Toluene Primary Reference Fuels Research Articles

On the super adiabatic flame temperature (SAFT) of toluene primary reference fuels

Toluene primary reference fuel (TPRF) mixtures containing iso-octane (i-C8H18), n-heptane (n-C7H16), and toluene (C6H5CH3) are commonly used as gasoline surrogates. We report the occurrence of super adiabatic flame temperature (SAFT) during the premixed combustion of TPRF mixtures in air. Both 1-D premixed flame and 0-D (well-stirred) reactor configurations have been considered for chemical kinetic analysis with Cantera version 2.5.1. A recently developed detailed gasoline surrogate mechanism (C3MechV3.3) has been used for the 0-D reactor analysis. On the other hand, the flame structures have been simulated with a skeletal mechanism. To ensure accurate analysis, the mechanisms used in this work have been validated at SAFT relevant conditions against experimental data from literature. For the first time in literature, post-flame heat release and brute force reaction sensitivity analysis have been performed to identify important reactions contributing towards SAFT in TPRF surrogates. The degree of superadiabaticity for 1-D and 0-D cases have been expressed with non-dimensional parameters ξ1D and ξ0D respectively. The results show that ξ1D and ξ0D increase with the increase in iso-octane content in TPRF compositions, while n-heptane and toluene have minor influence on SAFT. Key chemical observations to the SAFT mechanism in gasoline surrogate mixtures include: 1) The competition between the exothermic reaction zone and the endothermic post-flame region causes SAFT. 2) Both 1-D flame and 0-D reactor analysis show that the reactions involving H, OH, and C3 species have major influence on SAFT. 3) Iso-octane augments SAFT as its β-scission product iso-butene (i-C4H8) influences the post-flame endothermicity and C3 species formation.

Effects of ozone addition and octane sensitivity on combustion characteristics of gasoline compression ignition engines

Gasoline compression ignition (GCI) is one of the promising technologies that can achieve high efficiency and clean combustion. However, GCI engines often suffer from ignition instabilities at low loads with poor intake conditions. Various physical methods such as intake preheating are often employed to improve combustion instabilities, while the approaches based on chemical perspectives are not fully considered. In this work, the effect of ozone addition and octane sensitivity on GCI combustion was investigated based on a single-cylinder engine fueled with Toluene Primary Reference Fuels (TPRF). Four octane sensitivities (i.e., 0, 3.6, 5.3, and 7.2) at a research octane number of 75 and six levels of ozone seeding (i.e., 0, 5, 10, 30, 60, and 90 ppm) were considered. Pressure characteristics and combustion evolutions were compared and interpreted and underlying reasons for distinct combustion behaviors were clarified. The results show that both ozone seeding and octane sensitivity has a pronounced influence on the combustion characteristics of GCI engines. Specifically, with the increase of octane sensitivity, both low- and high-temperature combustion phasing is advanced, indicating the improvement in chemical reactivity during GCI combustion. Ozone seeding shows similar influences on GCI combustion and can reduce the disparities of combustion characteristics under various octane sensitivity conditions. Meanwhile, low concentrations of ozone seeding can exhibit significance in the promotion of GCI combustion while it becomes minimal when beyond a critical level. Chemical kinetic analysis shows that the introduction of ozone seeding promotes the production of reactive free radicals, which accelerates the oxidation of fuel molecules and then improves combustion at both low and high temperatures. The current work demonstrates the potential of higher octane-sensitivity fuel combined with low levels of ozone seeding in improving the combustion instability of GCI engines at low loads.

