Computational Studies of Isobaric and Hydrogen Internal Combustion Engines

  • Hammam H. Aljabri

Student thesis: Doctoral Thesis


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.
Date of AwardMar 2023
Original languageEnglish (US)
Awarding Institution
  • Physical Sciences and Engineering
SupervisorHong G. Im (Supervisor)


  • Isobaric combustion
  • Hydrogen combustion
  • pre-chamber combustion

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