Characterisation of Combustion Chamber Deposits Formed in Direct Injection Spark Ignition (DISI) Engines during an On-Road Vehicle Trial 2010-01-2155

Direct Injection Spark Ignition (DISI) engine technology is becoming increasingly common in the South African and global vehicle parcs. South Africa is in a unique position because a significant portion of all liquid fuels consumed are synthetically produced from coal and gas. These fuels are mainly supplied into the inland regions, particularly the Gauteng province, the economic heartland of South Africa and the most densely populated area in the country. It is important to understand the performance of synthetic fuels in the latest generation engines, in order to ensure that these fuels are fit for use in these new applications. The latest generation DISI gasoline engines (also known as Gasoline Direct Injection™ and Fuel Stratified Injection™) differ significantly in operation to older Port-Fuel-Injected (PFI) engines.

Although there is literature available on the relationship between crude-derived fuel composition and the composition of DISI engine deposits, no such information exists on the possible effects of synthetic fuel composition on such deposits. An on-road vehicle trial was conducted to assess and compare the effect of synthetic and crude-derived fuels on such engine deposits. During the on-road trial, four similar Volkswagen FSI vehicles were tested under similar conditions over a distance of 20,000 km in order to determine the relative performance of the fuels evaluated. Some of the fuels evaluated during this trial were synthetic fuels (one additized with a detergent additive and two unadditized Fischer-Tropsch fuels), while the fourth fuel was a conventional crude-derived fuel. Comprehensive characterization of these fuels by Nuclear Magnetic Resonance spectroscopy (NMR) and two-dimensional GC analysis (GCxGC-TOFMS) showed very significant differences in fuel composition between the synthetic and the crude-derived fuels. The engine deposits were characterized using various solid-state ₁₃C NMR techniques, thermogravimetric analysis (TGA), Fourier Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscopy (SEM).

This study confirmed that the methodologies developed for the characterization of deposits from PFI engines could also be applied to DISI engines, which is consistent with previous literature findings. The species identified in these DISI deposits were shown to be similar in nature to the deposits formed in PFI vehicles. Similarly, it was shown that the deposits formed during the combustion of synthetic gasoline were similar in composition to those formed during operation on a crude-derived fuel. Some differences in the quantity of the deposits formed were observed between the vehicles that ran on synthetic fuels and those that ran on crude-derived fuels.

Despite the differences in composition between the synthetic and crude-derived fuels, the bulk chemical composition of the deposits generated from these fuels are not significantly different. Moreover, the overall performance of these DISI-equipped test vehicles were unaffected by the fuel composition over the 20,000 km test period. However, it must be kept in mind that although vehicle preparation, driving conditions and fuel compositions were well controlled, ambient conditions were not controlled. Results are therefore considered to be comparative.