Intermediate Temperature Heat Release Research Articles

The effect of 1-octanol blending on the multi-stage autoignition of conventional diesel and HVO fuels

Alternative fuels can reduce the carbon intensity of diesel engine use, and a detailed understanding of the combustion process for these fuels can minimize resulting fuel consumption and emissions. 1-Octanol has been considered as an alternative diesel fuel, but little work evaluating its combustion process in detail exists. In this work, ignition of 1-octanol, both pure and in blends with conventional and renewable diesel base fuels, is analyzed in a constant volume combustion chamber and a modified Cooperative Fuels Research Octane Rating engine operating in homogeneous charge compression ignition combustion mode. 1-Octanol blending reduced the derived cetane number of either base fuel, but modest increases in chamber temperature at injection could compensate for even dramatic reductions in derived cetane number. It was further found that greater amounts of low-temperature heat release were required to reach main ignition as the 1-octanol blending volume fraction was increased in the constant volume combustion chamber. This finding is attributed to the changing local equivalence ratio, as the reacting spray diffuses further into the chamber and approaches the (lean) global equivalence ratio as the ignition delay grows. Finally, it was found that, despite lowering the derived cetane number, 1-octanol blending enhanced the reactivity of the conventional diesel fuel observed in the variable compression ratio engine. This finding is attributed to the higher rates of hydrogen abstraction from the weaker average C–H bonds in 1-octanol, as hydrogen abstraction governs intermediate temperature heat release. These findings clarify the effect of 1-octanol on physical processes and the various chemical regimes of heat release in compression ignition. The dependence of the effects of 1-octanol blending on the base fuel are emphasized, as these can inform the deployment of 1-octanol as a diesel blendstock. Application of these results can support the optimization of conventional combustion strategies for use with 1-octanol, as well as the development of 1-octanol-fueled low-temperature combustion strategies with the associated promise of very low criteria pollutant emissions.

The contribution of intermediate-temperature heat release to octane sensitivity

Carbon footprint reduction can be achieved by increasing the efficiency of combustion engines. For spark-ignition (SI) internal combustion engines (ICE), fuel octane sensitivity (OS)—defined as the difference between a fuel’s research octane number (RON) and motor octane number (MON)—can help identify ideal fuels for avoiding unwanted knocking conditions. Moreover, it will help to extend the range of operating conditions that can be used in next-generation high-efficiency engines. Presently, there is a controversy over whether high or low OS fuels are beneficial for increased efficiency of next-generation engines depending on application and technology, e.g. high OS fuels are advantageous for down-sized and turbocharged spark-ignition (SI) engines, whereas low OS fuels are superior in novel systems such as pre-chamber engines. This work aims to improve the understanding of the impact of OS on the combustion performance of engines by analyzing the contribution of intermediate-temperature heat release (ITHR) to OS. The heat release of several mixtures of toluene, iso-octane, and n-heptane with similar RON but varying OS were studied under homogeneous charge compression ignition (HCCI) conditions in a cooperative fuel research (CFR) engine. This method gives insight into the heat release of the auto-igniting fuel without the obfuscation of the spark-ignited flame. At RON conditions, ITHR was a marker of OS. Furthermore, a new parameter called ITHR sensitivity was defined as the difference between the ITHR measured at RON conditions and the ITHR measured at MON conditions. ITHR sensitivity decreased with increasing octane sensitivity. This shows that non-sensitive fuels experience a greater change in the amount of ITHR as compared to sensitive fuels when changing from MON conditions to RON conditions. Thus, for engines requiring high OS fuels, it is recommended that components with high ITHR sensitivity are used and vice-versa for engines requiring low OS fuels.

