Analysing the effects variable injection and exhaust valve timing have on a two-stroke diesel engine

Abstract

The two-stroke marine diesel engine is the most common propulsion system in large ships. They consume a large amount of fuel per day and therefore by improving the engine efficiency just slightly will make a difference in costs of fuel oil and in the environmental department. By tuning the engine with variable valve and exhaust timing, so it gets the highest efficiency over various loads, is one way to accomplice that. It was investigated if the engine would be able to run over the entire engine speed/load diagram and how the engine needed to be tuned for that. It was also investigated how the specific fuel oil consumption and nitrogen oxide emissions would be affected. The simulations involved changing different timings and flows of the engine and to be able to run at the speed limit, assumptions of how much mass flow of air the engine would need were done. All the simulations were done with a MATLAB code of a two stroke zero-dimensional engine which is validated. The results show that with a variable timing setup compared to a fixed one could save up to 1.8 % fuel in terms of specific fuel oil consumption and with a lower load the savings in of specific fuel oil consumption were less. In nitrogen oxides emissions the possible reduction could be as high as 1.3 %. It also showed that to run on the torque limit curve and propeller design curve a later exhaust valve closing and earlier start of injection were preferred but on the speed limit curve it was necessary to get a later exhaust valve closing but also a later start of injection.