Numerical Study of PAHs and Soot Emissions from Gasoline–Methanol, Gasoline–Ethanol, and Gasoline–n-Butanol Blend Surrogates

Soot formation is an intricate phenomenon, and soot propensity of a fuel is interwoven with the fuel composition, physical and chemical properties, and combustion environment. The present study examines the hypothesis that in addition to the chemical composition of the fuel, the sooting nature of the fuel is closely coupled with its chemical property known as octane sensitivity (S). With this motivation, the present study numerically investigates the effects of gasoline surrogate composition and its property, octane sensitivity (S), on polycyclic aromatic hydrocarbons (PAHs) and soot emissions. Four-component toluene primary reference fuel (TPRF)–alcohol blends, comprising iso-octane, n-heptane, toluene, and one of the three different alcohols- methanol, ethanol, and n-butanol, are used as gasoline surrogates. A total of 320 TPRF–alcohol mixtures, with S in the range of 1–10, are examined under laminar counterflow diffusion flame conditions. A detailed chemical mechanism coupled with a comprehensive soot model, which includes reactions for soot inception, surface growth, PAH condensation, and oxidation, is adopted. The analysis indicates that the toluene content in the fuel mixture has a prominent effect, while the alcohol content and octane sensitivity of the fuel have a weak correlation with the PAHs and soot. Thus, it is not clear if any of these three variables, namely, toluene content in the fuel, alcohol content in the fuel, and S, are individually sufficient to characterize the PAHs and soot across various blends. For this reason, a new variable (XCHO) based on the elemental composition of the fuel mixture is identified and it is shown that XCHO along with S of the fuel characterize soot emissions satisfactorily. Further, a reaction path analysis indicates that the efficacy of alcohols in reducing soot emissions follows the order: methanol > ethanol > n-butanol.

Numerical analysis of soot emissions from gasoline-ethanol and gasoline-butanol blends under gasoline compression ignition conditions

In the present work, computational fluid dynamics (CFD) simulations of a single-cylinder gasoline compression ignition (GCI) engine were performed to investigate the impact of blending two biofuels, ethanol and n-butanol, with gasoline on the trade-off between combustion phasing and soot emissions under low load conditions. In order to represent market gasoline (RD5-87), a four-component toluene primary reference fuel (TPRF) + ethanol (ETPRF) surrogate (with 20% ethanol by mole; E20) was formulated using a neural network based octane predictor such that the surrogate had the same ethanol content, Research Octane Number (RON) and Octane Sensitivity (S). In addition, a novel skeletal kinetic mechanism for ETPRF and TPRF + n-butanol (BTPRF) blends, incorporating polycyclic aromatic hydrocarbon (PAH) chemistry, was developed. A three-dimensional (3D) engine CFD formulation employing the skeletal mechanism, adaptive mesh refinement (AMR), finite-rate chemistry approach, and hybrid method of moments (HMOM) was adopted to capture the in-cylinder combustion phenomena and soot emissions. The engine CFD model was validated against RD5-87 experimental data for a broad range of start-of-injection (SOI) timings (-21/-27/-36/-45 crank angle degrees (CAD) after top-dead-center (aTDC)), with respect to in-cylinder pressure, heat release rate, combustion phasing, and soot emissions. The closed-cycle simulation results were analyzed to elucidate the non-monotonic trend of soot emissions versus SOI timing: SOI-36 > SOI-45 > SOI-21 > SOI-27. Thereafter, the validated CFD model was employed to simulate the combustion of a gasoline-ethanol blend with 45% (by mole) ethanol (E45) and a gasoline-butanol blend with 45% (by mole) n-butanol (B45) under the same operating conditions to study the effects of fuel composition and SOI timing on combustion phasing and soot emissions. The sooting propensity followed the trend: B45 > E20 > E45 at all SOI timings. Overall, it was observed that the autoignition propensity was primarily related to fuel chemistry. On the other hand, sooting propensity showed strong coupling with both fuel chemistry and physical properties, with greater impact of fuel physical properties at advanced SOI timings.

