Study shows viability of RCCI in a two-stroke engine; higher efficiency than direct-injection spark ignition
A team at the Engine Research Center (ERC), University of Wisconsin-Madison has demonstrated the viability of reactivity-controlled compression ignition (RCCI) in a two-stroke engine. (Earlier post.) A paper on their work is published in Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering.
RCCI is a dual-fuel combustion technology developed by Dr. Rolf Reitz and colleagues at the ERC. RCCI, a variant of Homogeneous Charge Compression Ignition (HCCI), provides more control over the combustion process and has been shown to have the potential to lower fuel use and emissions significantly. The RCCI process 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.
The process involves introduction of a low reactivity fuel into the cylinder to create a well-mixed charge of low reactivity fuel, air and recirculated exhaust gases.
For their study, the ERC team modified a two-stroke outboard engine to accommodate two direct fuel injectors for RCCI combustion in one cylinder, while the production direct-injection spark ignition combustion system was maintained in the other cylinder for comparison of performances at equivalent operating conditions.
The team found that using gasoline and diesel as the two RCCI fuels resulted in unstable combustion and rapid accumulation of particulate matter in the emissions-sampling equipment. However, RCCI experiments with gasoline and n-heptane (higher volatility) proved to be successful.
They found that the low-reactivity fuel fraction and the high-reactivity start-of-injection timing were independent combustion-control levers.
At 1500 rpm, an IMEP (indicated mean effective pressure) of 2.5 bar and NOx emissions of 1.25 g/kWh, reactivity-controlled compression ignition resulted in a lower coefficient of variation in the indicated mean effective pressure, lower carbon monoxide emissions and a significantly higher gross indicated efficiency than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (36.3% versus 27.0% and 25.7% respectively).
At 1200 rpm, IMEP of 2.0 bar and hydrocarbon + NOx emissions of about 16.5 g/kW h, the reactivity-controlled compression ignition efficiency was still significantly better than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (32.9% versus 25.2% and 26.6% respectively).
The researchers suggested that further design optimization might enable the use of a standard diesel fuel instead of n-heptane.
Scott R Miles and Jaal B Ghandhi (2016) “Investigation of reactivity-controlled compression ignition combustion in a two-stroke engine” Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering doi: 10.1177/0954407015624554