Off Topic Rear lights.

Currently reading:
Off Topic Rear lights.

The problem with de-frosting without the engine running is, no matter which method you use, the inside glass steams up, especially when a body gets inside. Wiping the inside of the screen always leaves smears, so I always prefer to let the demister do its job, but that takes a couple of minutes, even on full blast. I'm not denying the bore-washing phenomenon, just don't see an easy way to drive off quickly with safe vision. Not a problem for me now with the car garaged - apart from the 30s it takes me to close the garage door!
 
Ford sorted the problem years ago, electrically heated front screen, why all cars don't have this "safety" feature I don't know. You cannot start the car and drive off in the UK as, as said the inside fogs up with your breath, add oncoming headlights(dark) or low sun and you're blind, lets see what that does to the diesel emissions leaving it running from cold to warm up?
 
Ford sorted the problem years ago, electrically heated front screen

:yeahthat:

The Ford heated front screen is absolutely brilliant and does exactly what it says on the tin

why all cars don't have this "safety" feature I don't know.

Because Ford held the patent and they wouldn't licence it to anyone else.

You cannot start the car and drive off in the UK as, as said the inside fogs up with your breath, add oncoming headlights(dark) or low sun and you're blind, lets see what that does to the diesel emissions leaving it running from cold to warm up?

The solution is to leave both front windows fully open until the coolant is sufficiently warm to enable the heater to demist the screen. Brrrrr!
 
No, cold water should be used , if it feels warm then it's about 40+c on a 0c screen not a good result if you use sufficient cold there's not a freezing issue.


Core body temp is around 37.5'C but your hands and fingers are quite a lot lower than this, fingers can be as cold as 15'c without any problem at all so as long as the water doesn't feel 'cold' then it's not going to be too cold and if it doesn't feel hot then it's not going to do any harm, plunge cold hands in to water at 40'c and it will feel hot enough to burn


I've been de-icing with luke warm water for years, much kinder than chemicals and much less likely to damage than scraping.
 
Ford sorted it out, I have to agree.

Old knowledge here ...............
Never owned a Ford.

Back in the olden days, we hired a Ford Escort (Mk3 perhaps?)
Driving down the motorway and peering into the distance down the straight Somerset sections of the M5 ............. and they are very straight and very flat ............. the filaments of the heating matrix actually were a problem with visibility into the distance.

To be honest, I thought that the heating filaments were an issue I didn't like.
If the filaments are ok these days in that situation, I'd be ok about it, but my memory of them stops me even liking the idea of them.
I can see it now in my mind's eye.

No thank you ................ but it's old knowledge as I say.

Regards,
Mick.
 
Apparently not many people ordered option 52Y. That's an electric heater that supports the normal heater as long as the coolant is cold. It's probably not as effective as Ford's solution, but it's better than nothing.
 
The problem with de-frosting without the engine running is, no matter which method you use, the inside glass steams up, especially when a body gets inside.
I've been thinking about this.

I'm not sure that you're correct, though I would expect that SOME cars would do it. Maybe it's because for the past 15 years, I've only had vehicles with air con, so the inside of the cars are always dry as a bone. Maybe there isn't enough moisture inside the car to steam up the windows?

Our air con is on permanently with the engine running, and is never switched off.

No doubt if I were wet, or breathed directly onto the glass, it would mist up, but I can't remember ever having the windows steaming up when I've defrosted them.

I'll check again when we next have a frost, but a week or so ago it was minus5 degC when I defrosted the Clio ready for Mrs Mick F to go to work, and she didn't mist up at all .......... because I'd have seen and she would have had to have waited before driving away.

As I say, I'll check again, and check properly, next time.

Regards,
Mick.
 
Would having the air con always on not drastically impact on mpg and available bhp?

Yes, though the relative impact is less at lower temperatures.

Once the temperature falls below about 3C, the aircon compressor will automatically be disabled irrespective of the switch selection.

A/C is very effective at demisting the glass on humid days when the temperature is about 10C or so; we get a lot of days like that in the UK.
 
Would having the air con always on not drastically impact on mpg and available bhp?
Not much.
I suppose in very hot weather it might, but generally in UK, it isn't used to cool, but to "condition" the air.

If you want to experiment, drive gently and switch the air con off and on, and see if your speed varies.

When we first bought our Clio (brand new) in 2001, we were driving up and down the motorways from Cornwall to Lancashire due to family ill health. May have driven that journey four or five times a year for a couple of years.

I experimented for mile after mile with air con off and air con on, and the MPG figure didn't change at all that I could see.

However, if your car is sat in the hot summer sun, you could drive away and then turn the air con on, the power from the engine would plummet. Leave it for a while like that, and eventually all power would come back as the internal temp settles down to that demanded. The cooler you want to be, the more the power is required, but it still settles down.

Maybe in hot countries, the air con is an expensive necessity. Here in UK, it's generally almost free to run, and your car is always dry and clean smelling.

TTFN
Mick.
 
Last edited:
However, if your car is sat in the hot summer sun, you could drive away and then turn the air con on, the power from the engine would plummet. Leave it for a while like that, and eventually all power would come back as the internal temp settles down to that demanded.
Mick, as far as I know the automatic airco simply switches the airco compressor on and off. There is nothing in between on and off. If you need only 50 % cooling power, then it's half of the time fully on and half of the time fully off. This means that whenever the airco compressor is switched on, it takes a certain amount of engine power, regardless of how many degrees the temperature is above the setpoint.
 
Mick, as far as I know the automatic airco simply switches the airco compressor on and off. There is nothing in between on and off. If you need only 50 % cooling power, then it's half of the time fully on and half of the time fully off. This means that whenever the airco compressor is switched on, it takes a certain amount of engine power, regardless of how many degrees the temperature is above the setpoint.

