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Use of Higher than 10 volume percent Ethanol/Gasoline Blends in Gasoline Powered Vehicles4. EFFECT OF ETHANOL ON FUEL PROPERTIES Ethanol has been widely used in the U.S. as a gasoline component since the late 1970s when it was used as a fuel extender due to gasoline shortages. Later, when gasoline was more plentiful, ethanol began to see widespread use as an octane enhancer. When ethanol is added to gasoline, it modifies the fuel properties and affects the nature and quantities of exhaust and evaporative emissions from vehicles. This section covers the effects of fuel property changes related to the addition of ethanol to gasoline. Clear understanding of these effects is very important in evaluating the technical and performance related issues when considering the use of higher than 10 volume percent ethanol blends in gasoline. The principal changes in the fuel properties due to the addition of ethanol to gasoline include:
Table 6 shows the individual properties of ethanol and gasoline which play important roles in the properties of ethanol/gasoline blends. Table 6: Some Properties of Ethanol and Gasoline (8, 9)
4.1 Effect of Ethanol on Octane Number Octane number is a measure of the resistance of a fuel to autoignition. It is also defined as a measure of antiknock performance of a gasoline or gasoline component. The octane value posted on retail gasoline pumps is the average of the "Research" (RON or R) and "Motor" (MON or M) octane numbers, or "(R+M)/2". The standard tests for RON and MON are not completely applicable to ethanol. There is a great deal of scatter in RON and MON values reported for ethanol in the literature. Nevertheless, there is general agreement that ethanol has excellent antiknock properties allowing higher compression ratios and improved engine efficiencies (10). Since ethanol has higher octane than many gasoline components, when added to gasoline it increases the octane value of the finished fuel. The blending octane values shown in Table 6 indicate that ethanol boosts research octane to a greater extent than motor octane. Hence it is possible for an ethanol/gasoline blend, with the same posted octane rating as a non-oxygenated gasoline, to have a slightly lower motor octane level. Some engines respond more strongly to motor octane than research octane. At high speeds or under heavy load conditions, for instance when pulling a trailer up a hill, motor octane is a better indicator of antiknock performance. For these engines, a small reduction in motor octane could result in a slightly higher incidence of engine performance problems, such as engine knock, dieseling, or increased temperature. Over time, severe engine knock can lead to damaged pistons or other engine damage. Most late model cars in the U.S. and Canada have been equipped with electronic knock sensors that detect combustion knock and signal computers to retard the ignition timing and reduce the knock. The automatic ignitibn retardation could diminish engine power to an extent that might be recognized as a loss in performance by some drivers. It has been reported that ethanol/gasoline blends have superior resistance to combustion knock at low engine speed and deter run-on (the tendency of the engine to continue to run after the ignition has been turned off) largely because of their higher heat requirement for vaporization (11). Higher RON gasolines also reduce run-on. Although ASTM does not specify a minimum standard, it recommends that gasolines with a (R+M)/2 octane value of 87 have a minimum motor octane of 82. Some refiners have their own internal minimum motor octane performance values for their gasolines. 4.2 Effect of Fuel Volatility on Vehicle Performance A fuel's ability to vaporize or change from liquid to vapor is referred to as its volatility. Volatility is an extremely important characteristic of gasoline which affects many vehicle performance parameters. Table 7 lists the effects of gasoline volatility on vehicle performance. The effects of volatility on vehicle performance are also shown graphically in Figure 1. For example, if the volatility curve in Figure 1 moves down in the initial range of up to 30% evaporated, then more fuel would vaporize at lower temperatures and it may cause poor hot starting and vapor lock problems. If the curve moves upwards then vehicles may encounter poor cold starting problems. The effects of shifting the volatility curve in the mid and higher percent evaporated ranges are also clearly marked on Figure 1.
