Split-cycle engine
The Scuderi Split Cycle Engine design is a rethink of the conventional four-stroke Otto cycle internal combustion engine conceived by Carmelo J. Scuderi (1925-2002). While as of this writing no working prototype of the engine exists, computer simulations carried out by the Scuderi Group and the Southwest Research Institute showed promising gains in efficiency and reduced toxic emissions over a conventional four-stroke engine. It also has the innate capacity to power an air hybrid system.
Design
In a conventional Otto cycle engine, each cylinder performs four strokes per cycle: intake, compression, power, and exhaust. This means that two revolutions of the crankshaft are required for each power stroke. The Scuderi split-cycle engine divides these four strokes between two paired cylinders: one for intake/compression and another for power/exhaust. Compressed air is transferred from the compression cylinder to the power cylinder through a crossover passage. Fuel is then injected and fired to produce the power stroke. Because the engine produces one power stroke per crankshaft rotation, a Scuderi-cycle engine has the same total engine size (number of cylinders and displacement) as a comparable Otto-cycle engine.
The power cylinder fires just after the piston has begun its downward motion (after top dead center, or ATC). This is in contrast to engine design convention, which calls for combustion just before top dead center (BTC) in order to allow combustion pressure to build. The Scuderi-cycle engine can get away with firing ATC because its burn rate is faster, and so is able to build pressure more quickly. This property of firing ATC is a key feature of the design, as it enables the engine's higher efficiency and lower emissions.
Potential advantages
The engine could produce 80% less nitrous oxides (NOx) emissions. Most of the NOx in conventional engines is produced at the high peak temperatures reached during combustion. Since a conventional engine fires BTC, it further compresses the combusting gas, raising its temperature. By firing ATC, the Scuderi cycle produces lower peak temperatures and thus, lower toxic emissions.
A Scuderi cycle engine should have better efficiency (from 33% to 40%) when compared to conventional engines. Several factors contribute to this:
- Increased burn rates : The rapid transfer of compressed air to the power cylinder creates a great deal of turbulence, facilitating a faster burn. Whereas conventional engines require 22-24 degrees of crankshaft rotation for the charge to fully combust, the Scuderi engine requires only 10.
- Ability to run lean : Increasing the air:fuel ratio in an engine increases operating efficiency. However, conventional engines are limited in this regard by the need for a catalytic converter, which requires specific stoichiometry in the exhaust stream in order to function. Given its lower emissions, the Scuderi engine should not suffer from this limitation.
- Geometric optimization : Each of the two cylinders may be offset at different angles that reduce the friction inherent in their specialized tasks.
- Longer power stroke : The power stroke can be lengthened, over-expanding the combusted gas and increasing thermal efficiency by the Miller effect.
In a conventional engine, supercharging can be used to extract more power from an engine by adding a compressor that forces more air into the cylinder. In the Scuderi engine, the compression cylinder can be made larger than the power cylinder, producing the same supercharging effect without introducing additional mechanical complexity.
Additionally, it is expected that a Scuderi engine would produce high torque at low RPM and hence have a lower average operating engine speed.
Manufacturing engines based on this cycle should be easy because the design is compatible with existing engine manufacturing processes.
Potential drawbacks
Cool air never enters the power cylinder so heat would quickly build up causing lubrication oils to break down and components to fail. It may be necessary to use expensive exotic materials to line the cylinder walls, or otherwise provide special cooling, to address this issue. Water cooled cylinder walls would be required to resolve this.
The crossover valve would experience high accellerations so valve train durability could be an issue. This very well might limit the rpm of the engine design.
Auto-ignition and/or flame propagation could occur in the crossover passage. Auto-ignition occurs when the temperature of the compressed air becomes sufficient to ignite the fuel before a spark is fired; this causes knocking/pinging and is harmful to the engine. The Scuderi Group hopes to address this with proper placement and timing of fuel injection, and also proper crossover valve timing.
Perhaps to avoid these problems the engine could employ an intercooler to remove the heat from the compressed gas on its way to the power cylinder. This would decouple the temp increase from the pressure increase in a way that cannot be done in a standard engine.
Air-hybrid capability
The addition of an air storage tank and some controls would allow the Scuderi engine to function as an air hybrid. According to the Scuderi Group[1], the engine can produce and utilize compressed air by cycling through four separate modes:
- Regenerative Braking This mode disables the power cylinder while leaving the engine connected to the wheels. The compression cylinder continues to operate, filling the air storage tank with compressed air. In this way, the engine is able to recover energy from the momentum of the car as it brakes.
- High-efficiency mode This mode disables the compression cylinder, feeding the power cylinder with compressed air from the storage tank. The work of compression is eliminated, allowing more of the engine's power to be used to drive the car (thus reducing fuel consumption).
- Cruising Mode While cruising, not all of the compressed charge that the compression cylinder provides is needed by the power cylinder. This excess compressed air is utilized to refill the storage tank. When the tank is full, the engine enters high-efficiency mode.
- Turbocharged Mode A turbocharger can be placed in the exhaust stream to recover heat energy that would otherwise go to waste. The turbo would be used to feed compressed air to the compression cylinder, reducing the amount of work necessary to compress the charge. This mode is well suited to stationary applications such as electric generators.
The Scuderi group estimates that their engine could achieve 60% fuel efficiency by operating as an air hybrid.
References
- Scuderi Group pages on the engine's technology, particularly the theory of operation section.
- Theory of Operation (PDF)
- Green Car Congress article