Experimental and Numerical Study on the Effect of Nitric Oxide on Autoignition and Knock in a Direct-Injection Spark-Ignition Engine

<div class="section abstract"><div class="htmlview paragraph">Nitric Oxide (NO) can significantly influence the autoignition reactivity and this can affect knock limits in conventional stoichiometric SI engines. Previous studies also revealed that the role of NO changes with fuel type. Fuels with high RON (Research Octane Number) and high Octane Sensitivity (S = RON - MON (Motor Octane Number)) exhibited monotonically retarding knock-limited combustion phasing (KL-CA50) with increasing NO. In contrast, for a high-RON, low-S fuel, the addition of NO initially resulted in a strongly retarded KL-CA50 but beyond the certain amount of NO, KL-CA50 advanced again. The current study focuses on same high-RON, low-S Alkylate fuel to better understand the mechanisms responsible for the reversal in the effect of NO on KL-CA50 beyond a certain amount of NO. Experiments were conducted to measure the responses of KL-CA50 and trace-autoignition CA50, the latter being indicative of CA50 at which end-gas autoignition starts to become measurable from the apparent heat-release rate. Chemical-kinetics simulations were conducted to reveal the role of NO for end-gas autoignition, with a specific focus on sequential autoignition in a thermally stratified end-gas.</div><div class="htmlview paragraph">The simulation results reveal that the magnitude of low-temperature heat release (LTHR) generally increases with NO. However, the relative importance of NO for enhancing LTHR diminishes when the LTHR inherent to a fuel’s chemistry is strong, such as at lower temperatures in a thermal boundary layer. This rendered more uniform LTHR within a hypothetical thermal boundary and led to a more sequential (<i>i.e.</i> slower) autoignition event. It was also revealed that a change in compression ratio influences the importance of intermediate-temperature heat release (ITHR) due to changes of the temperature-pressure history of the end-gas. Together with the condition where end-gas autoignition occurs more sequentially, the shorter time spent in LTHR and ITHR regime can counter the increase in autoignition reactivity at high NO levels and allow KL-CA50 to advance.</div></div>

Probing intermediate temperature heat release in autoignition of C3-C4 iso-alcohol/gasoline blends

This work reports an experimental and modeling study on the blending effects of C3–C4 iso-alcohols, namely iso-propanol and iso-butanol, on the characteristics of intermediate temperature heat release (ITHR) for a research-grade gasoline (FACE-F) in a rapid compression machine operating at diluted/stoichiometric fuel loading, compressed pressure of 43 bar, and compressed temperatures from 700 to 970 K. Changes in ITHR behavior are characterized through ITHR extent and evolution at 0 to 30 vol% iso-alcohol blending levels. Experimental observations reveal the strong promoting and decelerating effects on ITHR extent and evolution, respectively, within the low-temperature regime for both iso-alcohols, with stronger effects seen for iso-propanol; within the intermediate-temperature regime, the influence of both iso-alcohols diminishes greatly. Comprehensive chemical kinetic modeling is undertaken using a recently developed gasoline surrogate/alcohol model in conjunction with a five-component gasoline surrogate (FGF-LLNL), with good agreement obtained with the experiments for all the blends. Sensitivity, rate of production and flux analyses highlight the importance of chemical kinetic interactions via both fuel-specific and non-fuel-specific reactions. Disabling all the chemical kinetic interactions between iso-alcohol and FGF-LLNL in carbonated/non-fuel-specific species leads to somewhat extended ITHR duration and reduced ignition reactivity. Direct comparison between FGF-LLNL/iso-propanol and FGF-LLNL/iso-butanol blends is also made, where iso-propanol and iso-butanol are found to influence ITHR characteristics via significantly different chemical kinetic interactions.

New insights into fuel blending effects: Intermolecular chemical kinetic interactions affecting autoignition times and intermediate-temperature heat release

