Maximum Amplitude Of Pressure Oscillations Research Articles

The sensitivity of pressure-based knock threshold values to alternative fuels: A comparison of methanol vs. gasoline

The heavy-duty industry typically uses a high cetane fuel to power their vehicles, currently typically fossil diesel. Switching to renewable fuels such as methanol requires a major changeover in engine technology. Due to the high octane number of methanol, the diesel engines will have to be replaced by more suitable spark-ignited engines. An accurate knock control system will then be needed to accommodate the high power density requirements of this sector. However, current pressure-based knock detection methods were primarily developed for gasoline fuel operation. Blindly copying these methods for a fuel which has vastly different knock properties can lead to suboptimal engine operation with an efficiency and power loss as result. In this work, experiments were conducted on a single cylinder port fuel injected spark-ignited CFR engine. The applicability of five different pressure-based knock metrics was reassessed for methanol fuel and their performance was compared to each other. Knock threshold values were established for both gasoline and methanol for the MAPO (maximum amplitude of pressure oscillations), AE (average energy) and MVTD (minimum value of third derivative) methods. The results show that the performance of the single peak metrics MAPO and IMPO (integral of modulus of pressure oscillations), and the time-averaged metrics AE and HRR (heat release rate) remains identical between methanol and gasoline operation. For the MVTD method however, the suggested low sampling rate of 1 sample/°ca is insufficient for methanol combustion since the fuel by nature already shows a narrow pressure peak. Overall, the HRR method outperforms the others through the addition of a noise constraint. The calculated knock threshold values, however, are drastically different between both fuels. Compared to gasoline, the thresholds for methanol were 1.25, 2.40 and 5.45 times higher for the MAPO, AE and MVTD methods respectively. This highlights the need for dedicated, fuel-specific, knock threshold values.

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.

Numerical analysis of combustion process and pressure oscillation phenomena in low-pressure injection natural gas/diesel dual fuel low speed marine engine

Dual fuel strategy is a promising way to achieve low-carbon emission for marine engines, but engine knock restricts the compression ratio and engine thermal efficiency by adopting premixed combustion mode. Engine knock of natural gas/diesel dual fuel low speed marine engine is not well understood under high compression ratio. The characteristics of combustion process and pressure oscillation phenomena were investigated by varying compression ratio, pre-chamber number, energy proportion of pilot diesel with a single-cylinder engine model with numerical simulation. The results show that, with increase in pre-chamber number using the same energy proportion of pilot diesel under compression ratio of 16, the maximum amplitude of pressure oscillation can be significantly reduced from 0.62 MPa to 0.23 MPa in main chamber, while both resonance modes follow (1,0) mode. With double pre-chamber, when the energy proportion of pilot diesel increases from 0.6% to 1.2%, due to the stronger intensity of jet flame, the heat release rate is accelerated as well as the accumulation of H2O2 radical become less before high speed spontaneous combustion, resulting in lower amplitude of pressure oscillation. When the energy proportion of pilot diesel increases from 1.2% to 3.0%, excessive amount of pilot diesel cannot strengthen the jet flame obviously while the amplitude of pressure oscillation increases slightly. These results suggest that about 1.0% in energy proportion of pilot diesel should be optimal by considering the thermal efficiency, pressure oscillation and NOx emission together.

Knock metrics experimental evaluation for pre-chamber SI engines

The present study analyzes the occurrence of pressure oscillations in engines operating with pre-chambers at stoichiometric conditions, aiming to assess whether the increase in the amplitude of pressure oscillations detected by means of cylinder pressure measurements is related to knock, or it is a result of mass transfer from the pre-chamber to the main chamber. Initially, a literature review of the phenomenon is conducted and several knock indicators are used to evaluate experimental data obtained with a commercial four-cylinder SI engine, equipped with pre-chamber ignition system prototype, at 1500 rpm and 4 bar of IMEP. Among the knock indicators analyzed the excitations of resonance modes, the angle and time-based maximum pressure rise rate, maximum amplitude of pressure oscillations, ringing intensity, and adapted knock index can be cited. Furthermore, a study of the predominant pre-chamber ignition mechanism is carried out, distinguishing torch and jet ignition, which is also correlated with knock occurrence. Therefore, the present study shows the importance of conducting an individual knock analysis for each pre-chamber configuration and presents adequate tools for such a study, in order to guarantee engine integrity while exploring the benefits of the system, without limitations of inappropriate knock thresholds.

