Interactions between the jets in a multi-hole injector and between the jet and the wall may affect the fuel-air mixing processes in a direct-injection Diesel engine. These interactions are the subject of the investigation in this work. It is known that in the case of free jets, for a given total mass and momentum flow rate, increasing the number of holes would result in an increase in the mixing rate. In the case of a multi-hole injector in an engine, however, if the number of holes are increased beyond an optimum value, the interaction between the jets themselves may result in a reduced mixing. In the limit of increasing the number of holes, a hollow-cone jet would result. The fuel-air mixing in the hollow-cone jet is shown to be slower than in a multi-hole injector with an optimum number of holes. It is also shown that the walls do not appear to have a significant direct effect on the mixing rate as the characteristic time associated with mixing appears to be much shorter than that associated with momentum loss to the walls.