Initial Combustion Period Research Articles

Effects of methane ratio on MPDF (micro-pilot dual-fuel) combustion characteristic in a heavy-duty single cylinder engine

In this study, the characteristics of micro-pilot dual-fuel combustion with respect to the fuel mixture ratio in a single cylinder dual-fuel engine have been investigated. In order to analyze the characteristics of micro-pilot dual-fuel combustion, a metal engine and an optical single cylinder dual-fuel engine were used. The fuel mixture ratio was varied for experimental purposes; the diesel was directly injected into combustion chamber and the methane gas was supplied via intake port. The present study reports that increasing the methane mixture ratio from 0 to 97.67% changes the diesel combustion to pre-mixed combustion. As a result, the peak cylinder pressure was increased from 184 to 198 bar, and the rate of heat release was greatly advanced. In the MPDF condition, the nitrogen oxides emissions were reduced by about 90%p, and the fuel conversion efficiency increased about 5%p because of the low combustion temperature of pre-mixed combustion. However, for the same reason, the hydrocarbon emissions were increased about 95%p. The fastest combustion speed was found form the results of methane mixture ratio between 40 and 80%. In the condition of diesel combustion and micro-pilot dual-fuel combustion, the combustion periods of middle and initial were increased, respectively, resulting in the low combustion speed. The standard deviation of peak cylinder pressure, which represents the combustion variation, was correlated with initial combustion period. While the condition of methane gas mixture ratio between 40 and 80% shows the lowest combustion variation, the highest combustion variation was occurred by MPDF condition. Through the optical engine experiment, it can be found that the cycle to cycle combustion variation is ascribed to the turbulent flow and the variation of ignition position. The combustion images show that the unpredictable characteristics of the ignition position and slow flame propagation speed caused the combustion variation in micro-pilot dual-fuel combustion.

Effect of Premixed n-Butanol Ratio on the Initial Stage of Combustion in a Light-Duty Butanol/Diesel Dual-Fuel Engine

The impact of premixed n-butanol mixture on the heat release rate was investigated based on a modified light-duty diesel engine. The results show that reactivity stratification is formed in the cylinder through n-butanol port fuel injection (PFI) and diesel direct injection (DI). The initial heat release rate of the diesel/butanol dual-fuel combustion is restrained due to the low ignitability of butanol and the high volatility. Because of the auto-ignition of diesel, premixed n-butanol undergoes a high-temperature reaction, which has an active influence on the heat releasing of diesel/butanol dual-fuel combustion. With the increase of the amount of premixed n-butanol injected, the heat release rate in the initial combustion period has a critical value. When the n-butanol injection quantity is less than 13 mg/cycle, the initial heat release rate of dual-fuel combustion is lower than the pure diesel combustion because the lean premixed n-butanol/air mixture limits the flame propagation. When the fuel injection rate of n-butanol is higher than 13 mg/cycle, the heat release rate is accelerated, leading to obvious flame propagation.

Comparison and analysis of the combustion process of three combustion modes of an ethanol - diesel direct injection engine

A diesel engine was modified into an ethanol - diesel direct injection engine. Diesel was injected through the engine’s original pump injection system, whereas ethanol was injected through an installed electronic controlled injection system. 100% of ethanol injection at 240°CA (referred to as B100), 50% of ethanol injection at 240°CA and 50% injected subsequently with diesel at 344°CA (referred to as B50L50) and 100% of ethanol injection with diesel at 344°CA (referred to as L100) were the three combustion modes. Based on test data, a 3D combustion chamber model was established for the analysis of the combustion modes. The result showed that B50L50 had the earliest combustion starting point, the highest combustion pressure and maximum average temperature in the cylinder, the largest maximum cumulative heat release and sufficient combustion. In addition, NO generation was the highest but the soot generation during the initial combustion period was not the highest even though it was high. The soot generation of B100 was the highest. For L100, the combustion starting point was the latest, after burning was severe, cylinder pressure and maximum temperature were lower, NO and soot generation were the lowest, but CO generation was the highest, comparatively.

