Analysis of performances of a dual-fuel turbocharged compression ignition engine

The paper describes research work on a full-scale dual-fuel 4-cylinder turbocharged compression ignition engine. Compressed natural gas (CNG) was applied as the main fuel. Selfignition of the air-fuel mixture was initiated from a diesel oil dose injected by a common rail system. The research was aimed to establish maximum CNG share in the mixture delivered into the cylinder. An excessive CNG share may result in "hard" engine operation. It may also lead to the occurrence of vibrations of piston-crank construction parts resulting in failure of this mechanism. These vibrations may originate from knocking combustion (selfignition of the air-fuel mixture in the zone of non-combusted mixture) or vibration excitation as a result of rapid pressure rise after selfignition. Boundary values of the CNG energy share were determined by analysing parameters related to the rate of pressure rise and rate of heat release as well as the engine head vibration amplitude represented by the voltage signal generated by the knock sensor. Boundary values of the above mentioned parameters were determined on the basis of measurements done on the engine fuelled in a standard mode. These parameters were registered at operating points corresponding to the maximum power and load. Then, there were done measurements of basic engine operating parameters at dual fuelling in chosen points of the load characteristic for the engine speed at which the engine fuelled in a standard mode had maximum torque. Load characteristics were done for three various diesel oil doses (constant over the whole range of engine load). Load changes were realized by changes ofCNG energy share in the fuel charge. Analysis of combustion process parameters and engine head vibrations showed that CNG energy share may reach 60%. Maximum torque is possible to obtain at 45% CNG energy share. 15% decrease of maximum torque was obtained.