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Old 9th December 2009, 13:47   #46
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Originally Posted by Samir Taheer View Post
I have to agree with MGB Evo. I've read in numerous places that switching your car off, for a few mins, is not really giving you any petrol savings. The car uses more petrol, whislt starting up, then it would during idle. Obviously, there is a time limit.

As for the swtich off, my old 800 (1985-86ish), if the ac is left on and you start the car, it takes a few seconds for the compressor to kick in. So it doesnt seem like its putting any load on the batter or engine.

@Filcord - why do you turn the a/c off when going uphill? So save fuel? I dont think it should make any difference if you turn it back on immediately.
If the time between the switch off and switch on is more than a minute, you will save fuel. I think diesel engines typically consume lesser amount of fuel when idling than petrol engines. Also with the modern day mpfi and electronic fuel injection the amount of fuel sprayed into the combustion chamber won't be a significant amount, unlike carb-operated cars.

Turning the AC off while climbing uphill or while overtaking a vehicle allows the engine to provide more power to the wheels. A better and faster pickup.
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Old 9th December 2009, 13:51   #47
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Sorry if I have posted in the wrong thread. My question concerns re-starting the car a/c. I usually put off the a/c when climbing a hill. Then when coming down or on level ground , re-start the a/c. Is it advisable to wait for two-three minutes before re-starting, like one does with a home a/c?

The reason one waits to re-start a home a/c or refrigerator is because of high head pressure in the refrigerant system.
Does this apply also to cars?
Switching off the AC will give you more power for a faster/better pickup when climbing uphill or overtaking a vehicle, when it's more important that the engine provide the maximum power to the wheels. If you have a powerful car (probably > 80bhp), it may not be required.

As for re-starting the AC, if you switched it off to gain more power to the wheels, as soon as you are finished with the power, you can re-start the AC. For e.g. when climbing uphill if you switched off the AC, you can re-start it when you start going downhill. There is no need to wait for a specific period.

Of course if you want to save fuel and still use the AC, you should restart the AC only if the inside temperature goes up, and switch it off as soon as the temperature drops to an acceptable level of coolness.
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Old 28th April 2011, 12:10   #48
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Default Re: Can we switch ON AC, while engine starts?

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Actually they do. But theres nothing the airlines can do. If they install signal jammers then it would jam their own communications as well. Moreover they cant just ask everyone to hand their cellphones and return them after the flight is over there are too many privacy concerns and other tensions involved. So they just politely ask.
This is an off topic post, but regarding the question of cellphones on airplanes, there's one thing I would like to point out. There is truth behind why you are not allowed to use a cellphone when landing or take off, read ' landing or take off' - because there are 2 radars operating in this frequency. These radars send signals that bounce off the airplane and tell the pilot how centered the plane is on the runway (while take off and while approaching touch down), so, it becomes extremely critical to ensure that these signals are not hampered in any way. Once the craft leaves the signal space of these radars you could 'theoretically' switch on phones, but airlines don't encourage you to do so.

For more information on this check out PAPI (Precision Altitude Path Indicator) on Wiki.

Mods : You can remove this post and put it under Miscellaneous if necessary.
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Old 29th April 2011, 19:08   #49
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Default Re: Can we switch ON AC, while engine starts?

If you are switching off ac at lights why switch it on at all. Any ways idling with ac on I don't think will consume more than 1ltr/hr. So comfort vs cost equation is in favor of leaving it on.
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Old 25th May 2012, 12:18   #50
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Default Re: Can we switch ON AC, while engine starts?

On a related question, I was looking for a conclusive answer from team-bhp but couldn't. Can some gurus state unambiguously if the car is parked with the engine on, with gears in neutral or with the clutch fully depressed does the air conditioner run on batteries or on the engine power?

I might be totally wrong, but my understanding was that the engine is connected to a crank shaft that rotates when the combustion cycle pushes the piston up and down and this crank shaft is connected to a flywheel through the clutch plates and everything that draws power from the engine (power steering and power brakes' hydraulics, A/c compressor, battery charger, etc.) takes its power in the form of rotary motion's kinetic energy from this flywheel. If that is true, if the clutch is fully depressed, the flywheel isn't being rotated by the engine power and hence cannot power A/c or music system.

