Knock Intensity Research Articles

Effect of ammonia reforming on combustion and emission characteristics of a 4-valve engine with an active pre-chamber

Ammonia (NH3), as a hydrogen carrier and carbon-free fuel, offers an attractive opportunity for engines to achieve carbon neutrality. Turbulent jet ignition (TJI) combined with ammonia reforming shows the great capacity in ammonia-fueled engines. In this study, the effects of reforming strategy in an ammonia-fueled TJI are numerically studied, addressing the reforming ratio and reforming region. The results show that when only using reformate in the pre-chamber, the promoting effect of jet flame is more effective on the initial combustion phase. There are still very high NH3 emissions due to the low reactivity in the main chamber. Further using reformate both in the pre-chamber and the main chamber, all the combustion stages (ST-CA10, CA10-50, CA50-90) can be shortened almost linearly with the increase of reforming ratio. Besides, the unburned NH3 can be reduced to an acceptable level when the reforming ratio reaches 200‰ (hydrogen energy ratio of 18.50%). The main reason is that the jet-induced strong flow field is coincident with the whole combustion stage. Further increasing the reforming ratio (pure hydrogen) in the pre-chamber, a high combustion efficiency and acceptable NH3 emission can be achieved at a low hydrogen energy ratio (7.08%). However, knocking combustion will happen at high reforming ratio with a low knock intensity. The results can provide some guidance for making the best-promoting benefit of the limited hydrogen in ammonia TJI engines with different reforming strategies.

Effect of ignition timing and hydrogen injection phase on HDI engine combustion and NOx emissions

ABSTRACT In order to improve the ITE and reduce NOx emissions in direct-injected hydrogen (HDI) engines under at medium and high loads, the effects of IT and hydrogen injection phase on the combustion and NOx emission characteristics of the HDI engine at high rpm and medium load were investigated numerically. The base engine is a four-cylinder turbocharged direct-injected gasoline (GDI) engine, which is modified into a HDI engine with direct-injected hydrogen fuel. The simulation is performed using GT-Power software. The operating condition of HDI engine is set to 5500 rpm and 50% load. Firstly, IT is optimized, and then based on the IT optimization results, the hydrogen injection phase is optimized. The results show that as the IT is advanced, the maximum pressure rise rate (MPRR) shows a gradual increasing trend, while the combustion intensity and the indicated thermal efficiency (ITE) increases firstly and then decreases, and the former reaches a maximum value at 25° CA BTDC, while the latter reaches a maximum value of 40.5% at 18°CA BTDC. In addition, the nitrogen oxide (NOx) emissions decrease as the IT is delayed. Based on 18°CA BTDC of IT, the results indicated that the later the moment of hydrogen injection, the higher the MPRR, resulting in the greater the knock intensity. The ITE is less affected by the hydrogen injection phase. When the hydrogen injection phase is delayed, NOx emissions increase slightly.

An experimental study of knock analysis of HCNG fueled SI engine by different methods and prediction of knock intensity by particle swarm optimization-support vector machine

Hydrogen and its careers have a lot of potential in transportation industry due to lesser emissions and better performance. Performance of hydrogen enriched compressed natural gas (HCNG) fueled engine is limited by knock. The purpose of this study is to evaluate and predict knock at different operating conditions by varying (0 %–40 %) hydrogen amount in HCNG, by varying (25 %–100 %) load on engine and by varying (700 rpm–1700 rpm) speed of engine. Knock is evaluated by knock ratio, knock intensity, exhaust temperature and in-cylinder heat transfer rate. Quasi-dimensional combustion model (QDCM) MATLAB program is used to calculate in-cylinder pressure theoretically. By increasing (0 %–40 %) hydrogen in HCNG 32.7 %, 7.6 %, 33.5 %, 21.5 % & 6.4 % increment observed in, in-cylinder pressure, knock intensity, knock ratio, in-cylinder heat transfer rate and exhaust temperature respectively. By increasing (25 %–100 %) load on engine 75.8 %, 69.8 %, 88.2 %, 77.8 % & 46.7 % increment observed in, in-cylinder pressure, knock intensity, knock ratio, in-cylinder heat transfer rate & exhaust temperature respectively. By increasing speed (1100–1700) rpm of engine aforementioned parameters decreases by 12.19 %, 38.9 %, 36.06 %, 47.2 % & 14.1 % respectively. Knock intensity is predicted by particle swarm optimization-support vector machine (PSO-SVM) algorithm effectively. To minimize the prediction error, 31 combination of different (1–5) inputs applied to predict knock intensity and found combination of 5 input variable is 67.2 % & 71.9 % more accurate than 1 input variable in-terms of mean squared error and mean absolute error respectively. Findings of this study can be used to train electronic control unit of engine and in the development of HCNG fueled engine.

