U. of Wisconsin RCCI combustion work progressing; modeled 53% gross indicated efficiency in a light-duty engine could result in 2x fuel savings compared to SI gasoline
(Part 2 of a series on low-temperature combustion)
Researchers at the University of Wisconsin Engine Research Center (ERC) led by Dr. Rolf Reitz are developing a dual-fuel compression-ignition engine low-temperature combustion (LTC) strategy called reactivity controlled compression ignition (RCCI) (Earlier post.) RCCI 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. RCCI results in efficient, premixed-charge combustion with near zero levels of NOx and soot.
In a recap of RCCI efforts he presented at the SAE 2011 High Efficiency IC Engines symposium earlier this year, Reitz noted that their latest work has shown that, with only small changes in injection parameters, the efficient and low-emission combustion characteristics of a heavy-duty engine using RCCI can be adequately reproduced in a light-duty engine. The team has modeled a 53% gross indicated efficiency for the improved light-duty engine.
In a single-cylinder heavy-duty research engine fueled with gasoline and diesel, RCCI delivered near zero NOx and soot and peak gross thermal efficiency of 56%; fueling with E85 and diesel resulted in an indicated efficiency of 59%. Thermal efficiencies were about 5–7% lower in a light-duty engine, due to the increased heat transfer losses associated with smaller engines. The higher heat transfer losses in the smaller light-duty engine are associated with higher swirl, larger surface-to-volume ratio, and lower piston speed.
Comparisons of the emissions and performance showed that both [heavy- and light-duty] engines can simultaneously achieve NOx below 0.05 g/kW-hr, soot below 0.001 g/kW-hr, ringing intensity below 4 MW/m2, and gross indicated efficiencies above 50%.
However, it was found that the peak gross indicated efficiency of the baseline light-duty engine was approximately 7 per cent lower than the heavy-duty engine. The energy balances of the two engines were compared and it was found that the largest factor contributing to the lower efficiency of the light-duty engine was increased heat transfer loss.
...It was found that by reducing the swirl ratio from 2.2 to 0.7, increasing the engine speed from 1900 to 2239 rev/min, and improving the combustion chamber geometry, the heat transfer losses in the light-duty engine could be reduced by the equivalent of 2% of the fuel energy/. The modeling showed that light duty engine could achieve 53% gross indicated efficiency, while maintaining near zero NOx and soot, and an acceptable ringing intensity.— Kokjohn et al. SAE 2011-01-0357
|Improving RCCI results in a light-duty engine. Source: ROlf Reitz. Click to enlarge.|
Making the optimizations in the light-duty engine improved combustion efficiency by 2.4% of the fuel energy.
In his symposium presentation, Reitz argued that RCCI offers great fuel flexibility and transient response. Proportions of low and high reactivity fuel can be changed dynamically, based on fuels used and next-cycle combustion feedback control, and the in-vehicle fuel blending reduces the need for multiple fuels at pump.
If one were to adopt RCCI in the 30% efficient SI gasoline engine, we believe that its 53% efficiency, which I picked as a number here representative of RCCI, would offer a 77% improvement in thermal efficiency, which is more than a factor of two savings in fuel.—Rolf Reitz
Low-load conditions. In their prior work, ERC has demonstrated RCCI a mid- to high-loads. Another new study (Hanson et al.), presented at SAE 2011 World Congress, reported on RCCI operation at load of 2 and 4.5 bar gross IMEP at engine speeds between 800 and 1700 rev/min in a heavy-duty engine. (This load range was selected to cover the range from the prior work of 6 bar gIMEP down to an off-idle load at 2 bar.)
The fueling strategy used port fuel injected gasoline and early cycle direct injection of either diesel fuel or gasoline doped with 3.5% by volume 2-EHN (2-ethylhexyl nitrate).
The team found that at 4.5 bar gIMEP, it was possible to maintain 54% gross indicated thermal efficiency with NOx and PM below US EPA 2020 limits. The results also showed that it is possible to operate at a near idle load of 2 bar gIMEP with a gross indicated thermal efficiency of 49% at 1300 rev/min and 44% at 800 rev/min.
The ERC team will present a paper on injection effects in low-load RCCI combustion (SAE 2011-24-0047) at the upcoming 10th International Conference on Engines & Vehicles, September 2011, Naples, Italy.
Alternative Fuels. Another study (Splitter et al.) presented at SAE 2011 World Congress considered the effect of properties of different fuel combinations in RCCI in a heavy-duty engine: gasoline-diesel dual fuel; E85-diesel dual fuel; and single fuel gasoline-gasoline+DTBP (di-tert butyl peroxide cetane improver).
The study found high gross indicated thermal efficiencies for all three, with 59% for E85-diesel, 56% for gasoline diesel; and 57% for gasoline-gasoline+DTBP.
Gasoline-diesel operation was demonstrated at engine loads up to 14.5 bar gIMEP, and E85-diesel was shown at loads up to 16.5 bar; neither strategy was load limited by engine pressure or combustion constraints.
Although not tested at loads higher than 9.6 bar IMEPg, a cetane-improved single-fuel (gasoline) strategy was demonstrated to offer near identical combustion and emission magnitudes and trends as the gasoline-diesel strategy; suggesting that testing at higher loads would be of interest.—Splitter et al.
The team plans further experimental and computational evaluations at higher and lower loads, with different fuels, and EGR effects.
|Central common-rail and new side-mounted GDI enable dual-fuel (RCCI) capability in the optical engine. Source: Mark Musculus. Click to enlarge.|
Collaboration with Sandia. ERC is collaborating with Sandia National Laboratory on several areas, including characterizing RCCI combustion using high-speed imaging diagnostics. The partners are combine planar laser-imaging diagnostics in an optical heavy-duty engine with multi-dimensional computer modeling (KIVA) to understand LTC combustion.
The dual-fuel system presents some challenges (and opportunities), noted Mark Musculus of Sandia’s Combustion Research Facility, in his presentation at the 2011 DOE Merit Review. UW and Sandia developed a new GDI side-injector system for gasoline fuels for the optical engine (in addition to the common rail) to expand its capability to dual-fuels.
The new system uses a Bosch 100 bar GDI injector mounted in place of a side window, combined with an 8-hole production Cummins XPI common rail injector (300-1,600 bar) in the cylinder head.
Rolf Reitz (2011a) Fuel Reactivity Controlled Compression Ignition (RCCI) - A Practical Path to High- Efficiency, Ultra-Low Emission Internal Combustion Engines (SAE 2011 High Efficiency IC Engines Symposium)
Rolf Reitz (2011b) Optimization of Advanced Diesel Engine Combustion Strategies. (2011 DOE Merit Review, ACE020)
Mark. P.B. Musculus (2011) Heavy-Duty Low-Temperature and Diesel Combustion & Heavy-DUty Combustion Modeling (2011 DOE Merit Review, ACE001)
Reed Hanson, Sage Kokjohn, Derek Splitter, Rolf Reitz (2011) Fuel Effects on Reactivity Controlled Compression Ignition (RCCI) Combustion at Low Load (SAE 2011-01-0361)
Sage Kokjohn, Reed Hanson, Derek Splitter, John Kaddatz, Rolf Reitz (2011) Fuel Reactivity Controlled Compression Ignition (RCCI) Combustion in Light- and Heavy-Duty Engines (SAE 2011-01-0357)
Derek Splitter, Reed Hanson, Sage Kokjohn, Rolf Reitz (2011) Reactivity Controlled Compression Ignition (RCCI) Heavy-Duty Engine Operation at Mid- and High-Loads with Conventional and Alternative Fuels (SAE 2011-01-0363)