Figure 1. The Effect of Volatility on Vehicle Performance Table 7: Effects of Gasoline Volatility on Vehicle Performance
Gasoline which is not volatile enough results in poor cold start and poor warm up driveability as well as unequal distribution of fuel to the cylinders in carbureted vehicles. These fuels can also contribute to deposits in crankcase, spark plugs, and combustion chamber. Gasoline which is too volatile vaporizes too easily and may boil in fuel pumps, lines or in carburetors at high operating temperatures. If too much vapor is formed, this could cause a decrease in fuel flow to the engine, resulting in symptoms of vapor lock, including loss of power, rough engine operation, or complete stoppage. Fuel economy could also deteriorate and evaporative emissions could increase. In order to assure that fuels possess the proper volatility characteristics, refiners adjust gasoline seasonally, providing more volatile gasoline in winter to offer good cold start and warm up performance. In summer, gasoline is made less volatile to minimize the vapor lock and hot driveability problems, and also to comply with environmental standards. The main parameters to establish volatility limits are Vapor/Liquid Ratio (V/L), Vapor Pressure, and Distillation Curve. ASTM provides standards for one or more test procedures to measure each of these parameters. The vapor/liquid ratio uses a test to determine the temperature required to create a V/L ratio of 20. This ratio can also be calculated for gasolines using a combination of distillation and vapor pressure characteristics. More volatile fuels require lower temperatures to achieve this ratio while less volatile fuels require higher temperatures to create the same ratio. The V/L ratio assists in defining a fuel's tendency to contribute to vapor lock. According to one of the ASTM test procedures, Reid vapor pressure (RVP) is measured by submerging a gasoline sample, sealed in a metal chamber, in a 1000F water bath. More volatile fuels will vaporize more readily, and give higher vapor pressure reading. Less volatile fuels will generate less vapor and therefore give lower reading. Because of the popularity of this test method, RVP has become a widely used term when referring to vapor pressure of fuels. The V/L ratio and RVP are measurements of a fuel's "front end volatility", or more volatile components which vaporize first. The distillation test is used to determine fuel volatility over the entire boiling range of gasoline. Gasoline consists of a variety of hydrocarbon components that evaporate at different temperatures. More volatile components evaporate at lower temperatures, less volatile ones at higher temperatures. The plot of these evaporation temperatures is referred to as a distillation curve. The ASTM specification sets temperature ranges at which 10%, 50%, and 90% of the fuel will be evaporated as well as at the temperature at which all the fuel would evaporate (referred to as "end point"). Each of these points affect different areas of vehicle performance as shown in Figure 1. The 10% evaporated temperature must be low enough to provide easy cold starting but high enough to minimize vapor lock and hot driveability problems The 50% evaporated temperature must be low enough to provide good warm up and cold weather driveability without being so low as to contribute to hot driveability and vapor lock problems. The mid boiling range of gasoline also affects short trip fuel econ9my. The 90% and end point evaporation temperatures must be low enough to minimize crankcase and combustion chamber deposits as well as spark plug fouling and dilution of engine oil. 4.2.1 Effect of Ethanol on Fuel Volatility Ethanol has a fixed boiling point and thus a constant volatility, while the volatility of gasoline can be tailored over a range by adjusting the relative amounts of different hydrocarbon components. Adding ethanol to gasoline depresses the boiling temperature of individual hydrocarbons. It depresses the boiling point of aromatic hydrocarbons slightly less than aliphatic hydrocarbons. The effect of ethanol addition on the shape of a distillation curve is shown in Figure 2 (12). As the data indicate ethanol/gasoline blend (10 vol% ethanol) has significantly lower temperatures for evaporation of the front end, which affects primarily the first 50% evaporated. If ethanol concentration in the blend is increased beyond 10 vol%, the volume of the fuel evaporating under 2000F will increase, and the distillation curve for these blends will be lower than the curve for 10 vol% blend shown in Figure 2. Vapor pressure is another important parameter of gasoline that is affected by the addition of ethanol. As shown in Table 6, the RVP of ethanol is much lower than the RVP of gasoline. However, blending ethanol into gasoline forms a non-ideal solution that does not follow linear relationships. Rather than lowering the vapor pressure, low concentrations of ethanol cause an increase in RVP as shown in Figure 3(12). The vapor pressure increase reaches a maximum around 5 vol% ethanol content, and then starts to come down with further increase in ethanol concentration. Thus blends with greater than 10 vol% ethanol will give a smaller increase in RVP. It has also been reported that with the addition of ethanol, gasolines with lower vapor pressures incur larger increases in vapor pressure than gasolines with higher vapor pressures (13). The data are shown in Figure 4.
Figure 2 Effect of Oxygenates on Distillation Curve When ethanol/gasoline blends are commingled with gasoline, as they might be during the routine fill up of vehicle fuel tanks, the RVP effects of ethanol are similar to those discussed above (14). The data in Figure 5 (15) show that mixing an ethanol/gasoline blend with gasoline of the same RVP results in substantially increased vapor pressure. Calculations of temperatures for specific V/L ratios of ethanol/gasoline blends using ASTM procedures developed for gasolines do not correctly predict measured values. Figure 6 shows how the addition of ethanol changes the temperatures at which various V/L ratios occur (16). For example, the reference gasoline reaches a V/L ratio of 20 at a temperature of 1600F. Adding 10% ethanol to the reference gasoline reduces this temperature to 1 380 F. The V/L ratio data for ethanol blends greater than 10 vol% is not readily available. However, it can be predicted that the temperature to achieve the V/L ratio of 20 for higher than 10 vol% blends will be less than the corresponding temperature for 10 vol% blend.
