Increasing fuel prices keep bringing attention to alternative, cheaper fuels. Liquefied Petroleum Gas (LPG) has been well known for decades as an alternative fuel for spark ignition (SI) passenger cars. More recently, aftermarket LPG systems were also introduced to Heavy Duty transport vehicles. These (port fuel) systems either vaporize the liquid fuel and then mix it with intake air, or inject fuel into the engine's intake ports.While this concept offers significant fuel cost reductions, for aftermarket certification and large-scale OEM use some concerns are present. Unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions are known to be high because of premixed charge getting trapped into crevices and possibly being blown through during valve-overlap. Apart from the higher emission levels, this also limits fuel efficiency and therefore cost savings.Direct injection of LPG offers a solution to the aforementioned concerns by directing the fuel to the desired areas of the combustion chamber. To investigate the potential of this concept, diesel has been blended with liquid butane, in different ratios. These blends were directly injected into a heavy duty test engine, using a standard common rail injector.Results show that even at low loads, a butane mass fraction of one-third can be used without negatively affecting emissions. For direct injected butane, hydrocarbon emissions were found to be an order of magnitude lower than when port fuel injected butane is used. At this butane fraction, smoke emissions decrease by 50%. Because a high combustion efficiency is retained, fuel cost savings can be significant. At higher loads, an even higher butane mass fraction can be used, further enhancing the fuel cost saving potential.