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Many of you have also noted the difference in mpg I get between winter and summer. This puzzled me for some time - I can see that it will take longer to warm up in cold weather, you do more idling while you defrost the car, use more electrical power for lights, fan hrw and so on, but none of these 'felt' like the whole story. Recent posts have suggested the addition of winter anti-waxing stuff is a more credible answer, so I'm happy to park the summer/winter mpg question.....

....which brings me to the wet / dry difference in mpg.

Anyone else noticed this? I'm pretty sure I get less mpg in damp conditions, (which probably means fog as well as rain).

Your comments, please.

Wind can substantially affect economy, especially when at motorway speeds - a 50mph headwind when travelling at 70mph gives the same resistance as travelling at 120mph on a still day. This is why I stick myself behind trucks when after economy, and when comparing economy

Wet roads increase rolling resistance. I can't remember how much, but back when doing the daily run to Manchester I'd take weather forecasts into consideration when working out when to refuel, so probably 3-5%. Rain also affects traffic which drastically affects economy.

Hot weather can also increase rolling resistance (friction against the road) as the rubber in the tyres gets softer. I've never been able to quantify this though, although the lack of grip you can get from summer tyres on a cold dry road suggests there's definitely a distinct change in friction. Further to this point, I run my tyres at a lower pressure in winter for more grip. In summer I tend to over-inflate for economy on long trips. My guess is that tyres pumped to the same pressure at rest will actually result in quite different pressures while on a long motorway journey depending on ambient and road temperature. Winter tyres produce greater friction than summer/all-season tyres.

Turbocharged cars have more power in the cold, especially ones with intercoolers (i.e. not hdi90). How this affects economy I don't know, but aside from the wet/dry issue, I never noticed any difference when regularly driving my tubby to within 0.05mpg per 500 miles. I did try to keep off the boost as much as possible though. This does support the seasonal diesel blend theory.

Dunno about you, but when not driving for economy I'm a lot heavier footed in the summer, especially when I've got the windows down so I can properly enjoy the (previously V6, now V8) roar

Was there not a theory a while ago. that turbo intercooler cars / trucks are better in damp conditions due to a lower air pressure and cooler ambient air temperature?

GingerMagic wrote:Was there not a theory a while ago. that turbo intercooler cars / trucks are better in damp conditions due to a lower air pressure and cooler ambient air temperature?

It's common knowledge that cold+wet conditions can increase engine peak performance when an intercooler's involved - that's why imprezas (evos too?) can come with factory-fitted jets to spray water on the intercooler. My question is whether it affects efficiency when you're not after peak output

In full power operation, the intercooler lets the turbo cram more air (cooler means denser, denser means more air gets in while the inlet valves are open) into the engine, which lets the ECU put more fuel in to achieve the right fuel/air mixture, which means more power. When you're just trundling along with extra-dense air, surely the car would just let less air in - either through throttle operation in petrols and/or wastegate operation, which my limited diesel knowledge tells me must be the only way the ECU could keep the fuel/air mixture good, the result of which would be... trivial if anything?

mjb wrote:It's common knowledge that cold+wet conditions can increase engine peak performance when an intercooler's involved - that's why imprezas (evos too?) can come with factory-fitted jets to spray water on the intercooler. My question is whether it affects efficiency when you're not after peak output

I think there are two separate effects here - spraying water on the outside of an intercooler increases its cooling performance, the water evaporates removing more heat from the intercooler than air flow alone can achieve. It's only going to make a difference if the intercooler would be too small without the sprays. On the other hand cool, damp conditions increase power output from petrol engines, whether N/A or not. Most of us spent our youth thrashing underpowered petrol cars and recognise they went better on damp miserable days. (Not sure about diesels though).

In full power operation, the intercooler lets the turbo cram more air (cooler means denser, denser means more air gets in while the inlet valves are open) into the engine, which lets the ECU put more fuel in to achieve the right fuel/air mixture, which means more power. When you're just trundling along with extra-dense air, surely the car would just let less air in - either through throttle operation in petrols and/or wastegate operation, which my limited diesel knowledge tells me must be the only way the ECU could keep the fuel/air mixture good, the result of which would be... trivial if anything?

I would agree with the above, but qualify that with the observation that engine management systems measure volumetric air flow and (sometimes) charge temperature. They cannot measure density, therefore it's difficult to see how the ecu could take advantage of higher air density.