A new detailed kinetic model for surrogate fuels: C3MechV3.3

There is an increasing demand for kinetic models of surrogate components to predict the combustion and emissions of real fuels. In this paper, a new surrogate fuel mechanism, C3MechV3.3, is proposed by the Computational Chemistry Consortium (C3). This mechanism is constructed based on a C0 – C4 core mechanism, with important species of interest in complex fuel surrogates such as the hexane isomers, n-heptane, iso-octane, nC8 – nC12 linear alkanes as well as polycyclic aromatic hydrocarbons (PAHs) and NOx as pollutants. This kinetic model consists of the latest chemistry subsets developed by the different partners in the context of the C3 effort. The proposed model was tested against a comprehensive set of experimental data for various fuels and blends over a wide range of temperatures, pressures, dilutions and equivalence ratios. Overall, the model shows good predictions for most of the experimental data. In particular, the focus of the validation is on natural gas/n-alkanes, primary reference fuel (PRF) and toluene primary reference fuel (TPRF) mixtures. Due to the large size of C3MechV3.3, a mechanism processing tool was developed to abstract species and reactions to generate any particular surrogate fuel and multi- fuel mechanisms of smaller size which can be used for preliminary mechanism reduction.

Investigation on the ignition delay prediction model of multi-component surrogates based on back propagation (BP) neural network

The ignition delay prediction model of three-component surrogates was established based on the back propagation (BP) neural network. The ambient temperature, ambient pressure, molar fraction of n-heptane, iso-octane and toluene were utilized as the input parameters. The ignition delay was utilized as the output parameter. The training and validation set only contained the 0-D simulation ignition delay of single- and two-component surrogates. But the trained BP neural network also showed a strong predictive ability towards the ignition delay of three component toluene primary reference fuel (TPRF) surrogates. Results show that the BP neural network with two hidden layers performs better than that with single hidden layer. With the optimization of genetic algorithm (GA) and particle swarm optimization (PSO) algorithm, the correlation coefficient is higher than 0.9996. The mean relative error (MRE) and the mean square error (MSE) are also maintained at a low level. The computational costs of 0-D simulation and BP neural network methods are compared. In 0-D simulation, the computational time of one case is 28 s. When the BP neural network is utilized, the computational time of 176 cases is just 3.2 s, which shows a significant improvement in the computation time. Through the mean impact value (MIV) analysis, the significance of each input variable to the output results is investigated. The input parameters of ambient temperature and molar fraction of iso-octane obtain the highest and lowest absolute value of MIV, respectively, indicating the major and minor effects on the ignition delay. Based on the predicted ignition delay of three-component TPRF surrogates, the research octane number (RON) and motor octane number (MON) can also be accurately predicted with the maximum deviance no more than 3 units. For real fuels of fuels for advanced combustion engines (FACE) gasolines, such as FACE A and FACE C gasolines, the surrogate fuel which has the same ignition delay at the specific pressure and temperature can be determined through the construction of the ignition delay look-up table of TPRF surrogates by the BP neural network. Following this method, the ignition delay of the real fuels can be matched accurately and the molar fraction of each component of the corresponding TPRF surrogates can also be acquired.

Effects of blending 2,5-dimethylfuran and dimethyl ether to toluene primary reference fuels: A chemical kinetic study

In the present work, renewable oxygenates 2,5-dimethylfuran (DMF) and dimethyl ether (DME) are considered as alternatives to fossil fuels for gasoline engines. The effects of blending DMF and DME on the combustion of gasoline surrogate are numerically studied. The gasoline surrogate is toluene primary reference fuel (TPRF) mixtures containing iso-octane, n-heptane, and toluene. Two skeletal mechanisms are proposed for the analysis: mechanism A with 504 species and 4212 reactions and mechanism B with 153 species and 740 reactions. These mechanisms are validated against a wide range of experimental data on ignition delay times and 1-D flames. The main findings of the present work are: 1) The ignition inhibiting effect of DMF dominates over the promoting effect of DME at 750 K and 25 bar when the total mole fraction of DMF and DME is less than 20 %. On the contrary, DME augments ignition with greater strength than the inhibition effect of DMF at 825 K and 25 bar. These observations are made for equimolar mixture of DMF and DME blended with TPRF in varying proportions. 2) The laminar burning velocity change stays within 10% of TPRF-air values for 50% TPRF/25%DMF/25%DME-air mixture for 0.6 ≤ φ ≤ 1. 6. 3) DME mitigates additional soot emission caused by DMF blending with TPRF. 4) For the first time in literature, it is shown that the laminar burning velocity and the maximum slope of OḢ mole fraction in the stoichiometric flames vary linearly with the research octane number (RON) for TPRF-air mixtures.

