End-gas Autoignition Research Articles

Characterization of pressure oscillations and combustion noise of hydrogen reactivity-controlled compression ignition (RCCI)

Knock and combustion noise are the main reasons limiting the increase in hydrogen (H2) replacement rate in H2 reactivity-controlled compression ignition (RCCI). This study aims to identify the knock-dominated mechanism and elucidate the characteristics of knock and combustion noise of H2 RCCI operated at a broad range of parameters through the combined analysis of the heat release profile and pressure oscillations. The results indicated that under inhomogeneous heat release conditions, the onset of pressure oscillations is mainly attributed to the local fast heat release at the initial combustion stage, but for the conditions with relatively homogeneous and high-reactivity mixture, the pressure oscillations are primarily excited by the end-gas autoignition. Compared with the single injection strategy, the double injection strategy with the first and second start of injection of −60 and −15°CA ATDC (SOI1/2 = −60/-15°CA ATDC) can effectively mitigate the pressure oscillations and shorten the propagation period of pressure waves. However, as the SOI2 is advanced to −30°CA ATDC, the pressure oscillations dramatically increase, because the enhanced fuel reactivity of premixed H2 exacerbates the end-gas autoignition. The results of frequency analysis of in-cylinder pressures show that with the increase in inhomogeneous combustion, the first resonance frequency reduces. As the intake pressure (Pin) is raised from 1.2 to 1.6 bar, the first resonance energy increases, since the elevated in-cylinder temperature associated with the piston compression during the compression stroke enhances the propagation and development of pressure waves. The high-order resonance is closely related to the autoignition of end gas with higher fuel reactivity. Overall, the combustion noise shows a good linear correlation with peak heat release rate for all the points investigated in this work, but the linearity of H2/polyoxymethylene dimethyl ethers RCCI is better than that of H2/diesel RCCI. Moreover, with the increase in Pin, the engine noise associated with the piston compression increases.

In-cylinder pressure and vibration analysis on port fuel injected gasoline engine

Research on knocking combustion is vitally essential since it impacts engine performance regarding noise and emissions, fuel consumption, power density, and durability. Because of end-gas auto-ignition, current SI engines may experience conventional knock, which restricts increasing the compression ratio to increase thermal efficiency. Therefore, this paper examined the in-cylinder pressure and vibration of a port fuel-injected gasoline engine under knocking and non-knocking conditions. The heat release rate was calculated from pressure traces where the crank angle resolutions were not constant, and the conversion from the time domain to the frequency domain of knock sensor signals by using the Fourier Transform was performed. The result reveals that more heat is released during knocking combustion than during non-knocking combustion. When there is a knock, the temperature variation increases. This would facilitate the use of heat transfer studies to detect knock occurrences. Moreover, knocking is caused by an ignition timing that is too advanced, 22°BTC, compared to the non-knocking spark case, 14°BTC. In the frequency domain, the amplitude of the knocking combustion is higher than the amplitude of the non-knocking combustion, indicating the engine’s noise and vibration.

Renewable Methanol as a Fuel for Heavy-Duty Engines: A Review of Technologies Enabling Single-Fuel Solutions

To meet climate targets, a global shift away from fossil fuels is essential. For sectors where electrification is impractical, it is crucial to find sustainable energy carriers. Renewable methanol is widely considered a promising fuel for powering heavy-duty applications like shipping, freight transport, agriculture, and industrial machines due to its various sustainable production methods. While current technological efforts focus mainly on dual-fuel engines in shipping, future progress hinges on single-fuel solutions using renewable methanol to achieve net-zero goals in the heavy-duty sector. This review examines the research status of technologies enabling methanol as the sole fuel for heavy-duty applications. Three main categories emerged from the literature: spark-ignition, compression-ignition, and pre-chamber systems. Each concept’s operational principles and characteristics regarding efficiency, stability, and emissions were analyzed. Spark-ignition concepts are a proven and cost-effective solution with high maturity. However, they face limitations due to knock issues, restricting power output with larger bore sizes. Compression-ignition concepts inherently do not suffer from end-gas autoignition, but encounter challenges related to ignitability due to the low cetane number of methanol. Nonetheless, various methods for achieving autoignition of methanol exist. To obtain stable combustion at all load points, a combination of techniques will be required. Pre-chamber technology, despite its lower maturity, holds promise for extending the knock limit and enhancing efficiency by acting as a distributed ignition source. Furthermore, mixing-controlled pre-chamber concepts show potential for eliminating knock and the associated size and power limitations. The review concludes by comparing each technology and identifying research gaps for future work.

