[W126 Coupe] Propane fuel?

[Nathan Goodlet]

"..Have you seen a US MB running LPG?.."


I have not seen any, but I know it could be done and done very well on an SEC. At one time I had in mind to do mine as a proper dual fuel, and figured the proper way would be to torch the fuel door and surround from a wrecked car and graft a second duplicate door on the opposite side, eliminating the biggest nuisance for a legal and official conversion, which is a permanently installed EXTERNAL filling and venting apparatus. Nobody would know, because they could never see a lid on both sides at once. You could not legally rely on opening the trunk to fill the tank, I don't think.

On an UN-official basis, and particularly with the cost of gas, I think there is an option, we may have discussed it here before a year or so ago. Using propane as an intermittent supplimentary/substitute fuel, a very low tech installation and no mods to the engine or stock fuelling at all, just a simple tube dumping propane into either the air cleaner, or the throttle body.

I can't be 100% sure, but I think the SEC is running completely on closed loop control of fuel at most speeds, if everything is working properly. I think that means that if you inserted a gaseous propane line into the air intake, the O2 sensor driven fuel control should always be adjusting for available oxygen in the exhaust, no matter how or by what fuel the osygen was consumed.

On propane conversions in the past, a significant negative at most speeds except for road cruise mode, you'd loose power because the propane displaced incoming air and essentially made the engine smaller. At today's gas prices, being able to make the 5.6 only consume the air of a 4.2 would be a good thing.

If I was still driving the 560, I'd consider installing a pyrometer in the exhuast, to see if exhaust temps were going way up. Because propane burns slower, it is possible that if spark advance is not changed for propane assist mode there could be a blowtorch effect on the exhaust system. I would also have like to see a visual display for the O2 sensor output. Between these 2 indications, I think you could quickly and safely determine if the theory was correct, if there was propane going into the intake, if the exhaust temp didn't go real high, and if the O2 sensor reading was still sensible, then the theory would be proven valid. 

I would have tried using a $125 nitrous oxide systems solenoid valve and jet holder, and get about 2 dozen sizes of jets to try, and connect the trunk mounted tank to the engine compartment with a metal line so it would absorb enough heat to completely evaporate, and set it up to open the valve on cruise control mode only, to guarantee a steady load state, unless or until I became convinced that the gasoline injection system was capable of automatically acting like a computerized fine tuning device, keeping the exhaust absolutely perfect would assure the combined mixture would be perfect.

For the purpose of this unofficial test, I would have used the barbeque tank, I happen to know it wedges in tight between the rear parcel tray deck and the trunk floor and would stay in place nicely there, if it was desireable and legal, you could put 5 or 6 of them there.   

Another thought I had was using a distribution block and 8 indivigual small lines to dispense the gas, 1/8" lines inserted into the idle air boots on a indivigual cylinder basis.

Nathan
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