Influences of C5 esters addition on anti-knock and auto-ignition tendency of a gasoline surrogate fuel

The research octane numbers and auto-ignition characteristics of a toluene primary reference fuel (TPRF), when blended with three C5 esters, γ-valerolactone (GVL), methyl butanoate (MB), and methyl crotonate (MC), were investigated on a cooperative fuel research (CFR) engine and a constant volume combustion chamber (CVCC). In fuel preparation, ethanol was used to improve the miscibility, and the total additive comprised 1/3 ethanol and 2/3 GVL/MB/MC on a molar basis. The experimental results first reveal that the addition of the three series of additives boost fuel octane rating, and their boosting effects rank as MB > GVL ∼ MC when fuels are blended on a molar basis. In contrast, the auto-ignition tendency of the three series of fuel blends, when blended on the molar basis, ranks as the MC blends > the GVL blends> the MB blends. Different from the similar reactivities observed for the MC blends and the GVL blends in the RON tests, the MC blends exhibit higher auto-ignition propensity than the GVL blends, probably because the higher enthalpy of vaporization of GVL causes a more significant cooling effect. Finally, different from the literature studies that reported similar reactivities for pure MB and MC, in this study MB shows lower reactivity than MC when blending with the TPRF gasoline surrogate.

Effects of intake conditions and octane sensitivity on GCI combustion at early injection timings

Gasoline compression ignition (GCI) engines favor low-octane fuels (RON = 60–80). However, low-octane fuels possessing low-temperature reactions strongly interact with in-cylinder thermodynamic conditions, which in turn significantly affect GCI combustion characteristics. In this work, using a toluene primary reference fuel (TPRF) with RON = 75, the effects of intake pressure and temperature on combustion evolutions, low-temperature heat release (LTHR), high-temperature heat release (HTHR), and indicated thermal efficiency were numerically investigated in a single-cylinder GCI engine, allowing for the role of octane sensitivity. An early injection timing was considered to obtain relatively homogeneous mixtures and large variations in intake pressure were employed to simulate low to medium loads. The results show that for a given equivalence ratio, the peak of the LTHR and HTHR and the main combustion phasing positively correlates with intake pressure. However, an opposite tendency in the peak of the HTHR is observed for a fixed amount of fuel injection. When octane sensitivity is elevated, the peak of in-cylinder pressure and heat release rate is reduced and ignition timing is retarded, especially for the HTHR. Elevating intake temperature facilitates high-temperature combustion while suppresses low-temperature reactions, which results in nonlinear variations in combustion phasing at a certain range of intake temperatures. The correlations of operating conditions and combustion evolutions are evaluated by pressure-temperature trajectory and Livengood-Wu integration, which indicates the significance of intake pressure in GCI combustion. Finally, combustion evolutions show that low-temperature reactions first take place at fuel-lean regions and hot flame pockets develop in fuel-rich regions, which suggests that low-temperature ignition is more sensitive to ambient temperatures while high-temperature ignition is more dependent on equivalence ratio.