Impact of wall heat transfer modelling in large-eddy simulation of hydrogen knocking combustion

This study assesses the performance of four wall heat transfer models in large-eddy simulations to simulate hydrogen knock in a cooperative research fuel engine. These models include Angelberger, Han and Reitz, O’Rourke and Amsden, and the GruMO-UniMORE. Simulations predict cycles with pressure out of the experimental range in some cases, especially under high knock intensity, with observable developing detonation. The Han and Reitz model is found to be the most suitable model for simulating knocking conditions, considering its performance and simplicity. This model is then used to investigate the wall heat transfer mechanism under knocking conditions. The results show that thermal boundary layers within the cylinder are disturbed through different mechanisms, contributing to auto-ignition close to the knockmeter cavity. The interaction between auto-ignition-induced pressure oscillations and the thermal boundary layer significantly increases the heat transfer rate during end-gas auto-ignition.

Investigation on flame propagation and end-gas auto-ignition of ammonia/hydrogen in a full-field-visualized rapid compression machine

Ammonia-hydrogen engine has attracted much attention due to its carbon-free nature. Given that ammonia has high knock resistance (indicated by its high research octane number of nearly 130), researchers are trying to increase the compression ratio (CR) to improve engines’ thermal efficiency. However, knocking cycles can still be detected in ammonia-hydrogen engines under elevated thermodynamic conditions. To further explore the ammonia-hydrogen knocking process, this study conducted a series of spark-ignition combustion experiments focusing on flame propagation and end-gas auto-ignition in a full-field-visualized rapid compression machine. Four ammonia-hydrogen blended fuels with hydrogen energy fractions of 0 % (H0), 10 % (H10), 20 % (H20), and 100 % (H100) were comparatively tested under the thermodynamic conditions of 30 bar and 750–985 K. The experimental results showed that the flame speed of H0, H10, and H20 is around 3–6 m/s under test conditions, much lower than that of H100, which exceeds 37 m/s at 30 bar/750 K. Strong end-gas auto-ignitions with maximum pressure amplitudes of 77 bar and 101 bar were recorded for H0 at 30 bar/985 K and H10 at 30 bar/915 K, respectively. The two auto-ignition events both exhibited detonation characteristics by clear wavefronts with speeds reaching up to 1620 m/s (H0) and 1812 m/s (H10). Based on the experimental results, simulations were carried out to analyze the chemical process during the end-gas auto-ignition. The simulated results showed that for H0 at 30 bar/985 K, the NO-, NO2-, and H2NO-related reactions contributed most to heat production, which improved the end-gas reactivity and promoted the occurrence of auto-ignition. For H10 at 30 bar/915 K, the H + O2 (+M) = HO2 (+M) became the most exothermic reaction, indicating the hydrogen addition significantly promoted the ammonia's end-gas auto-ignition. Additionally, the detonation events observed in this study can be well classified in the ε -ξdiagram.

