Alternative fuels: Types and their characteristics

It will be a long time before we are all travelling in electric cars or under hydrogen power. If Germany is to reach its ambitious goal of climate neutrality by 2045, alternative fuels will play an important role. Some of them are already widely used today in their pure form or as a mixture with fossil diesel and petrol fuels. Since 2007 the German Biofuel Quota Act (BioKraftQuG) has even stipulated a minimum share of biofuels as a percentage of total fuel sales in Germany.

Vegetable oils
Production/base materials
  • Pure oils pressed from plants, largely untreated. 
  • Mostly rapeseed; also soya, corn and sunflowers.
  • Oils are pressed directly from the plants. 
  • They are also extracted from the remaining press cake with solvent. 
  • This is followed by filtering and cleaning, which reduces solids and also components such as calcium, magnesium and phosphorus.
  • Not available at conventional filling stations. 
  • Usually only available from the producer (farmer). Manageable logistics.
  • Vegetable oils are much more viscous (thicker) than diesel and are not suitable as a petrol substitute.
  • They are harder to ignite and deliver slightly less energy per litre than diesel.
  • They must be preheated in winter. Up to 10% fossil diesel is often added to improve viscosity and ignition characteristics.
  • DIN 51623 Fuels for vegetable oil compatible combustion engines.
  • In tractor and agricultural machinery engines, in stationary CHP plants, rarely in truck engines.
  • Almost all types of diesel engines have to be “converted” (other seals, filters, injection system, etc.).
  • Manufacturer approval for running on vegetable oil should be available.
  • Operated with SAE 10W-40 engine oils - as for diesel engines. 
  • Significantly reduced oil change intervals. Oil service life only 25-30% (up to about 250 hours) compared to using diesel.
  • Vegetable oil does not fully burn, especially when the engine is cold. Highly viscous components condense and contaminate the engine oil as uncombusted vegetable oil. 
  • Engine oil may be mixed with a maximum of 5% vegetable oil. Otherwise there is a risk of piston ring sticking and deposits.
  • Oil analysis to determine vegetable oil content and oxidation characteristics are absolutely essential.



Biodiesel – FAME – RME
Production/base materials
  • Biodiesel is often referred to as fatty acid methyl ester (FAME) or RME (rapeseed methyl ester).
  • Based on oils and greases of vegetable or animal origin. 
  • These are “transesterified” into biodiesel in refineries specially designed for this purpose with the aid of methanol. 
  • B100 (pure biodiesel with 100% biodiesel) is available at many truck filling stations and service stations as it can be transported in tankers without intermediate cleaning.
  • Only a few refineries specialise in production, so availability is not unlimited.
  • Also sold at special pumps as an admixture to diesel, where it is usually called B7 or, depending on the concentration, also B5, B10 or B15.
  • Slightly higher viscosity and lower cold stability than diesel.
  • Reduces mileage slightly, but is usually somewhat cheaper (mainly due to different taxation).
  • Largely carbon-neutral (up to 68%), as combustion only releases the amount of carbon dioxide that plants or animals remove from the atmosphere as they grow.
  • DIN EN 590 applies to B0 to B7 diesel; DIN EN 14214 applies to admixtures for B0 to B7 and B100.
  • B100 is often subject to additional quality requirements from operators for viscosity, combustibility, water content, cold stability, density and solid foreign substances.
  • Predominantly as a standard additive to fossil diesel (B7).
  • Also in pure form (B100) for truck use (freight forwarders, municipalities).
  • No conversion of the diesel engines is required up to B10.
  • B100 should be approved by the engine manufacturer.
  • Mixed operation using B100 and diesel fuel is also usually possible without problems.
  • Both types of fuel can be refuelled alternately, subject to OEM approval. 
  • Any residues in the fuel system may be dissolved due to ester residues remaining in the RME/FAME (B100) from the refining process. Plastics, rubber and seals may also swell and non-ferrous metals in the fuel system can be attacked. 
  • As a precautionary measure, replace the fuel filter more frequently. Perform accompanying fuel and engine oil analyses. 
  • SAE 5W-30 to 10W-40 diesel engine oils approved by engine manufacturers can still be used.
  • When B100 is used, oil-change intervals in truck engine manufacturers’ specifications are often reduced to 30,000km instead of 120,000km.

