The century-old concept of the "split-cycle" engine may be on the verge of a comeback, thanks to an engineering company that has put a new twist on the old technology.
If the latest claims by Scuderi Group are accurate, the technology could shake up the auto industry and provide some competition for today's alternative fuel vehicles. Scuderi executives say its split-cycle concept is boosting fuel efficiency by 20 percent to 30 percent (and that figure could ultimately reach 50 percent) while reducing emissions.
One cylinder of Scuderi's split-cycle engine performs intake and compression, while the other handles power and exhaust. The engine completes all four strokes in one crankshaft revolution. (Figure courtesy of Scuderi Group.)
The concept, believed to have been patented as far back as 1912, has long lacked the technical appeal of the age-old Otto cycle engine. Split-cycle engines separate a cylinder's four strokes -- intake, compression, power, and exhaust -- between a pair of cylinders. One cylinder performs intake and compression, while the other is dedicated strictly to power and exhaust. In essence, one side of the engine acts like an air compressor, and the other side produces the power. Air travels between the sides through a "crossover passage."
In theory, this all sounds good, because it enables the engine to do all its work in one rotation of the crankshaft, instead of two, as a conventional four-stroke does. However, split-cycle efficiency was never very good until Scuderi changed a key part of the combustion process.
"We found that if you split the cycles and fire like a normal engine, the efficiency level will never match that of a conventional engine," Sal Scuderi, president of Scuderi Group, told Design News. "But if you fire after top dead center, the efficiency exceeds that of a conventional engine."
Scuderi Group also enhances that efficiency by adding a turbocharger to the equation. The turbocharger uses energy from the exhaust to reduce the size of the compression stroke. As it pulls more energy off the exhaust, the compression stroke gets smaller, and engine size shrinks.
Not likely: the Mechanically driven Supercharger not only introduce a power drag, but their compression efficiency is not that great (the Turbo driven supercharger is almost always more efficient). Either won't match the air compression and handling needs of the cylinder-piston over the whole range of RPM's and loads that the auto engine commonly faces.
The thing with a Turbo or Supercharger is that both have performance maps that do not match the performance map of the piston engine entirely. The Turbocharger produces way too much compressed air at high RPM's, and the mecahnically driven Supercharger does the opposite. Thus, the Turbo needs a waste gate to avoid overcharging the combustion chamber of the piston-cylinder engine, and presents some lag; the Supercharger has lower overall compression efficiency but does deliver pressure boost inmediately, and then runs off at higher RPM's. The idea is that the pressure boost needed by the cylinder-piston is not really well met by both devices; thus the concept shown could well be a better match for the cylinder charging needs, maybe better than the traditional Otto cycle. Please consider that the Otto cycle engine usually operates at very low effective compresion ratios, due to the throttle valve being open completely only when the accelerator pedal is pushed hard! This concept remainds me of the so called "Turbo-Compound" engines used in the 50's in the last piston driven airliners like the DC-7 Super and the latter Super Costellations... but were overly complex and unreliable, and were soon replaced by the jet turbine.
I've followed and studied alternative engines for many years. Many are pure scams, such as the MYT engine and quasiturbine. The split-cycle concept has some merit, but in the end has only a tiny advantage over conventional technology, and some large disadvantages. For example, conv. engines benefit from doing all 4 cycles in one cylinder, because the avg. temperature is lowered. The split-cycle will have a very hot power cylinder. The "one power cycle per revolution" is no advantage, if you view the mechanism as two pistons (which it actually is). In the end, it is just running a rather conventional 4-stroke process with a different mechanism. A better use of research dollars is to figure out how to clean-up turbodiesels (with proven superior efficiency), or incrementally improve conventional engines (reduce friction, direct injection, eliminate throttling losses, downsizing, hybrids). These approaches can give as much or actually more efficiency improvements. And...I need to mention that lower weight and better aerodynamics are probably the best path to better efficiency, as the Edison2 car has proven in actual results in the Automotive X-Prize competition (>100mpg, good real-world performance).
EV's are not a panacea anytime soon either, although someday maybe. I would like to see a renewable-based liquid fuel (biofuel or solar-synthesized fuel) take the place of gasoline. This could allow the most benefit with the least change to current infastructure and user convenience profiles.
