"Improving the efficiency of internal combustion engines is critical for meeting the global needs to reduce petroleum consumption and CO2 emissions.” (Sandia National Labs FY 2012 progress report). There is a great deal of research and development currently being pursued by government, industry and academia to improve engine efficiency and reduce emissions. This research has significant challenges and constraints. The improvements to efficiency and emissions must not compromise power or performance, not dramatically increase cost or complexity, and it must be reliable.
Two advanced areas of research that are expected to yield significant gains as identified by the Department of Energy’s Advanced Combustion Engine R&D subprogram are low temperature combustion modes (including homogenous charge compression ignition (HCCI) and its various derivatives), and thermal energy recovery.
A number of significant advances have been made in the areas of combustion, emissions controls, fuel injection, turbo-machinery, and other advanced engine technologies that continue to increase the thermal efficiency of ICEs with simultaneous reductions in emissions. (DOE Advanced Combustion Engine Research and Development Vehicle Technologies Office – Annual progress report 2012). However, despite these improvements and the millions of dollars being spent on R&D, meeting the goals set by governments around the world will be difficult to reach.
Internal combustion engines have been around for over 100 years. The elements that drive engine efficiency are well known and documented - most of which are already in use today at various levels of implementation. For example, three of the most basic features that drive engine efficiency are: high compression ratios, running at wide open throttle and the Miller Cycle over expansion. However, there are tradeoffs and limitations that prevent or limit these features from being fully implemented in today’s engines. Gasoline engines for example are limited by knock or premature detonation of the fuel therefore their compression ratios are limited. In addition, a gasoline engine controls its load and speed by the amount of air it intakes therefore, a gasoline engine must be throttled at part loads reducing its efficiency.
Diesel engines on the other hand can implement many of the features that improve efficiency however they have issues as well. “Current diesel engines already take advantage of the most important factors for efficiency – no throttling, high compression ratio and low heat rejection. However, diesel combustion creates significant emissions problems. “ (Argonne National Laboratories – Advanced Combustion Engine R&D – FY 2010 Annual Progress Report). In addition, diesel engines rarely utilize miller over expansion to improve efficiency.
The Miller Cycle over expansion is a prime example of a technology that is well known to improve engine efficiency but is not always utilized. The Miller Cycle requires the use of variable valves which add cost and complexity to the engine’s valve train, is limited in its range and typically reduces engine power as it increases efficiency. Even though the Miller Cycle is a proven process that increases the efficiency of an ICE, it is rarely used because of its limitations and tradeoffs. (site text book)
The design of Scuderi Split Cycle Engine provides more flexibility to implement the features that contribute to high efficiency in ICEs. By separating the compression stroke from the power stroke, those high efficiency features are either easier to implement or can be more effectively utilized.
The Scuderi Split Cycle Engine separates the high pressure strokes of compression and power. Most common initial reaction to this description is that the engine carries an extra compression cylinder for each power cylinder. This is simply not correct. The split cycle engine can be operated in various modes, but the basic method of operation is with the compression and power cylinders operating in a two stroke mode. Therefore the compression cylinder does intake and compression every revolution of the crankshaft and the power cylinder would do combustion and exhaust every revolution of the crankshaft. When operating in this mode the engine produces a power stroke every revolution. Since a conventional four stroke engine produces a power stroke every other revolution, it would require two of the conventional piston/cylinder arrangements to match the power strokes of a split cycle engine. Therefore, in a split cycle engine, a four cylinder engine still only has four cylinders. The difference is two would always be dedicated to compression and two always dedicated to combustion. (site SAE Paper for architecture)
The engine can also be operated with the compression cylinder in two stroke mode and the power cylinder in four stroke mode. In addition, with the use of an exhaust driven turbocharger, one compression cylinder can provide air flow to multiple power cylinders. This method of operating the split cycle engine provides miller over expansion benefits without the use of variable valves to control intake valve timing. This feature provides both efficiency and power density benefits. (site SAE Paper for miller)
There are features of the Scuderi Split Cycle Engine that are critical to its operation. These features are; separating the compression cylinder from the power cylinder, use of outwardly opening valves to prevent piston to valve interference, and direct fuel injection into the power cylinder. (Site SAE Paper for valves)
As stated above, three of the most important factors for efficiency in an internal combustion engine are no throttling, high compression ratios and Miller Cycle over expansion.
