Author Topic: Change in the weather.  (Read 345 times)

Jocko

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Change in the weather.
« on: June 16, 2018, 05:49:51 PM »
We all know that fuel consumption is poorer in the winter than the summer, but after enjoying several weeks of warm, dry, early summer weather, this past week has seen temperatures here down by 10C, with rain the past two days. What I have found is that the car is not getting anything like the mpg it was getting (going by ScanGauge E) when the temperatures were higher. My coasting points are all needing re-calibrated, as the car doesn't coast as freely (either in or out of gear) as it did when the weather was warmer. The joys of a Scottish summer!

peteo48

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Re: Change in the weather.
« Reply #1 on: June 16, 2018, 10:23:41 PM »
Of course the other factor, if it's raining, is the extra drag caused by water as your tyres have to push it out of the way.

Jocko

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Re: Change in the weather.
« Reply #2 on: June 16, 2018, 10:41:52 PM »
Yes, but that was only a light shower yesterday and rain today. As soon as the temperature dropped, a few days back, the coasting and with it the mpg dropped too. The rain just exacerbated the whole thing!

John Ratsey

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Re: Change in the weather.
« Reply #3 on: June 18, 2018, 10:10:33 PM »
I wonder if the underlying cause of worse mpg in cold weather is that car's mechanical systems takes longer to warm up when the temperature is cooler. It takes several miles on a warm day and a lot further on a cold day before the mpg reaches its potential. This effect is much more noticeable on short trips than long ones.  I wonder how much of the warm-up delay could be removed by a few engineering improvements such as a thermostatically controlled blind on the radiator grille to get the engine compartment up to optimum temperature as quickly as possible. Improving the warming up other parts of the drive system such as the wheel bearings may be more difficult. The standard fuel economy tests need to include starting from cold in, say, 5C temperature.

Jocko

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Re: Change in the weather.
« Reply #4 on: June 19, 2018, 06:00:42 AM »
What surprised me was that with an air temp change of probably less than 10C (no more than 22C in the sunshine to 12-14C now) made such a noticeable difference. And we are talking about an overnight change. You wouldn't think the countryside and roads would lose temperature so quickly. The car is garaged when it is not being driven, so it probably experiences an even smaller change in temperature. But still enough for the ScanGauge E (and my coasting experiences) to register a difference.
I think the biggest difference must be in the temperature of the Tarmac on the road surface, and the effect it has on the tyres. It effects F1 racing tyres, so must do the same for road tyres.

Barcam

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Re: Change in the weather.
« Reply #5 on: June 19, 2018, 08:34:34 AM »
If you do lots of short journeys then the engine takes longer to warm up and this makes difference.

culzean

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Re: Change in the weather.
« Reply #6 on: June 19, 2018, 09:25:18 AM »
I wonder if the underlying cause of worse mpg in cold weather is that car's mechanical systems takes longer to warm up when the temperature is cooler. It takes several miles on a warm day and a lot further on a cold day before the mpg reaches its potential. This effect is much more noticeable on short trips than long ones.  I wonder how much of the warm-up delay could be removed by a few engineering improvements such as a thermostatically controlled blind on the radiator grille to get the engine compartment up to optimum temperature as quickly as possible. Improving the warming up other parts of the drive system such as the wheel bearings may be more difficult. The standard fuel economy tests need to include starting from cold in, say, 5C temperature.

For the first couple of miles after a cold start every car does less than 25mpg,  the injectors have to squirt in more fuel (used to be called a choke) to keep the colder engine working properly.  Honda did look at the wheel bearings in MK1 and the result of using lower friction deep groove ball bearings instead of the tried and tested tape roller bearings was that they regularly failed before 40K.  The low viscosity gearbox oils (no thicker than engine oil) now used improve MPG due to lack of churning losses in gearbox and differential. I am sure tyre rolling resistance is worse when they are cold as well, when rubber is less flexible.
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

