In-cylinder Pressure Measurements Research Articles

A Feature Extraction Method for Prognostic Health Assessment of Gas Compressor Valves

Abstract This paper presents features derived from the pressure-volume (PV) diagram that is useful in estimating different valve faults in reciprocating compressors with a strong potential of remaining useful life prediction. The PV diagram is expected to deviate depending on valve wear conditions. The valve degradation types explored in this work are leakage, seat wear, and spring fatigue in the suction and discharge assemblies of a Dresser-Rand ESH-1 compressor commonly used in the petrochemical industry. The proposed method estimates the polytropic exponent on the compression and expansion phase as well as the discharge and suction valve loss power and uses them as features for a quadratic discriminant analysis. The features are created through in-cylinder pressure, suction pressure, discharge pressure, and crank angle measurements collected on a single-stage, dual-acting compressor operating on air with wear precisely machined and seeded into the poppets of the inlet and outlet valves. A very high classification accuracy is achieved in distinguishing the wear types, severity, and location with strong prognostic trends.

A Review of Recent Advancements in Knock Detection in Spark Ignition Engines

In gasoline engines, the combustion process involves a flame’s propagation from the spark plug to the cylinder walls, resulting in the localized heating and pressurization of the cylinder content ahead of the flame, which can lead to the autoignition of the gasoline and air. The energy release from the autoignition event causes the engine cylinder to resonate, causing an unpleasant noise and eventual engine damage. This process is termed as knock. Avoiding knock has resulted in limiting the maximum engine pressures, and hence limiting the maximum efficiencies of the engine. Modern engines employ knock sensors to detect resonances, adjusting the spark plug timing to reduce pressures and temperatures, albeit at the expense of engine performance. This paper sets out to review the different signals that can be measured from an engine to detect the start of knock. These signals traditionally consist of the in-cylinder pressure, the vibrations of the engine block, and acoustic noise. This paper reviews each of these techniques, with a focus on recent advances. A number of novel methods are also presented, including identifying perturbations in the engine speed or exhaust temperature; measuring the ion charge across the spark plug leads; and using artificial intelligence to build models based on engine conditions. Each of these approaches is also reviewed and compared to the more traditional approaches. This review finds that in-cylinder pressure measurements remain as the most accurate for detecting knock in modern engines; however, their usage is limited to research settings. Meanwhile, new sensors and processing techniques for vibration measurements will more accurately detect knock in modern vehicles in the short term. Acoustic measurements and other novel approaches are showing promise in the long term.

A Fast Calibration Method for Pneumotachograph with a 3L Syringe.

The pneumotachograph (PNT), a commonly used flowmeter in pulmonary function diagnostic equipment, is the required frequency calibration to maintain high accuracy. Aiming to simplify calibration steps, we developed a fast calibration system with a commercially available 3L syringe to provide a real output flow waveform. The acquisition of the real output flow waveform is based on the reliable measurement of in-cylinder pressure and the real-time detection of plunger speed. To improve the calibration accuracy, the tapping position for measuring in-cylinder pressure was optimized by CFD dynamic-mesh updating technique. The plunger speed was obtained by tracking the handle of the plunger with a smart terminal. Then, the real output flow was corrected using a compensation model equation. The calibration system was verified by the pulmonary waveform generator that the accuracy satisfied the requirements for respiratory flow measurement according to ATS standardization. The experimental results suggest that the developed method promises the fast calibration of PNT.

Optical Study on Spark Plug Gap in Extending Methane Lean Combustion Limits under High Ignition Energy Conditions

<div>Lean combustion has the potential to achieve high thermal efficiency for internal combustion engines. However, natural gas (NG) engines often suffer from slow burning rates and large cyclic variations when adopting lean combustion. In this study, the effects of spark plug gaps (SPGs) on methane lean combustion are optically investigated under high ignition energy conditions. Synchronization measurements of in-cylinder pressure and high-speed photography are performed for combustion analysis. The results show that large SPGs with high ignition energy exhibit great improvement in engine combustion stability and power capability. Under ultra-lean conditions, a large SPG with a high ignition energy of 150–200 mJ can extend the lean limit to 1.55. Combustion images indicate that this is contributed by the enlarged initial flame kernel, which promotes early flame propagation. Besides, an empirical criterion is adopted to quantify the underlying mechanism, and the results confirm that a more stable early flame development with a faster burning rate can be obtained by a larger SPG and higher ignition energy under lean conditions. Therefore, a large SPG is an effective way to improve combustion stability and thermal efficiency for NG engines.</div>

