Evaluation of Combustion Models for Medium Speed Diesel Engines

The greenhouse emission limits are getting constantly tighter due to the 2015 UNFCCC Paris Agreement, yet majority of the world energy is produced by combustion applications. The modern energy solution providers are pushed towards decreasing the emission rates and more efficient products. This leads to development process that needs to go more and more in details. In order to predict emission rates and optimize the engine design and performance, Computational Fluid Dynamics (CFD) with applied combustion simulation is an essential tool for nowadays engine manufacturers. This thesis is made for Wärtsilä Finland Oy, in order to improve diesel combustion CFD simulation prediction and performance. Two different combustion models, the Extended Coherent Flame Model 3 Zones (ECFM 3Z) and the Extended Coherent Flame Model with Combustion Limited by Equilibrium Enthalpy (ECFM CLEH) were evaluated with CFD simulations towards Wärtsilä 20 engine performance at 100% and 10% load operation point. Several applied sub-models were tested, and two different implementations for intermediate fuel oil (IFO) were evaluated. Both combustion models were tuned to replicate measured in-cylinder pressure curves. Evaluation is made towards engine performance values such as rate of heat release, cumulative heat release, engine efficiency and total piston work. Also other physical phenomena were compared between the models, such as emission rates, fuel film formation on the cylinder surfaces and heat transfer to walls. In order to visualize running simulation results, and compare them to measurements, a simple continuously updating plotting tool was implemented during the thesis. The results of this thesis are demonstrating that ECFM CLEH can predict at least as favorable results in Wärtsilä’s applications than ECFM 3Z, which has been in use for an decade. ECFM CLEH can provide the in-cylinder pressure curve with accuracy of +/- 1.5% at 100% load, whereas ECFM 3Z has the accuracy of +/- 3.5%. At 10% load the models had similar +/- 9.0% accuracy. ECFM CLEH performed well especially in expansion stage, but the early combustion stage could be improved by modifying the auto-ignition tables included in the model. ECFM CLEH predicted over 10% smaller total heat transfer to the walls and nearly 30% smaller local heat flux to the piston “hot spot” than ECFM 3Z, which is considered to predict too hot surface temperatures in Wärtsilä engines. As a future suggestion, it would be relevant to evaluate ECFM CLEH performance also for premixed gas combustion and dual fuel Wärtsilä engines