Effects of alcohol fuels on SACI engine and analyze of prediction of SACI engine performance by artificial neural networks

The combustion characteristics of spark assistant compression ignition mode under different loads and fuels were explored, as well as by constructing artificial neural networks(ANN) model, the prediction ability of different algorithms for engine performance was discussed. Burning methanol and n-butanol can help to improve IMEP and induce earlier spontaneous combustion. The knock intensity(KI) of engine fueled with methanol was highest followed by n-butanol and gasoline, but maximum amplitude of filtered pressure oscillation(MAPO) shows the opposite trend. KI showed great positive liner correlation with ignition timing. Methanol showed the most outstanding tolerance on the compression ignition state. Fueled with methanol can decreased equivalent brake specific fuel consumption(ESFC) up to 52.9 % compared with the initial SI gasoline engine. N-butanol can improve BSNOx, BSTHC and BSCO concurrently, however fueled with methanol will worsen BSNOx. ANN model for engine combustion, economy and emissions performance was built. The prediction accuracy of Bayesian regularization algorithm for engine performance predicting was highest, but it have no advantage in the calculating times when the amount of data was large while the Levenberg-Marquardt algorithm would be the ideal efficient. The mean square error of ESFC and emissions parameters under scaled conjugate gradients algorithm always poorest.

Effect of injection timings and injection pressure on knock mitigation with a compression stroke injection of hydrous ethanol in spark ignition

Direct injection fuel systems provide precise control over the amount of fuel injected and can enable higher compression ratio operation and earlier combustion phasing under knock-limited operation, particularly for fuels with a high cooling potential like hydrous ethanol, a blend of 92% ethanol and 8% water. Moving a portion of the total fuel mass from an intake stroke injection to a compression stroke injection can provide a knock suppression benefit, which can enable more efficient operation. In this work, the influence of injection pressure on this split injection spark ignition strategy is examined. The effect of injection pressure on two intake stroke injections were characterized, with an injection pressure of 200 bar improving combustion efficiency by ∼3 percentage points and advancing knock-limited CA50 by 1 crank angle degree over an injection pressure of 30 bar. Then, a compression stroke injection was introduced and swept into the compression stroke while maintaining the two intake stroke injections. Direct injections at an injection pressure of 30 bar enabled a small knock intensity reduction of ∼20%, whereas an injection pressure of 200 bar enabled a larger reduction of ∼90% in knock intensity. The spark timing advance permitted by the reduction in knock intensity with a compression stroke injection timing of −80 degrees after top dead center was 0.3 and 2.0 degrees at an injection pressure of 30 and 200 bar, respectively. Then, the second intake stroke injection was varied at 200 bar to evaluate how the stratification profile prior to the compression stroke injection impacted its ability to reduce knock intensity. It was found that compression stroke injections with an early second intake stroke injection was effective at reducing knock intensity throughout the compression stroke. As the second intake stroke injection was retarded, the early compression stroke injections became less effective at suppressing knock.