Detonation development in H2, CH4, and PRF-air mixtures under engine-relevant conditions: From a chemical perspective

The progression of highly boosted internal combustion engines (ICEs) faces impediments due to the super-knock phenomenon primarily associated with detonation development during end gas autoignition. Although it is well known that hot spot potentially triggers super-knock, quantitative prediction of super-knock and detonation development across diverse states and fuels remains a challenge. This study numerically investigates the transient autoignition process induced by a hot spot, considering different initial states and fuels (including alternative fuels H2, CH4, and conventional PRFs), with an emphasis on the chemical effects. It is found that the intense exothermic stage (IES) in a stoichiometric H2/air mixture greatly differs from those in hydrocarbon/air (CH4/air and PRF/air) and lean or diluted H2/air mixtures. Through sensitivity analysis, key elementary reactions dominating the autoignition are identified, attributing their impact on the IES primarily to their influence on the radical pool rather than on the heat release rate. It is found that only reactions H + O2⇌O+OH and H2O2(+M)⇌OH+OH(+M) significantly affect both excitation time and ignition delay time, suggesting that the kinetic pathway controlling the IES differs somewhat from that governing the overall ignition. For the autoignition induced by a hot spot, two types of detonation feature are identified: one corresponds to stoichiometric H2/air with broad detonation regime and short excitation time, while the other is associated with other fuel/air mixtures exhibiting relatively narrow detonation regime and prolonged excitation time. These findings highlight that the excitation time predominantly governs the detonation development. All hydrogen/air mixtures (stoichiometric, lean, and diluted) manifest a suppressed detonation intensity compared to hydrocarbon/air mixtures. Moreover, due to its significantly higher reactivity in the IES, stoichiometric H2/air has an additional normal detonation sub-mode for hot spot with larger temperature gradient. This sub-mode greatly expands the detonation regime and thereby results in a notably higher detonation propensity for the stoichiometric H2/air mixture.

Study on lean combustion of ammonia-hydrogen mixtures in a pre-chamber engine

Ammonia holds promise for achieving zero carbon emissions from internal combustion engines. The pre-chamber turbulent jet ignition, characterized by multipoint distributed ignition and rapid combustion, could be applied to ammonia combustion to extend combustion limits and stability. This study investigates the impacts of hydrogen energy fraction (Hfrac), equivalence ratio, and ignition timing on the combustion and emission characteristics of ammonia-hydrogen mixture at lean combustion conditions in a pre-chamber (PC) marine engine by CFD Simulation. In H2-fuel active PC mode, increasing the hydrogen content or achieving stoichiometric combustion within the PC will shorten the cold jet phase and accelerate the formation of a more intense hot jet. As the Hfrac and the equivalence ratio in the main-chamber increase, the combustion pressure, temperature, and heat release rate (HRR) rise, while the combustion duration shortens due to enhanced reactivity, potentially leading to end gas auto-ignition and knock due to lower auto-ignition energy of the mixture. NO primarily forms near the flame front, and both NO concentration and the generation area increase with an increase in hydrogen content and equivalence ratio. Fuel-NO is likely to be the dominant factor in NO generation with lean combustion conditions. N2O, as a product of NH3 low-temperature combustion, is mainly concentrated near the flame front. An increase in unburned NH3 emissions corresponds to an increase in N2O emissions. Delaying the ignition timing to reduce NOx emissions is primarily achieved by controlling NO emissions, even though there is a slight increase in N2O.

Low-carbon fuels for spark-ignited engines: A comparative study of compressed natural gas and liquefied petroleum gas on a CFR engine with exhaust gas recirculation

Decades of work on low-carbon fuels have established their potential for substantial emissions reductions; however, their adoption is still limited by infrastructure concerns and engine efficiency deficits. As infrastructures have begun to evolve, research on strategies that maximize engine efficiency through interactions with fuel properties must also now take center place. This paper compares the performance, emissions, and combustion characteristics of two forefront low-carbon fuels: compressed natural gas (CNG) and liquefied petroleum gas (LPG) in a cooperative fuel research (CFR) engine over a range of compression ratios and engine loads. The effects of exhaust gas recirculation (EGR), end-gas auto-ignition, and a novel combustion control tool, the combustion intensity metric (CIM), were also evaluated at different stoichiometric engine operating conditions. In comparison to LPG, CNG operation demonstrated an extended knock-free regime, allowing engine operation at higher engine loads and compression ratios, but LPG operation exhibited enhanced combustion characteristics with higher peak pressures and faster apparent heat release rates (AHRR). LPG operation achieved higher brake thermal efficiencies and lower equivalent CO2 emissions compared to CNG operation at the tested engine loads and compression ratios. LPG demonstrated significantly higher EGR tolerance limits compared to CNG, with a maximum of 28% EGR rate, compared to 23% for CNG. This improved EGR dilution tolerance was responsible for a 90% reduction in NOx emissions for LPG compared to a maximum of 70% with CNG. EGR dilution also exhibited more effective knock mitigation potential with LPG, suppressing knock intensity values by up to 98% and transitioning the engine operation towards normal combustion from heavy knocking conditions. The CIM was found to decrease burn durations and improve the quality of combustion by controlling the desired fraction of end-gas auto-ignition.

