On the emissions side, modeled drive cycle emissions results showed between 17 and 21% reduction in NO_x with multi-mode RCCI compared with diesel-only operation. However, if an engine has to switch to conventional diesel combustion (CDC) operation during high engine loads, the engine out NO_x will be very high and quickly can degrade the NO_x reduction potential of a multi-mode RCCI strategy, they found.

RCCI background. RCCI is a dual-fuel combustion technology developed by Dr. Rolf Reitz and colleagues at the University of Wisconsin-Madison Engine Research Center laboratories. RCCI, a variant of Homogeneous Charge Compression Ignition (HCCI), provides more control over the combustion process and has been shown to have the potential to lower fuel use and emissions significantly (e.g., earlier post, earlier post).

Dual-fuel RCCI injection strategy. Click to enlarge.

The RCCI process uses in-cylinder fuel blending with at least two fuels of different reactivity and multiple injections to control in-cylinder fuel reactivity to optimize combustion phasing, duration and magnitude. The process involves introduction of a low reactivity fuel into the cylinder to create a well-mixed charge of low reactivity fuel, air and recirculated exhaust gases.

The process, notes the ORNL team in their paper, has an advantage over many advanced combustion strategies in that the fuel reactivity can be tailored to the engine speed and load, allowing stable low-temperature combustion (LTC) operation to be extended over more of the light-duty drive cycle load range. However:

To date the potential vehicle fuel economy improvements that RCCI could allow are not well understood. … It is difficult to draw conclusions on drive cycle fuel economy and emissions performance for combustion strategies in the development stage that have demonstrated only a limited number of steady-state operating points. Vehicle systems simulation tools such as Autonomie developed by Argonne National Laboratory for DOE can be used to simulate vehicle operation using model-based simulations. The simulations use performance-based measurements, including fuel consumption and exhaust properties such as emissions and temperature, which are tabulated allowing for the generation interpolated response surfaces over the entire operating range of the engine maps.

—Curran et al.

The study.The ORNL work explorespotential fuel economy of multi-mode RCCI operation using vehicle systems simulations with experimental steady-state engine maps compared with a representative 2009 gasoline port-fuel injected (PFI) engine as a baseline.

The researchers used experimental steady-state RCCI operating points on a modified multi-cylinder GM 1.9L engine to develop an RCCI speed/load map consistent with a light-duty drive-cycle with sufficient detail to support vehicle simulations. The ORNL RCCI map shows an increase in RCCI operation over previous low-temperature combustion operation maps—but still does not cover the engine speed and load required to meet all power demands over the light-duty drive cycles with the self-imposed constraints on the engine experiments which led to the RCCI engine map.

Experimental RCCI map with UDDS drive cycle point overlain. Click to enlarge.		Experimental CDC map with stock pistons with UDDS drive cycle point overlain. Curran et al. Click to enlarge.

As a result, the simulations used a multi-mode RCCI/diesel operating strategy where the engine would operate in RCCI mode whenever possible, but would switch to diesel mode at the highest and lowest engine operating points.

Simulations were carried out in Autonomie using a 1580 kg passenger vehicle (mid-size sedan, i.e., Chevrolet Malibu) over numerous US federal light-duty drive cycles. The OEM pistons for the base engine (110 kW/148 hp and 315 N·m/232 lb-ft) were replaced with pistons modified for RCCI using CFD modeling by the University of Wisconsin. A certification grade gasoline was used for the low-reactivity fuel and a splash-blended B20 was used for the high reactivity fuel.

Four drive cycles were used:

• UDDS (LA4 or city test) to represent city driving conditions
• HWFET for highway driving under 60 mph (97 km/h)
• US06, an aggressive driving cycle that is also called the “supplemental FTP”
• New York City Cycle (NYCC) for stop-and-go traffic conditions

The team found that the amount of RCCI operation varied significantly with the different drive cycles. UDDS had 72% of the cycle by distance run in RCCI mode but only 55% by time since significant portions of the cycle were very low load with interspersed periods of idle. The HWFET had very little idling, and RCCI mode was achievable over 88% of the cycle by distance and 86% by time. For all of the drive cycles except HWFET, multi-mode operation had more diesel fuel than gasoline. The percentage of diesel fuel (B20) used during RCCI-only mode ranged from a low of 31% in the high load US06, which would have a high amount of time run in high load RCCI, which would be predominately gasoline, to a low of 43% in the stop-and-go NYCC.

Multi-mode RCCI operation fuel economy improvements ranged from 39% for US06 to up to 67% for NYCC. The improvements seen for the UDDS and HWFET were 59 and 53%, respectively. The fuel economy improvements compared with CDC operation ranged from 8 to a high of 15%.

The authors are planning a follow-up study is planned to create a multi-mode RCCI operating map using only the RCCI-modified pistons; for the current simulations, the multi-mode operating map switched between piston types. Another self-identified limitation of the study was the lack of transient engine performance for same mode and mode switching to calibrate the model that would account for transient performance.

Resources

• Scott J Curran, Zhiming Gao, Robert M Wagner (2014) “Reactivity-controlled compression ignition drive cycle emissions and fuel economy estimations using vehicle system simulations” International Journal of Engine Research doi: 10.1177/1468087414562258