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Split-cycle engine

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The Scuderi Split Cycle Engine is a design of internal combustion engine invented by Carmelo J. Scuderi. The Scuderi group who own the patents on the design, claim that computer simulations of the engine show promising gains in efficiency and reduced toxic emissions over conventional four-stroke Otto cycle designs. The group also claim it could be used as part of an air hybrid system.

As of April 2007 no working prototypes of the engine exist.

An animation from Scuderi patents showing how the engine cycles.


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. In a standard Otto cycle engine, the pistons fire every other revolution. However, the Scuderi engine fires every revolution.

In the Scuderi cycle, the power cylinder fires just after the piston has begun its downward motion (after top dead center, or ATDC). This is in contrast to Otto cycle design convention, which calls for combustion just before top dead center (BTDC) in order to allow combustion pressure to build.

Scuderi group claims

The separation of the intake/compression and power/exhasut cycles allows for a unique "boosting" effect to the power cylinder. The compression cylinder can be made with a larger bore and/or stroke than the power cylinder, thus the volume of air compressed into the power cylinder is greater than if the bore and stroke is uniform across the cylinders. In addition, since the compression cylinder is not required to withstand the violence of combustion, it can be made of lighter materials, thus reducing the overall weight of the crankshaft and piston assembly [citation needed].

The Scuderi-cycle engine can get away with firing ATDC because its burn rate is three times faster, and so is able to build pressure more quickly [citation needed]. This property of firing ATDC is a key feature of the design, as it enables the engine's higher efficiency and lower emissions [citation needed]. In an Otto cycle engine, ignition BTDC implies that the engine is momentarily fighting against the combustion flame front, while in the Scuderi engine the piston is moving with the flame front [citation needed]. Another feature offered by the Scuderi engine is the Miller effect [citation needed]. Finally, the Scuderi-cycle engine shares a similar infrastructure with a conventional internal combustion engine, so it can be packaged similarly in an automobile.

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:

  1. 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.
  2. 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).
  3. 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.
  4. 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% greater fuel efficiency by operating as an air hybrid.

Steam turbine capability

The Scuderi Group claim that a closed loop steam turbine, driving a Supercharger could also be added to the engine to recapture waste heat for greater efficiency.

Claimed advantages

The group claims a number of potential advantages of this configuration over the Otto cycle.

  • 80% less nitrous oxides (NOx) emissions. By firing at ATC, the Scuderi cycle produces lower peak temperatures and thus, lower toxic emissions. [citation needed]
  • Increased efficiency (from 33% to 40%) when compared to conventional engines.
    • Increased burn rates : The rapid transfer of compressed air to the power cylinder creates a great deal of turbulence, facilitating a faster burn. Otto cycle engines require 22 to 24 degrees of crankshaft rotation for the charge to fully combust, the Scuderi engine requires only 10. [citation needed]
    • The 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. [citation needed]
    • Geometric optimization : Each of the two cylinders may be offset at different angles that reduce the friction inherent in their specialized tasks. [citation needed]
    • Longer power stroke : The power stroke can be lengthened, over-expanding the combusted gas and increasing thermal efficiency by the Miller effect. [citation needed]
  • Additionally the system may be used as with Components available for use:

1.) Air Hybrid Capability - adding an empty air tank to store air for later use

2.) Steam Turbine Capabilty - adding a simplistic steam turbine closed-loop system will allow most of all lost heat energy to be recaptured and stored as air in the air tank

  • It would produce high torque at low RPM and as a result have a lower average operating engine speed, potentially reducing wear [citation needed].

Critism and skepticism

Unlike an Otto engine, 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. This need to reject high temperature waste heat could offset any efficiency gains over the Otto cycle. Efficiency could be improved by using compressed air from the first stage to cool the power cylinder, thus preheating the combustion air and not wasting the heat. However, if this method cannot remove enough heat, then water cooled cylinder walls may be required.

To avoid overheating the power cylinder, the engine could also employ an intercooled compressor cylinder to remove the heat from the compressed gas. This would serve to decouple the temperature increase from the pressure increase to a greater extent than in a standard engine.

The crossover valve would experience high accelerations 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.

The additional pumping losses through the transfer port will significantly reduce the efficiency of the engine. In a typical engine pumping losses are about 4% of the indicated power of the engine (Heywood P721). Most of those pumping losses occur in the valves. So if the pumping losses were to double, the efficiency would drop, from the claimed 42% to 38%, equal to that of a Prius' engine.

The above drawbacks may be solved or ameliorated, but until actual real world performance figures are released these represent possible causes for poor efficiency and reliability.

See also

References

News Articles (most recent articles)

Videos:

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