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Ammonia serves as an efficient hydrogen energy carrier, holding promise for achieving carbon neutrality in the internal combustion engine (ICE) industry. To boost the thermal efficiency of ammonia ICE, plasma-induced jet ignition (PIJI) is proposed with hydrogen as a combustion promoter. A zero-dimensional model of the PIJI ammonia-hydrogen engine, integrating SENKIN and ZDplaskin, reveals that plasma ignition consumes only 1/10 of the energy of spark ignition (SI) in the jet chamber. Active radicals resulting from the plasma reaction of O2 and NH3 expedite combustion in the jet chamber. By extending discharge duration from 3 ns to 4 ns, increasing discharge frequency from 30 kHz to 100 kHz, and strengthening the electric field from 350 Td to 450 Td, energy consumption is reduced by 80%, 50%, and 95% respectively. A low temperature exothermic phase is observed before the auto-ignition of the main chamber, irrespective of the ignition method. • A novel ignition method, plasma-induced jet ignition(PIJI), is proposed. • 0D plasma-induced ammonia-hydrogen jet ignition engine model is developed. • The ignition energy of plasma ignition is 1/10 of spark ignition. • Active radicals resulting from the collision of O 2 and NH 3 expedite combustion. [ABSTRACT FROM AUTHOR]

Lubricating oil-induced pre-ignition is a critical issue that requires attention in downsized gasoline engines and marine low-speed two-stroke natural gas engines. As a result, the ignition behavior of lubricating oil at high temperatures and pressures has been extensively studied. In some cases, when studying the ignition of oil droplets using a rapid compression machine, an explosion-like behavior of the oil droplets is observed, producing a soot cloud that can spread throughout the combustion chamber, especially when the ignition delay time of the ambient gas is short. To gain detailed insights into the mechanism of oil droplet explosion, the explosion process under initial pressures from 13 to 31 bar and temperatures from 700 to 1600 K was visualized using high-speed photography and microphotography on a rapid compression machine. The effects of temperature and shock waves were experimentally investigated, and droplet deformation after shock wave impact was calculated using a simple model. The results demonstrated that high temperature does not have a significant effect on droplet explosion under the conditions studied in this paper. The shock wave impact is the primary cause of the droplet's explosion. [ABSTRACT FROM AUTHOR]

Multiple ignition is beneficial to the improvement of engine thermal efficiency since it helps to shorten combustion duration and enhance lean-combustion stability. However, more researches are still needed to further understand knocking combustion under multiple ignition conditions, especially under lean-burn conditions. This study presents an investigation into the knocking characteristics under lean to stoichiometric conditions using multiple ignition on an optical rapid compression machine (RCM). Up to six ignition configurations were tested under three equivalence ratios (0.7/0.85/1). High-speed photography and high-frequency pressure acquisition were used to record the combustion processes. The results showed that the burning velocity increased nonlinearly with the increase of spark plug numbers while the knocking intensity (KI) first decreased and then increased, exhibiting a "U-shaped curve" trend. The burned mass fraction (BMF) at the instant of end-gas auto-ignition was also affected by ignition configuration. Generally, the smaller the BMF is at the instant of auto-ignition, the stronger the KI becomes. Furthermore, the key product during iso-octane oxidation, ·OOQOOH, was selected as an indicator to conduct chemical kinetics analysis on end-gas oxidation process. Results showed that the ignition configuration had strong impact on the heat release during flame propagation, which further influenced the reaction process of end-gas by influencing its low-temperature reactions. Under moderate spark plug number (around 3), the heat release during the flame propagation was most intensive, which further suppressed the low-temperature reactions of the end-gas and thus weakened auto-ignition intensity. [ABSTRACT FROM AUTHOR]

