Multi-valve engines

Multi-valve engines

History
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.

Solutions
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

FUENTE

http://www.autozine.org/technical_school/engine/Multivalve.html