Using a separate E85 direct injection boosting system combined with gasoline port fuel injection (PFI) makes the engine more efficient in its use of gasoline, and can be viewed as a more cost-effective alternative to a modern diesel, according to a Ford study presented by Robert Stein, currently of AVL, formerly of Ford, at the SAE 2009 World Congress.
Proposed by John Heywood and colleagues at MIT in 2005, the basic premise of E85 boosting is that ethanol (or other lower alcohols) suppresses knock due to the large evaporative cooling effect it has on the air-fuel mixture when injected directly into the cylinder, supplemented by ethanol’s inherent high octane number. Using the E85 boosting concept requires two fuel tanks and vehicle owner acceptance of dual fueling.
With knock suppressed, the compression ratio (CR) can be increased; in a turbocharged or supercharged engine, even higher boost pressure can be used. The resulting higher BMEP levels allow downsizing of the engine at equivalent vehicle performance. The MIT team spun off a startup—Ethanol Boosting Systems, LLC (EBS)—in 2006 to commercialize the concept. (Earlier post.) EBS has a collaborative R&D agreement with Ford, and participated in the study reported at the World Congress.
The vehicle owner will realize high fuel economy because gasoline, with its greater eating value per volume, is the fuel that is primarily used for most driving modes. Furthermore, by enabling higher CR, downsizing and downspeeding, E85 DI + gasoline PFI makes the engine more efficient in itsusee of gasoline.
Improved engine efficiency leverages the effect of the limited supply of E85, compared to simply displacing gasoline as in an FFV [flexible fuel vehicle]...this leveraging can be very substantial, and has the effect of dramatically improving the net energy balance of ethanol, and therefore its beneficial impact on reducing petroleum consumption.—Stein et. al. (2009)
In the study, Ford applied the E85 boosting approach to two engines: an early prototype 3.5-liter EcoBoost direct injection, turbocharged engine outfitted with PFI, and a 5.0L PFI engine in a Ford F-150 pickup truck.
Consumption of E85 varied under different operating and vehicle load conditions, and ranged from 1% to 48% for the 5.0L E85 DI + gasoline PFI engine in a pickup truck. Less E85 can be used by moderately retarding the spark timing, the researchers found, with a small effect on efficiency.
Among the conclusions reached in the study:
For a 5.0L E85 DI + gasoline PFI engine in an F-series pickup, the leveraging due to 12:1 CR is approximately 5:1 on the EPA M/H drive cycle—i.e., 5 gallons of gasoline are replaced by 1 gallon of E85. This leveraging effect will be significantly reduced for more aggressive drive cycles, they noted.
E85 usage in the scenario above is projected to be approximately 1% of the fuel for the EPA M/H cycle and 16% for the US06 aggressive driving cycle. With a 10 gallon E85 fuel tank, this rate of consumption would result in refueling intervals of approximately 20,000 miles on the M/H cycle and 900 miles on US06.
A 3.5L EcoBoost GDTI engine modified for E85 DI + gasoline PFI operation and constrained by a peak pressure limit of 125 bar demonstrated 27 bar BMEP at 2,500-3,000 rpm.
Achieving the full potential of an E85 DI + gasoline PFI engine requires an engine structure capable of at least 150 bar mean + 3 sigma peak pressure—comparable to a modern diesel.
An E85 DI + gasoline PFI engine is expected to have implementation advantages compared to a FFV GTDI engine operating on E85, including reduced dynamic range requirement for the DI pump and injectors, improved starting and emissions under cold temperatures, and potentially improved durability aspects (valve seat wear, bore wash, intake port/valve deposits.
There are a number of technical challenges associated with an E85 DI + gasoline PFI engine, including high peak cylinder pressures, combustion noise, and direct injector cooling.
The Ford team noted that the E85 DI + gasoline PFI can be viewed as an alternative to a modern diesel. Both engines use turbocharging, direct injection and an engine structure capable of high peak pressures; and both necessitate complex controls and calibration. A second tank is required for both: a urea tank for selective catalytic reduction (SCR) of NOx in the diesel, and the E85 tank for the DI + PFI engine.
There are a number of differences between the two, however:
If the vehicle owner does not fill the E85 tank, the engine can operate indefinitely with degraded performance using only gasoline. In comparison, a diesel SCR vehicle owner who does not refill the urea tank will experience a range of inducements including limited vehicle speed and eventually failure to start.
The DI + PFI fuel system of the E85 DI + gasoline PFI engine is less expensive than modern high pressure diesel injection system. The DI + PFI engine runs at stoichiometric air-fuel and uses a relatively inexpensive conventional three-way catalyst )TWC). The diesel requires more complex and expensive aftertreatment systems.
Because of these factors, the E85 DI + gasoline PFI engine will cost significantly less than a diesel engine, and will be able to achieve more stringent emission standards due to the extremely high conversion efficiency of a stoichiometric TWC aftertreatment system. The E85 DI + gasoline PFI engine also uses a renewable fuel in a leveraged manner to significantly reduce petroleum consumption and total net CO2 emissions.—Stein et al. (2009)
Robert Stein, Christopher House and Thomas Leone (2009) Optimal Use of E85 in a Turbocharged Direct Injection Engine (SAE 2009-01-01490)
D.R. Cohn, L. Bromberg, J.B. Heywood (2005) Direct Injection Ethanol Boosted Gasoline Engines: Biofuel Leveraging For Cost Effective Reduction of Oil Dependence and CO2 Emissions (LFEE 2005-001)