Investigation of In-Cylinder Pressure Measurement Methods within a Two-Stroke Spark Ignition Engine

<div>This work describes an investigation of measurement techniques for the indicated mean effective pressure (IMEP) on a 55 cc single-cylinder, 4.4 kW, two-stroke, spark ignition (SI) engine intended for use on Group 1 and Group 2 remotely piloted aircraft (RPAs). Three different sensors were used: two piezoelectric pressure transducers (one flush mount and one measuring spark plug) for measuring in-cylinder pressure and one capacitive sensor for determining the top dead center (TDC) position of the piston. The effort consisted of three objectives: to investigate the merits of a flush mount pressure transducer compared to a pressure transducer integrated into the spark plug, to perform a parametric analysis to characterize the effect of the variability in the engine test bench controls on the IMEP, and to determine the thermodynamic loss angle for the engine. The results indicate that as a spark plug, the measuring spark plug is not statistically different from the stock spark plug at the 95% confidence level. The results indicate a statistically significant, 4% difference in the measured IMEP between the pressure transducer in the measuring spark plug and the flush mount transducer. The results also suggest a statistically significant difference in performance between the modified and unmodified engine heads, verifying the suppositions of other researchers who suggested that even a small modification to a combustion chamber this size could measurably affect the engine performance. While run-to-run variation resulted in a 2% to 5% variation in IMEP, a sensitivity analysis determined that 1% to 3% of that variation arose from variability in the control variables, while the remainder was caused by variation in other engine operating parameters. Between 1000 rpm and 2000 rpm, where the engine was typically motored to determine the TDC, the thermodynamic loss angle was 0.3 crank angle degrees (CAD) to 0.7 CAD, larger than loss angles observed in automotive-sized gasoline engines. The results indicate that using tabulated thermodynamic loss angles to set the TDC location of the engine using a mono-directional peak pressure method would lead to a −1% to −2.5% bias in the IMEP.</div>

Advanced model-based closed-loop combustion control strategies with combustion rate shaping

Combustion rate shaping (CRS) is an advanced closed-loop concept which controls the crank-angle resolved combustion trace by adapting the fuel injection profile. The thermodynamic potential of this concept to enable real-time combustion control independent of fuel property variations, injector drift, cylinder tolerances, or changing air-path boundary conditions has already been demonstrated. The highest benefit of this control concept is achieved when it is directly applied on the engine control unit (ECU) in combination with an in-cylinder pressure measurement. This enables online optimization and adaption of the fuel- and air-path settings to the fuel used during vehicle operation. Therefore, the development of a CRS concept with real-time capability has been the main focus so far. To ensure a robust online real-time combustion control, a key development step was the identification of control variables that describe the entire combustion process and only minimally influence each other. For this purpose, the combustion phasing, the combustion gradient and the indicated mean effective pressure have been determined as parameters for the feedback control. However, the stringent requirements for higher efficiencies, emission robustness and combustion noise necessitate the extension of the CRS control concept. Therefore, in this work, the CRS control concept is extended with the aim of realizing a combustion with constant volume pressure increase to the peak firing pressure limit to achieve highest efficiency. In addition, intelligent control of the fuel injection profile is to be used to achieve a combustion temperature for lowest possible NOx raw emissions and a robust combustion noise excitation limitation. The optimized control strategy is implemented on a heavy-duty single cylinder engine test bench using a Rapid Control Prototyping (RCP) system, and the control performance is demonstrated.

