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Introduction to lpg as an automotive fuel and a guide to dif

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Old Oct 7th, 2011, 21:30   #1
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Default Introduction to lpg as an automotive fuel and a guide to dif

I wrote this quite some time ago when I was working mainly with my dad but the information is still relivant so reposting it here. The photos don't show right now but I will sort that later

What is lpg?
LPG is a natural gas and is available from two sources. It is an associated gas and exists wherever crude oil is to be found and therefore the greatest source of this energy is directly at the oil wellhead. Its second source is at the refinery where LPG is released from crude oil during the “cracking” process.
LPG consists of two individual gases each with own properties and uses. The two gases, propane and butane, can be used individually or mixed together as a blend and due to the fact that LPG is comparable with fully
vaporised patrol in general no alterations are necessary to a vehicle’s engine. LPG can be introduced directly to an engine with little or no difficulties.
LPG is transported as a liquid in order to gain the best advantage with regard to the vehicle range. To achieve this is carried in a pressure vessel, the pressure inside the storage tank under normal ambient conditions being
approximately 10 bar. When released into the atmosphere, LPG has an expansion factor of 250 times the volume of the storage tank in actual energy terms.
The LPG properties vary slightly from those normally associated with petrol. LPG is a dry gas and as such it has certain insulating properties. It will be found therefore that a higher voltage is necessary at the spark plug in
order to generate a spark. This increase in voltage can be as high as 10 – 15% above the level required for petrol operations. Modern ignition systems are kept in good working order to facilitate good running and starting characteristics on LPG.
LPG has a high octane number and slower combustion speed compared to petrol.

The gas fuelled engine pre-dates the horseless carriage in that the first 4 stroke engine was gas fuelled. During World War II numerous petrol engined vehicles were converted to run on town gas utilising large fabric bags on the roof of the vehicle to store the gas. It was also known for people unable to obtain petrol to have a cylinder of propane by the drivers seat with a rubber pipe running to the dry carburetor manually adjusting the fuel supply. Some enterprising individuals modified a variable regulator to become a demand one. This enabled butane or propane to be used in a crude fashion and became the forerunner to today's lpg systems. By the 1970s lpg had become very popular as an alternative fuel to petrol with some manufacturers such as Volvo offering it as a factory option. The system used was a basic mixer type system (also known as induction system).

So how do I use lpg and how does it work on my Volvo?
All lpg systems fill in the same basic way. The vehicle will be equipped on the outside with an lpg filling point.


To fill the vehicle the filling gun is connected to the fill point and locked on to it making a gas tight seal. The button on the pump is then held pushed in until the desired amount is reached or it automatically cuts off when the tank reaches the 80% full point. the filler is unlocked and removed. During this a small puff of gas comes out as the seal is broken, (Note: there are four different fill points used globally:
Dutch Bayonet - UK, Holland
Italian claw - Italy, France, Poland
Acme - Eire, Germany, Spain, USA & Australia
Todo - marine use only
For automotive use adaptors are available for travelling abroad.)
The fill point has a non-return valve in it to stop gas flowing back past it. A filling pipe from this goes to the tank, the gas goes through a non-return valve into the tank. The tank is a pressure vessel often shaped like a gas bottle or the shape of a wheel to fit the wheel well. The tank is constructed of thick steel to controlled standards and firmly secured to the vehicle structure in an approved manner.


The tank has a valve arrangement with the following features:
1) inlet valve with non-return and 80% capacity fill (to allow for expansion of gas)
2) pressure relief valve (to stop tank exploding from overheating or in a fire)
3) level gauge
4) outlet with electric safety shut off
From the tank outlet there is a supply pipe to the engine bay where it connects to another safety shut off solenoid which is then connected to the lpg evaporator/regulator often referred to as the vapouriser. This is connected to the engine's heater circuit to supply it with enough heat to fully vapourise the liquid gas without freezing. The vapouriser reduces the gas pressure from its 150 psi liquid form to a low pressure vapour. From here the vapourised gas is introduced to the engine.

How do I run the engine on lpg?
On the dashboard there will be a control switch which enables you to switch between petrol and lpg. The switch usually incorporates the lpg level gauge. When it is petrol mode the engine starts and runs on petrol normally. When in gas mode it will cut off the petrol so it can run on lpg although it may start on petrol prior to automatically switching.

There are various types of lpg systems:

Mixer Ring
On the mixer ring system the vapouriser has a diaphram or diaphrams that control internal valves when the diaphrams are moved by a varying suction force. This suction comes from the vacuum created in the mixer which is mounted in the carburetor mouth or throttle body. The mixer is fitted with a venturi (as per carburetor choke) that increases the air speed by means of creating a restriction that causes a vacuum behind it. The vacuum draws in the gas from the vapouriser. The more air that passes the venturi the more gas it sucks in. Control valves are used to adjust the flow rate to suit the vehicle.