Effect of knock on piston thermal load of a high compression ratio natural gas engine based on stepwise decoupling calculation

High compression ratio natural gas engine (NGE) is prone to knock at high load, which aggravates the thermal load of piston. Therefore, accurately evaluating the piston thermal load in the practical design is essential to enhance engine reliability and improve performance. The typical piston thermal load simulation adopts uniform wall boundary conditions, making it difficult to reveal the influence of turbulent flame and knock on the piston thermal load. In this paper, a stepwise decoupling method is applied to calculate the piston thermal load. The advancement of this method lies in its integration of chemical reaction mechanism with computational fluid dynamics (CFD) to calculate the spatial variations in heat transfer coefficient (HTC) and near-wall gas temperature at the piston top. Moreover, the conjugate heat transfer (CHT) method is adopted to solve the piston thermal load by coupling the thermal boundary conditions (TBC) obtained in the first step with the cooling oil boundary conditions. Based on this method, the effect of knock on piston thermal load of a high compression ratio NGE under different knock intensities is explored in this paper. It is found that turbulent combustion causes uneven distribution of thermal load at piston top. The decrease in knock intensity significantly reduces the gas temperature, heat flux and thermal load at piston top. Particularly under severe knock, the average temperature of piston is 22.9 K and 32.4 K higher than that of light knock and normal combustion, respectively. Furthermore, the main factor affecting the piston temperature field distribution is the gas temperature rather than the HTC. The gas temperature at piston top is overall the highest on the side near the inlet valve pit, which leads to a higher temperature on this side. Compared to light knock and normal combustion, when severe knock occurs, the thermal load on the exhaust valve pit increases significantly. This study provides an effective method for analyzing the heat transfer of the piston under knocking combustion, which is of guiding significance for controlling the thermal load of the high-temperature components in the combustion chamber.

Unlocking the potential of novel ternary non-edible seed oil biodiesel: impact on diesel engine knock intensity, performance, combustion, and emissions

The escalating demand for sustainable energy sources amid dwindling fossil fuel reserves prompts an exploration into innovative biodiesel alternatives for automotive applications. This study investigates a unique ternary mixture comprising non-edible seed oils from Azadirachta indica (Neem), Argemone mexicana (Satyanashi), and Melia azedarach (Bakayan) for biodiesel production and its potential as an alternative to conventional diesel. The ternary blend, extracted using a mechanical oil press, achieved a raw oil yield of 32%. Optimising process parameters, including a 93% biodiesel yield, was attained with a methanol to oil molar ratio of 6:1, 0.6% NaOH by weight, and a temperature of 60°C for 60 min. Performance testing of blends in a computerised single-cylinder, 4-stroke CI engine revealed notable improvements in brake thermal efficiency, with the B80 blend demonstrating the most significant reduction in brake-specific fuel consumption by 8.5%. The B100 blend exhibited the highest brake thermal efficiency at 37.6%. Emission analyses demonstrated reductions in CO and CO2 emissions, with the NOx reduction percentages varying for different blends. Notably, all blends exhibited comparable in-cylinder pressure and ignition delays with diesel, indicating the potential of the ternary seed oil mixture biodiesel as a viable diesel fuel substitute in CI engines.

Comparative knock analysis of HCNG fueled spark ignition engine using different heat transfer models and prediction of knock intensity by artificial neural network fitting tool

Hydrogen and its careers have a lot of potential in transportation industry due to lesser emissions and better performance. Performance of hydrogen enriched compressed natural gas (HCNG) fueled engine is limited by Knock. The purpose of this study is to investigate knock intensity (KI) by different convective heat transfer models. This study can be used to train electronic control unit (ECU) of engine operating on different loads from low to high speed. In present study, experimentation have been performed on HCNG fueled spark ignition engine at different operating conditions by varying hydrogen amount in HCNG, by varying load of engine, by varying exhaust gas recirculation (EGR) rate in air and by varying speed of engine to calculate in-cylinder pressure, knock intensity (KI) and heat transfer rate (HTR). Six convective heat transfer models (Assanis, Eichelberg, Han, Hohenberg, Nusselt and Woschni) have been incorporated in quasi dimensional combustion model (QDCM) to calculate the parameters of HCNG engine. Comparative and predictive analysis using artificial neural network fitting tool (ANNFT) of aforementioned simulated models have been performed with experimental findings to attain the efficient model for knock intensity of HCNG engine. Woschni model, predicts the minimum error of 0.37 % & 0.86 % b/w experimental and simulated in-cylinder pressure and indicated mean effective pressure respectively. Han model predicts the minimum error of 3.8 % b/w experimental and simulated knock intensity operating at 11.2 % EGR. Maximum error of 20.3 % at 2.4 % EGR is attained by using Woschni model in knock intensity calculation. The best suited algorithm for present data set is Bayesian regularization utilized in ANNFT to predict knock intensity effectively. The findings of this study can be utilized in the development of HCNG engine.

