sábado, 31 de marzo de 2018

Multi-valve engines

Multi-valve engines

Multi-valve engines started life in 1912 on a Peugeot GP racing car. It was then briefly used by the pre-war Bentley and Bugatti. However, mass production on road cars came as late as 1970s - Ford Escort RS1600 (1970), Triumph Donomite Sprint (1973), Chevrolet Cosworth Vega (1975), Lotus Esprit (1976), Fiat 131 Abarth (1976) and BMW M1 (1979) were the earliest adopters.

Triumph Donomite Sprint was one of the earliest road cars to feature multi-valve technology. Its 2-liter four-pot engine featured 16 valves but just one camshaft, unlike the DOHC designs popular on contemporary racing engines. The intake valves were driven directly by the camshaft, while the exhaust valves were driven by the same camshaft through rocker arms. Today, Honda's SOHC 16-valve engines still employ the same design.

By the mid-1980s, 4 valves per cylinder virtually became standard on high-performance cars, such as Ferrari 308 GTB Quattrovalvole (1982), BMW M635CSi (1983), Ferrari 288 GTO (1984), Mercedes 190E 2.3-16 (1984), Saab 9000 (1984, also the first to combine 4-valve and turbo on production car), BMW M5 (1985), Ferrari Testarossa (1985), Lamborghini Countach QV (1985) and Volkswagen Golf GTi 16V (1985), let alone those Group B rally specials.

However, it was the Japanese who came first to put multi-valve technology on mass production cars that everybody can afford. Honda Civic adopted 3-valve engines as standard in 1983 and 4-valve engines in 1987. Toyota mass-marketed its high-performance 1.6-liter 16V engine on Corolla coupe / Truneo (1983) and MR2 (1984), then equipped the bread-and-butter Corolla with 4-valve engines in 1987. They standardized multi-valve engines nearly a decade earlier than Western car makers !

Advantages and Disadvantages
Multi-valve engines have mainly 3 advantages. Firstly, it increases the coverage of valves over the combustion chamber, allowing faster breathing thus enhance power at high rev. Secondly, it allows the spark plug to be positioned in the center of combustion chamber, enabling quicker flame propagation, more even and more efficient burning. Thirdly, using more but smaller valves instead of two large valves means lower mass for each valve. This prevent the valves "float" from its designed position at very high rev, thus enabling the engine to rev higher and make more power as a result.

A comparison of the 4-valve head on BMW M3 V8 and the 2-valve head on Chevrolet small-block V8 finds the former has larger percentage of area covered by valves. The spark plug is positioned centrally in the 4-valve head, unlike the case of 2-valve head.

On the downside, multi-valve engines use more components, thus they carry more weight and higher costs. While these disadvantages can be largely overcome by mass production, another problem took some years to solve. The early multi-valve engines were not renowned for tractability. At low to medium rpm they actually produced less torque than the equivalent 2-valve engines. Why? Because the larger valve area resulted in slower air flow in the intake manifold. At low rpm, the very slow air flow led to imperfect mixing of fuel and air, resulting in knocking and reducing power. For a racing car or sports car, that might not be a big problem, but for regular passenger cars the lack of tractability is deemed to be unacceptable.

Toyota T-VIS
In response to the aforementioned drawback, Toyota introduced T-VIS (Toyota Variable Intake System) in the mid-1980s. T-VIS accelerated low-speed air flow in the intake manifold. The theory was quite simple: the intake manifold of each cylinder was split into two separate sub-manifolds which joint together near the intake valves. A butterfly valve was added at one of the sub-manifolds. At below 4,650 rpm the butterfly valve remained closed so to raise the velocity of air flow in the intake manifold. As fuel was injected at this section of manifold, better air-fuel mixing could be obtained. At high rev, the butterfly opened to allow maximum air flow.

The T-VIS was used on performance models like AE86, MR2 and Celica. However, for its mainstream passenger cars, Toyota dropped this feature and adopted a small-diameter intake manifold/port design for cost reasons. Many other car makers went the same way, sacrificing a bit top-end power for better low-speed tractability.