Effect of the Ethanol Injection Moment During Compression Stroke on the Combustion of Ethanol - Diesel Dual Direct Injection Engine

A set of GDI system is installed on a F188 single-cylinder, air-cooled and direct injection diesel engine, which is used for ethanol injection, with the injection time controlled by the crank angle signal collected by AVL angle encoder. The injection of ethanol amounts to half of the thermal equivalent of an original diesel fuel. A 3D combustion model is established for the ethanol - diesel dual direct injection engine. Diesel was injected from the original fuel injection system, with a fuel supply advance angle of 20°CA. The ethanol was injected into the cylinder during compression process. Diesel injection began after the completion of ethanol injection. Ethanol injection starting point of 240°CA, 260°CA, 280°CA, 300°CA and 319.4°CA were simulated and analyzed. Due to the different timing of ethanol injection, the ignition of the ethanol mixture when diesel fires, results in non-uniform ignition distribution and flame propagation rate, since the distribution and concentration gradients of the ethanol mixture in the cylinder are different, thus affecting the combustion process. The results show that, when ethanol is injected at 319.4°CA, the combustion heat release rate and the pressure rise rate during the initial stage are the highest. Also, the maximum combustion pressure, with a relatively advance phase, is the highest. In case of later initial ethanol injection, the average temperature in the cylinder during the initial combustion period will have a faster rise. In case of initial injection at 319.4°CA, the average temperature in the cylinder is the highest, followed by 240°CA ethanol injection. In the post-combustion stage, the earlier ethanol injection will result in higher average temperature in the cylinder and more complete fuel combustion. The injection of ethanol at 319.4°CA produces earlier and highest NOX emissions.

Experimental and numerical investigation of the effects of combustion chamber reentrant level on combustion characteristics and thermal efficiency of stoichiometric operation natural gas engine with EGR

In order to accelerate the flame propagation and hence to improve the thermal efficiency in the stoichiometric operation natural gas engine with exhaust gas recirculation, reentrant modification on the Cylindrical combustion chamber has been conducted in this study. The diameter ratio between chamber throat and cylinder bore (hereafter referred to as diameter ratio) has been gradually reduced from 0.64 to 0.46. The effects of the chamber reentrant level on the engine combustion and thermal efficiency were systematically investigated through experiments. Then the mechanisms of these effects were further clarified through both one-dimensional and three-dimensional simulations.The experimental results indicate that the combustion duration could be reduced continuously with higher chamber reentrant level, which is beneficial for the thermal efficiency improvement. However, this benefit will be gradually offset by the simultaneously increased knock tendency and heat transfer loss, which has been further validated through thermodynamic analysis on thermal efficiency with the help of one-dimensional simulation. As a consequence, the chamber with diameter ratio of 0.5, rather than 0.46, shows the best performance in this study, and the thermal efficiency can be improved by approximately 1.5% at full load condition and 3% at medium to high load conditions. In addition, through the experiments, it is also observed that the combustion duration with spark timing set at maximum brake torque point shows a good linear relationship with the diameter ratio. This function is greatly valuable to optimize the Reentrant combustion chamber since it could be obtained via few engine tests and then used for well predicting the effects of chamber reentrant level on thermal efficiency combined with only one-dimensional simulation. Through the three-dimensional simulation results, it is seen that the acceleration of the flame propagation with higher chamber reentrant level could be mainly attributed to the much increased flame surface wrinkle during the initial combustion period, which results from the increased turbulence level and its more obvious effect on the flame surface before top dead center during the combustion process.

G0700508 軽油着火希薄予混合気の燃焼過程に及ぼす燃料噴射条件および雰囲気条件の影響

This study investigates influence of fuel injection and ambient gas conditions on lean burn combustion process ignited with diesel fuel. Fuel injection pressure, ambient gas pressure and equivalence ratio were varied in a wide range and influence of those parameters was closely evaluated based on the high speed imaging and the pressure analysis. The result showed that the ignition delay period of diesel fuel was prolonged at high injection pressure and low ambient gas pressure. The initial combustion period was decreased with an increase of the ambient gas pressure when the injection pressure is high.

Experimental studies the burning process of gelled unsymmetrical dimethylhydrazine droplets under oxidant convective conditions

Gelled hypergolic propellants offer potential safety and performance improvements over conventional liquid and solid propellants. Understanding the combustion process of single gelled droplets is the first basic step to predict their behavior in the future combustion chambers. Experimental studies were performed to investigate the burning behavior of gelled unsymmetrical dimethylhydrazine (UDMH) droplets under oxidant convective conditions. The effects of oxidant convective conditions including velocity and temperature on the burning behavior were analyzed. The burning process was broken down into four stages: heating and swelling period, initial combustion period, violent combustion period, and stable and extinguished combustion period. Sometimes the droplet inside seemed to be porous or botryoidal. The microexplosion period lasted for a long time sometimes exceeding about 70% of the burning lifetime, and the phenomenon of gas jet combustion due to the burst steam from microexplosions was founded. The conversion of burning flame from a layered and enveloped flame structure to an escaped flame structure with increase in the convective velocity was observed. When the enveloped flame appeared, it was more helpful to the burning process, and the intensity of microexplosions and the burning rate constant were found to increase with the convective temperature. When the escaped flame appeared, it was disadvantageous to the burning process, and the intensity of microexplosions decreases with rise in convective velocity. Compared with the escaped flame, the intensity of microexplosions of the enveloped flame was lower, but the frequency was higher.

