Split-cycle engine

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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 no working prototype of the engine exists yet, computer simulations carried out by the Scuderi Groupand 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. In a standard Otto cycle engine, the pistons fire every other revolution. However, the Scuderi engine fires every revolution.

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.

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. 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. This property of firing ATDC is a key feature of the design, as it enables the engine's higher efficiency and lower emissions. 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. Another feature offered by the Scuderi engine is the Miller effect. 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:

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% greater fuel efficiency by operating as an air hybrid.

Steam Turbine Capability (Rankine Cycle)

The Scuderi Groups newest innovation that was discovered and patented recently. In a closed-loop system on top of Scuderi's split-cycle design and and the air hybrid design they have recently designed a way to include the Rankine Cycle in this design as another added component. The same way you can add an air tank to this engine to make it that much better you can also add on a closed-loop water system that captures the heat from the engine turning the water to steam and then turning a steam turbine that would then pump air into the air tank. Now, you have an engine with already staggering performance get even better performance by using the heat energy that is normally lost, to pump air into the air tank making the engine ultimately work less. This can all be done using with simplistic designs becuase the engine is running on a split-cycle platform. You can now look at this engine as a 3 in 1 engine in the sense that it is using the Otto cycle, Scuderi Cycle, and the Rankine Cycle all in one complete package. One more note on this new breakthrough is that because the closed-loop water system is extracting heat from engine it runs a lot cooler not allowing friction to wear and tear on the engine. The cooler temperatures also means better emissions. Of course, because more air energy is being recouped it also means more power, and efficency. Using this technology could totally eliminate the need for exhaust treatment systems.

Prototype

Currently, a prototype being developed by Southwest Research Institue at both locations in Texas and England. They are working on a two cyclinder split cycle gasoline engine. Working prototype will be completed near the end of 2007 to early 2008. Other versions of prototypes such as the Diesel version will began soon after the gasoline version is completed. The reasoning behind this is because both types use very similar designs so instead of paying to have the same things done twice it is being done once first and then it will be cheaper and easier to complete the next.

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.
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

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. In coventional Otto cycle engines, lower operating speed leads to less wear on certain engine parts and thus a longer lifespan. Manufacturing engines based on this cycle should be easy because the design is compatible with existing engine manufacturing processes.

Potential drawbacks

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.

References

Scuderi Group pages on the engine's technology, particularly the theory of operation section.
Theory of Operation (PDF)
Green Car Congress article

News Articles (most recent articles)

External links

Videos:

Sources and notes

Salvatore Scuderi is president of Scuderi Group, LLC and has released all information above either at various investor meetings and/or on company websites. Some material such as Steam Engine Turbine capability does not have attributions online because the technology has not yet been published on their website because of how recently it was patented.
Scuderi, Salvatore. "2007 Investors Meeting",Steam Turbine Capability, March 23, 2007