Fuel blending effects on chemically-dominated fuel properties, such as gasoline anti-knock quality, are influenced by fundamental chemical kinetic interactions between the blending agent and the base fuel. Historically, quantification of such interactions has focused on direct changes to the radical pool, including ȮH and HO2, while intermolecular chemical kinetic interactions pertaining to carbon-containing, non-fuel-specific intermediates are typically overlooked. In this regard, this work aims to derive new insight into intermolecular chemical kinetic interactions that are intrinsic to fuel blending effects, via a case study on blends of 0–30% ethanol (by volume) into FGF-LLNL (a multi-component gasoline surrogate for FACE-F research gasoline) using a rapid compression machine at a diluted/stoichiometric fuel loading, compressed pressure of 40 bar and low- to intermediate-temperature regimes that are representative of boosted SI engine operation.Ethanol blending effects on the intermediate temperature heat release (ITHR) of FGF-LLNL are characterized using experimental measurements, where ethanol is found to promote the extent of ITHR and suppress the transition from ITHR to main ignition. Chemical kinetic modeling is undertaken using recently updated gasoline surrogate and alcohol models. Sensitivity analyses on ITHR characteristics further corroborate the ethanol blending effects, and highlight the significant dependence of ITHR on both fuel-specific and non-fuel-specific reactions. An approach allowing comprehensive characterization of the complex intermolecular chemical kinetic interactions between constituents in a fuel blend is then proposed. Application of the approach to FGF-LLNL/E0–E30 reveals that ethanol perturbs the heat release and autoignition characteristics of FGF-LLNL not only by directly changing the ȮH and HO2 radical pools via fuel-specific reactions, but also indirectly through intermolecular interactions where participating intermediates can be produced and consumed by various sub-chemistries. Disabling the intermolecular interactions in carbon-containing species between ethanol and FGF-LLNL sub-chemistries leads to somewhat slower ITHR evolution and lower ignition reactivity. The role of the individual intermolecular interaction is also characterized using the proposed approach. Finally, implications of intermolecular interactions for future studies that aim to improve model performance are highlighted, where it is found that further investigations on the acetaldehyde sub-model and FGF-LLNL/ethanol interactive chemical reactions (e.g., FGF-LLNL+CH3O2 and RO2+ethanol) are needed for chemistry models to accurately predict fuel blending behavior.

The Role of Intermediate-Temperature Heat Release in Octane Sensitivity of Fuels with Matching Research Octane Number

Improving the efficiency of internal combustion engines is important for reducing global greenhouse gas emissions; the efficiency of spark ignition (SI) engines is limited by the knock phenomenon. As opposed to naturally aspirated engines, turbocharged engines operate at beyond research octane number (RON) conditions, and fuel octane sensitivity (OS = RON – motor octane number (MON)) becomes important under such conditions. Previous work by this group [ Energy Fuels 2017, 31, 1945−1960, DOI: 10.1021/acs.energyfuels.6b02659] elucidated the chemical kinetic origins of OS; this study is extended to provide a qualitative, as well as quantitative, definition of OS, based on fundamental ignition markers. A varying amount of toluene is blended with various primary reference fuels to match the ignition delay of the targeted research octane number fuels, allowing a range of octane sensitivities for each research octane number. This study establishes a correlation between OS and heat release rates at low, intermediate, and high temperatures. The significance and chemical origins of intermediate-temperature heat release in defining the OS of toluene blended in a mixture of iso-octane and n-heptane is also clarified. For the toluene–iso-octane–n-heptane mixtures considered here, low-temperature reactivity was not found to be a key marker of OS. The results also show areas of improved efficiency in beyond RON operating conditions, where high-sensitivity fuels could be beneficial.

Autoignition characterization of methanol, ethanol, propanol, and butanol over a wide range of operating conditions in LTC/HCCI

In this work, the autoignition properties of methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, and sec-butanol are studied in HCCI and compared. Experiments were performed, characterizing the sensitivity of these fuels to equivalence ratio, intake temperature, residual fraction, intake pressure, and engine speed. Under all of the low boost pressure operating conditions considered (1.15 bar), the order of reactivity of each fuel from low to high is: isopropanol, sec-butanol, ethanol ≈ n-propanol ≈isobutanol, methanol, n-butanol. n-butanol was less sensitive to changes in intake temperature than the other six fuels at low boost conditions. The required intake temperature to maintain a constant combustion phasing decreased significantly for n-butanol with intake pressure due to emergent intermediate temperature heat release (ITHR). At an intake pressure of 1.95 bar, sec-butanol showed some amount of ITHR which decreased its sensitivity to intake temperature. However, sec-butanol still maintained its high autoignition resistance, unlike n-butanol. Isobutanol did not show any ITHR at this intake temperature, though did show a decrease in intake temperature sensitivity. With the exception of n-butanol, the behavior and sensitivity of each alcohol fuel to the wide array of operating conditions considered were similar, with small differences in autoignition resistance. This means that these six fuels could potentially be blended and used interchangeably with little change to their autoignition characteristics.