Knock recognition of knock sensor signal based on wavelet transform and variational mode decomposition algorithm

Natural gas has become the most widely used alternative energy for engines because of its low emission pollution and sufficient energy reserve. In order to improve the thermal efficiency of the natural gas engine, it is necessary to increase the compression ratio, but knock restricts the increase of the engine compression ratio. The main knock detection method of engine is to use knock sensor. However, the body vibration detected by the knock sensor contains a lot of interference noise, which brings bad effect on the accuracy of knock detection. Thus, in this paper, a knock recognition method based on wavelet transform (WT) and variational mode decomposition (VMD) algorithm is proposed to process the knock sensor signal and extract the knock feature. Based on the test bench of natural gas engine, experimental investigation of knock combustion characteristics was conducted by adjusting the spark timing, and simultaneously collects cylinder pressure signal and knock sensor signal. Burg algorithm based on Auto-regressive (AR) model was used to extract the knock characteristic frequency of the research engine. Then, wavelet transform and variational mode decomposition algorithm were employed to process the knock sensor signal to remove the interference noise and extract the knock feature. Maximum amplitude of pressure oscillation (MAPO) was selected as the knock intensity evaluation index of cylinder pressure signal. Then, taking the evaluation result of the knock intensity of cylinder pressure signal as reference, the root-mean-square of the knock characteristic signal was used as knock intensity evaluation index of knock sensor signal to determine the knock intensity classification of the knock sensor signal. The results show that the proposed method can effectively extract the knock feature from the knock sensor signal and realize the accurate engine knock recognition. The recognition accuracy is higher than the most existing knock recognition methods.

Numerical modeling of knocking combustion in spark ignition engines

At present, around 85% of the 1.1 billion passenger cars in the world are powered by gasoline spark ignition (SI) engines. Engine downsizing coupled with boosted operation and transition to sustainable fuels is considered an attractive strategy to enhance power density and reduce fuel consumption of SI engines. However, these operating conditions result in severe thermodynamic conditions, thereby promoting the likelihood of abnormal combustion phenomena such as knock. The severity of knock can vary significantly, and the efficiency of engines at high loads is limited in practice by heavy knocking phenomena. Since, a thorough analysis of such recurrent but non-cyclic phenomena via experiments alone becomes highly cumbersome, in the present work, a multi-cycle large-eddy simulation study was performed to quantitatively predict cyclic variability in the combustion process and cyclic knock intensity variability in a direct injection spark-ignition engine. For both the mild knock and heavy knock conditions, the numerical results were validated against experimental measurements. Based on the simulation results, a correlation analysis was performed considering combustion phasing, peak cylinder pressure and maximum amplitude of pressure oscillation. Furthermore, a detailed three-dimensional spatial analysis illustrated the evolution of auto-ignition kernel development and propagation of pressure waves during knocking combustion.

Effects of knock intensity measurement technique and fuel chemical composition on the research octane number (RON) of FACE gasolines: Part 2 – Effects of spark timing