Jocko

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Re: Change in the weather.
« Reply #7 on: June 19, 2018, 10:08:38 AM »
If you do lots of short journeys then the engine takes longer to warm up and this makes difference.
I do two short journeys every week day, each 3.6 miles. Last week when it was 20C I was getting 60 mpg average (as indicated on my ScanGauge), this week at 14C I am lucky to average 55 mpg. The big thing I notice most though, is the point where I push the clutch in, to coast to a junction. I have to leave it a little nearer the junction, now the temperature has fallen. If I use the same points I used when it was warmer, I have to let the clutch out before I reach the junction and give it a little more throttle. I do a mix of coasting (always with the engine on) and DFCO to get the best fuel consumption I can. I am getting pretty good at it, but it takes concentration to get the best out of the car.

John Ratsey

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Re: Change in the weather.
« Reply #8 on: June 19, 2018, 10:34:00 PM »
I have to wonder whether the optimum fuel-air mixture for ambient temperatures above around 20C (the temperature range used for the standard fuel efficiency testing) has been carefully determined and programmed into the ECU but there's been less (no?) optimisation at lower temperatures and the mixture conservatively left a bit rich, at least when the engine isn't up to the optimum temperature, as the fuel consumption under these conditions doesn't affect the statistics. Manufacturers need to be pushed into minimising the warm-up period as not only does it adversely affect the real-life fuel economy but also the richer mixture is likely to result in a higher concentration of nasties coming out of the exhaust pipe.

Jocko

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Re: Change in the weather.
« Reply #9 on: June 20, 2018, 06:10:27 AM »
The ECU uses both the Engine coolant temperature, and Intake air temperature, as part of the process, and both are displayed by the ScanGauge E. But neither have any effect on coasting. That comes down to other things, mainly rubber on the road. Yesterday, it was a few degrees warmer here in Fife, and the coasting abilities were back up nearer last week's experience.

Jocko

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Re: Change in the weather.
« Reply #10 on: June 20, 2018, 06:59:31 AM »
It is lashing rain this morning so I decided to monitor the engine coolant temperature as I took Mrs Jocko the 1.8 miles to her work. On getting into the car, in the garage, the coolant temperature was 18C. By the time I reached the hospital it was 69C and as I drove home, the reverse of the route, it climbed to 82C. The normal steady operating temperature is 84C, only going above that when stuck in traffic in hot weather. According to the ScanGauge, I averaged 58.6 mpg for the journey. The benefit of starting work at 07:00 is that traffic is light. It is a different story when I pick her up at 11:00.

John Ratsey

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Re: Change in the weather.
« Reply #11 on: June 21, 2018, 09:36:19 PM »
Another potentially relevant thought is that warm weather tends to be associated with little or no wind. While a tail wind helps the mpg, on a round trip the overall effect of the wind is to reduce mpg as the increase in the aerodynamic drag with a head wind is greater than the decrease with a tail wind. And cross winds create turbulance and extra drag.

Nonetheless, I'm impressed by the mpg you are getting in urban conditions. Perhaps the garage and the smaller engine (faster to warm up) both help. A clear road and knowing where it's possible to coast to a halt helps a lot. Unpredictable roundabouts and traffic lights are bad for fuel economy.

Jocko

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Re: Change in the weather.
« Reply #12 on: June 22, 2018, 06:05:09 AM »
Knowing your route and light urban traffic is the single biggest aid to good mpg. Switching off at long lights also makes a big difference. Some you win, and some you lose, but most I win. I believe you have to be switched off for more than 6-7 seconds to benefit.
The wind makes a huge difference. I try to get in behind a big truck, if possible. Not too close (always obey the 2 second rule), but you can see the difference even 2 seconds behind. That is another thing that wet weather effects, as I have to leave a far longer gap.
My wife insists on opening her window, when it is warm, which effects mpg (especially at motorway speeds). Of course I am savvy enough (not brave enough!) to say anything. They reckon that below 50 mph you should open the window and switch off the air con (something I don't have), and above 50 mph do the opposite.

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