Effect of Blending Dimethyl Carbonate and Ethanol with Gasoline on Combustion Characteristics

We investigated the effects of blending dimethyl carbonate (DMC) and ethanol with commercial gasoline on combustion characteristics. Our experimental approach involved using a rapid compression and expansion machine (RCEM) to achieve elevated temperatures and pressures. The fuels containing different volumes of oxygenated hydrocarbons were burned at equivalence ratios of 1.0 or 0.7, an initial temperature of 340 K, and initial pressures of 0.10 or 0.05 MPa. To simulate knocking phenomena, we installed a rectangular channel in the combustion chamber of the RCEM and measured the pressure history inside the chamber. By analyzing the pressure history resulting from the end-gas autoignition, we evaluated the combustion duration and maximum pressure amplitude. Blending both oxygenated fuels with gasoline effectively reduced the maximum-pressure amplitude in the end-gas autoignition, with ethanol exhibiting a more pronounced suppression effect compared to DMC in the same volumetric mixing ratio. At an initial pressure of 0.10 MPa, the combustion durations of DMC/gasoline blends showed non-linear behavior, being shorter than those of pure gasoline and DMC and comparable to those of the ethanol/gasoline blends. However, the blending effect of DMC on combustion durations was greatly mitigated when the initial pressure was reduced to 0.05 MPa. Conversely, the combustion durations for ethanol/gasoline blends showed a nearly monotonic reduction with an increase in the ethanol blending ratio at both initial pressures of 0.10 and 0.05 MPa. Finally, we discussed the differential impact of the blending effect of oxygenated hydrocarbons on combustion characteristics.

Interaction of end-gas autoignition and cold wall in closed chamber

The presence of cold walls in spark-ignition (SI) engines could induce the interaction with end-gas autoignition. This interaction is numerically studied in the current work considering detailed chemistry and transport, with emphasis on near-wall flame structures and dynamics, as well as autoignition-affected wall heat flux. Two alternative fuels, hydrogen, and dimethyl ether (DME), are considered. In hydrogen/air mixtures, autoignition first develops in the end-gas and leaves an unburnt near-wall region. The combustion mode in this region then sequentially behaves as spontaneous ignition, laminar flame, and low-to-intermediate temperature reaction. Correspondingly, the contribution of diffusion and convection in the energy budget respectively increases and decreases as the reaction front moves towards the wall. End-gas autoignition also introduces strong pressure waves propagating back-and-forth, which subsequently lead to multiple local maxima on the temporal evolution of wall heat flux. Consequently, the maximum wall heat flux no longer changes monotonically with both the wall temperature and the quenching distance. Furthermore, the role of low-temperature chemistry (LTC) is studied using DME/air mixtures, which exhibits typical multi-stage heat release and Negative Temperature Coefficient (NTC) behaviors. It is also noted that, the influence of pressure wave is minimal when the unburnt mixtures have relatively low energy density, such as at off-stoichiometric conditions and/or lower initial pressures. Specifically: consistent to distinct three-stage homogeneous ignition at certain fuel-lean conditions (ϕ = 0.25) and intermediate initial temperature (typically around T0 = 600–750 K), in the corresponding 1D case three-stage end-gas autoignition respectively evolves into the near-wall cool, warm and hot flame; in the case at relatively low initial pressures, only the first-stage autoignition occurs in the end-gas, and as such LTC slightly increases the wall heat flux and advances flame quenching. As the reactivity of unburnt mixtures becomes higher, LTC and HTC respectively introduce strong and weak knocking combustion. Multiple weak and strong local maxima are observed in the temporal evolution of wall heat flux, both of which changes non-monotonically with the wall temperature.

