In the German-speaking world, a distinction is made between Brennstoff (fuel) and Kraftstoff (motor fuel). Often, both terms are incorrectly used as equivalent. But it’s clear: Brennstoff is used to generate thermal energy and Kraftstoff is used to generate mechanical energy. Heating oil and diesel fuel are very similar, both are so-called middle distillates. The decisive factor is: diesel for motor vehicles in accordance with EN 590 is produced specifically for the operation of internal combustion engines, domestic heating oil in accordance with DIN 51603–1 is specified for use in burners. Engine parameters such as ignition delay (cetane number) are therefore not relevant for heating oil. Nevertheless, the boiling curve specified for domestic heating oil is largely identical to that for diesel. This would make use in a vehicle theoretically and technically possible. In particular, older vehicles with mechanical injection systems or long-stroke diesel engines, such as those used in heavy-duty commercial vehicles, or medium-speed and slow-running diesel engines on ships can deal with “inferior” (not EN 590) fuel quality. A slightly higher ignition delay does not matter here, for example. However, the modern common rail injection systems in passenger cars in particular are sensitive to poorer fuel quality. In order to meet the requirements of today’s applicable exhaust gas regulations, very high injection pressures (up to 2,700 bar) are required to achieve fine atomisation of the fuel and thus optimum mixture formation. This results in extremely high component fits with minimal tolerances. Even the smallest impurities cause major problems here.
Motor oil under constant stress
The sulphur content in heating oil is still up to five times higher than in diesel fuel. This sulphur is oxidised in combustion and converted into sulphuric acid with the concomitant condensate when the temperature falls below the dew point and is introduced into the motor oil via blow-by. There, it must be neutralised by the alkaline reserve (base number, BN). This is thus additionally stressed when heating oil is used, the oil-change intervals are significantly reduced, as the modern low-SAPS oils, as they are standard for vehicles with particulate filters, are not designed for such “motor fuels”. Heating oil can also contain many more PAHs (polycyclic aromatics) than diesel fuel. Together with a possibly lower cetane number, these ensure more sooty combustion in the diesel engine. Once again, the motor oil becomes more stressed. Detergents, which normally dissolve dirt in the oil into fine particles, and dispersants, which hold particles in suspension and transport them to the filter, must be able to handle the additional soot quantity. If a diesel particulate filter is installed, this is more heavily occupied by the additional soot quantity. The result is a faster rise in back pressure, and the particulate filter needs to be regenerated more frequently. This leads to additional fuel entry into the motor oil, as the exhaust gas temperature for soot burn-off in the filter is increased “late” due to increased fuel and shifting of the injection time. Increased oil dilution occurs because the entire fuel is not converted and/or the regeneration cycle is interrupted unexpectedly, e.g. by short-distance operation. The result of the additional fuel input is: the lubricating capability of the oil decreases and increased abrasive wear occurs in bearings and other tribo surfaces.
“Hello – Customs control!”
As described above, heating oil is intended for the generation of thermal energy and is therefore subject to a reduced tax rate in Germany in accordance with the Energy Tax Act. Thus, use to generate mechanical energy and thus in vehicles is prohibited. Use in stationary or mobile emergency power diesel engines or power generators is permitted, as electrical energy is generated here via a generator! Anyone thinking about “charging” their electric vehicle while driving due to the equally high electricity costs with tax-advantaged heating oil and an emergency power generator should note that the tax exemption for heating oil is only valid if the power generator is “stationary” during its use! In order to make the tax reduction recognisable in the market, heating oil is therefore provided with marking substances. Heating oil is very easily recognisable by its red colour. Anyone who refuels their vehicle with heating oil is already committing tax evasion (this is known as readying - 'Bereithalten'), you don't even have to have started the engine! Tax evasion is a crime! Customs carries out regular checks.
Many have (unsuccessfully) tried to remove the red colour, and mixing the heating oil with normal diesel to fade the red colour is one "trick" that is often described. It’s important to know: In addition to the obvious red dye, there are other invisible marking substances that Customs can easily detect in the laboratory. So please: steer clear!