Why Does Porsche Use Boxer Engine | Stirlingkit
June 13, 2023When it comes to iconic sports car brands, Porsche stands out not only for its sleek design and high-performance vehicles but also for its unique choice of engine. Porsche has been utilizing boxer engines for decades, setting itself apart from other car manufacturers.
What is boxer engine?
Horizontally opposed engines, commonly known as "H-type engines," also known as flat engine, position the cylinders horizontally. The most common configuration of flat engines is the boxer engine, in which the pistons of each opposed pair of cylinders move inwards and outwards at the same time. Boxer engines are a type of flat engine. In this configuration, the engine pistons are evenly distributed on both sides of the crankshaft and move horizontally, to the left and right. This design reduces the overall height and length of the engine, lowers the center of gravity of the vehicle, and enhances its smooth operation. By installing the engine along the center line of the vehicle, the torques generated by the pistons on both sides counterbalance each other, significantly reducing vibration during driving. Moreover, the engine speed has been greatly improved, and noise levels have been reduced.
Although several brands, including Ferrari, Lancia, Alfa Romeo, and Chevrolet, have previously used horizontally opposed engines, the technology is currently predominantly utilized by Porsche and Subaru. Porsche takes pride in its expertise in horizontally opposed engine technology, which is evident in their models such as the Porsche 718 (Cayman, Boxster), and 911. However, other brands have weighed the advantages and disadvantages and eventually decided to discontinue the use of this technology.
photo from google
What's so good about boxer engines?
One of the primary advantages of a boxer engine is its low center of gravity, which aligns well with the requirements of sports cars. The engine's structure positions the pistons, cylinder block, and ground in parallel, resulting in reduced overall height and length. This compact engine layout takes up less space in the cabin and significantly lowers the vehicle's center of gravity. In fact, many modified cars lower the center of gravity by relocating the battery and replacing the shock absorbers, thereby enhancing vehicle handling. The compact engine layout also minimizes shaking during cornering, improving stability at high speeds.
A low center of gravity offers several benefits, including improved dynamics, braking, and handling stability. When it comes to handling stability, a low center of gravity enhances cornering stability and allows for higher cornering limits. The "flat" cylinder configuration not only lowers the car's center of gravity but also creates a flat and low front end, further enhancing driving stability.
photo from google
The second notable advantage is the excellent ride comfort, performance, and smooth acceleration offered by horizontally opposed engines. The balanced piston movement, typically with a 180-degree offset, reduces the balance weight requirements for the crankshaft and facilitates higher engine speeds. These engines can even maintain a low speed of 650 rpm while ensuring smooth operation. Additionally, compared to other engine types, horizontally opposed engines exhibit lower fuel consumption. Unlike inline engines, where pistons move up and down, in horizontally opposed engines, pistons move horizontally, eliminating the need to work against gravity. This structural advantage, coupled with the self-balancing nature of the horizontally opposed engine, results in smoother engine operation. Lightweight crankshafts and flywheels replace balance parts like balance shafts, enabling quicker acceleration and higher speeds.
Porsche exclusively utilizes the boxer engine in its sports car series, including the 718 and 911 models. These engines offer a low center of gravity and easy controllability, perfectly aligning with the requirements of these sports cars.
photo from google
Another distinctive feature of boxer engines is their ability to suppress vibration and noise. The symmetrical placement of cylinders allows for the cancellation of piston-generated vibrations. The opposing movement directions of the left and right pistons effectively counterbalance each other. It's akin to pulling both ends of an elongated rubber band together and releasing it slightly in the middle—there will be no bouncing on the hand. Conversely, if one end of the rubber band is released while pulling it taut, it will bounce off the hand. The symmetrical configuration of the horizontally opposed engine mitigates vibration-related issues.
So good, why don't other car companies use it?
there are inherent drawbacks and challenges associated with horizontally opposed engines that explain why other car companies do not widely adopt them.