The Research and Motor Octane Number (RON, MON) characterize a fuel’s knock resistance by rating the knock intensity of a sample fuel relative to that of Primary Reference Fuels (PRF) in a Cooperative Fuel Research (CFR) Engine. A fuel’s octane number is regulated to prevent damage from autoignition leading to knocking combustion in spark-ignition engines. The operational differences between the standard RON rating and modern engine operation are explored in a three-part publication series. The previous study focused on the effects of lambda and knock characterization. This second study primarily focuses on the effects of spark timing on RON determination. Following the findings from the first publication, the knock intensity was captured by the knockmeter and by the maximum amplitude of pressure oscillations (MAPO) at the lambda of peak knock intensity and stoichiometry. Knock-limited spark advance tests were conducted for a set of seven Fuels for Advanced Combustion Engines (FACE) from the Coordinating Research Council (CRC) with varying chemical composition, PRFs, and Toluene Standardization Fuels (TSFs). For retarded spark timings, pre-spark low-temperature heat release was found for low RON PRFs. Low RON PRFs also showed knocking characteristics before reaching the center of combustion suggesting that the use of knock-limited spark advance (KLSA) was preferred over the knock-limited combustion phasing. Primarily paraffinic fuels tended towards increased pressure oscillations while dominantly aromatic fuels experienced higher pressure rise rates. A MAPO-based KLSA correlated best to Octane Index at a negative K-factor suggesting beyond RON operation despite being at otherwise RON conditions. At stoichiometry, the MAPO-based KLSA did neither correlate to RON nor Octane Index. Good agreement was found between KLSA-based effective RON from this study to the MAPO-based effective RON from the first study.

Knock Detection in Combustion Engine Time Series Using a Theory-Guided 1-D Convolutional Neural Network Approach

This paper introduces a method for the detection of knock occurrences in an internal combustion engine (ICE) using a 1D convolutional neural network trained on in-cylinder pressure data. The model architecture was based on considerations regarding the expected frequency characteristics of knocking combustion. To aid the feature extraction, all cycles were reduced to 60{\deg} CA long windows, with no further processing applied to the pressure traces. The neural networks were trained exclusively on in-cylinder pressure traces from multiple conditions and labels provided by human experts. The best-performing model architecture achieves an accuracy of above 92% on all test sets in a tenfold cross-validation when distinguishing between knocking and non-knocking cycles. In a multi-class problem where each cycle was labeled by the number of experts who rated it as knocking, 78% of cycles were labeled perfectly, while 90% of cycles were classified at most one class from ground truth. They thus considerably outperform the broadly applied MAPO (Maximum Amplitude of Pressure Oscillation) detection method, as well as other references reconstructed from previous works. Our analysis indicates that the neural network learned physically meaningful features connected to engine-characteristic resonance frequencies, thus verifying the intended theory-guided data science approach. Deeper performance investigation further shows remarkable generalization ability to unseen operating points. In addition, the model proved to classify knocking cycles in unseen engines with increased accuracy of 89% after adapting to their features via training on a small number of exclusively non-knocking cycles. The algorithm takes below 1 ms (on CPU) to classify individual cycles, effectively making it suitable for real-time engine control.

Numerical study on knock characteristics and mechanism of a heavy duty natural gas/diesel RCCI engine

In this paper, the knock phenomenon of reactivity controlled compression ignition (RCCI) engine fueled with natural gas/diesel was numerically studied. The knock mechanism of the RCCI engine is explained and the strategy of suppressing knock is put forward. The knock characteristics were studied by setting monitoring points in different spaces positions of the cylinder. The results show that the pressure oscillation amplitude at the center and edge of the cylinder is large under the high load condition of RCCI engine. In addition, the knock mechanism was studied by using pressure difference method, maximum amplitude of pressure oscillation, important components, temperature isosurface, pressure fluctuation and heat release rate. The results show that the knock of RCCI engine is mainly caused by the end-gas auto-ignition. The pressure difference results show that the characteristic frequency is consistent with the natural resonance mode (0,1) of the cylindrical combustion chamber. On this basis, the effects of pilot oil injection timing and compression ratio on engine knock are further studied. It is confirmed that diesel knock and end-gas knock may exist simultaneously in the same cycle when RCCI engine knock occurs. And diesel knock occurs before top dead center, and end-gas knock occurs after top dead center. Proper adjustment of pilot oil injection timing and reduction of compression ratio can effectively suppress engine knock.