Figure 3 Effect of Ethanol Concentration on Blend Vapor Pressure
Figure 4 Effect of Base Gasoline RVP on RVP Increase Due to Ethanol Addition
Figure 5 Effect of Commingling a Gasoline and a Ethanol Blend of Same RVP
Figure 6 Effect of Ethanol on Measured Vapor to Liquid Ratio 4.2.2 Effect of Ethanol/Gasoline Blends on Vehicle Performance In order to a start cold engine, sufficient fuel must be present in vapor form in the engine cylinders to initiate and sustain combustion. Generally, for gasoline, increased RVP and lower front-end distillation temperature improve cold starting performance. However, ethanol/gasoline blends can behave slightly differently than gasoline. Ethanol blends will require more heat to vaporize than gasoline. For example, a blend containing 10% ethanol needs 16.5% more heat to vaporize completely than does gasoline. Some concerns have been raised about difficulty in starting vehicles using blends at extremely low temperatures (8). Other concerns about low temperature fuel characteristics of blends include, a) increased viscosity of ethanol/gasoline blends which may impede fuel flow and b) phase separation in the vehicle fuel system due to reduced solubility. At moderate temperatures, vehicle driveability becomes an important issue. Through many years of cooperative research the auto and oil industries have evaluated driveability from the viewpoint of the driver and have developed rating methods that quantify driveability. Factors that contribute to a good driveability rating include quick starting, stall-free engine warm up, smooth idle, hesitation-free response to throttle, surge-free operation during cruise, and freedom from vapor lock. Driveability is rated at idle, during acceleration, and under cruise conditions as the car is driven through a prescribed cycle. The Coordinating Research Council (CRC) has established procedures for measuring driveability (10). A number of extensive road test programs have been conducted by several organizations to assess the influence 0 oxygenated blends on the driveability of vehicles that embodied various technologies. Based on the results of several driveability programs, conducted at ambient temperatures ranging from 300F to 830F, commercial gasolines were at least 98 percent problem4ree. Among the blends tested, gasohol was closest to gasoline in the frequency of reported problems (17,18,19). In another study the performance of gasohol in three fleets totaling 108 vehicles from model years 1974 to 1981 was evaluated. Gasohol complaints were reported to be statistically higher than gasoline complaints with respect to starting, stalls, rough idle, hesitation, and loss of power (20). The primary fuel related concern that occurs at elevated ambient temperatures is vapor lock. Vapor lock is caused by premature vaporization of fuel, impeding subsequent fuel supply to the cylinders. The vapor forming tendencies of gasolines have traditionally been described by the temperature at which V/L = 20 (TV/L =20). A gasoline with a high volatility has a low TV/L = 20. Since splash blended ethanol decreases TV/L = 20, it suggests that it will tend to increase the incidence of vapor lock. Increasing the ethanol concentration beyond 10 vol% will also lower the temperature for V/L ratio of 20, and thus may increase the likelihood of vapor lock. The wide variety of existing fuel system types, and their diverse responses to blends, suggest that additional research is needed to establish the most meaningful predictors of blend performance at high operating temperatures. 4.3 Enleanment Effect of Ethanol Gasolines are mixtures of many hydrocarbon compounds that consist solely of hydrogen and carbon. Ethanol contains hydrogen, carbon, and oxygen. The exact air-to4uel ratio needed for complete combustion of the fuel to carbon dioxide and water is called its "stoichiometric air-fuel ratio". This ratio is about 14.7 to 1.0 (on weight basis) for gasoline. For ethanol/gasoline blends less air is required for complete combustion because oxygen is contained in the fuel and -because some of the hydrocarbons have been displaced. For example, a blend containing 10% ethanol would only require 14.0 to 14.1 pounds of air per pound of fuel. The effect of this type of fuel change on an engine is called "enleanment". The air-fuel ratio is an important factor in the design of engines and fuel metering controls. Most automobiles made after 1981 in the U.S. and from mid 1980s in Canada use some form of "closed loop" fuel system that continuously monitors and adjusts the amount of fuel delivered to the engine to maintain the stoichiometric air4uel ratio. These vehicles have adjustment ranges that accommodate oxygenated fuels and, when operating in the "closed loop" mode, d9 not experience any effects from oxygenated fuel. During cold start and at full throttle, these systems operate in an "open loop" mode that provides a rich fuel mixture that is necessary for these conditions. In the rich mixture, "open loop" mode, vehicles do experience enleanment effects from the oxygenated fuel. The driveability characteristics of the vehicle are not normally affected by switching between oxygenated and non-oxygenated gasolines, whether or not a vehicle is using a "closed loop" fuel control system. In a situation where a vehicle is not properly tuned and is operating in a "too lean" condition, switching to a fuel with increased oxygen would increase the risk of a driveability problem. The symptom most likely to appear in this situation is a hesitation during acceleration. 