Experimental investigation of knock control criterion considering power output loss for a PFI SI methanol marine engine

This study presents a new knock control criterion based on different loads and engine power output loss. By utilizing a modified port fuel injection (PFI) spark-ignition (SI) methanol marine engine, the suppressive effects of retarding ignition timing, reducing air-fuel ratio and delaying injection timing on knock intensity at different loads were investigated. Subsequently, a comparative analysis of three knock control strategies was conducted to determine their impacts on power output and thermal efficiency, thus proposing the optimal knock control strategy at different loads. The results show that at both low load (MAP = 73 kPa) and high load (MAP = 139 kPa), the high knock intensity in KLSA (knock-limited spark advance) +5 case is reduced to the safety limitation by retarding ignition timing and reducing air-fuel ratio, while retarding injection timing can effectively suppress knock only at low load. Furthermore, under KLSA+5 condition, there is a significant increase in the probability of severe knock for CA05 earlier than the knock boundary. As the knock tendency is mitigated, IMEP reduces monotonically. There is lower power output loss by optimizing injection timing and air-fuel ratio at low load, and optimizing ignition timing at high load, while the air dilution strategy consistently results in minimal thermal efficiency loss.

Quantitative analysis of the relationship between charge motion and knocking combustion in spark-ignition natural-gas engines under critical knocking conditions

Increasing the in-cylinder flow intensity is considered to have the potential to restrain engine knock and improve thermal efficiency. However, the effect of charge motion on the knocking combustion in natural gas (NG) engines is unclear. Therefore, swirl, tumble, and squish motions of different intensities were organized by changing the shape of the combustion chamber and initializing the in-cylinder flow velocity. Subsequently, the effect of charge motion on the knocking combustion of a spark-ignition NG engine under critical knocking conditions was investigated via numerical simulations. The results indicate that the knock intensity (KI) first increases and then decreases with an increase in swirl and tumble intensities and linearly increases with an increase in squish intensity. Swirl had the smallest impact on KI, followed by tumble, whereas squish had the greatest impact. The differences in flame propagation resulted in differences in KI under different charge motion patterns. The obvious non-uniformity of the radial and axial flame propagation is a potential cause of the significant increase in KI. Therefore, while increasing the flow intensity, matching a suitable combustion chamber to make the flame propagation more uniform helps achieve anti-knock and rapid combustion. Improving the swirl and tumble intensities can help NG engines achieve anti-knock and rapid combustion, whereas improving the squish intensity does not. This study provides an important reference for determining whether increasing flow intensity can improve NG engines’ knock characteristics and combustion performance.

Effects of in-cylinder water injection on knocking and combustion stability generated by expanding the lean burn limit

The integration of direct water injection (DWI) with the lean burn (LB) process can significantly enhance engine performance. This study reveals that an increase in the water-fuel (W/F) ratio suppressed knocking by extending the ignition delay time. Specifically, the knocking of stoichiometric combustion was suppressed, while that of deeper LB (excess air ratio (λ)˃1.5) was sensitive to a larger W/F ratio. The intensity of knocks during stoichiometric combustion decreased significantly (more than twofold) when the W/F ratio increased from 0.1 to 0.2. Under identical W/F conditions, a shorter ignition delay (CA0–10) and combustion duration (CA10–90) of the main combustion flame were identified as important factors for knocking when λ increased from 1 to 1.5. However, for λ values exceeding 1.5, CA10–90 emerged as the primary knocking determinant. Faster flame propagation and higher oxygen concentration caused by LB and an early spark promoted autoignition of the end-gas and increased the autoignition heat release rate, thus strengthening knock intensity (KI). Furthermore, a minimal amount of water (W/F ≤ 0.3) exerted negligible influence on the stoichiometric combustion stability. As the LB approached its limit, the average KI remained relatively consistent, with combustion stability emerging as the key factor in elevating the LB performance. A 0.3 W/F ratio and precise spark timing ensured that a balance exists between combustion stability and knocking, while simultaneously elevating λ to 2.0.

