Low Temperature Combustion Regime Research Articles

Chamfered-bowl piston for ultra-low particulate combustion with diesel and soybean biodiesel in a single-cylinder compression-ignition engine

Piston bowl geometries are critical to the combustion and emission characteristics of diesel engines. The present study investigated chamfered-bowl designs and biodiesel blending under a low-temperature combustion regime in order to achieve ultra-low particulate combustion (ULPC). The target soot level for ULPC was set at a filter smoke number value of 0.1 and engine experiments were conducted at the medium load condition. The injection visualization experiment provided the start of injection difference between the diesel and biodiesel fuels, while a three-dimensional computational simulation was conducted to examine the spray and combustion phenomena inside the engine cylinder. The experimental engine was a 1 l single-cylinder diesel engine equipped with a common-rail direct-injection system. The engine was operated at a speed of 1400 rev/min and the intake pressure was set at 200 kPa for a 50 per cent load condition. Two chamfered-bowl pistons were prepared for comparison with the conventional re-entrant bowl piston, and the experimental results showed that the chamfered-bowl pistons achieved significant soot reduction. In addition, computational analysis using a KIVA model showed that the chamfered bowls achieved wider fuel distribution and enhanced combustion. Biodiesel blending further reduced soot emission, and an increase in nitric oxides with biodiesel blends was also observed.

Characterization of Exhaust Gas Recirculation for Diesel Low Temperature Combustion

Exhaust Gas Recirculation (EGR) is common on most modern diesel engines resulting in significant reduction of NOx emissions. Heavy EGR application is an enabling technique for the advanced combustion modes operating in the low temperature combustion (LTC) regime, wherein simultaneous NOx and soot emission reduction can be attained. The primary effect of EGR is the dilution of the intake charge following the displacement of fresh air by the combustion products. However, the correlation between EGR and its effectiveness is non-linear due to the lean burn nature of boosted diesel engines. This correlation is further complicated when oxygenated fuels are used for combustion in the LTC mode. In this work, the intake oxygen concentration [O2]int is selected as a representative of EGR and its effectiveness in emission abatement is shown using an array of experimental results. An EGR characterization model is developed to quantify the dynamic interaction between [O2]int and engine operating variables, namely the intake boost, exhaust gas recirculation (EGR) amount, the fueling quantity and the fuel type. The model is validated on the research engine platform in steady state and transient tests. Finally, the control of EGR effectiveness is experimentally demonstrated to achieve ultra-low NOx emissions at different engine operating points.

Spray and Combustion Characteristics of Neat Acetone-Butanol-Ethanol, n-Butanol, and Diesel in a Constant Volume Chamber

Intermediate product of biobutanol production, acetone-butanol-ethanol (ABE) as an alternative fuel has drawn increasing attention in recent years due to its potential to eliminate various production costs. In this work, neat acetone-butanol-ethanol (ABE) with a component volumetric ratio of 3:6:1, n-butanol, and neat diesel were studied in a preburn type constant volume chamber with optical access. The ambient temperature and oxygen ranged from 800 to 1200 K and 21% to 11%, respectively, covering both conventional and low temperature combustion (LTC) regimes. Time resolved images of the spray and natural flame luminosity (indicator of soot) were captured by a high speed camera coupled with a copper vapor laser beam. The images show that the flame lift-off length and liquid penetration of n-butanol and ABE are much longer and shorter, respectively, than that of diesel under all tested conditions. This results in a longer “gap” between the liquid spray and the flame in ABE and n-butanol combustion that provides more space and time for the droplets to evaporate and mix with the ambient air, which is expected to decrease the local equivalence ratio at the combustion region. Indeed, the natural flame luminosity of ABE and n-butanol is reduced significantly under all tested conditions compared to that of diesel. For all the tested fuels, especially ABE, the combustion duration decreases with the reduction of ambient temperature due to a stronger premixed combustion, while it increases with the reduction of oxygen concentration due to the dilution effect. Therefore, ABE has a high potential to reduce soot emissions when used in diesel engines, but it would also suffer from combustion phasing retardation like butanol under LTC conditions with high EGR.

