So, you can see that the cylinder head also
contains the ahh valves and the valve mechanism. So let us look at the valves and the valve
mechanisms, before we go there you know like we are going to look at what are called us
the intake manifold and the exhaust manifold okay, so what are these intake manifolds and
exhaust manifolds. So let us quickly look at them. So if I want to take an air right from the
atmosphere, and let us say I have a multi cylinder engine. So I need to have a series of pathways right
and pipes and other flow parts to take the air from the atmosphere and into the cylinders
right into cylinder block. So, the function of the inlet manifold or
the intake manifold is precisely that. So the inlet manifolds essentially is a set
of pipes or flow pots, right that ahh basically facilitates the flow of air from the atmosphere
into the cylinders okay. So that is the function of an ahh intake manifold. So you can clearly see that ahh I have being
taken what to say from the atmosphere okay. So, it passes through a filter or a cleaner
to remove dust particles and then like comes in. And you can see that there is a channel of
pipes right or flow pots through which this air is distributed okay to the respective
cylinders okay. So, you see that these are the pass through
which they go to the corresponding cylinder. So, one second see the the this ahh picture
corresponds to a 4 stroke engine sorry 4 cylinder engine okay. So, that is what we have been considering
when we looked at the cylinder block and also the cylinder head. So, you can see that there are 4 parts and
you essentially ahh go to the corresponding cylinder 1 2 3 and 4, okay. So, in diesel engines you know like when you
come to emissions, we will also see that there is something called exhaust gas recirculation
that is what is called as EGR okay, we learn about that later on okay and we will see that
even the ahh what to say what comes out of the EGR is routed to the inlet manifold okay. We look at it when we come to emissions, similarly
we have what is called as an exhaust manifold. So the function of an exhaust manifold is
now very obvious right. So what what does it do, it is nothing but
a set of pipes that facilitates the removal of exhaust gases from the cylinders okay or
the combustion chambers okay. So that is the function of an exhaust manifold
okay right. So once again you can see that there are 4
parts 1 2 3 and 4 okay. So, through these 4 parts they exhaust gases
and how this intake manifold and exhaust manifold are interface to the cylinder, the you element
that controls and acts as a door between the manifold and the cylinder is the valve okay. So intake valve acts as a door between the
intake manifold and the cylinder, the exhaust valve acts as a door between the exhaust manifold
and the cylinder. So, obviously they need to be opened at the
appropriate time to do the corresponding intake or exhaust process okay. So now we are going to look at valves which
are also like one of the important components in an internal combustion engine okay. So let us look at the valve assembly okay. So which regulate in on how these components
are interfaced the manifolds and the this one. So, this is a ahh very simple schematic of
a valve assembly. So, this is a construction picture okay let
me also include a schematic and then things become clear. So, the valve assembly what is used in typical
automotive applications and by enlarge we use what are called as spring return valves. So, by enlarge in automotive applications
we use what are called a spring return valve as the name suggests, the return of the valve
is enabled by the restoring force of a spring okay, that is why it is called they are called
spring ridden valve okay, so how do they operate. So, in this picture you can clearly ahh see
how it works. So, this is our ahh cylinder okay and you
can see that this is the piston right. So, this is the cylinder block, this is a
cylinder head, cylinder head is shaded in red okay. So, you can clearly observed that there is
an exhaust valve and an intake valve okay which are actuated by the springs and a cam
which is connected to what is called as a rocker arm shaft okay ahh we are going to
what to say clearly look at this ahh operation what to say in a short time okay. So, what happens is that like when you want
to open the inlet valve and allow air to come in into the cylinder, this valve is pushed
up okay. So there is an opening which is created here. And air starts to flow within the cylinder,
okay, so that is what happens okay when you have the intake valve and when the intake
valve needs to be closed the contact between the intake valve stem and the rocker arm is
removed and the intake one goes and closes the gap due to the spring force, that is why
it is called a spring written valve. The same story for a for the exhaust valves
okay. So if you if we look at this component take
it apart and have an exploded view of the components. This is what will happen okay. So these are what are called as mushroom shaped
valves. So because this surface of the valve is shaped
like a mushroom, okay, it is like a curved surface, right. So these are what are called as mushroom shaped
valves okay. So, this is what is called as the valves stem
