This editorial piece emphasizes the importance of low-carbon, synthetic electro-fuels (e-fuels) in decarbonizing transport, which complements other decarbonization measures, including efficiency improvements, vehicle electrification and hybridization, as well as bio-based renewable fuels. Given the stringency of the emissions legislations worldwide, and the desire for many countries to achieve net-zero CO ₂ emissions by 2050, synthetic electro-fuels, produced using renewable electricity, water and captured CO ₂ , has the potential to deliver significant CO ₂ reductions. When e-fuels are designed to be chemically similar to conventional fuels, they can be compatible with existing vehicles and infrastructure. Therefore, e-fuels can be a drop-in solution to facilitate an accelerated decarbonization of the existing legacy fleet as well as next generation IC engines. Furthermore, e-fuels could unlock greater penetration of renewable electricity into the transport sector by enabling international trade of renewable electricity in the form of liquid energy carriers. This is particularly useful for matching lower cost renewable electricity producers from regions with high solar irradiation and wind potential, with existing and growing energy demand centers globally that have limited renewable electricity generation potentials. The paper highlights the potential of several e-fuels: hydrogen, ammonia, e-gasoline, alcohols, DMC, paraffinic fuels, OMEx (DME), and their use in internal combustion engines, considering both spark-ignition and compression-ignition combustion, with an insight on dual-fuel and reactivity-controlled CI combustion modes. Advantages in terms of combustion efficiency and pollutant emissions are also comprehensively discussed, together with the challenges related to their use and production. Finally, the paper remarks the importance of Life-Cycle-Assessment (LCA) to assess the climate change mitigation potential of alternative fuels and powertrains. LCA studies reveal that there is no silver-bullet to decarbonizing transport, neither fuels nor powertrain technologies, since the CO ₂ reduction potential of these technologies is highly dependent on how they are produced and operated throughout their entire life cycles.

CO2 emission constraints taking effect from 2020 lead to further investigations of technologies to lower knock sensitivity of gasoline engines, main limiting factor to increase engine efficiency and thus reduce fuel consumption. Moreover the RDE cycle demands for higher power operation, where fuel enrichment is needed for component protection. To achieve high efficiency, the engine should be run at stoichiometric conditions in order to have better emission control and reduce fuel consumption. Among others, water injection is a promising technology to improve engine combustion efficiency, by mainly reducing knock sensitivity and to keep high conversion rates of the TWC over the whole engine map.The comprehension of multiple thermodynamic effects of water injection through 3D-CFD simulations and their exploitation to enhance the engine combustion efficiency is the main purpose of the analysis. As basis for the research a single cylinder engine derived from a 1l turbocharged 3-cylinders engine is used to evaluate indirect and direct water injection. The entire engine flow field is reproduced and analyzed with 3D-CFD simulations and numerical models are employed to separate the influence of chemical and thermodynamic properties. Measurements are performed with different injectors for indirect/direct water injection in the single-cylinder engine in order to assess water break-up, wall wetting, spray interaction and penetration. Several injection strategies, such as varying start of injection, injection pressure, and water to fuel ratio, are tested at the single-cylinder engine test bench. Detailed gas phase chemistry is employed to link flame front speed with water concentration and knocking occurrence. These results are correlated with the 3D-CFD simulation of mixture formation, in-cylinder flow and water distribution for two different operating points (part load and maximum power) in order to study water behavior, with focus on the evaporation process, in-cylinder pressure and temperature profile, as well as the combustion development, during multiple engine cycles.

This work presents the assessment of direct water injection in spark-ignition engines using single cylinder experiments and tabulated chemistry-based simulations. In addition, direct water injection is compared with cooled low-pressure exhaust gas recirculation at full load operation. The analysis of the two knock suppressing and exhaust gas cooling methods is performed using the quasi-dimensional stochastic reactor model with a novel dual fuel tabulated chemistry model. To evaluate the characteristics of the autoignition in the end gas, the detonation diagram developed by Bradley and co-workers is applied. The single cylinder experiments with direct water injection outline the decreasing carbon monoxide emissions with increasing water content, while the nitrogen oxide emissions indicate only a minor decrease. The simulation results show that the engine can be operated at λ = 1 at full load using water–fuel ratios of up to 60% or cooled low-pressure exhaust gas recirculation rates of up to 30%. Both technologies enable the reduction of the knock probability and the decrease in the catalyst inlet temperature to protect the aftertreatment system components. The strongest exhaust temperature reduction is found with cooled low-pressure exhaust gas recirculation. With stoichiometric air–fuel ratio and water injection, the indicated efficiency is improved to 40% and the carbon monoxide emissions are reduced. The nitrogen oxide concentrations are increased compared to the fuel-rich base operating conditions and the nitrogen oxide emissions decrease with higher water content. With stoichiometric air–fuel ratio and exhaust gas recirculation, the indicated efficiency is improved to 43% and the carbon monoxide emissions are decreased. Increasing the exhaust gas recirculation rate to 30% drops the nitrogen oxide emissions below the concentrations of the fuel-rich base operating conditions.