Copper leaching effects on combustion characteristics and particulate emissions of a direct injection high pressure common rail diesel engine

In this work the impact of a helicoidally shaped copper duct on combustion characteristics and particulate emissions of a 2.2 L Direct Injection High Pressure Common Rail (DI HPCR) diesel engine were investigated. The copper duct is at the core of an electrical device constituted by magnetic coils powered at 12.5 V. This device is installed on the fuel line immediately before the high-pressure pump. Three operating conditions, low to medium loads, at 1500 and 2000 rpm have been considered. Combustion characteristics were analysed by post-processing in-cylinder pressure measurements, while an AVL Smoke Meter was used to monitor particulate emissions. The diesel fuel had initially a copper content less than 0.02 ppm; fuel was sampled at regular intervals during engine operation and inductively coupled plasma mass spectrometry (ICP-MS) analysis showed a monotonic increase of its copper content, up to 0.19 ppm (mg/kg). A soot reduction up to 28% was measured when the device was connected to the fuel line, compared to the baseline. No change in the combustion delay over time suggests that soot abatement was primarily due to a copper-mediated oxidizing effect. A preliminary analysis performed on heat release data acquired under the same engine operating conditions suggests an increase in the average value and standard deviation of the heat release peaks when the device is powered on. Additionally, four operating conditions were also investigated but the copper was replaced by a Teflon duct; the results indicates that if no metal is leached into the fuel, the magnetic field does not have any major effect on combustion and emission characteristics of the diesel engine.

Low-temperature oxidation studies on canola and coconut-derived biodiesel in a motored engine

This work reports results on the low-temperature oxidation of canola and coconut-derived biodiesel in a motored engine. A lean premixed fuel–air mixture of equivalence ratio 0.25 at an intake temperature of 125 °C was subject to reaction with pure compression. The extent of the reaction was controlled by changing the compression ratio in the range of 5.5–9.0. In-cylinder pressure measurements were used for heat release analysis, and detailed speciation analysis of the reaction products was done using a gas chromatography-mass spectrometry system. The canola-derived biodiesel shows minor low-temperature heat release, while coconut biodiesel exhibits a relatively higher heat release. It was found that the low-temperature oxidation chemistry for these biodiesels is similar to saturated single-component methyl esters. However, the heat release was somewhat suppressed due to unsaturated compounds in the two biodiesel fuels. The heat release suppression correlates well with the extent of unsaturation. The oxidation end products under the test conditions were similar for both fuels. The relatively higher reactivity of the coconut over the canola biodiesel was also seen in the speciation results, and its stable intermediate concentrations were much higher than the canola. The experimental results were compared to computations using an existing kinetic model. The onset of heat release was predicted with reasonable accuracy. However, the peak and accumulated heat release were significantly overpredicted.

Optical investigations on ignition and combustion characteristics of polyoxymethylene dimethyl ethers/methanol blends

ABSTRACT Internal combustion engines using carbon-neutral fuels are effective ways to achieve low CO2 emissions. Polyoxymethylene dimethyl ethers (PODE) and methanol are carbon-neutral fuels, both good alternative fuels for engine combustion. Dual-fuel combustion modes have been widely studied, but investigations on the ignition and combustion characteristics of PODE/methanol at blending mode are still lacking, especially in optical engines. In this study, the effects of methanol blending ratios (0, 10, 20%), intake temperatures (45, 85, 120°C), and injection timings (−60, −45, −30, −15 °CA aTDC) on the ignition and combustion behavior of PODE/methanol blends were optically studied. Synchronization measurements of in-cylinder pressure and high-speed photography were performed for combustion analysis. The results show that a reduced methanol blending ratio, increased intake temperature, and intermediate injection timing exhibit positive influences on flame initiation and combustion evolutions. Specifically, an optimized methanol blending ratio, intake temperature, and injection timing can shorten the ignition delay time by 11.2, 6.1, and 39.2 crank angles, respectively. Meanwhile, unlike methanol blending ratio and intake temperature, injection timing can significantly affect early-stage rapid combustion duration. Optical images show that multiple flame kernels are initially induced at local rich regions with shorter ignition delay time, while when ignition delay time becomes longer, flame kernels start to be discretely distributed throughout the combustion chamber. These observations demonstrate the significance of the competition between chemical reactivity and fuel stratification in ignition and combustion characteristics of PODE/methanol blends.

