DETROIT – With the first physical prototypes of the Scuderi Group’s split-cycle engine set to fire up later this year, the company is addressing concerns from potential customers with details of the unique powerplant’s inner workings.
In the design and modeling phase for about six years and on display at last week’s SAE World Congress here, the split-cycle layout portends massive improvements for the internal-combustion engine by separating the intake/compression and combustion/exhaust strokes into different cylinders on opposite sides of the engine.
Simulation work has been carried out by the Southwest Research Institute (SwRI) in San Antonio, with results pointing to a much more power-dense engine that uses less fuel and produces fewer emissions than conventional gasoline and diesel powerplants.
However, the lack of a working test unit means engine and auto makers thinking of licensing the split-cycle design have a number of questions about how the engine actually functions.
“Most of the questions (from OEMs) focus on the valvetrain operation,” Scuderi President Sal Scuderi tells Ward’s, noting auto makers in Europe and India are expressing the greatest interest in the concept.
Other auto makers, includingLLC, Motor Co. and Corp., also have seen the design, he adds, but declines to offer more detail due to several patents for the engine that still are pending.
SwRI will have the first working engines completed by October, Scuderi says. They initially will be gasoline-powered 1.0L 2-cyl. mills (turbocharged and normally aspirated), with later variants including compression-ignition diesel power and the company’s compressed-air-hybrid technology.
With the engine’s operation split into two cylinders, a crossover passage in the cylinder head is used to transfer the compressed intake charge to the combustion chamber.
This division has numerous advantages, Scuderi says, including the separation of hot and cold elements of the engine, along with providing greater control of internal pressures for optimum efficiency.
The size of each cylinder also can be varied with this arrangement. A long-stroke combustion cylinder produces an efficient, longer-burning Miller cycle (good for stationary generator applications), while a larger compression cylinder can serve as a built-in supercharger.
However, Scuderi says the optimum configuration for a vehicle likely will be a symmetrical design, which allows for better packaging and lower production costs. Other designs might be better suited for certain applications, but turbocharging more than compensates for the lack of built-in forced-induction due to the split-cycle’s anti-knock characteristics and tolerance for boost levels approaching 36 psi (2.5 bar).
As there is no traditional intake plenum on the engine, conventional poppet-type valves (two intake, one exhaust) allow air to enter and exit the engine and are operated by an overhead-camshaft assembly.
However, due to pressures of about 1,450 psi (100 bar) in the crossover chamber, small, lightweight titanium valves are used to transfer the air charge between cylinders. To further reduce air resistance, and to accommodate the split-cycle’s high compression ratio and miniscule piston-head clearance, the actuation of the four crossover valves (two per side) is reversed so that they open upwards into the cylinder head.
In addition, the high pressures, excess air supply and need for super-fast activation necessitates the valves be operated by compressed air, Scuderi says. A burst of air opens the valves upward, with a pneumatic return spring (similar to those used on 19,000-rpm Formula 1 racing engines) quickly returning the valve to its closed position.
Simulations have run the split-cycle engine up to about 5,000 rpm, he adds, but higher speeds are possible with more testing.
Two high-pressure fuel injectors per cylinder supply fuel via the crossover passage, with Mahle GmbH-sourced pistons and dual spark plugs handling combustion duties.
Scuderi says simulations have shown the split-cycle design produces more torque and significantly less oxides of nitrogen emissions than a comparable diesel engine. And with partial-load efficiency similar to that of a diesel, the engine is about 30% more efficient than a gasoline engine when not being worked hard.
Although the split-cycle is a radical departure from conventional engine design, many of the traditional parts and theories of operation apply, making production less daunting than it seems, Scuderi says, noting the concept could find its way into a passenger vehicle in about three to five years, depending on customer requirements.
In addition, the engine’s high level of flexibility and scalability allow it to operate in a wide range of applications, including power generators, transit buses and passenger cars.
As an example of the split-cycle’s potential, Scuderi points toMotors Ltd.’s new Nano minicar. The affordable “one lakh” or “people’s car” makes about 30 hp from its 0.66L 2-cyl. engine, whereas a 0.5L split-cycle mill could produce about 50 hp with significantly fewer emissions.
“Original interest in the split-cycle concept focused on its (air) hybrid element,” Scuderi says. “But (OEMs) are looking at it as a primary engine (replacement) now that the test results are coming out.”