A/C and fuel economy & How to use aircon optimally.

[venkatrx]

Originally Posted by Lukeskywalker

my car has a tacho (optra 1.6), agreed there is a load on the engine once the a/c is switched on, proof is increased rpm when idling, but my doubt is if i consciously make sure that i drive at a constant rpm say 2.5k, with or without a/c, my fuel consumed should be the same as the engine is revving at the same speed in either case. Pls enlighten my dumb soul

If rpm=2500 and your accelerator pedal pressed = constant (say 25% pressed) then the 'fuel consumption' is constant regardless of AC being on/off.

but remember that part of fuel consumed will go towards driving AC if it was on hence your 'fuel efficiency' will go down.

[speed0mania]

hey guys interesting debate!! but what i feel is that the rpm is directly proportional to the rpm but it isnt the only thing right if that was the case then overloading your car with an extra 100kgs and driving at the same rpm wouldn't give you any FE loss !! but as we all know that is not the case!! hence the car maybe at the same rpm but the engine is doing extra work to keep it at that same rpm range!! its simple a motor with no load might givea certain speed output but to give the same output when it is ladden with some load will consume more power!!
also the rpm goes up during idle as it is set to the minimum on idle (just sufficient from keeping the engine from stalling) when an additional load of the ac power is added to the alternator it has to increase the rpm so asto generate sufficient power for the ac and to keep the engine running !! we do not notice this while driving but if we equate the speed at a particular gear at any rpm with the ac on and off , we will see that at the same rpm and gear the speed is lower (maybe marginally depending on the car) when the ac is on!!
and as per what i understood the question to be originally ,
yes it is possible to save on the FE loss due to the ac if you drive with a lighter foot obviously you cannot go anyfurther if you usually drive with a light foot in the range of around 2000 rpm as too low a rev would give you a further loss inFE ! but if you usually drive above the 3k rpm mark then you could cut down on the loss by driving in the 2-3k range depending on your engine!!

[zen-ith]

Pardon me if this sounds naive but, my understanding is- At idling, when we switch on the AC, the increase in rpm to sustain the added load is done by the ECU (like the computation of how much RPM to increase depending on the speed of the blower presumably?). All that is ok in an mpfi (any car with an ecu). But what in my good old carb zen? Are we saying here that if i further reduce my idling rpm by some means, and then i switched on my AC, the engine would stall?

[naveendhyani]

Originally Posted by supremeBaleno

"The fuel consumption in the final speed of blower will be marginally higher than when the speed is at the lowest. This is due to the fact that the compressor cut off at higher blower speed is minimum or almost nil and hence the AC compressor load on the engine is relatively constant. Whereas in lowest blower speed the cut-off pf compressor is frequent and hence the load on engine is lesser intermittently."

seems very logical. now on after first 10 minutes of running on full blower my blower wud come down 2 slower speed

[Lukeskywalker]

Originally Posted by venkatrx

If rpm=2500 and your accelerator pedal pressed = constant (say 25% pressed) then the 'fuel consumption' is constant regardless of AC being on/off.

but remember that part of fuel consumed will go towards driving AC if it was on hence your 'fuel efficiency' will go down.

Even then, engine rpm is related to work done by the engine, higher rpm more work done, so keeping rpm constant means keeping the work done by engine constant too, right?
So, (correct me if i'm wrong) at constant rpm, your performance might go down, in terms of acceleration and pick up, but as you are holding the throttle constant, you are effectively controlling the amount of fuel being injected, hence the theory that you might have the same amount of fuel consumed with the a/c in "on" or "off" position. my hypothesis really

[speedsatya]

Originally Posted by Lukeskywalker

if i consciously make sure that i drive at a constant rpm say 2.5k, with or without a/c, my fuel consumed should be the same as the engine is revving at the same speed in either case. )

but the effort required to maintain a 2.5k rpm with AC will be more ...which means more fuel will be spent when AC is on ...and hence lower FE

[Ajay]

Originally Posted by supremeBaleno

Ivor and luke,
You guys are right about that. Running the blower (fan) at the lower speeds would give you more FE than running it full blast. I had this doubt myself and wrote to MUL. I am quoting below what their engineer replied to me :
"The fuel consumption in the final speed of blower will be marginally higher than when the speed is at the lowest. This is due to the fact that the compressor cut off at higher blower speed is minimum or almost nil and hence the AC compressor load on the engine is relatively constant. Whereas in lowest blower speed the cut-off pf compressor is frequent and hence the load on engine is lesser intermittently."

let us take the example of an auto climate control car. Assuming we set the temp at 21 degrees, wouldnt the compressor work till that temp is reached at first and then continue to maintain the same. If this is correct, isnt it true that the irrespective of the fan speed, the compressor will work on cooling the cabin to the required temp.? the higher the fan sped, the greater the reach and cooling capacity and when the fan speed is lower, the time required to cool the entire cabin is more which means the compressor has to run for a longer time.??