The effects of compression ratio and combustion initiation location on knock emergence by using multiple pressure sensing devices

Knock research is an essential study because knock limits spark-ignition (SI) engine power output, durability, noise, fuel consumption, and emission performance. The thermal efficiency of an engine can be improved by increasing the compression ratio, but at the same time it leads to the possibility of end gas autoignition inside the chamber. The objective of this work was to show the effect of three different compression ratios (CRs) 8.5, 9.5, and 10.5 on the mechanism of knock. In order to achieve this four equispaced spark plugs were fitted around the circumference of a special metal liner to initiate flame fronts from those plugs by firing with different spark approaches (e.g. spark timing, number, and location). In addition, six pressure sensors were installed at various locations to precisely record the autoignition event by monitoring pressure oscillations, combustion characteristics, and knock intensity. In-cylinder pressure and heat release rate (HRR) analyses were applied to distinguish the knocking combustion for the three different CRs. The results showed that a greater number of spark plugs produced more stable combustion even for a delayed spark timing (ST) than a single spark plug, but also increased pressure oscillations, HRR and knock propensity with higher CRs due to an increase in pressure and temperature inside the cylinder. The maximum amplitude of pressure oscillation (MAPO) method was used to determine the knock intensity, and it was observed that at the lowest CR of 8.5 three spark plugs gave higher MAPO values compared to four spark plugs for the same operating conditions. However, four spark plugs instigated higher MAPO values than three spark plugs for the higher CRs of 9.5 and 10.5 because, with the increase of CR and spark plug numbers, the pressure and temperature were increased during the combustion event, which was coupled with the compression effect on the unburned end gas mixture. Additionally, Fast Fourier transform (FFT) and wavelet methods were implemented to observe the frequencies induced during knock. It was found that sparking four plugs provoked various vibration modes in different amplitude ranges with the different CRs.

Experimental study of knock combustion and direct injection on knock suppression in a high compression ratio methanol engine

Methanol is a competitive low-carbon fuel for spark ignition engines. Though it has higher knock resistance than gasoline fuel, knocking combustion is still an obstacle for the application of methanol in spark ignition engines with relatively higher compression ratios. In this work, experimental investigations into knocking combustion and suppression were conducted on a single-cylinder, naturally aspirated, SI engine with a compression ratio of 15. Firstly, the knocking characteristics of the methanol engine with stoichiometric mixture under port fuel injection mode were evaluated. Results showed that knocking combustion occurred under medium and high loads. Especially, under the condition of indicated mean effective pressure (IMEP) = 0.7 MPa, some random heavy knock cycles with ultra-high knock intensity were observed. The frequency of heavy knock was 2.5% and the highest maximum amplitude of pressure oscillations reached 2.39 MPa. This heavy knock was similar to the super-knock phenomenon in boosted direct injection gasoline engines, and it can not be eliminated by retarding spark timing or adopting fuel-lean combustion. Then, the mechanism of the heavy knock was studied by thermodynamic condition analysis. Results showed that the heavy knock was triggered by pre-ignition which was induced by some hot spots in the combustion chamber. Finally, to make the best use of the charge cooling effect, the methanol direct injection strategy was applied to suppress the knocking combustion. When the engine worked with full opening throttle and adopt a single injection, the maximum knock intensity decreased to 0.43 MPa×°CA at −350 °CA ATDC start of injection which was 5.1% of the maximum value during port fuel injection mode. With the co-optimization of the start of injection and spark timing, the engine with a stoichiometric mixture could operate within the knock intensity limit at full load. Under this operating condition, the crank angle of 50% mass fraction burned located at 12 °CA ATDC and IMEP = 1.05 MPa was achieved. What’s more, adopting the optimal split injection instead of a single injection, the knock intensity was further reduced by 14.3% with a slight reduction of 0.02 MPa in IMEP.

Numerical investigation on knock characteristics and mechanism of large-bore natural gas dual-fuel marine engine