Soot measurements for diesel and biodiesel spray combustion under high temperature highly diluted ambient conditions

This paper presents the soot temperature and KL factor for biodiesel, namely fatty acid methyl ester (FAME) and diesel fuel combustion in a constant volume chamber using a two-color technique. The KL factor is a parameter for soot concentration, where K is an absorption coefficient and proportional to the number density of soot particles, L is the geometric thickness of the flame along the optical detection axis, and KL factor is proportional to soot volume fraction. The main objective is to explore a combustion regime called high-temperature and highly-diluted combustion (HTHDC) and compare it with the conventional and low-temperature combustion (LTC) modes. The three different combustion regimes are implemented under different ambient temperatures (800K, 1000K, and 1400K) and ambient oxygen concentrations (10%, 15%, and 21%). Results are presented in terms of soot temperature and KL factor images, time-resolved pixel-averaged soot temperature, KL factor, and spatially integrated KL factor over the soot area. The time-averaged results for these three regimes are compared for both diesel and biodiesel fuels. Results show complex combined effects of the ambient temperature and oxygen concentration, and that two-color temperature for the HTHDC mode at the 10% oxygen level can actually be lower than the conventional mode. Increasing ambient oxygen and temperature increases soot temperature. Diesel fuel results in higher soot temperature than biodiesel for all three regimes. Results also show that diesel and biodiesel fuels have very different burning and sooting behavior under the three different combustion regimes. For diesel fuel, the HTHDC regime offers better results in terms of lower soot than the conventional and LTC regimes, and the 10% O2, 1400K ambient condition shows the lowest soot concentration while maintaining a moderate two-color temperature. For biodiesel, the 15% O2, 800K ambient condition shows some advantages in terms of reducing soot concentration. Based on these results, the practical implementation of this combustion mode is outlined and a feasible option is proposed.

Combustion and emissions characteristics of Diesel and soybean biodiesel over wide ranges of intake pressure and oxygen concentration in a compression–ignition engine at a light-load condition

The objective of this study is to understand the effects of intake pressure and intake oxygen concentration on combustion and emissions of ultra-low sulfur diesel (“Diesel”) and 100% soybean methylene ether biodiesel (“B100”) and investigate on the low-temperature combustion (LTC) regime that has a potential to simultaneously reduce nitric oxides (NOx) and soot emissions. The present study is an experimental investigation with a 1-liter single-cylinder direct-injection diesel engine. Engine experiment was conducted under intake pressures and oxygen concentrations of 100–250kPa absolute and 5–19% by volume (vol%), respectively.Thermal efficiency and carbon monoxide (CO) contour maps revealed two distinctive regions divided at an intake oxygen level of 8vol% (or an equivalence ratio of 0.85). In the oxygen-sufficient region intake pressure exhibited dominant impacts on combustion duration, stability, and thermal efficiency. The NOx–soot contour map clarifies the location and size of the “soot barrier” in Diesel, which was not observed in B100. B100 achieved simultaneous NOx and soot reduction to 0.2g/kWh and 0.1 filter smoke number (FSN), respectively, at 150kPa intake pressure with 11vol% intake oxygen concentration. Corresponding thermal efficiency was approximately 43%. Increased intake pressure over 150kPa lowered thermal efficiency at the maximum brake torque (MBT) timing. The apparent net heat release rate traces indicated that most heat release event occurred before top dead center (TDC) and burn duration was elongated when intake pressure was above 150kPa.

Investigation of Low Temperature Combustion Regimes of Biodiesel With N-Butanol Injected in the Intake Manifold of a Compression Ignition Engine

