Porsche has consistently placed a strong emphasis on research and innovation throughout its history. It is no coincidence that in recent years, the Zuffenhausen-based manufacturer has heavily invested in the research and application of electric motors. However, Porsche has not abandoned its passion for internal combustion engines, as evidenced by its recent patent for a six-stroke engine. According to available documentation, this project aims to enhance thermodynamic efficiency while simultaneously improving engine performance. The six-stroke engine features a more complex combustion cycle than the traditional four-stroke engine, enabling better combustion, reduced fuel consumption, lower CO2 emissions, and improved performance.
This type of engine is not entirely new from a technological standpoint. In the early days of engine development during the 19th century, engines with more than four strokes were designed, such as the Ilmor/Schmitz five-stroke engine. Over time, this innovation extended to six-stroke engines, including the Griffin engine, the Dyer engine, the Bajulaz engine, and others. However, the practical applications of such engines were ultimately overshadowed by the widespread adoption of two-stroke and four-stroke engines.
Nevertheless, Porsche appears to be preparing to bring this type of solution to modern vehicles, potentially combining it with electric technology to improve overall efficiency and further reduce the environmental impact of its cars. Before diving deeper into these considerations, let us examine how the six-stroke engine works, its practical advantages, and how Porsche might use this powertrain.
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The Six-Stroke Engine: Technology and Functionality
The six-stroke engine is a combustion engine that differs from the traditional four-stroke engine by adding two extra phases to the combustion cycle. There are various types of six-stroke engines. The simplest is the “recovery six-stroke engine,” which is the solution Porsche has adopted. Another variation is the stepped six-stroke engine, which uses a second piston operating at twice the speed of the primary piston. These two engine types have also been combined to leverage the advantages of each principle. For brevity, we will focus only on the recovery-type six-stroke engine.
In a four-stroke engine, the cycle consists of four phases: intake, compression, combustion, and exhaust gas expulsion. In the six-stroke engine, two additional phases are added after the exhaust phase: a second expansion and another exhaust phase. This is achieved through an additional valve, known as a recovery or transfer valve. When activated, this valve allows the gases to flow into a recovery cylinder, enabling the fifth and sixth phases of the cycle.
The cycle of a recovery-type six-stroke engine begins, like any internal combustion engine, with the intake phase, where the cylinder is filled with an air-fuel mixture. This is followed by compression, where the mixture is compressed by the piston within the cylinder. Combustion is then initiated by the spark plug, causing rapid gas expansion that generates the energy needed to move the vehicle. In a traditional four-stroke engine, the next phase would be the expulsion of exhaust gases. However, in a six-stroke engine, some residual heat is used to power a second expansion, generating additional energy. Finally, an additional exhaust phase removes the spent gases.
Advantages of the Six-Stroke Engine
The primary advantage of this type of engine is its ability to extract more energy from the same amount of fuel. The recovery of residual heat before exhaust gas expulsion, as seen in four-stroke engines, increases the engine’s thermal efficiency. A six-stroke engine’s efficiency can improve by up to 40% compared to a four-stroke engine.
This improvement has environmental benefits as well. Exhaust gases emitted by a six-stroke engine generally contain fewer harmful residues than those from a four-stroke engine. Consequently, emissions released into the environment include lower amounts of CO2 and other pollutants.
Another significant benefit is better heat dissipation. The enhanced combustion process optimizes fuel energy, reducing the engine’s temperature and, thus, the need for powerful cooling systems. This also contributes to overall engine efficiency.
Drawbacks of the Six-Stroke Engine
Despite its advantages, the six-stroke engine has several drawbacks. One of the main challenges is the complexity of its design. Adding two extra phases to the cycle requires precise control of combustion and valve timing, making the system more complicated than traditional four-stroke engines. This increased complexity can lead to a higher likelihood of failures and elevated production and maintenance costs. Another significant challenge relates to engine durability and reliability. The introduction of additional expansion cycles may place greater stress on mechanical components, potentially reducing the engine’s lifespan. Moving parts like pistons and valves may experience accelerated wear, necessitating more robust and expensive materials to maintain optimal performance.
Additionally, there are concerns about compatibility with current technologies. Integrating six-stroke engines into existing vehicles may require substantial modifications to engine and transmission designs, increasing implementation costs for manufacturers. The scalability and applicability of this technology to mass-market vehicles also remain uncertain, as the cost-benefit ratio may not be favorable outside the realm of high-performance sports cars, such as those produced by Porsche.
Porsche’s Six-Stroke Engine
Porsche’s six-stroke engine is a fascinating development for enthusiasts. On February 23 of this year, Porsche filed a patent application in Germany for an engine developed in collaboration with the Technical University of Cluj-Napoca in Romania. The project involved Andre Kopp, Ovidiu Barac-Zbircea, and Nicolae Vlad Burnete. A similar patent was filed with the USPTO (United States Patent and Trademark Office) on September 12, registered under Application No. 18/585,308 , and described as a “two times three strokes” engine.
Inspired by the spirograph principle, Porsche’s six-stroke engine features two top dead centers (TDC) and two bottom dead centers (BDC). It consists of two cycles that complete a total crankshaft rotation of 1,080°, equivalent to three full revolutions. The inventors aim to combine the benefits of two-stroke and four-stroke engines while minimizing the drawbacks of both configurations. The first cycle, in particular, consists of:
- Intake: The piston moves from the first top dead center (TDC1) to the first bottom dead center (BDC1), allowing the air-fuel mixture to enter the cylinder through the open intake valve.
- Compression: The piston moves from BDC1 to TDC1, compressing the air-fuel mixture.
- Combustion and Expansion: At TDC1, the spark plug ignites the compressed mixture, causing the combustion gases to expand. This pushes the piston downward to BDC1, generating power.
At this point, by taking advantage of the planetary motion of the system, the expansion and compression phases are extended. The second cycle of Porsche’s six-stroke engine consists of the following stages:
- Second Expansion: Instead of expelling the gases immediately after combustion, the piston moves downward again, this time from the second TDC (TDC2) to the second BDC (BDC2). This harnesses the residual heat of the combustion gases for further expansion, generating additional power.
- Exhaust: The piston moves from BDC2 to TDC2, expelling the exhaust gases through the exhaust valve.
- Rest or Cooling: In some configurations, a cooling or rest phase is included for the cylinder. This can be used to extend the engine’s durability or reduce overheating.
According to the filed patent, Porsche’s six-stroke engine, compared to a four-stroke engine, manages to utilize the residual heat from combustion through the second expansion phase. This improves the combustion-to-expansion ratio, enhancing thermal efficiency. Consequently, fuel consumption is reduced, and fewer combustion residues are released into the environment. Moreover, the inventors claim that this type of engine offers advantages over other six-stroke engine designs. The implementation of a rest phase reduces the risk of overheating, while also improving the durability of the engine’s mechanical components.
The six-stroke engine currently represents a technical challenge for Porsche, as it does for any other manufacturer. For instance, the issues of vibrations arising from this particular configuration remain unresolved. Despite this, the recently filed patent once again demonstrates Stuttgart’s strong commitment to pursuing environmental sustainability without necessarily abandoning internal combustion technology, but rather revitalizing it. Speculating about the future, this could lead to a hybrid propulsion system based on the six-stroke engine and electric motors, an area in which Zuffenhausen has already gained significant expertise. Such a solution would further reduce emissions while preserving the principles of performance and sustainability, which have become the cornerstone of Porsche’s philosophy.
