Compatibility
High concentration ethanol require special engines. Ethanol use in spark-ignited engines is relative easy, since the properties of ethanol resemble gasoline. Flexible Fuel Vehicles (FFV) spark-ignition cars for high-oxygen content fuels, for example E85, are well-known and widely on market. Instead, ethanol use in the compression ignition engines is challenging. If ethanol is to be used in compression ignition engines, either the engine or the fuel has to be modified, the latter option requiring fuel additives.
Ethanol differs significantly from conventional diesel fuel regarding cetane number, heating value, self-ignition temperature, vaporization characteristics, and boiling point. The cetane number of neat ethanol is less than 12, whereas today the requirement for diesel in Europe is higher than or equal to 51.Thus, neat ethanol as such will not ignite in conventional diesel engine. Also the lubricity of ethanol is unacceptable for high-pressure injection pumps in diesel engines. The low boiling point of ethanol increases the risk of cavitation, and the high conductivity the risk of corrosion (IEA-AMF Task 46 Nylund et al. 2015, Peckham 2001, McCormick 2001).
In the past, Detroit Diesel manufactured glow-plug equipped heavy-duty engines for using methanol or ethanol. However, many problems like engine wear, and glow-plug failures and excessive fuel consumption were encountered. Thus, the production of these engines was discontinued. In Sweden another approach was selected: adding ignition improver and lubricity additive to ethanol to enable the usage of the diesel combustion process.
The following special engine technologies for alcohol use are commonly considered:
Flexible Fuel Vehicle (FFV) cars (E85)
High-oxygen-containing fuels, can be used in special flexible-fuel vehicles (FFV). Fuel containing ethanol up to 85% (E85) is used, for example, in Brazil, North America and in many European countries. In Brazil, FFVs are also designed for the use of hydrous E100 fuel. However, startability limit of neat ethanol is only approximately +12 °C, and startablitity problems may occur even at higher temperatures as demonstrated in the AMF Annex 44 (Fan & Donglian 2016) . In the US, so called P-Series fuel consisting of butane, pentanes, ethanol and the biomass-derived co-solvent methyltetrahydrofuran (MTHF) is permitted for use in FFVs. From the beginning, FFVs were designed for 85% methanol blend (M85), which is used today for example in China.
FFVs are basically spark-ignition gasoline cars with some modifications. For example, all materials in are compatible with ethanol, which is more aggressive towards materials than gasoline. Due to E85 fuel’s low heating value, fuel injectors in FFVs are designed for higher fuel flows than in conventional gasoline cars leading to higher volumetric fuel consumption. Feedback control in FFVs adjusts fuel delivery and ignition timing. Ethanol’s higher octane rating would enable an increased compression ratio to achieve better energy efficiency. However, even modern FFVs still represent a compromise as compared to dedicated ethanol cars.
The ignition of ethanol is poor. When using E85 fuel, excess fuel is injected during cold starts to achieve performance similar to gasoline cars. Therefore exhaust emissions tend to be high until the three-way catalyst (TWC) warms up (Lupescu 2009). Improved engine- and emissions-control technology is expected to reduce the exhaust emissions of FFVs in cold starts. Catalyzed hydrocarbon traps have been developed to store organic gases in cold starts until they can be removed when the TWC warms up (Lupescu 2009). Also intake port heating to reduce non-methane organic gas emissions has been studied (Chiba et al. 2010) and heated fuel injectors (Kabasin et al. 2009). All in all, the development of FFV cars is continuing in many areas.
Volumetric fuel consumption is higher for E85 fuel than for gasoline. The manufacturer’s figures for one FFV car equates to 33% higher volumetric fuel consumption for E85 than for gasoline, even though energy consumption as MJ/km is lower for E85 than for gasoline: 6.7 l (220 MJ) vs 8.9 l (205 MJ) per 100 km.
In AMF Task 52 report, the thermal efficiency from ethanol blended fuels was investigated using single cylinder Ricardo Hydra research engine. Tested fuels were neat gasoline and ethanol blends E20, E30, E50 and E85. The obtained thermal efficiency from ethanol blended fuels were in range of 30-34, which was a better efficiency than that for gasoline. Increasing ethanol content in fuels offered better antiknock property and allowed more advanced ignition timing.
