Engine Performance Using Gasoline Ethanol Blends
A number of researchers from academic institutions, state-funded laboratories, as well as the automobile industry, have studied automobile engine performances, combustion characteristics [5-8], research octane number (RON) changes [9], exhaust characteristics [8, 10], engine deposit formation [11], and cold start [12] in using anhydrous ethanol-gasoline blends. Also, engine performance using gasoline blended with hydrous ethanol which contains 4-5% water by volume is another area of interest [13, 14].
Blending anhydrous ethanol to gasoline can alter physical parameters of the fuel affecting engine performance. Some of the most important changes are potentially increasing (or decreasing)
Reid vapor pressure (RVP) [15], altering distillation properties [16] and preventing transport in pipelines due to risk of water-induced phase separation [17]. The net (or lower) heating value (NHV) of ethanol is also about one-third less than gasoline on a volume basis. While this difference reduces the volumetric fuel economy (miles per gallon or L/100 km) observed by consumers and travel range on a tank of fuel, ethanol actually gives a small improvement in the thermal efficiency of engine operation (miles per gallon of gasoline — equivalent or MJ/km). The physical properties of ethanol which are important in the application as additive to gasoline are shown in Table 16.7. Ethanol has both a higher octane rating and a higher heat of vaporization than typical gasoline [9].
The octane rating of a fuel is a measure of the fuel’s ability to resist autoignition and knock in a spark-ignited engine. Higher octanerated fuel is desirable as it enables improved engine efficiency. Two tests are generally used to quantify the anti-knock tendency of fuels: Research Octane Number (RON) and Motor Octane Number (MON). As a way of accounting for both, the Anti-Knock Index (AKI) which is the arithmetic average of RON and MON is commonly used to describe gasoline octane ratings in the United States.
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Table 16.7 Fuel properties of regular-grade gasoline and ethanol [9, 18].
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Ethanol improves octane ratings when added to gasoline. The RON and AKI of pure ethanol are approximately 109 and 99, respectively, much higher than regular or premium-grade US gasoline. When ethanol is added to the blend stock, the RON and MON increase of the resulting ethanol-gasoline blend is nonlinear with respect to volumetric ethanol content but has recently been shown to be essentially linear when evaluated using molar ethanol content [15]. The hydrocarbon composition of the blend-stock also influences the amount by which ethanol increases the octane ratings of ethanol-gasoline blends [16]. The heat of vaporization (HoV) of ethanol is nearly three-fold greater than gasoline on a liquid volume and mass basis. On a stoichiometric air + fuel mass basis and as a fraction of net heating value (NHV), the HoV for ethanol is about four-fold greater than gasoline. This factor likely contributes to the high RON of ethanol but to an unknown extent.
To achieve the desired fuel properties in the ethanol-gasoline blends like E10 and E15, the oil refining industry produces a "blend stock for oxygenate blending" (BOB) to which the appropriate amount of ethanol will be added prior to sale. Since key volatility properties such as vapor pressure and distillation temperature are changed when 10%v ethanol is added to the blend stock, it needs to be formulated to ensure that the final blend properties are within specifications for the appropriate geographical region and season. The need for volatility adjustment was the initial factor leading to the creation of BOBs and remains an important factor in refinery operations.
The Anderson group at Ford Motor Company, USA, have studied ways to increase the minimum octane number (Research Octane Number, RON) of regular-grade gasoline by means of the high octane rating of ethanol in a mid-level ethanol blend [9]. They suggested that higher RON would enable greater thermal efficiency in future engines through higher compression ratio (CR) and/or more aggressive turbocharging and downsizing.
Developing scenarios of future ethanol availability, Anderson’s group estimated that large increases (4-7 points) in the RON of US gasoline are possible by blending in an additional 10-20%v ethanol above the 10% already present. Estimated RON and RON + cooling ON of ethanol-gasoline blends for two blend stock RON values (88 or 92 RON) are shown in Figure 16.2.
