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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

31 May 2011

(Part 2 of a series on low-temperature combustion)

Rcci
In contrast to conventional diesel combustion, the highest temperature for RCCI combustion in center of chamber (adiabatic core). The high temperature in conventional diesel combustion is next to the piston bowl surface. Source: Rolf Reitz. Click to enlarge.

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
Rcci2
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.

Musculus
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.

Resources

  • 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)

May 31, 2011 in Emissions, Engines, Fuel Efficiency, Low Temperature Combustion | Permalink | Comments (17) | TrackBack (0)

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Reitz and team are doing very interesting work, however, it's time to progress beyond single cylinder testing indicated efficiency and put the work in meaningful terms of Multi-cylinder Brake Efficiency. There are a number of key ingredients in his recipe that are built into Indicated measurements which can not be assumed for the real world, especially means to provide the mixture of fresh air and diluent at the assumed efficiencies. These are serious hurdles which are not new and are unlikely to be overcome anytime soon. This is good progress but claiming victory before realizing results in a production type engine using real turbochargers is premature. Based on work presented at the same conference in paper 2011-01-0358, it is unlikely that peak BTE will exceed that of a the current conventional commercial diesel engine.

UA

UA, I agree with you about efficiency. A gross indicated efficiency of 47 % is no better than current engines. However, if just reducing swirl would give a significant increase in efficiency, this is a very simple modification to make. Here we assume that this could be done without any detrimental effects on combustion, which is the case on conventional engines. Without studying the papers, I would not think that turbochargers and EGR control necessarily will be a show-stopper. It is relatively easy to calculate and foresee the performance of these components on a multi-cylinder engine. Advantageous with low-temperature combustion is that low NOx and PM emissions can be combined with high efficiency.

All things being equal, shouldn't high temperature combustion be more efficient? Is there an expert reading this that can explain this? Unless fuel is burned more thoroughly or less fuel is used or expansion ratio is greater or some other mechanism, higher temperature should correlate with higher thermodynamic efficiency. Someone explain this to me.

Nick
Average temperature is important for efficiency but peak local temperature is important for NOx formation. As a young engineer, some 25 years ago, I came to the surprising conclusion that if you could equalize temperatures, no NOx would be formed in a diesel engine. Of course, I soon found that someone else had figured out this before me. The problem is to avoid the high local temperatures. So called “low-temperature combustion” (HCCI, PCCI, PPC, PPCI, etc…) try to achieve this. In fact, you could even increase the temperature somewhat from the current levels in those engines without forming NOx. It sounds like the Holy Grail… The problem is to come up with a practical solution to utilize this knowledge.

@Peter_XX:

Ah. Thanks for the info.

UA, you are being far too pessimistic, not realistic.

Now to incorporate this tech into all current production. As it gains momentum, the old tech products will look very bad and more quickly be phased out.

RCCI research is valid, but the industry has been commonly doing exactly that for a while now, under a different title, and methodology.

But it effectively ammounts to the same thing. Industry uses two "fuels":
a) incoming fuel and
b)recirculated preburned or partially burned fuel

Which is called Exhaust Gas Regeneration, EGR. By varying the ammounts of both fuel charge and EGR charge, it results in a blended RCCI effect, without the complication of dual fuels systems, dual injectors, et cetera.

-GdB
if you read the paper i referenced, it is very clear that RCCI and other LTC strategies are very sensitive to air charging system efficiency. The EGR levels result in relatively high levels of boost to provide sufficient fresh air to reach the target lambda. With LTC, residual exhaust gas energy is substantially reduced which is in essense the source of available energy to drive the turbine. The combination of low available exhuast energy and demanding compressor power requirements results in substantial negative engine backpressure that the engine must overcome. In the end, this means that pumping work to drive the turbocharger significant consumes gross power and erodes Brake Thermal Efficiency. Some will argue that this just means that turbochargers will have to get better, but as you may have guessed by my screen name, i happen to know a thing or two about this subject and i would not place bets on improvements that this technology requires anytime in the near future.

Regards,

UA

UA
You are, of course, right about the problems to get high enough boost pressure. Reduced exhaust temperature will increase the pumping work. However, the impact on exhaust temperature does not necessarily have to be so high. For example, decreasing swirl ratio increases exhaust temperature (due to reduced heat losses). Furthermore, two-stage turbocharging with intercooling is already a known measure to improve these conditions. My earlier point was also that it is fairly easy to calculate the impact on turbocharging. I am pretty sure that researchers at Wisconsin Uni. can master this. Another recent example of low-temperature combustion is the so called PPC, which has been developed by Lund University in Sweden. I did some calculations on that concept myself and actually found that the assumptions by these researchers were quite conservative. Whatever acronym is used for a combustion system like this, the main problem is – the combustion system itself.