Study of engine knocking combustion using simultaneous high-speed shadowgraph and natural flame luminosity imaging

Engine knocking is a classical but significantly impactful combustion phenomenon that arouses unfailing research interests. An in-depth understanding of the knocking mechanism holds the key to the next-generation high-efficiency spark ignition engines adopting a high compression ratio. In this study, a novel 72 kHz imaging system of simultaneous shadowgraph and natural flame luminosity (NFL) was implemented on an optical engine. The key combustion parameters that affect the engine knock intensity were investigated based on the heat release analysis. The shadowgraph image velocimetry (SIV) was utilized to measure the in-cylinder flow field. The end gas autoignition process in the engine knocking combustion was studied under knock intensity as high as 18.1 bar. The results indicate that there is an “optimum” combustion phasing that produces the highest knock intensity. The cases with more end gas mass at knock onset, though having lower mean temperatures, generally produce higher knock intensity. The knock intensity can be significantly different under the same end gas mass and mean end gas temperature, possibly due to the different temperature gradients in the end gas. The simultaneous shadowgraph and NFL imaging succussed in differentiating the density variation zone in the end gas caused by the low-temperature heat release from the hot flame zone before auto-ignition. An intense density variation zone is witnessed around the center of the cylinder under higher engine knock intensity by the shadowgraph images, showing a dark region that oscillates together with the local pressure oscillations. The coupling among the local density variation, pressure oscillation, and NFL intensity oscillation is observed during the engine knocking combustion. The velocity distribution of the flow field indicates that the engine knocking combustion enhances the in-cylinder flow, which may cause more convective heat transfer loss and flow loss.

Computational investigation of methanol pre-chamber combustion in a heavy-duty engine

This work explored the potential of methanol pre-chamber combustion (PCC) for heavy-duty engine applications. An optical engine experiment was conducted to visualize the jet flame development. The measured pressure traces and natural flame luminosity images were also used for the validation of three-dimensional computational fluid dynamics simulations. It was demonstrated that the main chamber (MC) combustion was successfully established by the reactive jet issued from the pre-chamber. Compared to methane PCC in our previous study, the distributed reacting jets were significantly thinner, in particular at the learner condition. The active PCC mode, which comprises enrichment of the mixture in the pre-chamber (PC) by means of direct methane injection, was effective in improving the engine performance. However, excessive PC fueling ratio (PCFR) resulted in lower thermal efficiency due to the higher wall heat transfer and combustion losses. In addition, the effects of various PC and piston geometries on the methanol/methane PC combustion were evaluated. The combination of an optimized PC and a flat piston yielded the highest thermal efficiency owing to the relatively lower combustion and wall heat transfer losses. At engine loads higher than 12.5 bar indicated mean effective pressure, exhaust gas recirculation must be implemented to avoid end-gas autoignition and reduce nitric oxides (NOx) emissions. As expected, the increase in (CR) further promoted engine work because of the higher expansion ratio. With CR of 13 and 14, higher thermal efficiency and lower NOx emission were simultaneously achieved under both intermediate and high loads when the engine was operating at the pure methanol PC combustion mode.

