Arrangement of Valves
Engine may be classified according to the location and type of the valve system employed
(Fig. 2.31). With both inlet and exhaust valves located on one side of the cylinder, a cross-section
view would be an L-shape. This type of valve arrangement is, therefore, called a L-head or
flat-head engine. The valves in this case are operated by a single camshaft. This is relatively
simple and dependable arrangement, but the design has two drawbacks. It cannot achieve high
compression ratio, and causes greater pollution as its exhaust gas contains high amount of HC
and CO. The reason is that the combustion chamber surfaces are large and relatively cool. This
prevents combustion of the layers of air-fuel mixture close to these surfaces. If one valve on each
side of the cylinder is used, as a modification to the above arrangement, it is called a T-head
engine and the arrangement uses two camshafts for the operation of valves.
Most current automobile engines have both valves in the cylinder head. This reduces the
cost of the engine block and allows better engine breathing by providing a large inlet port on
one side of the head and large exhaust port on the other side. The head is a large complex casting
that provides openings for valve ports, coolant, valve actuating devices, and lubricant. The added
cost and complexity of these type of cylinder head is offset by the reduced cost of the block and
by the added performance produced by better engine breathing. This type of engine is called an
I-head or overhead valve (OHV) engine. Engines with combined features of both the L-head and
the I-head engines have also been produced.
Fig. 2.31. Valve arrangements.
When one valve is in the head and the other valve is in the block, this is called a F-head
engine. This arrangement employs one camshaft. This has many of the advantages and
disadvantages of both L-head and I-head engines. The F-head engine has seen limited produc-
A modification of the I-head engine includes a third small valve located with the spark plug
in a pre-combustion chamber connected by a passage to the combustion chamber. During the
intake stroke a rich fuel mixture is inducted through the small valve and a lean mixture through
the normal intake valve. During compression, with all valves closed, some of the mixture is
pushed back into the pre-combustion chamber. Ignition occurs easily in the rich charge located
in the pre-combustion chamber. The hot burning gases rush from the pre-combustion chamber
into the lean charge in the main combustion chamber, igniting it. In this way a very lean charge
can be burned in the engine to minimize emis-
sion. Engines of this type are called stratified
charge I-head engines.
In I-head engine, with valves in the head, the
camshaft is usually located in the block. An
alternate location in some engines is that the
camshaft is placed above the valves on the head.
This is called an overhead cam engine. When the
camshaft is located in the block, the overhead
valves are driven through a lifter, push rod and
rocker arm assembly. When the camshaft is lo-
Fig. 2.32. Camshaft location.
cated on the head, the valves are actuated by some type of cam follower. The arrangements are
shown in Fig. 2.32.
An I-head V type engine is shown in Fig. 2.23. Figure 2.34 illustrates an F-head engine,
where the intake valves are in the head, and the exhaust valves are in the block. A six-cylinder
flat head engine is shown Fig. 2.35. An in-line six-cylinder OHV engine is shown in Fig. 2.36.
An overhead camshaft arrangement is shown in Fig. 2.37.
Fig. 2.33. I-head V-type engine.
Fig. 2.34. F-head engine.
Fig. 2.36. An in-line, six-cylinder OHVengine.
Main advantages of side valve engines are as follows :
(a) Overall engine height can be low.
(b) Less complicated head casting.
(c) A side valve engine generally retains its ‘tune’ longer than an overhead valve unit and
shows greater tolerance for fuel.
(d) No push rods and rocker arms etc. are required to operate valves, therefore fewer
moving parts are found.
(e) Special valve stem, oil-sealing arrangements are not required.
Fig. 2.37. An overhead camshaft engine.
On the other hand an overhead valve engine has the following main advantages :
(a) Greater accessibility.
(6) Better filing of cylinders as induction of gas in helped by the action of gravity and ports
can be made of better shape.
(c) Cylinder block casting is less complicated.