Effect of flash boiling injection on combustion and PN emissions of DISI optical engine fueled with butanol isomers/TPRF blends

Direct injection spark ignition (DISI) engines have been widely used in passenger cars due to their lower fuel consumption, better controllability, and high efficiency. However, DISI engines are suffering from wall wetting, imperfect mixture formation, excess soot emissions, and cyclic variations. Applying a new fuel atomization technique and using biofuels with their distinctive properties can potentially aid in improving DISI engines. In this research, the effects of isobutanol and 2-butanol and their blends with Toluene Primary Reference Fuel (TPRF) on spray characteristics, DISI engine combustion, and particle number (PN) emissions are investigated for conditions with and without flash boiling of the injected fuel. Spray characteristics are investigated using a constant volume chamber. Then, the combustion, flame propagation, and PN emissions are examined using an optical DISI engine. The fuel temperature is set to 298 K and 453 K for liquid injection and flash boiling injection, respectively. The tested blending ratio is 30 vol% butanol isomers and 70 vol% TPRF. The results of the spray test reveal that liquid fuel plumes are distinctly observed, and butanol blends show a slightly wider spray angle with lower penetration length compared to TPRF. However, under flash boiling injection, the sprays collapse towards the injector axis, forming a more extended single central vapor jet due to the plumes' interaction. Meanwhile, butanol blends yield a narrow spray angle with more extended penetration compared to TPRF. The flame visualization test shows that the flash boiling injection reduced yellow flames compared to liquid fuel injection, reflecting the improvements in mixture formation. Thus, improvements were noted in the heat release and PN emissions. Butanol addition reduced the PN emissions by 43% under regular liquid injection. Flash boiling injection provided an additional 25% reduction in PN emissions.

THIP: A new TPRF-like fuel surrogate development approach to better match real fuel properties

Simple surrogate formulations for gasoline are useful for modelling purposes and for comparing experimental results using a carefully designed fuel. Simple three component surrogate fuels based on toluene, n-heptane and isooctane, called toluene primary reference fuels (TPRF), are frequently used to match the RON and MON of actual gasoline fuels.In this work the main advantages of using an alternative three-component surrogate formulation, based on toluene, n-heptane and isopentane (THIP) are presented. Given a reference gasoline, good representations of RON and MON are achieved as well as better matching density, distillation properties and other volatility parameters associated with the lighter end of gasoline fuels. In particular, THIP surrogates can replicate Dry Vapour Pressure Equivalent (DVPE) values ranging from 10.4 kPa to 138.9 kPa, fully covering the EN228 Summer and Winter gasoline specifications. Moreover, Rapid Compression Machine (RCM) results demonstrate that THIP surrogates can reproduce the essential characteristics of the reference gasoline in terms of ignition delay time versus temperature. An existing chemical kinetic mechanism is shown to be able represent THIP surrogate well.

Flash boiling combustion of isomeric butanol and gasoline surrogate blends using constant volume spray chamber and GDI optical engine

Flash boiling spray has been regarded as a promising fuel injection technology. Many investigations have been focused on spray physics and mechanisms of flash boiling spray, but information on flash boiling of alcohols in GDI engines is very limited. In this paper, the spray characteristics, flame propagation, combustion analysis, and PN emissions of butanol isomers under flash boiling spray have been studied. A constant volume chamber with a high-speed camera used to investigate the flash boiling spray of butanol isomers and gasoline surrogate blends. The flash boiling spray was tested at 180 °C and compared to subcooled spray at 25 °C. The tested gasoline surrogates are primary reference fuel (PRF) and toluene primary reference fuel (TPRF). The flash boiling leads to the collapse of the spray six plumes into a single extended spray plume with longer spray penetration compared to subcooled injection. The spray study then extended to study the effects of flash boiling injection on flame development and PN emissions of the GDI optical engine. The results reveal that flash boiling injection leads to lower apparent flame speed and flame area compared to subcooled injection due to the lean-like burn condition, and yellow flame spots are diminished. The peak in-cylinder pressure and apparent heat release rate are reduced with longer ignition delay when the flash boiling injection is applied. The flame lasted longer under flash boiling conditions with improved late combustion phase, which results in higher heat released. The PN emissions reduced with flash boiling spray compared to subcooled conditions.