Performance, Emissions, and Combustion Characteristics of a Hydrogen-Fueled Spark-Ignited Engine at Different Compression Ratios: Experimental and Numerical Investigation

This paper investigates the performance of hydrogen-fueled, spark-ignited, single-cylinder Cooperative Fuel Research using experimental and numerical approaches. This study examines the effect of the air–fuel ratio on engine performance, emissions, and knock behaviour across different compression ratios. The results indicate that λ significantly affects both engine performance and emissions, with a λ value of 2 yielding the highest efficiency and lowest emissions for all the tested compression ratios. Combustion analysis reveals normal combustion at λ ≥ 2, while knocking combustion occurs at λ < 2, irrespective of the tested compression ratios. The Livenwood–Wu integral approach was evaluated to assess the likelihood of end-gas autoignition based on fuel reactivity, demonstrating that both normal and knocking combustion possibilities are consistent with experimental investigations. Combustion analysis at the ignition timing for maximum brake torque conditions demonstrates knock-free stable combustion up to λ = 3, with increased end-gas autoignition at lower λ values. To achieve knock-free combustion at those low λs, the spark timings are significantly retarded to after top dead center crank angle position. Engine-out NOx emissions consistently increase in trend with a decrease in the air–fuel ratio of up to λ = 3, after which a distinct variation in NOx is observed with an increase in the compression ratio.

Investigation of rapid flame front controlled knock combustion and its suppression in natural gas dual-fuel marine engine

In this study, a new-type knock mechanism for large-bore marine engines, which differs from the existing end-gas auto-ignition knock theory and detonation wave theory, is fully revealed, using a high time resolution dynamic pressure difference method. The results indicate that the disparate enhancement effects of the flame front on the high-pressure region and the low-pressure region result in an increasing pressure difference, eventually forming the knock. The distinctions between this knock and the conventional end-gas auto-ignition knock were examined. Subsequently, the different functions of swirl in the two knock mechanisms were investigated. The results illustrate that, increasing swirl in end-gas auto-ignition knock can accelerate the termination of flame surface propagation and reduce the end-gas temperature by shortening the end-gas heat accumulation, thereby decreasing the knock intensity. However, reducing swirl in the new-type knock can diminish the combustion intensity of the flame front, thereby reducing its enhancement of the pressure wave. Consequently, a reduction to one-fourth of the initial swirl level can reduce knock intensity by 91%. Particularly, a novel knock suppression method of positioning the jet flame in the opposite direction of the swirl is proposed, which can avoid the local rapid combustion to effectively eliminate knock while maintaining power.

Computational study of hydrogen engine combustion strategies: Dual-Fuel compression ignition with Port- and Direct-Injection, Pre-Chamber Combustion, and Spark-Ignition

This work attempted to identify the best strategy for hydrogen internal combustion (IC) engines. Computational simulations were conducted to evaluate the combustion and emission characteristics of four combustion strategies: H2/diesel dual-fuel combustion with port-(DF-PI) and direct-injection (DF-DI) of H2, pre-chamber combustion (PCC), and port-injection spark-ignition (SI). Two ultra-lean conditions with an overall lambda (λ) of 2.5 and 3.0 were considered. A pilot energy fraction of 3% was applied in both DF modes to limit carbon oxide emissions. Note that since the DF-DI mode is free of end-gas autoignition, a high compression ratio at 17:1 was applied in contrast to the other three cases (13:1). Of the four strategies, the DF-DI and SI operations tended to yield the higher optimal indicated thermal efficiency (ITE) than the DF-PI and PCC modes, due to the highest expansion ratio and usable earliest spark timing, respectively. However, the diffusion combustion-dominant DF-DI operation generated the highest nitric oxides (NOx) emission, owing to the wide stoichiometric high-temperature flame periphery. Because of the pilot-intensified combustion process, the DF-PI operation was easy to induce end-gas autoignition, which considerably limited its load extension and improvement of ITE. In addtition, for both the DF-PI and DF-DI operations, their NOx emissions were challenging to be eliminated even with a modeled exhaust gas recirculation rate at 50% using water injection, indicating that a DI water injection method or an after-treatment system must be implemented for the further reduction of NOx emission. In contrast, both the PCC and SI modes were able to fulfill the EU VI regulation limit of NOx emission with a relatively high ITE. Considering that these two modes only need a single fuel supply system, they are considered more promising for practical applications than the DF-PI and DF-DI modes.

