Optical diagnostics of early flame development in a DISI (direct injection spark ignition) engine fueled with n-butanol and gasoline
Introduction
Bio-fuels have the potential to mitigate net global CO2 emissions and also to reduce the consumption of fossil fuels [1]. When considering the two main categories of internal combustion engines (i.e. spark- and compression–ignition), alcohols, bio-methane and biodiesel are the main fuels used on a wide scale [2]. Other types of energy sources can be used with certain modifications, given that they feature specific properties which can differ significantly from those of fossil fuels [3], [4]. Their direct application in existing power units is in most cases difficult, mainly due to compatibility and stability issues [5].
Alcohols can be used as fuels for SI (spark ignition) engines due to their good antiknock quality, high flame velocity and the presence of oxygen in their molecule that results in lower emissions [6]. Amongst alcohols, butanol is an emerging renewable fuel which can be produced from biological sources [7]. It has several advantages over methanol and ethanol, including high tolerance to water contamination, reduced corrosive action on aluminum or polymer fuel system components, ability to blend in gasoline [8] or diesel [9], [10] at high fraction without modifying vehicles and better fuel economy due to higher energy density. However, the primary disadvantage of biobutanol is its quite low production rate; this needs to be put into perspective with regard to the actual conversion rates from biomass feedstock, which is paramount for biofuels in general [11], [12]. For this reason, many research groups and biotechnology companies studied several attempts to increase the butanol yield of the process to improve the economics [13].
Applications of n-butanol in SI engines have been investigated by several research groups during the last two decades. Pioneer works [14], [15], [16], [17] were carried out using medium low percentages of butanol in gasoline (30% by volume). The objective was to gain information on performance and emissions and the conclusion was that butanol blends allow increased engine thermal efficiency in agreement with the relative improvement in octane over gasoline. Later on, high blending ratios with gasoline have been tested [18], as well as the effect of exhaust gas recirculation [19]. More comprehensive studies that involved flame characterization through digital imaging revealed that boosted operation featured similar flame propagation and diffusive combustion near the valves region, with gasoline and its mixture with the alcohol [20]; localized fuel film also had an effect on knocking [21]. It has been found that when proper fuel evaporation can be ensured, the higher butanol laminar flame speed compared to gasoline [22] results in faster flame development [23] and improved performance [24], [25]. The effect of butanol on regulated and non-regulated emissions was found to be comparable to that of ethanol [26] and special control strategies such as dual injection [27] can be used for further reducing unburned hydrocarbon concentrations in the exhaust gas stream. Also, the flexibility of direct injection can be employed for controlling mixture stratification to a certain degree, and strategies developed for gasoline [28] can be applied to pure butanol fueling [29] with improvements in stability and reduction of emissions. As a general conclusion, the influence of butanol on pollutants emission and combustion efficiency is highly conditioned by engine type (i.e. port- or direct-injection) and operating conditions examined. More detailed analyses that also made use of optical investigations revealed that flame propagation does correlate with the basic fuel property of laminar flame speed [23] but also with local fuel concentration [29], suggesting a continuous process of flame-liquid fuel film interaction during combustion in DI (direct injection) SI engines. At low load the effect of fuel evaporation, as well as that of injection on fluid velocity and turbulent length scales was relatively reduced and chemical reaction rates were the dominant discriminant [30]. This needs to be put in the perspective of fuel break-up during injection, which was found to be quite similar when comparing butanol to gasoline [31], [32]; this further points to the fact that evaporation and impingement on the piston crown and cylinder liner needs to be considered during engine operation at increased injected quantities.
In this context, even if the use of alcohols for replacing gasoline in the SI engines has been widely investigated, there is a need to better understand the effects of butanol use, and in particular the influence of engine operating conditions on performance and specific combustion phenomena. To this end, an experimental investigation was performed on a transparent DISI engine fueled with butanol and gasoline, for two intake pressure settings (representative for low and high load), with stoichiometric and lean air–fuel mixtures. Emphasis was put on the effect of load, with regard to injected fuel quantity, which was found to be one of the main influences on the differences observed between the two fuels. High spatial and temporal resolution visualization was supported by well-known in–cylinder pressure measurements. Particular attention was drawn on flame characteristics and how they are correlated with overall thermodynamic parameters. Macro- and micro-scale flame parameters were evaluated for the different conditions, further underlying the effect of air–fuel mixture preparation on combustion development. Fuel properties were thus more clearly identified as paramount for efficient engine operation for various load settings and control strategies.
Section snippets
Experimental setup and procedure
Measurements were performed in an optically accessible single cylinder wall guided DISI engine (Fig. 1). It is equipped with the head of a commercial SI turbocharged engine with similar geometrical specifications (bore, stroke, compression ratio). A conventional elongated Bowditch piston is used [33], with a flat quartz 18 mm thick window providing a 68.5 mm diameter field of view. The combustion chamber is visible through an UV enhanced 45-degree mirror, mounted within the hollow piston.
