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FAQ Revised: Saturday 01 October 2005 11:58:23
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Table of Contents

1. FAQ Background
+ 1.1. What is this FAQ?
+ 1.2. What us uk.rec.cars.fuel.lpg about?
+ 1.3. I've got a question that isn't in the FAQ?

2. LPG Basics
+ 2.1. What is LPG?
+ 2.2. So how does it get to be liquid?
+ 2.3. Is Propane the same as LPG?
+ 2.4. What is Autogas, Motorgas, etc?
+ 2.5. What is CNG?
+ 2.6. Who sells LPG in ...
+ 2.7. What about biodiesel?

3. Conversions
+ 3.1. Can I convert my ...
+ 3.2. What about diesel engines?
+ 3.3. What about turbo engines?
+ 3.4. I've heard that running on LPG could ruin my engine, is this
true?
+ 3.5. How much will I save by converting to LPG?
+ 3.6. Will my car lose power/economy running on gas?
+ 3.7. Where can I find a converter?
+ 3.8. What about MOTs?
+ 3.9. Can I have a gas-only vehicle?

4. Technical Details
+ 4.1. So what's all this "open loop" and "closed loop" stuff?
+ 4.2. I've heard about variable mixers, what are they?
+ 4.3. What about gas injection?
+ 4.4. What about sequential injection?

5. New Questions and Answers
+ 5.1. What are the downsides?
+ 5.2. What about safety?
+ 5.3. Isn't it hard to find LPG to fill up?
+ 5.4. How does the vapouriser work?
+ 5.5. Can I do the conversion myself?
+ 5.6. I've heard there are different filler types. What are there?
+ 5.7. Is The Fuel Level Indicator always inaccurate.
+ 5.8. Why is there an audible "clang" from the boot when it
switches to Gas?
+ 5.9. I got my car converted to LPG by the fuel economy sucks. I
thought this was supposed to save me money. What can I do?
+ 5.10. How can I tell if the engine is running too lean?
+ 5.11. How can I tell if the engine is running too rich?
+ 5.12. Is it true that I need to change the ignition leads?


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1. FAQ Background

1.1. What is this FAQ?
This is the uk.rec.cars.fuel.lpg FAQ list, V 1.2. This was written by
Austin Shackles and converted to HTML by Andy Cunningham. The online
version can be found at http://www.cunningham.me.uk/lpg/faq.php, which
has all the links completed.


1.2. What us uk.rec.cars.fuel.lpg about?
This group exists for the discussion of matters relating to the
vehicular use of LPG in the UK, and related subjects such as gas supply
etc.


1.3. I've got a question that isn't in the FAQ?
Inevitably, some questions won't be in this FAQ - please feel free to
send comments to where they will be
considered for inclusion.



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2. LPG Basics

2.1. What is LPG?
Liquid (or Liquefied) Petroleum Gas.


2.2. So how does it get to be liquid?
It's stored at a pressure of around 7bar (100psi), at which pressure
the gas liquefies. This is done by pumping it into a pressure tank.


2.3. Is Propane the same as LPG?
Propane is a petroleum gas, as is butane. In the UK, most LPG sold for
automotive use is nearly all propane.


2.4. What is Autogas, Motorgas, etc?
Different names for LPG.


2.5. What is CNG?
Compressed Natural Gas (i.e. Methane). You can't put LPG in a CNG
vehicle, or vice versa.


2.6. Who sells LPG in ...
The LPGA has a list of suppliers in britain online at http://
www.boostlpg.co.uk/refueling_stations2.htm

For Eire, the Irish LPGA have a search engine at http://www.ilpga.com/
garages.html

For France, there's a search engine at URL http://
stations.gpl.online.fr/index.html



2.7. What about biodiesel?
Fuel for diesel engines derived from vegetable oil. Off topic on the
LPG newsgroup.



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3. Conversions

3.1. Can I convert my ...
Most petrol engined cars can be converted to run on LPG.


3.2. What about diesel engines?
Not so easy, but it can be done. Mostly, the sensible answer is to sell
the diesel car and buy a petrol one. It is possible to boost power and/
or reduce running costs by adding an LPG system to a diesel engine; you
can't, however, run solely on LPG like you can with a spark ignition
engine


3.3. What about turbo engines?
Petrol Turbo engine can be converted. You need to know what you're
doing, is all.


3.4. I've heard that running on LPG could ruin my engine, is this true?
LPG contains no additives to protect valves and valve seats. As a rough
guide, an engine which is suitable for unleaded petrol can be run on
LPG with no damage, provided it's tuned properly. Other engines may
need replacement hard valve seats fitted or a system can be fitted to
feed additive into the engine to protect valve seats.


3.5. How much will I save by converting to LPG?
How long is a piece of string? This question is almost impossible to
answer accurately without specific details. As a ball-park figure you
ought to achieve savings of at least 40% on your fuel costs when buying
from forecourt suppliers.