4.4 Effect of Ethanol on Fuel Economy The differences in the heating values between gasoline and ethanol, as shown in Table 8, would result in a theoretical decrease in fuel economy for ethanol/gasoline blends in the 2% to 3% range when compared to gasoline. Table 8: Theoretically Expected Effect of Ethanol on Fuel -Economy *
* Heating value of Ethanol is taken as 76,000 (Btu/gal) Because of its higher hydrogen to carbon ratio, ethanol produces a greater volume of gases per unit of energy burned than gasoline. This leads to higher mean cylinder pressures and more work performed during the expansion stroke. Ethanol also has a much higher heat of vaporization than gasoline. As the liquid fuel evaporates in the air stream being charged to the engine, the high heat of vaporization cools the air, allowing more mass to be drawn into the cylinder. This increases the power produced from a given engine size. Therefore, when burned in a gasoline-optimized engine, ethanol/gasoline blends will produce an increase in the volume of combustion products, and the effect of charge-air cooling. The combined effect of these will result in an efficiency increase of about 1 to 2 percent. Hence the overall practical fuel economy reduction for El 0 is expected to be very small compared to gasoline. 4.5 Water Solubility Phase Separation Separation of a single phase gasoline into a "gasoline phase" and a "water phase" can occur when too much water is introduced into the fuel tank. Water contamination is most commonly caused by improper fuel storage practices at the fuel distribution or retail level, or the accidental introduction of water during vehicle refueling. Water has a higher density than gasoline, so if water separates, it will form a layer below the gasoline. Because most engines obtain their fuel from at, or near, the bottom of the fuel tank, engines will not run once the water phase separates. Non-oxygenated gasolines can absorb only very small amounts of water before phase separation occurs. Ethanol/Gasoline blends, due to ethanol's greater affinity with water, can absorb significantly more water without phase separation occurring than gasoline. Ethanol blends can actually dry out tanks by absorbing the water and allowing it to be drawn harmlessly into the engine with me gasoline. If, however, too much water is introduced into an ethanol blend, the water and most of the ethanol will separate from the gasoline and the remaining ethanol. The amount of water that can be absorbed by ethanol/gasoline blends without phase separation, varies from 0.3 to 0.5 volume percent, depending on temperature, aromatics and ethanol content (21). If phase separation were to occur, the ethanol/water mixture would be drawn into the engine and the engine would most likely stop. Some vehicle manufacturers have expressed concern that ethanol/gasoline blends might absorb water vapor from the atmosphere, leading to phase separation. Such problems are of greater concern for engines with open-vented fuel tanks that are operated in humid environments, such as marine engines. Based on the prolonged experience of using 10% ethanol/gasoline blends, the blends are no more susceptible to phase separation than non-oxygenated gasolines. 4.6 Material Compatibility Some materials used in fuel systems tend to degrade over time, such as elastomers used to make hoses and valves. Other fuel system components are made of metals and plastics and must be compatible with the expected range of fuel composition. Some older elastomers were found to deteriorate more rapidly in the presence of alcohol. However, since the mid-1980s, all vehicles have used fluoroelastomers, which are specifically designed to handle all modern gasolines, including ethanol/gasoline blends. Permeation of fuel through elastomers can accelerate deterioration. In general, ethanol blends have higher permeation rates through elastomers than non-oxygenated gasoline. However, the higher permeation rates of ethanol blends are well within safety limits and are not expected to cause performance, deterioration, or safety problems. The experience of using ethanol blends in areas covered by the oxygenated gasoline program in the U.S. has not registered higher rates of materials degradation or failure than areas using conventional gasolines. |
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