Low Temperature Combustion Regime Research Articles

A numerical investigation of the influence of ozone on combustion to improve the performance of spark ignition engines

The aim of this work is to exploit the influence of using ozonized air to achieve stable and efficient combustion of lean mixtures in a gasoline-fuelled Spark-Ignition (SI) engine. The influence of ozone on the combustion of near-stoichiometric mixtures, which are typical of SI engines, has also been assessed. A Computational Fluid Dynamics (CFD) model has been employed to simulate compression, combustion, and expansion of a spark ignition, axisymmetric engine fuelled with iso-octane/air/ozone mixtures. The aim is to assess how ozone improves the engine performance under different engine speeds, ignition timings and equivalence ratios. The model has been validated against experimental data available in the literature. Parametric analyses have been carried out by considering three values of engine speeds (800, 1000 and 1200 rpm), three different ozone concentrations at Intake Valve Closure (IVC) (0, 100 and 200 ppm) and two equivalence ratios (0.9 and 0.7). The results show that ozone enables reactions in the Low Temperature Combustion (LTC) regime, modifies the mixture chemical composition and the auto-ignition tendency. Specifically, for all the cases under examination, the addition of ozone to the air/fuel mixture reduces the combustion duration, leading to an increase in terms of work output and a reduction of the specific fuel consumption. Moreover, the advantage of using ozone is greater for lean mixtures than for near-stoichiometric mixtures. Finally, for the near-stoichiometric cases, when the residence time of the mixture is high enough, auto-ignition occurs in the end gases.

A Numerical Investigation on Laminar Flame Speed of Syngas in Air with Ozone Addition

In the context of energy and propulsion systems, in recent years engineering research has increased its efforts to develop more efficient technologies with a lower environmental impact. Specifically, new combustion systems have been investigated, such as ultra-lean combustion and Homogeneous Charge Compression Ignition (HCCI) combustion, and sustainable fuels have been employed. With respect to the latter option, syngas and hydrogen have emerged as attractive choices. However, the low specific energy of syngas and the presence of diluents can cause flame failure and instability. On the other hand, it is well known that ozone-assisted combustion enhances the Laminar Flame Speed (LFS), reduces the ignition delay time, and improves the flame stabilization of fuels such as methane, n-heptane and iso-octane, since it affects the chemistry in the Low Temperature Combustion (LTC) regime. In this context, the aim of the present work is to evaluate the effect of ozone on the LFS of syngas/ozone/air mixtures. Specifically, 1-D simulations have been carried out to compute the LFS of CO/H2 and CO/H2/N2 syngas mixtures. The model has been validated against experimental data available in the scientific literature by considering four different reaction mechanisms. Then, the model has been used to perform a parametric analysis by considering different conditions in terms of syngas composition (three different CO/H2 ratios) and equivalence ratio (from 0.5 to 5). The results show that the addition of 2500 ppm of ozone in air results in an increase in LFS for all equivalence ratios examined. The four different mechanisms give comparable results in terms of LFS and LFS increase for CO/H2 syngas, and are also able to predict ozone effect on LFS enhancement for CO/H2/N2 mixtures. Finally, the results show that the influence of ozone, in terms of LFS percentage increase, is greater for CO/H2 ultra lean mixtures with a lower CO/H2 ratio.

Microgravity Spherical Droplet Evaporation and Entropy Effects.

Recent efforts to understand low-temperature combustion (LTC) in internal combustion engines highlight the need to improve chemical kinetic mechanisms involved in the negative temperature coefficient (aka cool flame) regime. Interestingly, microgravity droplet combustion experiments demonstrate this cool flame behavior, allowing a greater focus on chemistry after buoyancy, and the corresponding influence of the conservation of momentum is removed. In Experimental terms, the LTC regime is often characterized by a reduction in heat transfer losses. Novel findings in this area demonstrate that lower entropy generation, in conjunction with diminished heat transfer losses, could more definitively define the LTC regime. As a result, the simulation of the entropy equation for spherical droplet combustion under microgravity could help us to investigate fundamental LTC chemical kinetic pathways. To provide a starting point for researchers who are new to this field, this effort first provides a comprehensive and detailed derivation of the conservation of entropy equation using spherical coordinates and gathers all relevant information under one cohesive framework, which is a resource not readily available in the literature. Subsequently, the well-known d2 law analytical model is determined and compared to experimental data that highlight shortcomings of the law. The potential improvements in the d2 law are then discussed, and a numerical model is presented that includes entropy. The resulting codes are available in an online repository to ensure that other researchers interested in expanding this field of work have a fundamental starting point.

