Ammonia combustion in heavy-duty diesel engines commonly suffers from low reactivity, the difficult joint control of uNH3, NO+NO2 and N2O, and constrained thermal efficiency; optimizing the diesel injection strategy is therefore a key lever for simultaneous improvement. This study investigates the coupled effects of injector nozzle angle (INA145 degrees, INA90 degrees, INA60 degrees), pre-injection timing (PIT), and diesel pre-injection strategies (Single/ Dual-PI) on spray-wall interaction, mixture stratification, combustion dynamics and emissions in an ammoniadiesel dual-fuel heavy-duty engine operating at 50 % ammonia energy fraction. Results show that, compared with Single-PI, Dual-PI generally strengthens low-temperature heat release, advances the combustion center, and increases indicated thermal efficiency, while reducing smoke, CO, and THC. The wide-angle INA145 degrees combined with Late-PIT promotes premixed compression ignition in a hotter environment, which markedly lowers uNH3 and N2O but keeps NO+NO2 relatively high. In contrast, narrow-angle INA60 degrees with Early-PIT confines the spray to the bowl center, minimizes wall-wetting and yields a compact main heat-release phase with peak indicated thermal efficiency of about 49.9 % and low NO+NO2, at the expense of higher N2O. Medium-angle INA90 degrees provides an intermediate balance, with moderate ITE and a narrower NO+NO2/N2O band. By mapping these behaviors for three injector angles and both Single- and Dual-PI strategies under a fixed, practically relevant lean condition, the study identifies two actionable operating windows and clarifies the NO+NO2-N2O trade-off, providing engine-relevant guidance for calibrating low-carbon heavy-duty ammonia-diesel dual-fuel engines.

Low-pressure direct injection hydrogen engines show promise for achieving zero-carbon emissions in heavy-duty commercial vehicles, but the underlying mechanisms and key factors governing mixture formation and flame propagation remain poorly understood, especially for the hydrogen engines with side-mounted injectors and flatroof-and-shallow-bowl combustion chambers. This paper aims to address this knowledge gap by conducting three-dimensional computational fluid dynamics simulations. The results reveal that the bulk flow and wallguided hydrogen motion determine the mixture distribution. At spark timing, a band-shaped region with high excess air ratios (2) is observed, with two vortices formed near the spark plug. The combined effects of the high-2 region and turbulence influence the flame propagation, leading to a three-stage heat release process: the initial development stage, the primary heat release stage, and the after-burning stage. Owing to the presence of the high-2 region with lower turbulent intensity, flame propagation toward the end gas is decelerated. During the after-burning stage, the reduction of temperature, local equivalence ratio, and turbulent kinetic energy downstream of the flame front suppresses flame propagation in the end gas region, ultimately leading to local flame extinction and the formation of residual hydrogen.

The combustion and emission performance of ammonia/diesel dual-fuel (ADDF) engines is critically influenced by the diesel injection strategies and combustion regimes. However, there has been limited research performed on this topic. This paper has systematically compared the diesel single/double/triple injection strategies and the early/late compression ignition regimes, in terms of the heat release pattern, combustion performance, and pollutant emissions. The experiments are carried out on a 1.85 L PFI-DI single-cylinder ADDF engine. It's found that diesel pre-injection strategies can effectively improve the combustion and emission performance of an ADDF engine, whether in the ECI (early compression ignition) or the LCI (late compression ignition) combustion regime. From single to double and triple injection, ITE (indicated thermal efficiency) increases while PPRR (peak pressure rise rate) decreases, indicating smoother and more efficient combustion, and smoke opacity, THC, CO, uNH3 (unburned ammonia), NOx, and N2O emissions are all reduced. Compared to the LCI combustion regime, the ECI combustion regime produces a higher ITE, comparable PPRR, lower smoke opacity, THC, CO, and N2O emissions, and higher NOx emissions with the diesel double/triple injection strategies, but the result is reversed with the diesel single injection strategy. The experimental results indicate directions for the optimization of ADDF combustion.

