Ok, what I'm about to say will bore some people. If you arent interested in
lpg, stop reading now.
LPG, aka propane (UN number 1978), also comes in bottles. in fact, they are
the red ones you see powering temporary roadworks sign illuminations usually
on motorways. they are also to be found strapped to the back of a lot of
fork lift trucks. reason for this is that they can power a piece of
machinery indoors. the exhaust is non toxic. however, it is an asphyxiant,
so beware!
Note: the stuff you see in blue bottles is butane. it is technically LPG,
the working definition of which is a (hydrocarbon) gas that can be liquified
with a modest amount of compression, but no refrigeration. butane-the stuff
lighters run on-contains less energy per kg than propane, hence propanes
choice as a fuel. even so, (liquid) propane contains less energy than the
same mass of petrol, which is -partly- why it will not sent your wheeled
pride and joy as far as a gallon of the good stuff. (also partly explains
diesels economy, as a litre of diesel contains more energy than a similar
amount of petrol. yes, I know theres more to it than that, but this is a
simplified explanation) you can also expect a mild drop off in performance,
but nothing too dramatic. expect 10-15 mph drop off the top speed.
It also burns slower than petrol. Slower here is a relative term. propane
will still produce a healthy bang!!! because it takes longer for the flame
front to travel the full expance of the cylinder, the spark has to fire a
little earlier. figure 10-12 degrees earlier than the settings for petrol.
you can get little boxes that switch the timing automatically when the
system changes from one fuel to another. costs vary.
propane also burns hotter than petrol. not so you would notice, but it will
nicely burn off any carbon deposits in the combustion chmber area. this can
also mean around valve seats, uncovering hidden problems. hotter
temperatures also mean its a good idea to slightly shorten the intervals
between oil changes.
The electrical resistance across the spark plug gap is greater with a
propane/air mix than with a petrol/air mix. this means that your ignition
system has to work harder to fire that plug. The practical implications of
this are that the spark will find the easiest way out. if you have an old,
damp or even just oily set of leads, then at some point they will be bunched
up and touching. this is a paticular problem around the distributor of V8's!
this means that the spark will jump from one lead to another and fire the
wrong plug at the wrong time. especially if the gaps are a bit uneven. if
you are lucky, this will result in a missing cylinder. if you are unlucky,
it will fire a plug that is in the middle of an induction stroke, and the
flame front will travel backwards out of the engine. you wouldnt believe
what this will do to an air flow meter!. and if you happen to have your head
under the bonnet investigating when a 'mate' decides to gun it a bit to see
if he can clear it, then I guarentee that this will loosen any earwax you
may have!
I'm not trying to frighten you or put you off, but you need to know that it
isnt all plain sailing. usually you find out the above details only after
you have parted with lots of wonga. I would advise getting to know some
people-at least more than 1- and ask their experinces.
Mike
Oh, BTW, LPG cars are currently (I believe) exempt from comrade Kens
congestion charge.