Experimental and modeling study of C2–C4 alcohol autoignition at intermediate temperature conditions

C2–C4 alcohols are advantageous blendstocks identified by many research groups, including the U.S. Department of Energy Co-Optima Initiative, towards enabling efficient, boosted Spark-Ignition (SI) engines. Their use in advanced engine applications requires a comprehensive understanding of their intermediate-temperature autoignition behavior. This work reports an experimental and modeling study covering their fundamental autoignition characteristics in a twin-piston rapid compression machine at pressures of 20 and 40 bar, intermediate temperatures from 750 to 980 K, and two fuel loading conditions representative of boosted SI engines. Direct comparison between these alcohols is made, where the order of reactivity is established across different thermodynamic and fuel loading conditions. Changes in preliminary exothermicity (or intermediate-temperature heat release) displayed in single-stage autoignition across different alcohols and conditions are also quantified. This provides insight into fuel-to-fuel differences, and how these could affect advanced combustion concepts such as spark-assisted compression ignition. Kinetic models are used to simulate the experiments, and reasonable agreement is obtained. The sensitivity analysis results demonstrate the importance of accurately capturing the autoignition kinetics, particularly H-abstraction reactions on the parent fuels by OH and HO2, and the branching ratio associated with these.

Influence of intermediate temperature heat release on autoignition reactivity of single-stage ignition fuels with varying octane sensitivity

This study investigates the effects of intermediate temperature heat release (ITHR) on autoignition reactivity of full boiling range gasolines with different octane sensitivity through intake temperature and simulated exhaust gas recirculation (EGR) sweeps in a homogenous charge compression ignition (HCCI) engine. To isolate the ITHR effects, low temperature reactivity was suppressed through the use of high intake temperature and low intake oxygen mole fraction. For quantification of ITHR, a new method was applied to the engine data by examining the maximum value of the second derivative of heat release rate. Combustion phasing comparisons of fuels with octane sensitivity showed that fuel with less octane sensitivity became more reactive as intake temperature and simulated EGR ratio decreased, while fuel with higher octane sensitivity had a reverse trend. For all of the fuels that were tested, the amount of ITHR increased as the intake temperature and oxygen mole fraction increased. These ITHR trends, depending on octane sensitivity, were almost identical with the trends of combustion phasing, showing that ITHR significantly affects fuel autoignition reactivity and determines octane sensitivity.

Autoignition behavior of a full boiling-range gasoline: Observations in RCM and GCI engine environments

The understanding of, and capability to modulate fuel autoignition behavior are important aspects towards the development of advanced, low temperature combustion (LTC) engine concepts, which have potential to significantly increase engine efficiency while at the same time addressing emissions concerns. Proper combustion phasing and control of heat release rates via, for instance, multiple injections and exhaust gas recirculation, are significant challenges that can vary with fuel composition. Overall reactivity, which is a strong function of the in-cylinder temperature and pressure, is a prime driver for regulating combustion timing, while preliminary exothermicity, in the form of low- and intermediate-temperature heat release (LTHR, ITHR) can inhibit, or facilitate operational success, e.g., control combustion phasing. There are still gaps in understanding these features and how they interact with needs of advanced combustion engines, and how practical fuels can be rated for the new challenges of LTC engines.This work utilizes a rapid compression machine (RCM) and gasoline compression ignition (GCI) engine to probe the autoignition behavior of a California Reformulated Gasoline Blendstock for Oxygenate Blending (CARBOB) (RON = 86, MON = 81.4) under quasi- homogeneous charge compression ignition (qHCCI) operation. A wide range of thermodynamic conditions is explored (Tc = 700–1000 K, Pc = 15–90 bar) where ignition delay times and LTHR/ITHR are quantified. Both stoichiometric, dilute (11% O2) and lean (equivalence ratio (ϕ) = 0.23) fuel loading conditions are considered. The GCI engine is operated under fuel-lean conditions (ϕ = 0.23) using no external exhaust gas recirculation over a range of intake temperature (Tin = 30–70 °C), with the intake pressure adjusted (Pin = 1.2–1.3 bar) to achieve constant combustion phasing (CA50 = −4 °aTDC). Preliminary exothermicity is also quantified under these conditions.The stoichiometric, dilute RCM measurements reveal negative temperature coefficient (NTC) behavior for this fuel and indicate that rates and extents of LTHR and ITHR are both affected by the compressed conditions, with temperature imposing an inverse dependence, while increased pressure causes LTHR to increases, but ITHR to decrease. These are the first experimental quantifications of ITHR trends for a practical fuel under well-controlled, static conditions. When the lean RCM measurements are mapped to the in-cylinder conditions of the engine, it is found that under a constant combustion phasing scenario (as used in the engine), the fuel has little overall temperature sensitivity, while the starts of high temperature heat release (soHTHR) and top dead center (TDC) points seen along the engine compression trajectories correspond to isopleths of ignition delay times τRCM = 6 and 3 ms, respectively. In addition, the points of peak LTHR rate in the engine correspond to an isopleth of first-stage ignition τ1,RCM = 0.50–0.75 ms. Correlating the magnitudes of LTHR and ITHR measured in the RCM to the engine requires quantification of the preliminary exothermicity at the thermodynamic conditions of peat LTHR rate and soHTHR, respectively. The current measurements demonstrate how fundamental data from an RCM can be used to project expected trends in overall fuel reactivity to a qHCCI engine environment, and thus provide a possible avenue for characterizing fuels for LTC engines.

