[dinu2506]

Dear friends
I have a BMW X1 sDrive20D and I have observed a big dip in mileage figures when the car is running with AC on - 1.5 to 2km / lit difference in the city. I was under the impression that cars with big engines do not show a big difference in mileage with and without AC. My earlier Civic also did not have such a difference.
Would be good to get a feedback if this is normal? The AC seems to be working fine, with the climate control able to maintain the required temperature. Also engine performance with the AC on seems identical with the AC off.

[nitinbose]

This is quite an interesting area to talk about and has been much talked about.

You have mentioned a difference of 1.5-2km/lit. How long have you driven to arrive at this figure? At what speeds and gear were you driving at? Did you try the same on long highway trips?

Though personally, I feel its normal to have a drop in the fuel economy of about 10-15% with A/C ON.

Others may suggest their views and observations.

[EagleEye]

It all comes down to the size of the compressor in the car.
If the compressor draws more power, you will notice a greater drop in mileage. SUVs and bigger cars have to have bigger compressors to cool the larger cabins, so this is to be expected.

[dinu2506]

@Nitin, thats the problem, my run is predominantly very short hauls of less than 10km and that too in congested roads and not too good roads. I get around 10.5 km/lit without the AC and going down to 8.5 km/lit with the AC. I tried al modes of the AC, going to manual AC etc , but not much change. Switch off the AC and there is a definite improvement. I guess the combination of short hauls, slow movement and the auto box contributes to this. Maybe the new X1 with EcoPro mode makes a difference as I heard it also makes a difference to Ac settings.

I had gone on just one long distance trip and that was with full AC. I got an overall mileage of 12 km/lit which also included ghat sections (around 20%) and beautiful 4 laners. Before the ghat the computer was showing 14 km/lit

The only thing, my earlier Civic used to traverse the same route day in day out but I had not observed such a drastic variation in this car - maybe 0.5 km/lit diff.

[rocky080]

Originally Posted by dinu2506

Dear friends
I have a BMW X1 sDrive20D and I have observed a big dip in mileage figures when the car is running with AC on - 1.5 to 2km / lit difference in the city. I was under the impression that cars with big engines do not show a big difference in mileage with and without AC. My earlier Civic also did not have such a difference.
Would be good to get a feedback if this is normal? The AC seems to be working fine, with the climate control able to maintain the required temperature. Also engine performance with the AC on seems identical with the AC off.

Originally Posted by dinu2506

@Nitin, thats the problem, my run is predominantly very short hauls of less than 10km and that too in congested roads and not too good roads. I get around 10.5 km/lit without the AC and going down to 8.5 km/lit with the AC. I tried al modes of the AC, going to manual AC etc , but not much change. Switch off the AC and there is a definite improvement. I guess the combination of short hauls, slow movement and the auto box contributes to this. Maybe the new X1 with EcoPro mode makes a difference as I heard it also makes a difference to Ac settings.

I had gone on just one long distance trip and that was with full AC. I got an overall mileage of 12 km/lit which also included ghat sections (around 20%) and beautiful 4 laners. Before the ghat the computer was showing 14 km/lit

The only thing, my earlier Civic used to traverse the same route day in day out but I had not observed such a drastic variation in this car - maybe 0.5 km/lit diff.

Firstly could you give more details of the ODO reading of the car and the time of day you make these trips cause these mileage discrepancies gets ironed out as the car is driven more and the time of day also has a difference in the working duration of A/c Compressor.

In my opinion the difference in mileage seems to be justified as you mention that you drive in bad roads and congested traffic conditions and with the weather in Bangalore going to the dogs the A/C compressor has to work for longer duration to maintain the temperature set by you either in Auto or Manual mode. Also, remember that the X1 has a bigger cabin when compared to your civic so the duration of the compressor being on is more.

[nitinbose]

Originally Posted by dinu2506

@Nitin, thats the problem, my run is predominantly very short hauls of less than 10km and that too in congested roads and not too good roads. I get around 10.5 km/lit without the AC and going down to 8.5 km/lit with the AC. I tried al modes of the AC, going to manual AC etc , but not much change. Switch off the AC and there is a definite improvement. I guess the combination of short hauls, slow movement and the auto box contributes to this. Maybe the new X1 with EcoPro mode makes a difference as I heard it also makes a difference to Ac settings.

I had gone on just one long distance trip and that was with full AC. I got an overall mileage of 12 km/lit which also included ghat sections (around 20%) and beautiful 4 laners. Before the ghat the computer was showing 14 km/lit

The only thing, my earlier Civic used to traverse the same route day in day out but I had not observed such a drastic variation in this car - maybe 0.5 km/lit diff.

