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U. Wisconsin team reports gross indicated thermal efficiency of RCCI operation near 60%

In a paper presented at the 2013 SAE World Congress, a team from the University of Wisconsin reported a gross indicated thermal efficiency of Reactivity Controlled Compression Ignition (RCCI) operation of near 60%, given optimized combustion management and thermodynamic conditions. That 60% gross engine efficiency provides a pathway to meet the DOE Super Truck 50% brake thermal efficiency (BTE) engine goal as well as a pathway for reaching 55% BTE, the researchers concluded.

The findings also showed that improvements to boosting system efficiencies for low exhaust temperatures and overall reductions in friction are required to capitalize on the high gross efficiences offered by RCCI.

The paper was one of a set of RCCI-related papers presented at the World Congress, along with an RCCI presentation by Dr. Rolf Reitz at the SAE High Efficiency ICE Engine symposium immediately beforehand.

RCCI basics. RCCI is a promising low-temperature combustion strategy offering a pathway to high-efficiency clean combustion using in-cylinder blending of fuels with different auto-ignition characteristics. (Earlier post.)

A potential pathway to simultaneously address emissions and fuel economy regulations is through implementation of low-temperature combustion (LTC) strategies. Typically these strategies rely on long ignition delays to increase fuel mixing, reducing local equivalence ratios [φ] or temperature or both.

One of the most versed PTC strategies is homogeneous charge compression ignition (HCCI). This strategy relies on the autoignition of) a fully premixed air-fuel charge, affording operation with very lean mixtures (φ or φ'1< 0.3). The result is that near-zero in-cylinder levels of NOx and PM emissions are possible.

Additionally, as noted by Foster, the lean charge reduces combustion gas temperature, reducing the driving potential for heat transfer, and increasing the expansion γ, both increasing work potential. However, to capitalize on these advantages the combustion event must be knock-free.

...A method to increase HCCI combustion control and knock mitigation is through the addition of stratification...A different stratification approach is partial fuel stratification. This technique introduces controlled equivalence ration (φ) stratification into the chamber...Modeling results by Kokjohn and Reitz have shown that φ plus reactivity stratification further enhances the ignition gradient within the charge, enabling knock-free autoignition combustion phasings near TDC at mid-high load operation...This technique has correspondingly been called reactivity controlled compression ignition (RCCI).

—Splitter et al.

RCCI provides control by varying fuel reactivity using two fuels with different reactivities—e.g., the port fuel injection of gasoline (mixed with intake air, as in spark-ignition engines) and multiple direct-injections of diesel fuel into combustion chamber later during compression (as in diesel engines).

The 60% study. In the thermal efficiency study, Dr. Derek Splitter (now a post-doc at Oak Ridge National Laboratory) and his colleagues explored methods to obtain the maximum practical cycle efficiency with RCCI. The study used both zero-dimensional computational cycle simulations and engine experiments conducted using a single-cylinder heavy-duty research diesel engine adapted for dual fuel operation, with and without piston oil gallery cooling.

RCCI combustion with in-cylinder fuel blending using port-fuel-injection of a low reactivity fuel and optimized direct-injections of higher reactivity fuels had earlier been shown to permit near-zero levels of NOx and PM emissions in-cylinder, while simultaneously realizing gross indicated thermal efficiencies in excess of 56%.

The study considered RCCI operation at a fixed load condition of 6.5 bar IMEP an engine speed of 1,300 rpm. The experiments used a piston with a flat profile with 18.7:1 compression ratio.

The study found that a gross thermal efficiency (GTE) of 59.7% was possible with a high compression ratio; lean operation (Φ<0.3); and a 50% reduction in heat transfer and combustion losses.

The results of the study showed that the indicated gross thermal efficiency could be increased by not cooling the piston; by using high dilution; and by optimizing in-cylinder fuel stratification with two fuels of large reactivity differences.

Simulations using GT-Power demonstrated that the RCCI operation without piston oil cooling rejected less heat, and that ∼94% of the maximum cycle efficiency could be achieved while simultaneously obtaining ultra-low NOx and PM emissions.


  • Splitter, D., Wissink, M., DelVescovo, D., and Reitz, R., “RCCI Engine Operation Towards 60% Thermal Efficiency,” SAE Technical Paper 2013-01-0279, 2013, doi: 10.4271/2013-01-0279


Trevor Carlson

So what happens to this engine when you run out of diesel before gas or vice versa? In the real world you know this is going to happen and the engine better not grenade.

I'm assuming there would need be enough control to allow the engine to run rich enough and with timing to support full gas combustion or full diesel combustion. Of course doing so would dramatically hurt your fuel consumption as you'd be down in the 40% thermal efficiency range like current high compression IC engines.