The application of ammonia and hydrogen in internal combustion engines (ICE) is a promising zero-carbon technology. This paper investigates ammonia combustion in ICE using hydrogen multiple-injection jet-ignition (MIJI). The experiment was conducted on a single-cylinder heavy-duty engine with a compression ratio of 17 and a cylinder bore of 123 mm. In the passive jet ignition mode, the minimum hydrogen energy ratio for stable operation of the ammonia-hydrogen engine was 15 %. Using H 2 active jet ignition could improve the combustion stability significantly. It was found that engine performance was gradually improved by H 2 injection strategy in order of single-injection, double-injection to triple-injection. The triple-injection strategy could achieve a more ideal distribution of ammonia-hydrogen mixture: the first injection forms a homogeneous mixture to increase fuel reactivity; the second injection creates a stratified hydrogen mixture, facilitating rapid flame propagation; the third injection forms a relatively rich mixture in the jet chamber conducive to spark ignition. Through the optimization of the hydrogen injection strategy and spark timing, stable combustion (COV = 1 %) and high thermal efficiency (ITE = 42.5 %) in ammonia-hydrogen engines were achieved, with a minimum hydrogen energy ratio less than 3 %. • Both active and passive H2 jet ignition for NH3 combustion in ICE were tested. • NH 3 combustion using H 2 multiple-injection jet-ignition in ICE is achieved. • NH 3 –H 2 engine achieves high thermal efficiency at 3 % hydrogen energy ratio. • H 2 triple-injection strategy forms ideal distribution of ammonia-hydrogen mixture. [ABSTRACT FROM AUTHOR]

The ignition phenomena of stoichiometric methanol/oxygen/argon mixture are comprehensively investigated at p = 12–24 bar, T = 840–1000 K, using a rapid compression machine (RCM). A strong tendency of stochastic ignition followed by non-forcible flame propagation at a speed of ∼11 m/s is demonstrated. Such an event may be responsible for the pre-ignition or super-knock issue in methanol engines. Under engine relevant spark ignition conditions, different ignition regimes are observed in the end-gas, including thermal explosion, supersonic deflagration, and detonation, characterized by the Chapman–Jouguet velocity criterion. All modes originate from a similar, early auto-ignition ahead of the spark-triggered flame. The auto-ignition process is proved to be dominated by chemical kinetics, where the Livengood–Wu correlation is applicable. In addition, the transition mechanism of different ignition regimes is thoroughly validated against previous ignition theories. Basically, the detonation onset is closely related to the initial thermodynamic condition of the mixture. At a given temperature, the decrease of pressure induces the gradual substitution of detonation by supersonic deflagration and thermal explosion due to a smaller reactivity gradient in the end-gas. The ε − ξ diagram proposed by Bradley is adopted to interpret the transition mechanism of methanol, which turns out to be different from previous results of isooctane. A moderate burned mass fraction range of 0.35–0.45 is found when detonation is initiated. [ABSTRACT FROM AUTHOR]

• A heat transfer model considered flame-wall interaction was constructed. • The mechanism of iso-octane with 24 species and 25 reactions was developed. • The heat transfer model and reaction mechanism are validated at different loads. • The peak-pressure difference between the experiment and simulation is only 0.1 %. With the advancements in hybrid vehicle technology and the increasing stringency of fuel consumption regulations, engines are operating more frequently in the high-efficiency zone of medium to high load, aiming to improve fuel economy and enhance vehicle endurance. However, this can lead to elevated engine thermal loads, and require numerous computational fluid dynamics (CFD) simulations to optimize the combustion system of gasoline engines. To address these challenges, it is crucial to accurately calculate the heat transfer process in CFD calculations. Furthermore, it is necessary to optimize and simplify the skeletal reaction mechanism of gasoline-like fuels to predict the combustion process effectively and reduce computational costs. This investigation presents a new heat transfer model and a new iso-octane skeletal reaction mechanism. The heat transfer model takes into account the boundary layer structure and considers the influence of the chemical source term on the heat transfer process of boundaries. Consequently, it enables more accurate predictions of in-cylinder pressure and heat release rate under different engine loads. Moreover, the new skeletal reaction mechanism with 24 species and 25 reactions has been validated against simulation results obtained using the detailed LLNL reaction mechanism, specifically at high pressures and over a wide range of temperatures and equivalence ratios. These validations demonstrate the promising potential of the proposed reaction mechanism for gasoline engine applications, particularly under higher engine load conditions. The difference of peak-pressure between the experiment and simulation coupled with the new heat transfer model and the new reaction mechanism is only about 0.1 % under the high engine load. [ABSTRACT FROM AUTHOR]