The Influence of Intake Pressure and Ethanol Addition to Gasoline on Single- and Dual-Stage Autoignition in an HCCI Engine

Autoignition in HCCI engines is known to be controlled by the combustion kinetics of the in-cylinder fuel/air mixture which is highly influenced by the amount of low-temperature and intermediate-temperature heat release (LTHR and ITHR) that occurs. At lower intake pressures (typically 1.8 bar absolute) gasoline behaves as a two-stage heat release fuel. Furthermore, ethanol blending into gasoline strongly affects heat release characteristics, and this warrants further investigation. This paper experimentally investigates the conditions under which gasoline transitions from a single-stage heat release fuel to a two-stage heat release fuel as intake pressure is increased. Experiments were performed in single-cylinder HCCI engine fueled with two research-grade gasolines, FACE A and FACE C. These gasolines were tested neat, and with 10% and 20% (by volume) ethanol add...

Heat release analysis for rapid compression machines: Challenges and opportunities

Heat release analysis (HRA) is commonly used in combustion studies to derive understandings of chemical and physical processes in situations where direct measurement is not practical. In internal combustion engines, it is typically based on crank-angle resolved pressure diagnostics. However, it has not been applied extensively to rapid compression machine datasets. There are various challenges associated with rigorous application of HRA, including a reasonable accounting of physical processes that occur during the test period, such as heat loss. Limitations associated with transducer robustness and data acquisition system fidelity also exist. On the other hand, there is potential to extract a wealth of information from pressure-time histories via HRA, such as quantifying the evolution and trends of preliminary exothermicity, e.g., low- and intermediate-temperature heat release, across a range of thermodynamic conditions; detecting the existence of non-uniform ignition phenomena during a test; and providing additional targets for the evaluation and improvement of chemical kinetic models. This work discusses such opportunities, and some approaches towards resolving various challenges.

Optical diagnostics on the reactivity controlled compression ignition (RCCI) with micro direct-injection strategy

