(As part of the proposed Tier 3 rulemaking on vehicle emissions and gasoline sulfur content, the US Environmental Protection Agency (EPA) is proposing to allow vehicle manufacturers to request approval for an alternative certification fuel—such as a high-octane 30% ethanol by volume (E30) blend—for vehicles they might design or optimize for use on such a fuel. (Earlier post.)

Review of ethanol properties. In their paper, Robert Stein from AVL and James Anderson and Tim Wallington from Ford begin by reviewing properties of ethanol and gasoline blends. These include:

• Lower energy content than that of gasoline. The net heating value (NHV) of ethanol is approximately 33% less than that of gasoline on a volumetric basis. As a result, as the ethanol content of the fuel is increased, the fuel economy and driving range for a given fuel tank size are reduced. This penalty can be offset by exploiting improved thermal efficiency.

  The heat released per engine cycle and thus the full load torque of an engine is directly proportional to the mass of trapped fresh air per engine cycle and to the heating value of the fuel per unit mas of fresh air...typical gasoline and pure ethanol have nearly equal NHV per unit mass of air at stoichiometry, and therefore engine torque per unit mass of air would be equivalent at equal thermal efficiency.

  —Stein et al.

• Heat of vaporization (HoV). The heat of vaporization represents the amount of energy required to evaporate a liquid fuel. In a direct injection engine, they noted, the cooling of the charge and consequent knock relief provided by the evaporation of the fuel is proportional to the fuel flow per unit mass of air.

  The stoichiometric air-fuel ratio of ethanol is 9.0, and of gasoline, about 14.6 (depending upon the blend). More mass of ethanol is thus required at stoichiometry than of gasoline for a given mass of air.

• Vapor pressure. When ethanol is added to gasoline, the blend exhibits a vapor pressure that is higher than that of both the gasoline and the ethanol portions. Highest Reid Vapor Pressure (RVP) is seen at 10% v/v ethanol; further increasing ethanol content reduces the RVP, such that RVPs match that of base gasoline at ethanol concentrations of 30% to 55% v/v. This non-intuitive behavior is a consequence of molecular interactions between the gasoline components and ethanol, the authors explain.

• Distillation curve. As with the vapor pressure, the near-azeotropic behavior of an ethanol-gasoline blend affects the distillation characteristics for portions of the distillation curve.

  ...the near-azeotropic behavior of the ethanol-gasoline blends is visible as a more slowly rising curve with higher volatility than that of the base gasoline Ii.e., a greater volume distilled at a given temperature). For increasing ethanol content, this slowly rising curve expands to cover a larger portion of the distillation curve.

• Knock limit. The well-known improvement in knock resistance resulting from ethanol-gasoline blends is the consequence of three knock-related properties of ethanol: research octane number (RON); sensitivity, and HoV.

  Ethanol’s RON value of 109 provides high inherent or chemical knock resistance; the high sensitivity of ethanol results in longer autoignition delay time and great knock resistance as combustion phasing is retarded due to reduced unburned gas temperature; and the high HoV of ethanol results in substantial cooling of the charge especially with DI [direct injection].

  Further, the effect of charge cooling on reducing the rate of autoignition kinetics is amplified by the high sensitivity of ethanol. (Lower temperature provides a grater benefit in knock resistance with a high sensitivity fuel.)

  ...The increase in knock-limited torque with increasing ethanol content can be limited by the available boost pressure of the turbocharger system and the peak pressure capability of the engine structure, especially at high ethanol content.

Fuel efficiency. Given the properties of the fuel blend, improved fuel efficiencies can be generated by several approaches, including downsizing/downspeeding; increased compression ratio; reduced enrichment; and the leveraging of improved efficiency at part load.

• Downsizing/downspeeding. The increase in knock resistance with increasing ethanol content enables a substantial increase in full-load BMEP for a gasoline turbocharged direct injection engine (GTDI). This increase in BMEP can enable downsizing of the engine displacement and/or downspeeding through revised gear ratios, findal drive ratio, or shift scheduling.

  Both downsizing and downspeeding move the operating regime of the engine in the vehicle to a more efficient part of the engine speed-load map, providing improved vehicle fuel efficiency.

  The extent of downsizing/downspeeding can be limited in practice by attributes affected by the characteristics of the boost system, including transient response and the capability to provide sufficient boost and low and high speed.

• Increased compression ratio. Increased knock resistance enables an increase in compression ratio (CR), which provides a direct increase in thermal efficiency.

  The increase in thermal efficiency is non-linear with increasing CR, where the CR for maximum efficiency is a function of the engine displacement-per-cylinder and the bore-stroke ratio. Optimum CR occurs where the trade-off is balanced between increased expansion ratio vs. increased heat transfer and crevice volume losses and mechanical friction.

