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Scuderi Split-Cycle Engine Obtaining Better Combustion Consistency Under Low Load When One Crossover Passage Is Shut Off
23 June 2010
When operating under low load conditions, the pressure—and ultimately the efficiency—of the Scuderi Engine prototype increases, and becomes more repeatable from cycle to cycle, when one of the two crossover passages is shut off (thereby effectively doubling the pressure in the single open crossover passage), testing by Southwest Research Institute (SwRI) has shown.
The Scuderi Engine is a split-cycle engine that divides the four strokes of the Otto cycle over a paired combination of one compression cylinder and one power cylinder. Intake air is compressed in the compression cylinder and transferred to the power cylinder for combustion. The Scuderi Engine prototype (earlier post) has two crossover (Xovr) passages that connect the two cylinders, which separate the four strokes of the pistons.
Each crossover passage has a crossover compression (XovrC) valve on its upstream end and a crossover expansion (XovrE) valve on its downstream end, which control flow through the crossover passage. In one, cylinder intake and compression occur while combustion and exhaust take place in the other. Fuel is injected in the downstream end of the crossover passages just as the XovrE valve opens. Compressed air then travels at sonic velocity from the XovrE passage to the combustion cylinder, where combustion occurs.
One of the things that was always critical to us was whether or not we would be able to maintain our pressure [at low load]...one of the keys to the Scuderi engine is to maintain high pressure in the crossover passage. We have had several schemes to do that at low loads, low pressure...because we knew that as we went down in load and speed we would be losing pressure in the crossover passages.
—Stephen Scuderi, vice president and patent attorney for Scuderi Group
One concern the Scuderi Group had about the scheme of shutting down one crossover passage at low load was a potential degradation of the mixing of air and fuel, Scuderi said. In fact, he noted, the testing showed that mixing improved with one passage, resulting in a more consistent combustion process.
In tests of the prototype running at near idle load and speed (i.e., less than 2 bar load and 700 rpm) to loads of up to 4 bar and speeds of up to 1600 rpm, shutting down one of the two crossover passages yields a coefficient of variance (COV) as low as 1.4%, according to tests conducted by SwRI.
Typically, a conventional auto engine running on the Otto Cycle will produce COV rates ranging from 2.5 to 4.0%. COV is a measure of the variability between each combustion cycle. Lower COV figures correlate with higher efficiency and more complete combustion. Moreover, the very low COV in the Scuderi Engine indicates that the combustion mix (oxygen and fuel) is even more complete and robust than in a conventional engine.
This makes us very confident that, when fully mapped and analyzed, the efficiency of the engine will be even higher than previously predicted, which so far has been the pattern in the measured data we have acquired to date.
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