Isn't this the level of efficiency that Toyota Prius will have by 2012?

This is a higher efficiency than the future Prius engine in a fair comparison, since it concern only development of the combustion system, not all the other measures that will be used on the Prius engine. We have to compare apples and apples. Moreover, the diesel cycle is already very efficient, so it is more difficult to achieve further improvements on this than on a less efficient otto cycle. In addition, the part load efficiency will be higher for a diesel engine. Albeit the mentioned differences, it is likely that the gap between diesel and otto will decrease in the future. Note that this is only one of several different ideas on how to use low-temperature combustion. Eventually, industrial players have to decide on the concept that provides the best compromise between advantages and disadvantages. You should also note that the Prius engine will be kind of “proof of concept” in 2012 not a production-ready engine.

And so is this one....

HarveyD
Correct, both are concepts! That was my point. But your comment was that the Toyota Prius will have the new engine by 2012. It is difficult to exaggerate more. I could take up to 5 years and provide a fraction of the potential discussed when it is finally introduced. To give some perspective, Toyota increased the peak efficiency of the Prius engine by ~1% in the first 15 years.

Coming back to RCCI, this low-temperature combustion concept does not seem the most promising to me. Industry will eventually decide on what we will see in production. It is very likely that partial low-temperature concepts will be used first. Perhaps it is already used to some extent by a couple of manufacturers.

Peter,

To Harvey's point though... there is more reason to think Toyota will do what they've claimed than a research lab in Wisconsin which lives on government, foundation and industry grants.

Ralph
Well, it must be obvious to anyone that a University is not a car/engine manufacturer. As I already pointed out, industry will pick the low temperature combustion best suited for production. This research has been going on for years but I think there are better candidates that the RCCI concept from Wisconsin. I also think it was fair of me to point out that the Toyota engine was a concept, not something that is production-ready. Toyota has not even claimed to put this engine into production at a later timeframe than 2012. Once again, in defending HarveyD, your comments might wrongly indicate to readers on this site that Toyota would start to produce something like this in 2012.

By the way, do you trust a car company more than a University? I have not found anything fundamentally wrong in previous peer-reviewed publications from Wisconsin but I can find many errors in promotion material from Toyota (and other car companies as well). Toyota publishes a lot of publications but relatively few of them are peer-reviewed publications. The article at GCC was based on information from Toyota. This information was provided by Toyota in order to promote their products. In contrast to the paper from Wisconsin, Toyota has not provided enough information for a thorough review of their information. We should not compare scientific papers with promotion material.

Peter wrote:

"By the way, do you trust a car company more than a University?"

Toyota has a long track record for producing fuel-efficient drivetrains. Given that Toyota is also the leader in the production hybrid world and they're already selling a gasoline vehicle with 37% peak thermal efficiency, I'm far more apt to believe Toyota over a research lab who has not sold a single vehicle.

Don't believe the semi-scientific principle that the best determinant of future behavior is past behavior?

Regarding the merit of UofW's publications and who is more trustworthy...
-as a member of SAE, reviewer of papers, and a session organizer, I can confirm that any paper published by SAE has had typically 3 people review it, however, this doesn't mean that the work published inside is irrefutable. Everything needs to be taken with a grain of salt. It is not unheard of to see 2 papers on the same subject come to completely different and contradictory conclusions, even at the same conference in the same year. Both came to logical conclusions based on the work they did. Usually the difference is in the scope of the work and how thoroughly a hypothesis is tested and the assumptions made by the authors. Because they do not present their whole body of work to support their findings in their papers, it is often impossible to tell who is right. Moreover, it is unlikely that somebody doing cutting edge work has a peer with the detailed first hand experience to really challenge the work as a reviewer. If the paper was on a well explored and understood topic, it would not qualify as research. Also, for many research institutions (universities), researchers (professors) performance is measured by the quantity of their publications. This can have a big impact of whether someone will ever make tenure or not. Unfortunately, quality is sometimes left by the wayside in the process. Thus, in my opinion, university publications are no more (or less) trustworthy than those of a for profit manufacturer whose interests and motive are more obvious for all to see.

UA

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