Or is it that the crank shaft is also connected to one or more other rotors that still turns the A/c compressor, power brake cylinders etc.?
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Old 25th May 2012, 14:59   #51
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Every shaft has two end my friend.on one end clutch its mounted, and every thing related to sending power to the wheel goes through this flywheel.
The other end is used to drive all auxillary systems that are required for operation of engine itself, such as water pump, alternator,ac compressor, hydraulic pump, valve train. Alk these things are driven direct by crank rotation via a belt or chain.
Hence clutch or no clutch, other end of the crank shaft still rotates ac compressor.
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Old 25th May 2012, 17:00   #52
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Default Re: Can we switch ON AC, while engine starts?

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Originally Posted by APT View Post
On a related question, I was looking for a conclusive answer from team-bhp but couldn't. Can some gurus state unambiguously if the car is parked with the engine on, with gears in neutral or with the clutch fully depressed does the air conditioner run on batteries or on the engine power?

Or is it that the crank shaft is also connected to one or more other rotors that still turns the A/c compressor, power brake cylinders etc.?
As said by dustom, one end of the engine is connected to the gear box, other end drives the auxiliaries, including, timing gear, pumps etc. These essentials are running whenever engine is running. Ac compressor is coupled to the engine by an electromagnetic clutch. It gets coupled to the engine only when we switch on the ac (blower should be running). So function of engine is totally independent of clutch operation.
Its nice to ask questions, even if they seem to be very simple. We are here to share our knowledge and learn from others. I was going through this thread. And I found some people writing totally wrong views and that too confidently (some of the earlier posts), as if expert.. Please write what you are sure of. If not, kindly raise a query. There are so many mates out here to give the right advice/ answer.
Regarding car ac, here are few things I know
  • Car ac compressor is coupled directly to the engine by a electromagnetic clutch. It runs ONLY when engine is running.
  • The electromagnetic clutch gets activated only when blower is running (to safeguard the compressor).
  • The operation of ac is independent of car moving/ stationary, in gear/ neutral, clutch depressed or released.

It is always good idea to start the engine without any load (electrical). This way, battery will be happy to serve you for longer time. Same rule goes while switching off the engine.
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Old 25th May 2012, 17:23   #53
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Default Re: Can we switch ON AC, while engine starts?

Hi People,

Going bit off topic, i am facing a typical problem with compressor of my Santro ELRX late 2007 model, which has clocked 66k kilometers. On Idle, the magnetic of the compressor kind of switches ON and OFF, obviously there is no cooling during idle.

The very moment i press on the Gas irrespective in Idle or in Gear the the eletromagnetic cluthes engages and the cooling starts.

Initially i thought the problem is with the battery which is still the original one, hence i went to the battery shop to replace the battery but the problem still presisted, hence i did not replace.

The car starts in half self, and even the alternator light is also not coming.

The solution i see is to increase the idle RPM but would increase the fuel consumption.

Thanks

Amit
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Old 25th May 2012, 19:34   #54
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Default Re: Can we switch ON AC, while engine starts?

Amit, I suspect some problem with the electromagnetic clutch. It could also be some problem with the wiring. I suggest take it to a service centre. IMO, its a loose wiring or faulty solenoid. Do update your findings. Thanks
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Old 26th May 2012, 21:50   #55
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Default Re: Can we switch ON AC, while engine starts?