Production/base materials
  • Bioethanol, less commonly referred to as agro-ethanol, is made from residues of plants that still have residual sugar or starch content.
  • Alcohol is produced by fermenting these raw materials, which are usually not suitable for use in foodstuffs.
  • The starting materials are prepared as a mash to which yeast is added. The components result in a fermentation, which produces alcohol.
  • The alcohol content is then distilled from the fermented mass. The end-result is bioethanol with an alcohol content of up to 99.9%.
  • Produced on a relatively large scale, but pure bioethanol is not freely available.
  • Bioethanol can be used in petrol engines. E10 super petrol at filling stations contains 10% bioethanol.
  • Bioethanol is unsuitable for addition to diesel fuel. 
  • Pure bioethanol has a higher octane rating than petrol and a different ignition point.
  • Ethanol makes rubber (seals and hoses) and plastics softer or more brittle. 
  • Unlike petrol, it does not vaporise at temperatures below 13°C (preheating may be necessary in winter).
  • DIN EN 228 permits the addition of ethanol to petrol in Germany up to 10% by volume (E10). 
  • However, E10 must meet the quality requirements of DIN EN 15376 for super petrol.
  • Pure bioethanol or gasoline-ethanol mixtures containing more than 10% ethanol are no longer used as fuel. 
  • Bioethanol is usually added to the petrol. In accordance with DIN EN 228, this may contain up to 5% bioethanol (E5) without it having to be declared.
  • A higher concentration of 10% admixture is possible in super petrol (E10), but this has to be declared.
  • Bioethanol is also often used as a basis for producing fuel additives.
  • For years, petrol engines have been able to operate without problems with E5 and E10.
  • All engine oils listed in vehicle manufacturers’ approval lists can be used.
  • E10 can increasingly condense in the engine oil at low engine temperatures in short-haul transport. Its viscosity and lubricity are reduced as a result, and wear and tear occurs. 
  • An engine oil analysis in the OELCHECK laboratory may indicate mixing with unburned bioethanol. 

Natural gas – CNG – compressed natural gas – pressurised gas
Production/base materials
  • Like petroleum, natural gas is produced from fossil components.
  • It is extracted from underground deposits and consists predominantly of highly flammable methane.
  • In some cases it has to be cleaned of corrosive, toxic and/or non-combustible foreign gases.
  • Water is removed in a drying process.
  • For use as fuel it is compressed and stored or marketed in overpressure gas tanks.
  • Relatively well-developed filling station network.
  • Available at almost all motorway filling stations. 
  • Natural gas burns cleanly without harmful hydrogen sulphide (no acid rain). 
  • Neither nitrogen oxides, particulate matter nor soot are produced in any significant quantities.
  • The energy content of 1kg of natural gas is equal to that of 1.5l of petrol or 1.4l of diesel - hence it offers higher performance than other fuels. 
  • The octane rating is 125 (petrol is roughly 95). 
  • Due to the high knock resistance, the fuel-air mixture can be highly compressed. High efficiency is achieved with low consumption.
  • DIN EN 16723-2:2017-10 specifies natural gas for use in transport and biomethane for feeding into the natural gas network.
  • In principle, any petrol engine can be run on natural gas. Steel pressure tanks have to be installed. 
  • Can also be used in buses or suitably designed new vehicles. 
  • It is important to distinguish between bivalent vehicles that run on CNG and petrol, and monovalent vehicles that run purely on gas.
  • Internal combustion engines of vehicles powered by natural gas may also be used indoors (e.g. forklifts).
  • Natural gas-powered engines are used on a large scale for alternative generation of electrical energy, especially where waste heat can be used.


Biogas in natural gas quality: 

  • Biogas is produced by fermenting any kind of biomass. Its methane content is significantly lower than that of natural gas. It may contain a large proportion of carbon dioxide.
  • For biogas to be used like natural gas, it must be purified of harmful gases and its methane content increased to at least 96%.
  • Unpurified/unprocessed biogas can only be burned in specially designed engines (usually CHP). 