Deep reinforcement learning to discover multi-fuel injection strategies for compression ignition engines

Over the past several decades, regulation of compression ignition engine emissions has become increasingly stringent as concern about the environmental and health implications of these emissions has grown. These changing constraints have led to a series of new, alternative fuel injection strategies that aim to maintain power output while reducing in-cylinder generated emissions by operating in the low-temperature combustion (LTC) regime. These advanced injection strategies are created and retuned for individual combinations of engine geometry, fuel, and emissions constraints. Deep reinforcement learning has been shown to be an effective alternative to traditional optimization approaches for highly combinatorial control problems, such as discovering the optimal injection schedules for compression ignition engines. In this study, we deploy a previously presented deep reinforcement learning framework to iteratively optimize a series of engine geometries over a range of increasingly strict NO x emissions regulations. We then examine the resulting injection schedules. We discuss the potential for using this deep reinforcement learning framework for fuel selection screening and for discovering unique injection strategies for different engine geometries and future emissions standards.

CFD-based Data-driven Modeling of Reactivity and Stratification Dynamics for RCCI Engine Control

Reactivity Controlled Compression Ignition (RCCI) is a Low-Temperature Combustion (LTC) regime that provides thermal efficiency and emissions benefits compared to traditional Spark Ignition (SI) and Compression Ignition (CI) regimes. However, it is difficult to control combustion at these engines and run them at optimal conditions due to dependency of the their combustion on air-fuel mixture chemical reactivity and fuel stratification inside the combustion chamber. Modeling of reactivity and stratification can provide new pathways to control combustion in RCCI engines. In this study, a data-driven approach based on Computational Fluid Dynamics (CFD) results and a Linear Parameter Varying (LPV) method is proposed to model reactivity and stratification at RCCI engines. This work is illustrated for a real 2-liter 4-cylinder engine. The results show that the developed data-driven model (DDM) has acceptable prediction accuracy to estimate reactivity and stratification for RCCI engine control. The proposed method can be also implemented in other combustion modes in internal combustion engines.

The study of combustion characteristics for conventional and renewable Jet fuels at Low-to-intermediate temperatures in a rapid compression Machine

The combustion characteristics of jet fuels, including conventional (Jet-A1 and JP-5) and renewable (hydro-processed renewable jet, HRJ) jet fuels at low-to-intermediate temperatures were studied in a rapid compression machine (RCM). Compression pressures were varied at 10, 15, and 20 bar under a low temperature combustion regime (626 K-874 K). Fuel/oxidizer mixture conditions were created in order to investigate the auto-ignition delays responding to various equivalence ratios of 0.25 and 0.5. Longer auto-ignition delays were observed with a leaner fuel/oxidizer mixture (ϕ=0.25) by as much as 79.2% as compared to the richer mixture condition (ϕ=0.5). Doubling the compression pressures from 10 to 20 bar led to the reduction of the auto-ignition delays by as much as 58.8%. The negative temperature coefficient (NTC) regime was found to be strongly related to the changes in the equivalence ratio, where the NTC formation is clearly observed at lower equivalence ratio of 0.25 for all the tested fuels. In addition, the renewable jet fuel exhibited a shorter auto-ignition delay by as much as 54.5% as compared to Jet-A1 and JP-5 at equivalence ratio of 0.5. The higher hydrogen-to-carbon (H/C) ratio and lower concentration of cyclic molecules in the HRJ were among the reasons for the reactivity improvement compared to the petroleum jet fuels.