Ignition delay time measurements of diesel and gasoline blends

Blends of diesel and gasoline can be used to achieve certain desired ignition characteristics in advanced compression ignition engine concepts. In this work, ignition delay times were measured for two blends of diesel and gasoline in two shock tubes and in a rapid compression machine. These blends comprised of 50/50 and 25/75 volumetric% of diesel and gasoline, respectively. To ensure complete vaporization of the blends, the prepared samples were analyzed with nuclear magnetic resonance (NMR) and laser absorption. The analyses revealed full evaporation, and negligible decomposition/oxidation occurred during mixture preparation. Ignition delay measurements covered wide ranges of temperatures (710–1349 K), pressures (10 and 20 bar), and equivalence ratios (0.5, 1.0 and 2.0). The measured ignition delay times of the two dieseline blends are compared with experimental data of low- to mid-octane gasoline and low- to high-cetane fuels. The measured data are also compared with the simulated ignition delay times of primary reference fuel (PRF) and toluene primary reference fuels (TPRF) surrogates. Multi-component surrogates are proposed for the dieseline blends, and the measured ignition delays of the multi-component surrogates and the dieseline blends are in very good agreement.

RON and MON chemical kinetic modeling derived correlations with ignition delay time for gasoline and octane boosting additives

For regulatory and certification purposes the indices that characterize the ignition propensity for a fuel, Research Octane Number (RON) and Motor Octane Number (MON), are measured in a Cooperative Fuel Research (CFR) engine. In an effort to reduce the cost and time of CFR engine testing, computer simulation based work capable of predicting the octane numbers for any fuel blend has received increased attention. Notably, the works of Westbrook et al. [1], Badra et al. [2], and Kim et al. [3] simulated the second stage hot ignition delay time (IDT) for fuels and correlated their ignition delay time with experimental RON/MON measurements to determine the relationship between calculated IDT and RON/MON, thereby providing a means to analytically assess the apparent RON/MON and octane sensitivity (OS=RON-MON) values for the fuel. Using a similar methodology, the current study investigated RON-like and MON-like simulated engine compression histories calculated using GT-Power. The current work's calculated IDTs were determined for mixtures of primary reference fuel (PRF) gasoline and major blending components (i.e., iso-octane, n-heptane, ethanol, and toluene), and neat octane boosting additives (e.g., ethyl-benzene, iso-butanol, di-iso-butylene (DIB)) using 0-D closed homogeneous reactor chemical kinetic simulations driven by volume-time and pressure-time compression profiles. The primary goal of the current work was to explore any possible dependence of the fuel's predicted octane number with the compression profile used (i.e., volume-time profile and pressure-time profile). To investigate any possible dependence, the calculated IDTs for both the volume profile and pressure profile driven simulations were used to develop RON and MON correlation curves based on the PRF and neat fuel experimental literature data from McCormick et al. [16]. The model derived RON and MON correlation curves were used to analytically assess the octane numbers of PRFs, toluene PRFs (TPRFs), and both PRFs and TPRFs blended with ethanol for comparison with experimental data from Foong et al. [13]. The simulations indicated a synergistic blending effect on octane number for ethanol blended with PRFs and TPRFs. Overall, the volume profile driven RON/MON/OS assessments had less difference from the experimental PRFs and TPRFs data (i.e., on the order of approximately +/- 2-6 ONs) than the pressure profile driven RON/MON/OS assessments (i.e., greater than approximately +/- 6 ONs). Lastly, the volume and pressure profile driven RON and MON assessments for selected mixtures were compared to the RON and MON assessments completed in [1] which were determined with pressure profile driven simulations.