3.6. Will my car lose power/economy running on gas?
Probably. A good conversion shouldn't lose much over 10% of either. If
you're losing more than 20%, there's something wrong. Closed loop
systems are generally a bit more economical than open loop, but mostly
cost a bit more to install.


3.7. Where can I find a converter?
Best place to ask is probably on the newsgroup. The LPGA has a list of
approved installers, but the best installers aren't necessarily
members, nor are the members guaranteed to be good installers - they
should, however, conform to the LPGA code of practice.


3.8. What about MOTs?
At the moment, a dual-fuel vehicle is tested "as presented", so if you
take it to the MOT centre running on LPG that's how it will be tested.
There are moves to bring some aspects of gas installations into the MOT
test, such as tank security etc.


3.9. Can I have a gas-only vehicle?
Yes, but make sure you can get enough gas on board that you never run
out! The current position with gas supply is that you'd want about 300
miles range to be reasonably secure, more than that for long journeys
to out-of-the-way places on bank holiday weekends.



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4. Technical Details

4.1. So what's all this "open loop" and "closed loop" stuff?
To run an engine on LPG you need to supply a mixture of fuel and air to
the cylinders, just like you do with petrol. The most common systems
supply liquid LPG to a vapouriser (also called a regulator) where the
pressure is reduced and the liquid evaporates into a gas, which is
supplied to a mixer in the inlet tract. The vapouriser or regulator
also controls the flow of gas, as it's sensitive to air flow in the
inlet. Open loop systems apply a fixed set of adjustments to the
system, the same as a conventional petrol carburetter, so that
approximately the right amount of gas is supplied for all running
conditions. Closed loop systems use electronic control like an EFI
petrol system, and monitor the exhaust to adjust the tuning all the
time. This is more efficient, and safer for a car with a catalytic
converter, as incorrect mixture can destroy the cat.


4.2. I've heard about variable mixers, what are they?
A fixed mixer is like a fixed carb. Variable mixers, also called gas
carburetters, work like SU, Stromberg (or Ford VV) type carbs with a
variable venturi, and give better mixture control on an open loop
system than a fixed mixer, without the added cost of the electronic
controls for closed loop systems.


4.3. What about gas injection?
More expensive to buy and more difficult to fit. Gas mixes with air
much more readily than petrol does, so there's less advantage to using
fuel injection. It does offer advantages for newer modern engines with
plastic manifolds where backfires could damage the manifold.


4.4. What about sequential injection?
Sequential injection mixes pre-measured quantites of gas with air at
each cylinder intake. While it's more complex to fit, it's not as
expensive as gas injection, and provides for petter performance, and
better fuel management. This generally works better with the most
modern engine management systems.



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5. New Questions and Answers

5.1. What are the downsides?
There are several downsides to LPG which you need to be aware of before
converting or buying a converted vehicle:
+ Either a loss of boot space, no spare wheel, or both. Classic
Range Rovers (no air suspension) and Series I Discoveries can be
converted with under-sill tanks which don't full the boot. Another
option is to reduce the size of the normal petrol tank.
+ You can't go through the channel tunnel. LPG vehicles are not
permitted.
+ Other ferry or transport services may give you a hard time,
especially if you did a DIY conversion. Sea Cat will only accept
professionally converted vehicles.
+ You must tell your insurance company. Direct Line didn't
increase my premium for the LPG conversion. Your insurer might, but
if it's more than a few quid, try elsewhere.
+ Unless you have a good converter near home, expect to learn how
the thing works in case of problems.
Personally, I can live with all of these if it means I can do 300 miles
and still have change from GBP 50.


5.2. What about safety?
There are several safety factors built in to bear in mind. Firstly, the
tanks are much stronger than petrol tanks, and are designed to vent
outside the vehicle if crushed in in impact. They are strongly fixed in
place, typically bolted through the body work at either side.


5.3. Isn't it hard to find LPG to fill up?
No, the LPGA has a list of stations, and Nik Aves keeps this up-to-date
in an AutoRoute format file which can be downloaded.


5.4. How does the vapouriser work?
If you've ever done any scuba diving, then the answer is simple - just
like the regulator you used for breathing.

The Vapouriser basically depressurises the liquid gas so that it
evaporates, at a controlled rate. The rate of gas delivery is governed
by a mechanism inside the vapouriser, which in turn is controlled by a
diaphragm. The gas outlet into the engine (except for gas injection
systems) goes into one (or more) venturi(s) in the inlet tract. The
more air flow through the inlet tract, the more depression (suction) is
created in the gas outlet from the vapouriser. The diaphragm is open to
the atmosphere on most fitments and the diaphragm responds to the
difference in pressure between the gas outlet and atmospheric - more
suction in the pipe moves the diaphragm and lets more gas through the
vapouriser.