Experimental Evaluation of the Methane Number Measurement Procedure for Gaseous Fuel Rating

Methane Number (MN) is a fuel rating technique for gaseous fuels analogous to Octane Number. This study establishes and shares a repeatable and reproducible method for MN determination of a gaseous fuel using a modified Cooperative Fuel Research Engine (CFR). Adaptations required to convert a CFR engine for use in the MN test procedure are identified. The investigation includes allowable environmental parameters and operating variation limits. An essential aspect of the MN method involves identifying and quantifying Knock Intensity (KI) during engine operation. CFR engines, originally designed for gasoline testing, come equipped with their own knock measurement systems utilizing a capacitive detonation sensor. The original system is compared with a Fast Fourier Transform (FFT) approach that uses a piezoelectric pressure transducer. Quantification of methane number requires an accurate assessment of the reference fuel blend (CH4 + H2). A comparison is carried out between dynamic blending using mass flow meters and bracketing using certified gas bottles containing various CH4/H2 blends from a gas supplier.

Effect of using borax decahydrate as nanoparticles additive in blends of spirulina biodiesel/diesel on combustion characteristics and knock intensity

Abstract In this extensive investigation, the impact of borax decahydrate as a fuel additive in a diesel single-cylinder engine was rigorously examined. Borax decahydrate was introduced at concentrations of 5, 15, 25 and 35 g in 500 ml of biodiesel, forming five unique fuel mixtures with conventional diesel: 90% diesel + 10% spirulina biodiesel (SB10), SB10 + 1 g borax decahydrate (SB10B1), SB10 + 3 g borax decahydrate (SB10B3), SB10 + 5 g borax decahydrate (SB10B5) and SB10 + 7 g borax decahydrate (SB10B7). The investigation encompassed four diverse loading conditions and yielded insightful findings. Notably, at full load, SB10B3 exhibited a higher cylinder peak pressure than diesel, reaching 69.25 bar. Heat release rate profiles demonstrated superior efficiency for SB10 at 50% load, with a cumulative heat release rate of 950 J/°CA, which is lower than the 1050 J/°CA of diesel. Knock intensity (KI) evaluations revealed that, although SB10 and SB10B1 exhibited higher KI than diesel at full load due to elevated peak pressure, SB10B7 showed no knocking across all loads, indicative of reduced in-cylinder combustion. This meticulous numerical analysis emphasizes the potential of borax decahydrate as a catalyst and enhancer, providing valuable insights into the combustion dynamics of these alternative fuel blends and their viability for sustainable and efficient engine performance. In summary, out of all the blends, SB10B3 could be a potential diesel fuel replacement fuel for compression-ignition engines.

Experimental Investigation of Glycerol Derivatives and C1–C4 Alcohols as Gasoline Oxygenates

Certain oxygenated compounds, when blended with gasoline, have the ability to inhibit the occurrence and decrease the intensity of engine knock, helping improve engine efficiency. Although ethanol has had widespread use as an oxygenate, higher alcohols, such as butanol, exhibit superior properties in some respects. Besides alcohols, glycerol derivatives such as glycerol tert-butyl ether (GTBE), among others, also have the potential to be used as gasoline oxygenates. This work provides a direct comparison, performed on a modified Waukesha CFR engine, of C1–C4 alcohols and the glycerol derivatives GTBE, solketal, and triacetin, all blended with a gasoline surrogate in different concentrations. The tests focused on how these oxygenated compounds affected the knocking behavior of the fuel blends, since it directly impacts engine efficiency. The test matrices comprised spark-timing sweeps at two different compression ratios, at stoichiometric conditions and constant engine speed. The results showed that, in general, the C1–C4 alcohols and the glycerol derivatives were effective in decreasing knock intensity. n-Butanol and solketal were the noteworthy exceptions, due to their demonstrated inferior knock-inhibiting abilities. On the other hand, isopropanol, isobutanol, and GTBE performed particularly well, indicating their potential to be used as gasoline oxygenates for future engines, as alternatives to ethanol.