Modern Approaches
In recent years, the low-speed tractability problem can be dealt with a variety of solutions, such as variable intake manifold (which boosts low-end torque), variable valve timing (which may delay the opening of intake valves at low rev to accelerate air flow) and variable valve lift (which varies degree of lift hence air flow speed). However, the ultimate solution must be direct fuel injection. Fuel is now injected precisely into the combustion chamber rather than the intake manifold. Complete vaporization is realized by the high-pressure injector as well as swirl effect.

Number of Valves
3-valve engines
The earliest mass production multi-valve engines were 3-valves because of its simple construction - it needs only a single camshaft to drive both intake valves and the exhaust valve of each cylinder. Today, there are still some low-end cars using this cheap but less efficient design.

Surprisingly, Mercedes-Benz reverted from 4-valve to 3-valve technology on its modular V6 and V8 family in the late 1990s to mid-2000s. The change was not due to cost reasons, but the need for cleaner emission. Research done by Mercedes found great difficulties to comply with the cold-start emission limits required by forthcoming EU standards. By halfing the number of exhaust valves, the surface area of exhaust ports and manifolds can be largely reduced. This reduces the time taken to heat up the catalytic converter at cold start. With the advancement of emission control technology in the coming years, Mercedes eventually abandoned the 3-valve approach.

4-valve engines
Today, by far the majority of multi-valve engines employ 4 valves per cylinder.
Most 4-valve engines employ twin-cam (DOHC) for its obvious benefits, for instance, cross-flow intake/exhaust, low inertia and friction, and the allowance of independent intake and exhaust variable cam phasing. However, some cost-conscious engines (most by Honda and Mitsubishi) still employ single-cam (SOHC) to drive all valves. Like the aforementioned Triumph Donomite Sprint, these engines use rocker arms to transfer the motion from camshaft to exhaust valves. The resultant higher friction and inertia hampers revvability, but it does not matter for applications on family cars.

5-valve engines
Yamaha was the expert of 5-valve technology. Since the mid-1980s it had been using 5-valve technology on its high-performance motorcycles. In 1991, it helped Toyota to produce a 1.6-liter 20-valve engine for Corolla Levin (Trueno). This was perhaps the earliest 5-valve application on road cars. Closely followed that were Bugatti EB110 and LCC Rocket (powered by Yamaha bike engine). Meanwhile, having used 5-valve technology successfully in F1 cars, Ferrari applied it to F355 and F50. However, the only manufacturer ever put it to mass production was Audi (which also benefited Volkswagen group). For about a decade, most engines produced by Audi were equipped with 5-valve heads.

The 5V cylinder head of Audi. Audi devoted its production engines to 5V technology from the mid-1990s to mid-2000s, including the highly popular 1.8-liter 20V, 2.8 / 3.2-liter 30V V6 and 4.2-liter 40V V8.

All 5-valve engines have 3 intake valves and 2 exhaust valves per cylinder, still arranged as cross-flow. The exhaust valves are larger, but in terms of total area intake valves is larger. The intake valves do not necessarily open at the same time. For example, on Ferrari F355, the outer intake valves opened 10° earlier than the middle valve. This created swirl, enabling better air/fuel mixing, hence more efficient burning and cleaner emission.

In theory, 5 valves per cylinder may offer larger valve area than 4-valver for better breathing. The smaller and lighter intake valves also enable the engine to rev higher without worrying of "valve floating". The latter reason was especially crucial to high-revving superbike engines and racing motors. However, the overall advantage of 5-valve technology over 4-valver has always been arguable, because it involves more components hence more mass and friction. Cross-flow breathing is also less ideal than in the case of 4-valver. In 1993, Ferrari gave us a preliminary answer: its F1 motor returned to 4-valve heads. This was made possible as new pneumatic valve springs could solve the "valve floating" problem. In the road car department, Ferrari replaced the 5-valve 360 Modena with 4-valve F430 in 2005 and still capable of making more horsepower per liter. This broke the legend of 5-valve technology. By the mid-2000s, Audi started reverting to 4-valve engines as well, ending the short-lived fever of 5-valver.