Rapid compression machines (RCMs) are intensively used in auto-ignition investigations for their high similarity to real engines and their convenience for optical visualization. However, previous works using RCMs usually visualized the combustion processes from only a single view, which is limited in capturing critical details regarding ignition in other directions due to line-of-sight integration, given that ignition is actually three-dimensional. This work presents a comprehensive investigation of auto-ignition behaviors in a novel dual-window optical RCM. By simultaneously providing fully-optical accesses into the entire combustion chamber from both the end- and lateral-view, the auto-ignition of primary reference fuels (PRFs) under various initial conditions were captured and the impacts of vortices were examined. Images obtained from dual perspectives demonstrated that auto-ignition under vortex-minimized conditions always initiated from the end-walls and then propagated in the chamber axial direction, exhibiting obvious "near-wall initiation, off-wall propagation" characteristic. Such feature was insignificantly impacted by fuel reactivity, initial thermodynamic condition, and the existence of initial spark-ignited flame. Under vortex-existing conditions, however, the initiation and development of auto-ignition was highly dependent on the interaction between the turbulent field and the fuel reactivity. Only for cases with Da >1, were vortices observed to have distinct impacts on auto-ignition characteristics. At last, the influences of ignition delay time gradient were also discussed. [ABSTRACT FROM AUTHOR]

Super-knock is the main obstacle to improve power density and engine efficiency of modern gasoline engines. To understand the mechanism of super-knock, this study presents an investigation on the end gas combustion process of stoichiometric isooctane/oxygen/nitrogen mixture using a rapid compression machine (RCM), under the thermodynamic conditions close to those of production engines. The combustion process was captured by simultaneous high speed direct photography and pressure acquisition in the RCM. Three end gas combustion modes: no-auto-ignition, sequential auto-ignition, and detonation under different initial conditions were identified and characterized. The super-knock in engine was confirmed to be the result of detonation by comparing the pressure oscillation, thermodynamic state, and pressure rise relative to isochoric combustion with those of detonation observed in the RCM. The experimental results also indicate that the possibility of detonation occurrence increases with increasing initial pressure under the same compression ratio. However, comparing to the pressure, temperature has less effect on detonation formation. It was found that the end gas combustion mode is closely related to the mixture energy density. Generally, as the mixture energy density increases, the end gas combustion mode gradually transits from no-auto-ignition to sequential auto-ignition, and then to detonation. The first auto-ignition spots commonly appear in the mixture near the cylinder wall. The detonation was initiated by near-wall auto-ignition. [ABSTRACT FROM AUTHOR]

Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's ε - ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the ε - ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture. [ABSTRACT FROM AUTHOR]

The decarbonization movement has sparked interests in ammonia (NH 3) as a fuel for internal combustion engines in the transportation sector. Despite its promise, the poor combustion and emission performance are the main obstacles to widespread application. The purpose of this paper is to investigate the combustion and emission performance of a single-cylinder spark ignition engine fueled with NH 3 /gasoline blends under different operating conditions. The effects of NH 3 blending ratio, engine load, EGR ratio, engine speed, intake variable valve timing (VVT) phase and the composition and octane of the fuel were examined. The results indicated that blending NH 3 could effectively suppress knock, optimize combustion phase and improve thermal efficiency. Blending NH 3 had minimal impact on flame propagation in the main combustion duration (denoted by CA50 - CA10 and CA90 - CA50) under knock-free conditions but negative effects were exhibited on the development of initial flame kernel. The combustion stability under high NH 3 blending ratios were improved by increasing engine load, decreasing EGR ratio, etc. Furthermore, stable and efficient combustion of pure NH 3 with COV of 1.3% and indicated thermal efficiency of 41% was realized with a commercial ignition coil and without intake-charge heating. The fuel-type NO x exhibited a positive temperature correlation and negative pressure correlation. Accordingly, NO x emissions were decreased with spark timing (ST) advancing within a specific range of NH 3 blending ratios. However, it remained essentially unchanged or showed a slight increase with advanced ST at high NH 3 blending ratio. NH 3 emission was simultaneously affected by both the "crevice mechanism" and the "flame quenching mechanism". Particularly, the influence of the "flame quenching mechanism" intensified under higher NH 3 blending ratio and lower engine load conditions. N 2 O emission was regulated by the low-temperature oxidation path of NH 3. Increasing the combustion temperature could effectively decrease N 2 O emission. • Performance of SI engine fueled with NH 3 /gasoline was systematically investigated. • The fuel economy was improved through NH 3 addition under high load conditions. • Stable and efficient combustion of pure NH 3 with COV = 1.3% and ITE = 41.0% was achieved. • Factors influencing NO x , NH 3 and N 2 O emissions were identified. [ABSTRACT FROM AUTHOR]

