Autogas is the common name for liquefied petroleum gas (LPG) when it is used as a fuel in internal combustion engines in vehicles as well as in stationary applications such as generators. It is a mixture of propane and butane.
Autogas is widely used as a “green” fuel, as its use reduces CO2 exhaust emissions by around 15% compared to petrol. One litre of petrol produces 2.3 kg of CO2 when burnt, whereas the equivalent amount of autogas (1.33 litre due to lower density of autogas) produces only 1.5 * 1.33 = 2 kg of CO2 when burnt. CO emissions are 30% lower, compared to petrol and NOx by 50%. It has an octane rating (MON/RON) that is between 90 and 110 and an energy content (higher heating value—HHV) that is between 25.5 megajoules per litre (for pure propane) and 28.7 megajoules per litre (for pure butane) depending upon the actual fuel composition.
Autogas is the third most popular automotive fuel in the world, with approximately 16 million of 600 million passenger cars powered using the fuel, representing less than 3% of the total market share. Approximately half of all autogas-fueled passenger vehicles are in the five largest markets (in descending order): Turkey, South Korea, Poland, Italy, and Australia.
LPG as car fuel
To add up to the confusion, autogas often gets confused with CNG or LPG in metal bottles. Despite LPG being widely used as a fuel for cars, many people still cannot grasp the idea that cars really can drive on LPG, a gas far more known for cooking or heating. This leads to three possible undesirable outcomes if you would ask random people in streets where you can buy LPG for your car.
CNG is compressed natural gas, or methane. CNG is stored under a pressure of 200 bars and should not be confused with LPG, which has an operating pressure of around 10 bars. CNG and LPG systems are not compatible.
With an iron bottle of LPG one would need a compressor pump, to get the gas in into the car’s reservoir, but that is not considered a safe operating mode.
In order not to run out of autogas in areas where the use of LPG for cars is less common, motorists who depend on LPG as a car fuel should prepare their trip well on beforehand, rather than relying on access to mobile internet or wifi, to consult databases about LPG selling points.
Since the use of LPG as a fuel for cars is less common than petrol or diesel, quite often prices of LPG are not listed on the billboard with prices for diesel and petrol, making it more difficult to find LPG when travelling in areas that you have never been to. In some countries LPG filling points are often situated at the back or in a far away corner of a gas station, making them less visible and/or hard to reach if you are towing a caravan or trailer.
When travelling through areas where the use of LPG for cars is less common, one might come across LPG-pumps that haven’t been used in weeks, LPG-pumps that are switched off, staff of gas stations who have no clue about LPG, or gas stations where the staff forgot to order the refilling of their LPG reservoir, after running empty.
In rural areas many gas stations rely on automated distribution of fuel and payment by card, to cut the cost of employees in shops. In many countries, the sale of LPG is only permitted when the staff is present at the filling station. For budget travellers who cannot afford paying the extra money for unleaded petrol, it could be wise to contact LPG filling stations on strategic spots on beforehand, to check about availability, opening hours and prices.
The different autogas systems generally use the same type of filler, tanks, lines and fittings but use different components in the engine bay. Liquid injection systems use special tanks with circulation pumps and return lines similar to petrol fuel injection systems.
There are three basic types of autogas system. The oldest of these is the conventional converter-and-mixer system, which has existed since the 1940s and is still widely used today. The other two types are known as injection systems, but there are significant differences between the two.
A converter-mixer system uses a converter to change liquid fuel from the tank into vapour, then feeds that vapour to the mixer where it is mixed with the intake air. This is also known as a venturi system or “single point” system.
Vapour phase injection systems also use a converter, but unlike the mixer system, the gas exits the converter at a regulated pressure. The gas is then injected into the air intake manifold via a series of electrically controlled injectors. The injector opening times are controlled by the autogas control unit. This unit works in much the same way as a petrol fuel injection control unit. This allows much more accurate metering of fuel to the engine than is possible with mixers, improving economy and/or power while reducing emissions.
Liquid phase injection systems do not use a converter, but instead deliver the liquid fuel into a fuel rail in much the same manner as a petrol injection system. These systems are still very much in their infancy. Because the fuel vaporises in the intake, the air around it is cooled significantly. This increases the density of the intake air and can potentially lead to substantial increases in engine power output, to the extent that such systems are usually de-tuned to avoid damaging other parts of the engine. Liquid phase injection has the potential to achieve much better economy and power plus lower emission levels than are possible using mixers or vapour phase injectors.