Autoignition, knock, detonation and the octane rating of hydrogen

This paper presents an experimental and numerical study of hydrogen’s combustion, autoignition and octane rating. The experiments are conducted in a standard engine that is compliant for rating liquid fuels to the ASTM’s Research Octane Number (RON) method, with minor modifications to enable hydrogen fueling. The numerical analysis is undertaken using a combination of calibrated, two-zone combustion and kinetic modeling of the end-gas. Application of the standard RON method first shows that hydrogen has a RON of 62–64, which is significantly lower than that of standard gasolines. However, this standard RON method requires conditions that are not relevant to practical, hydrogen-fueled engines and, indeed, do not appear to feature autoignition and knock. A set of modified RON tests are therefore undertaken at more practical conditions. These indicate that hydrogen at standard knock intensity and λ=1,1.5 and 2 has a modified RON of 93.7,117 and greater than 120 respectively, spanning the range of λ that match the energy delivered by common SI engine fuels.Together, these results show that hydrogen is significantly more knock-resistant than standard gasolines when providing similar energy to the premixture. Indeed, it should have comparable knock resistance to some high-octane fuels. However, care must be taken when examining hydrogen autoignition, knock and abnormal combustion more generally. Hydrogen’s high flame speeds near stoichiometric conditions can produce high rates of pressure rise at lower compression ratios that can be mistakenly identified as autoignition, even with contemporary in-cylinder pressure measurement, while higher compression ratios near stoichiometric conditions can cause detonation. The identification and avoidance of conventional knock and other forms of abnormal combustion is therefore particularly important for hydrogen fueled engines. This also has significant implications for the development of standard test methods for determining the octane and methane numbers of hydrogen-rich fuel blends.

Combustion characteristics and detailed simulations of surrogates for a Tier II gasoline certification fuel

An experimental and numerical study of combustion of a gasoline certification fuel (‘indolene’), and four (S4) and five (S5) component surrogates for it, is reported for the configurations of an isolated droplet burning with near spherical symmetry in the standard atmosphere, and a single cylinder engine designed for advanced compression ignition of pre-vaporized fuel. The intent was to compare performance of the surrogate for these different combustion configurations and to assess the broader applicability of the kinetic mechanism and property database for the simulations. A kinetic mechanism comprised of 297 species and 16,797 reactions was used in the simulations that included soot formation and evolution, and accounted for unsteady transport, liquid diffusion inside the droplet, radiative heat transfer, and variable properties. The droplet data showed a clear preference for the S5 surrogate in terms of burning rate. The simulations showed generally very good agreement with measured droplet, flame, and soot shell diameters. Measurements of combustion timing, in-cylinder pressure, and mass-averaged gas temperature were also well predicted with a slight preference for the S5 surrogate. Preferential vaporization was not evidenced from the evolution of droplet diameter but was clearly revealed in simulations of the evolution of mixture fractions inside the droplets. The influence of initial droplet diameter (Do) on droplet burning was strong, with S5 burning rates decreasing with increasing Do due to increasing radiation losses from the flame. Flame extinction was predicted for Do =3.0 mm as a radiative loss mechanism but not predicted for smaller Do for the conditions of the simulations.

Time frequency analysis for the evaluation of ignition delay in conventional and PCCI combustion modes

• Time frequency analysis can detect both start of injection and start of combustion. • Time frequency analysis can provide an accurate evaluation of ignition delay in diesel engines. • Combustion development for PCCI and conventional diesel modes is investigated. In-cylinder pressure signal has been widely adopted to obtain simple offline and online diagnosis of the combustion process through one-zone analysis, which provides the apparent heat release of the mixture. The present paper also starts from the in-cylinder pressure measurement to apply time frequency analysis for an alternative evaluation of the combustion events. A comparison with traditional one-zone analysis is then performed. In particular, time frequency analysis is here applied for the investigation of all the characteristic events that define the fuel ignition delay, i.e. hydraulic start of injection, first, along with the corresponding start of combustion, next. The study here reported is relative to single-injection patterns and has been applied to both conventional and premixed low temperature diesel combustion modes. For the considered injection pattern, time frequency analysis, compared to single-zone combustion model supplemented by measurements of injection rate at the hydraulic rig and by engine measurements of fuel injection pressure in the injector supply pipe, provided precise evaluation of the relevant combustion events.