Assessment of The Impact of Multiple Injection Strategies on Combustion Noise, Smoke and Performance Characteristics of a CRDI Heavy Diesel Engine

This experimental research paper presents the impact of 5 different multiple injection strategies as 2-double comprising pilot-Main (pM), Main-after (Ma), 2-triple comprising pilot-Main-after (pMa), early-pilot-Main (epM) and quadruple comprising early-pilot-Main-after (epMa) in comparison with single injection on combustion noise (CN), Brake Specific Fuel Consumption (BSFC), Torque, Brake Thermal Efficiency (BTE) and Smoke emissions. The experimental evaluation was carried out on a 6-cylinder inline CRDI (Common rail Direct Injection) diesel engine in low temperature combustion (LTC) regime with high EGR of 45%, and fixed main injection Crank Angle (CA) using conventional diesel (BS-IV) fuel at full load and 8 speeds (low to high). The comprehensive study revealed that quadruple (epMa) injection strategy is superior in providing optimum results in comparison to other 4 multiple injection strategies in line to single injection. This strategy displayed better BSFC at medium to high speeds zone inline to reduce of combustion noise (radiated) level especially at low speeds around 2 dBA. The epMa exhibited the 2nd best smoke/soot concentration whereas pMa injection scheduling is best in lower smoke/soot generation. The epMa was found to be an optimum in torque performance though epM showed best torque performance in low to medium speed zone. The epMa showed the best BTE than all other. This work indicates the potentiality and further scope to achieve better performance and emissions simultaneously with quadruple injection strategy by parametric optimization of injection parameters.

Emission and performance characteristics of CRDI diesel engine using Quadruple injection strategy with different pilots and post injection timing

Multiple injections in association with suitable (High/heavy) exhaust gas recirculation (EGR) is a promising technique to achieve low temperature combustion (LTC) regime in modern CRDI (common rail direct injection) diesel engine. This helps for simultaneous reduction of NOx and Soot to meet the stringent emission norms without penalizing the fuel efficiency and overcome the drawbacks of EGR-LTC. There are limited insights available which discuss about the potentiality of latest Quadruple injection (early-pilot-main-post/after; epMa) strategy in combination with high EGR and different injection timing over triple injections (pilot-main-after; pMa) at wide operating conditions. This paper deals with the study on the effect of Quadruple injection (epMa) startegy on performance, emissions and noise with different Pilot injection timing and post injection dwell combinations (8 Nos) on a CRDI diesel engine with high EGR and fixed main injection schedule at 4 operating loads and 5 speeds. The study shows that Quadruple injection strategy with retarded early and advanced pilot (early 35° and pilot 19°CA BTDC) and advanced post injection dwell timing 1100 ms is superior in providing optimum results in emissions (NOx, PM, THC, CO) and combustion noise (CN)[1.55–2 dBA @Rig level and 0.7 dBA @ Vehicle level Pass by Noise (PBN)]. This gives the best results in brake specific fuel consumption (BSFC)[0.2 to 1.31%], Torque and brake thermal efficiency (BTE) performance w.r.t other combinations and base triple injections (pMa). In contrary, the quadruple injection strategy having advanced double pilots with delayed post injection dwell (early-39°, Pilot-19°CA BTDC and DtA 1300 ms); shows the best CN reduction. Best smoke results found with the combination of retarded pilots and advanced post injection dwell. This study shows the importance of injection timing specially the twin pilots (early, pilot) along with post injection dwell. Furthermore, it indicates the potentiality of newest Quadruple injection strategy over triple injection.

Exploring the Possibility of Achieving Partially Premixed Charge Compression Ignition Combustion of Biodiesel in Comparison to Ultra Low Sulfur Diesel on a High Compression Ratio Engine

