Fuel conversion efficiency of a port injection engine fueled with gasoline–isobutanol blends

Abstract

This paper describes the comparative advantages of using isobutanol as a fuel for SI (spark ignition) engines instead of ethanol. An experimental study of fuel conversion efficiency was performed on a port injection engine fueled with mixtures containing 10, 30 and 50% isobutanol blended with gasoline. Efficiency as well as performance levels were maintained within acceptable limits for all three types of fuel blends compared to running the engine on straight gasoline. These results show that isobutanol is an attractive drop-in fuel for SI engines, and can be blended with gasoline in much higher concentrations compared to ethanol, without any modifications to the fuel system or other engine components.

Introduction

SI (spark ignition) engines feature lower fuel conversion efficiency compared to compression ignition power units, but several improvements can be obtained by employing different control strategies. WOT (wide open throttle) operation combined with cooled EGR (exhaust gas recirculation) was found to ensure an important reduction of specific fuel consumption compared to throttled operation, at the same level of performance [1]. Retarded ignition, combined with throttling control during part load operation was used to maximize exhaust heat recovery while maintaining high conversion efficiency by reducing pumping losses [2]. A double injection strategy combined with an increased injection pressure was found to increase fuel conversion efficiency and reduce particulate emissions for a port injection engine working at WOT [3]. Part load fuel conversion efficiency was improved by reducing the pumping losses of a direct injection SI engine, using high levels of internal EGR combined with a modified two stage injection strategy [4]. New working cycles, similar to so-called ‘cylinder cut-off’ operation, were developed with the aim of improving part load fuel conversion efficiency [5].

Alternative fuels use in SI engines is also intensely studied. Measurements conducted on an experimental engine showed a similar level of combustion stability for gasoline and butanol during part load operation [6]. A significant problem with ethanol, as with all alcohols used as fuels, is the high latent heat of vaporization, causing poor cold start performance and high unburned HC (hydrocarbons) emissions [7]. Low concentrations of butanol blended with gasoline used in direct injection SI engines were found to ensure similar performance levels and emissions, when compared to low concentrations of ethanol mixed with gasoline, and showed only a slight drop in fuel conversion efficiency [8]. Other alternative fuels such as coal-bed gas require special strategies for ignition control as a result of varying gas composition [9], while the addition of hydrogen to natural gas used as a fuel in SI engines was found to greatly improve combustion stability during lean operation [10].

Governments around the world have been constantly increasing the minimum level of renewable transportation fuels blended into commercial gasoline [11], [12]. However, biofuels sustainability is an issue that is far from being resolved, with discussions covering rising food prices and land availability [13], [14], unfavorable energy output–input ratios [15], environmental and social topics [16]. An upscale in production could improve the energy related parameter [17], [18], but the overall benefits of employing large scale biofuels production could prove to be insignificant [19]. The actual level of carbon dioxide emissions reduction when producing and using biofuels is also an issue, as numerous factors must be considered when undertaking such evaluations [20]. Even if these debates are still ongoing, given the legal framework that mandates the use of renewable energy sources, already in place in many regions, with strategies spanning beyond the next decade [11], there is a need to study the effects of using biofuels in the transport sector. Ethanol seems to be the biofuel of choice for SI engines. However, given its higher heating value (Table 1) and very low level of corrosion, isobutanol is much better suited as a drop-in fuel compared to ethanol. Also, this four carbon alcohol is far less miscible with water, thus eliminating the risk of contamination with humidity contained in ambient air.

As with all alcohols, cold start performance is significantly reduced when the engines are operated in cold weather. Isobutanol has a very low saturation pressure, and air-fuel mixtures cannot be ignited at low temperatures, making it impossible to start the engine if ambient temperatures drop below ∼20 °C [22], [23]. While this is a disadvantage when compared to ethanol, isobutanol’s low saturation pressure eliminates the problem of increased RVP (Reid vapor pressure) specific for short chains alcohols mixed with gasoline [24]. Therefore, isobutanol can be blended into gasoline in greater concentrations without exceeding imposed RVP limits.

Some authors perform a second law analysis when evaluating fuel conversion efficiency [25], [26], however, most engineers working in the field of power production, find a first-law analysis much more straight forward and intuitive [27]. Such an analysis is performed in this study, on a passenger car powered by a port injection engine (Table 2) fueled with gasoline–isobutanol blends. A ‘drop-in and drive’ scenario was investigated, where alcohol was simply added to gasoline and the resulting mixture was used to fuel the vehicle, with no modifications to the engine or fuel system. Given that the fuel system fitted to the vehicle used during the experimental trials is also used on many other SI engines, similar results should be obtained on a wide range of other passenger cars powered by port injection engines. Based on measurements on a chassis dynamometer, fuel conversion efficiency was calculated for the entire load and speed range of the engine used during the experimental trials. By performing the measurements on a passenger car rather than just the power unit, conditions very close to real world engine operation in automotive applications were ensured.