This work revealed the unique knock characteristics and mechanism of large-bore natural gas dual-fuel Marine engine with flame jet and strong swirl by a novel three-dimensional pressure difference method to further reduce the knock tendency. The knock characteristics were investigated by pressure monitoring points distributed in different spaces. The results indicate that, owing to the large cylinder bore, almost no pressure oscillations were found in the center position, whereas, severe pressure oscillations appeared at the edge. The pressure difference results illustrate that the movement of the high-pressure area and the low-pressure area tends to the low-frequency resonance mode (1,0), which manifests as an axial rotation following the swirl motion direction. Furthermore, the pressure difference method combined with the maximum amplitude of pressure oscillation (MAPO) method and energy distribution slices was adopted to explore the knock mechanism. The results show that the large-bore natural gas engine has a special knock mechanism which can neither be attributed to the end-gas self-ignition theory nor completely flame acceleration theory. The knock mechanism is that the flame jets from the prechambers cause the uneven initial pressure distribution, and the uneven pressure distribution is strengthened by the uneven heat release on the flame surface. This knock mechanism was confirmed by changing the pilot fuel mass and equivalence ratio subsequently. By the way, a knock suppression method by reducing the pilot fuel was proposed based on the knock mechanism.

Optical diagnostics and multi-point pressure sensing on the knocking combustion with multiple spark ignition

Engine knock is an abnormal combustion phenomenon that limits the thermal efficiency and service life of spark-ignition engines. A better understanding of the knock mechanisms and characteristics is beneficial to knock alleviation and engine efficiency improvement. In this study, a metal liner with four evenly-spaced spark plugs in the periphery of the combustion chamber is designed to initiate knock from different positions. Four spark strategies are applied to the single-cylinder optical research engine and six pressure sensors are utilized to analyze the local pressure oscillations in the cylinder. The knocking combustion is investigated by simultaneous 72 kHz high-speed imaging and 6-point pressure sensing. The experimental results indicate that using multiple spark-ignition could promote knock intensity, advance the start of auto-ignition and introduce more acoustic resonance modes. The center pressure sensor is more sensitive to the first radial resonant mode (0, 1) of the knock pressure oscillation, while the side sensors are more sensitive to the first and second circumferential resonant modes (1, 0) and (2, 0). The knock onset judged by natural flame photography is earlier than that by pressure analysis because the auto-ignition event happens first and induces the subsequent pressure fluctuation. Natural flame luminosity analysis demonstrates that the initial auto-ignition sites only cause weak pressure oscillations, and the instantaneous combustion of the remaining end-gas increases the heat release rate significantly and gives rise to more violent pressure oscillations. Statistically, the maximum amplitude of pressure oscillation follows an exponential relationship with the peak mean flame luminosity. The end-gas resides in the gaps among the flame fronts generated by different spark strategies while the first auto-ignition sites are not evenly distributed in the end-gas zone. This fact gives insights into the local temperature non-uniformity of the end gas zone that affects the spatial distributions of the initial auto-ignition sites in the cylinder.

Statistical study on engine knock oscillation and heat release using multiple spark plugs and pressure sensors

Engine knock has been an inherent problem that limits engine efficiency improvement and threatens engine service life. To trigger the controllable knock, a specialized liner with installing four side spark plugs mounted on the cylinder head are used in this study to produce various in-cylinder flame propagation. Various spark strategies (e.g., spark timing, spark number, spark location) are applied to generate different auto-ignition sites and knock characteristics. Up to five channels of pressure signal are collected to analyze the knock intensities regarding different spark strategies. To investigate the knock-induced fluctuations and heat release, we use the multiple correlations to evaluate the maximum amplitude of pressure oscillation (MAPO) respecting different influential factors and building a multiple linear regression (MLR) model for MAPO prediction and validate it against the experiment results. Besides, the Wiebe function and curve-fitting techniques are applied to estimate the energy released by auto-ignition in each cycle, and its relations with knock intensity are assessed. The results show that the average growth of pre-oscillation plays an important role in MAPO value (correlation coefficient, r_s = 0.965), the established MLR model possesses high accuracy for MAPO prediction. The pressure oscillation regarding the 1st resonance mode is extracted from the band-pass filtered signal, representing the main pressure oscillating process. Compared with the single spark ignition, triggering more spark plugs could boost the fuel consumption rate, while shortening the knocking combustion duration and reducing the heat release fraction contributed by auto-ignition. At the same CA50 (9 CAD aTDC), the four spark ignition leads to the least heat release fluctuations than other spark strategies. With advancing the CA50, the heat release fraction produced by knock increases at first then reduces due to the thermodynamic conditions and the flame propagation. Activating two or three spark plugs, the rising rate of the heat release rate goes up, but the knock-induced heat release fraction decreases. Moreover, the primary acoustic resonance mode (1, 0) gives more regular knock vibrations than the mixing form (including mode (1, 0), (0, 1), (2, 0), etc.), the average traveling distance of acoustic wave in the time gap of receiving two continuous signals is very close to the cylinder bore. The results give insights into the pressure wave propagation during knock and the relations between knock conditions, pressure oscillations, and heat release.