The process of vapourising the gas absorbs a lot of heat, and if left
to it's own devices the vapouriser will rapidly freeze solid due to
moisture in the air creating frost and ice in and around it. [it's most
impressive, frost *everywhere*]. To combat this, the vapouriser has a
water feed from the engine coolant, which keeps it from freezing. It's
important to have anti-freeze in the coolant, even in summer, or the
vapouriser can freeze before the coolant gets warm in the engine - once
the water in the vapouriser freezes (which can take less than a
minute), no coolant will circulate in it and you'll have to run on
petrol until the warmth in the engine bay thaws the vapouriser again.
The only other way around this is to run on petrol until the engine is
warm before switching to gas.



5.5. Can I do the conversion myself?
Yes, you can. I'd only recommend it if you are a fairly experienced
mechanic and are used to tackling large projects. As a guide line, my
conversion took more or less 3 days, by an experienced mechanic working
in a fully equipped professional garage.

If you are planning on doing the conversion yourself you can do most of
the work in stages with the vehicle driveable in between. It's only
when you start interfacing the electronics to the car, and adding the
mixer and coolant hose connections that you are going to be interfering
with the running of the vehicle.

Two words of warning. First of all, get hold of a copy of the standard
from the LPGA web site and stick to it, and secondly, get the system
professionally leak tested before the first filling.


5.6. I've heard there are different filler types. What are there?
The standard fitting in the UK is a "bayonet" style fitting. The LPGA
discourages the use of adaptors for fitting, but personally, I think
that the smaller connector used for a screw-fit adaptor makes the
fillers much more discrete. Compare my twin fillers on the bumper at on
my web site with the great hole in the side of some conversions.

The downside to an adaptor is that there is a slightly larger volume of
gas which escapes when you disconnect the filler, and it takes a few
seconds to screw in and remove the adaptor.



5.7. Is The Fuel Level Indicator always inaccurate.
Yes, it is. Learn to live with it. I have always used the trip meter as
a backup to the fuel guage anways, even though I've mostly driven
fairly new cars.


5.8. Why is there an audible "clang" from the boot when it switches to Gas?
The solenoid on the outside of the tank works a valve inside the tank
by generating a magnetic field. Inside the tank a ball bearing is
attracted by the rest of the tank, and it sounds louder because the
tank acts as a sounding board.

When you switch back to petrol (or turn the ignition off) the ball
bearing falls back into place inside, but without the same force, so
there isn't the same noise.



5.9. I got my car converted to LPG by the fuel economy sucks. I thought
this was supposed to save me money. What can I do?
There are a couple of things to check. First of all, you can turn the
lower screw on the vapouriser IN, in full turn increments, to tweak the
peak rate gas flow. This is easy and requires no special tools.


5.10. How can I tell if the engine is running too lean?
"Too lean" means that the air/fuel mixture doesn't contain enough fuel
- so there is not enough to generate the power needed.

The perfect mixture where all the oxygen and all the fuel combine will
give optimum combustion, and this is what any tuning exercise is trying
to acheive. Chemists refer to this combination as a stoichometric
mixture.

If anyone can help describe typical symptoms, this would be
appreciated.



5.11. How can I tell if the engine is running too rich?
The perfect mixture where all the oxygen and all the fuel combine will
give optimum combustion, and this is what any tuning exercise is trying
to acheive. Chemists refer to this combination as a stoichometric
mixture.

Typical Symptoms of over-lean fuel mixture include: fast erratic idle,
lack of power, engine overheating, unexpectedly good fuel consumption,
poor running when cold/poor cold starting on gas startup. If it's too
rich: "lumpy" idle (like a car with a manual choke does once warmed
up), may make black smoke, poor fuel consumption, very easy cold
starting on gas startup, and "pinking".

Note that you may not see all of these symptoms; it depends on the
setup of the particular car.

You can also check this state of affairs by using a digital voltmeter
across the Lambda sensor. The voltage on a healthy vehicle should vary
between 0.2V and 0.8V with 0.2V being no oxygen (too rich) and 0.8V
being too much residual oxygen (too lean). Bonus points (i.e., a beer)
to anyone who can devise a circuit diagram to monitor this with two
LEDs.



5.12. Is it true that I need to change the ignition leads?
LPG doesn't ignite as easily as petrol, so a good spark is needed to
get it to burn. On older vehicles, an ingition amplifier is sometimes
needed, but on modern electronic ignition systems, it's just a case of
having good quality spark plugs and ignition leads. I upgraded my car
to include Magnecor ignitions leads, which are bright blue and look
cool to anyone peering in the engine bay!



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Updates in this revision:

* Fixed outdated reference to Irish LPGA. (AndyC)
* Added explanation of Lambda sensor voltages received by E-mail

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Copyright © 2001-2003 Andy Cunningham and Austin Shackles

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