Understanding the difference in combustion and flame propagation characteristics between ammonia and methane using an optical SI engine

As a carbon-free hydrogen-carrier fuel with a high content of hydrogen, ammonia (NH3) exhibits unfavorable combustion properties such as low flammability and large cyclic variations, requiring further investigation in actual engines. In this work, for the first time, using a single-cylinder optical spark ignition (SI) engine with a high compression ratio, engine performance as well as combustion and flame propagation characteristics were investigated. Synchronization measurement of in-cylinder pressure and high-speed photography was performed, and the combustion characteristics were compared between ammonia and methane under the same operating conditions. The results show that the combustion stability and power capability of ammonia are significantly lower than that of methane. Further analysis combining combustion phasing and flame propagation characteristics indicates that the burning time loss mainly comes from the initial flame development process rather than the main combustion duration which results in a deficit in engine performance. The average probability maps of the flame location indicate that the flame front propagation of the two fuels emerges with obviously different characteristics. Quantitative flame solving shows that NH3 flame propagation features a higher flame stretch sensitivity which is macroscopically reflected in the cyclic variations. Furthermore, the quantification of flame response to turbulence was solved and observed much higher effects for NH3 flame than methane, which indicates NH3 flame propagation is mainly driven by the flame response to turbulence while the flame propagation of methane exhibits higher sensitivity to temperature than ammonia. The current study shall provide insights into the application of ammonia in the internal combustion engine.

Ignition Sensitivity Analysis for Energy-Assisted Compression-Ignition Operation on Jet Fuels with Varying Cetane Number

<div class="section abstract"><div class="htmlview paragraph">Local deposition of thermal energy can be used to assist the combustion process of low cetane number (CN) fuels in compression-ignition engines, here termed energy-assisted compression ignition (EACI). In the current work, a commercial ceramic glow plug, operated beyond its conventional operation range, was used as the ignition assistant (IA) and sensitivity of fuel jet ignition to operation parameters was studied for two fuels using EACI in an optical engine. A design-of-experiments (DoE) study was devised to determine which engine parameters influenced the energy-assisted pilot injection ignition process the most. The DoE was constructed with four parameters: injection pressure, injected mass, injection timing, and ignition assistant temperature. The fuels used were F24 (Jet-A with military additives) with a cetane number of 48 and a cetane number 35 fuel mixture consisting of 60% F24 and 40% of an alcohol-to-jet fuel (ATJ), blended on a volumetric basis. Simultaneous OH chemiluminescence and schlieren imaging were employed along with in-cylinder pressure measurements to assess ignition delay, jet development, and the combustion process. A Gaussian Kriging emulator was utilized to perform the sensitivity analysis. The results indicate that ignition of the fuel jet directed toward the ignition assistant is mainly driven by the ignition assistant temperature, with around 90% of the sensitivity for both fuels ascribed to ignition assistant temperature. However, for ignition assistant surface temperatures greater than ~1350 K ignition delay is approximately constant with increasing temperature. Ignition delay from the time the fuel reaches the ignition assistant to start of combustion, here termed hot surface ignition delay, was analyzed to gain a better understanding of the EACI ignition process. The hot surface ignition delay for ignition assistant temperatures <i>></i>1350 K ranged from approximately 200 to 500 <i>μ</i>s, independent of fuel cetane number. This indicates that ignition for ignition assistant surface temperatures <i>></i>1350 K is likely the result of high-temperature ignition chemistry.</div></div>

Fault diagnostics and adjustment of the fuel system of a medium-speed diesel engine using heat release characteristics

Adjustment of the diesel fuel equipment is an important condition for maintaining the technical condition of the diesel engine in operation at the level declared by the manufacturer. This paper presents the results of an experimental study of the method previously proposed by the authors for tuning the fuel system of a diesel engine based on the analysis of diagrams of net heat release in the cylinder. Net heat release diagrams are calculated based on the first law of thermodynamics, written for a closed system, using only cylinder pressure and cylinder volume. The method allows some instability of the load mode during testing and does not require simultaneous measurement of in-cylinder pressure in all engine cylinders. The method is adapted for typical operating conditions and in-cylinder pressure measurement through the indicator channel. The object of the study was a medium-speed six-cylinder diesel engine with a high-pressure hydromechanical fuel pump. The article also considers the possibility of using net heat release diagrams to identify injector malfunctions that lead to poor fuel atomization.