Combustion and emissions of isomeric butanol/gasoline surrogates blends on an optical GDI engine

The combination of distinctive physicochemical properties of biofuels and recent engine technologies offer benefits in terms of efficiency improvement and emissions reduction. The recent development in butanol bio-based production pathways encourage researchers to study their combustion characteristics. This paper experimentally evaluates the combustion and emissions of a gasoline direct injection (GDI) engine fueled with the butanol isomer/gasoline surrogate blends, in which primary reference fuel (PRF) and toluene primary reference fuel (TPRF) are selected as the gasoline surrogates, respectively. The flame propagation behaviors, in-cylinder pressure, apparent heat release rate, along with PN emissions from this optical GDI engine are discussed. First, butanol addition to the gasoline surrogates is found to slow down flame propagation, reduce peak cylinder pressure and heat release rate, and extend ignition delay and combustion duration. Further, among the four butanol isomers, n-butanol and tert-butanol are the most and least reactive fuels, respectively, whereas iso-butanol and sec-butanol show reactivities in between, as supported by the measured flame propagation and pressure traces, calculated heat release rates, as well as time scales describing the combustion progress, e.g. ignition delay and combustion duration. Finally, butanol addition reduces the PN emissions from the GDI engine, and the PN emissions reduction capacity of the four butanol isomers ranks as sec-butanol > iso-butanol > n-butanol > tert-butanol. Also, compared to the PRF/butanol blends, the PN emissions are reduced to a less extent when butanol is blended with TPRF.

QSPR Studies on the Octane Number of Toluene Primary Reference Fuel Based on the Electrotopological State Index

The quantitative structure–property relationship (QSPR) models for predicting the octane number (ON) of toluene primary reference fuel (TPRF; blends of n-heptane, isooctane, and toluene) was investigated. The electrotopological state (E-state) index of TPRF components was computed and weight-summed to generate the quantitative descriptor of TPRF samples. The partial least squares (PLS) technique was used to build up the regression model between the ON and weight-summed E-state index of the investigated samples. The QSPR models for the research octane number (RON) and motor octane number (MON) of TPRF were built. The prediction performance of the obtained PLS models was assessed by the external test set validation and leave-one-out cross-validation. The validation results demonstrate that the proposed PLS models are feasible for predicting the ON, both RON and MON, of TPRF. In addition, several other QSPR models for the ON of TPRF were developed by employing the stepwise regression and Scheffé polynomials methods, and the prediction performance of these models were compared with that of the PLS models. The comparison result shows that the proposed PLS models are slightly better than multiple linear regression models and Scheffé models. It is demonstrated that the combination of the E-state index and PLS is an easy-to-use and promising method for studying and forecasting the ON of TPRF.

Experimental study on the effect of pre-ignition heat release on GCI engine combustion

Interactions between fuel and engine are basic issues for gasoline compression ignition (GCI) engine, and recent engine experiment results show that the gasoline fuel with low-octane number RON = 60–80 is optimal for GCI engines. However, gasoline fuel with low-octane ratings exhibits significant low-temperature combustion behavior, which will significantly affect the entire combustion process. In this study, different toluene primary reference fuels (TPRF) with identical research octane number (RON) but various octane sensitivities were employed. Optimized start of injection (SOI) was obtained based on indicated mean effective pressure (IMEP), combustion stability limits and engine emissions. Under the optimized SOI, the role of fuel sensitivity in combustion processes was investigated. With addressing low-temperature heat release (LTHR) before main ignition, the effect of intake conditions and fuel sensitivity on combustion phasing was clarified. The results indicate that the LTHR only occurs when the SOI is sufficiently advanced, and an earlier injection timing can achieve stable combustion, higher IMEP and lower NOX emission. More obvious LTHR promotes the advancement of combustion phasing, and the magnitude of LTHR is negatively correlated with intake temperature but positively correlated with intake pressure. The effect of increasing LTHR magnitude on combustion phasing is similar to that of increasing intake temperature. Finally, the effect of octane sensitivity on LTHR was also studied, which shows that the impact of LTHR between different sensitivity fuels is mainly reflected in combustion phasing, and the onset of LTHR is more advanced for low sensitivity fuels.

