Abstract: This thesis aims to contribute to the development of the isobaric combustion engines by exploring multiple injection strategies, by means of computational simulations using a commercial software Converge. A single injection case validated with experimental data in terms pressure trace and heat release rate was used as a baseline reference. The adjustment of the turbulent kinetic energy dissipation constant is found to have the most significant influence in reproducing the pressure and heat release rate histories observed in the experiment. As a first attempt to achieve isobaric combustion, a multiple injection strategy using a single injector was explored with up to four consecutive injections. Considering that the computational simulations were unable to reproduce the experimental data due to a number of uncertainties in the implemented models, the present study attempted to identify the main causes of the discrepancies through various parametric studies. First, different liquid fuel properties were examined and it was found that, while the physical properties of the fuels have a notable effect in terms of evaporation and atomization, such variations were not sufficient to reproduce the experimentally observed heat release cycle. Next, the effects of the uncertainties in the kinetic mechanisms were assessed by the reaction multiplier, an artificial adjustment of the rate constants, and it was found that the reaction multiplier affected the ignition of the first injection, but not the subsequent injection events. As such, the use of reaction multipliers to reproduce the experimental data was found to be unsuccessful. The effect of thermodynamics properties was also examined by employing real-gas equations of state, such as Redlich-Kwong and Peng-Robinson, and the results showed little difference at the conditions under consideration. Finally, advancing the start of injection was found to have the most significant effect on pressure trace and heat release rate to lead to a substantial improvement in the numerical prediction. The results suggest that the key uncertainties in modeling of the present engine combustion are likely the accurate timing of the start of injection combined with the exact injection rate shape profile.

There is an urgent call for action to address the energy efficiency, climate, and local air quality concerns associated with transport because of CO2, particulates, nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC) emissions. This has driven the international policy agenda towards reducing greenhouse gas (GHG) with a major emphasis on CO2 emission. Fossil fuel combustion is considered a main contributor to the emission of CO2. The transport sector with a particular emphasis on ground transport is considered the fastest growing sector among all emission sources. To meet climate change goals, governments around the world may need to implement strict regulations on the transport sector. Governments around the world have indeed set stricter emissions standards for vehicles as a way to reduce greenhouse gas emissions from the transport sector. These standards can be achieved through various methods, such as requiring more efficient engines, alternative fuels, or the adoption of electric vehicles. On the other hand, in recent years, a lot of effort was put into promotion of electric vehicles as zero emissions vehicles. This statement should be reconsidered, since the greenhouse impact of electrical vehicles is not negligible. Conversely, in some cases, an electrical vehicle can have an even higher emission impact than modern vehicles with sophisticated internal combustion engines. In fact, the pollutant emissions discharged at the tailpipe outlet will be so low as to be hardly measurable, and their practical impact on air quality will be negligible. In terms of particulate matter emission for example, the impact of tire and brake wearing is already much higher than that due to the ICE (tire wear produces around 50 mg/km of particulates), reaching values around 10 times the emission from the engine (5 mg/km). This implies that today’s conventional ICE-powered-car is equivalent to fully electric and hybrid cars with regard to particulate emissions, when tire and brake and other contributions (e.g. road dust) are accounted for. All the data indicate that ICEs will never cease to exist and the majority of cars will be powered by ICEs in the future. These factors sparked my work on the simulation of ICEs. The first project was mainly focused on high-pressure isobaric combustion, which is a promising concept that has the potential to introduce high efficiency. This work started with the development and validation of the computational models for full cycle combustion engine simulations to capture the flow and combustion characteristics and their interactions with the intake and exhaust flows through the valves and ports. The computational models were extensively validated against the optical engine experiment data, to ensure the fidelity needed for predictive simulations. Upon identifying the numerical models, a comparative study of isobaric and conventional diesel combustion was conducted. The results revealed the superiority of the isobaric combustion mode compared to the conventional diesel combustion especially at high load conditions. On the other hand, the isobaric combustion led to high soot levels compared to the conventional diesel combustion due to the undesirable spray-to-spray interactions resulting from a single central injector with multiple consecutive injections which introduced a fuel-rich zones. For the same injection technique, a study of the effect of injection pressure and the number of holes were numerically investigated as means to reduce the soot levels. To further decrease the soot emissions, multiple injector configurations were used and the results showed more than 50% drop in the soot levels and an increase in the indicated thermal efficiency due to the lower heat transfer losses. The successful injection strategies for low-emission isobaric combustion mode have further motivated research about fuel flexibility. The potential of using fuels from different sources with varying reactivity was explored by utilizing the high pressure combustion. Various primary reference fuels (PRFs) were employed at the same middle engine load, varying from PRF0 up to PRF100. Different injection methods from a single to four injections were studied. The results demonstrated that various PRFs showed significant discrepancies when using a single injection method, owing to the different fuel auto-ignition capability. On the other hand, excellent fuel flexibility was achieved by employing a small pilot injection, under this condition various fuels led to similar engine combustion performance and emissions. Exhaust gas recirculation (EGR) was used as a way to reduce NOx emissions where 50% EGR was employed. To reduce soot emissions, various volume fractions of three shorter-chain alcohols (methanol, ethanol, and n-butanol) were blended with the baseline fuel (n-heptane). The methanol-blended fuels yielded the lowest soot emissions, but the worst fuel economy was obtained due to the highest heat transfer losses. By increasing the nozzle number and introducing an adequate amount of isochoric combustion, the fuel economy for pure methanol combustion was effectively promoted. The second project was focused on ultra-lean hydrogen combustion using CONVERGE CFD as computational framework. The problem of numerically detecting engine knock and the methods to mitigate such a problem were addressed. Different combustion modes such as port fuel injection spark ignition (PFI SI), homogenous charge compression ignition (HCCI), and pre-chamber (PC) were investigated. The effects of the chemical mechanisms in terms of ignition delay time and laminar flame speed were studied. Starting with the simple combustion mode using PFI SI, high engine knock tendency was observed. The effects of compression ratio, air-fuel-ratio, and spark time were examined as means to reduce engine knock. Upon mitigating the engine knock issue, a comparative study of the PFI spark ignition and the PC modes was conducted. The results revealed that the current used design of the PC introduced high turbulence levels, which resulted in high heat transfer losses to the engine piston. In general , all of these studies (isobaric and hydrogen combustion) were aimed to increase the overall engine efficiency and reduce the emissions.