• Effect of NH 3 blending was investigated in a gasoline engine. • Optimized combustion performance was obtained with NH 3 blending. • Emissions from the gasoline/NH 3 were analyzed in detail. • The NO x emission laws were revealed by the chemical reaction analysis. Mitigating climate change involves the greater adoption of carbon–neutral and renewable energy sources within the transportation sector. Ammonia (NH 3), as a carbon-free and sustainable fuel, has garnered growing interest in recent years. The present study aims to investigate the impact of NH 3 blending on combustion and emission characteristics of a stoichiometric spark-ignition gasoline engine, with a particular emphasis on nitrogen-based emissions. The experimental investigation was complemented by chemical kinetic calculations. The results showed that NH 3 blending could effectively suppress engine knock, optimize combustion phase and improve thermal efficiency. For pure gasoline, advancing the spark timing resulted in increased NO x emissions. However, when NH 3 was blended, NO x emissions decreased with advancing spark timing, indicating a negative correlation with pressure. The NH 3 emission was attributed to the 'crevice mechanism' as well as the absorption/desorption in the lubricant oil film on the cylinder wall. Chemical kinetic analysis revealed that the NO x emission from NH 3 blended combustion is closely related to reactive radicals such as OH, H and O. The reduction in NO x emissions under high-pressure conditions was primarily attributed to the consumption of these reactive radicals via three-body reactions. Interestingly, NO x emissions initially increased with increasing NH 3 blending ratio but eventually followed a decreasing trend. This can be attributed to the lower combustion temperature, lower concentration of reactive radicals, and enhanced de-NO x reactions. [ABSTRACT FROM AUTHOR]

Ammonia-hydrogen engines are promising for heavy-duty, long-distance transportation, while the lower hydrogen energy share (X H2) in ammonia-hydrogen fuels facilitates hydrogen storage and use. In this study, a numerical investigation of engine performance based on an engine with ammonia port injection and different hydrogen jet ignition strategies at a fixed X H2 of 3% was conducted. The results show that too-early hydrogen injection results in lower hydrogen concentration and weak ignition in the pre-chamber, while too-late injection results in less hydrogen entering the main chamber, making more ammonia not mix well with the hydrogen, both of which increase the combustion duration. Hydrogen injection at the bottom dead center before the compression stroke mixes the hydrogen with the ammonia preliminary and leads to the formation of a high-temperature, slightly lean-burn region during the combustion, which worsens NO emission but reduces N 2 O emission. Compared with hydrogen single injection, splitting the same mass into two equal-mass injections favors higher peak pressures, shorter combustion durations, and higher indicated thermal efficiency (ITE). Injections at 360 and 90 °CA before the top dead center, respectively, achieve at least 1% relatively higher ITE with similar NO, NH 3 , and N 2 O emission performance compared with the single injection. • Study of a large bore H 2 active jet ignition NH 3 –H 2 engine at 3% H 2 energy share. • Multiple injection favors shorter combustion durations and higher engine efficiency. • Injection at the bottom dead center likely to lead to high NO and low N 2 O emissions. • Injecting hydrogen too early or too late both prolongs the combustion duration. [ABSTRACT FROM AUTHOR]

