Emissions under the dual-fuel concept, at 11 bar IMEP. The green line is the US 2010 baseline. The horizontal axis shows the percentage of gasoline in the blend. Source: Rolf Reitz. Click to enlarge.

The University of Wisconsin concept proposes the use of dual fuel tanks, with port fuel injection of gasoline and direct injection of diesel, with the in-cylinder mixing of the fuels. Mixing ratios vary based on real-time operating conditions.

(Another paper presented at DEER 2009 by Christopher Gehrke of Caterpillar described their work with a pre-blended gasoline-diesel fuel with a derived cetane of 25-26 which did not have the same level of positive results as the UW approach. “There may be some benefit in pursuing this fuel blending-type strategy...but injecting the fuels through the same nozzles may not be the way to go.”)

Low temperature combustion (LTC) strategies (MK, PCCI, HCCI) are one way researchers are looking to reduce engine-out emissions while maintaining high engine efficiency. LTC has disadvantages, however, including difficulty at high load and no direct control of combustion timing. This has lead numerous researchers to look for a hybrid between low temperature diesel combustion and homogeneous charge combustion, Reitz said.

A lot of very interesting work has been done by Shell Global Solutions lab, Dr. Kalghatgi, for example, where he argues that as long as the equivalence ratios are low enough, in other words that the combustion process fuel preparation is mixed enough so that one stays in the low emission window, one can see some interesting results.

—Rolf Reitz

Partially premixed combustion increases ignition delay to add mixing time. There are two ways to achieve this partially premixed combustion strategy: one is with high EGR rates to reduce PM formation with low combustion temperatures, the other is by exploiting the properties of fuels. Kalghatgi and his group explored both the use of low-cetane fuels and exhaust gas recirculation (SAE 2007-01-0006). Other researchers have looked at high EGR rates (Akihama et al. SAE 2001-01-0655), and optimizing fuel reactivity (Bessonette et al., SAE 2007-01-0191).

Bessonette et al. found that they were able to extend HCCI load range by varying composition: 16 bar BMEP required 27 cetane fuel, while 3 bar BMEP required 45 cetane fuel. In other words, Reitz said, at high loads, you achieve best operation with gasoline-like fuels, while at low-loads, diesel-like fuel produces the best results.

Using the proposed “fast-response fuel blending”, the fuel mix might be as high as 85% gasoline to 15% diesel under heavy loads; under lighter loads, the percentage of diesel would increase to a roughly 50-50 mix.

For a small engine to even approach these massive engine efficiencies is remarkable. Even more striking, the blending strategy could also be applied to automotive gasoline engines, which usually average a much lower 25 percent thermal efficiency. Here, the potential for fuel economy improvement would even be larger than in diesel truck engines.

—Rolf Reitz

The US consumes about 13.5 million barrels of oil per day in transportation. Hypothetically, if the such dual-fuel engines with 53% thermal efficiency could be applied across the entire fleet, the US could reduce its oil consumption by 4 million barrels per day—about one-third of all oil destined for transportation, Reitz said.

The work is funded by DOE and the College of Engineering Diesel Emissions Reduction Consortium, which includes 24 industry partners.

Resources

• Sage Kokjohn, Reed Hanson, Derek Splitter and Rolf Reitz (2009) High-Efficiency, Ultra-Low Emission Combustion in a Heavy-Duty Engine via Fuel Reactivity Control (DEER 2009)

• Improving Fuel Efficiency with Fuel-Reactivity-Controlled Combustion (Reitz et al., ERC Symposium 2009) Rolf D. Reitz,

• Christopher Gehrke (2009) Development of Advanced Combustion Technologies for Increased Thermal Efficiency (DEER 2009)

• Gautam T. Kalghatgi, Per Risberg, Hans-Erik Angstrom (2007) Partially Pre-Mixed Auto-Ignition of Gasoline to Attain Low Smoke and Low NO_x at High Load in a Compression Ignition Engine and Comparison with a Diesel Fuel (SAE 2007-01-0006)

• Reed Hanson, Rolf Reitz, Derek Splitter (2009) Operating a Heavy-Duty Direct-Injection Compression-Ignition Engine with Gasoline for Low Emissions (SAE 2009-01-1442)

• Paul W. Bessonette, Charles H. Schleyer, Kevin P. Duffy, William L. Hardy, Michael P. Liechty (2007) Effects of Fuel Property Changes on Heavy-Duty HCCI Combustion (SAE 2007-01-0191)

• Kazuhiro Akihama, Yoshiki Takatori, Kazuhisa Inagaki, Shizuo Sasaki, Anthony M. Dean (2001) Mechanism of the Smokeless Rich Diesel Combustion By Reducing Temperature (SAE 2001-01-0655)