Slight drop in MPG

zcacogp · Dec 28, 2008

Chaps,

A quickie.

Sad people like me monitor their MPG very closely. (An obsession ... humour me, eh?)

Mine has dropped slightly of late. Around 10% Why would this be?

All factors are the same:

- Car regularly serviced.
- Similar roads driven on (comparison is made across a range of driving, and how much of each sort is noted - commuting/long distance/heavy traffic etc)

I am comparing across the last 6 - 9 months, so it is possible that the greater use of winter electrics (lights/demister/heater etc) may have an effect, but can they really make this much difference?

Also, I have not changed the izzy cap and rotor arm in the 3 years I have owned the car. How do I know when they should be changed/would this make this much difference/where is the best place to buy replacements from (guessing Beryt/ECP etc).

Oli.

RC18B 911 turbo · Dec 28, 2008

The cold will effect MPG as the fuel is thinner and the engine will pull more in (or at least it was something along those lines that I was told) that or you need to ease off the gas a bit[

].
Anyway who buys cars like these for MPG I haven't a clue what mine does[

].

DavidL · Dec 28, 2008

I thought cold air was more efficient for combustion so I suppose would give better MPG?
Or produces more power from more efficient combustion and therefore the better "quality" air supply uses more fuel, which of course encourages a heavier right foot which also uses more fuel.[

]
Shows how much I know about it!!

A5DSR · Dec 28, 2008

My 320d MPG has also dropped off a fair bit in the l;ast few months.
Maybe that's why the fuel is getting cheaer, it's being watered down...[8D]

Fen · Dec 28, 2008

All the cars I have had with trip computers have shown a drop in mpg in the colder months. I put it down to denser air requiring more fuel, however I'm not really convinced as the mixture to produce a given amount of go shouldn't change so I'm not sure denser air would have an effect unless you're using full throttle.

knightma · Dec 28, 2008

The reason cold dense air causes greater fuel consumption in winter is due to the additional air drag it causes on the car. Another reason for greater winter fuel consumption is the fact that engines and gearboxes start from lower temperatures thus create more losses for longer until they are warmed up. I've also always monitored mpg on my cars and a moving average mpg calculation of about 6 tank fulls (to average out fluctuations in driving conditions) always produces an almost perfect sine wave with a peak every summer and trough every winter.

Fen · Dec 28, 2008

I hadn't thought of the increased air drag angle and for me it makes sense as my driving was always mostly motorway and A-road (the reason why cold fluids never made sense to me as the reason as my car always did a reasonable length of journey once started). I suspect it may not be the same for Oli living in the smoke however.

Black_JPN · Dec 29, 2008

Good excuse to bite the bullet and just upgrade eh Oli? [

]

On topic - I monitor mine too now but it's been so long since I put fuel in (we're talking months, car hasn't been used much) that I wouldn't be able to prove the weather was affecting it. My driving varies alot too which also varies the mpg greatly.

sawood12 · Dec 29, 2008

I think it is purely due to the fact it takes longer for the engine to warm up so you are on choke for longer. I'd be surprised if the point about denser air requiring more fuel I don't think is true mainly because the fuel density is also increased so you need less fuel by volume. Also the fact that with colder air you are making more power so you're not demanding as much fuel, I think this would cancel out the effect.

Interesting on the increase in drag though.

Fen · Dec 30, 2008

I experienced the effect on journeys of 1.5 hours and upward, which I think completely discounts the warming up aspect as the time to warm up is insignificant compared with the running time, even in winter.

Think about your statement about more power, Scott. How exactly would denser air make more power while burning less fuel? You need more fuel to mix with the denser air as you need to keep the ratio of fuel to air the same, and when it's denser that means you get more air for a given volume.

peanut · Dec 30, 2008

Apparently another possible reason that could account for a noticable decrease in MPG is a faulty FPR fuel pressure regulator causing an over pressuring of the fuel system.
You'd need to do a fuel pressure check on the fuel rail to check
Usually though this would be accompanied by other symptoms including occasional stumble or spluttering running rich with some smoke from exhaust on startup and hard acceleration and possibly a noisy fuel pump.
I read this on the Clarkes Garage excellent site

sawood12 · Dec 30, 2008

ORIGINAL: Fen

Think about your statement about more power, Scott. How exactly would denser air make more power while burning less fuel? You need more fuel to mix with the denser air as you need to keep the ratio of fuel to air the same, and when it's denser that means you get more air for a given volume.

I'm not saying you're burning less fuel - your burning the same fuel by mass to maintain the AFR but burning less fuel by volume. This is because the fuel is also more dense due to the temp, so to maintain your AFR you need less fuel by volume (i.e. less injector duty) such that you deliver the same mass of fuel. And of course when it comes to MPG its all about how much fuel you use by volume and not mass. In cold weather you're getting more fuel for the same volume as it is more dense.

Another thing is the increased demand on the electrical system in cold wintry weather, using the lights more, using the fan more, using the heated rear window etc - it all adds up to more load on the engine.

sawood12 · Dec 30, 2008

By the way, i'm not sure what the trade off is between the air being more dense and the fuel being more dense i.e. I don't know if the both effects cancel eachother out in that in cold weather you need less air flow into the engine for a given amount of power demand due to denser air, and correspondingly less fuel due to it also being more dense.

zcacogp · Dec 30, 2008

Chaps,

Lots of interesting responses, thanks. Mark - that is a very interesting point about the heavier air being harder to force a car through. That is a factor I have never considered. Thanks.

