|Cylinder pairs in the Scuderi Split Cycle. Scuderi divides the four strokes of the Otto cycle across two paired cylinders, a compression and a power cylinder.|
The Scuderi Group, a company that designed and is developing a new internal combustion engine based on dividing the conventional four-stroke engine cycle across two paired cylinders, is enhancing that basic design with the addition of an air-hybrid capability.
The group, which just announced that it has raised more than $8 million in funding—including a $1.2 million award from the US government—will introduce the new air-hybrid split-cycle engine at the upcoming SAE World Congress exhibition in Detroit.
In the conventional four-stroke engine, a single piston slides up and down in a cylinder through the intake, compression, power and exhaust strokes—each power stroke requires two revolutions of the crankshaft. This basic design has not changed for more than 100 years.
The patented Scuderi Split-Cycle Engine 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 via a gas passge to the power cylinder for combustion.
The gas passage includes a set of uniquely timed valves, which maintain a pre-charged pressure through all four strokes of the cycle. Shortly after the piston in the power cylinder reaches its top dead center position, the gas is quickly transferred to the power cylinder and fired to produce the power stroke.
The design does not imply the doubling of the size of engines. In the four-stroke cycle, the engine fires every other revolution; in the split-cycle design, it fires every revolution. This is similar to a conventional two-stroke engine, but without the corresponding emissions and fuel consumption issues.
In other words, a four-cylinder split-cycle engine (two sets of paired cylinders) will produce the same number of power strokes as a four-cylinder four-stroke engine (four independent cylinders), but with enhanced efficiency, according to Scuderi.
In the Scuderi split-cycle, an intake charge is drawn into the compression cylinder, the piston of which then pressurizes the charge and drives it through a crossover passage, which acts as the intake port for the power cylinder.
A check valve prevents reverse flow from the crossover passage to the compression cylinder, and a crossover valve prevents reverse flow from the power cylinder to the crossover passage. The check valve and crossover valve are timed to maintain pressure in the crossover passage at or above firing conditions during an entire four-stroke cycle.
Combustion occurs (soon after the intake charge enters the power cylinder from the crossover passage and after the power cylinder piston passes through its top dead center position (ATC).
The crossover passage valves are critical to the efficiency of the engine. Although there have been paired-cylinder designs in the past, they were not able to match the thermal efficiency levels of the conventional four-stroke engine.
Part of that inefficiency resulted from the over expansion of the charge in the power cylinder, necessitating re-compression. By maintaining pressure in the crossover passage at combustion pressures, and firing after top dead centre, the Scuderi engine avoids that problem, according to the company.
According to the Scuderi Group, some of the benefits from the non-hybrid split-cycle design would include:
Fuel efficiency improvements of 15% to 30% initially with further improvements possible;
Potential reduction of NOx emissions of 50% to 80%;
Lower average operating engine speed reduces engine wear and tear;
High torque at low RPM means higher power at lower engine speeds;
Compatibility with existing engine manufacturing processes and tooling;
Same total engine size (number of cylinders and displacement) as comparable conventional internal combustion engines.
Because the compression cylinder is only concerned with delivering a pressurized charge to the power cylinder, enlarging the compression cylinder could take the place of a charging system such as a supercharger or turbocharger—the basic role of which to is force more air into the charge.
This specialization of cylinder function also opens the door to the air-hybrid concept to be announced in Detroit.
The Group believes that the Scuderi Air-Hybrid Engine will allow internal combustion engines in a variety of applications (diesel and gasoline, personal and commercial) to be 60% fuel efficient (compared to today’s 33%, almost doubling mileage, in other words) and reduce toxic emissions by 80%, while making it easier and less expensive to incorporate the technology into today’s automobile manufacturing process.
Scuderi identified a number of potential issues with the design during the modeling phase. These include:
Sustained elevated temperatures in the power cylinder could lead to thermal-structural failures of components and problems with lube oil retention. Scuderi may need to utilize different materials or techniques for the cylinder walls. Lower temperature coolants may be necessary. Lubrication may also be an issue, although one addressed with advanced synthetic or solid lubricants.
Possible valve train durability issues with crossover valve due to high acceleration loads, and possible valve-to-piston interference in the expansion cylinder. Both this and the preceding concern may be addressed by new valve designs.
Auto-ignition and/or flame propagation into crossover passage. Scuderi is looking to the placement and timing of fuel injection as well as crossover-valve timing to address these.