Figure 3
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An index to relate fuel properties to HCCI auto-ignition would be valuable to predict the performance of fuels in HCCI engines from their properties and composition. The indices for SI engines, the Research Octane Number (RON) and Motor Octane Number (MON) are known to be insufficient to explain the behavior of oxygenated fuels in an HCCI engine. O...
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... auto-ignition temperature was extracted from the start of combustion described above. Corresponding auto-ignition temperatures for fuel T1 are shown by black circles in Figure 2 and Figure 3. The blue squares mark the maximum rate of heat release for the LTHR and the red triangles mark the minimum rate of heat release between LTHR and the main combustion. ...
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Citations
... To identify the conditions at which autoignition occurs for a given air/fuel mixture in SI without interference from flame propagation, stoichiometric autoignition experiments should be conducted. However, the excessive pressure gradient could result in the breakdown of the engine; so, instead of running at stoichiometric conditions, understanding fuel combustion behavior could be achieved by running with very lean mixtures, i.e. running under homogeneous charge compression ignition [18,19,20,21]. Such an approach, with the pressure-temperature diagram, was conducted by the authors [22,23]. ...
... The high intake temperatures presented in the current study were not expected to result in large amounts of low temperature reactions. Low temperature reactions are also expected to decrease with increased ethanol content [17,21,22,23]. The amount of low temperature reactions seen in these experiments were visible in the heat release rate, but are too small to be accurately quantified. ...
... Ethanol is commonly blended with gasoline in part because of its low autoignition reactivity and high heat of vaporization, which together result in ethanol's relatively high octane number. Octane number measurements have been conducted for ethanol blended with different commercial gasolines [9,10], primary reference fuels (PRFs) (mixtures of n-heptane and isooctane) [4,[11][12][13], and toluene reference fuels (TRFs) (mixtures of PRF and toluene) [4,[11][12][13]. Significant non-linear blending has been reported in some cases. ...
... Ethanol is commonly blended with gasoline in part because of its low autoignition reactivity and high heat of vaporization, which together result in ethanol's relatively high octane number. Octane number measurements have been conducted for ethanol blended with different commercial gasolines [9,10], primary reference fuels (PRFs) (mixtures of n-heptane and isooctane) [4,[11][12][13], and toluene reference fuels (TRFs) (mixtures of PRF and toluene) [4,[11][12][13]. Significant non-linear blending has been reported in some cases. ...
... The development data is made up of ON measurements from ASTM standards for TRFs [1,2], Morgan et al. for TRFs [14], Knop et al. for TRFs [15] and Foong et al. [4] for TRF/ethanol mixtures. The validation data is made up of ON measurements from the present work for TRFs and Lund for TRFs and TRF/ethanol mixtures [11][12][13]. It is noted that all of this data is restricted to fuels with a RON between 80 and 120, which spans the ONs of plausible, production gasolines. ...
... In [18] Shibata et al. noted that a small change in chemical composition can change the HCCI combustion characteristics, such as the amount and phasing of LTR. Ethanol and toluene have a quenching effect on LTR and it is strongest for ethanol [19]. ...
