The "Little radiator" out front is the Condenser. The other heat exchanger
inside the car (that gets cold) is the Evaporator.
An air conditioning system is actually a misnomer. What you actually have
is a "Heat Pump".
It takes energy to raise the phase of a liquid to a gas. You put heat into
the liquid, (in this case a low-boiling point refrigerant such as R-12 or
R-134a) which causes the molecules to vibrate more and push themselves
apart, resulting in a gas. This process is know as evaporation and is the
same thing you do when you boil water on the stove. When it is allowed to
condense it releases the "trapped" energy in the form of heat (remember you
cannot destroy energy on change it's form). The refrigerant is prodded into
doing this with changes in pressure. Also remember there is really no such
thing as cold, only absence of heat!
When you turn on your air conditioner, you switch power on to the
electromagnetic clutch on the compressor unit. This enables the engine to
transmit kinetic energy into the compressor and begin pumping (squeezing)
the liquid refrigerant. The refrigerant runs in a loop, and the compressor
sucks from the evaporator located inside the car. This is like a
mini-radiator with tubes containing the refrigerant over which hot air is
blown by a fan. The refrigerant picks up the heat from inside your car here
(when it's working correctly) and begins to boil helped along by the low
pressure caused by the suction, and in doing so, changing from a liquid into
a gas. It now has some of the unwanted heat from inside the car "trapped"
in it's phase change. It exits out of the compressor at high pressure (it's
being squeezed) and runs into the condenser located out in front of the main
radiator. It is almost just like the radiator except a little bit smaller,
and much higher pressure. The pressure at the condenser can hit around 300
PSI (20 bar) on a hot day! The super-heated gas refrigerant quickly loses
it's trapped heat from all the air blowing over the coils and returns to a
liquid again. If you aren't moving, the engines fan and sometimes an
electrical fan will insure the air stays moving over the condenser. That is
the sound you are likely hearing when you switch it on. At the outlet of
the condenser there is also a small "tank" to hold some excess refrigerant
called an "accumulator". Because the volume of refrigerant can change with
the outside temperature, you need a storage location for the excess. This
is also where they filter and chemically clean the refrigerant. It's
important to not have any water or contaminants in there, so it's kept
clean. They also add a small amount of light-weight oil to the system to
lubricate the compressor. The liquid then travels on to it's last stop
before repeating the cycle again, which is the "expansion valve". This is
the proverbial "pinch in the hose" that allows the pressure to stay high on
the outside of the car (losing heat) and low in the inside of the car
(absorbing heat). The expansion valve on some cars is just a small hole
(orifice) that's been carefully chosen to keep the pressure balanced, or
sometimes it's a bit more complex with a pressure or temperature sensing
valve. Most Peugeots use a Pressure modulated expansion valve (PEV) also
known as a "Suction throttling valve" (STV), it has a little needle valve
that's driven by pressure. Basically a pressure regulator of sorts. Once
the liquid refrigerant has squeezed through this small restriction, it finds
itself back inside the evaporator again boiling and sucking up heat, and the
cycle is complete.
They also usually have several electrical thermostats in the system, one to
adjust the desired interior temperature, and the other monitors the
evaporator to keep it from going below freezing. If you have your fan speed
set on low, it can easily go below freezing and turn into a big block of ice
(especially if it's humid), and that will block air flow, so the thermostat
temporarily stops the compressor if it's too cold. A properly running
system will reduce the air temperature of the inlet air to the outlet air
from around 25 to 40 degrees Fahrenheit (-4 to 5 Celsius) depending on the
refrigerant type, outside air temp, and engine speed. If it's 100 degrees
Fahrenheit (38 Celsius) outside, your interior may never get below about 75
(24) unless you use recirculation. On many American cars they do away with
the recirc air control and simply label it "MAX AC" because many people
think just the opposite, that they want the outside air in because the car
is hot. Basically on hot days you want to run fresh air mode initially,
until the car temperature is lower than the outside air, then switch to
recirc.
