COMBUSTION PROMOTION IN HYDROCARBON AND CARBON-FREE FUELS FOR INTERNAL COMBUSTION ENGINE APPLICATIONS

This work investigates the use of combustion promoters to enhance the performance of both conventional spark-ignition (SI) engines and advanced concepts such as homogeneous charge compression ignition (HCCI). The study focuses on two main aspects: ozone-assisted combustion of gasoline-like fuels and promoter-assisted combustion of carbon-free fuels such as ammonia (NH3) and hydrogen (H2). Based on the literature review, the first part of this work proposes the use of ozone (O3) to enable stable combustion in SI engines fueled with commercial gasoline under ultra-lean conditions, where high cycle-to-cycle variability typically limits performance. A combined numerical and experimental approach is adopted. Laminar flame speed (LFS) and chemical kinetics of a ternary gasoline surrogate (iso-octane/n-heptane/toluene), named Toluene Reference Fuel (TRF), are analyzed under engine-relevant conditions to understand the impact of O3 on reactivity, flame structure and the main reaction pathway. Results indicate that, consistent with commercial gasolines, the LFS of TRF is highest for stoichiometric to slightly rich mixtures but undergoes a sharp reduction under lean conditions. When O3 is added to the TRF mixture, the LFS increases, especially under lean conditions. However, for reactants temperatures above 550 K at atmospheric pressure, the presence of O3 promotes the formation of a cool flame and the temperature at the flame base rises. This behavior leads to a sharp increase in LFS. The same effect is observed with an increase of the reactants pressure. Indeed, when the reactants temperature is sufficiently high, a rise in pressure promotes the ozone decomposition reaction, O3 + N2 → O2 + Ȯ + N2, leading to a non-monotonic LFS behavior due to cool flame formation. The results indicate that the Ȯ radicals produced from this decomposition promote H-abstraction from the fuel, producing ȮH radicals. The formation of a cool flame can lead to knock in SI engines. However, reducing the equivalence ratio increases the Ignition Delay Time (IDT). Experimental results from the SI engine reveal that, under low engine speed (800 rpm) at partial-load under stoichiometric conditions, the presence of O3 promotes Low-Temperature Combustion (LTC) regime, leading to detonation. In contrast, at a higher engine speed (1400 rpm), no significant effects were observed. Consequently, the equivalence ratio was reduced, under full load, to achieve the ultra-lean mixture condition. Engine experiments confirm that small ozone additions improve combustion stability under ultra-lean operation, reducing Coefficient of Variance (CoV) on Indicated Mean Effective Pressure (IMEP of 6.8 bar) from 4.97% to 3.3% while maintaining stable load. Excessive ozone concentrations, however, promote knock. Additionally, ultra-lean operation enables a reduction in fuel consumption and emissions. The second part of this work regards the combustion process of carbon-free fuels. NH3 shows several challenges when used as fuel in internal combustion engines, due to its high minimum ignition energy, a lower heating value of 18.8 MJ/kg, high ignition temperature and low LFS. Based on literature review, oxygen (O2) is selected as a combustion promoter. Therefore, the use of O2 was experimentally investigated in SI engine fueled with NH3/air mixtures at partial load at 1000 rpm. The main results indicate that reducing the intake pressure (pin = 0.94 bar) leads to unstable combustion with a high CoVimep. Under O2-assisted combustion, small amount of the combustion promoter (22.4% by vol.) in the intake air reduces the CoVimep and stabilizes the IMEP at 5.8 bar. When the intake pressure was further decreased to 0.86 bar (low-load condition), an O2 concentration of 32.5% is required to stabilize combustion by removing the misfires, resulting in an IMEP of 4.3 bar. Simulations are carried out using the 0-D dual zone SI engine model to predict the effects of O2 on emissions. As expected, the higher in-cylinder pressure, resulting from increased flame propagation under O2-assisted combustion, led to a rise in in-cylinder temperature. This temperature increase promotes the formation of nitrogen oxides (NOx), which in these experiments consisted primarily of nitric oxide (NO). 1-D numerical of LFS are also performed to estimate the increase in flame propagation at spark timing conditions. The results highlight that the chain-branching reaction O2 + H ↔ Ȯ + ȮH plays a crucial role even under O2-assisted combustion increasing the Ȯ, ȮH and HȮ2 radical pool. For NH3 oxidation, the H-abstraction is mainly driven by the reaction NH3 + ȮH↔ NH2 + H2O. The role of other reactions, which involve Ȯ and H, NH3 + Ȯ ↔ NH2 + ȮH and NH3 + H ↔ NH2 + H2 is marginal. Based on these findings, the auto-ignition of NH3/H2/air mixtures is experimentally investigated in HCCI engine (compression ratio of 16.4:1) using the aqueous hydrogen peroxide (H2O/H2O2 = 70/30 by vol.) as combustion promoter. Aqueous hydrogen peroxide (H2O2aq) can play a key role in terms of ȮH radicals increase due to the decomposition reaction: H2O2(+M) ↔ 2ȮH(+M). Experimental results show that the injection of 0.68% by vol. of H2O2aq respect to the NH3/H2/air mixture significantly reduces the IDT by promoting fuel oxidation and accelerating the heat release rate, thereby maintaining stable engine operation. When H2 content is reduced from 12% to 8% by vol., it needs a double quantity of H2O2aq to guarantee combustion stability. Furthermore, the addition of H2O2aq improves the thermal efficiency not only by improving combustion but also by lowering exhaust gas temperature. Finally, promising results are achieved by extending the lean limit of the mixture, reducing the equivalence ratio from 0.6 to 0.5. Simulations of the IDT under engine-relevant conditions reveal that the H2O2 decomposition, via H2O2(+M) ↔ 2ȮH(+M), promotes the H-abstraction from the fuel by ȮH radical (NH3 + ȮH ↔ H2O + NH2 and H2 + ȮH ↔ H + H2O). Moreover, a sharp increase in the HȮ2 radical is observed, attributed to the third-body reaction H + O2(+M) ↔ HȮ2(+M), the reactions HNO + O2 ↔ NO + HȮ2 and NH3 + HȮ2 ↔ NH2 + H2O2, which consumes H2O2. However, this latter reaction, at higher temperatures, shows a reversed trend, with any H2O2 formed undergoing rapid thermal decomposition. Kinetic analysis also reveals that the consumption of HȮ2, mainly through NO + HȮ2 ↔ NO2+ȮH, contributes to an increase in the ȮH radicals.