The current research focus on fuel effects on low temperature reactions (LTR) in Homogeneous Charge Compression Ignition (HCCI) and Partially Premixed Combustion (PPC). LTR result in a first stage of heat release with decreasing reaction rate at increasing temperature. This makes LTR important for the onset of the main combustion. However, auto-ignition is also affected by other parameters and all fuel does not exhibit LTR. Moreover, the LTR does not only depend on fuel type but also on engine conditions. This research aims to understand how fuel composition affects LTR in each type of combustion mode and to determine the relative importance of chemical and physical fuel properties for PPC. For HCCI the chemical properties are expected to dominate over physical properties, since vaporization and mixing are completed far before start of combustion. The HCCI experiments were carried out in a Cooperative Fuel Research (CFR) engine, while the PPC experiments were carried out in a single cylinder high speed direct injected (HSPDI) diesel engine. A Gaussian profile was fitted to the data and used to determine the fraction of LTR for each type of combustion. The fuels used in this study are blends of ethanol, n-heptane, and isooctane (ERF), blends of toluene, n-heptane, and isooctane (TRF), and blends of toluene, ethanol, n-heptane and isooctane (TERF). The fractions of ethanol and toluene were varied at three levels to examine the influence on LTR for both HCCI and PPC. The results showed that increasing ethanol and toluene concentration in the ERF and TERF blends increases the LTR fraction in PPC while they decreased the LTR fraction for HCCI combustion. Ethanol had a stronger impact than toluene. Increasing isooctane and decreasing n-heptane concentration increased the LTR fraction for PPC while it decreased LTR fraction for HCCI. The lack of agreement between fuel effects in PPC and HCCI indicate that the processes behind LTR are more complex in PPC than in HCCI. It is not certain that a fuel with more pronounced chemical prerequisites for LTR will produce more LTR. The strong relation between LTR and ID for PPC indicate that the ignition quality is central for the fraction of LTR in PPC.
... In previous work by the authors [5,6,7], pressure effect on auto-ignition temperatures was studied. This work extends this analysis to include the effect of engine speed. ...
Homogeneous charge compression ignition (HCCI) is a promising concept that can be used to reduce NO x and soot emissions in combustion engines, keeping efficiency as high as for diesel engines. To be able to accurately control the combustion behavior, more information is needed about the auto-ignition of fuels. Many fuels, especially those containing n-paraffins, exhibit pre-reactions before the main heat release event, originating from reactions that are terminated when the temperature in the cylinder reaches a certain temperature level. These pre-reactions are called low temperature heat release (LTHR), and are known to be affected by engine speed. This paper goes through engine speed effects on auto-ignition temperatures and LTHR for primary reference fuels. Earlier studies show effects on both quantity and timing of the low temperature heat release when engine speed is varied. In this study, these effects are further explored by looking at the auto-ignition temperatures and the pressure and temperature evolution in the cylinder.
Four primary reference fuels (PRF, blends of n-heptane and iso-octane) were used, from PRF70 to PRF100. All fuels were tested in a CFR engine with variable compression ratio, running in HCCI operation. Engine speed was varied from 600 to 1200 rpm. An equivalence ratio of 0.33 was used, and a constant combustion phasing of 3 degrees after TDC was maintained by changing the compression ratio for each operating point. Different pressure and temperature evolutions were achieved by varying the inlet air temperature in three steps from 50°C to 150 °C.
At higher engine speeds the LTHR decreased or disappeared. Auto-ignition temperature increased at higher engine speeds due to the shorter residence time in the LTHR temperature zone. The temperature range where LTHR was detected was shifted to higher temperatures with increased engine speed.
... All emission numbers are based on 50 subsequent measurement points with an interval of about a second between each. The heat release calculations are described in more detail in [12,13]. ...
HCCI combustion can be enabled by many types of liquid and gaseous fuels. When considering what fuels will be most suitable, the emissions also have to be taken into account. This study focuses on the emissions formation originating from different fuel components. A systematic study of over 40 different gasoline surrogate fuels was made. All fuels were studied in a CFR engine running in HCCI operation. Many of the fuels were blended to achieve similar RON's and MON's as gasoline fuels, and the components (n-heptane, iso-octane, toluene, and ethanol) were chosen to represent the most important in gasoline; nparaffins, iso-paraffins, aromatics and oxygenates. The inlet air temperature was varied from 50°C to 150°C to study the effects on the emissions. The compression ratio was adjusted for each operating point to achieve combustion 3 degrees after TDC. The engine was run at an engine speed of 600 rpm, with ambient intake air pressure and with an equivalence ratio of 0.33. NO x emissions were low for all operating points, and ethanol and toluene addition was found to decrease NOx emissions for higher octane fuels. CO emissions were related to in-cylinder temperature by formation from HC and oxidation to CO2. HC emissions were found to be mainly dependent on the compression ratio used in each case. Ethanol addition was shown to reduce HC emissions for combustion at a given compression ratio. Toluene addition was found to increase HC emissions at a given CR when high concentrations (40-60 vol.%) were added. Low concentrations of toluene showed a small decrease of or no effect on HC emissions.
