At present, the emissions from an internal combustion engine exhaust is reduced by exhaust after treatment devices. However, after treatment devices like SCR which is used to control NOx, results in additional weight, high costs and rejects toxic gases like ammonia etc. To overcome this problem, a new combustion technique should be developed to improve the primary combustion processes inside the combustion chamber itself to reduce these exhaust gas emissions. This work presents the results of such a technique that is applicable to direct injection, Diesel engines. The technique is based on the porous medium combustion (PMC) technology, which is developed for steady state household and industrial combustion processes. Based on the adiabatic combustion in porous medium (PM), a porous medium in engine piston as a concept is proposed here to achieve improved combustion efficiency and low emissions. Using a commercial code CONVERGE the entire cycle is simulated and presented here. Temperature evolution of the PM and its effects are also discussed in detail. The study is carried out on a single cylinder, four stroke, water cooled, direct injection Diesel engine. The results show that NO_x is reduced by 77% and soot by 80%. However remaining pollutants CO and HC increased by 91.6%, 86.5% which can be reduced by a simple two way catalytic converter. Also mean temperature during whole cycle is reduced by 300 K which is the main reason for reduction of emissions like NO_x. Introducing porous medium inside the piston bowl creates homogeneous mixture. The homogeneous mixture formation and 3-D self-ignition without flame front in the PM material creates an effective combustion. Moreover, the heat recuperation in PM during the previous cycle is used for warming up the compressed air and evaporate the fuel to create a homogenous mixture in the respective forthcoming cycle.