Autogas Liquefied petroleum gas intended for use in vehicle internal combustion engines (LPG, Liquefied Petroleum Gas). It’s not to be confused with compressed natural gas (CNG, Compressed Natural Gas) or liquefied natural gas (LNG, Liquefied Natural Gas) as fuel.

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Table of contents

Liquid gas as fuel

Liquid gas (butane/propane) is used as a fuel for gasoline engines. It was already very widespread in Italy in the 1970s and has remained so to this day. The entire bus fleet of Wiener Verkehrsbetriebe also runs on LPG.

It burns more environmentally friendly than gasoline. Emissions of nitrogen oxides are about 20% of gasoline combustion, CO2 emissions are reduced by 15%, and unburned hydrocarbons by 50%. In addition, LPG exhaust gases can be converted into vehicle catalytic converters at lower temperatures due to their better chemical utilization. Apart from natural gas forklifts, LPG-powered forklifts are the only vehicles with combustion engines that can be operated indoors. Diesel trucks, on the other hand, emit soot and high levels of nitrogen oxides.

In direct comparison with gasoline, depending on the gas system (see below), there is a volumetric (liters vs. kg) additional consumption of 5 to 20 % for LPG, because LPG has a lower density. For comparison: Super gasoline with 95 octane has a density of ca. 0.76 g/cm³, liquid gas (around 108 octane) between 0.51 and 0.56 g/cm³, depending on the mixing ratio. At first glance, this results in an additional consumption of 40%, but this is offset by the higher calorific value of LPG (46.1 MJ/kg (12.8 kWh/kg) versus 43.6 MJ/kg (12.1 kWh/kg)) is theoretically reduced to 33. Due to the consumption of starting gasoline (and its neglect in the consumption calculation), the theoretical additional consumption of 33 % decreases to practically 5 to 25 %, depending on the average distance. The higher the short-distance share, the lower the typical “additional consumption” from.

LPG was first used in cabs (z. B. all cabs in Istanbul), later also in the private motor vehicle sector, in the 1980s very widespread in Austria due to favorable prices. However, due to the higher taxation for private vehicles, it became uninteresting again in Austria. However, the Wiener Linien have converted all buses to run on LPG because LPG remained tax-free for public vehicles.

Today, LPG is established as a fuel in most European neighboring countries (such as B, F, GB, I, NL, PL…). In Germany, the number of passenger cars and gas stations is growing rapidly. The reason is the low tax. According to the Energy Tax Act, liquid gas (LPG/autogas) as a fuel will be taxed at 16.6 cents/kg – 1.29 cents/kWh (= approx. 9 cents/liter) until the end of 2018.

In Germany, a liter of LPG currently costs between 52 and 76 cents at the gas station (as of 15. November 2007) – although there is a clear north-south divide here. LPG filling stations exist z. Zt. 3284 (14. December 2007). By August 2008, the number of filling stations in Germany is expected to reach 4,000.

In Switzerland, liquid gas is to be tax-privileged as of 2008 and thus increase by ca. 0.30 francs (0.40 francs gasoline equivalent) cheaper. At the moment it costs between 1.20 and 1.69 francs/liter (as of 7. November 2007). [1]

Vehicle conversion

Conversion of passenger cars to liquefied petroleum gas (LPG) is relatively uncomplicated. Almost any vehicle with a gasoline engine can be converted for about 1500 to 3500 euros, depending on the number of cylinders, power and the exhaust emission standard to be achieved. The unladen weight of an LPG system is about 40 kg.

There are various installation options for the tank:

  • In the spare wheel well (34 to 85 liters), the spare wheel is then replaced by a breakdown spray.
  • In the trunk (60 to 200 liters), usually in cylinder form.
  • Underfloor tanks are also possible.