Simulation of the NO<sub>X</sub> Emissions for a CNG Direct-Injection Engine

The distribution of temperature and NOX concentration field under different crank angle of a CNG direct-injection engine converted from a four-valve gasoline vehicle engine are calculated by the computation model of the CNG direct-injection engine work process using CFD software. The results show that the temperature field spreads around the center of the spark plug in initial combustion period. The position of maximum temperature in combustion chamber moves from the spark plug to the wall and temperature tends to uniform distribution as the piston goes down along with the spread of flame front and the completion reaction after the flame. Temperature distribution is vital to the generation of NOX which changes along with high-temperature zone. The change principle of quantity of NOX in the cylinder is that at first, the quantity continues increasing, then declines, and finally remains stable. Besides this, the generation amount of NOX increase along with the speed.

A trial of ignition innovation of gasoline engine by nanosecond pulsed low temperature plasma ignition

Application of nanosecond pulsed low temperature plasma as an ignition technique for automotive gasoline engines, which require a discharge under conditions of high back pressure, has been studied experimentally using a single-cylinder engine. The nanosecond pulsed plasma refers to the transient (non-equilibrated) phase of a plasma before the formation of an arc discharge; it was obtained by applying a high voltage with a nanosecond pulse (FWHM of approximately 80 or 25 ns) between coaxial cylindrical electrodes. It was confirmed that nanosecond pulsed plasma can form a volumetric multi-channel streamer discharge at an energy consumption of 60 mJ cycle−1 under a high back pressure of 1400 kPa. It was found that the initial combustion period was shortened compared with the conventional spark ignition. The initial flame visualization suggested that the nanosecond pulsed plasma ignition results in the formation of a spatially dispersed initial flame kernel at a position of high electric field strength around the central electrode. It was observed that the electric field strength in the air gap between the coaxial cylindrical electrodes was increased further by applying a shorter pulse. It was also clarified that the shorter pulse improved ignitability even further.

Identification of intermediates in an n-heptane/oxygen/argon low-pressure premixed laminar flame using synchrotron radiation

This study was performed with the use of tunable vacuum ultraviolet (VUV) lasers for molecular beam sampling mass spectrometry (MBMS) to detect the intermediates of a premixed n-heptane/oxygen/argon laminar low-pressure flame. In this experiment, 24 intermediate species were identified and quantified by the photoionization mass and efficiency spectra at a stoichiometric mixture of n-heptane/oxygen highly diluted by argon gas. The mole fraction profiles of these species at different flame positions are presented. The results show that the main intermediates are ketone and ether in the initial combustion period and these species then further oxidize to hydrocarbons. Of all the species, the concentrations of ethylene and polycyclic aromatic hydrocarbons (PAHs) are the highest concentration up to 10 −2 and the lowest concentration to 10 −5 in the mole fraction, respectively, at selective flame position. In addition, alkine like propyne exhibits very similar mole fraction profiles with that of benzene in the n-heptane flame. The results provide the strong evidence that benzene production correlates to alkine production in the flame and reinforces, at least qualitatively, the role of propyne species in the benzene formation mechanism. Finally, the distribution of 24 typical species in the flame is correlated with temperature, which is important in establishing the oxidation mechanism of hydrocarbon fuels.

Cycle-resolved measurements of the fuel concentration near a spark plug in a rotary engine using an in situ laser absorption method