In this study, the in-cylinder soot and NOx trade off was investigated in a compression engine by implementing premixed charge compression ignition (PCCI) coupled with low temperature combustion (LTC) for selected regimes of 1–3 bars IMEP. In order to achieve that, an omnivorous (multifuel) single cylinder diesel engine was developed by injecting n-butanol in the intake port while being fueled with biodiesel by direct injection in the combustion chamber. By applying this methodology, the in-cylinder pressure decreased by 25% and peak pressure was delayed in the power stroke by about 8 CAD for the cycles in which the n-butanol was injected in the intake manifold at the engine speed of 800 rpm and low engine loads, corresponding to 1–3 bars IMEP. Compared with the baseline taken with ultra-low sulfur diesel no. 2 (USLD#2), the heat release presented a more complex shape. t 1–2 bars IMEP, the premixed charge stage of the combustion totally disappeared and a prolonged diffusion stage was found instead. At 3 bars IMEP, an early low temperature heat release was present that started 6 deg (1.25 ms) earlier than the diesel reference heat release with a peak at 350 CAD corresponding to 1200 K. Heat losses from radiation of burned gas in the combustion chamber decreased by 10–50% while the soot emissions showed a significant decrease of about 98%, concomitantly with a 98% NOx reduction at 1 IMEP, and 77% at 3 IMEP, by controlling the combustion phases. Gaseous emissions were measured using an AVL SESAM FTIR and showed that there were high increases in CO, HC and NMHC emissions as a result of PCCI/LTC strategy; nevertheless, the technology is still under development. The results of this work indicate that n-butanol an be a very promising fuel alternative including for LTC regimes.

Characterization of the effect of fatty ester composition on the ignition behavior of biodiesel fuel sprays

The effect of fatty ester composition on the combustion behavior of biodiesel fuel sprays is investigated for soy-based methyl esters, canola-based methyl esters and canola-based butyl esters. The purpose of this study is to aid fuel engineering efforts by identifying molecular structure features of biodiesel which can influence the compression ignition process during direct biodiesel utilization in diesel engines. Combustion behavior is classified through ignition delays and maximum apparent heat release rates measured in a rapid compression machine, and these measurements have been taken at the low temperatures (676–815K) and reduced oxygen concentrations (12% and 18%) corresponding to the low temperature combustion regime of advanced diesel engines. Ignition delay measurements show that the appearance of first heat release from the fuels is decreasingly sensitive to oxygen concentration as the reaction zone temperature is increased. Growth in the ester alkoxy chain length from one to four carbon atoms had minimal effect on the ignition delay. For tests in 12% oxygen, the increased polyunsaturated of soy methyl esters relative to canola methyl esters leads to longer ignition delays. This is not observed using 18% oxygen where physical transport processes are more important. The canola butyl esters exhibited the largest peak values for heat release rates, with the distinction becoming clear at reaction zone temperatures above 750K. Normalization on ignition delay and input energy bases indicates that variation of the maximum apparent heat release rates among the fuels is primarily due to unique mixing times and fuel heating values.

Comparison of Compression Ignition Engine Noise Metrics in Low-Temperature Combustion Regimes

Comparison of Compression Ignition Engine Noise Metrics in Low-Temperature Combustion Regimes

A generalized renormalization group turbulence model and its application to a light-duty diesel engine operating in a low-temperature combustion regime

A generalized renormalization group closure model based on the “dimensionality” of the flow strain rate is proposed and applied to engine flows in this study. In the model, the model coefficients C1, C2, and C3 are constructed as functions of the flow strain rate. Computations were made for compressing/expanding flows in a diesel engine under motoring conditions. It was found that the generalized renormalization group model performs better than the standard renormalization group k−ε model in terms of its predictions of turbulent kinetic energy and model length scales. Computations in a diesel engine operating under low-temperature combustion conditions were also investigated to further assess the performance of the present model. Predictions of the spatial distributions of unburned hydrocarbons were significantly improved and agreed well with available experimental images. Engine-out unburned hydrocarbon emission was predicted to be smaller with the generalized renormalization group model than with the standard renormalization group k−ε model.

