Modern Diesel cars, fitted with state-of- the-art aftertreatment systems, have the capability to emit extremely low levels of pollutant species at the tailpipe. However, diesel aftertreatment systems can represent a significant complexity, packaging and maintenance requirement. Reducing engine-out emissions in order to reduce the scale of the aftertreatment system is therefore a high priority research topic. Engine-out emissions from diesel engines are, to a significant degree, dependent on the detail of fuel/air interactions that occur in-cylinder—both during the injection and combustion events—and also to the induced air motion in and around the bowl prior to injection. In this paper the effects of two different piston bowl shapes are investigated – one with a stepped bowl lip, and the other without. Experiments are performed in a single-cylinder high speed light duty engine fitted with a production cylinder head and fuel injector in order to quantify the effect of the bowl shape and spray targeting—varied by changing the injector nozzle tip protrusion—on emissions and fuel consumption. Multi-dimensional CFD modelling is used to detail the effect of these geometry changes on the in-cylinder flow and fuel/air mixing processes thereby guiding the interpretation and understanding of the experimental results. The results suggest that an integrated approach with experimental and numerical studies working in parallel is essential to understanding how these changes affects engine-out emissions, fuel consumption, and other parameters. The results show that the interaction of both the combustion system geometry with fuel injection hardware and strategy is what is most important, and that individual changes to one of these must be matched by changes to the other, this can then be used to seek further design improvements.