Partially replacing diesel with carbon-free ammonia fuel is gaining more interest in tackling the issue of greenhouse gas emissions in freight transportation applications. But in ammonia-diesel dual-fuel engines, the combustion and emissions are of particular complexity, with complicated trade-offs among the rough combustion, the thermal efficiency, and the CO2/N2O/NO+NO2/NH3 emissions. To this end, this paper conducts systematical experimental tests on a modified PFI-DI single-cylinder heavy-duty diesel engine with ammonia injected in the intake manifold and diesel directly injected into the cylinder, containing ammonia energy fraction ranging from 0% to 50%, engine loads from IMEP 0.5 to 1.3 MPa, and sweeping diesel injection timing. Results show that the heat release of ammonia-diesel dual-fuel combustion (ADDC) possesses two exothermic stages, and as the ammonia energy fraction increases, the heat release process of ADDC in the first stage is intensified and that in the second stage is just the opposite, which causes rougher combustion but lower indicated thermal efficiency (ITE). Increasing the ammonia energy fraction also makes NO+NO2 and CO2 emissions reduced but NH3 and N2O emissions increased. N2O emissions with stronger greenhouse gas impact usually offset the reduction of CO2. Advancing diesel injection timing can improve ITE, but the cost is the rapidly increasing PPRR and NO+NO2 emissions. Besides, as the engine load increases, the ignition delay gets shortened and the combustion process becomes concentrated and efficient, the resistance to ammonia in ADDC gets better, and NH3 emissions are reduced at the cost of more NO+NO2 emissions.

Pre-injection can effectively improve combustion and thermal efficiency while reducing unburned ammonia (uNH3) 3 ) and N2O 2 O emissions in ammonia/diesel dual-fuel engines. Nevertheless, the pre-injected diesel often impacts the cylinder liner or piston pinch area, adversely affecting combustion and emissions. In this study, various narrow injector nozzle angles (90 degrees/60 degrees) degrees /60 degrees ) of diesel fuel are applied on investigating engine performance and emissions. Experiments are conducted at 1200 r/min, IMEP 1.2 MPa, and AEF (ammonia energy fraction) 50 %, consisting of varying INA (injector nozzle angle) of 145 degrees/90 degrees/60 degrees, degrees /90 degrees /60 degrees , PIR (pre-injection ratio) from 20 %-80 %, PIT (pre-injection timing) from-70 degrees CA ATDC to-30 degrees CA ATDC, and sweeping MIT (main injection timing). Results show that larger INA prefer a lower PIR and delayed PIT to reduce the pre-injected fuel hitting the liner, and smaller INA prefer a higher PIR to reduce the main-injected fuel hitting the piston wall. ITE of INA145 degrees degrees peaks at a moderate PIR and delayed PIT while ITE of INA90 degrees degrees and INA60 degrees degrees peaks at a high PIR and early PIT. Decreasing INA combined with high PIR and early PIT helps to reduce uNH3, 3 , N2O, 2 O, and GHG emissions, at the cost of a slight decrease in ITE.

Hydrogen shows great potential for its use in internal combustion engines as a carbon-free fuel. Most experimental studies focus on light-duty engines, while experimental studies of heavy-duty direct-injection hydrogen engines are still rare. In this study, a dedicated low-pressure-direct-injection combustion system is designed on a 2.15 L single-cylinder hydrogen engine. Based on this, the combustion and heat transfer characteristics of the engine are investigated at IMEP 10 bar and 15 bar, with various excess air ratios (2 = 1.8-3.0) and spark timings (-16 to - 4 degrees CA ATDC), and the applicability of classical empirical heat transfer models in hydrogen engines are further examined. As the spark timing advances, the combustion phases are linearly advanced, while the combustion duration remains almost unchanged due to the high reactivity of hydrogen. The 2 range for stable combustion is 2 = 2.0-3.0 at IMEP 10 bar and 2 = 2.2-2.8 at IMEP 15 bar, relatively narrower than the reported results in light-duty engines. Knock occurs under conditions of 2 = 1.8 with IMEP 10 bar and 2 = 2.0 with IMEP 15 bar. At 2 = 3.0, combustion instability occurs at both 10 bar and 15 bar IMEP. It is noted that pre-ignition occurs alongside combustion instability at 2 = 3.0 and IMEP 15 bar. Selected heat transfer models fail to accurately predict the heat transfer laws of the engine, and the modified Shudo equation shows good performance.