Ammonia, ethanol, and methanol are promising carbon-neutral fuels in the future. Therefore, the use of alcohol fuels to enhance ammonia combustion in engines deserves further study. The performance of a spark ignition engine with high compression ratio at various loads was experimentally investigated to determine the impact of fuel composition. Specifically, ethanol and methanol were chosen as the primary fuels, while gasoline was assigned as the control group. Gaseous ammonia was introduced through the intake port. The results show that ammonia increases indicated thermal efficiency (ITE) in different ways when blended with different fuels, with the gasoline group obtaining higher ITE (2.8 % relative increase) by optimizing combustion phase, and the ethanol and methanol groups directly enhancing ITE (1.2 % and 0.8 % relative increase) with the same combustion phase due to reduced heat transfer loss. Methanol group shows a more ideal combustion duration under low loads and large ammonia blending ratios and achieves higher ITE due to higher oxygen content. Replacing gasoline with ethanol and methanol significantly reduces greenhouse gas (GHG) under similar NO x emissions, and the advantage of blending carbon-containing fuels with ammonia to reduce GHG is more significant under high loads and large ammonia blending ratios. • NH 3 raises fuel economy by optimizing combustion phase when blending with gasoline. • NH 3 raises fuel economy by reducing heat transfer loss when blending with alcohol. • Methanol performs better than ethanol for low loads and large NH 3 blending ratios. • NH 3 blending may raise greenhouse gas emissions due to worsened N 2 O emissions. [ABSTRACT FROM AUTHOR]

Ammonia is a carbon-free fuel with significant potential to minimize carbon emissions. However, ammonia has weak combustion properties, necessitating more study to improve its combustion performance in engines. A numerical simulation was conducted to evaluate the impact of fuel composition and injection-ignition synergy strategy on the performance of an ammonia-hydrogen spark ignition engine with liquid ammonia direct injection and hydrogen port injection. Specifically, two distinct injection modes were investigated: injection after intake valve close (IAIVC) and injection before top dead center (IBTDC). The outcomes reveal that the IBTDC mode generates a strong stratification of ammonia near the top dead center, resulting in ammonia-rich combustion, then leading to enriched hydrogen production and finally enhancing ammonia combustion and shorting the combustion duration. Liquid ammonia in-cylinder direct injection reduces the combustion temperature and decreases NO emissions. Optimizing the injection timing and spark timing based on a split injection strategy results in lower fuel consumption and emissions. Specifically, NO emissions decrease from 30.5 g/kWh to 21.7 g/kWh at a similar ITE (≈43.5%), and ITE increased from 43.3% to 44.3% for similar NO emission (≈30.0 g/kWh), respectively, with the reduction in both NH 3 and N 2 O emissions. • Ammonia-rich combustion in engines can produce hydrogen. • Ammonia pyrolysis to produce hydrogen shortens combustion duration. • Late injection of ammonia improves thermal efficiency and reduces emissions. • Liquid ammonia in-cylinder direct injection reduces NO emissions. [ABSTRACT FROM AUTHOR]

Super-knock has been the main obstacle to improve power density and engine efficiency of modern highly turbocharged gasoline engines. Previous researches show that pre-ignition is the inducement of super-knock, while detonation is the root reason of how super-knock could damage engines dramatically. Lots of studies have been conducted to eliminate pre-ignition for suppressing super-knock indirectly. This work applies the fuel of methane to suppress detonation and then to suppress super-knock directly using a rapid compression machine (RCM). Furthermore, 1-D simulation model was set up to investigate the mechanism why methane could suppress detonation and super-knock. Finally, through single cylinder engine tests, this suppression strategy in engine practical usage was validated. The experiment and simulation results show that replacing the fuel from iso-octane or gasoline to methane while keeping other conditions identically could transfer detonation combustion mode to flame propagation. The peak pressure could be reduced dramatically and there is no pressure oscillation. Therefore, methane mixture could suppress detonation and then to suppress super-knock effectively, even if pre-ignition already exists. It could be an effective and practical control strategy to protect modern highly turbocharged engines. • First study concerning using methane to eliminate detonation for suppressing super-knock directly. • Replacing the fuel from iso-octane to methane could transfer the combustion mode from detonation to flame propagation. • 1-D direct simulation, RCM and engine experiments show that methane is an effective super-knock suppression strategy. [ABSTRACT FROM AUTHOR]