The range in gas mode is 350 to 1000 km, depending on tank size and consumption. During conversion, the gasoline tank is retained so that the vehicle can be operated with either gasoline or liquid gas (bivalent drive). Switching between gasoline and LPG operation can be done automatically or manually while driving. If the Switchover point automatically selected, this should be done by a water temperature sensor. This ensures that the switchover only takes place when the engine is at operating temperature and the liquid gas can burn optimally. The installation of the gas tank is therefore also associated with a considerable increase in range, provided that gasoline is also carried accordingly.

Some retrofitters also recommend the installation of additive admixtures in the intake tract to reduce wear on the valves and valve seats under the changed fuel type and combustion conditions, since the combustion temperatures are considerably higher than those of gasoline with the same fuel-air mixture. In some vehicles, the valve seats are much less resistant to temperature than in other vehicles, so that engine damage may occur in gas operation under certain circumstances. Higher combustion temperatures and the resulting engine damage can be avoided from the outset if an installed gas system is appropriately “richer” in the upper load range is adjusted.

Technology of LPG vehicles

Vehicles running on LPG are either started on gasoline and then, depending on the system installed, switched to gas operation by a switch or automatically to avoid any warm-up problems, or they run directly on LPG. A distinction is made between Venturi systems, sequential systems and LPI systems.

The first two types of system have in common that the pressurized liquid gas in the tank is fed to the engine in gaseous form via a vaporizer and pressure regulator. Since the gas cools down considerably during evaporation, the evaporator is heated with cooling water. For this reason, most LPG systems are only operated from approx. 30 °C cooling water temperature is switched to gas in order to prevent the gas system from icing up at low outside temperatures.

Since 2004 LPI systems are also offered. These systems use a fuel pump to convey liquid gas under pressure in a ring line, from where it is injected in liquid form through metering valves into the intake tract. Due to the heat absorbed for evaporation, there is a cooling effect of the intake air, which can be 5 to 15 Kelvin depending on the load. This results in a slight increase in power (see turbo engine/intercooler) or a lower loss of power in relation to evaporator systems, since gasoline, d. h. the original operating fuel is also injected in liquid form and the cooling effect already occurs as a result. Therefore, it is not possible to speak of a general increase in performance in relation to gasoline.

The LPG combustion takes place with lower pollutant emissions and increased running smoothness. The latter is due to the high anti-knock properties of 105 to 115 octane.

On the subject of safety, the ADAC writes: “There are no indications from practical experience that there is an increased safety risk with these vehicles, 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 various safety systems: for example, the filling line connection is equipped with a check valve that prevents gas from escaping in the event of a pipe rupture. The fuel line to the engine compartment is protected by a solenoid valve that closes immediately in the event of a loss of power. If the pressure loss is too high, the gas control unit immediately switches to 0 volts, which closes the valve. If, in the event of an accident, the vehicle’s power supply should cease to function, the solenoid valve described will be closed in any case due to the loss of power.

In case of fire, most tanks are tested up to an overpressure of 30-35 bar (downpressure ca. 60-90 bar). Depending on the type of tank (1-hole/4-hole), either a separate pressure relief valve or a pressure relief valve integrated into the multi-valve is installed. This opens at a pressure of approx. 25-28 bar, which ensures that the gas is released in a controlled manner in the event of a fire and that the tank cannot burst.

Venturi technology

Venturi technology is the oldest and cheapest solution. A vacuum-controlled metering valve is installed in the intake manifold and gas is metered in a vacuum-controlled manner. Due to the narrowing of the intake cross-section, a slight loss of power and increased consumption must be expected. In addition, this technology can lead to recombustion in the intake tract. This phenomenon, known as backfire, can occur if the gas that is constantly present in the intake tract with this technology is ignited due to a fault in the ignition system. Pressure relief valves installed in the intake manifold and/or in the air filter box, which open in the event of an explosion and allow the pressure to escape, can prevent damage caused by backfire. Venturi technology is approved up to the Euro 2 exhaust emission standard (or. z. T. also D3) without loss of a tax class suitable.

Partial sequential systems

Partially sequential systems use a more precise electronically controlled metering valve that directs the gas into the intake manifolds of the cylinders by means of a star-shaped gas distributor. There is no cross-sectional narrowing in the intake tract and thus no loss of power. These systems often have their own programmable map sensor for gas operation. Therefore, even older cars and passenger cars up to the Euro 3 emission standard can be equipped with this system.

