Porsche, renowned for its innovative automotive engineering, has recently patented an engine cycle that deviates from the conventional four-stroke system. This new design, referred to as a 2 x 3-stroke by Porsche but arguably better described as a 4 + 2-stroke system, introduces a fascinating layer of complexity and potential benefits. This article delves into the intricacies of this patented Porsche 6-stroke engine, examining its mechanics, potential advantages, and the engineering challenges it presents.
The standard four-stroke engine cycle comprises intake, compression, combustion (expansion), and exhaust. Porsche’s innovation adds a two-stroke event within this familiar framework. This is achieved through an intricate crankshaft configuration that allows for two distinct top dead center (TDC) and bottom dead center (BDC) positions within a single cycle. Following the initial expansion phase, the engine initiates a second compression and expansion sequence – the core of the two-stroke addition. However, the process is even more nuanced than a simple add-on.
Drawings from the US patent office
Patent illustrations from Porsche reveal a ‘scavenging’ phase strategically placed between the first expansion and the second compression. This scavenging process is enabled by the unique crankshaft mechanism, which allows the piston to descend to a ‘lower’ BDC compared to the four-stroke portions of the cycle. This deeper descent uncovers a series of scavenge ports that are typically hidden. In two-stroke terminology, ‘scavenging’ refers to the process of clearing exhaust gases from the cylinder and charging it with fresh air. In this Porsche design, just before these ports are exposed, the exhaust valves open, releasing some of the initial combustion gases and reducing cylinder pressure. As the piston continues its downward motion and uncovers the scavenge ports, pressurized air is forced into the cylinder, pushing the remaining exhaust gases upwards and out through the exhaust system – a process known as ‘uniflow scavenging.’ Subsequently, both the exhaust valves and scavenge ports close, setting the stage for the second compression and ignition at TDC.
This second combustion event is crucial to understanding Porsche’s objectives. It is speculated that the primary goal is not necessarily to boost power output, but rather to reduce harmful emissions. By re-burning gases that might otherwise be trapped within the cylinder after the first combustion, the engine could achieve cleaner operation. Two-stroke engines are known to be compatible with ‘homogeneous charge compression ignition’ (HCCI), a combustion method recognized for its efficiency and reduced emissions, operating without a conventional spark or flame. While some exhaust gases are expelled during the initial exhaust phase, a significant portion can be retained and re-combusted in the second event. However, preventing fresh air from escaping into the exhaust system is critical for the effective operation of standard after-treatment systems, which could potentially limit power gains from this design.
Despite the theoretical advantages, the Porsche 6-stroke engine concept introduces considerable engineering challenges. The most apparent is the mechanical complexity of the crankshaft, which incorporates a hypo-cycloidal gearing system, reminiscent of intricate Spirograph patterns. Piston-controlled ports, while integral to the scavenging process, present challenges related to piston ring wear. Ensuring durability and reliability in this area would require significant engineering effort, although precedents exist in large, reliable marine two-stroke engines that utilize uniflow scavenging. Furthermore, piston-controlled ports typically require careful design to minimize oil consumption. The system also necessitates boost pressure to facilitate effective scavenging. Adding to the complexity, the camshafts in this 6-stroke design would need to operate at one-third engine speed, with the exhaust valves opening twice per cycle.
Considering these complexities, a fundamental question arises: why pursue such an intricate design? The automotive industry harbors a strong bias against traditional two-stroke engines, often due to emissions concerns and perceived refinement issues. However, Porsche’s 6-stroke concept, while significantly more complex, could be seen as a sophisticated evolution, addressing some of the inherent limitations of both four-stroke and conventional two-stroke engines. Whether the potential emissions benefits outweigh the considerable engineering hurdles remains to be seen, but Porsche’s patent undoubtedly represents a fascinating exploration into the future of internal combustion engine technology.