My view is that you're pretty okay. Maybe you could try changing your driving style a bit, i.e. avoid harsh acceleration (something hard to resist in a BMW ), harsh/sudden braking. You should see your fuel economy improve. The civic could not offer you such driving pleasures hence you did not observe the difference. This driving practice would also be a default behavior.

[dinu2506]

Originally Posted by rocky080

Firstly could you give more details of the ODO reading of the car and the time of day you make these trips cause these mileage discrepancies gets ironed out as the car is driven more and the time of day also has a difference in the working duration of A/c Compressor.

The car has not run much, the current ODO reading is 3600km. I make this run once in the morning to office at 7:30am. I do not need to use the AC this time and it returns around 11 to 11.5 km/lit. If I use the AC, it drops to 9.5 km/lit. In the evening, around 5pm, I do the return journey on the same route. The traffic is more, so is the temperature. I manage around 10km/lit without the AC and drops to around 8 to 8.5km/lit with the AC.

The car does not have sun film, wanted to know, will installing a front film make a difference?

[rocky080]

Originally Posted by dinu2506

The car has not run much, the current ODO reading is 3600km. I make this run once in the morning to office at 7:30am. I do not need to use the AC this time and it returns around 11 to 11.5 km/lit. If I use the AC, it drops to 9.5 km/lit. In the evening, around 5pm, I do the return journey on the same route. The traffic is more, so is the temperature. I manage around 10km/lit without the AC and drops to around 8 to 8.5km/lit with the AC.

The car does not have sun film, wanted to know, will installing a front film make a difference?

I am no BMW expert, but i guess since the car has run only 3600 Kms this difference you are noticing should drop once the engine completes its run-in period and settles down. Adding a sun film with sufficient darkness will help but i would not suggest it as it is illegal to stick anything on the car glass.

[ACM]

This is exactly as expected. 10-15% drop is very normal. Also for diesels the efficiency is higher in longer drives (once the engine has heated up to the optimum temperature.).

Nothing wrong with the drop in efficiency.

[2000rpm]

Congested traffic is the reason. Try the same on a highway stretch and I am sure the difference will be very less. In fact from my experience, smaller engine cars show a higher percentage drop in fuel efficiency when running in congested traffic conditions.

Let me try to explain you mathematically.
Suppose you have 100 units oif energy with
AC consuming 20 percent
Engine consuming 80 percent

On Highway conditions, all 80 percent of engine energy is used in travel. Hence the AC consumption figures will indicate the figures it is supposed to (i.e 20%). Assume the car requires one unit of energy for one Km of travel. That will give you 80 kms travel for 100 units of available energy.

Now in traffic conditions, out of the available 80 units, say 20 units are used in idling at signal, 20 units are used in travel in low gears (1st and 2nd). Assume 2 units of energy consumed to travel one Km in lower gears. Hence your total travel distance will be

40 (consuming remaining 40 units)
10 (consuming 20 units from low gear driving)
0 (consuming 20 units of idling)

Effectively you travel 50 kms only with the AC not adding to any of its consumption!!!

Hope I have not made it too complex :P

[dinu2506]

I understand, the only reason is that I have not observed this in any of my previous cars. Its close to a 20% drop in efficiency, and thats whats bothering me. In absolute terms, with my amount of driving I don't lose much, but the percentage drop seems significant.

[VW2010]

10km is hardly a distance to measure your cars fuel efficiency. The difference is minimal in reality. Unless you see a huge difference of 4-5Km/L you should be concerned.

The difference you see is pretty normal irrespective of size of the engine for the 10KM you are driving in congested traffic.


Instead of checking this in a traffic why not take the care out on the free with and without AC for a stretch of 10-20 Kms and check the same. The variables should be minimal including traffice etc.

[FlatOut]

I think this may partly be explained in the difference between the two engines. The BMW's is a diesel which is most efficient at low/part load.

So, when commuting through traffic the BMW engine is probably running very efficiently and any additional load such as the A/C compressor will have a greater effect on the economy than with a petrol engine which is least efficient when at low load and in the stop-start cycle.

It isn't quite as simple as this, but I feel this goes some way to the difference you've experienced, dinu2506. The way particular engines are tuned, their cubic capacity, gearing, tuning etc all will have an effect on this sort of result. As will the drag of the compressor, of course.

[Sahil]

Don't rely on the figures you see in your MID. Only way you can figure out for sure is to drive a tankful with AC and then a tankful without AC. On similar roads.

[dinu2506]

The mid is relatively accurate in this car. Cross checked with tankful to tankful