okay and what happens is that you take this valve and inserted from the bottom of the
cylinder. So, if you we saw the bottom view right if
you have the cylinder head you just inserted like this okay, so what happens then the stem
comes to the top of the cylinder. So, let us say this is the thickness of the
cylinder head, the stem part of the stem projects on top, you split the valve seat here, so
that the valve sits on the valve seat okay. The valve seat is put at the bottom surface
of the cylinder head where the valve head this is what is called as a valve head okay
which is the shape of a mushroom will sit. So the stem will go through the thickness
of the cylinder head and on top you put this first assemble the spring seat. Then what you do is that you put the oil seats,
then you assemble these water called as dual coil springs, there are 2 coil springs to
increase the stiffness of the springs, okay. You put the dual coil springs, put the spring
retainer and assemble everything through a quarter okay. So that is how it is assembled. I hope it is clear how this exploded view
is assembled okay, so you insert the stem from the bottom of the cylinder head, the
valve had sits on the valve seat at the bottom, then you essentially put the spring seat and
2 coil springs and then put the spring retainer and hold everything through this quarter okay,
that is how it assembles. Now, you can immediately see that the spring
seats are very important why. Because when we analyze coil springs, we assume
that if I press a coil spring, its displacement is instantaneous. And if I release a coil spring once again
it is restoration is also practically instantaneous. But in real life we are not going to have
that effect point number 1, right. Second point is that although if you hold
the coil spring between my 2 pumps, right, if I press it it made the displacement will
not exactly be on this axis all the time right. The spring may move a little bit on the surface. So we do not want that right ahh the the offset
vibrations right, now the later vibrations. So in order to prevent that and hold the spring
in its position, we use the spring seats and spring retainer okay. So the those are the main functions of the
ahh spring seats and spring returns. So let me show you another schematic to clearly
illustrate how they are fitted okay. So, now, you can immediately observe as to
how these components are assembled and then actuated okay. So, in this ahh schematic you know like we
show the entire assembly let us first discuss this schematic then we will go to the other
one. So, if you look at this schematic right. So, you can observe that the the there is
a this is a valve stem, this the interior valve assembly as I told you this the thickness
of the valve cylinder head right. So, you can see that the valve stem projects
are and then like you hold the what to say springs right here and then you essentially
keep them compressed like this. Now, you can have what is called as a cam
which directly operates on this valve stem and then whenever you want the cash stem to
be open the profile of the cam is change such that the valve stem is pushed on. So, this will open and when the profile of
the cam changes the there will be a small gap actually introduced within the cap surface
in the valve stem. And due to the restoring force of the spring,
the valves stem will move up okay, that is that is the function. In some cases a rocker arm is used okay. Particularly when you have multiple valves,
right you can you can have more than one inlet valve and one outlet valve per cylinder. Then you can also use a rocker arm wherein
this what is called as a rocket arm, the cam does not directly act on the valves stem but
then it acts on the rocker on which is piloted here. And then it pushes it okay, so then this end
of the rocker arm applies a force on the valve stem and the same action happens okay, so
this is how the valve assembly works. Now, there are a couple of important issues
associated with this operation. The first thing is that let us say I open
the valve okay the cam pushes the valve or the rocker arm pushes the valves come down,
okay the valve was open. Now, the profile of the cam changes such that
the contact between the rocker arm or the cam is momentarily lost or becomes the force
becomes lower right, then what happens you know like this spring should push the ahh
the stem assembly instantaneously. So that like there is a contact but that never
happens in practice, right. Because these components are not welded together,
right. So, essentially what happens is then there
can be some vibrations right, there can be some impact between the 2 and that can create
some noise okay. So, that is present in the spring return valves,
where you have water what is called as some significant valve train noise particularly
when they operated high speeds. So, how do how have people solved it, they
use what are called tappet valves. So, what they do is, they put what is called
as a tapet on top of the valve stem. A tap it is nothing but an oil filled container,
which is this put there, okay. So the contact between the cam or the rocker
arm does not happen on the valve stem but on the oil fill tappet. So that dampens of the vibrations, okay. So it includes essentially increase the dampens. This is like a mass spring damper system right. You have inert here right, that is why you