One of the major concerns is related to lubrication issues and the engine's low torque performance. The horizontally opposed engine's structure is complex, and it presents challenges in achieving proper oil lubrication. The horizontal cylinder placement results in shorter cylinder bores and crankshaft moment arms compared to V-shaped or L-shaped arrangements. As a result, the engine's low-torque performance is naturally weaker. Gravity's influence causes oil to accumulate at the bottom of the piston, leading to inadequate lubrication on one side of the cylinder. This results in reduced lubricating oil at the top and excessive oil at the bottom, significantly compromising the lubrication effectiveness.
Moreover, the precision manufacturing required for horizontally opposed engines increases maintenance costs. The wider body design of these engines also hampers layout flexibility. The horizontal reciprocating motion of the piston, influenced by its own gravity, causes uneven wear on the upper and lower inner surfaces of the cylinder liner. Friction between the cylinder liner and piston ring assembly accounts for a significant portion of the engine's overall friction loss. This disparity in wear patterns can lead to problems such as cylinder scuffing and cylinder explosions, as observed in the Subaru EJ257 engine sold in the North American market.
While the issues of uneven lubrication and wear can be addressed, solutions such as specialized lubricating oils come at a higher cost. For instance, Porsche 911 models employ three oil pumps to ensure sufficient oil pressure, and some advanced cars even have five pumps, increasing the cost of engine oil. The problem of uneven wear has also been mitigated through advancements in material technology.
photo from google
Furthermore, horizontally opposed engines are more complex and expensive to produce. For a horizontally opposed 4-cylinder engine with double overhead camshafts, additional camshafts and intake and exhaust systems are required compared to an inline 4-cylinder engine. The different layout and piston distribution also result in varied mold components on both sides, necessitating separate mold production and increasing manufacturing costs. Precise calculations and designs are required for factors like intake and exhaust angles, combustion chamber airflow shape, and valve structure to ensure engine efficiency and durability, further raising the design difficulty and manufacturing expenses. Additionally, the assembly process for horizontally opposed engines is intricate, particularly when splicing the two middle cylinders together symmetrically. A slight assembly error can lead to piston wear in the cylinder, reducing the engine's service life.
The wider body and longitudinal engine layout of horizontally opposed engines make simple tasks like disassembling and installing spark plugs more cumbersome. Additionally, due to the lower adoption rate, maintenance costs for horizontally opposed engines tend to be relatively higher, considering that many parts need to be imported.
Another challenge is the high strength requirements for the engine's motion pair. The motion of opposing pistons reaching top dead center or bottom dead center simultaneously places significant tensile stress on the crankshaft in the 180-degree alignment quadrant. Therefore, careful strength design of the crankshaft and connecting rod is crucial for the kinematic pair of horizontally opposed engines.
Ultimately, other car brands choose not to widely use boxer engines because they do not see a compelling need to overcome these challenges and drawbacks associated with this particular engine configuration.
photo from website https://www.sportsubaru.com/
Development of the Porsche Boxer Engine:
Porsche's development of the horizontally opposed engine began with a 2.0L air-cooled 6-cylinder engine. From its inception, the Porsche 911 was designed as a high-performance sports car, incorporating advanced and costly technology into its engine. With its air-cooled design, rear longitudinal engine placement, and rear-wheel or four-wheel drive layout, the engine compartment of the 911 was limited in space. Initially, Porsche allocated only 2.7L of space for the engine.
As the 1970s approached, increasing horsepower requirements and the introduction of catalytic converters and unleaded gasoline necessitated an expansion of the engine volume. Drawing on their accumulated technology and racing experience, Porsche made breakthroughs by strengthening the cylinder block, reducing the cylinder wall thickness, and increasing the cylinder bore. The adoption of high-strength metal materials and electronic fuel injection greatly improved the engine's maximum speed and output. Despite minimal increase in overall engine size, the optimized layout of the 911 engine with a volume exceeding 2.7L did not require widening the rear, maintaining compatibility with regular cars. In essence, the rear engine compartment's limited space, coupled with its slip-back and short shape, posed challenges for accommodating a larger engine, ultimately leading to the adoption of a flat-four configuration.