The same thing that happens when you run out of diesel with a regular diesel engine. It stops running.

Or if it runs out of gasoline then it runs like a diesel engine.

But more likely it would use a limp home mode like an SCR engine when either fuel stream dropped below a certain value.

Trevor Carlson

I'm thinking of how you'd keep an 18.7 CR gas engine from detonating even with the maximum "limp home" methods. It would need to run rich, λ>1 with maximum ignition timing retard (it would need a spark plug which is not clear if the above concept engine has one) and extra late intake valve closing.

Diesel only operation wouldn't be as hard unless it was really cold and the engine wasn't started already.

A dual fuel engine would need to have the ability to run on either fuel independently. For instance users may find the cheapest gas stations do not have diesel available, and so they would fill up at different stations at different points of their trip. Some stations have both handles at the same pump. This would be the ideal arrangement for minimum refueling time.

Roger Pham

@Trevor and RFH,
This is practically a diesel engine that can also run on port-injected gasoline and using diesel direct injection for ignition. As such, diesel fuel must always be available for it to run, although it can run without gasoline, albeit with high exhaust emission! The advantage of this over conventional diesel is very clean emission that can do away with expensive SCR and DPF post combustion exhaust treatment.

Subtitute the gasoline for NG and you'll be able to burn very low-cost fuel at higher than diesel efficiency, AND the ability to be flexible-fuel vehicle that can run even when no NG is available. Diesel fuel is far more available than CNG. Current spark-ignited NG engine is not nearly as efficient as diesel engine, more on par with current gasoline engine. This is of profound importance because CNG tank is far heavier and bulkier than gasoline tank, so, if you can nearly double the engine's fuel efficiency by this method, the savings in the cost, the weight, and the space occupied by the CNG tank will make CNG vehicles far more practical! Fill up with local CNG to realize huge fuel savings, while resort to diesel fuel for longer trips where CNG may not be available. Add a hybrid drive train to this engine technology and the CNG tank can get even smaller.

Of course, the NG can be substituted for by the use of synthetic methane made with 2/3 of energy from renewable or nuclear H2 and 1/3 of energy from waste biomass, and we will have a zero-CO2 transportation solution using ICE in a hybrid drive train.

Rafael Seidl

60% gross thermal efficiency from an ICE is actually phenomenal, even giant marine two-stroke diesels are limited to around 52% in the optimal operating point. Of course, having to manage two fuel tanks is a major hassle, especially in a mobile application.

However, plenty of stationary small-scale gensets already run on natural (or other) gas with diesel fuel pilot injections. Given the large differences in the reactivities of those fuels, perhaps the combustion strategy developed here could be applied in that context first.


Also Diesel fuel has LOWER octane than gas, so you cannot inject much of it until the engine is ready for ignition.

A well written article with little hype or Scuderia.

This technology sounds like it is a few years away - depending on whether the cost can be controlled and the real world efficiency.

And no, just making them by the millions will not make this technology economical.

Roger Pham

Diesel fuel is only used in trace quantity for triggering the ignition of the homogeneous gasoline fuel and air mixture. In the rare occasions of diesel-fuel-only situation, perhaps a multiple injection of diesel fuel will be needed to lower exhaust emission of NOx and PM.

This is lower cost than diesel because of the lack of requirement for expensive emission control system associated with diesel engine. This is a hybrid between diesel and gasoline engine. The diesel injector replaces the spark ignition system in the conventional port-injection gasoline engine. Since most of the fuel is already well mixed, the diesel fuel injector may not need to be of expensive ultra-high pressure type. A lower cost, moderate pressure diesel injector may work just fine to meet current emission requirement without further NOx treatment. Turbocharging will be required to permit downsizing on par with GDISI.

However, to meet proposed future Tier III emission, perhaps high-pressure diesel injector will be needed in order to create superfine mist in order to allow for proper combustion when cooled EGR will be required for stoichiometric combustion and 3-way cat to bring down NOx to almost zero.
In this case, the higher expense of this technology will be offset by significant saving in fuel cost.

Jim McLaughlin

There are already multiple technologies that combine natural gas and diesel. Clean Air Power and Westport have different tradeoffs. Both can run on diesel only but at the moment, only Westport can comply with US10/Euro VI emissions regulations and has limited power in diesel only mode, as well as limited time allowed. Clean Air power is simpler but unburned methane is above US10/Euro VI limits and a catalyst to scrub it is impractical at the moment. These are both in commercial use today. So combining gasoline and diesel does not seem far fetched.

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