixers can be used with a simple converter, because the mixer itself provides the adjustability of the mixtures. It is this type of system that is used by GasResearch Australia - a simple converter working with a complex mixer, called in this case a gas carburettor.
GRA Gas Carburettors
The GasResearch Australia carburettors use a venturi with a shallow angle of incidence, so the air is not stalled. A partial vacuum is formed by the flow of air through the venturi, with the magnitude of this vacuum being proportional to load. In other words, at higher airflows there is more vacuum generated within the carburettor, helping to draw gas into the intake air stream. Venturis are available in four different sizes to suit different horsepower engines.
The carburettor has three major gas supply systems built into it. Firstly - idle, where an idle mixture screw is used. The idle port has an internal diameter of 1.2mm, and can be drilled out to 1.4mm in rare cases where the idle mixture stays lean even with the screw fully adjusted. Gas is fed to this idle port through an external rubber hose.
A progression port is used to adjust mixtures in light cruise conditions. The moving throttle blade uncovers the progression port, with gas then being drawn from this port into the intake air. The amount of gas that flows through this port is determined by the diameter of a jet placed in the rubber hose that supplies it. Jets are available in 0.1mm increments from 1.8 to 2.5mm. The larger the jet used, the richer will be the light cruise mixtures.
A metering rod controls mixtures at higher loads. This consists of an aluminium rod whose external diameter varies in two or three distinct steps. The metering rod slides within a cylinder, with the steps in the rod moving past a gas supply orifice and so opening it to a greater or lesser extent. The rod is connected by a spring-loaded shaft to a rack, which in turn is moved by a pinion turning in conjunction with the throttle blade. That means that as the throttle blade opens, the metering rod is slid along in its cylinder, opening up the gas delivery path in a series of steps.
The smaller the diameter of each of the sections of the metering rod, the greater the room there is for the gas to flow past the rod into the intake pipe, and so the richer the mixtures. The different diameter steps allow the air/fuel ratio to be adjusted for variations in high load. A rod with three different diameters has the facility to provide a leaner high-load cruise than one with just two steps.
Tuning
We followed the tuning of a XD Falcon Ford 351, being carried out on the chassis dyno of Darlington Auto Tune in Adelaide (Australia).
The engine was a Cleveland V8 with 10:1 compression, roller rockers, a Crane hydraulic cam and standard heads. The car had been fitted with twin GasResearch carbies being fed by dual LG Motor Gas B2 mixers. The cost of the gas carby kit was A$1450, which included the carbies, converters, intake manifold adaptor, high flow lock-off valve, primer and hardware. The airfilters, gas cylinder and gas supply plumbing were extra.
The dual carby systems use a sequential system, where at idle and low loads one carburettor's throttle is not opened at all. During the tune-up, the idle mixtures were easily adjusted using the idle screws. Just off idle, the mixtures were quite lean and so the progression jet on one of the carbies was swapped for one of larger diameter.
On load the engine was still running lean, and so the metering rod from the carby that opened first was machined in a lathe to allow more gas to flow past it. A number of different metering rods were trialed in the second carburettor until mixtures were satisfactory throughout the load and rev range.
At this point the dyno indicated that the Falcon had a peak power of 160kW (214hp) at the wheels with a power curve that was generally smooth.
GRA VN Commodore
GasResearch Australia has a complete power-up kit for the VN-onwards Holden V8 Commodore. Development was not aimed at making the car a "Harry Hot-Rod with 300kW and zip driveability", but rather one at home in the humdrum of commuting while still having the balls to cut it when needed.
Early on in the project it was decided that cam and compression changes would be needed to get the results being aimed for. The development of the kit was carried out in a number of stages.
In dead standard trim the car had a flywheel peak power of 170kW at 4500rpm. The next stage was to convert the car to dual petrol/LPG operation. The LPG vapour system used closed loop (oxygen sensor) control, allowing the car to meet the relevent emissions standards. The gas carby replaced the EFI throttle body, meaning that when run in petrol mode, the carby throttle acted only as an air valve. The engine was run in this form with standard ignition timing. It dropped in peak power by 6 per cent, with the new max power figure being 160kW.
The next step was to change the diameter of the gas carb venturi to reduce its restriction. This involved reducing the size of the venturi (allowing more room around the outside of it for intake airflow) until the vacuum signal being generated by the venturi became too weak. The inside of the venturi was then bored out. The new carby was fitted to the engine and the distributor was adjusted, moving the whole ignition timing map. Peak power at 167kW was still lower than that achieved on petrol, but at the bottom end, the power curve now matched or exceeded 'petrol' performance.
The rear section of the exhaust and the muffler were then replaced by 2½ inch items. The Mem-Cal (ECU software) was re-written with dual petrol and LPG maps, allowing an appropriate LPG ignition timing advance map to be used. These changes resulted in an improvement in peak power over the starting point of 5 per cent on petrol and 2 per cent on LPG.
It was time to get serious. The engine was opened up, bored by 0.020 inches and special 9.6:1 pistons slotted into place. A GasResearch Voodoo Stage II cam was also fitted. On petrol this combo gave severe detonation, and so all further dyno runs were made using just LPG. On LPG, the engine in this form developed 188kW (up 11 per cent on standard) but gave a poor idle.
To achieve an idle with less hunting, a cam with wider lobe centres was fitted. This dropped peak power marginally (to 187kW) but the engine now idled at 750 rpm without problems.
The final form of the package added a complete 2½ inch exhaust, K&N filter and the cold air induction duct from a VR SS Commodore. The new air intake was drilled to avoid a pressure build-up in the airbox. With these mods, the LPG V8 made 195kW at 4700 rpm, an increase of 15 per cent over standard petrol power.
Contact:
GasResearch Australia
03 9702 4555
Darlington Auto Tune
08 8277 4222
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