Closed Loop Mixer Ring
To meet the demands of the modern engine (late 1980s onwards) lpg manufacturers utilised the lambda signal to operate a variable valve to replace the manual valve which allowed continual gas ecu fuel adjustments.
(Lambda sensors are fitted to the exhausts on modern fuel injected engines to measure fuel mixture.) Mixer systems are ideal for old carburetor engines and still quite suitable for the earlier petrol injection engines. They are commonly fitted in today's markets. This would apply to all early Volvos up to the 940s (excluding turbos)
1) The main flaw with these systems is backfires (can be minimised by ensuring all engine electrics are kept in good condition and mixture kept correctly set). Don't forget even a petrol fuelled carburetor engine can backfire.
2) For a more modern engine the open loop system is totally unsuitable and the closed loop is likely to incur problems in post 2000 cars. It is not approved for fitment to these or earlier cars with plastic manifolds. We do not recommend mixer systems for any turbo car.
3) Not quite as fuel efficient as newer injection systems.

Gas Injection
This development was required due to the backfire issues with the mixer ring and the common introduction of plastic manifolds which could be shattered by backfires and also the more stringent requirements of the more modern injection systems. Instead of introducing gas at the throttle body gas injection introduces the gas at the bottom of each inlet duct via small pipes combined with an electronic control systems.
There are four different gas injection systems:
Distributor - Method of operation: The first generation of multipoint gas injection systems was the distributor system as used by Volvo and Vauxhall as OE fitment. As per all vapour injection systems which have followed it uses the same rear end tank system as traditionally used.
The first notable change was the vapouriser which instead of working on suction pushes vapour out under pressure (1-2 bar approx). The gas travels from the vapouriser via a large bore pipe to an electronic solenoid/stepper motor that pulses up and down quickly to adjust the fuelling. Out of the other end of the stepper motor is a pipe per cylinder which introduces the gas to each cylinder. As per closed loop mixer system it is controlled by an electronic control ecu which is driven by engine rpm and lambda readings. Further adjustment of the system also incorporates corrections for gas temperature and could hold off changeover to lpg until engine has reached a preset temperature.
Flaws with system:
1) Engine management system problems with fuel trims as lpg system is unable to read petrol injection times and keep these within trim. This can result in poor starting on petrol, poor running on petrol after initial switch back and also engine check light issues.
2) The engine running may not always be as smooth as kinds due to not being able to introduce fuel at the correct times as this is a group injection system.
3) Very time consuming for the installer to set-up. Quite often resulted in cars being given to customers running poorly.
4) Reliability problems due to fine control solenoid/stepper motor gumming up, wearing and causing very bad running problems. This is very common even on factory fit systems and can be expensive to repair.
Liquid injection - method of operation:
This was a completely new system with nothing in common with any previous gas system. It is used mainly by Ford on their factory conversions. The system is very different in that the gas is pumped out of the tank under high pressure to the gas injectors in the manifold. The ecu controlled lpg injection fire the lpg into the inlet ducts in its liquid state. All other systems introduce gas as a vapour.
1) These systems have been renowned for running problems and given lpg a bad name.
2) The main total failure cause has been the in-tank pump with replacement cost in excess of £1000.
3) Not many people know the system and Ford offer no support.
The future: There is a possibility that development could resolve many of the issues with this system and it is possible it could be a common usage system in the future. (Jan 2008).

First generation multipoint vapourising lambda controlled:
As per the distributor system the vapouriser pushes out gas under low pressure The big changes with this system is the gas is delivered by separate injectors, one per cylinder.


The ecu for this type of system was totally new and was the first system to have any integration with the petrol injection times. It could read one of the petrol injectors which it could use as a trigger point for firing the lpg injectors and could also be used in a bid to control petrol ecu's trims. This was done by reading petrol injection times to build a map and sending false readings to via the lambda probe to adjust fuel trims. The lpg fuelling is controlled by a compensated fuel map in that there is a table of rpm and manifold pressure. this system is also capable of making further minor adjustments for variation in temperature and lambda readings. This was the first system worthy of use on turbo engines. This system still has a place in today's market.
1) This system is not very installer friendly because it needed mapping to each carby the installer. Not everyone had the necessary skills to complete this totally successfully.
2) The petrol ecu integration was not easy to do and not always successful in keeping good petrol running and satisfying the petrol management system.
3) A lot of these early systems had reliability problems with injectors as this was new technology.

Second generation multipoint vapourising lambda controlled:
These use primarily the same parts as the first generation. They were a big turnaround for converting OBD & OBDII compliant vehicles. This was achieved by reading each individual petrol injector and using the signal to drive the corresponding lpg injector. By using the system the lpg was fully integrated to the petrol system with the petrol ecu effectively running the gas. For this kind of system the gas ecu is totally different to the first generation. The petrol injectors were cut by the ecu rather than separate emulators and by tables able to convert petrol injection times to the correct lpg injection times required by the engine. Installation made much easier for the installer. Initial set-up is automated with each update by the manufacturer making it more and more accurate due to continual development of the systems. All the installer need then do is fine tune the calibrations. Most of these systems no longer use the lambda for corrections as the petrol ecu takes care of this. Only a few of these latest systems use the lambda as a means to alter the mapping. To achieve the correct mixture the ecu has a basic map of rpm over petrol injection time. As fine tuning for this ecu will compensate for engine temperature, gas vapour temperature and gas pressure at the injectors. The other new feature that comes with this is the gas pressure sensor that enables auto switch back to petrol under low gas pressure conditions (empty tank or extreme load demand). This feature removes the risk of engine damage from running to lean.
Although many of the different manufacturers have had various quality issues there is only one major problem - COST.