Prediction of knock intensity and validation in an optical SI engine

To provide fundamental insights into the underlying mechanisms of knock occurrence in a spark-ignition (SI) engine, optical diagnostic measurements were performed at advanced spark timing under boosted conditions. Employing high-speed imaging, deflagration-to-detonation transition (DDT) processes were recorded. The states and locations of the unburned mixture more susceptible to autoignition were identified by statistically analyzing the recorded images of the spatial distribution of flame propagation and end-gas autoignition under distinct combustion modes. The relationships between the unburned mixture fraction (UMF) and two metrics associated with the knock intensity levels, namely the maximum amplitude of pressure oscillation (MAPO) and the peak in-cylinder pressure, Pmax, were examined. In contrast to common beliefs, we found that the knock intensity represented by MAPO is not strongly correlated with UMF because UMF alone is inherently insufficient to represent the thermo-chemical properties of the bulk mixture inhomogeneities and its chemical reactivity at the onset of end-gas autoignition occurrence. Instead, a much stronger correlation was found between UMF and a newly proposed normalized-peak-pressure metric, allowing an a prior prediction of Pmax, which is a critical parameter associated with the propensity of engine failure if Pmax exceeds the strength limit of the engine. To quantitatively predict MAPO under SI-engine conditions, a refined model was proposed by imposing the pressure and temperature traces into the zero-dimensional (0-D) reactor model, such that the thermochemical properties of the transient mixture state at the onset of end-gas autoignition are properly incorporated into the predictive criteria. The model was validated against a large experimental dataset consisting of 619 cycles under various operating conditions. The results show that the model can reliably predict the end-gas autoignition and the knock intensity levels dictated by MAPO regardless of the stochastic nature of the knock development process.

Effects of turbulence–heat loss interactions on detonation development in end gas and its resulting knock intensity

The main objective of the present work is to investigate the end-gas autoignition and detonation development in a confined space with the presence of wall heat loss by two-dimensional numerical simulations with a hydrogen/air mixture. The effects of turbulence–heat loss interactions, initial temperature, equivalence ratio, and wall temperature on end-gas combustion modes are analyzed in detail. The results show that with the presence of wall heat loss, end-gas autoignition takes place in the hot core regions away from the walls, and the autoignition fronts touching the wall can lead to a much larger wall heat flux than that induced by main flame–wall interactions. In the base cases, increasing the turbulence intensity promotes the end-gas autoignition mode transition from thermal explosion-detonation to thermal explosion-deflagration and finally to no-autoignition, whereas detonation takes place in all cases regardless of the turbulence intensity after the initial temperature or equivalence ratio is raised. However, in these cases with a low equivalence ratio, the detonation propagation is unstable, which can be easily decoupled spontaneously after it encounters the cold flow. It is further found that for the cases with unstable detonation propagation, the burned mass fraction (BMF) dominates the knock intensity, whereas for the cases with stable detonation propagation, the maximum pressure in a chamber will extremely depend on the local and instantaneous interactions between the pressure/shock waves, but the effect of BMF becomes minor.

A theoretical study on the relationship between pressure rise and the Damköhler number of end-gas auto-ignition in spark-ignited engines

The pressure rise caused by end-gas auto-ignition in spark-ignited engines is discussed using numerical simulation and theoretical approaches. The main objective of this study is to explain the mechanism by which the end-gas expansion during auto-ignition suppresses the pressure rise in spark-ignited engines, and theoretically to demonstrate using asymptotic analysis that the pressure rise depends on the Damköhler number. A one-dimensional direct numerical simulation (DNS) of end-gas auto-ignition is performed, and the modelling assumptions for it are discussed based on the DNS results. The Damköhler number, defined as the acoustic time scale and the characteristic time scale of the chemical reaction, is introduced in the modelling. The end-gas auto-ignition model is solved numerically, and it is shown that the pressure rise increases with the Damköhler number. Additionally, it is shown that the tendency of the pressure rise is due to the balance between the propagation rate of the expansion wave generated in the end gas and the reaction rate at auto-ignition, which varies with the Damköhler number. To derive the analytical solution of the relationship between the pressure rise and Damköhler number, the end-gas auto-ignition model is simplified based on the numerical results. The simplified model for end-gas auto-ignition is then solved using Newton’s method, and the analytical solution of the pressure rise is derived.