Abstract Super-knock is the main obstacle to improve power density and fuel efficiency of highly boosted gasoline engines. Previous investigations show that pre-ignition and detonation are the two key combustion processes of super-knock. The former is the inducement and the latter is the root reason of how super-knock could damage engines dramatically. Lots of studies have been conducted for suppressing super-knock through eliminating pre-ignition. Using a rapid compression machine, this study explores the stoichiometric propane or methanol mixture to suppress super-knock by eliminating detonation. Under the same pressure at the end of compression, the same fuel energy density and the same charged fresh air, the peak pressure of propane mixture could be reduced dramatically and the pressure oscillation could be eliminated, when compared to iso-octane mixture. The combustion could be transferred from detonation to flame propagation. For methanol mixture, the combustion process could be transferred from detonation to weak auto-ignition with low peak pressure and negligible pressure oscillation. These indicate that both propane and methanol mixture could suppress detonation and thus suppress super-knock effectively, even if pre-ignition occurs. It could be an effective and practical control strategy to protect modern highly turbocharged spark ignition engines. Highlights • First study using propane or methanol mixture to eliminate detonation for suppressing super-knock directly. • Using propane mixture could transfer the combustion mode from detonation to flame propagation. • Using methanol mixture could transfer the combustion mode from detonation to weak sequential auto-ignition. • Eliminating detonation with the propane or methanol mixture is conducted by preventing the generation of shock wave. [ABSTRACT FROM AUTHOR]

Dedicated exhaust gas recirculation (D-EGR) is to generate H 2 via fuel-rich combustion and viewed as a potential technique to meet future emission regulations without further after-treatment. In this study, firstly, the H 2 /CO formation through fuel-rich combustion in a single-cylinder natural gas spark ignition engine was quantitatively characterized by gas chromatography. Then, the effect of H 2 /CO addition on stoichiometric natural gas combustion performance and emission characteristics at 15% and 20% EGR levels was investigated. Finally, reaction path analysis and the brute-force sensitivity of ignition delay were conducted to evaluate the effect of H 2 addition on reaction process. The yields of H 2 and CO approximately linearly increased from ∼2% to ∼10% as the equivalence ratio varied from 1.1 to 1.5. The H 2 /CO addition accelerated the flame speed of mixture and significantly shortened the combustion duration, significantly improving the indicated thermal efficiency and the total unburned hydrocarbon with the acceptable penalty of increased NOx and CO emissions. Numerical results revealed that OH + H 2 = H + H 2 O and H + O 2 = O + OH were the most sensitive reactions with the presence of H 2 . This study delivered a quantitative basis for the optimization of D-EGR fueling strategies in natural gas engines. [ABSTRACT FROM AUTHOR]

Increasing the compression ratio of gasoline engines is a promising method to increase the engine's fuel economy. However, engine knock caused by auto-ignition is still a large obstacle to improving thermal efficiency and engine load for high compression ratio hybrid engines. Spark induced compression ignition (SICI) is an effective way to utilize auto-ignition to solve the aforementioned problems. Meanwhile, ammonia, a carbon-free fuel, with an outstanding antiknock property, has the great potential to be used in SICI mode. In this study, the effects of ammonia addition on knock suppression, combustion characteristics, thermal efficiency, and emission performance were investigated in a high compression ratio (15.5), four-valve, single-cylinder gasoline engine under SICI combustion mode. In experiments, gasoline was directly injected into the cylinder while ammonia was injected into the intake port. The results show that blending ammonia could resist engine knock and improve thermal efficiency. Within the knock limitation, the duration of flame propagation under ammonia blending conditions could be shortened and meanwhile, the auto-ignition becomes weakened compared with pure gasoline. Benefiting from combustion phase optimization, the thermal efficiency and engine load could be increased or maintained at optimal ammonia blending ratio. The maximum increase of thermal efficiency and engine load is 2.46% and 0.2 MPa respectively. Moreover, the increased engine load can extend the limit of the ammonia blending ratio. For nitrogen emissions, blending ammonia results in NO x emission deterioration due to the formation of fuel-type NO x. NO x emission has a weak dependence on the ammonia blending ratio, and the trend of NO x emission varied with spark timing is opposite to pure gasoline conditions, which is closely related to the pressure sensitivity of fuel-type NO x. Ammonia slip was also detected in the engine exhaust because of the incomplete combustion. • This is the first study revealing the effect of NH 3 addition on SICI mode. • NH 3 addition with spark timing adjustment could suppress knock without reducing flame speed. • Thermal efficiency increases through combustion phase optimization when blending NH 3. • There is a negative correlation between pressure and fuel-type NO x. [ABSTRACT FROM AUTHOR]