Numerical investigation on knock intensity, combustion, and emissions of a heavy-duty natural gas engine with different hydrogen mixing strategies

With the continuous development of hydrogen energy technology, mixing hydrogen has become a potential method to enhance the performance of natural gas engines and reduce emissions. Aiming to offer guidance for optimizing strategies for mixing hydrogen from the perspectives of knock control and thermal efficiency enhancement, a three-dimensional model of a commercial heavy-duty natural gas engine is constructed based on the actual boundary conditions from a high load bench test. In this study, simulations were conducted under conditions where the hydrogen volume fraction ranged from 0% to 40%. The study explored the effects of two injection strategies, port fuel injection of hydrogen-enriched natural gas (PFI) and port fuel injection of natural gas combined with direct injection of hydrogen into the cylinder (PFI + DI), on engine knocking, performance, and emissions. Results indicate that for various injection strategies, there is an increasing trend in knock intensity (KI) with the higher hydrogen mixing ratio. When the hydrogen volume fraction exceeds 20%, the timing of direct hydrogen injection significantly affects KI. This effect is mainly caused by the distribution of unburned hydrogen at the knock onset crank angle (KOCA). Compared to other injection strategies, the PFI + DI strategy when the direct hydrogen injection time at 120°CA BTDC results in a high concentration of hydrogen near the spark plug at the ignition moment, combined with good mixture uniformity in the cylinder. This leads to the shortest CA0-10 and CA10-90, and achieves a maximum indicated thermal efficiency (ITE) of 44.68% under 20% hydrogen volume fraction. Compared to the original natural gas engine, the ITE increased by 2.9% and the NOx emissions increased by 166%, while the HC and CO emissions decreased by 88% and 53%.

A Pressure-Oscillation-Based RON Estimation Method for Spark Ignition Fuels beyond RON 100

Knock in spark ignition (SI) engines occurs when the air–fuel mixture in the combustion chamber ignites spontaneously ahead of the flame front, reducing combustion efficiency and possibly leading to engine damage if left unattended. The use of knock sensors to prevent it is common practice in modern engines. Another measure to mitigate knock is the use of higher-octane fuels. The American Society for Testing and Materials’ (ASTM) determination of the Research Octane Number (RON) and Motor Octane Number (MON) of spark ignition fuels has been based on measuring cylinder pressure rise at the onset of knock since its inception in the 1930s. This is achieved through a low-pass filtered pressure signal. Knock detection in contemporary engines, however, relies on measuring engine vibrations caused by high-frequency pressure oscillations during knock. The difference between conditions in which fuels are evaluated for their octane rating and the conditions that generate a knock intensity signal from the knock sensor suggests a potential difference between octane rating and the knock limit typically identified by a contemporary knock sensor. To address this disparity, a modified RON measurement method has been developed, incorporating pressure oscillation measurements. This test method addresses the historical lack of correlation between RON and high-frequency pressure oscillation intensity during knock. Using toluene standardization fuels (TSFs) as a reference, the obtained results demonstrate excellent high-frequency knock intensity-based RON estimations for gasoline. The method is able to differentiate between two fuels that share the same ASTM RON, associating them with a RON-like metric that is more aligned with their performance in a modern SI engine. This alternative method could potentially serve as a template for an upgrade to the existing ASTM RON method without significantly disrupting the current approach. Additionally, its capability to evaluate fuels beyond RON 100 opens the door to assessing a wider range of fuels for antiknock properties and the intensity of fuel oscillations during knocking combustion.