Pre-injection is an effective means of improving combustion and thermal efficiency while reducing uNH3 and N2O emissions in ammonia/diesel dual-fuel engines. However, the coupling effects of the multiple controlling variables such as PIR (pre-injection ratio), PIT (pre-injection timing), MIT (main injection timing), and diesel injection pressure on combustion and emissions remain to be investigated. To this end, experiments are conducted at 1200 r/min, IMEP 12 bar, and AEF (ammonia energy fraction) 50%, and the ammonia is injected into the intake manifold at -360 degrees CA ATDC. Results show that there are two ignition modes, called pre-injection-controlled ignition and main-injection-controlled ignition, depending on PIR and PIT. As PIR increases, ITE (indicated thermal efficiency) first increases and then decreases, and for different PIRs, delayed PIT is always good for ITE. When PIR is 60% and PIT is -50 degrees CA ATDC, ITE peaks at 50.7%. In terms of emissions, the pre-injected diesel mixes with the ammonia/air charge, increasing the chemical reactivity of the premixed gas and resulting in a significant reduction in uNH3 and N2O emissions. However, due to the wall-wetting and dilution effect, the pre-injected diesel is difficult to burn completely, resulting in increased THC and CO emissions. The variation rule of ITE is identical to that of N2O and uNH3 emissions, but opposite to that of NOx emissions.

Combustion and emission characteristics of a heavy-duty single-cylinder ammonia engine with a hydrogen-fueled active pre-chamber ignition system are investigated experimentally at the condition of IMEP 10 bar, 1000 rpm, where effects of spark timing, excess air ratio (lambda), and hydrogen energy ratio are tested. Three distinguishable combustion phases are observed, including pre-chamber combustion, turbulent-jet-controlled combustion, and ammonia-chemical-kinetics-controlled combustion. Advancing spark timing makes combustion phases earlier, increasing pressure rise rate, combustion pressure, and NOx emissions. The optimal spark timing for the highest indicated thermal efficiency (ITE) is -12 degrees CA ATDC in the experiment. lambda range for stable combustion is 1.1-1.5 and ITE peaks at lambda = 1.3. The hydrogen energy ratio, 7.9%similar to 10.5%, has little influence on the engine performance. However, a low hydrogen energy ratio could increase the risk of misfire. The optimal hydrogen energy ratio is about 9 similar to 10%.

This study examines the interactions of wall temperature and impingement distances and their effects on the impinged diesel spray ignition and combustion characteristics experimentally in a modified constant-volume combustion chamber. Four wall temperatures of 570 K, 620 K, 700 K, and 800 K, and four impingement distances that correspond to severe wall-wetting, slight wall-wetting, critical wall-wetting, and non-wall-wetting separately are tested. The results indicate increasing wall temperature can promote auto-ignition and the following combustion process, denoted by shorter ignition delays and higher flame area (FA) and spatially integrated natural luminosity (SINL) values, but the promotional effect generally weakens with the increase of the impingement distance. When the wall temperature is high enough, the spray/wall impingement can accelerate fuel evaporating and mixing with the entrained air, in which case decreasing the impingement distance is positive to auto-ignition and combustion. This trend is basically the opposite in low-wall temperature cases where the cooling effect plays a leading role. Besides, the normal distances from the ignition points to the wall decrease with the increment of the wall temperature, and the changes are more remarkable at lower impingement distances, which means that increasing the wall temperature may enhance auto-ignition and combustion in the near-wall region, probably leading to an enlarged temperature gradient in the boundary layer and strengthened heat transfer.

根据撞壁距离L与自由喷雾液相贯穿距Lliquid之比,将喷雾撞壁模式分为严重湿壁、轻微湿壁、临界湿壁、未湿壁.通过高温高压定容弹试验和 OpenFOAM 仿真,研究了高、低壁温条件下,4 种喷雾撞壁模式对着火特性、火焰发展特性以及燃油分布特性的影响.结果表明:提高壁面温度可以促进喷雾着火及燃烧过程,且其促进作用随L/Lliquid 增大而减小;高壁温条件下,一定程度的喷雾撞壁有利于油气混合和燃烧,低壁温条件下则相反.