Agree the pump is on/off never seen a variable compression(rate) compressor, our 500 does not have ac buy our oyher has as has all the cars I've ever driven and I agree don't ever turn it off, though it's always been on a 2l car
 
Last edited:
Mick, as far as I know the automatic airco simply switches the airco compressor on and off. There is nothing in between on and off. If you need only 50 % cooling power, then it's half of the time fully on and half of the time fully off. This means that whenever the airco compressor is switched on, it takes a certain amount of engine power, regardless of how many degrees the temperature is above the setpoint.
So, how do you explain how the car slows down considerably with the temp set to fully cold when the internal temp is very hot?

The way I see it, if the demand is high, there must be more load on the pump.
If the demand is low, the pump won't take so much energy even though it's running.

I don't actually know how these things work, I can only go from my experience of having air con for the past fifteen years.

Thanks,
Mick.
 
So, how do you explain how the car slows down considerably with the temp set to fully cold when the internal temp is very hot?

The way I see it, if the demand is high, there must be more load on the pump.
If the demand is low, the pump won't take so much energy even though it's running.

I don't actually know how these things work, I can only go from my experience of having air con for the past fifteen years.

Thanks,
Mick.

Whatever the temp is(both inside AND outside) the system just runs the compressor longer(greater demand/load), so in winter it may only click on for 5-10 seconds, on a really hot summers day when stationary it will stay compressing continuously with the fans running at the same time and may do so for a while after moving again, also note the condition of the system alters things, it must be full of gas and the condenser must be in a good state, not clogged falling to bits(fins).
All cars will slow when it cuts in, more powerful heavier cars suffer less slowing+ reduction in power, a smaller lighter car will suffer far more in that respect, but a cool dry car is worth everything than hot bothered and steamed up.
 
Last edited:
Sorry John,
I don't agree.

The more work the compressor has to do, the more energy required to run it.
It's not a matter of on and off, it's a matter of load.

Mick.
 
Mick, like you I have my aircon on all the time - I've also experimented with mpg tests with it on or off and can't detect a difference, and have done it with various cars. However, unlike you I find the 500 fairly prone to steaming up until the interior warms a bit. Perhaps my body naturally exhudes more moisture!
 
To understand this you have to have a proper grasp of how aircon works.

In most basic terms the compressor is dumb and will compress the gas on demand, the cleaver thing about aircon is that it's actually the gas itself that decides how the system works and this is dictated by the ambient air temperature.

If you compress a gas it gets hot, expand a gas it gets cold. So in air conditioning the compressor squeezes the gas and it moves along to the condenser, where the now hot compressed gas is cooled, on a hot day it is harder to cool the hot gas, on a cold day it is much easier and efficient to cool the hot gas, is this is the fist step in understanding why aircon is very efficient in the winter.

The gas travels around the system where it will meet the expansion valve on expanding the gas gets cold and this is how the air con works, the exchange of heat here is the next step in the process of efficiency. At the evaporator, If the air temperature is hot then more of that heat gets transferred to the gas, if the air is cold then less transfer of heat takes place. Heat = energy so the more heat transferred into the system the more energy the air conditioning has to dissipate on the next round.

So the gas with more heat goes back to the compressor, the extra heat carried makes it harder to compress the gas, the gas gets hot again and then cooled in the condenser, now the condenser has to shift more heat, from the process and so the cycle continues.

In cold weather the gas does gather as much energy from the evaporator, it dissipates more energy at the condenser and the compressor does not have to work as hard to compress the gas, in an aircon system the resistance on the compressor, is the relative pressure of the gas and so the colder the weather the more efficient and less energy needed from the engine to run the system.

The compressor is controlled by pressure sensors in the system which cut the compressor if the pressure gets too high or low at various points.

If the weather is hot and there is a lot of energy in the gas then the system pressure will rise and so the system will cut out earlier.

As the pressure drops the aircon will start up again.

In cold weather there may not be enough energy going into the gas to keep the pressure high enough so again the system will cut out to let the gas move round the system and pressure at the low pressure side to come back up again.

A correctly gassed system is very efficient in this process, a system with too much or too little gas can cause big problems for mpg or power.

All aircon systems cut off at wide open throttle to allow more power to the engine. So if you demand the power it will be there.

In the summer the impact that aircon has on the efficiency of the engine and power, when the system is most demanding on resources, is still much less than driving around with windows open and the aerodynamic drag this causes at above 30mph if you're stopped in traffic just turn off the aircon and open a window, except you'll lose all that nice cool air the aircon made, so if you have stop start let the car turn off and on as it sees fit to maintain the status quo.

Over a prolonged period of time you might see a couple of MPG difference with aircon on rather than off, but if opening windows you will see s more dramatic change.

In truth carrying an extra passenger or a load of unnecessary junk in the boot will have more impact than using the aircon.

Hopefully this all makes sense
 
A very complete and informative explanation Andy. I think modern a/c systems are pretty effecient, which is why we don't notice an impact on mpg - I admit I haven't conducted one of my comparison tests in very hot weather - it's unbearable driving with the a/c off!
I also think modern engine management systems do a good job of masking any perceived change in performance as the compressor load changes. Some years ago we had a Ford Ka (first shape) with a/c, and the performance suffered noticably with it switched on. Haven't noticed it with any of our cars since then.
 
For lenses front or rear for surface scratches I use AutoGlym Resin polish, polishes them out quite easily.
 
The more work the compressor has to do, the more energy required to run it.
Yes, of course it needs more energy, but more energy doesn't mean more power. The same power for a longer time is also more energy.
 
Back
Top