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.