can close instantaneously right. So, you have a spring return spring. So, what people do that they introduce a damper. And as we know from a basic dynamic systems
also that if you put a damper you and you increase the damping ratio, the oscillations
will be subdued, right. So, that is what people do right by putting
a tappet okay. So, then what happens is then there is another
issue okay. So what is the next issue. If you keep on increasing the speed as we
discussed at 3000 rpm for the crankshaft. you We had 10 milliseconds, right for the
valve to open and close in 1 stroke, right. So, let us say I increase the speed you know
like let us say to 4000 rpm. Now what is going to happen. How much time will I have, around 6, what
to say now the thing will reduce by 30% right. So, I will have sorry 33 one third right. I will have two thirds of the time right to
essentially do the opening and closing operation. Now, you see that it has decreased. So, to just to take round numbers, let us
say I increased from 3000 rpm to 4500 rpm that means that I have increased by 50%. So, what is going to happen 10 milliseconds
also is going to decrease. So, in other words if you increase the speed
the time available is going to decrease. So, what happens is that you need some response
time see as we as we also learned from our dynamic system schools you know like that
uniform mass spring damper system. There is a raise time, there is a response
time and so on right things do not close in immediately. So what happens is it like as the speed increases
and the time available for your ahh opening and closure comes down your valve maybe commanded
to open the valve assembly, the camshaft the cam will be commanding the valve to be open
and it will command it to close but before it closes completely the next cycle will start. The control mechanism will say oh, hey open
the valve once again. So, what will happen is that an issue called
as valve float arises, okay. So what is valve float. That is the valve will seem to be floating. That is it never closes completely and never
opens completely because by the time it opens it goes to opening completely the command
has reversed to closing it, but the but the the time it starts to close, the next cycle
would have started and it would have been commanded to open. So that is going to be a challenge right,
what people can do that is have done you can never eliminate valve float in the spring
written valves because that is how they work right, it is inherent to them. So, what people have done is that like they
have increased the stiffness of the spring okay. So, people can either use high stiffness springs
you replace things when they become soft with the operation you can increase the preloads
as you can see in this before and after curve right. And then like you can people also introduce
these shims okay, Shims are nothing but accurately machine washes okay on which you mount the
springs. So that you even avoid any lateral vibrations
and then you ensure that the what to say spring does it is operation very, very optimally
okay as far as restoring the valve to its original position is concerned, okay. So all these tools have led to the reduction
in valve float, okay. But in a spring written valve, you can never
completely eliminate valve float okay. So that is a that is an limitation, then if
you want to completely eliminate valve float, what is it that we can do. We have to use a different closing mechanism,
right. So because imagine a door for example, you
know most of the doors that we have today are these automatic closing ones right. So let us say you open a door I get the handle,
I pull the door open right. So, then what happens, if I go outside I leave
the handle and the door closes slowly. But on the other hand, I want to close the
door also fast enough, what do I do. I can grip the handle on the other side and
pull it right not depend on any spring or any automatic closing mechanism and so on
right, same story. So what happens is then, there is something
called as a desmodromic valve used in a very, very few road vehicles where you want very
accurate control, particularly high speeds, and so on okay. What is called as desmodromic valve, okay
desmodromic okay valve. So what happens is that in a desmodromic valve
you do not have this return spring, but you would have a rock around for pushing it down,
you will have a rock around for pulling it up okay. So, in both sites you will essentially ahh
open and close the valve without a spring because with the spring you have complaints
and then you have these dynamic effects coming in right. So, essentially when you have this rigid contact,
right you can push and pull to the best possible extent okay, so that way, this desmodromic
valve uses a rocker arm to essentially both open and close the valve okay, and this ahh
eliminates valve float, okay. So this mechanism essentially can completely
remove valve float okay. So we looked at the ahh valve assembly, will
continue from here in the next class and then like look at other components you know like
ahh like camshaft and other components that make up an engine and then we will continue
right okay. Thank you.