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.
Stable HCCI combustion requires a proper level of fuel reactivity. This study shows that adding ethanol as a renewable fraction to low octane gasoline decreases the reactivity of the gasoline, while adding 2-ethyl hexyl nitrate (2-EHN) can enhance the reactivity of the blend and counter the effect of ethanol.
The experimental apparatus consisted of a modified CFR engine for HCCI combustion equipped with two port fuel injectors and an intake air heater. Gasoline blended with ethanol (10% v/v) was used as the base fuel. Different percentages of 2-EHN (0.25%, 0.50%, 1%, and 2.5%) were added to the base fuel as an ignition improver. The blends were tested at operating points defined for HCCI number at two different engine speeds (600 and 900 rpm) and three different intake temperatures (50, 100, and 150 °C) to investigate the effect of 2-EHN on the auto-ignition behavior of the fuel.
Combustion, emissions, and performance parameters of HCCI combustion of the blends were measured. The presence of 2-EHN in the blends improved the auto-ignitability of the blends in a nonlinear manner. It was also found that 0.25% of 2-EHN can compensate for the effect of ethanol on the required compression ratio and remove the quenching effect of ethanol on low temperature heat release. The results show that for the same fuel, a higher compression ratio is needed to maintain the combustion phasing constant at a higher engine speed.
Petroleum-derived gasoline is currently the most widely used fuel for transportation propulsion. The design and operation of gasoline fuels is governed by specific physical and chemical kinetic fuel properties. These must be thoroughly understood in order to improve sustainable gasoline fuel technologies in the face of economical, technological, and societal challenges. For this reason, surrogate mixtures are formulated to emulate the thermophysical, thermochemical, and chemical kinetic properties of the real fuel, so that fundamental experiments and predictive simulations can be conducted. Early studies on gasoline combustion typically adopted single component or binary mixtures (n-heptane/isooctane) as surrogates. However, the last decade has seen rapid progress in the formulation and utilization of ternary mixtures (n-heptane/isooctane/toluene), as well as multicomponent mixtures that span the entire carbon number range of gasoline fuels (C4–C10). The increased use of oxygenated fuels (ethanol, butanol, MTBE, etc.) as blending components/additives has also motivated studies on their addition to gasoline fuels. This comprehensive review presents the available experimental and chemical kinetic studies which have been performed to better understand the combustion properties of gasoline fuels and their surrogates. Focus is on the development and use of surrogate fuels that emulate real fuel properties governing the design and operation of engines. A detailed analysis is presented for the various classes of compounds used in formulating gasoline surrogate fuels, including n-paraffins, isoparaffins, olefins, naphthenes, and aromatics. Chemical kinetic models for individual molecules and mixtures of molecules to emulate gasoline surrogate fuels are presented. Despite the recent progress in gasoline surrogate fuel combustion research, there are still major gaps remaining; these are critically discussed, as well as their implications on fuel formulation and engine design.
In low temperature combustion (LTC) engines, premixed fuel-air mixture is created in the cylinder, and combustion starts by auto-ignition due to compression of the fuel–air mixture during compression stroke. The LTC process involves various physical processes (atomization, evaporation and mixing) and complex chemical reactions occurring in the cylinder. Fuel properties and fuel composition play an important role in all the physical and chemical processes involved in LTC process. Autoignition depends on the evolution of cylinder pressure and temperature with time and autoignition chemistry of the fuel–air mixture. Autoignition chemistry depends on the fuel composition and fuel-air mixture quality (equivalence ratio). This chapter discusses the autoignition characteristics (autoignition chemistry, impact of fuel molecular structure on autoignition, fuel autoignition quality), fuel effects on autoignition and several fuel indices developed for LTC engines. Fuel design and fuel properties/quality required for LTC engines are also discussed in the present chapter.