(d) Higher compression ratio can be obtained.
(e) Design of combustion chamber can be easily varied than with a side valve engine.
The valves are made to open or close very slowly to provide quiet operation under high-speed
conditions. The timing of opening and closing of both the valves is controlled by the design of
the cams on the engine camshaft. In practice, the events of four-stroke cycle do not start and
finish exactly at the ends of the strokes. For better breathing and exhausting, both the inlet and
exhaust valves have lead, lag and overlap periods. These early and the late opening and closing
events can be presented on a valve-timing diagram. The opening and closing points of the valves
in relation to piston and crankshaft positions are called the valve timing.
Lead is the opening of the valve before the piston has reached either TDC or BDC.
Lag is the closing of the valve after the piston has reached BDC and closing or opening
(some cases) after the piston has reached TDC. f
Overlap is the period during which both the valves are open.
The amount of lead or lag during valve opening or closing and the amount of overlap depend
on the design of the engine (particularly the port arrangement, and inlet and exhaust systems)
and on the required performance characteristics the engine. The opening point of the inlet valve
and the closing point of the exhaust valve depend upon the following conditions :
(a) The velocity of the flow of exhaust gases along the exhaust manifold, which in turn
depends upon engine speed, throttle opening, the length and diameter of the exhaust
pipe, and the flow restriction in the silencer.
(b) The pressure in the inlet manifolds, which is dependent upon engine speed and throttle
(c) The position of the ports with respect to the combustion chamber and to each other.
Inlet Valve Timing.
In most engines the inlet valve opens slightly before the piston starts downward on the
suction stroke and closes after the piston has started
upward following completion of the suction stroke.
This is necessary to permit the valve to be open
sufficiently for full induction of the charge. The inlet
valve remains open until the piston reaches a point in
the next upward stroke (the compression stroke) so
that the pressure in the cylinder equals that of outside.
This period varies in different designs of the
automobile engines ranging from 28 to 71 degrees of
crankshaft rotation. Figure 2.38 illustrates the valve
timing data for the inlet valve of a typical automobile
engine, where the valve starts to open 5 degrees before
TDC i.e. during 5 degrees of the exhaust stroke,
remains open during the 180 degrees of the normal
suction stroke and in addition during 45 degrees of the
beginning of the compression stroke. This gives the
inlet valve a total opening of 230 degrees of crankshaft
Exhaust Valve Timing.
The exhaust valve opens before the completion of the expansion stroke. Due to this the gases
have an outlet for expansion, which removes the
greater part of the burned gases, reducing the amount
of work to be done by the piston on its return stroke.
This compensates for the waste of the some of the force
of the expansion due to opening of exhaust valve.
However, the valve should not be opened too early.
During the next upward stroke i.e. exhaust stroke,
the remaining gases are forced out of the exhaust
valve. The gases are slightly under compression and
some amount of compressed exhaust gases are left in
the clearance space when the piston is at TDC. There-
fore, for best performance of the engine the exhaust
valve remains open for a short time after the commen-
cement of the suction stroke. The chances of drawing
the exhaust gas back into the cylinder due to this
opening of exhaust valve is small as the compressed
exhaust gases are at higher pressure than that in the
Fig. 2.38. Inlet valve timing diagram.
Fig. 2.39. Exhaust valve timing diagram.
manifold and the movement of piston is very little, for 10 to 15 degrees movement of the
Figure 2.39 shows the exhaust valve timing data where the valve opens 45 degrees before
BDC and closes 12 degrees after the TDC. Considering the normal opening of 180 degrees in
the exhaust stroke, the total exhaust valve opening becomes 237 degrees.
Thus it can be seen in Figs. 2.38 and 2.39 that the 5 degrees pre-admission of the inlet valve
causes it to overlap 5 degrees with the exhaust valve. The closing of exhaust valve 12 degrees
after the TDC produces 12 degrees overlap. Hence the total overlap is 17 degrees.