6-valve or more ?
Until now, anything more than 5 valves per cylinder is still a dream... a wild dream actually.

In 1985, Maserati announced this V6 engine with a total of 36 valves. The engine coupled to light-pressure twin-turbo to produce 261 horsepower... from just 2 liters of displacement ! Unfortunately it was cancelled before reaching production.

NR750, Honda's GP motorcycle in the early 1990s, even featured 8 valves each cylinder ! Interestingly, the piston was in oval shape to accommodate all valves. It also needed two con-rods to support. In fact, the V4 engine was actually a V8 with each of the two adjacent cylinders combined - just to take the racing regulations' loophole that banned more than 4 cylinders.

Copyright© 1997-2011 by Mark Wan @ AutoZine




martes, 27 de marzo de 2018



-Los retenes son piezas de sellado elaboradas con materias primas de primera calidad en caucho. Se hacen también con siliconas y resinas de alta performance. Las siliconas y resinas dan al retén alta resistencia a la temperatura, aceites y corrosión.
-El reten es una pieza adicional de la máquina o motor, cuya misión consiste en el sellado de ejes y la protección de los elementos involucrados.

Los retenes se utilizan en bancada, distribución y árbol de levas, válvulas, rueda e industriales, para lavarropas, kit, juntas para salida de caños de escape, laterales de bancada, tapones para pernos de teflón, guarniciones.

Estos retenes por su utilización son hechos de silicona y también de nitrilos.
Unas de las formas de clasificar los retenes es por el diámetro del eje donde estrá montado. La siguiente tabla, a modo de ejemplo, nos ilustra una forma de lo que hay que saber al momento de comprar unos retenes para autos, en este caso.

En los retenes de válvula debemos considerar la adaptación a la composición química de los diversos tipos de aceite que aparecen en el mercado evitando que se eleve la temperatura a la que debe soportar para rendir mayor cantidad de horas de servicio.
Entre las calidades de retenes de válvula automotrices mencionaremos:

- Nitrilos
- Siliconas
- Teflón

Y en las variedades tenemos:

- Arandelas
- Capuchones
- Con aros
- Alma de acero
- Con apoya resortes

Esto considerando las medidas que requieren los distintos fabricantes de motores.


Estos retenes se usan en industrias, en electrodomésticos; en lo automotriz: para rueda delantera, trasera, piñón, salida de caja. También en palier, selectora, directa, compresor, bomba de inyección, bomba de vacío.

Según sean automáticos y semiautomáticos, de carga frontal y horizontal, tendrán instalados distntos tipos de retenes. Son fabricados con nitrilos y siliconas.


 8- retenes
9- rodamientos

Existen principalmente dos tipos de retenes característicos especialmente diseñados para la mecánica automotriz.

I. Retén totalmente recubierto de goma: el más usual, ya que debido a su acoplamiento elástico permite en el mecanizado del alojamiento mayor rugosidad, tolerancias menos estrechas y más margen de dilatación. No forma oxido en el ajuste y la carcasa metálica está protegida contra la oxidación.

II. Retenes dobles: para una estanqueidad más segura. Importante en su uso la existencia de grasa o aceite entre ambos labios, ya que la pérdida de tal lubricación ocasionaría calentamiento. Utilizados para la separación de dos fluidos.

Prácticamente los demás tipos de retenes se reducen a los anteriormente expuestos, con ligeras variaciones de forma o tipos de materiales.

La misión del reten:

* Impedir la entrada a la máquina de sustancias perjudiciales (agua, polvo etc.).
* Retener los elementos beneficiosos de lubricación (grasa, aceite, agua...)
* Evitar que dos fluidos que se encuentren en dos compartimentos diferentes lleguen a mezclarse.
* Bloquear la salida de gases o líquidos de trabajo.

Es muy extensa la gama de retenes que se pueden suministrar. En todos los materiales existentes en el mercado (nitrilo, silicona, vitón etc.). De 5 a 500 mm. de diámetro. Para trabajar con amplio margen de temperatura:

* NITRILO -30 a 120 ºC
* SILICONA -50 a 180 ºC
* VITON -30 a 230 ºC

La calidad de los retenes que se comercializan en el mundo, deben estár reconocida y homologada en diversos países según normas técnicas en vigencia.