디젤 저온연소 운전 영역에서 흡기압이 엔진 성능에 주는 영향

One of the effective ways to reduce both <TEX>$NO_x$</TEX> and PM at the same time in a diesel CI engine is to operate the engine in low temperature combustion (LTC) regimes. In general, two strategies are used to realize the LTC operation-dilution controlled LTC and late injection LTC - and in this study, the former approach was used. In the dilution controlled regime, LTC is achieved by supplying a large amount of EGR to the cylinder. The significant EGR gas increases the heat capacity of in-cylinder charge mixture while decreasing oxygen concentration of the charge, activating low temperature oxidation reaction and lowering PM and <TEX>$NO_x$</TEX> emissions. However, use of high EGR levels also deteriorates combustion efficiency and engine power output. Therefore, it is widely considered to use increased intake pressure as a way to resolve this issue. In this study, the effects of intake pressure variations on performance and emission characteristics of a single cylinder diesel engine operated in LTC regimes were examined. LTC operation was achieved in less than 8% <TEX>$O_2$</TEX> concentration and thus a simultaneous reduction of both PM and <TEX>$NO_x$</TEX> emission was confirmed. As intake pressure increased, combustion efficiency was improved so that THC and CO emissions were decreased. A shift of the peak Soot location was also observed to lower <TEX>$O_2$</TEX> concentration while <TEX>$NO_x$</TEX> levels were kept nearly zero. In addition, an elevation of intake pressure enhanced engine power output as well as indicated thermal efficiency in LTC regimes. All these results suggested that LTC operation range can be extended and emissions can be further reduced by adjusting intake pressure.

On the Chemical Kinetics of Ethanol Oxidation: Shock Tube, Rapid Compression Machine and Detailed Modeling Study

Abstract Auto-ignition characteristics of ethanol were experimentally investigated using two Shock Tube (ST) facilities and a Rapid Compression Machine (RCM). Ignition delay times for stoichiometric ethanol-air mixtures were measured for temperatures between 775–1300 K in a High Pressure Shock Tube (HPST) at pressures close to 80 bar by probing pressure time histories and CH* emission. In some experiments the HPST was additionally employed for schlieren imaging to visualize ignition behavior by probing density gradients during ignition for ethanol-air mixtures. The ignition delay experiments in HPST were complemented by RCM measurements for extending the temperature regime to the Low Temperature Combustion (LTC) regime, down to 705 K, providing kinetic model validation data over a very wide temperature and pressure range. The current results also extend the earlier shock tube measurements performed in the same laboratory for pressures around 40 bar for temperatures down to 800 K [Heufer et al., Shock Waves 20 (2010) 307]. Furthermore, a Rectangular Shock Tube (RST) was solely used for additional schlieren imaging experiments to acquire information on ignition modes in stoichiometric ethanol-air mixtures around 10 bar. An improved chemical kinetic model was developed based on the Li et al. mechanism [Li et al., “Ethanol Model v1.0”, Princeton University, 2009] which was updated with evaluated rate parameters from the literature and validated through results obtained from the aforementioned facilities. The model predictions were compared to previously published low-pressure, premixed flat flame molecular beam mass spectrometry speciation data [Kasper et al., Combust. Flame 150 (2007) 220; Wang et al., J. Phys. Chem. A 112 (2008) 9255] where reasonable agreement is obtained considering the uncertainties in experiments and model. However, the model provides excellent agreement for the auto-ignition results obtained in the RCM and the high temperature shock induced ignition delays. Significant disparities with the model predictions are obtained for the shock tube results at temperatures below 1000 K as it transitions from the intermediate to the low temperature regime. The reasons for these deviations are assigned to strong fuel specific “pre-ignition” effects observed in ethanol auto-ignition, in contrast to other investigated fuels, which was satisfactorily explained through schlieren experimental results. To our knowledge this work is first of its kind that combines results from complementary experimental methods from three different facilities providing a holistic description on the auto-ignition behavior of ethanol. Furthermore, this paper reports ignition delay measurements for ethanol in air, at the highest pressures applicable to practical combustors.

Increased particle emissions from early fuel injection timing Diesel low temperature combustion

A clean premixed Diesel combustion strategy, called low temperature combustion (LTC), was able to achieve very low nitrogen oxide emissions (<35ppm) through use of exhaust gas recirculation (12.1% inlet oxygen), and reduced particulate matter (PM) emissions (<0.05 FSN) through advanced fuel injection timing (−24°aTDC). When varying the injection timing by relatively small increments, large changes in PM mass and number emissions were measured within the premixed LTC regime. A discrepancy is investigated between expected reductions in PM emissions by simple fuel–air premixing and combustion temperature metrics, and actual PM emissions measurements when advancing the fuel injection timing earlier than −24°aTDC. For these earlier injection timings, particle numbers were seen to increase in two distinct particle size modes, whereas only one particle size mode existed at the minimum PM emissions −24°aTDC injection timing. Additional parameters from a 1D free fuel spray model were used to suggest new information that could explain the cause of these unexpected increases in PM. Using 0D and 1D calculations, the engine-out particle size and number emissions are analyzed to better understand their sensitivity to changes in the fuel injection timing within the early injection timing LTC regime.