This thesis aims to contribute to the research and development of a new highly efficient split-cycle engine concept – the double compression-expansion engine (DCEE) – by expanding the knowledge of combustion processes suitable for this and, potentially, other modern engines, via experimental and computational studies. In this work, first, the importance of continued improvement of internal combustion engines is demonstrated by comparing the life-cycle CO2 emissions of different modes of transport, including walking and bicycling. Then, an isobaric combustion concept is proposed for use in modern high-pressure combustion engines, such as the DCEE. Isobaric combustion is compared to conventional diesel combustion at different pressure levels, fueling, and EGR rates, and shown to reduce cylinder wall heat transfer losses by 20 %, simultaneously improving the NOx emissions by a factor of two. An in-situ injection rate measurement technique is developed and applied to improve the understanding of the complex injection strategies required for isobaric combustion. It is also shown that isobaric combustion is possible to achieve with a single fuel injector, but using multiple injectors may offer additional benefits of even lower heat losses, better heat release control, and improved soot and NOx trade-off. Then, an alternative combustion system to the diesel isobaric is proposed – a hydrogen direct-injection (DI) compression-ignition (CI) combustion concept, which has the advantage of ideally eliminated CO2 and soot emissions. DICI H2 combustion is found to differ significantly from conventional diesel, most importantly, in terms of the injected and retained momentum, and in-cylinder flow patterns and fuel-air mixing. Thus, a completely different optimization path must be taken for H2 engines, which involves maximizing the free-jet mixing phase of combustion while minimizing the momentum-dominated global mixing phase. This is achieved computationally in this work by adapting the combustion chamber shape to the H2 jets and modifying the injector nozzle, which proved effective. Finally, hydrogen combustion is computationally compared to diesel in the context of the DCEE on the basis of thermodynamic system parameters and detailed energy breakdown, and proved superior. Brake thermal efficiencies in the range of 56 % are demonstrated for the entire DCEE powertrain fueled with hydrogen.

Extensive analysis of the physical and chemical effects controlling the operation of combustion modes driven by auto-ignition is presented in this thesis. Specifically, the study integrates knowledge attained by analyzing the effects of fuel molecular structure on auto-ignition, quantity or quality of charge dilution, and in-cylinder temperature and pressure on burning characteristics in single and multiple injection strategies employed in compression ignition (CI), partially premixed combustion (PPC) and homogenous charge compression ignition (HCCI) engines. In the first section of the thesis, a multiple injection strategy aimed to produce heat at a constant pressure, commonly known as isobaric combustion, has been studied. Then, to eliminate the complexity of spray-to-spray interactions observed with isobaric combustion, the second section of the thesis is focused on compression ignition (CI) through single injection. In the final section, the presentation will move towards moderate conditions with high dilution, in which combustion becomes dominated by chemical kinetics. At these conditions, there is emerging evidence that certain fuels exhibit unusual heat release characteristics where fuel releases heat in three distinctive stages. Overall, the thesis discusses factors controlling the auto-ignition for CI, PPC and HCCI engines that can provide valuable insights to improve their operation. Isobaric combustion in CI engine involves large interactions between physical and chemical effects. Injection of spray jets into oxygen-deprived regions catalyzes the mechanism for soot production – urging to employ either multiple injectors, low reactivity fuel or an additional expansion stage. Fuels – regardless of their auto-ignition tendency – share the same combustion characteristics in the high load CI, where auto-ignition is controlled by only the injector’s physical specifications. Such observation is a showcase of the fuel flexible engines that has the potential of using sustainable fuels – without being restrained by the auto-ignition properties of the fuel. The thesis provides evidence from experiment and simulation that three-stage auto-ignition is indeed a phenomenon driven by chemical kinetics. Three-stage auto-ignition opens the perspective to overcome the limitation of the high-pressure rise rates associated with HCCI engine.