hello and welcome back to educational mechanics, I'm Matt and in this video
we're going to look a bit deeper into the air braking system and in particular at the quick
release valve giving you an understanding of how it works and why we need them
we'll first look at them on a vehicle layout showing how they work in a circuit
and then look a bit deeper into the valve to see exactly how it works
a quick recap on what we already know but if you want a bit of a refresher have a
look at our introduction to air brakes video there's also a link down in the description
on our air braking system we've got an engine driven compressor that supplies a reservoir
which holds compressed air ready for use the reservoir supplies the driver's controls
the driver controls where the air goes using the foot control valve for the service brakes
and the hand control valve for the secondary and parking brakes. Whether you have a quick
release valve fitted in the system or not a timely release of the brakes is just as
important as the brake application and allowing compressed air to escape from a system takes
more time than putting it in there as the air travel time increases as the pressure decreases
when you take your foot off the foot pedal or apply the part brake you want that action to
happen quickly to achieve this a quick release valve is fitted in a circuit between the air
supply and the brake chamber this can be both in the service and the secondary lines let's
arrange some components onto a vehicle layout to bring it into context we just can have the
foot control valve some air lines and i'll put some s-cams in so you can see the brakes working
we'll put some more components on shortly so the driver applies the foot brake which sends
air down to the actuators to apply the brakes when the driver releases the foot brake the air
needs to come back to the foot control valve to exhaust releasing the brakes now we know that
it takes longer for air to leave a system than it does getting in there this would mean that there
could be a considerable lag between removing your foot from the pedal and the brakes actually coming
off but that's not so bad though is it it just takes a bit longer for the brakes to be released
although what damage are you doing by trying to drive off with the brakes still applied
let's now put the secondary and parking brakes in and see what happens there
okay to apply the part brakes the driver operates the hand control valve
which exhausts the air from the secondary circuit allowing the power springs to apply the brakes
however with a delay between applying the park brake lever and the park brakes actually coming
on if you wanted to apply the park brakes in a hurry well you'll have to wait oh dear
let's rearrange this system with some quick release valves
and see what difference they can make in speeding up these laggy brake operations
right then what we've done here is put quick release valves near the axle
close to the brake actuators let's go through those drivers actions again
when the driver applies the foot brake compressed air travels down through the
quick release valve to the actuators to apply the brakes no real difference there
when the driver releases the foot brake the air only needs to come back as far as the
quick release valve to be exhausted it no longer has to travel back up to the foot control valve
so when the driver releases the pedal the brakes release much quicker
and that's how we've solved the problem of the service brakes not releasing quickly and where
the quick release valve gets its name amazing now let's put one in the secondary circuit and see how
that works right then when the driver releases the park brake compressed air travels down through the
quick release valve into the secondary chamber of the actuator compressing the power spring
releasing the brakes when the driver applies the park brake the air only needs to travel as far as
the quick release valve to be exhausted allowing the power spring to apply the brakes much quicker
okay so let's have a look inside the quick release valve and see how it works
here we have a cross section of a quick release valve now quick release valves come in different
shapes and sizes by different manufacturers but they all pretty much work the same way
there are two main ports on a quick release valve the first is the supply port which is
connected to the control valve or relay which is where the compressed air comes into the
second is the delivery port which goes to the brake chambers this quick release valve has
two delivery ports as it's mounted on an axle so one for the left actuator and one for the right
we also have an exhaust port which is open to atmosphere
the quick release valve as with most other valves has three states of operation on off
and hold when the driver operates the brakes air enters the quick release valve at the supply port
the first thing to happen is the air pressure acts on the rubber diaphragm with pressure above the
diaphragm and nothing below is pushed downwards sealing off the exhaust port with pressure still
above and nothing below the outer edges of the diaphragm are bent downwards away from the valve
body allowing the air to flow through the valve to the delivery ports and onto the brake chambers
the valve is now in the on position now this is happening very quickly and what happens next
is that air pressure has built up in the brake chamber and the delivery line
this pressure backs up and is felt on the underside of the diaphragm with equal pressure
above and below the diaphragm the outer edge of the diaphragm can reseat against the valve body
with the exhaust and the inlet closed the valve is now in the hold position when the driver releases
the brake application the air pressure above the diaphragm is released back through the control
valve air pressure below the diaphragm lifts it opening the exhaust port allowing air in the brake
chambers to exhaust quickly through the quick release valve exhaust port rather than having
to travel all the way back to the control valve let's have another look at the quick release valve
operation as the driver applies different amounts of brake and effort so the driver puts their
foot lightly on the brake pedal and we'll say the supply pressure is about 30 pounds per square inch
which means that air will be delivered through the valve
and at about 30 pounds per square inch the QRV will hold
the driver's foot now only has two ways to go either on more or off more if the driver applies
a bit more force to the brake pedal the supply pressure will increase the pressure above the
diaphragm will be higher than under the diaphragm making it flex allowing more air to be delivered
to the brake chamber until the pressure equalizes and the diaphragm goes back to its normal position
once again we find ourselves in a hold position right this time the driver is going to remove
some force from the pedal but not all of it the supply pressure above the diaphragm reduces
with a higher pressure below the diaphragm than above it the diaphragm is forced up opening the
exhaust valve allowing air from the brake chambers to be exhausted through the quick release valve
with the inlet closed and the exhaust open the valve is in the off position
exhausting air from the chambers when the delivery line is exhausted enough and the
pressure under the diaphragm has decreased enough the pressure above acts on top of the diaphragm
forcing it down sealing off the exhaust and places the quick release valve once again
in the hold position the drive is now going to completely remove their foot from the brake pedal
the supply is exhausted through the foot brake valve and the pressure under the diaphragm from
the brake chambers lifts the diaphragm opening the exhaust port allowing the
air to be exhausted through the valve the quick release valve is once again in the off position
so looking at that the quick release valve is always either in the on the off or the hold
positions and this happens very quickly let's have another look a bit quicker
and there we have it the quick release valve there to minimize the brake lag
and speed up the release of the service brakes and application of the secondary brakes
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we'll have a look at more air brake components in other videos see down in the description for the
air brakes playlist and check out the channel for more videos around all types of vehicle mechanics