At specific operating conditions, the auto-ignition in the unburnt mixture that precedes the occurrence of knock in conventional SI engines happens in two stages. In a previous publication, the authors demonstrated that the low-temperature heat release significantly influences the auto-ignition behavior of the mixture, thus severely impairing the prediction capabilities of the Livengood-Wu integral that the majority of the commonly used 0D/1D knock models are based on. Consequently, a new two-stage auto-ignition prediction approach for modeling the progress of the chemical reactions was introduced. It was demonstrated that the proposed auto-ignition model predicts the occurrence of two-stage ignition and accurately considers the significant influence of low-temperature heat release on the mixture’s auto-ignition behavior at various operating conditions. However, the correct prediction of local auto-ignition is not sufficient for the reliable calculation of the knock boundary, as the occurrence of this phenomenon does not necessarily result in knock. Based on the proposed two-stage auto-ignition prediction approach, this paper presents a new knock model for the development of future SI engine concepts in a 0D/1D simulation environment. In addition to not considering low-temperature ignition, the commonly used knock models based on the Livengood-Wu integral assume that no knock can occur after a pre-defined, constant MFB-point. The evaluation of measured knocking single cycles however has revealed that the latest possible MFB-point where knock can occur changes significantly with parameters such as engine speed and EGR rate. Hence, a cycle-individual criterion for the occurrence of knock accounting for the current operating conditions is needed. To this end, an approach based on the unburnt mass fraction in the cool thermal boundary layer at the time of auto-ignition is proposed. Besides the operating conditions, this knock occurrence criterion also considers the flame propagation and the cylinder geometry. Additionally, the already published submodels integrated into the two-stage auto-ignition prediction approach are expanded by the influence of injected water. Thus, as the new knock model also accounts for EGR, fuel composition and properties as well as air-fuel equivalence ratio effects, it fulfills all requirements for the simulation of future engine concepts. The approach contains no empirical measurement data fits and has just one calibration parameter that does not change with the operating conditions. Finally, model validation against measurement data on a handful of different engines at various operating conditions is performed. It is demonstrated that the new model can estimate the knock boundary very accurately with errors in the predicted center of combustion below 2°CA and thus contributes to the cost-effective development of future SI engine concepts in a 0D/1D simulation environment.

Water injection is investigated for turbocharged spark-ignition engines to reduce knock probability and enable higher engine efficiency. The novel approach of this work is the development of a simulation-based optimization process combining the advantages of detailed chemistry, the stochastic reactor model and genetic optimization to assess water injection. The fast running quasi-dimensional stochastic reactor model with tabulated chemistry accounts for water effects on laminar flame speed and combustion chemistry. The stochastic reactor model is coupled with the Non-dominated Sorting Genetic Algorithm to find an optimum set of operating conditions for high engine efficiency. Subsequently, the feasibility of the simulation-based optimization process is tested for a three-dimensional computational fluid dynamic numerical test case. The newly proposed optimization method predicts a trade-off between fuel efficiency and low knock probability, which highlights the present target conflict for spark-ignition engine development. Overall, the optimization shows that water injection is beneficial to decrease fuel consumption and knock probability at the same time. The application of the fast running quasi-dimensional stochastic reactor model allows to run large optimization problems with low computational costs. The incorporation with the Non-dominated Sorting Genetic Algorithm shows a well-performing multi-objective optimization and an optimized set of engine operating parameters with water injection and high compression ratio is found.

Hydrogen and its unique properties pose major challenges to the development of innovative combustion engines, while it represents a viable alternative when it is based on renewable energy sources. The present paper deals with the holistic approach of hydrogen combustion modeling from a 0D/1D reactor evaluation with Cantera up to complete engine simulations in the 3D-CFD tool QuickSim. The obtained results are referenced to the current literature and calibrated with experimental data. In particular, the engine simulations are validated against measurements of a single-cylinder research engine, which was specifically adapted for lean hydrogen operation and equipped with port fuel injection and a passive pre-chamber system. Special attention is hereby given to the influence of different engine loads and varying lambda operation. The focus of this work is the complementary numerical investigation of the hydrogen flame speed and its self-ignition resistance under the consideration of various reaction mechanisms. A detailed transfer from laminar propagation under laboratory conditions to turbulent flame development within the single-cylinder engine is hereby carried out. It is found that the relatively simple reaction kinetics of hydrogen can lead to acceptable results for all mechanisms, but there are particular effects with regard to the engine behavior. The laminar flame speed and induction time vary greatly with the inner cylinder conditions and significantly affect the entire engine’s operation. The 3D-CFD environment offers the opportunity to analyze the interactions between mixture formation and combustion progress, which are indispensable to evaluate advanced operating strategies and optimize the performance and efficiency, as well as the reliability, of the engine.