That ends my "tutorial", and maybe will shed some light on the mysteries of
this system.
As far as troubleshooting, the #1 problem is refrigerant leaks! If the
refrigerant is low, there will never be enough pressure to maintain the
phase changing, and the system will quit cooling. On most Peugeot systems
there is a small glass window on the top of the accumulator known as a
"sight glass". This window is located on the outlet of the tank and if the
system has a sufficient charge, you will see liquid in there, not gas! (a
few tiny bubbles are ok, but large voids mean you have low refrigerant)
The system will continue to work when you see voids in the liquid, but
poorly, and once there is no visible liquid, it will cease to work. Some
systems have a low-pressure sensor that will stop the compressor if there is
low refrigerant. You will notice this by the compressor only running
briefly when first switched on, staying off for a while and then briefly on
again.
I've also seen defective electrical components cause problems (big surprise,
eh?!). Bad connections, bad relays, and dysfunctional thermostats. If in
doubt, jump a wire from the battery positive to the wire going directly to
the front of the compressor. Its should engage. Make sure the interior fan
is running full blast, then watch the sight glass. If you see solid liquid,
the system is probably fully charged, but you have an electrical fault.
Simply work back from the compressor.
Refrigerant used to be almost always R-12 in cars. This is a CFC-based
chemical and has been outlawed by international treaty. The new stuff being
used is R-134a, which is a HCFC, but you still don't wanna vent it into the
atmosphere if you can help it. If you are is an R-12 system, you can
retrofit the system to R-134a yourself, but it's a pain to do properly, as
standard 134a is not compatible with all types of compressor oil and seals
in most Peugeot systems. The better choice is to get a modified refrigerant
such as "Freeze-12" that is also compatible with the existing stuff. One
brand I know of is called "Freeze-12". http://freeze12.com/
Good luck on your cool endeavors! =)
-Phil
----- Original Message -----
From: "Eduardo Kaftanski" <e@nn.cl>
To: "John" <john@npagroup.com>
Cc: <peugeot-L@yahoogroups.com>
Sent: Tuesday, April 12, 2005 4:51 AM
Subject: Re: [Peugeot-L] Air Con - 405 TD
>
> On Tue, Apr 12, 2005 at 11:10:50AM -0000, John wrote:
>>
>>
>> Hi all.. ok so my new GLX is my first ever car with air con ;) (well,
>> in my defence, it's use in England is limited to one day a year)
>>
>> It doesn't work...
>>
>> can someone give me a brief run down of the system, what components are
>> there, and how it interacts with the normal cooling system? (if it does)
>
> There is a compressor, run by the engine by a belt and an electromagnetic
> clutch, a radiator, mounted forward of the normal one, some piping,
> an evaporator/dissecant (silver car but the battery) and a condensor (a
> small radiator under the dash, thats what gets cold)
>
>>
>> The light comes on, when you activate the aircon, and sometimes there
>> is the sound of a motor running under the bonnet, which goes off when I
>> depress the switch again. (after engine is off)
>>
>
> The sound you hear is the engine fan. It comes one when you turn the
> aircond
> on to cool the front radiator.
>
> If that works, and the air does not come out cold, the AC must be
> discharged.
> If you dont use it, the seals get dry and brittle and leak the gas thats
> inside the system. You should take it to an AC shop where they will hook
> it up to some gauges and tell you if its charged up.
>
>
>
> --
> Eduardo Kaftanski
> eduardo@linuxcenterla.com
> Red Hat Certified Engineer/Instructor/Examiner
> Gerente Ingenieria LinuxCenter S.A.
> Mariano Sanchez Fontecilla 310, 2do piso, Edificio Birmann24, Las Condes,
> Chile
> http://www.linuxcenterla.com +56-2-4834000
>
>
>
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Received on Tue Apr 12 09:29:56 2005
