Page 1 In automotive engineering, a multi-valve (or multivalve) engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and can spin at higher rpm than a 2-valve engine, yielding more power.

Page 2 Adding more valves increases valve area and improves the flow of intake and exhaust gases, thereby enhancing combustion, volumetric efficiency, and power output. Multi-valve geometry allows the spark plug to be ideally located within the combustion chamber for optimal flame propagation. Multivalve engines tend to have smaller valves with a lower reciprocating mass, allowing more power from higher rpm, without the danger of valve bounce. Page 3 3-valve cylinder head 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.

Page 4 4-valve cylinder head This is the most common type of multi-valve head, with two exhaust valves and two similar (or slightly larger) inlet valves. This design allows similar breathing to a 3-valve head, and as the small exhaust valves allow high rpm(Exhaust valve lift does not have air separation problems, so the lift can be higher, allowing smaller exhaust valve size)., this design is very suitable for high power outputs. . 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. This could be a bit cheaper, but introduce more friction and hurt high speed power. T herefore the sportiest Honda and Mitsubishi still use dohc.

Page 5 5-valve cylinder head Less common is the 5-valve head, with two exhaust valves and three inlet valves. All five valves are similar in size. This design allows excellent breathing, and, and as every valve is small, high rpm and very high power outputs are available.

It has been questioned whether a 5-valve configuration gives a worthwhile benefit over 4valve designs. In theory the three inlet ports should give both good cylinder-filling and high turbulence (both desirable traits). Some manufacturers have reverted to the cost-effective 4-valve design which is simpler and cheaper to manufacture than the 5-valve head.

Page 6 Originally, 5-valve design doesnt 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. Page 7 The exhaust valves are larger, but in terms of total area intake valves are larger (The larger the intake valve size, the more the engine will breath).

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 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. Starting in 1922, many Bugatti engines used three valves per cylinder actuated by a single overhead camshaft (sohc) and multi-valve engines continued to be popular in racing and sports engines. Page 8 Although most multi-valve engines have overhead camshafts, either sohc or dohc; but a multivalve engine may be a pushrod ohv design.

Page 9 Drawbacks and Solutions 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. 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. Page 10 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

However, for later mainstream sedan engines, Toyota dropped this idea and adopted a small-diameter intake manifold / port design.

Many other car makers also went the same way, sacrificing a bit top end power to improve low speed flexibility. Today, the introduction of variable intake manifold can solve this problem.

Page 11 Many things needed to be Considered Nissan has produced the 1988-96 KA24E engine with three valves per cylinder (two intakes, one exhaust) that are also actuated by single overhead camshaft. Mercedes and Ford are currently producing V6 and V8 engines using this configuration. Ford claims an 80% improvement in high rpm breathing without the added cost of a second camshaft per bank of cylinders. The Ford design uses one spark plug per cylinder located in the center, but the Mercedes design uses two spark plugs per cylinder located on opposite sides, leaving the center free to add a direct-to-cylinder fuel injector. Thus there are many considerations to deciding how many valves an engine should have besides just the added cost Vs adding breathing capability. Page 12 Recent Research "Multiple-valve engines are designed to run very high rpm.". The fact is, the multi-valve engine does not run appreciably more rpm than the two-valve engine. Rpm is notonly the thing. There are more important reasons for the multi-valve engine. These reasons go way beyond rpm. Rpm-Related Benefits Here are three rpm-related benefits of the multi-valve cylinder head. All are related to the valve's reduced mass. Page 13

First, smaller valves have less inertia. This means they are easier to control at higher rpm, so higher rpm is possible.

There is less tendancy for the valves to "float," that is, loft off the cam lobe slightly, which is one of the many barriers to high rpm in an engine. Second, smaller valves require less spring pressure. Many multi-valve engines have just one spring per valve instead of the traditional two. Without the second spring, spring harmonics problems associated with high rpm are eliminated, removing yet another obstacle to high rpm. Page 14 And third, a bunch of smaller valves use up the combustion chamber's ceiling more completely than can fewer, larger valves. The result is more efficient use of the chamber's potential flow area. The scattering of little valves all over the combustion chamber's ceiling are essentially one big valve in effect. Therefore high flow is possible, and i f the rest of the engine can support that flow then high rpm is possible.