Micro direct-injection (DI) strategy is often used to extend the operation range of the reactivity controlled compression ignition (RCCI) to high engine load, but its combustion process has not been well understood. In this study, the ignition and flame development of the micro-DI RCCI strategy were investigated on a light-duty optical engine using formaldehyde planar laser-induced fluorescence (PLIF) and high-speed natural flame luminosity imaging techniques. The premixed fuel was iso-octane and an oxygenated fuel of polyoxymethylene dimethyl ethers (PODE) was employed for DI. The fuel-air equivalence ratio of DI was kept at 0.09 and the premixed equivalence ratio was varied from 0 to 1. RCCI strategies with early and late DI timing at –25° and –5° crank angle after top dead center were studied, respectively. Results indicate that the early micro-DI RCCI features a single-stage high-temperature heat release (HTHR). The combustion in the low-reactivity region shows a combination of flame front propagation and auto-ignition. The late micro-DI RCCI presents a two-stage HTHR. The second-stage HTHR is owing to the combustion in the low-reactivity region that is dominated by flame front propagation when the premixed equivalence ratio approaches 1. For both early and late micro-DI RCCI, the intermediate-temperature heat release (ITHR) of iso-octane, indicated by formaldehyde, takes place in the low-reactivity region before the arrival of the flame front. This is quite different from the flame front propagation in spark-ignition (SI) engine that shows no ITHR in the unburned region. The DI fuel mass is a key factor that affects the combustion in the low-reactivity region. If the DI fuel mass is quite low, there is more possibility of flame front propagation; otherwise, sequential auto-ignition dominates. The emergence of the flame front propagation in micro-DI RCCI strategy reduces its combustion rate and peak pressure rise rate.

Influence of a non-equilibrium discharge impact on the low temperature combustion stage in the HCCI engine

A method for the combustion organisation in the cylinder of an HCCI engine is proposed. This method is based on the selective impact of electrical discharge in the form of a high frequency corona. The discharge is switched on at a certain crank angle and affects the mixture for a short time (a few rotation degrees of the crankshaft). The aim of the given study was to clarify how the discharge affects the low temperature combustion (LTC) stage to control the inflammation in an HCCI engine. It was shown that the best discharge impact timing is the beginning of the low temperature heat release (LTHR) or intermediate temperature heat release (ITHR) stages. The discharge does not directly ignite the mixture but only “pushes” the beginning of these stages and promotes their more rapid passing, controlling the heat release. It is mostly due to the dissociation of fuel and oxidiser, instead of heating the area of impact. Consequently, a non-equilibrium discharge can control the combustion timing, which is especially important in the case of a cold start or use of a lean mixture that is characterised by misfiring. It has been shown that the combustion timing after the top dead centre depends on the energy deposited into the injected mass of the fuel–air mixture treated by the discharge and the discharge switching on time. The dependence of the concentrations of NOx, CO, and unburned hydrocarbons on a specific deposited energy and the initial conditions is presented. The chemical kinetic mechanism of a selective discharge effect is revealed. The numerical simulation has been performed for the equivalence ratio ϕ = 0.33–0.5 and the intake temperature Tin = 360–400 K in a propane-air mixture.

Pressure and temperature effects on fuels with varying octane sensitivity at high load in SI engines

The octane sensitivity (S), defined as the difference between the Research Octane Number (RON) and the Motor Octane Number (MON), is of increasing interest in spark ignition (SI) engines because of its relevance to knock resistance at boosted high-load conditions. In this study, a set of three fuels was designed to hold RON nearly constant (RON=99.2–100) and to vary S (S=0, 6.5, and 12). These fuels were operated at the knock-limited spark advance (KLSA) at nominal engine loads of 10, 15, and 20bar indicated mean effective pressure (IMEP) in a single cylinder SI engine with side-mounted direct injection fueling, at λ=1 stoichiometry. At each load condition, the intake manifold temperature was swept from 35°C to 95°C to alter the temperature and pressure history of the charge. At the 10bar IMEP condition, knock resistance was inversely proportional to S, with the S=0 fuel being the most knock resistant. As load increased the trend reversed and knock resistance became proportional to fuel S, with the S=12 fuel being the most knock resistant. The fuel S knock resistance reversal with load is attributed to changing fuel ignition delay. At increased load, intermediate temperature heat release (ITHR) for the S=0 fuel was observed several crank angles prior to spark command, and ITHR magnitude was proportional to intake temperature. As intake temperature continued to increase, the S=0 fuel transitioned from ITHR to low-temperature heat release (LTHR) prior to spark command. At the highest load and intake temperature, 20bar IMEP and 95°C, the S=0 fuel exhibited distinct LTHR and negative temperature coefficient (NTC), and the intermediate S value fuel (S=6.5) exhibited distinct ITHR behavior several crank angles prior to spark command. However, for all tested conditions, the S=12 fuel exhibited neither ITHR nor LTHR. To understand the measured trends, chemical kinetic modeling was used to elucidate the fuel-specific dependencies on in-cylinder pressure–temperature history. An island of low temperature reactivity was identified at temperatures between 700K and 825K and at pressures higher than 17bar. The size and magnitude of this island was found to be fuel-specific, decreasing with increasing S. The combined findings illustrate the commonality and utility of fuel S, ITHR, LTHR, and NTC across a wide range of conditions and the associated implications of fuel S in boosted modern gasoline direct injection SI engines relative to the RON and MON tests.

