Modern car aerodynamics tackles numerous challenges. Specialists aim not only for minimal air resistance but also track axis lift force, crucial as today’s cars reach speeds surpassing those of airplanes taking off. Ensuring engine and brake disk cooling, cabin ventilation, and strategically placed air intake/exhaust holes are necessary. Aerodynamics governs cabin noise levels, preventing dirt-capturing air streams on windows, mirrors, lights, and handles. Windshield cleaning quality must remain consistent at higher speeds.
Tasks are diverse, interconnected: creating brake cooling intakes may increase drag. Solving this puzzle requires expertise. We’ll explore two main aspects: air resistance and downforce.
Aerodynamic Resistance
Air resistance force, proportional to speed squared, impedes acceleration. How does this relate to car parameters? Converting to mechanical work terms reveals power loss in cubic dependence on speed. Even increased engine power struggles with the last speed increments.
Reducing drag is vital, not just for aerodynamics but for the automotive industry’s environmental push.
Drag Force Formula:
S – cross-sectional area (m²), V – air flow velocity (m/s), \rho – air density (1.23 kg/m³), – aerodynamic drag coefficient.
To influence force at a set speed, modify either or . Two solutions exist: reduce car cross-sectional area (creating a narrower, lower body) or optimize body streamline, focusing on the aerodynamic resistance coefficient . The latter option is crucial as car sizes increase, contradicting the trend in the automotive industry. This trend intensifies with crossovers entering sports car segments, where aerodynamic requirements are high.
The only viable option is optimizing the body streamline, with the coefficient of aerodynamic resistance as the perfection criterion (sometimes denoted as C_w in literature).
The value of Cx is determined experimentally. In a streamlined body like an elongated water drop, Cx is 0.04; in a sphere, it’s 0.47; in a cube with perpendicular edges to the flow, it’s 1.05. If turned at a 45-degree angle, Cx decreases to 0.8. Most cars have a Cx in the same range, but the lower limit rises to about 0.25.
Factors affecting a car’s Cx include internal resistance (12% of the total), resulting from air passage through the underhood and passenger compartment. Frictional resistance between airflow and body surface (10%) and shape resistance, mainly manifested by overpressure in front and rarefaction behind, also play a role.
Internal resistance faces challenges due to powerful modern engines requiring more air for cooling. Transitioning to efficient electric motors remains a future technology for significant improvements.
Surface friction resistance contributes 10% to Cx. A layer of air adjacent to the surface collides with micro-roughnesses, forming a boundary layer. Maintaining laminar flow requires smooth body surfaces, reduced gaps, and slight curvature.
Shape resistance, the main factor in Cx, arises from pressure in front and rarefaction behind. Achieving a shape that smoothly cuts through air, avoiding flow detachment, is challenging. Engineers face the difficulty of creating a dirigible-like shape with minimal edges and a gradually tapering rear part.
Retrospective
Driven by two electric motors with a total power of 67 hp and weighing 1000 kg, it achieved a maximum speed of 105 km/h.
In the early 20th century, when cars were nascent and barely surpassed 40 km/h, aerodynamics wasn’t a concern. With a Cx value around one, these models couldn’t compete in streamlining even with a notorious brick-like shape. Enthusiasts, mainly record and “concept car” developers, paid attention to this.
Shapes were initially borrowed from navigation and aviation. Camille Jenatzi’s 1899 car, the first to exceed 100 km/h, likely had imperfect aerodynamics, given its considerable 67 hp power, driver exposure, and exposed suspension elements.
A successful streamlined attempt was the 1913 Alfa Romeo 40-60 HP. Its airship-shaped body, concealing the passengers and integrating the chassis, achieved 139 km/h with 70 hp, signifying exceptional aerodynamics for its time.
Tropfenwagen
Tropfenwagen (1921) stood out with a low Cx of 0.28 and a unique W-shaped 6-cylinder engine in the tail. Engineer Edmund Rumpler’s 1921 creation marked a shift in approach. The “drop car” featured narrowed front and rear parts, a curved roof, and an oval cabin, achieving a remarkable Cx of 0.28, despite protruding wheels increasing resistance by 50%. Unfortunately, it lacked demand.
By 1921, the approach evolved. Edmund Rumpler’s Tropfenwagen in 1921 signaled a shift. Its unique design, with a narrowed front and rear, a curved roof, and an oval cabin, achieved a remarkable Cx of 0.28, even with protruding wheels increasing resistance by 50%. Despite this, the extravagant car faced low demand.
Ideal Aerodynamic Shape – Cx = 0.16:
One of the ideal shapes for a car’s aerodynamics is a Cx of 0.14-0.16. While other shapes are possible, their Cx values will hover around 0.15.
Rear Shapes
Comparison of rear end shapes: 1 – shortened form typical for 20-40s serial cars; 2 – “optimal” 1934 proposed form; 3 – ideal form. The 2nd variant with a steep rear cut is preferable to the sloping form 1; the flow breaks away much later.
Meanwhile, the Göttingen Institute of Aerodynamic Research derived an “ideal” shape with Cx at 0.16. It resembled a modern Porsche 911 but with a more pointed front and rear.
For sports cars like Adler Triumph (1934), this form worked, but for “civilian” cars, it seemed almost useless, irrationally using the internal volume of the long “tail.”
Long attempts at serial production of such ideal shapes saw success in the Tatra-87 of 1940, with a larger rear end inclination but a reduced Cx of 0.38.
Kamma Avitomobile
In 1934, researchers concluded that an extended rear part is ineffective if it doesn’t match the ideal shape. The Kamma K5 car (Cx 0.37) later exemplified this concept.
By the 20-40s, serial cars saw a decrease in average Cx from 0.8 to 0.55, primarily through layout and stylistic changes. Post-war, Citroen DS (1955) achieved a Cx of 0.38, and Porsche’s 1959 356 reached 0.39, standing out amid a 0.5 average. The double integral over general region calculator was also used for the first time.
By the 70s, aerodynamics gained priority. Engineers shifted from creating optimal shapes to optimizing designer proposals. Despite limited intervention, this approach kept Cx at 0.45 in the ’70s and allowed consistent improvement with supercomputers.
Front Spoiler
Front spoiler: Small spoilers reduce overall aerodynamic resistance by preventing air flow under the car. The nose has few requirements, but directing airflow correctly is crucial to avoid flow disruption.
Hatchbacks and Station Wagons
The main element influencing aerodynamics is the rear body part, where optimization possibilities abound. Hatchbacks and station wagons face challenges due to rear end designs, with inclined fifth doors impacting streamlining.
Steeply cut shapes (station wagons) create large discharge zones, impacting movement resistance and causing rapid rear window contamination. Sloping hatchbacks, like Audi A5 Sportback and Porsche Panamera, achieve better streamlining at small inclinations.
Sedan
Sedans and coupes tend to have the best streamlining performance. Stepped rear ends, like those in sedans, alleviate issues, maintaining a small rarefaction zone behind the rear window.
Conclusion
The lower boundary of Cx was found long ago, and progress is explained by reduced research costs. Achieving forward progress requires reconsidering aerodynamics’ role, exploring new forms, proportions, and prioritizing engineering thought over design imagination. Potential lies not only in the 1920s ideal shape (Cx 0.16) but also recent studies confirming that streamlining and rational layout are not mutually exclusive.