Exploratory research of partially premixed charge compression ignition (PCCI) in conjunction with direct fuel injection was done. A single-cylinder commercial diesel engine was used. In this work, the evaluation of the engine vibrations, pollution, efficiency, and combustion properties has been performed on a PCCI diesel-fueled engine. A part of the fuel was converted into vapor inside the intake manifold by using an innovative premixing chamber with an electronic fuel injector. At the same time, the main fuel quantity was injected directly inside the engine cylinder before the top dead center (TDC) to control the engine phasing. A unique approach based on the fast Fourier transform (FFT) of the cylinder vibration data was applied for combustion vibrations and acoustic investigation. To further clarify their relationship, the influence of combustion characteristics on acoustic and vibrations metrics was investigated. The results demonstrate that combustion noise remains a crucial issue for adopting this novel combustion approach in the automotive industry. The studies revealed that partial premixing reduces nitrogen oxide (NO X ) pollutants significantly. This is thought to be the outcome of the PCCI combustion, which occurs before the typical mixing controlled phase, lowering regional gas temperatures. The experiment findings also revealed that partial premixing has an intrinsic tradeoff between NO X emissions and inefficient combustion products (carbon monoxide (CO) and unburned hydrocarbons (UHCs)). It was also shown that incomplete combustion and non-optimized spontaneously igniting of the premixed charge resulted in a minor reduction in combustion efficiency (CE).

Direct Dual Fuel Stratification (DDFS) is a new low-temperature combustion (LTC) approach that employs dual fuel direct injection in the combustion chamber. In this method, a relatively small proportion of diesel fuel was pre-injected and utilized to activate the burning of the premixed charge followed by direct injection of a diesel/ ethanol mixture (75% diesel, 25% ethanol by volume) into the combustion chamber near TDC. The DDFS approach generates a lower reactivity charge in the central sector of the combustion chamber and a stronger reactivity charge along the wall. For the examination of combustion vibrations and acoustic, a new technique dependent on the fast Fourier transform (FFT) of the cylinder vibration statistics was used. Whereas the PCCI combustion technique is more likely to cause knocking than the typical diesel CI, the results show that charge stratification in DDFS combustion does have a good effect in reducing vibration intensity. Owing to the improved controllability of the start of combustion and burning period, DDFS combustion attained a brake thermal effi-ciency (BTE) of 51%, which was larger than CI and PCCI. DDFS shows ultra-low nitrogen oxide (NOx) (below 1 g/ kW-h), mild carbon monoxide (CO) (below 6 g/kW-h), and unburned hydrocarbons (UHC) emissions. Because of the higher thermal efficiency and a decreased carbon source due to presence of ethanol, DFFS combustion could produce lower CO levels by up to 34% when compared to PCCI.

Nitrogen oxides can be introduced into the combustion engine through exhaust gas recirculation (EGR) and therefore change the reaction channel of hydrocarbon fuels. In this paper, we firstly evaluate the con-version of NO to NO2 with the presence of oxygen and later apply the results into the simulation. Shock tube experiments are carried out at equivalence ratio of 0.5, 1 and 2, and total NO/NO2 concentration of 10 0, 50 0 and 10 0 0 ppm. Experimental results show that for n-heptane equivalence at 0.5 and 1 and temperature lower than 1250 K, the addition of NO/NO2 promote the ignition and the promoting effects strengthen as the concentration of NO/NO2 increases. At a higher temperature , however, promoting ef-fects of NO/NO2 are negligible. The kinetic analysis suggests that reactions: NO + HO2 q NO2 + OH and NO2 + H q NO + OH may work as a "reaction cycle" to effectively produce the OH radicals and promote the ignition.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

In recent studies around low-temperature combustion (LTC) engines, it has been found that the incomplete oxidation products (IOP) within exhaust gas recirculation (EGR) presents non-negligible chemical effects. However, IOP generally has complex ingredients, and little literature available conducts quantitative and comparative studies on the effects of these ingredients. In this paper, CH3CHO, which is considerably abundant, and NO, which is most active, are selected as typical ingredients of IOP, and their effects on ignition delay times of n-heptane/ethanol fuel blends are studied experimentally in a shock tube covering the equivalence ratios of 0.5, 1, 1.5, temperature range of 900–1200 K, and pressure of 10 atm. Experimental results show that both CH3CHO and NO/NO2 (part of NO is inevitably converted to NO2) promote ignition, and the promoting effect of NO/NO2 is stronger than CH3CHO . It is consistent for both CH3CHO and NO/NO2 that the higher the concentration or, the lower the equivalence ratio, the greater the promoting effect. The difference is that the promoting effect of CH3CHO weakens with temperature increasing, while the promoting effect of NO/NO2 strengthens. Besides, the promoting effect of NO2 is weaker than that of NO, and NO2 even begins to inhibit ignition at temperatures below 960 K. In addition, it follows chemical kinetic analysis that both CH3CHO and NO/NO2 accelerate the decomposition of fuel by producing more OH radicals during the ignition stage. Although the additives are in trace amounts, the OH produced by their early involvement in reactions can effectively accelerate the ignition, which indicates the non-negligible chemical effects of IOP in EGR.