Cycle-resolved measurements of the fuel concentration near a spark plug in a commercial rotary engine were performed. An in situ laser infrared (IR) absorption method was developed using a spark plug sensor and a 3.392-μm He–Ne laser as the light source. This wavelength coincided with the absorption line of hydrocarbons. The newly developed IR spark plug sensor had a higher signal-to-noise ratio than its previous version due to the optimization of its quartz lens and two optical fibers. The new sensor provided quantitative cycle-resolved fuel concentration measurements around the spark plug with a high temporal resolution. At lean preset air/fuel ( A/ F) ratios, fuel was mixed with the surrounding air gradually by the rotor motion in the plug hole of the rotary engine. Strong mixture inhomogeneities were measured during the compression stroke; the magnitude of these inhomogeneities decreased throughout the compression stroke. Cycle-resolved measurements were made to investigate the effects of the fuel concentration near the spark plug on the combustion characteristics of the commercial rotary engine. There was a strong correlation between the fuel concentration measured with the spark plug sensor and the combustion characteristics during the initial combustion period, which occurred faster when conditions were slightly richer than stoichiometric near the spark plug. The indicated mean effective pressure (IMEP) was slightly related to the A/ F ratio near the spark plug. It was possible to measure the cycle-resolved A/ F ratio near the spark plug and investigate its cycle-to-cycle fluctuations to achieve stable operation using the newly developed spark plug sensor.

Analysis of Combustion in a Small Homogeneous Charge Compression Assisted Ignition Engine

Combustion analysis has been conducted on a small two-stroke glow ignition engine, which has similar combustion characteristics to homogeneous charge compression ignition (HCCI) engines. Difficulties such as unknown ignition timing and the polytropic index have been addressed by combining both heat release and mass fraction burn analyses. Results for all operating conditions have shown good correlations between the two methods. The engine has been fuelled with a mixture of methanol, nitromethane, and lubrication oil. The effect of nitromethane on combustion is difficult to determine, since altering nitromethane content also changes the air-fuel ratio under the current experiment set-up. However, it is still possible to show that nitromethane shortens the combustion periods beyond the uncertainty created by the mixture strength and cycle-by-cycle variations. The results further show that a faster combustion does not necessarily give a higher indicative mean effective pressure (i.m.e.p.) in this engine. This is because the start of combustion can shift away from its optimum value when nitromethane is added. The initial combustion period is found to be between 10 and 30° CA (crank angle); the main combustion period is between 25 and 50° CA. These combustion periods are comparable to a traditional spark ignition engine. In a very rich mixture, the ‘hot’ glow plug has been found to change significantly the combustion characteristics. Further study would be recommended to elucidate the effect of glow plugs. Lastly, in the case of poor combustion, cycle-by-cycle analysis shows that a misfire or partial burn cycles are always followed by high i.m.e.p. and fast burn cycles.

Analysis of cycle-by-cycle variation in a direct injection gasoline engine using a laser-induced fluorescence technique

To analyse the cycle-by-cycle variation of combustion in a direct injection gasoline engine equipped with a fan-shape spray nozzle and operated with exhaust gas recirculation (EGR), the fuel mixture distribution was measured at a time of spark and during the combustion period by the laser-induced fluorescence (LIF) technique. It was found that in the case of advanced or retarded injection timing, the initial combustion period tends to extend and the indicated mean effective pressure (i.m.e.p.) becomes low when lean mixtures appear at the spark position and at the spark timing. This suggests that the cycle-by-cycle variation of combustion under these conditions is dominated by the fuel concentration at the spark position and spark timing. In contrast to this, for the best injection timing, which allows the lowest cycle-by-cycle variation, the i.m.e.p. fluctuation is affected not by the initial combustion period but by the main combustion period. The observation of LIF images revealed that the i.m.e.p. fluctuation at this condition is strongly correlated to the unburned mixture quantity at the side area of the piston cavity during the latter half of the combustion period. It was shown by a computational fluid dynamics (CFD) calculation that the combination of a uniform spray pattern and a compact cavity shape is effective to reduce the over-lean mixture region in the edge of the piston cavity, which is responsible for the cycle-by-cycle variation of combustion at the condition of best-tuned injection timing.

The influence of spark ignition characteristics on stable combustion in a spark ignited engine (4th report, A study on cycle-to-cycle combustion fluctuation)

As an initial combustion period involving the growing process of a initial flame kernel, is easily affected by the cycle-to-cycle air-fuel ratio and gas motion, combustion always fluctuates unstably in a spark ignited engine. In this study, by measuring the phenomena of spark ignition and engine combustion simultaneously, it is clarified that a decrease in cycle-to-cycle spark ignition fluctuation is achieved by increasing spark current. The spark ignition, however, is mainly fluctuated by cycle-to cycle gas motion in-cylinder during ignition timing. When a long durational spark which has a high spark current is given to stabilize the engine combustion, a new aspect has been obtained showing that the initial combustion period can be made to shorten and reduce its fluctuation. Especially, this characteristic of spark ignition is effective for a lean burn, which is fundamentally unstable combustion.