Diesel engines already are able to achieve no throttling and high compression ratios. Diesel engines are able to run at wide open throttle because their combustion process does not depend on a specific fuel and air ratio. In addition, Diesel engines compress only air, introducing the fuel only when the piston is close to top dead center. They do not have a premature detonation issue the way gasoline engines do. As a result, they can reach much higher compression ratios. The combination of no throttling and high compression ratio enables diesel engines to obtain higher efficiencies than gasoline engines, especially at part load operating conditions. However the Miller Cycle has only been applied to diesel engines on a limited basis. MAN Diesel & Turbo has recently applied Miller to some of their large diesel engines. The problem with Miller over expansion in diesel engines is that the reduced temperatures resulting from the over expansion can cause increased smoke emissions. MAN was able to overcome the smoke emissions issue with the use of variable valves however, that adds cost and complexity to the engine. Implementing Miller over expansion has enabled MAN to meet the 2011 IMO Tier II emissions regulations. However, the Miller cycle is only applied to diesel engines on a limited basis in the part load operating ranges of the engines.
Gasoline engines have two features that prevent them from operating with no throttle and high compression ratios. First, a gasoline engine must maintain a specific fuel/air ratio known as stoichiometric in order for the three way catalytic converters to work properly. Because of this a gasoline engine’s load and speed is controlled by the amount of air the engine pulls in during its intake stroke. Since the air flow into the engine must be controlled to maintain the stoichiometric fuel/air ratio, the engine must be throttled when operating at part load conditions. Second, the fuel and air mixture is in the engine during compression stroke, therefore if the compression ratio is too high the fuel will prematurely detonate due to the heat of compression. Today’s advanced engines have found ways to improve gasoline engine performance by utilizing variable valves to reduce the throttling losses and direct fuel injection to obtain higher compression ratios. However, diesel engines still maintain a significant advance on efficiency, especially when running at part load. Application of Miller over expansion is becoming more common in engines with variable valves. However, it is only applied at part load operation and it is limited to only about 20% reduction of the compression stroke.
The Scuderi Split Cycle Engine has the advantage when trying to incorporate the features of no throttling, high compression ratios, and miller over expansion to a gasoline engine, and adding miller over expansion to a diesel engine without the use of intake valve timing.
When the Scuderi Split Cycle Engine is operating with an exhaust powered turbocharger one compression cylinder will be able to provide compressed air to multiple power cylinders. This configuration enables the engine to operate at miller over expansion under all loads and speeds and it is the optimum design for adding miller to diesel or gasoline engines.
Applying miller to diesel or gasoline engines normally requires variable valves that control intake valve timing. In diesel engines this can cause smoke conditions due to reduced temperatures of the compressed air. With the Scuderi Engine the miller over expansion is a function of its architecture and does not require variable valves and it does not cause a drop in the compressed air temperature.
One of the major advantages of The Scuderi Split Cycle Engine is its design flexibility. By separating the cycles, it simply has more features to adjust than a conventional engine. Operating gasoline engines over the entire load and speed range at wide open throttle is a significant challenge. However, the Scuderi Group has several methods it is currently exploring to be able to achieve the goal of no throttling for gasoline engine operation. Currently, the Scuderi Split Cycle Engine can utilize the ability to go from two stroke operation to four stroke operation as discrete steps in reducing engine output to match reduced loads without throttling the engine. In addition, the Scuderi Group is currently exploring other techniques that are only possible with a split configuration that could enable a much wider range of engine operation without the need to throttle the engine.
High compression ratios can also be achieved in gasoline engines with the Scuderi Split Cycle Engine for two basic reasons. First during the compression stroke there is no fuel mixed with the air so there is no premature detonation issue during compression. Second, in the Scuderi Split Cycle Engine, direct injection of the fuel is done before the high pressure air enters the cylinder. The fuel is substantially vaporized and then compressed by the piston. The high pressure air enters the cylinder when the piston is at or near top dead center. The time for air and fuel mixing is very short virtually eliminating any knocking issues before the fuel is ignited by the spark plug. The expansion ratio for combustion is approximately 16 to 1.
The Scuderi Split Cycle Engine is able to better utilize the key efficiency drivers of high compression ratios, and Miller over expansion and is improving no throttling operation for gasoline engines and is applying the Miller over expansion to diesel engines. In addition, the implementation of these features is done without expensive or exotic components and in fact the direct injection process used by the Scuderi Engine is done with low pressure injectors. This will not only reduce parasitic losses caused by pumping fuels to high pressures but will also reduce the cost of the fuel delivery system.