Safe operation of dual-fuel engines using constrained stochastic control

Premixed combustion strategies have the potential to achieve high thermal efficiency and to lower the engine-out emissions such as NOx. However, the combustion is initiated at several kernels which create high pressure gradients inside the cylinder. Similarly to knock in spark ignition engines, these gradients might be responsible of important pressure oscillations with a harmful potential for the engine. This work aims to analyze the in-cylinder pressure oscillations in a dual-fuel combustion engine and to determine the feedback variables, control actuators, and control approach for a safe engine operation. Three combustion modes were examined: fully, highly, and partially premixed, and three indexes were analyzed to characterize the safe operation of the engine: the maximum pressure rise rate, the ringing intensity, and the maximum amplitude of pressure oscillations (MAPO). Results show that operation constraints exclusively based on indicators such as the pressure rise rate are not sufficient for a proper limitation of the in-cylinder pressure oscillations. This paper explores the use of a knock-like controller for maintaining the resonance index magnitude under a predefined limit where the gasoline fraction and the main injection timing were selected as control variables. The proposed strategy shows the ability to maintain the percentage of cycles exceeding the specified limit at a desired threshold at each combustion mode in all the cylinders.

Large-eddy simulation study on cycle-to-cycle variation of knocking combustion in a spark-ignition engine

The cycle-to-cycle variation in the knock intensity is commonly encountered under abnormal combustion conditions. The severity of these abnormal combustion events can vary significantly, and the efficiency of engines at high loads is limited in practice by heavy knocking phenomena. Since, a thorough analysis of such recurrent but non-cyclic phenomena via experiments alone becomes highly cumbersome, in the present work, a multi-cycle large-eddy simulation study was performed to quantitatively predict cyclic variability in the combustion process and cyclic knock intensity variability in a direct injection spark-ignition engine. To account for the turbulence-chemistry interaction effects on flame propagation, the G-equation combustion model was used. Detailed chemistry was solved outside the flame front with a toluene primary reference fuel skeletal kinetic mechanism. For both the mild knock and heavy knock conditions, the numerical results were validated against experimental measurements. Based on the simulation results, a correlation analysis was performed considering combustion phasing, peak cylinder pressure and maximum amplitude of pressure oscillation. Furthermore, a detailed three-dimensional spatial analysis illustrated the evolution of auto-ignition kernel development and propagation of pressure waves during knocking combustion for three typical cycles with different knock intensities. It was found that an early occurrence of auto-ignition in the end gas was prone to high knock intensity. Although multiple auto-ignition kernels were observed in different cycles, the degree of coupling between chemical heat release and pressure waves varied, thereby leading to different maximum amplitude of pressure oscillation values.

Quantitative evaluation of n-butane concentration on knock severity of a natural gas heavy-duty SI engine

Knocking combustion of a natural gas spark ignition engine with the addition of n-butane was investigated. Specially, the effects of n-butane concentration on knock severity were quantitatively evaluated with engine full-load at various engine speeds. The results show that with an increase in n-butane concentration, the peaks of in-cylinder pressure and heat release rate increase, combustion duration is shortened, and CA50 is advanced. At high speeds, n-butane concentration has a more pronounced influence on combustion and knock. With increased n-butane concentration, cyclic pressure traces oscillate more and more violently, the maximum pressure rise rate (MPRR) and the maximum amplitude of pressure oscillation (MAPO) in some cycles increase significantly. Here the MPRR threshold of light knock cycle is defined at 1.0 MPa/CAD, and the one of heavy knock cycle is 4.0 MPa/CAD. Meanwhile the MAPO threshold of light knock cycle is set at 0.1 MPa, and the one of heavy knock cycle is 0.3 MPa. Based on these thresholds and the probability of knock severity, light and heavy knock conditions are identified. Moreover, the combustion close to light knock may increase thermal efficiency, and heavy knock has caused the erosion of aluminum piston crown and top land in the experiment.