The fuel is transferred into a vehicle tank as liquid by connecting the bowser at the filling station to the filler fitting on the vehicle.
The type of filler used varies from country to country, and in some cases different types are used within the same country.
The four types are:
ACME thread. This type has a threaded fitting onto which the bowser nozzle is screwed before the trigger is pulled to establish a seal before fuel transfer. This type is used in Australia, US, Germany, Belgium, Republic of Ireland. Some LPG filling stations in the United Kingdom also use ACME.
‘Dutch’ Bayonet. This type establishes a gas-proof seal by a push and twist action. This type is used in the United Kingdom, Netherlands and Switzerland. Some LPG filling stations in Norway also use Bayonet. Spain was using a longer version of Bayonet when LPG was still for taxi’s only, but has switched to Euronozzle when LPG sales were made available to the general public.
‘Italian’ Dish. This type is used in Italy, France, Portugal, Poland, Czech Republic, Slovakia, Austria, Hungary, Slovenia, Croatia, Serbia, Albania, Greece, Bulgaria, Romania, Ukraine, Russia, Lithuania, Latvia, Estonia and Sweden.
Euronozzle. This new type of adaptor was developed to minimize or eliminate the small amount of gas that escapes when disconnecting the filler hose from the vehicle. The Euronozzle is (or was) supposed to become a unified new filling system for the entire European continent, but investments in such a change-over have failed to start. As of 2018 Spain remains the only country in Europe that has adopted the Euronozzle adaptor, a decision that was made when Spain had to redevelop a filling station network for LPG from practically zero.
Adaptors that allow a vehicle fitted with a particular system to refuel at a station equipped with another system are available.
The fill valve contains a check valve so that the liquid in the line between the filler and the tank(s) does not escape when the bowser nozzle is disconnected.
In installations where more than one tank is fitted, T-fittings may be used to connect the tanks to one filler so that the tanks are filled simultaneously. In some applications, more than one filler may be fitted, such as on opposite sides of the vehicle. These may be connected to separate tanks, or may be connected to the same tanks using T-fittings in the same manner as for connecting multiple tanks to one filler.
Fillers are typically made of brass to avoid the possibility of sparks when attaching or removing the bowser that might occur if steel fittings were used.
Hoses, pipes and fittings
The hose between the filler and tank(s) is called the fill hose or fill line. The hose or pipe between the tank(s) and the converter is called the service line. These both carry liquid under pressure.
The flexible hose between the converter and mixer is called the vapour hose or vapour line. This line carries vapour at low pressure and has a much larger diameter to suit.
Where the tank valves are located inside an enclosed space such as the boot of a sedan, a plastic containment hose is used to provide a gas-tight seal between the gas components and the inside of the vehicle.
Liquid hoses for LPG are specifically designed and rated for the pressures that exist in LPG systems, and are made from materials designed to be compatible with the fuel. Some hoses are made with crimped fittings, while others are made using re-usable fittings that are pressed or screwed onto the end of the hose.
Rigid sections of liquid line are usually made using copper tubing, although in some applications, steel pipes are used instead. The ends of the pipes are always double-flared and fitted with flare nuts to secure them to the fittings.
Liquid line fittings are mostly made from brass. The fittings typically adapt from a thread in a component, such as a BSP or NPT threaded hole on a tank, to an SAE flare fitting to suit the ends of pipes or hoses.
Vehicles are often fitted with only one tank, but multiple tanks are used in a some applications. In passenger car applications, the tank is typically either a cylindrical tank, made from steel, mounted in the boot of the vehicle or a toroidal tank (also steel) or set of permanently interconnected cylinders placed in the spare wheel well. In commercial vehicle applications, the tanks are generally cylindrical tanks mounted either in the cargo space or on the chassis underneath the body. Increasingly, the tank is an aluminium Conformable Tank, which is lighter, has more capacity and cannot rust.
The tanks have fittings for filling, liquid outlet, emergency relief of excess pressure, fuel level gauge and sometimes a vapour outlet. These may be separate valves mounted into a series of 3 to 5 holes in a plate welded into the tank shell, or may be assembled onto a multi-valve unit which is bolted into one large hole on a boss welded, or in the case of an aluminium tank, extruded as part of the tank shell.