Diesel engines for additized ethanol (ED95)
In Sweden ethanol is used in the diesel combustion process in Scania’s ethanol engine by using ignition improver and lubricity additive. The first ethanol bus equipped with Scania’s engine started service in 1985, and in 2000 there were 407 buses running in Sweden. Now more than 600 buses have been supplied by Scania. Today ethanol buses are running also in other countries, for example, in Mexico, Australia, and Denmark.
The modifications of diesel engines for ethanol-use include e.g. increased compression ratio, a special fuel injection system and a special catalyst to control aldehyde emissions. Scania's current 3
generation ethanol engine is an adaptation of Scania's latest 9-litre diesel engine with air-to-air charge cooling and exhaust gas recirculation, EGR. The ethanol version features, among other things, elevated compression ratio (28:1) to facilitate ignition, higher fuel delivery to compensate lower energy density of the fuel, and special materials for the fuel system. The engine is available with Euro V and EEV emission certification (Scania 2007, Nylund 2011).
Dual injection, or pilot injection, is a combination of two individual fuel systems with the direct injection into the combustion chamber. By using a pilot injection of diesel to help to ignite a later injection of neat ethanol. Up to 90% ethanol can be used at high loads and 50-60 % at low and medium loads. A range of ethanol percentages can be used, as well as neat diesel. This technology creates opportunities for controlling the combustion to a very high degree, for example, more effectively aiming at the highest efficiency or the lowest NOx and PM emissions. This technique is a variant of what is called partial premixed controlled combustion (PCCI). Lubrication additives and/or improved materials might be needed for this technique. The main advantages are high engine efficiencies, high displacement of fossil diesel, and low NOx and PM emissions (Larsen 2009).
Ignition of the ethanol can also be secured by a spark-plug, a glow-plug or by hot recirculated exhaust gases. New or combined combustion systems may offer possibilities, as well. (Petterson 1994). One idea could be to produce small amounts of hydrogen on-board, and use this as an ignition enhancer. The desired effect could be achieved possibly by introducing the hydrogen mix into the intake manifold late during the intake stroke. Ethanol gives inherently low particulate emissions, but there is probably going to be a need for NOx control by EGR, lean NOx or NOx storage catalyst or perhaps SCR, and a control of unburned fuel and aldehydes by oxidation catalyst (Nylund 2004b, Larsen 2009).
Material compatibility and safety issues are handled extensively in the Guidebooks, e.g. Engelen et al. (2008), RFA (2010) and E85 Handbook (2008).
Low-level ethanol/gasoline blends in engines (E10)
Ethanol/gasoline blends have been used for decades in some countries, for example “gasohol” has been used since the 70s in the U.S. and Brazil. The U.S. Environmental Protection Agency (EPA) accepts gasoline that contains 10-15 vol-% ethanol for 2001 and newer light-duty vehicles, subject to certain conditions (US EPA 2011). In Brazil, ethanol content of gasohol has gradually risen to 25 vol-% for use in slightly modified cars. In Europe, maximum limit for ethanol content in gasoline is 10 vol-% (E10) since 2011, but maximum 5 vol-% ethanol containing gasoline is available for cars, which are not compatible with E10 (Directive 2009/30/EC).
Mid-level ethanol blending ratios are under research in many regions. Some results have shown that gasoline engines from 2001 through 2009 may fail when operated on E15 and E20 (CRC 2012). Many sites also require modifications into dispensing system for selling E15 (Gregory 2012). On the other hand, some results show that E15 has no adverse effect in the 1994-2004 model year U.S. light-duty vehicles (Ricardo 2010).
Compatibility of ethanol with conventional cars depends on technology and material choices. Modern cars with a closed-loop fuel control system can compensate the leaning effect of oxygen containing gasoline and modern gasoline fuelled cars are often built up with materials that tolerate ethanol. However, fuel injection system of other than FFVs for E85 use is not compatible with high concentration ethanol. Therefore maximum oxygen content for gasoline is defined in legislation and standards. (Read more). Renewal of car population is a slow process. JEC (2011) has estimated that in 2020 about 10% of European car fleet is older than model year 2005. Older cars and non-road engines are less compatible with ethanol than modern cars.