A real-time pressure wave model for knock prediction and control

This article develops a control-oriented real-time zero-dimensional knock pressure wave model for spark-ignited engines based on the reaction-based two-zone combustion model developed earlier. This knock pressure wave model is capable of predicting the in-cylinder pressure oscillations under knocking combustion in real-time and can be used for the model-based knock prediction and control. A pressure wave equation including the knock deadening behavior is proposed, simplified, and used to calculate the pressure perturbations generated by knock combustion. The boundary and initial conditions at knock onset are analyzed and the analytic solution of the pressure wave equation is obtained. The model is calibrated and validated over two engine operating conditions at knock limit. The chemical kinetic-based Arrhenius integral and the maximum amplitude of pressure oscillations are used as the evaluation methods for knock onset and intensity prediction, and the knock frequency is studied using a fast Fourier transform of the filtered in-cylinder pressure oscillations. The simulation results show that the proposed knock pressure wave model is able to predict the knock onset timing, frequency, and intensity accurately under two engine operating conditions. Furthermore, the knock characteristics associated with gas mixture properties at intake valve closing is analyzed based on the experimental data, and their effect to knock cycle-to-cycle variation is also studied for the proposed model.

Numerical Investigation on Knock Combustion in a Diesel–Dimethyl Ether Dual-Fuel Engine

Numerical investigation on knock was conducted in a direct-injection diesel engine with port premixed dimethyl ether (DME) fuel. Knock combustion at various engine operating conditions with a fixed speed was simulated, and the effects of premixed DME ratio, injection timing, and exhaust gas recirculation (EGR) rate on knock suppression were studied based on the multidimensional computational fluid dynamics (CFD) analysis. A reduced diesel–DME kinetic mechanism coupled with the CFD model was used to obtain local pressure and radical concentration at various monitoring locations within the computation domain. A method with a band-pass filter was then employed to deal with pressure data, and maximum amplitude of pressure oscillation was adopted as an evaluation index of the knock intensity. The analysis of species concentration was also used to quantify knock here. The research results indicated the profiles of pressure and major intermediate species could provide useful information to quantify and predict knock for DME–diesel dual-fuel premixed compression-ignition combustion. Moreover, using EGR was an effective way to reduce the knock intensity and delay the knock onset. A lower DME premixed ratio and retarded injection timing could also mitigate the knock.

Effects of equivalence ratio and pilot fuel mass on ignition/extinction and pressure oscillation in a methane/diesel engine with pre-chamber

For reducing emissions and saving energy, pilot-ignited methane/diesel engines are attracting considerable attention as an alternative to traditional diesel engines. However, this dual-fuel engine suffers from poor ignition characteristics and knock occurrence under specific operating conditions. Thus, the present study comprehensively aims to address the effect of the CH4 equivalence ratio and pilot fuel amount on the ignition/extinction process, in-cylinder pressure oscillation, and engine performance in a low-speed dual-fuel engine with pre-chamber. The results show that the fuel-mixture ignition delay is prolonged with a decrease in the CH4 equivalence ratio, but it decreases as the pilot fuel amount increases. However, beyond a certain amount, an additional increase in pilot fuel will not affect the methane combustion, indicated engine power, and emissions. It was also found that extinction occurs with a CH4 equivalence ratio of 0.3, because the reaction of the low-temperature intermediate species CH2O becomes almost stagnant. The increase in CH4 equivalence ratio can improve the engine power, but it also induces pressure oscillation and promotes nitrogen oxide production. Moreover, the pressure oscillation or maximum amplitude of pressure oscillation (MAPO) increases with the increase in pilot fuel amount, although it has no significant change when the pilot fuel proportion is beyond 1.5%. The present study can provide a theoretical basis for improving the combustion stability of dual-fuel engines under low engine load and avoiding the risk of knock under moderate to high engine loads.