The present day requirement for high performance from a medium power engine has resulted in generally higher maximum crank shaft speeds, typically in the region 5500 ? 6000 rpm. This can be explained by recalling the factors governing the power output of an engine since, if neither piston displacement nor mean effective pressure can be further increased. Then the only other way to raise the maximum power is to permit higher crank shaft speeds. For this reason it becomes increasingly more important to avoid erratic operation of valves. As a result, in automobiles we use multi valved engines which are operated directly from an over head cam shaft.
In the engine terminology, the designation ?multiple valves? signifies that each cylinder has more than one inlet and one exhaust valve. Since the early 1980?s, there has been a trend in combustion chamber design towards providing two inlets and two exhaust valves for each cylinder, instead of the customary one of each. Four- valved combustion chambers were actually race proven even before world war I and thereafter were mainly used in application to racing car and few high- performance sports car engines, the reason for their use being that of obtaining maximum power output.
What then is the reason for now using four-valve combustion chambers in the newly produced cars, especially considering the additional application and cost incurred with such an arrangement. The answer to this question has to be more comprehensive and lies in the present day requirement for high performance from a medium power engine, which has resulted in generally higher maximum crankshaft speeds, typically in the region of 5500-6000 rpm. This can be explained by recalling the factors governing the power output of an engine since, if neither piston displacement nor mean effective pressure can be further increased then the only other way to raise maximum power is to permit higher crankshaft speeds. For this reason it becomes increasingly more important to avoid erratic operation of the valves. As a result, in automobiles, we use multi-valved engines, which are operated directly from an overhead camshaft. .
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Multi Valve Engine
In multi-valve engines, each cylinder has more than two valvesMulti-valve engines started life in 1912, adopted by a Peugeot GP racing car. It was briefly used by the pre-war Bentley and Bugatti. Honda S600 was probably the earliest production road-going 4-valve car. Most poppet valve, four-stroke internal combustion engines have at least two valves per cylinder
Multi-valve engines started life in 1912, adopted by a Peugeot GP racing car. It was briefly used by the pre-war Bentley and Bugatti. However, it was not applied to production cars until the 60s - Honda S600 was probably the earliest production road-going 4-valve car. In the 70s, there were several more 4-valve cars introduced, such as the Lotus Esprit (1976), Chevrolet Cosworth Vega (1975, engine made by Cosworth), BMW M1 (1979) and Triumph Donomite Sprint. The latter introduced the first single-cam 4-valve engine, using rocker arms to drive valves.
In the early 80s, when Ferrari had just adopted Quattrovalvole V8, Honda was introducing 3-valve engines to its mainstream bread-and-butter models. In the mid-80s, both Honda and Toyota made 4-valve engines standard in virtually all mainstream models. The Western car makers did that some 10 years later !
Improving breathing is one of the keys for power enhancement. Unquestionably, in the 2-valve era valves used to be the bottleneck, hence the need for more valves.
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 a few car cars using this cheap but inefficient design, such as Fiat Palio and all Mercedes V6 and V8 engines. Mercedes uses that because of emission rather than cost reason.
A typical 2-valve engine has just 1/3 combustion chamber head area covered by the valves, but a 4-valve head increases that to more than 50%, hence smoother and quicker breathing. 4-valve design also benefit a clean and effective combustion, because the spark plug can be placed in the middle.
4 valves are better to be driven by twin-cam, one for intake valves and one for exhaust valves. Honda and Mitsubishi models prefer to use sohc, driving the valves via rocker arms like the aforementioned Triumph. This could be a bit cheaper, but introduce more friction and hurt high speed power. Therefore the sportiest Honda and Mitsubishi still use dohc.
It is arguable that whether 5 valves per cylinder helps raising engine efficiency. Audi claimed it does, but fail to provide evidence to support. In fact, its 5V engines are no more powerful and torquey than its German rivals with 4 valves per cylinder.
Originally, 5-valve design doesn’t guarantee covering more head area than 4-valver. Nevertheless, if the head of combustion chamber is in irregular shape like the picture shown, the valves may cover larger area. Ferrari F355 make use of this to enhance high-speed breathing. Is there any disadvantage? Yes, faster breathing also harm low-speed torque if no counter measure is taken. Therefore it is more suitable to sports cars.
All existing 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 are larger. In F355, by arranging the outer intake valves open 10° earlier than the center valve, it got the swirl needed for better air / fuel mixture, hence more efficient burning and cleaner emission.
The advantage of 5-valve engine is still under questioned. Not only few car makers used it (VW group, Ferrari and the bankrupted Bugatti), but Formula One cars also no longer favour it. Even the Ferrari F1 cars which was once famous for 5V engine has switched back to 4-valve design a few years ago.
Drawback and Solution - e.g. Toyota T-VIS
Most early 4-valve engines were not good at low-to-middle speed torque, simply because the larger intake area resulted in slower air flow. Especially at low speed, the slow air flow in the intake manifold led to imperfect mixing of fuel and air, hence knocking and reduced power and torque. Therefore 4-valve engines were regarded as strong at top end but weak at the bottom end, until the technology of variable intake manifold became popular recently. The aforementioned Chevrolet Cosworth Vega performed particularly weak at low speed.
In response to this, Toyota introduced T-VIS (Toyota Variable Intake System) in the mid-80s. T-VIS accelerated low speed air flow to the manifold. The theory was quite simple: the intake manifold for each cylinder was split into two separate sub-manifold which joint together near the intake valves. A butterfly valve was added at one of the sub-manifold. At below 4,650 rpm the butterfly valve would be closed so that raising the velocity of air in the manifold. As a result, better mixing could be obtained at the manifold (excluding direct-injection engines, fuel injection always takes place in the manifold).
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