Ignition Characteristics of Diesel and Canola Biodiesel Sprays in the Low-Temperature Combustion Regime

The objective of this work is to investigate the spray ignition characteristics of diesel and canola-derived biodiesel in a rapid compression machine at the low temperatures (676–816 K) and reduced oxygen concentrations (12 and 18%) that are used in low-temperature combustion strategies of advanced diesel engines. A method for testing ignition delay times and apparent heat release rates at a series of temperatures is developed and characterized, whereby a given temperature is attained prior to the fuel spray by altering the charge cooling time after the end of compression. A single-zone heat release model is formulated and used with the experimental pressure data to calculate two unique ignition delay periods. For diesel, the total ignition delays measured with this approach are found to correlate very well with diesel ignition delay data published in the literature. When the first ignition delay period is compared, which approximates the initial time of heat release after the start of injection, it is no...

The Effects of Exhaust Back Pressure on Conventional and Low-Temperature Diesel Combustion

Modern diesel engines are seeing increasing system and after-treatment complexity which can lead to significant increases in the exhaust back pressure (EBP). This increases the amount of trapped residuals, raising the charge temperature but reducing the oxygen concentration. In this work, these effects of the EBP on diesel engine performance and emissions under conventional and low-temperature diesel combustion (LTC) regimes were investigated. Increasing the EBP resulted in higher pumping work for both combustion modes. While for conventional diesel combustion the effect of the EBP on combustion and emissions were not significant, for LTC the higher back pressures influenced the combustion and emissions formation processes. At low-load conditions, the increase in the charge temperature advanced combustion; at intermediate-load conditions, the reduction in the oxygen concentration delayed it. Smoke emissions were significantly reduced by a higher back pressure at intermediate-load conditions.

A Numerical Study on Diesel Engine Size-Scaling in Low Temperature Combustion Operation

A numerical study was conducted for diesel engine size-scaling in the low temperature combustion regime. Conventional scaling models were found to be inadequate for the low temperature combustion regime where chemical time scales become very important. Equal-time scaling was suggested to ensure the same chemical time scale for the scaled engines. With this approach, combustion phasing was well matched. However, peak in-cylinder pressure was higher in the upscaled engine, which is attributed to more homogeneous mixture conditions due to smaller surface to volume ratio (i.e., less wall friction). The overly enhanced mixing can be compensated with lower injection velocity.

Modeling of flame lift-off length in diesel low-temperature combustion with multi-dimensional CFD based on the flame surface density and extinction concept

Low-Temperature Combustion (LTC) is becoming a promising technology for simultaneously reducing soot and NOx emissions from diesel engines. LTC regimes are evaluated by the flame lift-off length – the distance from the injector orifice to the location of hydroxyl luminescence closest to the injector in the flame jet. Various works have been dedicated to successful simulations of lifted flames of a diesel jet by use of various combustion modeling approaches. In this work, flame surface density and flamelet concepts were used to model the diesel lift-off length under LTC conditions. Numerical studies have been performed with the ECFM3Z model, n-Heptane and diesel fuels to determine the flame lift-off length and its correlation with soot formation under quiescent conditions. The numerical results showed good agreement with experimental data, which were obtained from an optically accessible constant volume chamber and presented at the Engine Combustion Network (ECN) of Sandia National Laboratories. It was shown that at a certain distance downstream from the injector orifice, stoichiometric scalar dissipation rate matched the extinction scalar dissipation rate. This computed extinction scalar dissipation rate correlated well with the flame lift-off length. For the range of conditions investigated, adequate quantitative agreement was obtained with the experimental measurements of lift-off length under various ambient gas O2 concentrations, ambient gas temperatures, ambient gas densities and fuel injection pressures. The results showed that the computed lift-off length values for most of the conditions lay in a reasonable range within the quasi-steady lift-off length values obtained from experiments. However, at ambient temperatures lower than 1000 K, the lift-off length values were under-predicted by the numerical analysis. This may be due to the use of the droplet evaporation model as it is believed that evaporation has a strong effect on the lift-off length.