LNG (liquefied natural gas): 

  • Natural gas becomes liquid when cooled to temperatures below -162°C. This reduces its volume by a factor of 600. Natural gas that has been liquefied by refrigeration compressors is called LNG. It is predominantly used in ships and special commercial vehicles.
  • Significantly higher combustion temperatures occur in gas-powered engines than in petrol engines. The engine and its oil are subjected to extreme thermal loads. 
  • Unlike petrol, natural gas does not contain additives which have an active cleaning effect, among other things.
  • There is an increased tendency to oxidation, with the risk of ash containing hard deposits forming.
  • Engine manufacturers generally require the use of low/mid-SAPS engine oils, which have a limited sulphate ash content.
  • Some engine oils have been specially developed for natural gas and biogas engines.
  • Regular checks on the oxidation tendency and initial pH (i-pH) are recommended.

LPG – liquefied petroleum gas – car gas
Production/base materials
  • Liquefied petroleum gas is primarily obtained as a by-product when extracting natural gas and crude oil, and when distilling crude oil.
  • It may consist of propane, butane or a mixture of the two. 
  • As a fossil fuel, it can also be used in petrol engines to generate energy.
  • Filling station network is very limited. 
  • LPG is gaseous at ambient temperature and pressure. However, it can be liquefied at low pressure, usually 6 bar at room temperature. Once condensed, the volume is reduced by 260 times. 
  • It has a high knock resistance of over 100 octane.
  • LPG burns almost sulphur-free, no soot is produced. Emission values are lower than those of petrol. 
  • DIN EN 589 defines the quality requirements.
  • As fuel for petrol vehicles equipped with a gas tank for LPG operation. This is why it is also known as “autogas”.
  • Internal combustion engines powered by LPG or natural gas may also be used indoors (e.g. forklifts).
  • Also versatile use for heating, cooking or as a heat transfer medium in trade and industry.
  • The engine oil can become increasingly diluted by condensed LPG gas fractions, especially with many cold starts, such as in short-haul transport. 
  • LPG, like natural gas, contains no additives or active cleaning agents such as are added to petrol. 
  • The engine oil has to neutralise the oxidation products that arise when running on LPG, which lead to deposits.
  • Usually low-or mid-SAPS engine oils are used, which tend to have a low sulphate ash content.

BtL – biomass to liquid – synthetic fuels from biomass
Production/base materials
  • Raw materials are solid biomass (straw, wood, plant waste) or specially cultivated crops.
  • Thermochemical combustion of biomass produces synthesis gas.
  • This is converted into liquid hydrocarbons in a synthesis (usually using the Fischer-Tropsch process*). 
  • The synthetic fuel produced can be converted into diesel or petrol by applying distillation processes used in petroleum refining.
  • Not available in all countries.
  • Are added to conventional fuels.
  • BtL fuels are chemically only slightly different from fossil petrol or diesel fuels. 
  • They usually have a higher cetane rating (around 70) than diesel (50-60).
  • Special properties can be tailored during distillation.
  • EN 15940:2016 Paraffinic fuels.
  • Currently not used in pure form.
  • DIN EN 228 or DIN EN 590 allow undeclared addition of BtL to specified petrol and diesel fuels.
  • No conversion of engines is necessary (also for exclusive use).
  • The products recommended by manufacturers can be used as engine oil.
  • Oil analysis depends on the load and mileage, as for operation with fossil fuels.

GTL – gas to liquids – synfuel
Production/base materials
  • With the help of steam, adding pure oxygen to natural gas produces a synthesis gas. 
  • Conversion to hydrocarbons takes place by means of Fischer-Tropsch synthesis*. 
  • This produces long-chain paraffins that are converted into liquid fuels, primarily diesel, by cracking and distillation (fractionation). 
  • Permanently available in many countries because gaseous natural gas is more difficult to transport and store than GTL.
  • Burns cleaner than fossil diesel and produces significantly fewer emissions.
  • Almost sulphur-free and contains no aromatic compounds.
  • Easier to ignite than fossil diesel (higher cetane rating).
  • Cold behaviour similar to fossil diesel.
  • EN15940:2016 standard for paraffinic diesel fuels.
  • Pure GTL is used in diesel engines on commercial vehicles or mobile machines and devices (construction machinery).
  • Low-emission inland waterway vessels are other major users.
  • GTL is added to diesel fuels such as Shell V-Power Diesel.
  • Leading diesel engine manufacturers have given approval for operation only with GTL.
  • No conversion of diesel engines necessary.
  • Engine oil analyses to monitor the oil change interval, as for operation with fossil diesel fuel.

OELCHECKER Summer 2020, pages 8-11