Compositional effects on the ignition of FACE gasolines

As regulatory measures for improved fuel economy and decreased emissions are pushing gasoline engine combustion technologies towards extreme conditions (i.e., boosted and intercooled intake with exhaust gas recirculation), fuel ignition characteristics become increasingly important for enabling stable operation. This study explores the effects of chemical composition on the fundamental ignition behavior of gasoline fuels. Two well-characterized, high-octane, non-oxygenated FACE (Fuels for Advanced Combustion Engines) gasolines, FACE F and FACE G, having similar antiknock indices but different octane sensitivities and chemical compositions are studied. Ignition experiments were conducted in shock tubes and a rapid compression machine (RCM) at nominal pressures of 20 and 40atm, equivalence ratios of 0.5 and 1.0, and temperatures ranging from 650 to 1270K. Results at temperatures above 900K indicate that ignition delay time is similar for these fuels. However, RCM measurements below 900K demonstrate a stronger negative temperature coefficient behavior for FACE F gasoline having lower octane sensitivity. In addition, RCM pressure profiles under two-stage ignition conditions illustrate that the magnitude of low-temperature heat release (LTHR) increases with decreasing fuel octane sensitivity. However, intermediate-temperature heat release is shown to increase as fuel octane sensitivity increases. Various surrogate fuel mixtures were formulated to conduct chemical kinetic modeling, and complex multicomponent surrogate mixtures were shown to reproduce experimentally observed trends better than simpler two- and three-component mixtures composed of n-heptane, iso-octane, and toluene. Measurements in a Cooperative Fuels Research (CFR) engine demonstrated that the multicomponent surrogates accurately captured the antiknock quality of the FACE gasolines. Simulations were performed using multicomponent surrogates for FACE F and G to reveal the underlying chemical kinetics linking fuel composition with ignition characteristics. A key discovery of this work is the kinetic coupling between aromatics and naphthenes, which affects the radical pool population and thereby controls ignition.

Experimental investigation of butanol isomer combustion in Homogeneous Charge Compression Ignition (HCCI) engines

Longer chain alcohols, such as butanol, possess major physiochemical advantages over ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. In this study, two butanol isomers (n-butanol and isobutanol) are investigated as potential fuels for Homogeneous Charge Compression Ignition (HCCI) engines. Wide ranges of intake pressure and equivalence ratio are investigated and the results are presented in comparison to ethanol and gasoline as reference fuels. Under all tested conditions, the butanol isomers require lower intake temperatures for a fixed combustion phasing, indicating higher HCCI reactivity. Both isomers show single-stage ignition behavior at all test points and behave similarly in regard to the combustion stability. Engine operation using n-butanol is slightly more stable under all conditions and misfiring occurs slightly later under very lean and naturally aspirated conditions. Similar to gasoline, n-butanol shows a higher heat release rate (HRR) at the beginning of combustion. The intermediate temperature heat release (ITHR) lowers the coefficient of variation (CoV) of IMEPg (gross indicated mean effective pressure), especially at retarded combustion timing and lean mixtures. However, the knock resistance of n-butanol is lower compared to isobutanol and the other tested fuels. The exhaust emissions of the two butanol isomers are in the same range as the two reference fuels. Overall, the results indicate that butanol is suited for use as a fuel in HCCI engines, either in neat form or in blend with gasoline.

