Abstract
One of the most direct ways to reduce the pollutant emissions generated by an internal combustion engine is the reduction of the specific fuel consumption (bsfc) of that engine. Besides reducing the friction and pumping loses, another method of reducing the specific fuel consumption is to increase the cylinder volume and decrease the number of cylinders maintaining the overall displacement of an engine.
Opposed pistons engine provide the advantage of being fully balanced with a reduced number of cylinders. The engine offers the potential of running high rotational speed due to the specific architecture that the stroke per cylinder is divided between the two opposed pistons.
The scope of this work is to develop an efficient DI combustion chamber by performing experimental testing with a 100 mm bore opposed piston opposed Diesel engine. The engine has been tasted in a dynamometer cell under controlled condition by varying the combustion chamber and optimizing injection spray architecture. Additional testing has been performed with a transparent experimental combustion chamber to visualize spray penetration, mixture formation and start of combustion quality for different swirl levels, injection pressures and injection strategies.
Besides the inherent advantages, one challenge of this architecture is the location of the fuel injector. In an opposed piston engine, the combustion chamber is defined by the volume between two moving pistons and cylinder liner and the injector is mounted perpendicular to cylinder axes. This configuration can be referred to as “side injection”. Side injection can offer advantages in mixture formation due to greater distance that fuel sprays can travel before reaching the adjacent walls in comparison with a conventional engine combustion chamber where the injector is located in the center of the cylinder. The penetration of the fuel spray is greatly affected by the shape and diameter of the combustion chamber because these parameters direct influence the swirl level at the end of the compression stroke. A very high swirl level is preventing fuel jet to penetrate the high tangential velocity layers of air at the outer diameter of combustion chamber. Since the spray is taken by the swirl, the center of the combustion chamber remains under lean condition resulting in poor air utilization in the combustion process. The number of sprays for each injector is influenced by the holes diameter and injection pressure required so the spray can penetrate toward the center of combustion. Testing has been made with one and two injectors per cylinder and also with different injection strategies. Pilot and post injection shows significant improvement in reducing the auto ignition delay and smoke level and the application of a very early (80-100 CAD BTDC) pre-pilot injection of about 15% from the entire fuel injected per cycle/cylinder shows the potential of a low temperature combustion by improving the bsfc while reducing also the NOx formation.
Keywords: Diesel, DI combustion chamber, opposed piston engine, side injection, mixture formation, spray penetration, optical combustion chamber, injection strategies.