Re: LPG in cars

[Tom Jennings <tomj@xxxxxxx>]

On Tue, 10 May 2005 farna@xxxxxxx wrote:

> Okay, I know this is obvious to you Tom, and anyone else who
> knows anyting about LPG/CNG, but I'll say it anyway: Yours just
> changes from liquid to vapor in the carb.


Oh sorry, I didnt' state this "obvious" little detail :-)

It goes liquid to vapor in a device external to the carb, the (ta
da) converter. It's basically a two-stage regulator, like a
welding regulator; big diaphragm, spring, little valve seat, etc.
Biggest difference is that it's got a water jacket.

Really, no vehicle engine larger than a few hundred cc's can
really get away with vapor off the top of the tank for a reliable
system.  Analogous to a gravity-fed fuel system, the flow is small
and unreliable, it depends literally on the weather and amount fo
fuel in the tank!

It's also a safety issue; a real LP car system has three passive,
failsafe, fuel shut off features under the hood:

One, an explicit shutoff and filter, usually engine-vacuum driven.
A few In Hg opens the valve.  Stalled motor, the liquid propane is
shut off from the tank, like electric fuel pump run from
oil-pressure.

Two, the converter. It outputs NEGATIVE pressure -- if you pull
the vapor hose off the carb, even with a supply of LP (see One),
nothing comes out. The carb has to *draw* vacuum on it, but only
-0.5 inches of water.

Three, the carb isn't venturi-based; it's got a funny air-valve.
There's a tiny idle air bleed, but engine pumping opens the
air-valve which lets in fuel; no engine pumping, no fuel to
manifold.


The tanks also are models of robust, dumb-as-a-bag-of-hammers
simple safety, spring loaded shear valves, all sorts of stuff.

I inspect my fuel lines once a year or so (really) and listen and
spray soap for leaks.