Comprehensive study of biodiesel fuel for HSDI engines in conventional and low temperature combustion conditions

In this research, an experimental investigation has been performed to give insight into the potential of biodiesel as an alternative fuel for High Speed Direct Injection (HSDI) diesel engines. The scope of this work has been broadened by comparing the combustion characteristics of diesel and biodiesel fuels in a wide range of engine loads and EGR conditions, including the high EGR rates expected for future diesel engines operating in the low temperature combustion (LTC) regime. The experimental work has been carried out in a single-cylinder engine running alternatively with diesel and biodiesel fuels. Conventional diesel fuel and neat biodiesel have been compared in terms of their combustion performance through a new methodology designed for isolating the actual effects of each fuel on diesel combustion, aside from their intrinsic differences in chemical composition. The analysis of the results has been sequentially divided into two progressive and complementary steps. Initially, the overall combustion performance of each fuel has been critically evaluated based on a set of parameters used as tracers of the combustion quality, such as the combustion duration or the indicated efficiency. With the knowledge obtained from this previous overview, the analysis focuses on the detailed influence of biodiesel on the different diesel combustion stages known ignition delay, premixed combustion and mixing controlled combustion, considering also the impact on CO and UHC pollutant emissions. The results of this research explain why the biodiesel fuel accelerates the diesel combustion process in all engine loads and EGR rates, even in those corresponding with LTC conditions, increasing its possibilities as alternative fuel for future DI diesel engines.

Evaluation of low-temperature diesel combustion regimes with n-Heptane fuel in a constant-volume chamber

The concept of Low Temperature Combustion (LTC) has been advancing rapidly because it may reduce emissions of NOx and soot simultaneously. Various LTC regimes that yield specific emissions have been investigated by a great number of experiments. To accelerate the evaluation of the spray combustion characteristics of LTC, to identify the soot formation threshold in LTC, and to implement the LTC concept in real diesel engines, LTC is modeled and simulated. However, since the physics of LTC is rather complex, it has been a challenge to precisely compute LTC regimes by applying the available diesel combustion models and considering all spatial and temporal characteristics as well as local properties of LTC. In this paper, LTC regimes in a constant-volume chamber with n-Heptane fuel were simulated using the ECFM3Z model implemented in a commercial STAR-CD code. The simulations were performed for different ambient gas O2 concentrations, ambient gas temperatures and injection pressures. The simulation results showed very good agreement with available experimental data, including similar trends in autoignition and flame evolution. In the selected range of ambient temperatures and O2 concentrations, soot and NOx emissions were simultaneously reduced.

Regulated emissions and speciated hydrocarbons from smokeless low-temperature combustion diesel engines with ultra-high exhaust gas recirculation and exhaust oxidation catalyst

With ultra-high exhaust gas recirculation (EGR) suppressing the in-cylinder soot and nitrogen oxides (NO x) formation as well as with the exhaust oxidation catalyst removing the engine-out total unburned hydrocarbon (THC) and carbon monoxide (CO) emissions, clean diesel combustion in terms of low regulated emissions (NO x, particulate matter, THC, and CO) can be established in an operating range up to 50 per cent load. However, unregulated emissions such as aldehydes, aromatics, and 1,3-butadiene, which are seen as a severe threat to human health, are concerned when operating the engine with ultra-high EGR. In this study, the THC emissions from a diesel engine operated with ultra-high EGR low-temperature combustion were speciated using Fourier transform infrared spectroscopy. Some unregulated toxic emissions including aldehydes, aromatics, 1,3-butadiene, and some low molecular hydrocarbons dramatically increase in the ultra-high EGR low-temperature combustion regime. The exhaust oxidation catalyst is effective to remove aldehydes and some unsaturated hydrocarbons, but aromatics and methane generated from the ultra-high EGR operation are hardly reduced, particularly at higher loads.

Detailed Unburned Hydrocarbon Investigations in a Highly-Dilute Diesel Low Temperature Combustion Regime

Detailed Unburned Hydrocarbon Investigations in a Highly-Dilute Diesel Low Temperature Combustion Regime