Spectroscopic analysis of the phases of premixed combustion in a compression ignition engine fuelled with diesel and ethanol

The premixed charge compression ignition (PCCI) combustion represents a possible solution for decreasing the pollution with respect to diesel engine, while maintaining the efficiency rate at values that are comparable, and in some cases higher, than those of a diesel engine. This paper investigates the operation of an optical-access compression ignition engine (bore: 82mm, stroke: 90mm) running at PCCI combustion with neat bio-ethanol and European commercial diesel fuel injected in the intake manifold and into the cylinder, respectively. In its original configuration, the engine burned diesel and this case was used as reference of compression ignition combustion. Then, different amounts of bio-ethanol were injected varying the energizing time of the injector set in the intake manifold. This allowed to create PCCI combustion with high levels of pre-combustion mixing, and to ensure low equivalence ratio and low flame temperatures too. Moreover, both the amount and the start of diesel injection was varied to investigate their effects on the several combustion phases. UV–Visible imaging and spectroscopic measurements were performed in the engine and the autoignition of the charge, the combustion process and the chemical species involved were detected and analysed. In particular, optical diagnostics allowed to observe how the mixture burned: no luminous flame emission in the visible range was recorded; while in the ultraviolet wavelength range numerous species, like HCO, HCOH, OH, and CO and others were detected. Varying the in-cylinder premixed ratio the combustion retarded and the rate of heat release passed from single to three-phase premixed combustion, so revealing a phase due to intermediate temperature reactions. The same behavior was observed varying the start of injection and the quantity of diesel that affected the premixed ratio. Spectroscopic measurements revealed that during the intermediate temperature heat release large amount of OH radical governed the start of combustion of the charge. It was also observed that the preignition combustion was mainly due to the stratified mixing of the diesel fuel close to the bowl wall. Finally the presence of OH radical was monitored for the whole combustion process.

Computational study of the pressure dependence of sequential auto-ignition for partial fuel stratification with gasoline

Fuel stratification is a potential strategy for reducing the maximum pressure rise rate in HCCI engines. Simulations of Partial Fuel Stratification (PFS) have been performed using CONVERGE with a 96-species reduced mechanism for a 4-component gasoline surrogate. Comparison is made to experimental data from the Sandia HCCI engine at a compression ratio 14:1 at intake pressures of 1bar and 2bar. Analysis of the heat release and temperature in the different equivalence ratio (φ) regions reveals that sequential auto-ignition of the stratified charge occurs in order of increasing φ for 1bar intake pressure but in order of decreasing φ for 2bar intake pressure. Increased low- and intermediate-temperature heat release at 2bar intake pressure compensates for decreased temperatures in higher-φ regions due to evaporative cooling from the liquid fuel spray and decreased compression heating from lower values of the ratio of specific heats. At 1bar intake pressure, the premixed portion of the charge auto-ignites before the highest-φ regions and the sequential auto-ignition occurs too fast for useful reduction of the maximum pressure rise rate compared to HCCI. Conversely, at 2bar intake pressure, the premixed portion of the charge auto-ignites last, after the higher-φ regions. More importantly, the sequential auto-ignition occurs over a longer time period than at 1bar intake pressure such that a sizable reduction in the maximum pressure rise rate compared to HCCI can be achieved.

Intermediate temperature heat release in an HCCI engine fueled by ethanol/n-heptane mixtures: An experimental and modeling study

This study examines intermediate temperature heat release (ITHR) in homogeneous charge compression ignition (HCCI) engines using blends of ethanol and n-heptane. Experiments were performed over the range of 0–50% n-heptane liquid volume fractions, at equivalence ratios 0.4 and 0.5, and intake pressures from 1.4bar to 2.2bar. ITHR was induced in the mixtures containing predominantly ethanol through the addition of small amounts of n-heptane. After a critical threshold, additional n-heptane content yielded low temperature heat release (LTHR). A method for quantifying the amount of heat released during ITHR was developed by examining the second derivative of heat release, and this method was then used to identify trends in the engine data. The combustion process inside the engine was modeled using a single-zone HCCI model, and good qualitative agreement of pre-ignition pressure rise and heat release rate was found between experimental and modeling results using a detailed n-heptane/ethanol chemical kinetic model. The simulation results were used to identify the dominant reaction pathways contributing to ITHR, as well as to verify the chemical basis behind the quantification of the amount of ITHR in the experimental analysis. The dominant reaction pathways contributing to ITHR were found to be H-atom abstraction from n-heptane by OH and the addition of fuel radicals to O2.