Fully sequential systems

Fully sequential systems (currently state of the art) have a separate metering valve for each cylinder. These modern systems often no longer have their own autonomous map computer, but convert the injection map for gasoline stored in the on-board computer to equivalent gas quantities. Therefore, conversion and programming is easier, but requires sequential or group sequential gasoline injection. Modern vehicles have had this technology since the mid-1990s. The introduction of the Euro 3 and Euro 4 emission standards with EOBD (Euro On Board Diagnostics) then made sequential fuel injection mandatory. The Euro 4 emission standard is achieved without any problems or. undercut (at least that is what the retrofitters claim). In any case, exhaust gas confirmation must be obtained via the currently valid (or. The exhaust emission standard must be met by the vehicle, otherwise it will not be possible to obtain approval (TuV) in Germany (or at least not in Germany). very difficult, i.e. expensive) to obtain. A certificate of correct installation and leak test according to VDTuV 750, etc. is also required. to be demanded. (This is also necessary for the aforementioned systems and is often not available for systems installed abroad).)

LPI systems

LPI is the abbreviation for Liquid Propane Injection, which means liquid gas injection. Sequential gas injection in liquid form is probably the latest (so-called) 5. generation of automotive gas systems. Although this technology was introduced in the early 1990s, its technical implementation is still problematic. Compared to vaporization systems, these systems are generally more expensive, and the LPG pumps/tanks are relatively noisy and susceptible. The manufacturers advertise with combustion chamber cooling, since the LPG is injected liquid into the engine. Even if the LPG is injected into the intake manifold well before the intake valves of the combustion chambers and the LPG should already vaporize in the intake manifold, the charge air flow of the engine is still cooled and thus the delivery rate is increased, whereby it should not make a significant difference whether the vaporization of the LPG takes place in the intake manifold or in the combustion chamber. This does not apply to systems with evaporators. Here, the cooling effect of the evaporating LPG seeps into the cooling water and cannot be used to increase the delivery rate.

The name LPI has been registered by the Dutch manufacturer Vialle. ICOM, the manufacturer, therefore refers to the LPG injection technology as JTG.

In the case of the Vialle system, a separate map is generated by means of a separate control unit – various fields should already be predefined.

The ICOM system uses LPG injectors that are similar in characteristics to gasoline injectors. (The characteristics are not always perfectly the same, with the result that some converted vehicles run either too rich or too lean in certain load ranges in gas operation, and the slow adaptation of the engine control unit is thereby misadjusted, so that this in turn results in problems in the gasoline starting behavior.) This allows to use the injection times of the gasoline control unit, the gas control unit works only as a switch between gasoline and gas injection nozzle. There is no need to adjust the gas control unit, but the gas injectors must be calibrated when the system is installed.

Refueling technology

For refueling an LPG vehicle, three different connection systems have been introduced worldwide. Depending on the country, a corresponding adapter is required for the use of the fuel dispenser. It is the ACME connection ( Europe adapter ), screw connection), the dish connection (“Italy adapter”, Dish Coupling) and the bayonet connection (“NL-Adapter”). The plan is to have a single European connection called Euronozzle.

Overview of the connection systems currently in use:

ACME Belgium, Germany, Ireland, Luxembourg, (partly: Austria and Switzerland)
Bayonet connection Great Britain, Netherlands, Spain, Norway
Dish connection France, Greece, Italy, (partly: Austria and Switzerland), Poland, Portugal, Czech Republic, Hungary, ..
Euronozzle planned


Filling stations in Europe

In Germany there were already more than 3100 [2] autogas filling stations in November 2007. The trend is strongly upward; z.Zt. (November 2007) another 360 filling stations are planned. By July 2008, 4000 autogas refueling stations are forecasted. [3] (graphs showing the development of the number of autogas and natural gas filling stations in Germany.)

