Have you noticed that the recently launched Maruti Baleno's ads proudly tell that the car uses four-valves per cylinder. The ads for the Maruti Wagon-R also highlight this point arrogantly. In the recent past Honda advertised their Hyper-16 valve City. Daewoo went gaga over their four-valve head Nexia. Even HM went berserk over their three-valves per cylinder Lancer. What is this obsession with valves? After all what difference does it make, whether the engine has two, three, four or five-valves as long as it does the duty of driving the car? Does it really help to have four valves in the engine? Does it make the engine any better? If these are some of the questions plaguing your mind then just read on.

Read more about 'How Car Engine Works' on - www.howthingswork.com (by Marshall Brain)

Going into the working details of the engine; it is an energy converter which burns fuel (petrol, diesel or something else) inside the cylinder and converts the heat energy recovered into mechanical energy. The more the heat energy, the more is the power developed by the engine. So the obvious aim of every engine designer is to produce as much heat inside the cylinder as possible. It can be done in two ways, one in which you pump in more fuel inside the cylinder, the other and preferred way is to burn the existing fuel efficiently and completely.

The burning process is a chemical reaction, which requires a large amount of oxygen. Generally in normal engines, this quantity falls short of requirement resulting in an incomplete combustion process. Thus the normal engine is inefficient in producing power and wastes fuel a lot. The valve is an important part of a four-stroke engine. It acts as a regulator of the intake and exhaust passages of the engine, opening and closing them when the need arises. Thus the amount of air that can be filled inside the cylinder is linked to the valve.
Thus the major aim of all engine designers is to try and fill as much air inside the cylinder as possible.
This can be done in any of the following ways: -

Increasing the cross-sectional area (Bore) of the cylinder
The easiest way of allowing more air inside the cylinder is to increase the area of cross-section of the cylinder. Increasing the bore of the cylinder may have two effects. In the first case where we are fixing the cylinder volume then any increase in the bore will have to be substituted by a corresponding decrease in the stroke length so that the cylinder volume remains the same. This may be feasible to some extent, but after that it becomes impractical as a very short-stroke engine will be low on torque and high on revs, which again, after a limit is undesirable.

In the second case if we don't impose any restrictions on the cylinder volume, i.e. increasing the bore without any decrease in the stroke length, then there is an increase in the dimensions of the engine leading to an increase in the size of the engine bay leading to an increase in the size of the car leading to an increase in the weight of the car leading to a lowering of the power-to-weight ratio leading to..well; No Advantages! In a nutshell, increasing the bore does no wonders to the performance of the car.

Fitting a Turbocharger
A Turbocharger in effect supplies pressurized air to the cylinder so that for the same valve size and opening time, more air is filled inside the cylinder. This seems to be a very viable option, but then a turbocharger is very expensive adding to the total vehicle cost, and is difficult to install. It too has its limitations in terms of engine design. So in certain cases a turbocharger may not be a viable option which brings us to the third possible option.

Increasing the utilized area of cross-section of the cylinder
The third option seems the most viable. You see, when we are using only two-valves per cylinder (Fig.1), i.e. one intake and one exhaust, then a large portion of the bore area is left unused. Now reduce the diameter of the valves so that you can accommodate three valves per cylinder (Fig.3). The utilized area of cross-section is increased. Making the valves still smaller can accommodate another valve so that we get a four-valve layout (Fig.2) utilizing still more area of cross-section. Similarly we can have a five-valve (Fig.4) or six-valve layout.

So the more the number of valves, the more cross-sectional area of the cylinder is used, the more will be the volumetric efficiency of the engine, the more power it will produce. However, this has certain limitations. You cannot increase the number of valves to more than five or six per cylinder as more than this will lower the strength of the cylinder head. To add to this the gain in the utilized area of cross-section is negligible. Also more than five or six valves in a cylinder will lead to mechanical complexities which make them practically impossible. Accommodating a large number of valves and their assembly on the cylinder head will not be feasible. In fact all manufacturers today are using two, three, four or five valves per cylinder configurations. Maserati did for a while flirt with six valves per cylinder format but has since reverted to the standard layouts. Honda experimented with an oval piston, eight-valve per cylinder engine but it never went past the laboratory stage, it being too mechanically complex.