Diesels usually run very weak mixtures, particularly when lightly loaded, so there tends to be enough oxygen. I was wondering if the air filter would restict flow more when air with high water vapour content was being drawn it. Clearly this would result in lower MAF reading, less fuel injected, less power, so more accellerator required for a given speed. But does all this affect efficiency? Or is it the water content reducing combustion chamber temperature, which could affect thermodynamic efficiency?

Intercooled diesels can be affected by over-cooling of the charge air, the air being pushed into the cylinders at partial loads can be too cool and 'quench' the burn.....this in turn makes the driver squeeze the throttle more and for longer.

There's supposed to be an ideal charge temp of about 15 deg C from memory but it is very often less than this. In Canada drivers of big rigs will by-pass or block some of the IC to get 'hotter' charges, the principle is the same for cars really.

Petrol engines like cold intakes because the fuel/air gets forcefully ignited it can benefit from the additional Oxygen present, a Diesel engine's ignition process is 'frustrated' by cold temps and doesn't occur as readily.

A lot of what I said above came from a very long chat with a Diesel 'Tuner' at the NEC Jap Show a few years back.

Doggy wrote:On the other hand cool, damp conditions increase power output from petrol engines, whether N/A or not. Most of us spent our youth thrashing underpowered petrol cars and recognise they went better on damp miserable days.

Sure it's not just because it's easier to break traction in the wet? Can't say I've noticed any effect on my not so underpowered cars

I would agree with the above, but qualify that with the observation that engine management systems measure volumetric air flow and (sometimes) charge temperature. They cannot measure density, therefore it's difficult to see how the ecu could take advantage of higher air density.

O2 sensor results in the ECU being able to calculate whether it's running rich or lean. It knows with reasonably high precision how much fuel's going in, and has the map sensor telling it how much volume is going in, so it should be able to make a good estimate at the density

HDi's don't have oxygen sensors, it's a gasoline thang.

I have noticed a considerable difference with my 2.0. hdi depending on the ambient temperature. We went to the north of Scotland a couple years ago and on the way up it was almost freezing, the average consumption for journey up was around 60 mpg, I must add here I do drive very gently and rarely over 60 mph or 2000 rpm.

Halfway through the week the temperature suddenly shot up to 25C and stayed there for the rest of our trip, on the journey home we averaged 67.8 mpg for 650 miles. While I realise that the figures displayed were probably not accurate I can confirm that we got over 1000 miles from a tankful.

I also noticed the first few miles from a cold start returned around 50 mpg but when hot improved to about 60mpg.

I think there are a number of factors involved, the engine of course is more efficient when warm, with higher ambient temperature the rolling resistance is less as the oil and grease thins, something that I saw mentioned recently that I hadn't considered before is that the air is less dense when warm so reducing the wind resistance.

Peter

Funnily enough, on the way home from work today I was getting much better MPG.....!
Bring on the summer

I definitely noticed an improvement today too, but hoped it was because of some maintenance to my vacuum system over the weekend. Now I'm not so sure.

Summer doesn't do any harm, that's for sure.

Doggy wrote:I definitely noticed an improvement today too, but hoped it was because of some maintenance to my vacuum system over the weekend. Now I'm not so sure.

What did you do to it? Just out of interest

Welly wrote:Intercooled diesels can be affected by over-cooling of the charge air, the air being pushed into the cylinders at partial loads can be too cool and 'quench' the burn.....this in turn makes the driver squeeze the throttle more and for longer.

Is this an argument in favour of the intercooler bypass air doser?

rwb wrote:Is this an argument in favour of the intercooler bypass air doser?

I was thinking of rigging up a manual control for the throttle butterflies to test that theory, (hence my question about i-pad OBD dashboard apps a while back).

I'll have to decide soon whether to bother or just remove remove the bypass in the interests of fewer potential boost leaks and maybe better reaponse.
(It's geting really uncomfortable sitting on this fence).

PeterN wrote:I have noticed a considerable difference with my 2.0. hdi depending on the ambient temperature. We went to the north of Scotland a couple years ago and on the way up it was almost freezing, the average consumption for journey up was around 60 mpg, I must add here I do drive very gently and rarely over 60 mph or 2000 rpm.

Motorways...., empty north of Scotland roads.... steady speeds, featherlight touch on the throttle.., coasting downhills.. ?

One night about 35 years ago, on a dark winter evening, nothing on the telly, at a complete loose end....
I got into my Triumph TR5PI and drove from Cheltenham down the M5 to Gordano Services at Bristol, turned round and came back.
All at 50mph.

I managed almost 30mpg.

I promised myself that I would never, ever again treat a fast car in such an irresponsible manner.