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.