This thesis has been motivated by an interest in spark-ignition gas engines, which are becoming one of the most critical contributors to marine propulsion systems in the early future. However, they were lacked attention from emission aspects, especially the methane slip is not yet investigated during the transient sea conditions. To draw an ideal conclusion of the impact of the transient conditions on the lean burn gas engine response and particularly the amount of the methane slip, full-scale experiments with sufficient instruments are needed. Moreover, the unsolvable challenge is determining the sources of methane slip while there is a time delay in measuring the methane slip during the rapid transient conditions [68]. To cope with these issues and recognize the methane slip primary sources, developing numerical methods has been ideal for reducing the project risk and cost and providing a clear methane slip trend.
Different possible sources for methane slip in the internal combustion engine are proposed [69–71], where the three main sources of methane slip for the lean burn gas engine are gas exchange, crevice volume, and quenched flame [72,73]. They are the primary sources in steady-state, but what if the load suddenly changes.
Does the percentage of the load variation influence the amount of methane slip?
On this basis, the following research objective can be formulated:
• Research objective 1
What are the primary sources of methane slip in lean burn spark-ignition engines, especially during transient conditions?
The lean burn gas engine load fluctuation is caused by external disturbance represented by the propeller torque in waves [74]. The disturbance is the wavelengths, wave heights, and wave directions. In order to evaluate the fluctuations, a coupled modeling of engine-propeller is needed. This idea is the basis for the second objective of the project:
• Research objective 2
What is the influence of wave characteristics on transient loads and marine gas engine response in terms of combustion efficiency and emissions?
The operating of spark-ignition engines on a lean mixture has several positive features such as improvement of combustion efficiency and reducing the fuel consumption [75, ], but the engine response may suffer from instability [77,78]
and even misfire [79]. To offset the disadvantages of the lean mixture, improving the engine stability gets even more attractive, and becomes our third research objective:
• Research objective 3
How to stabilize the engine during transient conditions and improve the combustion?
The maximum efficiency occurs around the full load, and there is a decrease in engine efficiency during part-load operating. This reduction for SI engines normally occurs due to an increase in pumping loss and throttle closure [80,81].
Emission rise due to the incomplete flame propagation of lean mixture [82,83]
provided our third research objective as follow:
• Research objective 4
How is the engine response concerning sea transient conditions in part-loads?
Answering these questions has constructed the main goals and scope of this research. As a consequence, nine journals and conference papers are published.
1.4 Publications
The following publications constitute a part of the thesis:
1. Authors: Tavakoli S., Jensen M. V., Pedersen E., Schramm J., Title:
Unburned Hydrocarbon Formation in a Natural Gas Engine Under Sea Wave Load Conditions. Journal: Marine Science and Technology. Editor:
Springer. Year: 2020. Status: Published. Type: Journal Paper. https:
//doi.org/10.1007/s00773-020-00726-5.
2. Authors: Saettone S., Tavakoli S., Taskar B., Jensen M. V., Pedersen E., Schramm J., Steen S., and Andersen P. Title: The importance of the engine-propeller model accuracy on the performance prediction of a marine propulsion system in the presence of waves. Journal: Applied Ocean Research. Editor: Elsevier. Year: 2020. Status: Published. Type: Journal Paper. https://doi.org/10.1016/j.apor.2020.102320.
J., and Pedersen E. Title: Modeling and Analysis of Performance and Emissions of Marine Lean-Burn Natural Gas Engine Propulsion in Waves.
Journal: Applied Energy. Editor: Elsevier. Year: 2020. Status:
Published. Type: Journal Paper. https://doi.org/10.1016/j.
apenergy.2020.115904.
4. Authors: Tavakoli S., Schramm J., and Pedersen E. Title: Strategies on Methane Slip Mitigation of Spark Ignition Natural Gas Engine During Transient Motion. Journal: SAE Automotive Technical Papers. Editor:
SAE.Year: 2021. Status: Published. Type: Journal Paper. DOI:https:
//doi.org/10.4271/2021-01-5062.
5. Authors: Tavakoli S., Schramm J., and Pedersen E. Title: Influence of Turbocharger Inertia and Air Throttle on Marine Gas Engine Response.
Journal: Journal of Fluid Flow, Heat and Mass Transfer (JFFHMT).Editor:
AVESTIA. Year: 2021. Status: Published. Type: Journal Paper. DOI:
10.11159/jffhmt.2021.013.
6. Authors: Tavakoli S., Schramm J., and Pedersen E. Title: Effects of Propeller Load Fluctuation on Performance and Emission of a Lean-Burn Natural Gas Engine Operating at Part-Load Condition. Status: Under revision.Type: Journal Paper.
7. Authors: Tavakoli S., Maleki K., Schramm J., and Pedersen E. Title:
Emission Reduction of Marine Lean-Burn Gas Engine Employing a Hybrid Propulsion Concept. Journal: International Journal of Engine Research.
Editor: SAGE. Year: 2021. Status: Published. Type: Journal Paper.
https://doi.org/10.1177/14680874211016398
The work conducted during the Ph.D. also resulted in the following papers:
• Authors: Tavakoli S., Pedersen E., and Schramm J.Title: Natural Gas Engine Thermodynamic Modeling Concerning Offshore Dynamic Condition. Book:
Proceedings of the 14th International Symposium, PRADS 2019, September 22-26, 2019, Yokohama, Japan- Volume II.Status: Published. Type: Conference Paper.
• Authors: Tavakoli S., Schramm J., and Pedersen E.Title: The Effect of Air Throttle on Performance and Emission Characteristics of an LNG Fuelled Lean Burn SI Engine in Steady and Unsteady Conditions. Book: In Proceedings of the 5th World Congress on Momentum, Heat and Mass Transfer (MHMT’20). Year:
2020.Status: Published.Type: Conference Paper
Project Objective3
Paper 4
Paper 5
Paper Objective 7
4
Paper 6 Objective
1 Paper
1 Paper
4
Objective 2 Paper
2 Paper
3
Figure 1.5:Interconnection between the papers and the objectives of the PhD project.
The interconnection between the papers and the objectives of the Ph.D. project is presented in Fig.1.5. The author tries to address all the knowledge required in each objective and the relevance of the published papers to the four main objectives.
Furthermore, the output of the current work is arranged in an intuitive diagram shown in Fig. 1.6. The starting point was developing an engine model platform. Next, the implemented equations were confirmed by using available measured data. The verified model was the original configuration of all the investigations in this study. Based on the imposed torque, two separate paths were considered: transient condition around the full load and transient condition around the part-load. The full load concept is presented in papers number 1, 2, 3, 4, 5, and 7 with harmonic sinusoidal torque. The part-load concept is only presented in paper number 6, where the modeling is for an actual transient condition. In the full load concept, papers 2 and 3 have experienced a wide range of waves, while papers number 1 and 4 presented an additional coupled emission model for methane slip. Strategies for controlling combustion and improving the engine response are presented in papers number 4, 5 and 7.
Engine model (1-7)
Validation (1-7)
Full load modeling (1,2,3,4,5,7)
modeling (6)
A range of waves (2,3)
Control (4,5,7)
Figure 1.6:An intuitive overview of the current work. The numbers in parentheses refer to the relevant paper.