In the last years the interest for Compressed Natural Gas (CNG) as alternative fuel in transportation is grown, in particular due to its large availability and lower environmental impact with respect to gasoline or diesel fuel. In this scenario, the application of the Dual Fuel (DF) Diesel-Compressed Natural Gas (CNG) combustion concept to light duty engines can represent an important route to increment the diffusion of natural gas use. Many studies have proven the benefit of DF with respect to conventional diesel combustion in terms of CO2, NOx, PM and PN emissions, with the main drawback of high unburned hydrocarbon, mainly at low/partial engine loads. This last aspect still prevents the application of DF mode to small displacement engines. In the present work, a 2.0L Euro 5 compliant diesel engine has been set up to operate in DF mode and tested on a dyno test bench. Thus, an extensive experimental campaign has been devoted to the engine optimization, in order to assess the achievable reduction of unburned hydrocarbon emissions, keeping the peculiar benefits of DF combustion. The experimental test plan has been conducted both in steady state and transient operating conditions over current and future homologation cycles. The effects of many engine control parameters (e.g. CNG substitution ratio, diesel pilot injection strategies, EGR, boost pressure) on DF combustion evolution have been deeply evaluated, thus allowing to define an optimized engine calibration at low and medium engine load. The research has not only confirmed the expected benefits of DF combustion technology in light duty engines, but has proven the capability of the engine calibration optimization to significantly reduce THC emission. Furthermore, such optimization allowed a consistent CO2 reduction (more than 10%) over NEDC and WLTC cycles, keeping the same NOx values with a very strong reduction of PM and PN emissions.