In Austria there are currently 6 public filling stations and 1 is planned. [4] . A justification to a parliamentary question from January 2006 shows why LPG is not promoted as a fuel in Austria. [5]

In the Switzerland can be refueled at 13 gas stations. Another 7 are planned. [6] Due to a tax reduction of LPG to 1. January 2008 at approx. 0.40 francs, one can assume a further expansion of filling stations.

In many European countries (the Netherlands, Belgium, Italy, Poland, the Czech Republic, France, etc.) the LPG system is used.) as well as Turkey a surface covering gas station net exists.

Service stations worldwide

You can refuel autogas in many countries. The largest consumption of LPG is in South Korea (22%), followed by Japan (9%), Turkey (8%), Mexico (8%), Australia (7%). [7] LPG is also widespread in Russia, Armenia, China, USA, Canada.

Prices and costs

Prices for LPG in Germany are from 0,52 to 0,76 € per l (on average from 0,67 € per l), abroad from 0,30 to 0,64 € per l.

When comparing prices with gasoline, however, it must still be taken into account that the consumption per liter increases by 15% on average when operating on LPG, since LPG has a considerably lower density than gasoline and thus a lower calorific value per liter. Therefore, when comparing the fuel costs between gasoline/super and LPG can be roughly assumed to be 60-70. Gasoline engines can reach or even undercut the fuel costs of an equivalent diesel engine in LPG operation.

Autogas filling stations for company fleets can be purchased from about 5.000 and supplied with about 40 to 45 cents per liter. A calibrated gas pump including data transmission for cash register systems costs about 15.000 to 20.000 €.

Billing problems when refueling

A defined Mass of gas costs z. B. in the 11 kg (net) propane gas cylinder 11 to 18 €. Parameters such as pressure and temperature are not specified. At an autogas filling station, the Volume (liters) measured. However, the density and thus the mass of the gas depends on the temperature. At a higher temperature, less gas mass is filled in. Here, there is no legal regulation at which standard pressure and standard temperature the specified number of liters applies.

Comparison of liquefied petroleum gas (LPG) and natural gas (CNG) for motor vehicles

