Thread: Lpg
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Old 03-10-2013, 03:54 PM   #15
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Join Date: May 2008
Location: San Francisco, CA
Posts: 171
My Ride: 2000 323i
This sounds like a discussion for another country so my experience my not apply, but here in the US a CNG or LPG conversion doesn't quite involve those extreme tradeoffs. We have quite a few non-taxi vehicles that are CNG and they drive identical to petrol gas vehicles.

The conversion is EXTENSIVE for a car like the e46. Probably the biggest investment will be in the DME. Because of the nature of the fuel and that it puts out fewer BTUs of heat energy as compared to petrol gas. Injector pulse width and ignition timing has to be dramatically adjusted. Somebody is going to have to manually adjust all those values. Otherwise you'll need tanks, plumbing, dryers, and pressure transducers. The tanks surprisingly don't weigh as much as you think. For safety reasons, here in the US they're all composite (fiberglass). You can get carbon fiber ones if you wish. You also will need A LOT of tanks to get equivalent range between fuel ups. Expect to lose your entire trunk to tanks, and even then you won't have the full 400 mile range.

They also have some weird quirks. Because the gas pressure increases as ambient temp increases, they have to be fitted with safety bleeders. They don't activate very often so you don't have to worry about money automatically be ejected. However, when they activate they sound like a broken fan belt and they shake the whole car. There's also a dryer that separates the water from the gas, which will make a similar noise during purging. And in the engine, the pressure has to drop from the tank pressure (3000 psi) down to the fuel injector optimized pressure (50 psi). The pressure transducer, which is the part that drops the pressure, forms ice on its surface due to the extreme cold of dropping pressure.

I am in agreement about the concerns with premium fuel. I have no clue how anyone would figure this out, but its a big concern in a high compression motor like the S54.

Incidently, one of the big tuners converted a 330i e90 to LPG and it worked great, so its do-able.

Not sure if this helps, just wanted to add in some less speculative info to the discussion
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