Assessing the impact of FAME and diesel fuel composition on stability and vehicle filter blocking 2019-01-0049
In recent years, there has been an impetus in the automotive industry to develop newer diesel injection systems with a view to reducing fuel consumption and emissions. This development has led to hardware capable of higher pressures, typically up to 2000 - 2500 bar. Simultaneously, regulatory requirements have resulted in a change in fuel composition with a move towards the development and use of renewable fuels around the world. Over the past few decades the percentage of biodiesel, in the case of fatty acid methyl esters (FAME), in the global fuel supply has increased to meet the European EN 590 limit now set to 7% for diesel fuels sold within the EU and with aspirations to increase this level. The unsaturation in FAME is widely thought to increase the propensity to oxidation than other regular hydrocarbon species present in diesel resulting in in-vehicle fuel degradation leading to solid formation in the fuel. Another growing concern is the theory that the increasing temperatures and pressures experienced by the fuel in the new injector system technologies has the ability to cause fuel degradation.
The majority of studies into diesel degradation are conducted under non-representative laboratory conditions, or are extrapolated from the deposits found in filters from vehicles with failed injectors. In this study, the cause of this degradation was investigated by using a novel High Pressure Common Rail (HPCR) non-firing rig designed to mimic a diesel common rail system, simulating realistic, albeit accelerated, operating conditions. The degree of deposition on the system fuel filter was monitored, for both petroleum diesel, EN 590, a reference non-biodiesel fuel RF79 (B0), Bx (where x is percentage volume/volume of FAME) and surrogate diesel fuel components.
A systematic study of synthetic surrogates demonstrated that, as well as FAME, any base fuel component, under sufficiently high pressures and temperatures experienced in the HPCR are prone to degradation irrespective of the concentration of the component in the original fuel. The most unstable component acts as the instigator, thus promoting fuel oxidation. The other components in the fuel such as FAME, aromatic and cycloalkane portions will also oxidise and eventually polymerise to form solids blocking the filter. This also demonstrates that while a large body of work on the oxidative instability of biodiesel in the chemical laboratory is indicative of instability this does not mimic what is seen under more realistic vehicle conditions and the focus on FAME instability is misleading.
Kesavan Gopalan, Christopher J. Chuck, Christopher Roy-Smith, Christopher D. Bannister
University of Bath, BP Formulated Products Technology
International Powertrains, Fuels & Lubricants Meeting