The concept of a highly premixed dual fuel triple-direct-injection (TDI) combustion model aimed at providing adjustable in-cylinder stratified charge from two distinctive fuels, ethanol and diesel fuels, is presented.

A reduced chemical mechanism for ammonia/n-heptane blends, which consists of 495 reactions and 74 species, is proposed in this study. This reduced mechanism is developed using the direct relation graph with error propagation (DRGEP) method based on the latest detailed kinetic mechanism of ammonia/n-heptane blends, which includes the important interactive reactions between n-heptane and ammonia (n-C7H16 + NH2 double left right arrow C7H15 + NH3). The reduced mechanism was first validated against experimental ignition delay times (IDTs) and laminar burning velocities (LBVs) for different ammonia/n-heptane blends and can capture well these data. Moreover, both the detailed and reduced mechanisms were used to simulate the combustion and emission characteristics of a homogeneous charge compression ignition (HCCI) engine fueled by different ammonia/n- heptane blends. The simulation results show that the predicted auto-ignition timings and peak pressure values the reduced mechanism match reasonably well with those of the detailed mechanism for different ammonia/n- heptane blends. Also, the predicted time-history profiles of the important pollutants, including NO, NO2, N2O and HCN, show reasonably good agreement between the detailed and reduced mechanisms, and the maximum discrepancy of the peak concentration is within a factor of two. The important reaction pathways of pollutant formations at HCCI engine conditions are discussed in detail. Furthermore, the current reduced mechanism also used to simulate the combustion process of an ammonia/diesel dual-fuel engine. Overall, the engine com-bustion phasing, peak pressure, and heat release can be well simulated using the current mechanism, and the computational efficiency is acceptable.

The measurement of engine vibrations components is used to identify combustion phases in a single-cylinder PCCI diesel engine in this work. A single channel Knock transducer was used to measure engine vibration caused by combustion pressure forces and knocking tendency. The transducer is used to obtain the engine's frequency spectrum in the temporal domain. Categorization discrete wavelet analysis is used to isolate the observed vibration data. Spectral analysis and filtration of fragmented sections of the source signal are used in this approach. Fast Fourier transforms (FFT) and short-time Fourier transform (STFT) are used to investigating the discrete parts of the observed signals (STFT). In order to study engine events, time is given in milliseconds in the time-frequency domain. The engine knocking is determined from the observed signal using time-frequency analysis. The outcomes reveal that the engine events derived from vibration signals are closely connected to the premixing fuel ratio, as expected.

The objective of this study is to explore various strategies i.e. reactivity stratification, thermal stratification, injection timing, and exhaust gas recirculation as key factors for prolonging the PCCI/RCCI/DDFS engine operational spectrum and suppressing the ringing intensity. Since most of the current motivating strategies can be lumped into the category of premixed Low-Temperature-Combustion (LTC), the potential of this paper is to demonstrate the robustness of LTC in addressing several challenges related to premixed charge compression ignition engine, viz., lack of phasing regulation of combustion at heavy engine loads and excessive pressure rises. With a premixed preparation strategy, however, emission species are seen to decrease, yet the challenges of pre-combustion charge preparation are present. This can deduct a noticeable extent of fuel energy. To avoid the steep trend of pressure rise problem that restricts LTC engines with their operating range, higher levels of fuel blending and regulating the timing of auto-ignition are used. This review demonstrates the ongoing progress in premixed LTC techniques aim to control of nitrogen oxides (NOx), reduction of particulate matter (PM), and hydrocarbon (HC). In comparison to normal LTC operation, the duel fuel charge stratification might increase indicated thermal efficiency (ITE) by 5% to 7% across the survey. For various charge compositions, tests were made in homogeneous charge compression ignition (HCCI) and reactivity controlled compression ignition (RCCI). These results are provided and analyzed in comprehensive detail, with extensive correlations to model predictions and a thorough kinetic analysis. Furthermore, injection strategies were reviewed to accomplish highly efficient preparation of the combustion charge, reduce the combustion environment temperature, and enhance premixing. By highlighting relevant components of this rich and quickly growing field of knowledge, this study aims to give an insight to and summary of current studies on LTC combustion. As a result, this document serves as a fundamental collection of information and recommendations on the current state of combustion research on various fuels.