Utilizing large amounts of exhaust gas recirculation (EGR) and high fuel injection pressure are common strategies used to promote the fuel and air mixing process, the authors note. However, the application of high EGR reduces in-cylinder oxygen content, causing deterioration of combustion efficiency that leads to increased hydrocarbon (HC) and carbon monoxide (CO)emissions. One way to avoid the disadvantages of high EGR rates is to use a lower-cetane, higher-volatility fuel.

The resistance to autoignition of the low-cetane fuel may provide a sufficient ignition delay for mixing and faster vaporization by high volatility can increase mixing rate. The combination thus can allow thorough mixing without high EGR rates. Accordingly, some studies have explored an LTC strategy with gasoline-like fuels, the authors note.

However, because of gasoline’s resistance to autoignition, strategies like intake boost, intake heat or fuel stratification had to be used for stable operation in this LTC combustion mode. Therefore, a fuel with a moderately reduced octane rating, such as a blend of diesel and gasoline, recently termed “dieseline”, has been suggested for LTC, in particular at light loads. In an LTC strategy with 1500 bar injection pressure and an EGR rate higher than 40%, the blend of 50% diesel and 50% gasoline could maintain stable operation at idling conditions and the benefits in emissions were also observed at heavy loads.

Following Weall et al’s work, in this paper dieseline was prepared by adding a moderate amount of gasoline (20% by volume) into diesel fuel and was tested using a previously developed, late-injection premixed LTC mode. Fuels with gasoline proportion higher than 20% was not investigated due to the increased combustion instability. Different from prior work, the EGR rate was limited to less than 40% to avoid the production of high concentrations of incomplete combustion products and decreased combustion efficiencies found in previous LTC research. A novel high-efficiency and low-emission dieseline LTC mode and the extension of its operational range are the focus of this work.

—Han et al.

In their study, they used a single-cylinder engine based on a four-cylinder direct-injection production diesel engine sold by General Motors in Europe. The compression ratio of this engine was decreased from 19:1 to 15:1 by replacing the original piston with one featuring a larger volume combustion bowl.

Two fuels were used: a baseline ultralow-sulfur certification diesel fuel, which had a cetane number of 42; and a dieseline blend of 80% of the baseline diesel and 20% commercial 93 octane, (RON + MON)/2, unleaded gasoline by volume. Dieseline has a lower cetane number and higher volatility.

In their study, they found that:

• The longer ignition delay and higher volatility of dieseline produces a more homogeneous fuel air mixture before the start of combustion. The more premixed charge extends smokeless combustion to higher loads and produces lower NO_x emissions across the load sweep, with relatively high combustion efficiency maintained by light use of EGR.

• Intake boost shifts the entire dieseline LTC operation range to higher loads. Increased air quantity as a result of intake boost allows more fuelling so the high load limit is increased and the light load limit also moves upward due to the increased NO_x emissions at light to mid loads.

• Slightly increased EGR reduces NO_x emissions, extending dieseline LTC to lighter loads without heavily deteriorated combustion efficiency.

• Within the context of the study, they found that intake boost poses opposing effects on HC and CO emissions. Given an equivalence ratio, increased intake pressure results in reduced HC emissions because reduced ignition delay with intake boost restricts the possibility of unburned fuel to enter the cold boundary zones.

• On the contrary, CO emissions increase with intake pressure at high equivalence ratios because the decreased bulk cylinder temperatures contribute to CO formation.

The University of Michigan/General Motors Collaborative Research Laboratory in Engine Systems provided financial and technical support for the study.

Resources

• Dong Han, Andrew M. Ickes, Dennis N. Assanis, Zhen Huang and Stanislav V. Bohac (2010) Attainment and Load Extension of High-Efficiency Premixed Low-Temperature Combustion with Dieseline in a Compression Ignition Engine. Energy Fuels, Article ASAP doi: 10.1021/ef100269c