· F1 = Goma exterior
· F2 = Chapa exterior
· F5 = Goma exterior, labio antipolvo, sin muelle
· F13 = Goma interior, labio exterior
· F14 = Goma interior, labio antipolvo exterior
· F18 = Goma exterior, labio antipolvo
· F19 = Chapa exterior, labio antipolvo
· F20 = Goma exterior, doble labio, dos muelles
· F24 = Goma exterior, sin muelle
· F31 = Goma exterior, corta aceites derecha
· F32 = Goma exterior, labio antipolvo, corta aceites derecha
· F33 = Goma exterior, corta aceites izquierda
· F34 = Goma exterior, labio antipolvo, corta aceites izquierda
· F35 = Goma exterior, corta aceites bidireccional
· F36 = Goma exterior, labio antipolvo, corta aceites bidireccional
· F36 = Goma exterior, labio antipolvo, corta aceites bidireccional
· F37 = Goma exterior, estriado exterior
· F38 = Goma exterior, labio antipolvo, estriado exterior



· Son elementos mecánicos cuya función es retener el paso de fluidos entre 2 superficies en movimiento, una respecto a la otra como la figura mostrada (un eje y su alojamiento).
· Su aplicación principal es impedir la filtración de lubricantes.
· Cuando es requerido debe detener el ingreso de polvo u otros contaminates del exterior hacia dentro del sistema.
· Además, debe desgastarse mas rápidamente que las piezas con las cuales trabaja y que cuestan mucho más.

El diseño básico de los retenes, es mostrado en la figura y se compone de:

El labio primario o de retención, encargado del sello, es la parte que estará en contacto con el elemento en movimiento.
El anillo metálico que da consistencia al reten y permite su montaje y fijación.
El resorte, encargado de aportar un constante apriete entre el labio primario y el eje.
Y finalmente, el labio secundario o guarda polvo, encargado de evitar la contaminación del labio de retención.


Nitrilo (NBR)
El caucho nitrilo tiene excelentes propiedades mecánicas y alta resistencia al desgaste. Compatible químicamente con aceites, grasas vegetales y minerales, agua, etc. Su resistencia a la temperatura es de -40º a 120 ºC.
Desarrollado para ser utilizado frecuentemente con lubricantes EP, tiene muy buena resistencia al óxido y al ozono. No recomendable para sellos de agua. Resistente en rangos de temperatura de -20º a 150 ºC.

Silicona (SI)
La silicona utilizada para la fabricación de retenes, tiene grandes características para resistir altas y bajas temperaturas. Es un buen aislante, resistente a la intemperie. Adecuada para temperaturas entre -50º a 180 ºC.


Durante la instalación y para asegurar un adecuado funcionamiento de los retenes se deben tomar en cuenta los siguientes criterios:

➤Examinar el eje y eliminar cualquier rugosidad, restos de mecanizado, y en general cualquier impureza de su superficie. Los cantos deben ser redondeados o biselados. Caso de no ser posible, debe preverse un casquillo de montaje con bordes redondeados, y un diámetro exterior ligeramente superior al eje.

➤Aplicar grasa al labio del retén. Si tiene labio guardapolvo, poner grasa entre los dos labios. Normalmente, el lado del resorte es el que se debe estar encarado al aceite a retener.

➤Cualquier pequeño corte producido en el labio principal del reten en el momento de su montaje, será una fuga segura en el momento de funcionamiento, por lo que se debe evitar el contacto con el mismo.

➤ Cuando se instala el reten en su alojamiento, debe realizarse con una presión uniforme en toda su circunferencia, cuidando además que su introducción sea totalmente perpendicular al eje.

➤Se recomienda la utilización de útiles de montaje del tipo reflejado en la figura. El diámetro exterior del útil debe ser ligeramente inferior al del alojamiento. El diámetro exterior del reten debe haber sido engrasado previamente al montaje.