Modern fill valves are usually fitted with an automatic fill limiter (AFL) to prevent overfilling. The AFL has a float arm which restricts the flow significantly but does not shut it off entirely. This is intended to cause the pressure in the line to rise enough to tell the bowser to stop pumping but not cause dangerously high pressures. Before AFLs were introduced, it was common for the filler (with integral check valve) to be screwed directly into the tank, as the operator had to open an ullage valve at the tank while filling, allowing vapour out of the top of the tank and stopping filling when liquid started coming out of the ullage valve to indicate that the tank was full. Modern tanks are not fitted with ullage valves.
The liquid outlet is usually used to supply fuel to the engine, and is usually referred to as the service valve. Modern service valves incorporate an electric shut-off solenoid. In applications using very small engines such as small generators, vapour may be withdrawn from the top of the tank instead of liquid from the bottom of the tank.
The emergency pressure relief valve in the tank is called a hydrostatic pressure relief valve. It is designed to open if the pressure in the tank is dangerously high, thus releasing some vapour to the atmosphere to reduce the pressure in the tank. The release of a small quantity of vapour reduces the pressure in the tank, which causes some of the liquid in the tank to vaporise to re-establish equilibrium between liquid and vapour. The latent heat of vaporisation causes the tank to cool, which reduces pressure even further.
The gauge sender is usually a magnetically coupled arrangement, with a float arm inside the tank rotating a magnet, which rotates an external gauge. The external gauge is usually readable directly, and most also incorporate an electronic sender to operate a fuel gauge on the dashboard.
There are a number of types of valve used in autogas systems. The most common ones are shut-off or filter-lock valves, which are used to stop flow in the service line. These may be operated by vacuum or electricity. On bi-fuel systems with a petrol carburettor, a similar shut-off valve is usually fitted in the petrol line between the pump and carburettor.
Check valves are fitted in the filler and on the fill input to the fuel tank to prevent fuel flowing back the wrong way.
Service valves are fitted to the outlet from the tank to the service line. These have a tap to turn the fuel on and off. The tap is usually only closed when the tank is being worked on. In some countries, an electrical shut-off valve is built into the service valve.
Where multiple tanks are fitted, a combination of check valves and a hydrostatic relief valve are usually installed to prevent fuel from flowing from one tank to another. In Australia, there is a common assembly designed for this purpose. It is a combined twin check valve and hydrostatic relief valve assembly built in the form of a T-fitting, such that the lines from the tanks come into the sides of the valve and the outlet to the converter comes out the end. Because there is only one common brand of these valves, they are known colloquially as a Sherwood valve.
The converter (also known as vaporiser or reducer) is a device designed to change the fuel from a pressurised liquid to a vapour at around atmospheric pressure for delivery to the mixer or vapour phase injectors. Because of the refrigerant characteristic of the fuel, heat must be put into the fuel by the converter. This is usually achieved by having engine coolant circulated through a heat exchanger that transfers heat from that coolant to the LPG.
There are two distinctly different basic types of converter for use with mixer type systems. The European style of converter is a more complex device that incorporates an idle circuit and is designed to be used with a simple fixed venturi mixer. The American style of converter is a simpler design which is intended to be used with a variable venturi mixer that incorporates an idle circuit.
Engines with a low power output such as; scooters, quad bikes and generators can use a simpler type of converter (also known as governor or regulator). These converters are fed with fuel in vapour form. Evaporation takes place in the tank where refrigeration occurs as the liquid fuel boils. The tanks large surface area exposed to the ambient air temperature combined with the low power output (fuel requirement) of the engine make this type of system viable. The refrigeration of the fuel tank is proportional to fuel demand hence this arrangement is only used on smaller engines. This type of converter can either be fed with vapour at tank pressure (called a 2-stage regulator) or be fed via a tank mounted regulator at a fixed reduced pressure (called a single stage regulator).
The mixer is the device that mixes the fuel into the air flowing to the engine. The mixer incorporates a venturi designed to draw the fuel into the airflow due to the movement of the air.
Mixer type systems have existed since the 1940s and some designs have changed little over that time. Mixers are now being increasingly superseded by injectors.
