As PHEVs equipped with diesel engines begin to come on the market, a new potential problem has come to light: diesel fuel/biodiesel blends show a tendency to form precipitates, which can cause the fuel to become unstable.
To make the problem more complex, different countries have different quotas for biodiesel blend ratios. Substances such as HVO (hydrotreated vegetable oil) and GTL (gas-to-liquid, an alkane mixture) are becoming more popular as diesel substitutes or blend stocks.
Now, researchers in Germany have devised an onboard sensor that can monitor fuel quality in diesel PHEVs.
In “Novel concepts for onboard determination of fuel quality in plug-in hybrid cars,” published in the journal Fuel, M. Eskiner and colleagues explain that aged fuels, especially blends, can form precipitates that can be considered as oligomers. Unsaturated hydrocarbons undergo autoxidation, forming alkyl and peroxy radicals which can react with oxygen and eventually degrade into aldehydes, alcohols, ketones, acids and high molecular oligomers.
Problems due to aged fuels can occur at the fuel pump, in injection systems or in exhaust after-treatment. Fuel pumps and injection systems are very sensitive to an increase in viscosity due to oligomerization. The increased viscosity leads to poorer nebulization during injection, leading in turn to an increase in soot or unburned fuel due to incomplete combustion.
“In a PHEV, on short-distance rides, the combustion engine is not necessary, because of frequent battery recharging,” explains Eskiner. “Consequently, the storage time of the fuel in the tank is much longer than usual. For this reason the long-term fuel stability in a PHEV is of particular importance.”
The Coburg University team introduced a direct measuring technique based on dielectric spectroscopy, focusing on 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 is suited for mass production and could theoretically be used as a fuel tank sensor in a PHEV.
Source: Green Car Congress