Plasma electrolytic oxidation thermal barrier coating for reduced heat losses in IC engines

The work involved the development and on-engine testing of a new “thermo-swing” barrier coating for reduced wall heat transfer and increased thermal efficiency in future diesel engines utilizing aluminium alloy pistons. Such swing coatings, of low thermal conductivity and low specific heat capacity, have recently been proposed to produce a dynamic thermal barrier layer that rapidly changes the temperature of the upper surface of the piston crown in response to the adjacent in-cylinder gas temperature. The new coating tested in this work was formed directly from the piston substrate material using an optimised plasma electrolytic oxidation process, with a silica top coat subsequently applied to entrap air within coating pores. Benchtop laser flash measurements were undertaken to quantify coating thermal properties and provide the required empirical data for future thermal simulation. Coatings of varying features were tested in a bespoke thermodynamic single cylinder diesel engine instrumented for precision measurements of in-cylinder pressure, fuel consumption and legislated engine-out emissions. The optimum coating applied across the full piston crown and bowl enabled up to 3% improvement in indicated thermal efficiency under idealised part load operating conditions. The coating reduced heat transfer during combustion, leading to elevated engine-out NOx. By retarding combustion phasing slightly from the optimum, the NOx increase could be mitigated while still retaining most of the fuel consumption benefit, with the remaining benefit associated with reduced heat transfer during the remaining power stroke. The emissions of other key pollutants (CO, unburned hydrocarbons and soot) were less affected under the part load conditions tested.

Study of combustion and emission characteristics of a diesel engine fueled with diesel, butanol-diesel and hexanol-diesel mixtures under low intake pressure conditions

A significant number of landmasses on earth are above sea level where the ambient pressure is below 1 atm. With economy and population growth, the rapid expansion of human activities at these high-altitude regions is expected. However, the research on diesel engine combustion and emissions under low intake pressure conditions are limited, especially when alcohol-diesel mixtures are used as fuel. In this work, the combustion and emission characteristics of a common-rail diesel engine fueled with diesel, butanol-diesel and hexanol-diesel mixtures under low intake pressure conditions were investigated. Mixtures were prepared with two blend ratios, which were 20% and 40% by volume. Experiments were conducted under five different intake pressures (1.2, 1.0, 0.8, 0.7, and 0.6 atm) and two engine loads (0.45 MPa and 0.55 MPa IMEP). The combustion characteristics and engine performance were analyzed using measurements of in-cylinder pressure, HRR, ignition delay, combustion duration, ISFC and ITE. NOx, soot, CO, and HC emissions under different operating conditions were also measured and discussed. Results revealed lower intake pressure led to higher ISFC and lower ITE. Soot and CO emissions increased significantly once intake pressure dropped below 0.8 atm, while NOx emissions decreased with intake pressure reduction. Under the same conditions, adding renewable alcohol extended the ignition delay and reduced the combustion duration. These effects were more pronounced for combustion of butanol-diesel mixtures than combustion of hexanol-diesel mixtures. Combustion of hexanol-diesel mixture with 20% blend ratio (H20) had similar or lower ISFC and higher ITE than diesel combustion when intake pressure was lower than 1 atm due to its fuel oxygen content and similar fuel properties as diesel. The engine fueled with H20 could still operate under intake pressure of 0.6 atm and engine load of 0.45 MPa IMEP, but the engine misfired when other fuels were used. In terms of emission characteristics, adding alcohol significantly reduced soot and CO emissions, especially when intake pressure was below 1 atm. However, combustion of alcohol-diesel mixtures produced more NOx emissions than diesel combustion under the same conditions.