Vapour phase injectors
Most vapour phase injection systems mount the solenoids in a manifold block or injector rail, then run hoses to the nozzles, which are screwed into holes drilled and tapped into the runners of the intake manifold. There is usually one nozzle for each cylinder. Some vapour injection systems resemble petrol injection, having separate injectors that fit into the manifold or head in the same manner as petrol injectors, and are fed fuel through a fuel rail.
Liquid phase injectors
Liquid phase injectors are mounted onto the engine in a manner similar to petrol injectors, being mounted directly at the inlet manifold and fed liquid fuel from a fuel rail.
Electrical and electronic controls
There are four distinct electrical systems that may be used in autogas systems – fuel gauge sender, fuel shut-off, closed loop feedback mixture control and injection control.
In some installations, the fuel gauge sender fitted to the autogas tank is matched to the original fuel gauge in the vehicle. In others, an additional gauge is added to display the level of fuel in the autogas tank separately from the existing petrol gauge.
In most modern installations, an electronic device called a tachometric relay or safety switch is used to operate electrical shut-off solenoids. These work by sensing that the engine is running by detecting ignition pulses. Some systems use an engine oil pressure sensor instead. In all installations, there is a filterlock (consisting of a filter assembly and a vacuum or electric solenoid operated shut-off valve) located at the input to the converter. In European converters, there is also a solenoid in the converter to shut off the idle circuit. These valves are usually both connected to the output of the tachometric relay or oil pressure switch. Where solenoids are fitted to the outputs of fuel tanks, these are also connected to the output of the tachometric relay or oil pressure switch. In installations with multiple tanks, a switch or changeover relay may be fitted to allow the driver to select which tank to use fuel from. On bi-fuel, the switch used to change between fuels is used to turn off the tachometric relay.
Closed loop feedback systems use an electronic controller that operates in much the same way as in a petrol fuel injection systems, using an oxygen sensor to effectively measure the air/fuel mixture by measuring the oxygen content of the exhaust and control valve on the converter or in the vapour line to adjust the mixture. Mixer type systems that do not have a closed loop feedback fitted are sometimes referred to as open loop systems.
Injection systems use a computerised control system which is very similar to that used in petrol injection systems. In virtually all systems, the injection control system integrates the tachometric relay and closed loop feedback functions.
Optional valve protection
Many LPG equipment installers recommend the installation of so-called valve protection systems. These can consist in the most simple case of a bottle containing valve protection liquid. The liquid is drawn into the air intake system and distributed into the engines’ cylinders along with the fuel and air.
More sophisticated systems can consist of a piggyback ECU that is synchronised with the LPG injector ECU. This results in a more precise injection of valve protection fluid.
Converter-and-mixer system operation
The designs of converters and mixers are matched to each other by matching sizes and shapes of components within the two.
In most areas of the world the word “converter” is not commonly used. ‘Regulator’ or ‘reducer’ or ‘vaporizer’ are more popular.
Because it has 3 main functions:
Reducer: reduces the high pressure of incoming liquid phase LPG down to atmospheric pressure.
Regulator: regulates the gas flow according to the requirement of the engine.
Vaporizer: vaporizes the liquid form LPG into gas form by using the hot coolant circulation of the engine.
In European style systems, the size and shape of the venturi of the carburettor is designed to match the converter. In USA style systems, the air valve and metering pins in the mixer are sized to match the diaphragm size and spring stiffness in the converter. In both cases, the components are matched by the manufacturers and only basic adjustments are needed during installation and tuning.
An autogas carburettor may simply consist of a throttlebody and a mixer, sometimes fitted together using an adapter, the venturi is not needed.
Cold start enrichment is achieved by the fact that the engine coolant is cold when the engine is cold. This causes denser vapour to be delivered to the mixer. As the engine warms up, the coolant temperature rises until the engine is at operating temperature and the mixture has leaned off to the normal running mixture. Depending on the system, the throttle may need to be held open further when the engine is cold in the same manner as with a petrol carburettor. On others, the normal mixture is intended to be somewhat lean and no cold-start throttle increase is needed. Because of the way enrichment is achieved, no additional choke butterfly is required for cold starting with LPG. Some evaporators have an electric choke valve, energising this valve, before starting the engine, will spray some LPG vapour into the carburetor to help cold start.