The horizontally opposed engine has become an iconic Porsche technology. By combining the horizontally opposed engine with direct fuel injection, Porsche achieved comprehensive power enhancements. This engine configuration found its full potential in the 911 models and was successfully applied to the Boxster and Cayman, two smaller sports cars. The use of horizontally opposed engines has been passed down through generations, becoming a cultural heritage of Porsche. Its unique tuning style and performance characteristics make it an integral part of the brand's identity.
Porsche Boxster/Cayman Engine Structure:
The standard and S-series models of the Porsche Boxster, Cayman, and 911 feature all-aluminum crankcases made using a special chill casting process. This process allows Porsche to reduce weight while increasing the rigidity of the housing. The result is a closed crankcase with integral crankshaft bearings, connected by a single closed cylinder waterway.
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Porsche Boxster/Cayman Engine Piston Linkage:
In the Porsche Boxster and Cayman engines, forged aluminum pistons are used to achieve high-performance goals safely. Improvements to the piston rings minimize frictional losses. The power is transmitted to the crankcase through newly designed connecting rods with enhanced hardness and fatigue strength. The optimized piston and connecting rod design, along with the low compression height, enable the creation of related components with the best power-to-weight ratio. The lower inertial force reduces the load on the crankshaft drive mechanism and contributes to smoother engine operation.
Porsche Boxster/Cayman Engine Intake Control Mechanism:
Porsche has optimized the design of the intake port and valve seat ring in the Boxster and Cayman engines based on extensive simulation results. These measures ensure proper intake airflow and facilitate the manufacturing process. The result is high characteristic output power and torque values. The intake and exhaust camshafts have also been fine-tuned to enhance gas exchange, resulting in higher maximum torque and power output. Additionally, Porsche boxer engines employ the VarioCam camshaft control mechanism, utilizing a "vane controller" and "hydraulically operated plunger" to adjust the timing of the intake valves. This system employs map control to regulate intake valve lift within a range of maximum and minimum levels.
photo from google
The Longevity of Porsche's Boxer Engine: The Real Reason
Horizontally Opposed Engine Structure:
The horizontally opposed engine design concentrates the weight under the engine, resulting in a low center of gravity. This inherent advantage significantly enhances the vehicle's handling. For Porsche, a brand dedicated to sports performance, this characteristic is simply irresistible.
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Turbo Response Speed:
While the horizontally opposed engine contributes to this advantage, the true hero is Porsche's VTG (Variable Turbine Geometry) system. This system adjusts the angle at which the exhaust airflow interacts with the turbine blades, thereby influencing the turbine's responsiveness. In simpler terms, the engine's electronic control unit (ECU) can optimize turbo response speed and boost values to match different power demands.
Weight Distribution of Horizontally Opposed Structures:
Porsche models equipped with horizontally opposed engines are typically rear-mounted or mid-mounted. Due to the engine's layout, which maintains a 50:50 weight distribution between the left and right sides, the weight is evenly distributed on the rear axle. This weight distribution contributes to improved traction, especially when combined with the rear placement of the boxer engine.
photo from google
Smoothness of the Horizontally Opposed Engine:
Porsche places great emphasis on sleek body lines, a significant factor in the enduring success of the 911 series. However, this design approach leaves limited space in the engine compartment. Despite the challenges posed by strict space requirements, Porsche manages to tightly fit the horizontally opposed engine due to its compact size advantage.
Overall, the horizontally opposed engine layout enables a compact and lightweight engine to generate substantial power. Its compact design allows for a lower engine placement, resulting in a lower center of gravity. This, in turn, greatly enhances the handling of Porsche sports cars, particularly in corners. With the weight distribution favoring the drive axle, traction is significantly improved when the boxer engine is mounted at the rear. Moreover, the horizontal layout offers improved fuel efficiency as engine power increases. Thanks to its lightweight construction and low center of gravity, the Porsche 911 engine delivers exceptional performance while remaining suitable for everyday driving.
Porsche has discovered that the horizontally opposed engine aligns perfectly with its pursuit of control and performance. This is why the brand has remained committed to this engine configuration. Ultimately, it's not a matter of good or bad, but rather a question of suitability and alignment with Porsche's performance-oriented philosophy.
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