Many researchers have shown that LTC strategies such as Homogeneous Charge Compression Ignition (HCCI) and Premixed Charged Compression Ignition (PCCI) are promising techniques for simultaneous NO_xand soot reduction. Due to the existing fuel infrastructure, most HCCI and PCCI research has been conducted using either gasoline or diesel fuel. However, in their neat forms, each fuel has specific advantages and shortcomings for LTC.

For instance, gasoline has a high volatility; thus evaporation is rapid and a premixed charge can be obtained using port fuel-injection. However, because gasoline resists auto-ignition, it becomes difficult to achieve combustion at low-load conditions. Conversely, diesel fuel has superior auto-ignition qualities; however, this can result in difficulty controlling the combustion phasing as engine load is increased.

...experiments...suggested that the best fuel for HCCI operation may have auto-ignition qualities between those of diesel fuel and gasoline, depending on the engine speed and load. Based on these results and experiments...the RCCI strategy was developed to allow the fuel reactivity to be optimized at all speeds and loads, providing an increased operating range for premixed LTC.

—Hanson et al.

Engine experiments were performed at ORNL using a 1.9L, 4-cylinder 2007 model light-duty diesel. The engine used all stock hardware, except for the high pressure EGR heat exchanger, gasoline port ful injection system and optimized pistons for RCCI. The team replaced the OEM ECU with a Drivven engine controller to allow full access for control of all parameters and sub-systems, including the PFI system.

RCCI experiments were performed using port fuel-injection of gasoline, E20 or E85 and direct injection of ultra low sulfur diesel (ULSD) or B20. E20 testing was conducted over a wide speed/load map representative of operation in a light-duty vehicle over the FTP75 cycle. B20 testing was carried out at loads of 2.0, 2.6 and 4.2 bar BMEP and at engine speeds of 2,000, 1,500 and 2,300 rpm, respectively. Among the findings were:

• E20 increased brake thermal efficiency on average compared to gasoline. The team suggested the increase in BTE was caused by a net gain between lower heat transfer and exhaust losses while having higher combustion inefficiency and increased volumetric efficiency (VE). From these gains in VE and reduced thermal losses, the global average increase in BTE was 1.33%

• With E20, the maximum pressure rate rise (MPRR) and HRR were reduced, which allowed for a 2 bar increase in the peak load from 8 to 10 bar BMEP.

• Use of E20 decreased the required PFI fuel fraction, which increased NO_x emissions.

• Increased volumetric efficiency from the use of E20 lead to lower pumping losses.

• Use of B20 allowed for a reduced PFI fraction; unlike the use of E20, this decreased NO_x emissions.

• Use of B20 in reased combustion efficiency due to reduced HC, but had higher CO. This gain in combustion efficiency helped to increased BTE by up to 1.68%.

• The increased DI fuel fraction from the use of B20 increased MPRR similar to the E20 results.

• The use of E85 and B20 allowed the peak BTE of RCCI to be increased from 40% with gasoline/diesel operation to 43%

• The engine peak BTE was similarly increased from 42% to 43%. The OEM engine reached peak BTE at 16 bar BMEP vs. 11 bar BMEP with RCCI.

...it is important to note that the present results are not necessarily optimal, as this was the first attempt at using biofuels in a MCE [multi-cylinder engine] with RCCI. Future work should be done to optimize the injection strategy for the best NO_xHC trade off, and thus improve some of the lower combustion efficiency points seen in the data.

In addition, the global equivalence ratios could benefit from being lower at high load. Work to improve the VE via increased airflow from improved cylinder head port design and/or using advanced two-stage turbocharging would be beneficial. Higher airflow rates would allow lower NO_x emission at high loads and possibly also to allow higher peak loads to be reached.

Understanding cylinder-to-cylinder thermal effects in the MCE will be needed to help develop closed loop combustion control strategies. Closed loop control of combustion phasing will be necessary for implementation in mass production to deal with varying intake temperatures.

Finally, with advanced air handling systems, the use of EGR can be investigated as a means for further emissions reduction and for increase dilution at high loads for combustion phasing delay, possibly allowing for even higher peak loads and efficiencies.

—Hanson et al.

Resources

• Hanson, R., Curran, S., Wagner, R. and Reitz, R. (2013) Effects of Biofuel Blends on RCCI Combustion in a Light-Duty, Multi-Cylinder Diesel Engine. SAE Int. J. Engines 6(1) doi: 10.4271/2013-01-1653