ABSTRACT The combustion characteristics of biodiesel operating in the Low Temperature Combustion (LTC) regime, via Partially Premixed Charge Compression Ignition Combustion (PPCI), is relatively unknown. Therefore, this study compares engine performance and emissions of ultra-low sulfur diesel (ULSD) and waste cooking oil biodiesel through an attempted PPCI methodology. The Fuel Injection Timing (FIT) was gradually altered from conventional combustion timing (12.0° for biodiesel and 12.5° for ULSD) to 35.0° before top dead center. Results indicated that the fuel consumed to maintain the required load condition increased gradually for both ULSD and biodiesel due to the start of combustion shifting further into the compression stroke for each injection advancement. Nitrogen oxides (NOx), particulate matter (PM), carbon monoxide, and total hydrocarbons were marginally lower for biodiesel compared to ULSD. There was limited success in reducing both NOx and PM emissions simultaneously by varying the FIT alone for both fuels. The high compression ratio of the engine utilized was a major limiting factor in achieving LTC. Importantly, the Negative Temperature Coefficient (NTC) behavior of biodiesel and its impact on ignition delay was successfully captured. The operating conditions used in this study are closer to a production engine as compared to shock tube and constant volume combustion systems typically used to validate chemical kinetic (CK) models. Moreover, data surrounding NTC and its influence on ignition delay with biodiesel in CI engines is limited in the literature. Thus, the results presented provide valuable data that could be utilized to help validate CK models investigating the behavior of biodiesel fuels.

The impact of intake pressure on high exhaust gas recirculation low-temperature compression ignition engine combustion using borescopic imaging

In diesel engines, high levels of exhaust gas recirculation can be used to achieve low-temperature combustion, resulting in low emission levels of both nitrogen oxides (NO x) and particulate matter. This work studied the effects of varying the intake manifold pressure on in-cylinder combustion processes and engine-out emissions from a light-duty single cylinder diesel engine under conventional and high exhaust gas recirculation low-temperature combustion regimes. The work was conducted at a part-load cruise condition of 1500 r/min and at an indicated mean effective pressure of approximately 600 kPa. Exhaust gas recirculation rates were varied between 0% and 65% at absolute intake pressures of 100–150 kPa. Very low NO x emissions were achieved (<10 ppm, ∼0.05 g/kW h) for intake oxygen mass fractions below about 11%, independent of boost pressure. Smoke emission levels were lower than for non–exhaust gas recirculation combustion at oxygen mass fractions below ∼9%, depending on the boost pressure. High intake pressures reduced fuel consumption by 15% and combustion by-product emissions by 50%–60% compared to low boost. For the low intake boost case, little visible flame was apparent through borescope imaging. At higher boost pressures, intense flame luminosity was observed within the piston bowl early in the expansion stroke. Spatially averaged soot luminosity based on photomultiplier tube data showed that peak soot luminosity was five times greater and occurred 8 °CA earlier for high boost. This work demonstrates how the combination of appropriate boost pressures and exhaust gas recirculation rates can be used to mitigate the emissions and thermal efficiency penalties of high-dilution low-temperature combustion to achieve near-zero NO x operation.

Single-cylinder engine evaluation of a multi-component Diesel surrogate fuel at partially-premixed and low-temperature combustion modes

To investigate and develop efficient and clean combustion systems, advanced CFD modeling tools need accurate kinetic models capable of predicting the chemical and physical processes that take place in the combustion chamber during and after fuel injection and air mixing. Given the complex composition of market Diesel fuels, simpler surrogate fuels composed of a limited number of pure substances are used to model the physical, chemical, and combustion properties of Diesel fuel. The surrogates must closely reproduce the market Diesel fuel properties and duplicate the real-world engine combustion and emissions behavior. Previous work created a multi-component surrogate fuel that consisted of normal-hexadecane/2,2,4,4,6,8,8-heptamethylnonane/decahydronaphthalene/1-methylnaphthalene. The surrogate fuel properties closely matched the market Diesel fuel. The surrogate fuel was then evaluated under conventional Diesel engine combustion conditions and obtained excellent results replicating the real-world combustion and emissions of the market Diesel fuel. In this work, the surrogate fuel was investigated using a contemporary Diesel engine, operating at a low-load where partially-premixed and low temperature combustion conditions were achieved. For comparison and validation, the original market Diesel fuel was also tested under the same operating conditions. The experimental results from EGR and combustion phasing sweeps under partially-premixed and low temperature combustion showed the surrogate fuel closely duplicated the ignition delay, low temperature and high temperature heat release of the market Diesel fuel. The exhaust CO, NOx and HC emissions along with the particle size distributions were also closely matched. This investigation demonstrates that the four-component surrogate accurately represents the market Diesel fuel under low load, premixed and low temperature combustion regimes. Therefore, the surrogate fuel formulation should be suitable for continued work such as kinetic mechanism development, 3-dimensional spray and combustion simulation, and further exploratory research under premixed and low temperature combustion conditions.