  • Chemical
  • LPG is propane/butane, the octane number is between 105 – 115, depending on the butane content.
  • CNG consists mainly of methane (about 84 – 99 vol.-%), the octane rating is 120 – 140.
  • Physically
  • LPG is stored in liquid form at about 5 – 10 bar pressure (approx. 400 g of gaseous fuel per liter of gross tank volume).
  • CNG is stored in gaseous form at about 200 bar (approx. 160 g of expanded fuel per liter of compressed gas tank volume) and reduced to 7 bar by a high-pressure regulator.
  • Engine performance: each gas (LPG and CNG) displaces air from the cylinder, thus less oxygen is available for combustion. In the part-load range relevant for everyday use, engine performance can even increase slightly due to more homogeneous and better cylinder filling. In turbocharged engines such as turbos or supercharged vehicles, the loss of power can be compensated for by a higher boost pressure and thus a higher oxygen charge. The high octane number of well over 100 of LPG and CNG favors this measure. For classic naturally aspirated engines, a loss of power depending on the fuel must be expected with both fuels.
  • LPG: One molecule of propane consumes five molecules of oxygen during combustion, while one molecule of butane consumes six and a half molecules. Depending on the gas composition, up to 4 % of the cylinder charge consists of LPG. Old venturi systems resp. their nozzle (permissible only up to Euro 2) can cause up to 10% power loss due to additional throttling losses in the intake tract. With newer sequential evaporator systems, a power loss of up to 3.8% can theoretically be expected, but this is not noticeable in practice. A liquid-injection system cools down the air in the intake tract, improving cylinder filling (analogous to the intercooler). Therefore, often no power loss is measurable anymore or even a slight power gain can be observed.
  • CNG: One molecule of natural gas consumes two molecules of oxygen during combustion, so significantly more natural gas (typically 12%) must be metered into the cylinder, and even more atmospheric oxygen is displaced as a result. This oxygen is therefore not available for combustion and a loss in performance of 12 to 15 % must be expected. The loss of power can be especially noticeable in low-powered vehicles and high full-load start-ups.
  • Carbon dioxide emission
  • LPG CO2 emissions are 1980 g per liter.
  • LPG reduces CO2 emissions by around 15% compared with gasoline combustion.
  • CNG reduces CO2 emissions by around 25 % compared with gasoline combustion. For engines optimized for CNG combustion, greater CO2 reduction is possible by increasing compression ratio and thereby increasing efficiency. In reserve mode under gasoline, however, consumption is higher (higher than when a vehicle optimized for Super Plus has to run on regular gasoline).
  • Tank technology
  • LPG can be retrofitted in wheel well tanks without trunk loss. Cylinder tanks are available in volumes of up to more than 200 l; wheeled trough tanks are offered in volumes of up to more than 100 l, depending on the size of the trough. The net tank volume is 80 % of the gross volume. This is necessary for safety reasons (expansion in a hot vehicle). The test pressure of these tank systems (TuV) is 40 bar with usual operating pressures of 8 to 12 bar.
  • CNG is carried in cylinder tanks located in the trunk or, as is now common in series-produced vehicles, underfloor or in tanks located on the roof (commercial vehicles). CNG tanks have a test pressure of over 300 bar, the burst pressure is 600 bar.
  • Distribution
  • CNG: Currently being aggressively promoted in Germany primarily because of its better climate balance, although the number of filling stations is still lower compared with LPG (as of 1. January 2007 42.759 vehicles in Germany (source: Federal Motor Transport Authority), Switzerland, Austria, Italy, Argentina) CNG filling stations currently number 750 in Germany (as of April 2007 [2] ).
  • Distribution
  • Like gasoline and diesel fuels, LPG is usually transported by road to the filling station, thus polluting road traffic. However, the filling station operator is not necessarily bound to a regional gas supplier.
  • CNG, on the other hand, consists of natural gas and finds its way to the consumer via pipelines. Filling stations receive the natural gas from the network of the local gas supply company; the compression to the tank pressure of more than 220 bar takes place on site. Rural CNG filling stations without a connection to a natural gas network are difficult or impossible to find. expensive to supply. Here bio natural gas filling stations are a reasonable alternative.
  • Practical consequences
  • Although LPG is effectively somewhat more expensive as a fuel than CNG (December 2005 [2] ), conversions are significantly cheaper and achievable ranges generally higher due to the lower pressure and greater worldwide distribution. If a gasoline engine is retrofitted to run on gas, LPG is often the best choice for reasons of cost and space.
  • CNG is effectively slightly cheaper than LPG as a fuel (December 2005 [2] ), but requires thicker-walled and thus heavier tanks due to the higher pressure, and often allows only shorter ranges, from 240 to 300 km depending on consumption. If a vehicle is equipped with underfloor tanks ex works, ranges of more than 400 km can be achieved with a single tank of CNG. New purchases and conversions are often subsidized by the local gas utility company.

Safety regulations

LPG storage tank systems are systems requiring monitoring in accordance with the Industrial Safety Ordinance and must therefore be inspected by an approved monitoring body at regular intervals (gas system inspection) before they are put into operation. The provisions of the Ordinance on Industrial Safety and Health with regard to explosion protection must also be observed.

Special measures are required when working below ground level (cellar and garage). a.), as LPG is heavier than air and can collect as a “lake. Floor openings (manhole covers, hatches, basement outlets) must also be included in the safety consideration.

The transport of LPG is regulated by the ADR regulations.

Already since 1998 within most garage regulations (GaVo) of the individual Lands of the Federal Republic it became generally accepted that it is permitted to drive with autogas passenger cars in underground garages. Restrictions only apply in Berlin, Bremen and Saarland. However, garage owners sometimes still prohibit entry by means of information signs. In this case, the garage owner’s domiciliary rights must be observed.

Even within Austria, the regulations are not uniform, as they fall within the competence of the federal states.

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