We’re further analyzing the soot to learn more about its potential impacts on light-duty engines and air quality.

—John Storey, lead author

The bulk of the experimental results presented in the ORNL study are from a 2007 GM DI 1.9-L diesel engine modified to operate in the RCCI combustion mode. The light-duty diesel was equipped with the original high-pressure common rail injection system, high-pressure exhaust gas recirculation (EGR), variable geometry turbocharger and variable swirl actuation. The intake manifold was modified for PFI. Both stock pistons and pistons with a wider, shallower bowl were used. The modified piston yielded an increase in both combustion efficiency and indicated efficiency for RCCI operation in single-cylinder engine experiments.

Originally developed by researchers at the University of Wisconsin led by Dr. Rolf Reitz, RCCI is a promising low-temperature combustion (LTC) technique utilizing two fuels with different reactivities to produce low NO_x and PM while maintaining high thermal efficiency. (Earlier post.)

RCCI uses early direct injection (DI) of a high-reactivity fuel such as diesel fuel and a well-mixed low-reactivity fuel such as gasoline or methane. Unlike conventional dual-fuel combustion modes, RCCI completely decouples the end of injection of the DI fuel and the start of combustion. This allows for a well-mixed and dilute charge.

NOx and soot formation in local equivalence ratio (Φ) versus local temperature space. Storey et al. Click to enlarge.		RCCI concept. PFI: port fuel injection; DI: direct injection. Storey et al. Click to enlarge.

The low-temperature combustion (LTC) process results in ultra-low NO_x and soot emissions compared to CDC by avoiding the traditional NO_x and soot formation islands … RCCI is an LTC strategy that uses a slight amount of fuel stratification, which may have impacts on the PM formation and oxidation processes.

… With the highly premixed LTC mode with two fuels of differing reactivities, compositions and boiling ranges, it would be expected that any PM formed would be different both physically and chemically.

—Storey et al.

Other research into RCCI emissions has particulate matter has shown that, despite a near zero smoke number, a significant particulate mass can be collected on filter media used for particulate matter certification measurement. In addition, particulate matter size distributions reveal that a fraction of the particles survive heated double-dilution conditions.

RCCI operation, with both low FSN [soot] and NO_x emissions, offers the promise of meeting the [US EPA] emission limits with much less aftertreatment than conventional diesel; RCCI PM, however, tends to have a high condensed organic phase, which will most likely get measured as PM mass. Thus, understanding the physicochemical characteristics of the PM will be critical to mitigating the PM emissions.

—Storey et al.

The mass collected on the RCCI filter is larger than the mass collected from PCCI operation, despite the obvious lack of soot. Expressed another way, there is no direct correlation between FSN (soot) and measured PM mass for RCCI as there is for conventional combustion modes with more carbonaceous soot components. Storey et al. Click to enlarge.

Physical measurements at ORNL and by others show that the measured PM from RCCI combustion is dominated by semi-volatile hydrocarbon (HC) species that are mostly removed with heat in an evaporator tube or by catalytic stripping.

At all of the engine operating points investigated, there was a consistent solid particle fraction peaking in size between 40 and 60 nm—a mode that is very low in concentration and does not show up in conventional soot measurements. The majority of the PM mass is in the organic phase.

The nature of the RCCI PM indicates a fundamentally different PM formation process than that which occurs during CDC might be occurring. The highly pre-mixed nature of RCCI combustion does not provide the localized rich zones that are normally attributed to soot formation in CDC. Additional evidence of a different PM formation process is provided here with results showing no significant difference in PSD [particulate size distribution] with RCCI using biofuel–conventional fuel combinations.

Typically, in conventional combustion of diesel or gasoline, an oxygenate such as biodiesel or ethanol will reduce PM because the oxygen is thought to interfere with the soot formation process. However, there was little difference observed in the size and concentration of PM during RCCI combustion with biofuels. Thus, the formation of RCCI PM appears to occur through the condensation of semi-volatile HCs on a small solid core (carbonaceous, ash or other), but additional research into the in-cylinder formation pathway of this measured PM is still needed.

… Until the requirements for PM emission control are determined, speculating on the potential health effects of RCCI engine PM is premature. Without transient engine experiments in a hardware-in-the-loop setting or actual vehicle experiments, the data are insufficient to confirm the ability of an RCCI engine or vehicle to meet PM regulations without any aftertreatment of PM, especially in light of research pointing to the need for a multimode CDC-RCCI strategy to cover the full speed and load range. An oxidation catalyst associated with the vehicle aftertreatment train may be sufficient to remove the semi-volatile organics responsible for the PM mass. The need for particulate filters associated with such multimode combustion is still not well understood.

—Storey et al.

Resources

• John ME Storey, Scott J Curran, Samuel A Lewis, Teresa L Barone, Adam B Dempsey, Melanie Moses-DeBusk, Reed M Hanson, Vitaly Y Prikhodko, and William F Northrop (2016) “Evolution and current understanding of physicochemical characterization of particulate matter from reactivity controlled compression ignition combustion on a multicylinder light-duty engine,” International Journal of Engine Research doi: 10.1177/1468087416661637