Modelling of diesel spray flames under engine-like conditions using an accelerated Eulerian Stochastic Field method

This paper aims to simulate diesel spray flames across a wide range of engine-like conditions using the Eulerian Stochastic Field probability density function (ESF-PDF) model. The ESF model is coupled with the Chemistry Coordinate Mapping approach to expedite the calculation. A convergence study is carried out for a number of stochastic fields at five different conditions, covering both conventional diesel combustion and low-temperature combustion regimes. Ignition delay time, flame lift-off length as well as distributions of temperature and various combustion products are used to evaluate the performance of the model. The peak values of these properties generated using thirty-two stochastic fields are found to converge, with a maximum relative difference of 27% as compared to those from a greater number of stochastic fields. The ESF-PDF model with thirty-two stochastic fields performs reasonably well in reproducing the experimental flame development, ignition delay times and lift-off lengths. The ESF-PDF model also predicts a broader hydroxyl radical distribution which resembles the experimental observation, indicating that the turbulence–chemistry interaction is captured by the ESF-PDF model. The validated model is subsequently used to investigate the flame structures under different conditions. Analyses based on flame index and formaldehyde distribution suggest that a triple flame, which consists of a rich premixed flame, a diffusion flame and a lean premixed flame, is established in the earlier stage of the combustion. As the combustion progresses, the lean premixed flame weakens and diminishes with time. Eventually, only a double-flame structure, made up of the diffusion flame and the rich premixed flame, is observed. The analyses for various ambient temperatures show that the triple-flame structure remains for a longer period of time in cases with lower ambient temperatures. The present study shows that the ESF-PDF method is a valuable alternative to Lagrangian particle PDF methods.

The role of correlations in uncertainty quantification of transportation relevant fuel models

Large reaction mechanisms are often used to describe the combustion behavior of transportation-relevant fuels like gasoline, where these are typically represented by surrogate blends, e.g., n-heptane/iso-octane/toluene. We describe efforts to quantify the uncertainty in the predictions of such mechanisms at realistic engine conditions, seeking to better understand the robustness of the model as well as the important reaction pathways and their impacts on combustion behavior. In this work, we examine the importance of taking into account correlations among reactions that utilize the same rate rules and those with multiple product channels on forward propagation of uncertainty by Monte Carlo simulations. Automated means are developed to generate the uncertainty factor assignment for a detailed chemical kinetic mechanism, by first uniquely identifying each reacting species, then sorting each of the reactions based on the rate rule utilized. Simulation results reveal that in the low temperature combustion regime for iso-octane, the majority of the uncertainty in the model predictions can be attributed to low temperature reactions of the fuel sub-mechanism. The foundational, or small-molecule chemistry (C0C4) only contributes significantly to uncertainties in the predictions at the highest temperatures (Tc=900K). Accounting for correlations between important reactions is shown to produce non-negligible differences in the estimates of uncertainty. Including correlations among reactions that use the same rate rules increases uncertainty in the model predictions, while accounting for correlations among reactions with multiple branches decreases uncertainty in some cases. Significant non-linear response is observed in the model predictions depending on how the probability distributions of the uncertain rate constants are defined. It is concluded that care must be exercised in defining these probability distributions in order to reduce bias, and physically unrealistic estimates in the forward propagation of uncertainty for a range of UQ activities.