The temperature of the engine is critical to the tuning of an autogas system. The engine thermostat effectively controls the temperature of the converter, thus directly affecting the mixture. A faulty thermostat, or a thermostat of the wrong temperature range for the design of the system may not operate correctly.
The power output capacity of a system is limited by the ability of the converter to deliver a stable flow of vapour. A coolant temperature lower than intended will reduce the maximum power output possible, as will an air bubble trapped in the cooling circuit or complete loss of coolant. All converters have a limit, beyond which mixtures become unstable. Unstable mixtures typically contain tiny droplets of liquid fuel that were not heated enough in the converter and will vaporise in the mixer or intake to form an excessively rich mixture. When this occurs, the mixture will become so rich that the engine will flood and stall. Because the outside of the converter will be at or below 0 °C when this happens, water vapour from the air will freeze onto the outside of the converter, forming an icy white layer. Some converters are very susceptible to cracking when this happens.
LPG injection for diesel vehicles
LPG may be used for a supplemental fuel for diesels of all sizes. Diesel contains 128,700 BTU per US gallon, where propane contains 91,690 BTU per US gallon. If LPG is 30-40% less expensive, there may very well be a saving. Any actual savings are dependent on the relative cost of diesel versus LPG. In Australia, where diesel costs substantially more than LPG, savings of 10 to 20% are claimed.
The above systems add small quantities of LPG with the primary aim of improving economy, but much larger quantities of LPG can be injected in order to increase power. Even at full output a diesel engine runs about 50% lean of stoichiometric to avoid black smoke production, so there is a substantial amount of oxygen in the intake charge which is not consumed in the combustion process. This oxygen is therefore available for the combustion of a substantial addition of LPG resulting in a large increase in power output.
Concerning safety risks, the ADAC writes: “There is no practical evidence that these vehicles present an increased safety risk, even from those countries where a relatively large number of LPG cars are registered. Crash – and fire tests show that LPG cars are no more dangerous than comparable gasoline vehicles. “Autogas tanks and their pipe connections are equipped with different safety systems: So is the filling line connection with a check valveprovided, which prevents the escape of gas at a Rohrabriss. The transport line into the engine compartment is secured with a magnetic valve directly at the tank removal, which immediately closes when the power supply is interrupted. If the pressure loss is too high, the gas control unit interrupts the power supply to the solenoid valve. Should the vehicle power supply no longer work in the event of an accident, then the solenoid valve described is definitely closed due to the lack of power supply.
In the case of a fire, most tanks are tested up to an overpressure of (30… 35) bar (bursting pressure approx. 60… 90 bar). Depending on the tank type (1-hole / 4-hole), either a separate pressure relief valve or a pressure relief valve integrated in the multi-valve is installed. This opens at a pressure of approx. (25… 28) bar, which ensures that the gas is drained in a controlled manner in case of fire and the tank can not burst. For increased security, a redundant safety valve can also be installed. This consists of a total of 2 pressure relief valves, z. B. one in the primary multi-valve and another in the secondary multi-valve, or 2 pressure relief valves in the main multi-valve.
Venturi technology is the oldest and cheapest solution. Here, a Venturi nozzle is mounted in front of the throttle in the intake passage, which automatically adds gas to the intake air, which is requested from a vacuum-controlled evaporator. The working principle is similar to a carburetor. In principle, this technology works without any regulation, only the evaporator is adjusted to a specific fuel-air mixture. Currently regulated Venturi systems still have a control unit, the u. a. the existing lambda probeevaluates and optimizes the mixture by fine control of the amount of gas. Due to the inherent constriction of the intake cross-section, slight loss of power and increased consumption are to be expected in Venturi plants. Many air flow meters work on older vehicles according to the same principle, above all, the experience of Umrüsters is in demand. In addition, it can come with this technique to a back-burning in the intake system. However, this phenomenon known as backfire is not a coincidence, it only occurs in case of faulty technology, eg. B. by too lean or much too rich mixture (analogous to carburetor), by defective or worn ignition systems or leaking inlet valves. In the intake manifold and / or in the air filter box built-in pressure relief valves, Those who open in the event of an explosion and release the pressure can prevent damage from Backfire. The (regulated) venturi technique is up toEmission standard Euro 2 (or sometimes also D3) without loss of tax code.