Combining in-cylinder pressure and 1D simulation tools to understand the combustion characteristics of natural gas in pre-chamber ignition systems for energy generation

Recent outlooks suggest a long-term relevance of internal combustion engines for both power generation and transportation. Nonetheless, stringent pollutant reduction requirements combined with new CO2 mandates draws a challenging scenario, requiring intensive research and development activities to develop and optimize combustion modes to fulfill these requirements. Among the recent advancements, the active pre-chamber ignition system has been considered as a potential alternative to achieve a highly efficient and clean combustion process. Its combustion development is dictated by the pre-chamber ignition system which will provides a high energy flow jet inside the main chamber to enable a multi-site oxidation of the global lean mixture. The comprehension and quantification of the flow and combustion characteristics of the pre-chamber are of utmost importance to optimize the engine operation and pre-chamber design. Nonetheless, restrictive space for instrumentation at experimental side requires alternative numerical methods to aid the quantification of the state parameters and combustion process of these systems. In this sense, this research proposes a novel methodology combining in-cylinder pressure measurement and 1-D simulations as a tool to determine the state, flow and combustion development of different active pre-chambers operating with natural gas in a heavy-duty engine for power generation. The methodology is developed considering 3 different pre-chamber geometries, operating at different spark timings and equivalence ratios. The results suggest that the methodology is able to quantify the state conditions prior to the spark discharge as well as the evolution of the combustion process by means of considering the perturbations of the mass flow from the pre-chamber inside the main chamber energy balance. Moreover, the methodology allowed to quantify variations of equivalence ratio as small as 0.1 and combustion durations variations of 1 CAD.

Optical study on the effects of the hydrogen injection timing on lean combustion characteristics using a natural gas/hydrogen dual-fuel injected spark-ignition engine

Lean combustion has the potential to achieve higher thermal efficiency for internal combustion (IC) engines. However, natural gas engines often suffer from slow burning rate and large cyclic variations when adopting lean combustion. In this study, using a dual-fuel optical engine with a high compression ratio, the effects of direct-injected hydrogen on lean combustion characteristics of natural gas engines was investigated, emphasizing the role of hydrogen injection timing. Synchronization measurement of in-cylinder pressure and high-speed photography was performed for combustion analysis. The results show that the direct-injected hydrogen exhibits great improvement in lean combustion instability and power capability of natural gas engines. Visual images and combustion phasing analysis indicate that the underlying reasons are ascribed to the fast flame propagation with hydrogen addition. Regarding the direct injection timings, it is found that late injection of direct-injected hydrogen can achieve higher thermal efficiency, manifesting advanced combustion phasing, and increased heat release rate. Specifically, the flame propagation speed is elevated by approximately 50% at −100 CAD than that of −250 CAD. Further analysis indicates that the improvement of engine performance is ascribed to the increased volumetric efficiency and in-cylinder turbulence intensity, manifesting distinct flame centroid pathways at different injection timings. The current study provides insights into the combustion optimization of natural gas engines under lean burning conditions.

Effects of high ignition energy on lean combustion characteristics of natural gas using an optical engine with a high compression ratio

Natural gas engines often suffer from slow-burning rate and large cyclic variations, which significantly affects engine performance and thermal efficiency. In this study, using an optical spark-ignition engine with a high compression ratio, the role of high ignition energy in lean combustion characteristics of natural gas was investigated. Synchronization measurement of in-cylinder pressure and visualization images were performed to analyze the correlations of engine performance and combustion characteristics. The results show that elevating ignition energy can improve combustion instability and thermal efficiency, manifesting more concentrated heat release and greater influences on the combustion duration from 10% to 50% in burned mass fraction than that of 50%–90% range. The combustion visualizations show that ignition energy mainly affects initial flame kernel formation and early flame development, and improved flame speed and combustion instability can be observed even when excess air coefficient reaches 1.4 at sufficiently high ignition energy. Furthermore, an empirical criterion based on burned mass fraction and combustion visualizations was adopted to quantify early flame development. The shortage of ignition delay time and the acceleration of initial flame propagation are observed with the increases of ignition energy, which leads to lower cyclic variations and a faster burning rate at the early combustion stage.