Effects of Intake Manifold Conditions on Dual-Fuel CNG-Diesel Combustion in a Light Duty Diesel Engine Operated at Low Loads

The use of compressed natural gas (CNG) in light duty applications is still restricted to conventional spark ignition engines operating at low compression ratio, so overall efficiency is limited. A combustion concept that has been successfully applied on large stationary engines and to some extent on heavy-duty engines is dual-fuel combustion, where a compression-ignited diesel pilot injection is used to ignite a homogeneous charge of methane gas and air. CNG is injected in the intake ports during the intake stroke and later in the cycle the premixed air-CNG mixture is ignited via a pilot diesel injection close to top dead center. However, this concept has not been applied to a significant extent on light duty engines yet. The main reasons are linked to high temperature methane oxidation requirements and poor combustion efficiency at diluted conditions at low loads. Therefore, in this paper an experimental investigation of the effects of different intake manifold conditions on the dual-fuel combustion process is presented, based on performance and emissions of a light duty diesel engine rebuilt for dual-fuel operation operated at low loads and lean conditions. The main goal is to understand how intake temperature and pressure affects the combustion process and to identify possible control strategies for those parameters over the low load range of operation. Results show that intake air temperature plays an important role in the flame propagation process at highly diluted conditions and higher air temperature allows a sharp reduction in total unburned hydrocarbon emissions (TUHC). Reduced intake air pressure can expand the operating range of lean CNG-Diesel dual-fuel engines by means of greater combustion efficiency, despite higher pumping losses. Maximum gross indicated efficiency recorded during the experiments was 42%. It was possible to run below 4g/kWh TUHC emissions and with high enough exhaust temperature for high efficiency methane oxidation in the aftertreatment system beyond 5 bar IMEPg. Loads ranged between 3 bar and 8 bar IMEPg.