Driveability is affected by volatility of fuel. Volatility is specified by vapor pressure and distillation properties of gasoline (Read more). These properties are controlled in legislation and standards to ensure proper cold-starting, warm-up behavior, driveability, and to avoid unnecessarily high evaporative emissions. Modern cars with multi-point fuel injection systems are generally less sensitive towards volatility properties of gasoline than old cars. With old cars, front-part of distillation was critical for cold-start performance and evaporative emissions, whereas mid-range distillation was important for warm-up performance, acceleration and short-trip economy. End-part of distillation influenced oil dilution, deposits and long-trip economy.
When ethanol is added to gasoline, it influences the front part of distillation (Read more) and may affect hot-weather drivability. Vapor in this part of distillation contains an even higher fraction of ethanol than would be expected based on its share in the gasoline, due to the azeotropic behavior of the ethanol/gasoline blend. Modern vehicles using multi-point injection technology are less susceptible to hot-weather driveability problems than older vehicles. The EN228 specification appears adequate for controlling drivability in European vehicles, but further studies are needed if the standard’s volatility limits are modified to allow ethanol blends of 10 vol-% and higher. (Stradling et al. 2009).
Cold-weather drivability is affected by mid-range volatility, defined in Europe by the E100 value. This parameter is linked to exhaust emissions under cold-start conditions. With splash blends of ethanol in gasoline, cold-weather drivability may improve somewhat due to the higher volatility of the blends, but the opposite was observed when fuels were tailored at the same volatility levels. Ethanol’s high latent heat may affect drivability with a cold engine, and may also have a leaning effect under open-loop engine conditions. Stradling et al. (2009) suggest that minimum E100 limits in the EN228 gasoline specification would ideally vary with the ambient temperature at different volatility classes. The CRC in the USA has developed new fuel parameters (Driveability Indices) that include ethanol offset terms for US vehicles. Ethanol would prevent icing problems, which are, however, probably relevant only for some old vehicles.
Ethanol is more aggressive towards materials than gasoline. This may lead to early wear or damage to materials and components, swelling of elastomers and corrosion of metals. Ethanol may dissolve lubricant layers between metal parts, which may increase wear. The high electrical conductivity of ethanol may lead to a risk of electrolytic or galvanic corrosion with materials designed for hydrocarbons (Walwijk et al. 1996). Corrosion inhibitors are recommended for ethanol, and compatibility of additives should be secured (RFA 2010, Engelen et al. 2008).
In Brazil, there is long experience of blends of around 20% ethanol with gasoline. Examples of modifications to cars relating to this fuel include cylinder walls, cylinder heads, valves, valve seats, pistons, piston rings, intake manifolds, carburetors, electrical systems and nickel plating on steel lines.
Consideration must be given to alcohol/gasoline blends in the distribution chain. Materials used in tankers, tanks, lines, seals, hoses, and refueling equipment must be alcohol-resistant.
Material recommendations for ethanol and ethanol/gasoline blends are listed in Table 1. Recommendations are listed by Engelen (2008) and RFA (2010). The material cautions defined for the E85 fuel are included, as well (E85 Handbook 2013). Recommendations focus on materials of vehicles and/or fuel distribution systems. In particular, aluminum should be avoided. First-generation direct-injection fuel systems with aluminum rails do not tolerate ethanol (ETP 2011). Corrosion product of aluminum forms a gel. Water may suppress the reaction between alcohol and aluminum (Kameoka et al. 2005). Engelen et al. (2008) point out that manufacturers of products should be consulted for before introducing ethanol or ethanol/gasoline blends. Ethanol may also damage conventional paints, which is a considerable risk during refueling.
E25 dispensers may not be compatible with E85 (E85 Handbook 2013). For example, blender pump dipenser for E85 should have filters with nominal rating of 50% for 5 microns and absolute rating of 99% for 10 micron particles, whereas 10-micron filter is typically used for gasoline and low-level ethanol blends, and 30-micron filter for diesel fuel.
Table 1. Recommendations for materials considered for use in ethanol and ethanol/gasoline blends in fuel distribution system by Engelen et al. (2008) and recommendations for E85 (E85 Handbook 2008).
