Developers of the Scuderi "air Hybrid" split-cycle engine are encouraged by the novel powerplant’s performance in a dynamometer program at Southwest Research Institute (SwRI) in San Antonio, where it has been under test since mid-2009.
“We’re raising the maximum engine speed of our testing progressively," reported Nick Scuderi, Vice President of Sales. "We started the program at 1000 rpm and we're now running at 3000 rpm. We expect to be at 4000 rpm when we arrive at the SAE World Congress in April."
The test program has focused on moving incrementally from low-load to high-load operation. “We’re also close to completing the first engine-control map and should have it finalized by Congress as well,” he noted.
Speaking with AEI during the recent SAE Hybrid Vehicle Technology Symposium, Scuderi reported the “base” naturally aspirated 1.0-L port-injected gasoline prototype, which resembles a large parallel twin, is delivering “stable combustion." He claimed its 2-4% cycle-to-cycle coefficient of performance is competitive with current Otto-cycle gasoline engines.
The company also has begun modeling a turbocharged version. “We’re using 2.5-3 bar (36-43 psi) inlet pressure, but because it’s split-cycle we’re not encountering knock,” he said.
Nick Scuderi and his engineer brothers Sal (the company’s chairman) and patent-attorney Stephen are developing the engine invented by their late father, Carmelo Scuderi. The Massachusetts-based Scuderi Group unveiled its proof-of-concept prototype at the 2009 SAE Congress.
The Scuderi engine is a modern take on the split-cycle combustion system, which dates back to 1914. The operating concept divides the four strokes of the combustion cycle among two paired cylinders—the left cylinder functions as an air compressor, handling intake and compression, while the right cylinder handles combustion and exhaust.
A crossover passage between the cylinders transfers the compressed charge into the combustion/exhaust cylinder.
The addition of a pressurized tank for air storage allows the Scuderi engineers to convert the split-cycle design into what the company calls an "air hybrid." The air tank stores compressed air (up to 200 bar/2900 psi) and uses it for combustion—effectively a "battery" that can rapidly capture and release energy, according to Scuderi.
Electronic controls allow the engine to perform as both a combustion device and air pump, both functions complementing each other and allowing multimode operation. In Regenerative Braking mode, the engine controls turn off the power cylinder and divert the compressed air flow to the storage tank. Vehicle momentum continues turning the engine, compressing the air and storing it in the tank.
In High-Efficiency mode, the compression cylinder is turned off. Pressurized air from the storage tank supplies the power cylinder, reducing compression losses to minimum levels. Cruising mode sends only part of the compression cylinder's charge to the power cylinder; the remainder of the charge is diverted to the air tank for later use.
When the air tank is fully pressurized, the compression cylinder shuts off and the vehicle operates in High-Efficiency mode.
Scuderi engineers claim that by optimizing the split-cycle concept, the engine has potential to reduce NOx emissions by up to 80% and CO2 emissions by up to 50%, compared to similar displacement gasoline engines.
Previous split-cycle efforts suffered poor volumetric and low thermal efficiencies, as Fiat discovered during its own development program in the 1950s. The Scuderis claim their engine solves the breathing problem on the compression side by using a pair of pneumatically actuated valves that open outwardly, enabling a very close (<1 mm) piston-to-cylinder head clearance without valve interference.
This arrangement effectively pushes nearly 100% of the compressed air from the compression cylinder into the crossover passage, eliminating the breathing issues of previous split-cycle designs.
The engine requires just one crankshaft revolution to complete a single combustion cycle. Ignition is timed to occur between 11 and 15 degrees after top-dead center. Scuderi claims this is key to the engine’s high BMEP (up to 22 bar/320 psi), thermodynamic efficiency, and reduced toxic emissions compared with similar-displacement conventional engines.
Firing after TDC is the key to the split-cycle’s combustion strategy. To enable the ATDC ignition, Scuderi uses a combination of high air pressure in the crossover passage—“it’s at sonic velocity,” said Nick Scuderi—along with extremely high mixture turbulence in the power cylinder and rapid flame speeds.
“We’re running 18-23° spark advance in the naturally aspirated version, and we’ll run about 12° on the turbo,” Scuderi reported. The result is a combustion rate that is claimed to be four times faster than that of a conventional four-stroke.
Separating the compression and combustion sides of the cycle means compression ratio is not limited by the combustion process—the Scuderi engine runs effective compression ratios of 75:1 on the compression side and 50:1 on the power side. Peak cylinder pressure on the compression side of the naturally aspirated version reaches more than 50 bar (735 psi)—equal to that of a conventional engine during combustion.
Compression-side cylinder pressures on the turbo version currently being modeled are over 130 bar (1900 psi).
The test engine at SwRI is displaying “exceptional” turbulence characteristics and inlet charge atomization, a result of its long valve opening duration during combustion, Scuderi said. The engine-technology company has partnered with Bosch for fuel system and engine control hardware and software.
Scuderi noted that several OEMs have ongoing interest in the program. “I think by the end of this year we might see our first license,” he said.