Series-production plug-in hybrid electric vehicles equipped with diesel engines are already available in the market or planned for the future (e.g., Volvo V60, Audi Q7 e-tron quattro, Peugeot 508 RXH Hybrid4, Range Rover SDV6 Hybrid Diesel). However, fossil diesel fuel/biodiesel blends show a tendency to form precipitates—which can be considered as oligomers with aging. This tendency of fuel blends to form precipitates during long term storage could impact the operation of diesel plug-in hybrid vehicles (PHEV).
Making the problem more complex is the existence of different quotas of biodiesel blend ratios in different countries. Further, HVO (hydrotreated vegetable oil) and GTL (gas-to-liquid)—a mixture of mainly branched and unbranched alkanes—are becoming more popular as neat diesel substitutes or blend stocks. Now, researchers in Germany hav devised a novel sensor for the onboard determination of fuel quality in diesel PHEVs to address these issues. (A variation of the approach could be used for gasoline PHEVs.) A paper on their work appears in the journal Fuel.
… fuel degradation must be considered: Firstly, aged fuels differ in their emission behavior; secondly, fuel degradation, especially blends, showed a certain tendency to form precipitates which can be considered as oligomers. The cause of undesired fuel instability are unsaturated hydrocarbons: They undergo autoxidation forming alkyl and peroxy radicals which can react with oxygen to form peroxide or hydroperoxide. In the next reaction step these peroxides and hydroperoxides degrade into low molecular oxidation products such as aldehydes, alcohols, ketones, acids and high molecular oligomers. Problems due to aged fuels can occur at the fuel pump, injection systems or exhaust aftertreatment. Fuel pump and injection system are very sensitive to an increase in viscosity due to oligomerization. The increased viscosity leads to poorer nebulization during injection. This can lead to an increase of soot or unburned fuel due to an incomplete combustion. Enabling factors for fuel aging are oxidative and thermal stress.
Since the introduction of plug-in hybrid vehicles (PHEV), an additional factor could be added—the long-term stability of fuels. In PHEV, the combustion engine is only used when the power supply from the battery is too low. On short distance rides, which are the majority of all driven kilometers (about 80% below 50 km) the combustion engine is not necessary, because of frequent battery recharging. Consequently, the storage time of the fuel in the tank is much longer than usual. For this reason the fuel long-term stability in PHEV is of particular importance.—Eskiner et al.
The team from Coburg University of Applied Sciences and Art introduced a direct measuring technique based on dielectric spectroscopy (which measures the dielectric and electric properties of a medium as a function of frequency), focusing the in situ determination of degradation products in fossil diesel fuels.
Besides using a lab-scale sensor, they also developed a smart micro sensor that, in principle, is suited for mass production and that could be applied as a fuel tank sensor in a PHEV.
It has been shown by determining the permittivity that the stability of the suspension is strongly dependent on the concentration of alkanes/aromatics and C18:2 (as a substitute for biodiesel). An antagonistic effect was found in the range of 15 to 22.5% C18:2. For purposes of comparison, the FTIR signal was measured, which correlates with the sensor signal. The small size of the sensor system is an option for on-board control in PHEV. Works are ongoing to expand this method by a further sensor system based on a spectroscopic method (fluorescence and near-infrared spectroscopy). This is to achieve a redundancy and to identify varying fuel compositions more specifically. This strategy prevents engine damages since fuel degradation is detectable at an early stage and the combustion engine can be started before precipitation occur. It is also a promising approach to apply this method for gasoline powered PHEV: Here for example the vapor pressure during long-term storage of varying fuel mixtures (e.g. ethanol content) could be determined.—Eskiner et al.
M. Eskiner, F. Ammer, D. Then, J. Staufenbiel, M. Rossner, O. Schröder, J. Krahl (2017) “Novel concepts for onboard determination of fuel quality in plug-in hybrid cars,” Fuel, Volume 209, Pages 224-231 doi: 10.1016/j.fuel.2017.07.102