The AC compressor is driven solely by engine power through the belt. It will run only when the engine is ON.
There is a clutch system in the compressor which is electrically controlled and it get engaged only when we switch the AC on. So if your engine is running and AC is switched off, compressor pulley is driven by the engine but as the clutch remains disengaged, there is no compressor action. As soon as you switch the AC switch ON, the compressor clutch engages and compressor becomes active. Its starts drawing power and loading the engine. This is evident with drop in pickup that is observed.
Now the question regarding cranking with AC on. There are many electrical loads that are cut off when engine is cranked. Stereo, wiper etc are some of them. And guess what, AC is also one of the loads that get cut off when you crank the engine. This is so that battery can dedicatedly cater to starter load. Thus compressor remains inactive while cranking irrespective of whether you have kept AC on or off. So no issues with battery but as soon as the cranking is over compressor loads the engine which is still waking up. This should be avoided and preferably AC should be switched on once car is already in motion.
Lastly, if the engine is off, only the blower can be switched on, not the compressor. So battery will drive blower fan but you will not get any cooling as compressor remains deactivated.
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Old 27th May 2012, 10:28   #56
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Default Re: Can we switch ON AC, while engine starts?

As long as your battery is healthy, the extra load of the AC compressor will not hamper the starter motor. Now if the battery is on its way out, then any extra load will affect the starter, as the current is being shared between the AC clutch and the starter motor.

In case the AC clutch is also isolated while cranking, all is fine, but if not then the AC will present extra load.

As far as I am concerned, I tend to switch the AC off before reaching home, rest of the time it is on in summers, so I do start with AC on.
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Old 27th May 2012, 10:53   #57
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Default Re: Can we switch ON AC, while engine starts?

In my car, the ECU cuts in the compressor clutch about 2 seconds *after* the engin is fired. Additionally, all heavy electrics (headlamps, power windows) are also automatically turned off while the starter motor is operating.

So usually, I do not bother with turing the compressor and lights off while craking the engine
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Old 5th July 2012, 16:33   #58
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Default Re: Can we switch ON AC, while engine starts?

Respected MODS : I don't know whether this is of any value to this thread or not. I simply want to share my experience so that it may be of some help to carry this discussion forward.

Today I managed to stall my car with aircon ON in slow moving traffic. In a hurry I tried to fire the engine without switching the aircon OFF( so that the blower was still ON at speed 2). Normally, my WagonR starts at half crank,but this time it didn't. The engine fired up and then died(The clutch was pressed down all this time,so it couldn't be the normal clutch released too fast or wrong gear issue). This happened twice(and not to mention the innumerable honkings I got from behind! ).
Then I switched off the blower and cranked.Voila! It started at half crank.

Gurus,does this throw any light on whether or not we should have the AC on during cranking?
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Old 5th July 2012, 19:22   #59
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Default Re: Can we switch ON AC, while engine starts?

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Originally Posted by powerwidFE View Post
Respected MODS : I don't know whether this is of any value to this thread or not. I simply want to share my experience so that it may be of some help to carry this discussion forward.

Today I managed to stall my car with aircon ON in slow moving traffic. In a hurry I tried to fire the engine without switching the aircon OFF( so that the blower was still ON at speed 2). Normally, my WagonR starts at half crank,but this time it didn't. The engine fired up and then died(The clutch was pressed down all this time,so it couldn't be the normal clutch released too fast or wrong gear issue). This happened twice(and not to mention the innumerable honkings I got from behind! ).
Then I switched off the blower and cranked.Voila! It started at half crank.

Gurus,does this throw any light on whether or not we should have the AC on during cranking?
Points to a voltage problem in the electrical circuits could be
. Battery
. Ignition wire frayed
. Ignition coil

Get the above checked from a reliable garage.

My K10 is 2 years old, and I have never faced any problem starting with both the AC and the fan on. In fact in all my cars the fan is constantly on, as I like the slight breeze blowing on my face (now a days you cannot enjoy the breeze with windows open due to excessive pollution in the city), hence the fan.
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Old 5th July 2012, 20:31   #60
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Default Re: Can we switch ON AC, while engine starts?

I had had the AC Switch in the ON position once while on the traffic signal. While starting the engine again the switch was OFF temporarily along with the blower and stereo system. Once it started the electrical resumed.

This means there is a auto cut-off system that cuts-off everything while cranking the engine to decrease the load on the starter motor and the battery.

Hope this helps.

Anurag.
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