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.
I agree with most of your comments. However, the "Once the avg. temperature gets too high, lubrication breaks down and engine reliability will suffer" seems a bit unfair. Saab managed to make more than a few 2 cycle engines [similar heat issues] with lifetimes warranties over 40/50 years ago. I would think that the temp problems are about the same and that modern synthetic lubricants could handle the bottom end.
Although I have a bias opinion, because of the DRE, I do not believe cooling the power cylinder is going to be a problem.
We build race engines that have no water jackets between each cylinder to allow for very large cylinder bores. These engines are used in sprint cars that run nearly wide open throttle for the entire race and produce more than 700 hp. The average amount of heat that is developed in each cylinder to produce this much HP has to be far higher than that of a power cylinder on a split cycle engine even when you consider the cool intake air bringing the average temperature down. Yet we can do this lap after lap with no cooling issues.
Today’s direct injection engines also introduce oil under the pistons to assist with cooling.
Also if the engine is in fact more efficient it will have less heat to deal with.
When running superchargers the compressor discharge temperature can exceed 200 degrees F. Since this is higher than the normal operating temperature of an engine it has a negative cooling effect. Yet they too can be run for long periods of time without adverse effects.
OK...I'll concede that of all my criticisms of the split-cycle concept, the temperature comments are the ones I'm least certain about. Although I still contend overheating will be a challenge based on my knowledge - it may indeed be engineerable. For example, the wankel engine has compression / combustion / exhaust always in the same location of the engine (except for the rotor). I know that this was a big development challenge for Mazda (I once owned a racing rotary), but they worked it out with special coatings on the walls, carbon seals, and (check it out) a huge radiator vs. engine size. Efficiency was (and is still) not good, unfortunately.
The main question for the split-cycle concept is "what does it bring to the table that a miller or atkinson cycle engine doesn't?". I contend - almost nothing, and it has some disadvantages to boot.
I really like what Mazda is trying to do with their Activ engines - they are exploring limits of optimization from several angles: gasoline engines with high 14:1 compression ratios, and diesel engines with low 14:1 compression ratios.
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