Fig.1 Two-valve layout Area utilized for intake =16% Area utilized for exhaust =12.96% Total area used=28.96%	Fig.2 Four-valve layout Area utilized for intake = 20.48% Area utilized for exhaust =15.68% Total area used=36.16%
Fig.3 Three-valve layout Area utilized for intake =23.04% Area utilized for exhaust =20.48% Total area used=43.52%	Fig.4 Five-valve layout Area utilized for intake = 23.52% Area utilized for exhaust =15.68% Total area used=39.20%
The above diagrams approximately illustrate the increase in the utilized area as the number of valves is increased. The figures used are just to illustrate the point and are not representative of actual dimensions. As seen there is an increment in utilized area as we switch from a two- valve to a three-valve configuration. This is more so because normally in a three-valve configuration we use a large diameter Intake valve. The increment in utilized area goes on decreasing with subsequent increases in the number of valves.

Now the question arises which configuration among three, four and five-valves per cylinder is the best. All are better than the two-valve per cylinder layout. Manufacturers normally favor a three-valve layout in an under-square engine (where the bore is less than the stroke), five-valve layout in an over-square engine (where the bore is greater than the stroke) and a four-valve layout in a square engine (bore and stroke are nearly equal) In these cases the factor, which influences the selection is the swirl and tumble effect created by an increase in the number of valves. (This aspect is too complex and advanced to be discussed within the confines of this article, I will deal with it in one of my later articles) In short, a three, four or five-valve layout can be equally good for different engines.

But, does that mean that multi-valve engines are always preferred. Not necessary! Look at it this way. When you improve the breathing of the engine, the power gets improved. This is because the engine is now able to draw more air, but at the same time it also draws in more fuel. So the fuel efficiency of the engine comes down. Thus if a company is planning to sell a car by virtue of its fuel efficiency, then it may rather not prefer a multi-valve unit. This has happened many times, for example in the 1996 Honda Logo; Honda opted for a SOHC, two-valve per cylinder engine in order to achieve better fuel efficiency. So if economy is your USP then two-valve units seem to make more sense.

Or so it may seem. A counter argument for this theory is that "why not opt for a smaller multi-valve engine?" In fact a smaller multi-valve engine will have many advantages as compared to a larger volume two-valve unit. It will weigh less, take less space so that you will have a lot of freedom in design. Pray why didn't Honda opt for a smaller unit of say 600-800 cc capacity and four valves per cylinder. But then the choice of engine is governed by many factors and not just by performance or fuel-efficiency. For a company like Honda, which has innumerous models and engines in its bin, it makes more sense to opt for an already existing engine rather than develop a new unit from scratch. Thus Honda will be able to save a lot in terms of development costs. Additionally in the future when the customer's demand for performance increases then the same engine can be fitted with a four-valve head. This would not have been possible with a smaller engine capacity, which due to its smaller size would have limitations in power delivery. In the Indian context the Tata Indica engine at 1400-cc capacity uses a substantially bigger engine. The result is that the Indica is one of the thirstiest cars in its segment. However, when in the future the customers' demand for power increase then Telco can use the same engine with a multi-valve head. Looking at it this way the Indica in its present form produces 60 bhp, add an MPFI system and you get 75 bhp. Put in a multi-valve head and the engine can easily produce 85-90 bhp. What this means is that opting for a larger engine saves Telco developmental costs it would have incurred in the future. On the other hand the other players like Daewoo and Hyundai have opted for comparatively smaller units for their cars making them more fuel-efficient. However being small these engines cannot be coaxed into producing more than 60-65 bhp. Thus there is always a trade-off between many factors and performance or fuel-efficiency alone does not decide the choice of engines.