Electronic feedback control systems may be a necessary requirement for conversion certification in your locality.  If you do a lot of driving, you may want to do this upgrade to just improve your LPG fuel consumption.  Since the fuel mixture is controlled by the shape of the gas valve in the mixer, there is no real way to adjust it as you would a gasoline carburetor.  There is an idle mixture control and a power mixture control.  The power mixture control has a minimal effect at cruising speeds and affects the full throttle power fuel mixture by restricting the flow of propane entering the mixer.

Electronic control systems typically measure the exhaust oxygen (O2) content and control the fuel mixture by adjusting the output of the propane converter's pressure regulator.  These systems are called closed loop systems because the controller continually measures the exhaust O2 and adjusts the fuel mixture.  Open loop systems are like old-style carburetors in that the carburetor solely controls the fuel mixture and there is no fuel mixture readjustment as the engine is running.  Open loop propane converters produce a low (measured in inches of water) but constant output pressure.

Tom Jennings upgraded his 1963 Rambler to a closed loop system and noticed an immediate improvement in his fuel consumption and drivability.  He used the Autotronics 4046 Close-Loop Controller and found that their instruction manual left out a lot of small but important details.  If you are considering trying this system, read the Tom Jennings experience first.

Impco ADP

Impco has created an electronic feedback control system to control the fuel mixture supplied to an engine by their mixers.  The propane system is very similar to a conventional open-loop arrangement except for a few differences.  The ADP system relies on an exhaust oxygen sensor to monitor whether the fuel mixture is rich or lean and tries to maintain a stoichiometric fuel ratio.  A stoichiometric fuel mixture is one where there is exactly the right amount of oxygen to react completely with the fuel.  That is, the fuel mixture is neither rich nor lean but ideal.

In addition to the exhaust oxygen sensor, an ADP system must obviously have the ADP electronic control unit (ECU), which ideally needs to be installed in the passenger compartment of the car.  The ADP ECU receives the feedback signal from the exhaust O2 sensor and sends a control signal to the feedback converter.  The converter is what actually provides the ideal fuel mixture to the engine.  The converter does this by adjusting the fuel pressure supplied to the feedback mixer.

The feedback mixer is identical to the standard mixer except for the gas valve cone.  The gas valve cone has a different shape than the standard mixer's gas valve cone.  Impco would not say how it is different but from a control point of view, it would make sense for it to be shaped to supply a constant ratio of fuel to air.  A standard mixer may be retrofitted with a feedback gas valve or a mixer may be ordered as a feedback mixer.

Impco was kind enough to provide us with the ADP installation manual in PDF form.  As the manual is quite large, Impco has provided it in four parts and you may download it for yourself:

DualCurve 5952 Digital Fuel Controller

DualCurve reports that the 5952 Digital Fuel Controller is designed to work with Air Valve carburetion, such as IMPCO, Nolfe, OHG, or others.  You must go to their web site and select Electronic Mixture Controls from their product list to learn more.  To convert newer model vehicles in USA, it is required to install a system with EPA certification.  DualCurve has a installation manual you may download for yourself:

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Last modified: November 12, 2004