A two-Stroke is VERY different... and it's Specific Fuel Consumption (efficiency) is lower than in a 4-Stroke. By optimizing the charging cylinder to perform a charging task only, and then making the power cylinder optimum for producing the downstroke better, can suspect this concept couls have advantages. But is is definitely NOT a two-stroke. (the better eficiency Two-strokers are those that burn diesel and have an integral blower to sweep the burnt gasses out of the combustion chamber, but the ubiquitous gasoline two-strokers are notably less efficient and produce a lot more contaminant gasses in their operation. Please analize in detail the article's drawing, then the alleged advantages can be visualized. Anyway, the idea of using a crankshaft driven compressor in order to handle the air, and a power cylinder to drive the same crankshaft, with both relative sizes optimized to match the volumetric needs of the engine, always working at the same exac RPM, looks like an elegant solution to me. It needs to be built and througly tested before anyone can claim it a success. But the idea looks like a good one. Other elegant engineering ideas were not as sucessful as desired, like the variable compresion engine from Volvo (If I remember correctly), that was so difficult to run in practice or expensive to produce so that today none are seen in the streets.
IF I AM NOT MISTAKEN ,THIS HAS BEEN IN PRODUCTION BY A MOTORCYCLE COMPANY( STEYR,DAIMLER,PUCH )FOR MANY YEARS.IT IS A 2 STROKE VERSION.WHCH EXECPT FOR IT BEING A BALANCE NIGHTMARE , WAS QUITE SUCCESSFUL
Amclaussen, good summary of Turbo vs. Supercharger. You said:
The idea is that the pressure boost needed by the cylinder-piston is not really well met by both devices; thus the concept shown could well be a better match for the cylinder charging needs, maybe better than the traditional Otto cycle.
However, the piston-driven compressor in the prototype is mechanically matched to the engine rpm, and it's volumetric output is thus rpm dependent, and has parasitic power draw, which is the same as a supercharger. The qualitative issue is whether it is a more eficient air pump across all rpm ranges than a external supercharger. Also, I tend to surmise that the gains are from what is essentially a two-stage forced induction, turbocharger and then internal compressor. Seems I have seen people trying this with external turbo pumping into external supercharger into engine with encouraging results.
I have written letters E Mails to my congressman, president, EPA etc and received absolutly no response to the idea that perhaps the US should reconsider our emission requirements for NOx and other emissions in light of new knowledge and better understanding of global warming and effects of these various regulations.
The politicians are not interested in dealing with the rulings of an entrenched bureaucracy and perhaps making them revaluate their regulations
FYI Based on the fleet milage obtained in Europe and in the US, if the US adopted the european emission regulations, the US fleet average mileage could be in the order of 50% better. In other words we would use a lot less foreign oil if we changed our automotive emission requirements. thus reducing drastically the CO2 emissions among other things
We all need to push to make sure our government revalutes things in the light of new information.
That's exactly the point: The engine airflow needs need to be matched by the section feeding the air in. Let's remember so called "affinity Laws" that govern flow, pressure and power in dynamic machines. Very different from positive displacement or piston devices. (Flow proportional to speed, Pressure proportional to speed squared, Power proportional to speed elevated to the third power...)
In a turbo you need to match a very different performance map (that's because a dynamic compressor -or expander- uses -or produces- power to the cube of it's RPM, therefore the operation line is curved, where the line for a piston engine would be straight). The Turbocharger in a typical engine does not produce any real power surge at lower RPM's, it needs a relatively large exhaust flow to be eable to speed-up and start producing appreciable pressure into the inate manifold, and at high RPM's, the pressure and flow will be way too much over the piston engine needs, requiring a wastegate and controls to avoid overboosting the engine, overheating piton tops and valves or blowing the engine! By coupling the a compression cylinder to the same crankshaft, the matching of the flows and power appears to be easier to be established and kept over the RPM range of the engine... At least that's the main advantage I guess this design could have. Now, a piston compressor is always more efficient than a dynamic (centrifugal or axial, or screw compressor types). by placing the compression piston on the same crankshaft would be advantageous compared to having an external supercharger, not only for its intrinsic compression efficiency, but taking together the unavoidable transmission losses at the Supercharger belt and pulleys or gears. I hope I made it clear, sorry for the lenghty explanation, English isn't my native language.
The more I look to the idea, the more advantageous it seems.. unless I'm ignoring some catch! Someone ready to find it?
"Now, a piston compressor is always more efficient than a dynamic (centrifugal or axial, or screw compressor types)."
THAT is the part that I question. Any emperical data to qualify that? It seems almost all industrial air compressors have adopted screw type designs over the older piston type design, why would they do that if it were less efficient?
"However, split-cycle efficiency was never very good until Scuderi changed a key part of the combustion process."
"But if you fire after top dead center, the efficiency exceeds that of a conventional engine."
With all the backyard mechanics working of engines over the years, and with the racing industry squeezing every drop of out of engines while looking for an edge, I'll be shocked if a simple change in timing leads to something as big as they are claiming.
And I agree that balancing the airflow is a concern.
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