  Increased displacement-per-cylinder and lower bore-stroke ratio provide reduced surface-to-volume ratio and lower ratio of crevice volume to clearance volume, and hence the optimum CR is higher. Thus, the benefit of increased CR enabled by increasing ethanol content is engine design specific.

• Reduced enrichment. Enrichment of the fuel mixture at high-speed, high-load conditions is currently used to reduce exhaust temperature to avoid damage to engine or emissions aftertreatment components. However, enrichment also degrades thermal efficiency. Increased ethanol content enables improved efficiency by reducing or eliminating the need for fuel enrichment.

• Part-load CO₂. Ethanol has about 7% lower CO₂ emissions at part load than gasoline due to lower burned gas temperature, which results in reduced heat transfer losses and improved thermal efficiency, and due to a higher hydrogen-to-carbon ratio.

• Volumetric fuel economy. Leveraging the fuel efficiency effects can partially offset—or perhaps completely offset for E20-E30—the volumetric fuel economy decrease from ethanol, they suggested.

They study found that the effects of higher ethanol content on gaseous emissions are generally neutral for modern vehicles. Emissions of PM, toxic compounds, and off-cycle emissions due to enrichment would drop significantly.

Based on the above consideration, a mid-level ethanol blend (greater than E20 and less than E40) appears to be attractive as a long-term future fuel for the US, especially if used in vehicles optimized for such as fuel. To provide high knock resistance, this fuel should be formulated using a blendstock that retains the octane rating of the current blendstock used for regular-grade E10 gasoline.

Further work is needed to recommend a specific ethanol blend level and corresponding octane rating, including analysis of fuel efficiency and CO₂benefits for representative powertrain/vehicle applications, and assessment of the potential ethanol supply and required infrastructure.

—Stein et al.

Evaluating fuel economy and CO₂ emissions of E10, E20 and E30 in an EcoBoost engine

In a paper also presented at the World Congress, a Ford team (Jung et al.) used engine dynamometer testing to compare E10, E20, and E30 splash-blended fuels in a Ford 3.5L EcoBoost direct injection (DI) turbocharged engine. They tested the engine with CRs of 10.0:1 (current production) and 11.9:1.

They they then used the resulting data in a vehicle simulation of a 3.5L EcoBoost pickup truck. Among the findings of their study were:

• E20 (96 RON) fuel enabled an increase in CR from 10:1 to 11.9:1 with similar knock behavior to the baseline E10 (91 RON) fuel. Based on the simulation, the combination of E20 and 11.9:1 CR offered a 4.8% improvement in EPA metro/highway (M/H) CO₂emissions with comparable full load performance.

  The increased efficiency offset the decreased energy content of the E20 fuel, resulting in similar volumetric fuel economy (mpg US) with the baseline E10 and 10:1 CR.

• The higher compression ratio improved engine efficiency by ~4-5% in the part-load region where the engine is not knock-limited. However, they noted, modern turbocharged downsized engines typically operate for considerable times at the knock-limited higher load regions. Using E10 with the higher CR significantly degrades performance.

• E30 (101 RON) provided additional knock benefits at high loads compared to E20, and could have enabled a further increase in CR (which was not tested in this study).

  At 11.9:1, E30 delivered a 5.1% benefit in EPA M/H CO₂ and a 3.1% degradation in EPA M/H fuel economy versus E10 and a CR of 10:1. US06 Highway cycle CO₂emissions were improved by 7.5% for the E30/11.9:1 CR combination, while fuel economy and range were essentially unchanged.

• If E20 and E30 were used in the baseline 10:1 CR engine, there would be a 1.1% to 1.5% benefits for CO₂ relative to E10, but 2.8% to 6.6% reductions in fuel economy and range.

Resources

• Stein, R., Anderson, J., and Wallington, T., “An Overview of the Effects of Ethanol-Gasoline Blends on SI Engine Performance, Fuel Efficiency, and Emissions,” SAE Int. J. Engines 6(1): 2013, doi: 10.4271/2013-01-1635

• Jung, H., Leone, T., Shelby, M., Anderson, J. et al., “Fuel Economy and CO2 Emissions of Ethanol-Gasoline Blends in a Turbocharged DI Engine,” SAE Int. J. Engines 6(1): 2013, doi: 10.4271/2013-01-1321

• Jung, H., Shelby, M., Newman, C., and Stein, R., “Effect of Ethanol on Part Load Thermal Efficiency and CO2 Emissions of SI Engines,” SAE Int. J. Engines 6(1): 2013,doi: 10.4271/2013-01-1634