Partial Sequential Plants
Partial sequential systems use an electronically controlled metering valve, which injects the gas by means of a star-shaped gas distributor in the intake manifold of the cylinder. A cross-sectional constriction in the intake tract and thus a loss of performance does not take place. Likewise, the backfire risk is lower because the gas is supplied immediately in front of the inlet valves and therefore there is no relevant ignitable mixture in the intake system. These systems often have their own programmable map generator for gas operation, which merely picks up the signal from the lambda probe, the speed (eg camshaft sensor) and the throttle position from the vehicle. Therefore, even older vehicles up to the emission standard Euro 3 can be equipped with this system. However, the semi-sequential investment is now offered quite rarely. These are compared to a Venturi considerably more expensive to purchase and more complicated to set by the map to be programmed. Therefore, the price often exceeds the residual value of the vehicle to be converted.
Fully sequential investments
Fully sequential plants (state of the art in 2009) have their own metering valve per cylinder. These modern systems often no longer have their own autonomous map calculator, but convert the determined by the gasoline control unit injection duration under gasoline in an equivalent injection duration for gas. Instead of the gasoline nozzle, a gas nozzle is actuated, the gas control unit determines only pressure and thus load-dependent correction factors. Therefore, retrofitting and above all programming is easier, but requires existing sequential or group sequential gasoline injection. Modern vehicles have had this technology since the mid-1990s. The introduction of emission standards Euro 3 and Euro 4 with EOBD (Euro- On-Board Diagnosis) then made the sequential gasoline injection mandatory. The emission standard Euro 4 is easily reached or undercut (manufacturer information). In any case, an emission confirmation on the currently valid (or the vehicle corresponding) to require exhaust emission standard, otherwise a decrease (TÜV) in Germany is not (or very difficult, so expensive) to obtain. Likewise, a certificate of correct installation and tightness test in accordance with VDTÜV 750, etc. is required. (This is also necessary for the aforementioned systems and often not available for installations installed abroad).
LPI is the abbreviation for L iquid P ropane Injection and translated means liquid propane injection, ie LPG injection. The sequential gas injection in liquid form probably represents the newest (so-called) 5th Generation of the autogas systems. This technique was already introduced in the early 1990s. These systems are usually a bit more expensive compared to evaporation systems. The LPG pumps and tanks are relatively noisy and were then vulnerable in the first series versions. Meanwhile, there are special LPG pumps which have been homologated according to the applicable ECE 67R-01 directive and are designed for operation with LPG. Since the pump is also a separate component of a LPG system, it must be marked with a corresponding test number according to 67R-01. Only this is the approval of the pump for LPG recognize beyond doubt.
The manufacturers advertise with combustion chamber cooling, since the LPG is injected liquid into the engine. Even if the autogas is injected into the intake manifold clearly before the intake valves of the combustion chambers and the LPG should evaporate already in the intake manifold, the charge air flow of the engine is nevertheless cooled by the heat required for the evaporation and thus the degree of delivery is increased. This does not apply to systems with evaporators. Here, the necessary heat of evaporation is taken from the cooling water and can not be used to increase the degree of delivery.
The ICOM system uses LPG injectors whose characteristics are similar to gasoline injectors. As a result, the injection times of the gasoline control unit can be used. The gas control unit works only as a switch between gasoline and gas injector. Only the gas injection nozzles must be calibrated during installation. A complicated setting of the gas control unit is thereby eliminated as in the evaporator systems. Further advantages are the non-existent maintenance costs for filter replacement or readjustments of the software for the driving operation.
The refueling of a vehicle with LPG must be done under pressure, so that the fuel remains liquid. To make a pressure-tight connection exist several connection systems (ACME, DISH, bayonet, Euronozzle); suitable LPG tank adapters are usually kept in the vehicle, but can also be borrowed at many petrol stations.
The handle of the fuel nozzle is locked in the open position after the connection. As a result, not only (as with other fuels), the line is opened, but also made a tight seal between the sealing elements of each terminal. Refueling does not begin until a deadman’s button on the pump is pressed and held (occasionally a footswitch is also offered for this feature). This should ensure a continuous supervision of the refueling process. By means of a level indicator in the tank, the filling process is automatically ended at maximum filling.
Source from Wikipedia