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.

Flame characteristics and abnormal combustion of ammonia-diesel dual-fuel engine with considering ammonia energy fractions

Carbon neutrality is a significant challenge but also an essential opportunity for internal combustion engines (ICE), and fueled with low-carbon fuels is the key for ICE to reduce carbon emissions. Ammonia is a carbon-free fuel, whereas ammonia-diesel dual-fuel (ADDF) combustion can provide efficient and clean combustion of ammonia. In this study, the flame characteristics and abnormal combustion of ADDF engines are studied by considering ammonia energy fractions and diesel injection timings. The results show that a high ammonia energy fraction led to an extended period of combustion, while the CA05 first advances and then decreases with the decrease of ammonia energy fraction. This phenomenon indicates that an easily ignitable ammonia-air mixture is also important for ammonia-diesel dual-fuel ignition. Besides, an optimal SOI is needed to maximize the promoting effect of ignited fuel on ammonia combustion, and the optimal SOI changes with the ammonia energy fraction. Flame images confirm that the weightiness of the auto-ignition and flame propagation changes with the SOI, and the auto-ignition kernels gradually reduce and flame propagation plays a more and more important role as the SOI advances. At high loads, knocking combustion will also occur for ADDF engines, although the knock intensity is low. Besides, due to the enhancement effect of the knocking pressure wave, the flames show a strong white light at the knocking onset. The current results can help understand the combustion process and thus find the optimization method of ADDF engines.

Experimental study of spatial distribution of knock events in a turbocharged spark-ignition engine

An experimental investigation on the spatial distribution of knock events in a turbocharged spark-ignition engine for hybrid vehicle applications was conducted by using a multichannel fiber optic method. The knock positions were detected under different conditions to investigate the influence of crucial engine design and operating parameters on the knock characteristics including the spatial distribution in the combustion chamber and its relationship to knock intensity. The measured data reveal that the spatial distribution in the engine with a port fuel injection (PFI) system is mainly located on the exhaust side with insignificant influence of engine speed and load, which is attributed to the elevated thermal load around the exhaust valves. However, the knock events under gasoline direct-injection (DI) conditions were found to occur in more scattered locations with more occurring on the engine front-end and rear-end sides. These results indicate that the in-cylinder fuel-air mixing process may have a significant impact on the knock occurrence spots under DI conditions. The knock positions of the engine with different excessive air ratios, injection timings, and intake-valve timings were also detected, indicating that engine operating parameters have complex influences on the knock-region distribution in a DI engine. In addition, experiments were also carried out in two different cylinders to verify the cylinder-to-cylinder variations in knock regions which may be caused by the engine cooling design. Furthermore, no apparent correlations were observed between the knock position and the knock intensity by analysis of the experimental data.

Study of water injection on suppressing knock in a high compression ratio and supercharged hybrid gasoline engine

This study investigated the effects of a direct water injection from both macroscopic characteristics and chemical kinetics aspects in high compression ratio engines, specifically regarding knocking concerns. On the one hand, −80 °CA is thought to be the ideal water injection timing (WIT) in terms of lowering mixture temperature, knock suppression, and water droplet dispersion. The mixture temperature is lowered to below 760 K by −80 °CA of WIT. When ignition timing (IT) is maintained at 2 oCA or −3 oCA, respectively, the water-fuel ratio (WFR) of 0.4 and 0.6 is the threshold for the impact of water droplet mass on combustion. The knock intensity decreases with increasing water-fuel ratio, and the pressure rise rate threshold of 0.6 bar/°CA is a knock-occurrence criterion. On the other hand, spontaneous combustion of end mixtures first occurs beneath the exhaust valve, transporting burned gases and raising temperatures. The IC4H7 concentration is higher in the mid-to-high temperature areas near the cylinder liner. Elevated concentrations of CH2O indicate high-temperature reactions, while increased levels of CH3 and HCO signify auto-ignition onset. Whether the IT is 2 oCA or −3 oCA, both IC4H7 concentration and OH concentration observe a declining trend with the increase of WFR.