All of these things would seem to equate the multi-valve head with high rpm. Despite this, high rpm is not the reason for the multi-valve head.

The truth, the benefits of the multi-valve head are much more involved than simple rpm. That is its effect on combustion

Page 15 There are three benefits of the multi-valve cylinder head that relate to combustion. First, the multi-valve head is, most of all, an attempt at the ideal combustion chamber shape. Years ago, it was believed that the hemispherical combustion chamber was the best shape. From a purely geometrical standpoint, it is. T The hemi's constant radius theoretically encourages a very uniform rate of travel for combustion's flame, resulting in more predictable consumption of the air/fuel charge. The problem is, a hemi shape results in a very large chamber, one too large for adequate compression without using high-domed pistons. Those pistons mess up the chamber's uniform shape, trapping end-gases far from combustion's flame, encouraging detonation. Furthermore, the sheer volume of the hemi chamber requires so much ignition advance to get the job done that too much time is spent heating up the engine and not enough in making power.

Page 16 Current combustion chamber technology dictates very flat combustion chambers. As it happens, the multi-valve chamber is a natural for this flat shape many smaller valves permit a flatter chamber shape than do fewer larger ones. Moreover, the reduced volume of this chamber results in a shorter flame period, which means ignition timing doesn't have to be advanced as far. This is why modern engines run so much less ignition timing than older ones do.

The result is free power because for whatever number of crankshaft degrees that are saved, the engine is no longer fighting itself, with the piston compressing an already-burning mixture. Remember also that the piston is part of the combustion chamber. With their flat-topped pistons, high compression is possible in these chambers without detonation. There are no nooks and crannies in which end-gases can hide. This chamber's flatter pistons also preset to the chamber less surface area and therefore absorb less of combustion's heat. More of combustion's heat is put to the useful work of pushing on the piston. Hence, more power. Page 17

The second combustion-related benefit of the multi-valve combustion chamber centers on the valves. With two or three intake valves opening at the same time instead of just one, more port area is exposed at a given valve lift. That is, the port is opened to a given amount in fewer crankshaft degrees than is possible with just one intake valve. There is more instantaneous port opening. The effect of this is interesting. The air/fuel mixture waiting in the port has a more immediate urgency to enter the cylinder when the intake port opens. The pull on it is more sudden. The result is increased cylinder filling, which results in higher combustion pressure and torque. On the road, this means improved low-rpm power and enhanced throttle response.

Part 18 The third combustion-related benefit of the multi-valve chamber focuses on the ports.

More specifically, the benefit is port velocity, that is, airspeed. The multi-valve head has higher intake port velocity than the two-valve head. This is perhaps the most significant benefit of all. The typical multi-valve design has small divided ports between the valves and the intake system. These smaller ports increase the average port velocity, which makes the air/fuel mixture enter the cylinder more homogenous -- better mixed. The result is better combustion, which translates directly to increased power, especially at low rpm. But that's just the beginning. Page 19 Port velocity is also increased in the multi-valve engine by the effect of the valve's combined peripheral area. peripheral area increases with the number of valves. Even with identical total valve area, two intakes have more peripheral area and therefore flow more than does one intake. Three intakes have even more, and so on. Therefore, velocity increases. That is, more flow with the same area results in more velocity. The result is, again, increased combustion efficiency. So multi-valve heads burn better, and the more intake valves there are, the better they burn. Page 20 The multi-valve cylinder head therefore offers many more benefits having to do with combustion than with rpm. Okay, but if multiple intakes are such a good thing, then why don't some engines have six, seven, or even eight valves per cylinder? In fact, they do. Maseratti produces six-valve chambers, and Honda's stupendous NR750 is an eight-valver. Yamaha's 5-valve Genesis design is of course the most famous. The company has gone on record as saying they experiemented with more than five but settled on that number as offering the most return for the investment . The reason is, although the phenomenon continues to increase with the number of valves, it isn't as dramatic at eight valves per. The benefit begins to level off after five valves. The practical choice for Yamaha was to focus on that number.