Recently, some studies have shown high efficiencies using
controlled auto-ignition by blending gasoline and diesel to a
desired reactivity. This concept has been shown to give high
efficiency and, because of the largely premixed charge, low
emission levels. The origin of this high efficiency, however, has
only partly been explained. Part of it was attributed to a lower
temperature combustion, originating in lower heat losses. Another
part of the gain was attributed to a faster, more Otto-like (i.e.
constant volume) combustion.
Since the concept was mainly demonstrated on one single test
setup so far, an experimental study has been performed to reproduce
these results and gain more insight into their origin. Therefore
one cylinder of a heavy duty test engine has been equipped with an
intake port gasoline injection system, primarily to investigate the
effects of the balance between the two fuels, and the timing of the
diesel injection. Besides studying trends in the dual-fuel regime,
this also allows to find best points to compare with conventional
diesel combustion.
Results show that compared to more conventional combustion
regimes, this dual-fuel concept can escape from the common
NOx-smoke trade-off, reducing both to near-zero values.
Although hydrocarbon emissions are somewhat increased, indicated
efficiencies are significantly improved. The absolute efficiencies
are not as high as reported in other work, but the increase does
confirm the potential of the concept. The increase in indicated
efficiency is shown to originate from a higher thermal efficiency,
because short burn durations at high gasoline fractions enable for
CA50 to be phased closer to TDC, without combustion occurring too
much before TDC.
Pressure rise rates are as low as with conventional diesel
combustion, when using the same Exhaust Gas Recirculation (EGR)
percentage. Although the dual fuel concept has a much higher rate
of heat release, this is phased better after TDC. A dedicated set
of experiments has also shown that the late-cycle diesel injection
is dominant in combustion phasing and that control has to be found
in this diesel injections.