Numerical investigation on combustion regulation for a stoichiometric heavy-duty natural gas engine with hydrogen addition considering knock limitation

Aiming to further improve the thermal efficiency and reduce NOx emissions in the stoichiometric hydrogen-enriched natural gas (NG) engine, a detailed 3-D simulation model of stoichiometric operation heavy-duty NG engine is built based on the actual boundary conditions from high load bench test. The superimposed methods for knock regulation, combustion and emission control, including Miller valve timing, hydrogen volume fraction and EGR rate were proposed and investigated comprehensively. It reveals that the typically bimodal characteristic of heat release rate (HRR) curve is caused by knock, which seriously restricts the performance improvement of stoichiometric NG engine under high load condition. To predict and control the occurrence of the second peak of HHR accurately, a new parameter BI is defined. Moreover, the Miller timing with 20°CA of the intake valve late closing shows better combustion performance within the knock limit, accompanied by a slight increase in NOx emissions. Additionally, the 5% hydrogen blend, coupled with the Miller cycle, can further enhance the indicated thermal efficiency (ITE) of the NG engine due to the stronger effects on acceleration of laminar flame propagation velocity than the promotion of end-gas auto-ignition. Besides, the great potential of EGR rate for balancing NOx and ITE is also confirmed in the heavy-duty hydrogen-enriched NG engine adopting Miller cycle. Compared to the original indexes, combing with the regulation strategies of intake valve late closing (20°CA), hydrogen addition (5%) and EGR (17%) are proved to increase the indicated thermal efficiency by 1.89% and reduce NOx emissions by 11.47% within the knock limit.

Performance of octane index in LTGC engines from beyond MON to beyond RON

This work analyzes the inability of the Octane Index (OI) to characterize the autoignition resistance of gasoline-like fuels in a Low-Temperature Gasoline Combustion (LTGC) engine. Experimental studies in the Sandia LTGC engine at naturally aspirated ‘beyond-MON’ and high-boost ‘beyond-RON’ conditions showed a poor correlation between OI and autoignition reactivity. These experiments were successfully replicated in CHEMKIN, allowing detailed studies of the factors affecting the OI correlation. Systematic investigations were conducted to determine why the OI performs less well for LTGC combustion compared to end-gas autoignition (i.e. knock) in spark-ignition (SI) engines. A pseudo-MON condition that follows the pressure–temperature (P-T) trajectory of the real MON test was tested numerically, using 10 different fuels, leading to the conclusion that the OI correlation performs very well for an SI engine running at close-to-MON conditions. The effects of each difference between SI and LTGC combustion were then analyzed for four P-T trajectories: ‘beyond-MON’, ‘MON test’, ‘between RON-MON’, and ‘beyond-RON’ conditions. These differences included changes in: equivalence ratio, engine speed, and residence-time history along the P-T trajectory for piston-only compression, as used for LTGC, compared to compression by the piston plus flame propagation for SI combustion. For LTGC combustion, the OI correlation did not characterize the autoignition quality of gasoline-like fuels well for any P-T trajectory, but the factors causing this poor performance varied with conditions. For example, at naturally aspirated LTGC conditions (beyond-MON), changes in equivalence ratio were the primary cause, followed by differences in the P-T trajectory and changes in residence time. Whereas, for intake-boosted beyond-RON conditions, differences in the P-T trajectory had the largest effect, followed by changes in residence time. Finally, the parameter K that characterizes the OI correlation showed values that contradict the OI theory under realistic LTGC conditions, suggesting that K might be meaningless for LTGC engines.