Effects of butanol blending on spray auto-ignition of gasoline surrogate fuels

Effects of butanol isomers on spray auto-ignition of two gasoline surrogate fuels, primary reference fuels (PRF) and toluene primary reference fuels (TPRF), were experimentally studied on a constant volume combustion facility. Combustion pressures, heat release rates, ignition delay (ID) and combustion delay (CD) times were compared and discussed in the context of changed fuel compositions and ambient temperatures. Experimental results revealed that butanol addition inhibits the spray auto-ignition propensity of PRF80 or TPRF80 surrogate fuels, which have the same research octane number of 80, and extends both the ID and CD times. The four butanol isomers extend the ID times of gasoline surrogates in an order of i-butanol > s-butanol ~ n-butanol > t-butanol, and the shortest ID times of the t-butanol/gasoline surrogate blends are attributed to their earliest first-stage heat release. Generally, the CD extension effects of the four butanol isomers are ranked as i-butanol > t-butanol ~ s-butanol > n-butanol, but the effects of s-butanol and t-butanol are slightly sensitive to the gasoline surrogate composition and ambient temperature.

Kinetic study on the effect of di-tert-butyl peroxide and 2-ethylhexyl nitrate on the reactivity of toluene primary reference fuels

ABSTRACT Fuel additives have shown to provide an important contribution to control the auto-ignition process of the homogeneous charge compression ignition (HCCI) and reactivity-controlled compression ignition (RCCI). In this work a kinetic modeling study has been conducted to study the effects of di-tert-butyl peroxide (DTBP) and 2-ethylhexyl nitrate (EHN) on the reactivity of toluene primary reference fuels (TPRF). The kinetic analysis at engine-like conditions showed that for TPRF, the first-stage heat release was similar for the two additives. However, in the case of EHN the OH precursor nitrous acid (common isomer HONO) was formed in significant amounts during the induction period which eventually lead to a faster high-temperature ignition. This was in contrast to primary reference fuels where the ignition enhancing effect of DTBP was more significant compared to EHN as a result of a more pronounced first-stage heat release when adding DTBP and a lesser amount of nitrous acid formed when adding EHN compared to a TPRF. Using a TPRF as gasoline surrogate fuel could reproduce the trends in RON as function of added EHN and DTBP for a commercial gasoline, indicating the importance to include fuel sensitivity (RON-MON) in a surrogate fuel.

CFD Analysis of Port Water Injection in a GDI Engine under Incipient Knock Conditions

This paper investigates, through computational fluid dynamics (CFD) simulations, the knock resistance improvements that can be obtained in a turbo-charged GDI engine with water injection. In a first step, water and gasoline injector models are validated comparing the results with experimental data from constant volume chamber tests. Then, multi-cycle simulations are performed using the G-equation turbulent combustion model focusing on spray evolution and wall film dynamics. The main intent is analyzing the effectiveness of different water injection timings and injection pressures in a port water injection (PWI) installation. Combustion rates are validated against experimental engine data, with and without water injection. Afterwards, in order to predict autoignition behavior with different spark advance (SA) timings, the extended coherent flamelet model (ECFM) combined with a tabulated kinetic ignition (TKI) dataset is used. End-gas autoignition delays are calculated using a reduced mechanism for toluene primary reference fuel (TPRF), which revealed essential for capturing actual gasoline ignition characteristics. Results indicate that the water atomization quality, i.e., injection pressure, is significant in a PWI installation allowing a reduction of the water wall film formation in the ports. Water injection timing needs also to be carefully chosen for optimized performance. As the injected water allows the SA to be increased, the overall benefits on indicated mean effective pressure and fuel consumption are quantified under the same knock safety margin, matching adequately well the available measurements.