[amit mohan]

Hi Luke,

the compressor is connected to the driveshaft so as soon as it is switched on the ac draws power to run the compressor and hence the increase in rpm to maintain idling or even while driving no matter what FE wud be compromised by how much is hard to say.

secondly when the fan is at lower speed the air from the vent is cooler(have experianced it) and since the air is cooler the thermostat keeps switching on and off to maintain temp. whereas vice versa fan at top speed is pushing more air thru the fins and hence to maintain temp. the thermostat keeps the compressor on hence the resultant lower FE on full blower and higher FE on lower fan speeds.

[greatmana2000]

Originally Posted by zen-ith

Pardon me if this sounds naive but, my understanding is- At idling, when we switch on the AC, the increase in rpm to sustain the added load is done by the ECU (like the computation of how much RPM to increase depending on the speed of the blower presumably?). All that is ok in an mpfi (any car with an ecu). But what in my good old carb zen? Are we saying here that if i further reduce my idling rpm by some means, and then i switched on my AC, the engine would stall?

As far as my understanding goes ,the ECU dosent do anything . when the Ac is switched on ,the accelerator position is slightly increased ,so as to sustain the AC load .

For a Maruti 800 car the AC compressor load is approximately 7hp . so deducting 7 hp from 45 bhp does make a lot of difference in the acceleration and revving levels .

Running the fan motor (blower) also consumes energy as ,the blower runs from battery and the alternator charges the battery . when the alternator is charging the battery the alternator needs more energy ,than when the alternator is not charging the battery.

The engine revs faster when the AC is switched on as the compressor poses a load on the engine ,and for the engine to stop stalling under load ,the engine is set to run a bit faster ,especially when the AC is switched on and you are driving the car ,the car might stall when you try starting from a stand still .

just my 2 cents ..

[greatmana2000]

Originally Posted by supremeBaleno
Ivor and luke,
You guys are right about that. Running the blower (fan) at the lower speeds would give you more FE than running it full blast. I had this doubt myself and wrote to MUL. I am quoting below what their engineer replied to me :
"The fuel consumption in the final speed of blower will be marginally higher than when the speed is at the lowest. This is due to the fact that the compressor cut off at higher blower speed is minimum or almost nil and hence the AC compressor load on the engine is relatively constant. Whereas in lowest blower speed the cut-off pf compressor is frequent and hence the load on engine is lesser intermittently."


supremebaleno ..Are you sure you got a reply from a maruti engineer ?if so ,i wonder what kind of engineers produce maruti cars ...lolll .

The blower has different speeds as ,a car has varying passengers . there might be various no of configurations . for example there may be only front seat occupants or rear seat occupants . inorder to reach the rear seat occupants ,the blower has higher /faster speeds .

Infact ,faster blower speed would cut in the compressor earlier ,as the cooling by convection is much better and heat transfer is much faster ,as more volume of air passes than when the blower is at a slower speed . when the blower is at a slower speed ,it gives more room for condensation to take place,thats the reason for water droplets ....

[Ajay]

Originally Posted by amit mohan

Hi Luke,

secondly when the fan is at lower speed the air from the vent is cooler(have experianced it) and since the air is cooler the thermostat keeps switching on and off to maintain temp. whereas vice versa fan at top speed is pushing more air thru the fins and hence to maintain temp. the thermostat keeps the compressor on hence the resultant lower FE on full blower and higher FE on lower fan speeds.

Doesnt the thermostat detect the temperature when the blower is at full speed coz if it pushes in more air, the faster the cabin is cooled off isnt it?

[ivor]

Originally Posted by greatmana2000

Originally Posted by supremeBaleno
I am quoting below what their engineer replied to me :
"The fuel consumption in the final speed of blower will be marginally higher than when the speed is at the lowest. This is due to the fact that the compressor cut off at higher blower speed is minimum or almost nil and hence the AC compressor load on the engine is relatively constant. Whereas in lowest blower speed the cut-off pf compressor is frequent and hence the load on engine is lesser intermittently."

supremebaleno ..Are you sure you got a reply from a maruti engineer ?if so ,i wonder what kind of engineers produce maruti cars ...lolll .

Actually, this makes a lot of sense and this is something that is validated on my old Zen. On a manual AC, it doesn't matter how many are in the car, when the blower is on at full blast the compressor never cut off.

Sorry, greatmana2000 but the engineer makes far more sense than your explanation, which seems to be referring to auto climate control.

[ivor]

Originally Posted by Ajay

Doesnt the thermostat detect the temperature when the blower is at full speed coz if it pushes in more air, the faster the cabin is cooled off isnt it?

At least in my old Zen, it didn't matter what the cabin temperature was--if the blower was at full blast, the compressor never cut off

[Ajay]

Originally Posted by ivor

At least in my old Zen, it didn't matter what the cabin temperature was--if the blower was at full blast, the compressor never cut off

In my car which is not auto climate controlled as well, The compressor cut off is quite fast when the blower is at full speed.

[ivor]

Originally Posted by Ajay

In my car which is not auto climate controlled as well, The compressor cut off is quite fast when the blower is at full speed.

Do you have a Zen too? Perhaps what I described is restricted to Marutis--or perhaps your thermostat is set to cut-off at a particular temperature while my thermostat was set to never cut-off when blower was at full blast.