Yes I know not many people keep their car in a garage these days, but
I was curious when I noticed that my average miles per gallon was consistently lower early in the day. It is actually well established that cold fuel reduces fuel efficiency (in fact in the US pre-heat kits are available.) So in the name of science I have just done some research (on the M3, on my way to work) and the results are here, and appear to indicate that going from 15C to 25C increases mpg from around 70 to around 80 mpg !
freejoules.mysite.orange.co.uk/page5.html
If 83 mpg sounds high, the A2 is (or was-it's out of production) I think the only aluminium bodied mass-produced car, and very light.
So perhaps garages will become popular again after all..
James Page
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Luckily the A2 will fit in virtually any garage, unlike a lot of modern cars!
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My experience with engines and fuel is more related to aircraft than cars. I have certainly noted that I get better fuel consumption (more mpg) in my car when the air is warm. Cold fuel will be more dense so if you need Z ozs of it to drive 100 yards you should go more lots of 100 yards off a tankful of cold fuel, = improved consumption. But, does a fuel injection system work on mass or volume? On that basis, why would anyone want to pre-heat their fuel? A fuel tank of a given capacity will contain a larger mass of cold fuel than of warm fuel. Going to hide in my shell proof bunker while waiting for demolition by Number Cruncher and other seriously clever people!
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>>But, does a fuel injection system work on mass or volume?
For a petrol engine, both!
From a cold start, the lambda sensor in the exhaust is too cold to work, so the fuel injection system, whether it measure air volume flow, air mass flow, or manifold absolute pressure, injects a known *volume* of fuel derived from a so called open loop map stored in the ECU's memory. During this mode of operation, the engine can effectively be "on choke", with enrichment determined by the signal from, among other things, the coolant temperature sensor.
As the lambda sensor heats up, eventually, it's output signal changes. The ECU detects this change, and switches into closed loop mode. The lambda sesnor doesn't have complete authority, but, what it does do, is to apply a correction from the open loop map to make the engine run, in an averaged sense, at stoichometric. This means that the ratios of air to fuel are being balanced in a mass sense, despite the ECU only being able to know about the fuel volume ot has injected.
659 states the big reasons why cold running causes higher consumption. I can only add that having done qualification testing of aeroplane gearboxes at -40C, where the oil is more like butter!, the effect of lubricant drag when cold is not at all insignificant.
Even fluids like brake/clutch fluids behave very differently when cold. I know of one recent(ish) case of a manufacturer having trouble with cars in Scandanavia; hydraulic clutches not engaging again when customers bring up the clutch pedal.
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If 83 mpg sounds high the A2 is (or was-it's out of production) I think the only aluminium bodied mass-produced car and very light.
>
Several all Aluminium cars around the Audi A8 & the Jaguar XJ being the two obvious candidates. Hence the XJ is more economical than the smaller S-Type being 200+ Kg lighter.
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I totally second the OPs theory. My mpg in the summer is better than in the winter by a good ten per cent. This is also the case if I garage in the winter and leave outside in the summer.
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There are lots of variables here.
A modern diesel measures air mass flow, boost pressure and temperature, atmospheric pressure and - importantly, fuel temperature as a means of determining the fuel per shot for a given work demand. What it cannot (yet?) do is measure the calorific value of the fuel.
Winter diesel sacrifices calorific value to the need to lower its cloud point (the point at which wax crystals start to develop), so any diesel running on Winter fuel will be less efficient.
All of the lubricants throughout the vehicle are viscosity/temperature dependent. Lower the ambient temperature and there is more drag in the engine, transmission, driveshaft joints and wheel bearings. If you make some measurements, this can be very significant.
Running tyre inflation pressures are higher in Summer due to the action of our friend the Gas Laws. Rolling resistance is therefore less.
There are other things too, but it's getting late...
659.
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Friction is probably the biggest factor. A cold engine. gearbox, wheel bearings etc. will produce a lot of friction and hence requires more energy/fuel to overcome the friction. A warm or hot engine, gearbox, wheel bearings etc. produces less friction and hence uses less energy/fuel.
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If it were true friction the various bits wouldn't last long. It's lubricant drag.
659.
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In reply to '659' I should have pointed out that in all my tests (which started 4 June) the engine was warm at the time I reset the mpg ie I'd been driving for around half an hour before the start of the measurement. And I presume no winter disesel was involved in my case.
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surely driving in the cold is better for mpg, the air is denser giving a lean mix, but then you have to equate for the use of the choke until the engine warms up so i dont think it makes any difference what so ever
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Our garage, which is integral with the bungalow, is insulated to the same standard as the rest of the property. At one point I insulated the up-an-over door, but removed it after it appeared that the extra weight was affecting the electric door opener. I don't notice that all this has any effect on fuel consumption. Fuel consumption is worse in winter but I assume this is mainly due to the greater time the "choke" (or whatever it's called nowadays) is operating.
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Yes - Fuel is more dense in the winter and therefore contains more energy by volume. In theory it should therefore be more economical in the winter. However, friction losses and the need to use more choke appear to negate the benefits, so that summer driving is more economical despite the fuel containing less energy by volume.
I wonder if there is an optimum temperature at which to park a car? Water is at its most dense at 4 Celcius - shoudo we garage our cars at that or at 15 degrees.
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Sorry, small correction to my last post- for the data on my website I had been driving for around 5-15 minutes before starting the test (although I would have thought engine oils would be well warm by then).
I did once try to measure tyre pressure effects but they seemed to be small - certainly nothing like the 10% seen here.
In reply to Espada, density change is also v small and also even if the volume does increase slightly that doesn't make it more economical. You're suing the same amount of fuel (more if engine takes same volume..) and you'll need to fill up sooner !
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Agree with 'warm car' theory - I garage all the time, but notice a difference on a 'standard run' , if, for any reason I leave car outside.
Also, don't refiners change the diesel formulation slightly for winter/summer? I notice a definite change somewhere around early May (independent of local temps.) when my fuel consumption suddenly decreases (all other factor being as equal as they can be).
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Warm air is less dense than cold air, so the drag will be less on a warm day. Not by much, but perhaps enough to make a difference to mpg at higher speeds where drag is an issue.
Edited by oldnotbold on 11/07/2008 at 17:56
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