Why should a turbocharged engine have 16 valves,what is the purpose , could the number be reduced?
Answers:
A turbocharged engine doesn't need to have 16 valves, however, more valves means more fuel which means more performance.
Considering the turbo forces more air into the combustion chamber the additional valves allow more air in and the exhaust gasses out efficiently. However you can do it in a 2v system. Generally they have larger intake and exhaust valves. I am assuming you are directing this toward a compact cat. The DOHC design is extremely efficient for them. If you look at a retro-mod like a 60's - 70's car with twin turbo's you will find them to only be a 2V system. The difference again is the size of the Valves.
You get more horsepower out of engines with 4 valves per cylinder for 2 reasons: (1) Better flow of gas/air mixture into the cylinders, and (2) Less restriction on the outflow of exhaust gases once the cylinder has fired. So the BEST situation would be a turbocharged engine with 4 valves per cylinder, but there's no rule that says you absolutely must have 4 valves per cylinder in order to install a turbocharger. If you install a turbocharger on a standard 2-valve-per-cylinder engine, it will give you as much power as (if not, a little more than) a standard engine with 4 valves per cylinder that has no turbocharger.
My motorcycle was designed as an 8 valve unit. However, manufacturers started bringing t out 16 valve designs. Rather than a complete redesign they just enlarged the valve which upped the power for half the price.
Personally I think the more moving parts, the more can go wrong.
A turbocharged engine can benefit from 16 valves because these extra values allows it to breathe better, thus giving it more horsepower. It's not a requirement however.
A turbo doesn't need a 16v cylinder head. 16v's have a larger flow area than 8v's so the turbo can push more air in efficiently.
I agree with Sarge. If you think of an engine as an air pump, any improvement in air flow, such as four valves per cylinder, improves efficiency and power regardless of whether the engine is turbocharged or not. Inefficient intake or exhaust cycles, such as caused by only two valves, will negate part of the boost - partially defeating the purpose of the turbo. Turbo engines, drive trains, etc. are designed from the ground up due to the unique requirements of the heat and power generated by the intake boost. I would assume that two intake valves per cylinder, for more even entry of the air/fuel mixture, improves the combustion process.
An engine needs to breathe in (induction) and out (exhaust) The more efficiently it can do this the better. You function better when your nose isn't blocked don't you?
16 valves enables a greater volume of mixture in and out of the engine. having 16 valves also reduces the surface area of the combustion chamber of the cylinder head, this allows for cooler running. An 8 valve would work but not as efficiently
A four-stroke engine requires at least one intake valve and one exhaust valve per cylinder.
Most modern petrol cars have two of each - two smaller valves can be opened faster than one larger valve, aiding gas flow into and out of the engine. In a four-cylinder engine that means 16 valves in total (4 cylinders, 4 valves per cylinder).
The fact that the engine is turbocharged isn't really that relevant to the number of valves - you can turbocharge a 2v/cyl engine as well as a 4v/cyl engine. The reason most modern turbocharged engines have 4 valve/s cylinder is that the manufacturer starts with an already high-efficiency engine before adding the turbocharger.
Some VW/Audi engines (turbocharged and non-turbocharged) have 5 valves/cylinder (3 intake, 2 exhaust).
Some other manufacturers have used 3 valves/cylinder (e.g Mercedes-Benz).
Some turbocharged engines do not have 4 cylinders, so don't have 16 valves (e.g. smart's 3-cylinder turbo, BMW's new turbo 6-cylinder 335i engine, Fiat's 20v turbo 5-cylinder engine in the old coupé).
Engine breathing is important, but another reason for 4 valves per cyl is the weight of the valves themselves. The smaller valves can be opened and closed faster, with lighter springs than the valves in a 2v system with heavy springs. This reduces wear on the camshaft/followers and also reduces the risk of valve float at higher rpm's
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