Keeping the blend stock RON at 88 (which provides E10 with approximately 92.5 RON), they estimated RON would be increased to 94.3 for E15 to as much as 98.6 for E30. Even further RON
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Figure 16.2 Estimated RON and RON + cooling ON of ethanol-gasoline blends for two blend stock RON values (88 or 92 RON). (Reprinted with permission from reference [9]; copyright 2012 Elsevier). |
increases may be achievable assuming changes to the blend stock RON and/or hydrocarbon composition. For example, an increase in blend stock RON from 88 to 92 would increase the RON of E10 from 92.5 to 95.6. Furthermore, even higher RON of 97.1 for E15 and 100.6 for E30. Potential compression ratio (CR) increases are approximated for the different estimates of future octane numbers, including the effect of increased evaporative cooling from ethanol in direct injection engines. Furthermore, for ethanol and blend stock RON scenarios considered, CR increases were estimated to be on the order of 1-3 CR-units for port fuel injection engines as well as for direct injection engines in which the greater evaporative cooling of ethanol can be fully utilized [9].
Canakci and coworkers have recently studied the effect of ethanol-gasoline blends on the engine performance, combustion characteristics and emissions of spark ignition (SI) engines [5]. In this experiment, a vehicle having a four-cylinder, four-stroke, multipoint injection system SI engine was used. The tests were performed on a chassis dynamometer while running the vehicle at two different vehicle speeds (80 km/h and 100 km/h) and four different wheel powers (5, 10, 15, and 20 kW). The measured emission values with the use of E5 and E10 have been compared to those of pure gasoline. The experimental results revealed that when the test engine was fueled with ethanol-gasoline blends, CO, CO2, unburned HC and NOx emissions decreased for all wheel powers at the speed of 80 km/h. However, when the vehicle speed was changed to100 km/h, more complex trends occurred in the exhaust emissions for the fuel blends, especially for the wheel power of 15 kW. It was also seen that the air-fuel equivalence ratio increased with the increase of ethanol percentages in fuel blends when compared to a pure gasoline case [5].
In a similar study, Karavalalis and coworkers studied the impacts of ethanol fuel level on emissions of regulated and unregulated pollutants [19]. In this study they investigated the impact of ethanol blends on criteria emissions (THC, NMHC, CO, NOx), greenhouse gas (CO2), and a suite of unregulated pollutants in a fleet of gasoline-powered, light-duty vehicles. The vehicles ranged in model year from 1984 to 2007 and included one flexible-fuel vehicle (FFV). Emission and fuel consumption measurements were performed in duplicate or triplicate over the Federal Test Procedure (FTP) driving cycle using a chassis dynamometer for four fuels in each of seven vehicles. The test fuels included a CARB phase 2 certification fuel with 11% MTBE content, a CARB phase 3 certification fuel with a 5.7% ethanol content, and E10, E20, E50, and E85 fuels. In most cases, THC and NMHC emissions were lower with the ethanol blends, while the use of E85 resulted in increases of THC and NMHC for the FFV [19]. The CO emissions were lower with ethanol blends for all vehicles and significantly decreased for earlier model vehicles. Results for NO emissions were mixed, with some older vehicles showing increases with increasing ethanol level, while other vehicles showed either no impact or a slight, but not statistically significant, decrease. The CO2 emissions did not show any significant trends. Fuel economy showed decreasing trends with increasing ethanol content in later model vehicles. There was also a consistent trend of increasing acetaldehyde emissions with increasing ethanol level, but other carbonyls did not show strong trends. Further, Karavalalis and coworkers reported that the use of E85 resulted in significantly higher formaldehyde and acetaldehyde emissions than the specification fuels or other ethanol blends. Benzene, toluene, ethyl benzene, xylenes (BTEX) and 1,3-butadiene emissions were lower with ethanol blends compared to the CARB 2 fuel, and were almost undetectable from the E85 fuel. Furthermore, they reported that the largest contribution to total carbonyls and other toxics was observed during the cold-start phase of the federal test procedure [19].
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