Results and discussion
Pressure traces averaged on 200 cycles were analyzed for both fuels and the three load settings (Fig. 2). Higher peak pressure is evident for the alcohol with throttled operation. At WOT, on the other hand, peak pressure was higher for gasoline, as was the case in the final stages of combustion; this effect was more pronounced for lean conditions.
Fig. 3 reports several thermodynamic parameters; with related MFB (mass fraction burned) determined using Rassweiler & Withrow methodology [41].
Conclusion
High spatial resolution UV–visible visualization was applied in an optically accessible SI engine to investigate the potentiality of n-butanol as gasoline replacement at low speed for different load and air–fuel mixture conditions. Experiments demonstrated that gasoline can be replaced with the alcohol fuel with no penalties on performance during throttled, stoichiometric operation. With wide open throttle, the increase in indicated mean effective pressure was reverted, with gasoline providing
Acknowledgments
The authors are grateful to Mr. Alfredo Mazzei for the technical support for the experiments and to Eng. Stefano Valentini for the engine design.
References (49)
100% sustainable energy
Energy
(2014)- et al.
A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines
Renew Sustain Energy Rev
(2015) Internal combustion engines: progress and prospects
Renew Sustain Energy Rev
(2014)- et al.
Deterioration of automotive rubbers in liquid biofuels: a review
Renew Sustain Energy Rev
(2015) - et al.
Review of scientific research regarding PPO, tallow and RVO as diesel engine fuel
Fuel
(2015) - et al.
Experimental studies on the effect of injection timing in a SI engine using dual injection of n-butanol and gasoline in the intake port
Fuel
(2014) - et al.
King Grass: a promising material for the production of second-generation butanol
Fuel
(2015) - et al.
Technical feasibility study of butanol–gasoline blends for powering medium-duty transportation spark ignition engine
Renew Energy
(2015) - et al.
Study on performance and emissions of a passenger-car diesel engine fueled with butanol–diesel blends
Energy
(2013) - et al.
Influence of properties of various common bio-fuels on the combustion and emission characteristics of high-speed DI (direct injection) diesel engine: vegetable oil, bio-diesel, ethanol, n-butanol, diethyl ether
Energy
(2014)
State of the art of biofuels from pure plant oil
Renew Sustain Energy Rev
Underlying factors to consider in improving energy yield from biomass source through yeast use on high-pressure homogenizer (hph)
Energy
Progress in the production and application of n-butanol as a biofuel
Renew Sustain Energy Rev
Butanol-a single-cylinder engine study: availability analysis
Appl Therm Eng
NOx emission from a spark ignition engine using 30% iso-butanol–gasoline blend: Part 1 – preheating inlet air
Appl Therm Eng
NOx emission from a spark ignition engine using 30% iso-butanol–gasoline blend: Part 2 – ignition timing
Appl Therm Eng
Fuel conversion efficiency of a port injection engine fueled with gasoline–isobutanol blends
Energy
Emission characteristics of a spark-ignition engine fuelled with gasoline-n-butanol blends in combination with EGR
Fuel
Optical diagnostics of the combustion process in a PFI SI boosted engine fueled with butanol–gasoline blend
Energy
In-cylinder spectroscopic measurements of knocking combustion in a SI engine fuelled with butanol–gasoline blend
Energy
An experimental and mechanistic study on the laminar flame speed, Markstein length and flame chemistry of the butanol isomers
Combust Flame
Characterisation of flame development with ethanol, butanol, iso-octane, gasoline and methane in a direct-injection spark-ignition engine
Fuel
Combustion of n-butanol in a spark-ignition IC engine
Fuel
Combustion process investigations in an optically accessible DISI engine fuelled with n-butanol during part load operation
Renew Energy
Cited by (28)
Valorization opportunities and adaptability assessment of algae based biofuels for futuristic sustainability-A review
2023, Process Safety and Environmental ProtectionImpact of coolant temperature on the combustion characteristics and emissions of a stratified-charge direct-injection spark-ignition engine fueled with E30
2022, FuelCitation Excerpt :Second, less fuel evaporation is harmful to the distribution of the mixture in the combustion chamber. According to previous work, differences in evaporation performance can affect the stratification of the mixture, resulting in different early flame morphology [60]. Due to the faster flame growth speed of ethanol-gasoline blends competed to pure gasoline [61], it is possible that the combustion process of E30 may become more sensitive to the quality of mixture formation since the flame can propagate faster from the spark plug into regions of ongoing fuel–air mixing.
Bio-butanol as a new generation of clean alternative fuel for SI (spark ignition) and CI (compression ignition) engines
2020, Renewable EnergyCitation Excerpt :Regalbuto et al. [69] investigated the butanol isomer combustion in SI engines. In Ref. [69], three isomers (bio-butanol, iso-butanol, and sec-butanol) of four butanol isomers (bio-butanol, iso-butanol, and sec-butanol) were used. The three isomers were mass-mixed using 30% butanol and 70% gasoline as raw materials.