Energy Optimization and Fuel Economy Investigation of a Series Hybrid Electric Vehicle Integrated with Diesel/RCCI Engines

Among different types of low temperature combustion (LTC) regimes, eactively controlled compression ignition (RCCI) has received a lot of attention as a promising advanced combustion engine technology with high indicated thermal efficiency and low nitrogen oxides ( NO x ) and particulate matter (PM) emissions. In this study, an RCCI engine for the purpose of fuel economy investigation is incorporated in series hybrid electric vehicle (SHEV) architecture, which allows the engine to run completely in the narrow RCCI mode for common driving cycles. Three different types of energy management control (EMC) strategies are designed and implemented to achieve the best fuel economy. The EMC strategies encompass rule-based control (RBC), offline, and online optimal controllers, including dynamic programing (DP) and model predictive control (MPC), respectively. The simulation results show a 13.1% to 14.2% fuel economy saving by using an RCCI engine over a modern spark ignition (SI) engine in SHEV for different driving cycles. This fuel economy saving is reduced to 3% in comparison with a modern compression ignition (CI) engine, while NO x emissions are significantly lower. Simulation results show that the RCCI engine offers more fuel economy improvement in more aggressive driving cycles (e.g., US06), compared to less aggressive driving cycles (e.g., UDDS). In addition, the MPC results show that sub-optimal fuel economy is achieved by predicting the vehicle speed profile for a time horizon of 70 s.

Ignition and combustion characteristics of n-pentanol–diesel blends in a constant volume chamber

Pentanol is considered as one of the most promising alternative biofuels due to its excellent physicochemical properties. The objective of this work was to compare the ignition and combustion characteristics of different n-pentanol/diesel blends in an optical constant volume combustion chamber. The tested fuels included 20% (D80P20) and 40% (D60P40) of n-pentanol blended with diesel in volume, and pure diesel (D100). Broadband chemiluminescence technique was used to measure the timing and location of spray ignition. A high-speed CCD camera with two ND8 dimmer lenses was used to capture the incandescence radiated from the soot particles during combustion. A wide range of experimental conditions was investigated. The ambient temperature ranged from 800K to 1200K and the oxygen concentration ranging from 10% to 21%, covering both the conventional and low temperature combustion regimes. The results show that pure diesel has shorter ignition delay and distance comparing to pentanol blends. A larger blending proportion of pentanol D60P40 advances the ignition phase more than the D80P20 in low oxygen concentration conditions. Due to the fuel-borne oxygen and the dilution effect, the natural flame luminosity is reduced significantly with the increase of pentanol ratio in most conditions except under the intermediate temperature region of 1000K. In that condition, the shorter ignition delay and flame lift-off length of pentanol blends cause a slightly increase in the natural flame luminosity. The natural flame luminosity images showed that the oxygen-contained structure of pentanol could accelerate soot oxidation under all conditions. This indicates that pentanol blends could decrease final soot emissions in internal combustion engines.

Flame extinction and low-temperature combustion of isolated fuel droplets of n-alkanes

A recent set of experiments carried out onboard the International Space Station (ISS) have shown that large n-alkane droplets, after the radiative extinction of the visible flame, can burn quasi-steadily in a low-temperature regime, up to a diffusive extinction accompanied by the formation of a vapor cloud. The experiments have also demonstrated that small droplets are unable to exhibit radiative extinction, but instead burn to completion or disruptively extinguish.In this work, we applied a mathematical model able to reproduce the experimental data in terms of vaporization rates, standoff ratios, extinction diameters. A detailed kinetic mechanism (with ∼450 species and ∼17,000 reactions) was taken into account, in order to correctly reproduce the low-temperature combustion regime. The role of several parameters (initial diameter of the droplet, composition of the gaseous environment, and pressure) on the extinction of the hot-temperature flames was numerically investigated. Predictions are found in good agreement with experimental measurements, showing that only droplets with an initial diameter larger than a critical diameter undergo radiative extinction. A linear relationship between the squared critical diameter and the molar fraction of oxygen in the atmosphere was found and demonstrated on the basis of scaling arguments.

