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