Fuel Saving Tips

almillar

OK since we're onto reducing the weight of the car and someone mentioned spare wheels above, I've taken inspiration from lots of new cars on the road and removed my spare wheel. It's heavy! Much heavier than a compressor and can of gunk (make sure you buy the correct size for your tyres). All I need is to be able to get home, and my spare is sitting in the garage if needed. Maybe you should take the spare with you on long journeys though.

thescouselander

Delivery_Driver wrote: »

Say the aircon only uses 5 bhp to run, doesn't seem much if your car is 150bhp.The thing is, your car is only making 150bhp at peak power with full throttle which is a tiny amount of the time if ever for some people.Cruising down the motorway you might only be using 50bhp, so that 5bhp suddenly becomes a significant chunk of the power used and will affect fuel economy.

Unless you're running some sort of refrigerated trailer unit the air con compressor is unlikely to use that much power. 1-2 bhp is more realistic and even then the duty cycle could be less than 50%

Delivery_Driver

thescouselander wrote: »

Unless you're running some sort of refrigerated trailer unit the air con compressor is unlikely to use that much power. 1-2 bhp is more realistic and even then the duty cycle could be less than 50%

Is this something you know a lot about as the searches I have done all seem to throw up figures of 5+bhp but its a tricky one to find definitive answers for.?
The gist seems to be as although the area of the car is small compared to a building, there is less insulation & more glass(so the car heats up quickly in the sun) and the need to cool the car very quickly compared to a building which would maintain its temperature better.

Ebe_Scrooge

10JH wrote: »

Nice collection here - http://www.theecologist.org/green_green_living/home/1083620/top_10easy_ways_to_make_driving_greener.html

They seem to really know their stuff. "Turn the heating down in winter". That's going to save heaps of fuel, seeing as the engine is producing the heat whether you like it or not, all the heater controls do is divert the heat into the cabin. :rotfl: Or are they worried about the amount of electricity used to power the blower fan ? Now we really are getting into the "is it even measurable in practical terms" territory :-)

LandyAndy

almillar wrote: »

OK since we're onto reducing the weight of the car and someone mentioned spare wheels above, I've taken inspiration from lots of new cars on the road and removed my spare wheel. It's heavy! Much heavier than a compressor and can of gunk (make sure you buy the correct size for your tyres). All I need is to be able to get home, and my spare is sitting in the garage if needed. Maybe you should take the spare with you on long journeys though.

:rotfl::rotfl:And your shredded tyre after you've managed to stop the car is going to let you do that isn't it?

poppasmurf_bewdley

To save petrol, I was always told to drive as though you have a glass of water balanced on your bonnet and you can't spill a drop.

Try it - it works!

thescouselander

Delivery_Driver wrote: »

Is this something you know a lot about as the searches I have done all seem to throw up figures of 5+bhp but its a tricky one to find definitive answers for.?
The gist seems to be as although the area of the car is small compared to a building, there is less insulation & more glass(so the car heats up quickly in the sun) and the need to cool the car very quickly compared to a building which would maintain its temperature better.

To be honest I don't have the definitive figures but I do work on equipment that requires air conditioning so I could probably go and get some info from someone who knows for sure.

5HP would be equivalent to around 3.7kw so assuming the heat pump could move around 3 times that energy it would be shifting over 11kw of heat from out of the car. That seems a lot to me.

I guess you'd also have to account for the power required for the blower in the car and the cooling fan for the radiator outside although for the latter it probably wouldn't need to run if the car was travelling at speed.

Perhaps I'll see what I can find out tomorrow.

vikingaero

In all the aircon v. open window tests I've seen, the a/c has nearly always taken away 10% of the MPG. The open window MPG penalty was around 5-7%.

thescouselander

The only practical test I have seen is the one I did myself. Basically I set the cruise control to 70 mph on a flat section of motorway and set the MPG readout to instantaneous. As far as I can tell the MPG readout displays to 1 decimal place but changes in increments of 0.3 mpg. Anyway, when the AC is switched on the instantaneous readout didn't change so whatever the MPG penalty was it was too small for the computer to detect. Not exactly scientific I know but if the impact of the AC was significant surely it should have shown up.

reeac

thescouselander wrote: »

The only practical test I have seen is the one I did myself. Basically I set the cruise control to 70 mph on a flat section of motorway and set the MPG readout to instantaneous. As far as I can tell the MPG readout displays to 1 decimal place but changes in increments of 0.3 mpg. Anyway, when the AC is switched on the instantaneous readout didn't change so whatever the MPG penalty was it was too small for the computer to detect. Not exactly scientific I know but if the impact of the AC was significant surely it should have shown up.

At last, someone who agrees with me regarding air con. and fuel consumption! When we first got a car with a fuel computer we happened to be making regular weekend trips from our home in Berkshire to Suffolk [M4, M25, A12] and back and I checked fuel consumption for complete 135 mile trips with and without air con.. I was unable to detect any consistent effect as it was less than the effects of such variables as traffic density. I'm well aware that air con. uses energy and therefore fuel but it's a pretty small effect at least in a big car [it was an XJ6 4.0] - probably less than 3%.