Safety features:
The modern lpg converted car is extremely safe. To prevent the escape of gas while the engine is not running their is a lock off solenoid on the tank to stop any gas coming out the tank, a lock off solenoid in the engine bay to isolate the pipe. These are controlled by the switch that requires ignition key live and engine rpm signal to maintain the solenoids in the open position. In the event of the engine stalling the gas will shut off. Should the pipe be fractured the tank will automatically be shut off.
In the event of a fire the gas in the tank will expand and increase in pressure. When the pressure nears the maximum safe limit the pressure relief valve will blow off the excess pressure. This prevents any explosion.
Tanks fitted to the inside of a vehicle are fitted with dual containment on the pipe and the valves are enclosed in an airtight box vented to outside. Should there be a leak the escaping gas will be ducted to outside away from the exhaust. All other pipework has to be fitted outside the vehicle adequately secured and protected.

There are still systems being produced and sold in the UK which may meet the requirements of the average commuter but will not meet the requirements of an enthusiast. The most common issues occurring to date are unacceptable injector life and performance. With higher power engines many of the vapourisers supplied have insufficient flow to supply enough fuel under high loads. For the performance enthusiast you will need a system that has a vapouriser with sufficient fuel supply and delivery rate, combined with injectors that are capable of delivering the required dose of fuel. Most running issues are down to some installers failing to rigidly adhere to manufacturers installation specifications. These two factors can make the choice of installer and equipment difficult for the ordinary customer.

Mike and Dai
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Old Oct 8th, 2011, 05:44   #2
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Default sticky

i think this post deserves to become a sticky.

great write up dai
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Old Oct 8th, 2011, 15:16   #3
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Originally Posted by brodgar View Post
i think this post deserves to become a sticky.

great write up dai
Hear hear

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Old Dec 22nd, 2011, 23:23   #4
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Gr8 write up and balanced information:
Honesty is not a Spare Wheel that you pull out when in trouble. It's like a Steering Wheel that keeps you on the right path if controlled properly throughout the life's journey...
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Old Jan 25th, 2012, 17:51   #5
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Very informative. Thanks.
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Old Mar 20th, 2012, 14:16   #6
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Many thanks for this very useful post. I made an appointment earlier today with a dealer to go along this afternoon to look at a phase1 V70 classic 170bhp fitted with lpg, non-factory after market system, pristine, full service history and very low mileage, they have all the paperwork present and say that it runs perfectly on lpg

However, I am wary, this is a two grand car (a bit different to the 240 converted to gas I almost bought last week for £700, would have bought it if the huge tank hadn't been mounted in the estate)

Having read of the 'unacceptable injector life' and seen the high costs of repairs to these systems, I'm going to give this beautiful car a miss and continue to look for a phase 1 diesel
[IMG]Volvo2 by Strider'swoman, on Flickr[/IMG]

Current '96 945 2.3 lpt - Aurigas, tailgate spoiler, sports grille, lpg fuelled
Previous '88 764 TD, '92 945 TD, '88 745, '81 244 DL
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Old Mar 21st, 2012, 23:12   #7
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Originally Posted by Laney760 View Post

Having read of the 'unacceptable injector life' and seen the high costs of repairs to these systems, I'm going to give this beautiful car a miss and continue to look for a phase 1 diesel
The short injector live is limited to a small number of makes and production dates. Replacment injectors are not that greatly expensive so nothing to worry about.

It is only the factory fit systems that are expensive to reair but they do not actualy have any injectors.

You are never going to get close to the Low running costs of LPG with a dirty DERV.
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Old Oct 14th, 2012, 08:36   #8
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cheap injectors are only about £30 for four i have them fitted to my wifes rover 25 they are not brilliant but they are ok aparently they only last 40000 miles but for £30 im not all that bothered
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Old May 19th, 2014, 19:47   #9

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Was reading here:
and one of the issues that it mentions is additional lube to stop the valves burning.
I read somewhere else that this valve burning problem shortens engine life.
Any suggestions on how to avoid/mitigate this?
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Old May 19th, 2014, 20:29   #10
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Valves will only burn if the fuel mixture is very lean (on petrol or LPG)

Some engines suffer with micro welding of the valve to the seat that results in the faces wearing away and the valve traveling further and further up until the valve clearance is fully closed up and beyond that the valve will not even close.

There are number of products to stop/reduce this effect with the most common being Flashlube
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