Vapour Injected LPG

[Joe Chiasson]

> 
> <snip>You sound like you know what you are talking about.  Could you
conpare
> and contrast Liquid propane Gas and Compressed Natural Gas for this type
of
> application?  Do they make pulsed (as apposed to continuous) injectors
for CNG?
> 
> GT
> 914-6
> 

Yeah Chrysler's (Bosch) CNG injectors, and Servojets (hear these need some
kind of voltage amplifier to work, not sure) are pulsed injectors. 
Chrysler's CNG propgram has been on and off again over the last little
while but I think you may be able to get the injectors at the dealer.

As clare stated CNG tanks are heavier and carry less fuel because the NG is
stored ad a vapour in most conventional systems.  Thus your looking at a
range of about 150-200 miles for conventional coversions.  But they do have
LNG storage tanks for vehicles, you get this big crogenic cylinder storing
liquified NG which certainly increases your range (note still introduced
into the engine compartment as a vapour).  CNG has low power for the same
reason vapour LP does, it displaces too much air.
CNG if implemented properly will always have better emmisions, it is a
cleaner fuel (mostly because its in a gaseous form it dosen't transport
junk with it), and a cleaner burning fuel.  Infrastructure controls the
popularity of this fuel, when you have a limited range and refueling
stations are far and few you could get yourself into problems.  However if
you heat your home with NG you can get yourself a compressor.  The biggest
problem I see with CNG is its storage pressure of 2000 - 3000 psi. 

Propane is stored as a liquid underpressure.  But the operating range of a
propane system is form 0 to 312.5 psi.  All propane tanks have a pressure
relief valve that either relives the tanks pressure at 250 psi, or 312.5
psi, sepending on what design guidline the company is building to.  Because
of this drastically lower pressure propane tanks can be made lighter and
larger.  Under chasis tanks can give you up to 350 miles depending on the
application of course, and a bed mount tank in a pick up can get you up to
500-600 miles for a cylinder with a diameter the height of the bed.
Injecting as a liquid you do not displace as much air, you cool the intake
charge, and you get the most bang.  The refueling infrastructure in Canada
is amazing for propane, almost any where you can find gasoline you can find
LP.  The cost is excellent too for most of canada,  the western provinces
propane is <50% the cost if gasoline per Litre,  in the middle provinces
its around 60-65% that of gasoline.  The infrastucture in the US in some
areas is quite sufficient yet prices jump all over the place and sometimes
they are higher per gallon than gasoline.  If done properly a liquid system
will have no hot driveability propblems, yet hot starting propblems are
another issue.  Because of all the engine heat soak back most of the
underhood liquid will flash to vapour around 20-40 minutes after shutting
off the engine.  This vapour needs to be purged out of the system back to
the tank(or into the manifold if you can controll it properly) before the
vehicle will start.  The chrysler system which uses a siemens dekka II
bottom feed injector flushes propane through the lined on the key stroke
cooling the tip of the injector and allowing it to start.  The Vialle
system, starts on gasloline and switches. So basically you are running into
the inconvience of a cold starting diesel.  Having to wait for the glow
plugs to warm up in a diesel is analogous to waitng for the vapour to purge
in a liquid system.

A CNG system is probably the most expensive conversion due to the cost of
the tanks and the higher design criteria.  For the average application it
has the least range and least power, better emissions.

A Liquid Injection LP system is in the middle, mostly due to the cost of
the tank (if you a pump inside) or an external pump.  You have to have a
pump if you are going to do a liquid system, under heavy engine loads tank
presure can be sufficient, but under any other driving conditions the
systems need that extra head above tank pressure.  For the average
application it has the most power, more range, and emmisions are comparable
to Vapour injection.

Vapour systems, the multipoint injection systems are of course are more
expensive than TBI and carbureted. Depending on the system these can be
comparably to the cost of liquid, some (GFI) alot more expensive, some
(IMPCO) pretty damn cheap. Power is less than liquid but more than CNG. 
Range all depends on how the system is configured.  If it is configured to
meet emmission standards it less than liquid, but if you lean it right out
you can get some good range, but sacrifice emissions.

joe