In terms of the air/fuel and burning thing ... in theory, colder air is denser, and therefore there is more mass of oxygen for a given volume. This means you can throw more fuel in, and get more power out. BUT the air:fuel ratio should always be stochiometric, therefore shouldn't change, and the amount of fuel required is a function of the driving. So, similar driving should mean similar fuel use. So, all you get for colder air is the ability to make more power if you want to, from a given engine capacity. (Where the air is compressed, and run through an intercooler, I'd expect the potential for more power to be quite significant - i.e. you turbo boys should notice your cars getting quite a lot more lively when it's cold outside. Is this so?)

Yes, fuel will get more dense at lower temperatures, but I'd bet that the coefficient of thermal expansion of fuel is much, much lower than the same coefficient for air. I.E. air will vary in density much more with temperature than fuel will.

Helpful to know that others see a slight economy drop in the winter as well. All may still be well in the world of my S2 ...

Oli.

sawood12 · Dec 30, 2008

Yes you are right Oli, the more air you get in the more fuel you put in, and this is what i'm saying, but on the basis of mass and not volume. When you are working out how much fuel to add to air to achive a certain AFR you do it on the basis of mass and not volume. This is why cars have Mass Airflow Meters because you want to measure the mass of the air coming in rather than the volume which is what the barn door AFM measures.

So consider this - you want to maintain a constant speed of 70mph. When it is colder you wont have to depress the accelerator as much because the air is more dense you get the mass of air the engine requires with less throttle, therefore you are injecting less volume of fuel as the fuel is also denser as you get the same mass with less volume so you are using less fuel.

You buy the fuel on the basis of volume and not mass and MPG is a measure of volume of fuel used over a mile and not the mass of fuel used, so I reckon that the effect of temperature on fuel used once the engine is upto temp is negligable as due to the temperature the engine is using less air and fuel by volume - unless of course you are not trying to maintain 70mph and are cashing in on the extra performance potential of cooler air, then of course you will use more fuel, but we're talking back to back comparison of the same driving style in different temps here (I hope).

Also from a common sense point of view it doesn't make sense that the engine will be more fuel efficient in warm climate than a cooler one (within reasonable limits of course). All engines work more efficiently when operating in a cooler environment. I think the increase in fuel consumption in winter is to do with something else other than the chemistry of combusion i.e. a greater demand on the electrical system, or more aerodynamic drag or whatever. I wish I could remember my rudamentary High School Phisics. If my text books were'nt up in the loft i'd look it up. In fact I might just look it up when I'm putting all the Christmas decs back in the loft.

Fen · Dec 30, 2008

I agree with that last post pretty much 100%.

My experience quantifies the difference as around 10% based on several cars over a number of winters so there is a significant cause or combination of causes however.

As an aside I have often tried t fill up in the early morning rather than evening (i.e. on the way to work rather than on the way home), especially in the summer as logically youshould be getting more bang for your litre of cool fuel as opposed to warm. Probably not enough to make a real difference, mind.

zcacogp · Dec 31, 2008

Scott,

We may be in danger of violent agreement here ...

Yes, the air is burned by mass, not volume. Yes, the fuel is burned by mass not volume, but the density increase is negligible (an assumption - challenge me on this if need be. This assumption would mean that Fen's early-morning-fill-up trick is time wasted, but that's by-the-by.)

So, a given volume of air is of greater mass. So your ratio of air volume:fuel volume will go down. So, yes, for a given load (70mph on your favourite motorway) the throttle will be slightly less close to the carpet.

BUT the energy required will be the same. So the volume of fuel will remain the same (given my assumption above.) The volume of air is not relevant, as it's the mass of air that is relevant to the equation. And you can get more mass of air in, therefore more performance, but that's not relevant as we are still only doing 70mph on our favourite motorway, not trying to break the sound barrier*. (There will be a minimal gain as because you are moving less air through the inlet tract the pumping loss will be fractionally smaller, but this will be infinitesimally small.)

So, yes, I am in agreement with you in that any difference in fuel economy in cold to hot climates has to be to do with other factors. Electrical system load / greater air drag / richer mixture to make up for cold starting / lower air pressure in the tyres leading to more drag etc etc etc.

And Fen, I'm with you on the 10% estimate. Pretty closely, actually.

Oli.

* - Yes, I drive a pants S2, so breaking the sound barrier is a realistic possibility. Life must be difficult when you are a slow Turbo owner, eh? (Had to get that dig in somewhere!) [

]

Diver944 · Dec 31, 2008

I'm no science geek but are we not forgetting that our cars have a simple air flow meter with a flap that measures air flow and adjusts fuel according to it's deflection. It has no clue about air density and when its at full deflection it will dump in the same amount of fuel regardless of how dense the air is.

zcacogp · Dec 31, 2008

Paul,

Good point. What other sensors are there in the engine management system? An air temperature sensor would register differently, as would an air pressure sensor (barometer). Do they have either of these?

They also have knock sensors, which will detect when there is too little fuel going in and make it richer when necessary. Air will be measured by volume, and I assume the system would then put in a minimum amount of fuel such that the knock sensor doesn't get triggered. If the air is cold and dense, the minimum amount of fuel necessary will be greater (as in, it will put more in to prevent triggering the knock sensor.)

Oli.