The combustion of iso-octane droplets with initial diameters from 0.5 to 5 mm: Effects on burning rate and flame extinction

The burning characteristics of iso-octane droplets in the standard atmosphere are reported over a large range of initial droplet diameters (0.5mm<Do< 5mm) for near one-dimensional droplet flames as promoted by low gravity. A ground-based drop tower and a space-based platform (International Space Station) were used to provide an environment to examine the influence of Do that encompasses regimes where radiation does not have an effect on burning to where it does. For Do< 2.7mm, the droplets burned to completion without extinction. Larger droplets evidenced extinction due to radiative emissions from the flames to the ambience that produced a two stage burning process. The burning rate (K) was nearly constant in the first stage and then gradually decreased. Concurrently, radiation emissions dropped by two orders of magnitude and ‘flickering’ flames were noted during the transition. After extinction, burning entered a regime where K gradually decreased with time and approached values commensurate with evaporation in a heated environment (i.e., no combustion). Energy balances based on scale analyses related K and flame temperature (Tf) to Do: a balance at the flame including radiation losses led to the scaling K ∼ Do−n in reasonable agreement with the data for Do> 2mm; an energy balance at the droplet surface showed that Tf quickly dropped from ∼1600K to ∼700K after flame extinction, followed by a gradual reduction of Tf to near ambient temperature. The trends in the data are also discussed for potentially revealing a possible low temperature combustion regime.

Impact of aromaticity and cetane number on the soot-NOx trade-off in conventional and low temperature combustion

This paper investigates whether or not two persistent diesel dogmas, namely “the higher the cetane number (CN) the better” and “the lower the aromaticity the better”, still ring true when a compression ignition engine is operated in the low temperature combustion (LTC) regime. The transition from conventional, high temperature combustion (HTC) to LTC is realized in a step-wise approach by increasing the level of exhaust gas recirculation, reducing the compression ratio and by lowering intake pressure. The fuel matrix spans a range of both aromaticity and CN. All experiments are conducted on a modified DAF heavy-duty compression ignition engine.Two main conclusions can be drawn from the results. First, at equal aromaticity, there is no discernible benefit of a high CN with respect to the soot-NOx trade-off in the HTC mode. In fact, when operating in the LTC regime, a high CN even results in a penalty in aforementioned trade-off. Second, at equal CN, increased aromatic content always has a negative impact on the soot-NOx trade-off, irrespective of combustion mode. Accordingly, our results demonstrate that, with respect to the soot-NOx trade-off, the first and second dogma is valid in neither and both of the combustion modes, respectively.

Intake condition requirements for biodiesel modulated kinetic combustion concept to achieve a simultaneous NOx and soot removal

The fuel oxygen contained in oxygenated fuels can help reduce harmful engine-out emissions and improve the combustion process in compression-ignition engines. The use of soybean methylene ether biodiesel in the low-temperature combustion (LTC) regime has the potential to suppress soot formation and nitrogen oxides (NOx) emissions even further, which eventually alleviates the burden of the after-treatment system.In the present study, the effects of the intake pressure and injection timing on the combustion and emissions of the modulated kinetic (MK) combustion concept with ultra-low sulfur diesel and 20% biodiesel blended fuel (BD20) were investigated in a single-cylinder CI engine. The intake pressure was varied from 100kPa to 250kPa for the intake oxygen concentration range of 11–17%.The engine test results indicate that simultaneous reductions in both the NOx and soot emissions were realized under the MK LTC combustion regime. At the best operating point, BD20 achieved the simultaneous NOx and soot removal at a lower intake pressure and lower EGR level than diesel, which led to better fuel economy. In addition, BD20 achieved acceptable levels of combustion stability and noise level.

A Study of Combustion Inefficiency in Diesel Low Temperature Combustion and Gasoline–Diesel RCCI Via Detailed Emission Measurement

Reduction of engine-out NOx emissions to ultra-low levels is facilitated by enabling low temperature combustion (LTC) strategies. However, there is a significant energy penalty in terms of combustion efficiency as evidenced by the high levels of hydrocarbon (HC), carbon monoxide (CO), and hydrogen emissions. In this work, the net fuel energy lost as a result of incomplete combustion in two different LTC regimes is studied—partially premixed compression ignition (PPCI) using in-cylinder injection of diesel fuel and reactivity controlled compression ignition (RCCI) of port injected gasoline and direct injected diesel. A detailed analysis of the incomplete combustion products was conducted. Test results indicated that carbon monoxide (CO), hydrogen, and light hydrocarbon (HC) made up for most of the combustion in-efficiency in the PPCI mode, while heavier HC and aromatics were significantly higher in the RCCI mode.