The effect of rain on fuel consumption : Mon Mar 05, 2012 7:59 pm  

We went up northland at the weekend and on a steady drive up on Friday morning, the computer showed an average of 52.5 mpg. The return trip on Sunday gave an average of 45.5 mpg.

Both journeys were undertaken at similar speeds and with similar traffic flows, the only difference was the rain on Sunday. In fact until the conditions dried up around Birmingham, the average was down around 41 mpg.

It reminded me of a long-term experiment I was involved in, back in the late 80s, when we were trying to determine the effectiveness of air-management systems (spoilers for want of a better word), when fitted to trucks. For three months we ran two identical tractor/trailer combinations around British roads. The trucks and trailers were identical in power, size and weight, one ran with various different combinations of air-management, the other with none at all. We swapped the drivers around and made sure that they were far enough apart to mitigate any slipstream effect. The best difference we could see was a 4% improvement in fuel economy.

The one thing that did become apparent was in periods of heavy rain, both trucks suffered an increase in fuel consumption of up to 15%. This is down to the amount of tractive effort that is wasted by simply moving water through the tyre treads, instead of propelling the truck on its way.

So, if you want to drive cheap, don't drive in the wet.

Re: The effect of rain on fuel consumption : Mon Mar 05, 2012 8:13 pm  

I'll bear that in mind

The consumption on my Peugeot 307 is pathetic solely because since I got it in October its never done a long run anywhere and has only ever driven to my daughters place of work in the centre of Leeds and to mine just outside of the centre.

The consequences of this is that its 9 miles each way and it takes anything from 30 to 60 minutes to do (each way) with queues being part of at least 50% of the journey.

Average mpg over six months is 27.7, average speed is 14mph, most of the £20 worth of fuel per week is burned straight out of the exhaust while sitting in a queue.

Back to the bike in April, never took more than 20 mins to do the journey to my office.

Re: The effect of rain on fuel consumption : Mon Mar 05, 2012 9:10 pm  

I am getting a healthy 17mpg at the moment .

Never mind the rain, its the school run that is killing me. My 4 mile trip from Wigan into Bolton is 15 minutes tops on a clear day. as soon as the schools are back it takes around 35-40 easy.

Sitting in traffic just kills any attempt at achieving a decent mpg.

Re: The effect of rain on fuel consumption : Mon Mar 05, 2012 10:06 pm  

Mines just massively increased back to around 40mpg from around 20mpg after I had a new water pump fitted.

Re: The effect of rain on fuel consumption : Mon Mar 05, 2012 11:37 pm  

With spiralling fuel prices, could it not therefore make it potentially economic for individuals and companies who clock up lots of miles in vehicles to purchase different sets of tyres to suit predicted conditions.

Even today there is the argument that its probably cheaper over the lifetime of a set of tyres, to pay extra for a good set, rather than buying cheaper ones. Not only will the cheaper ones be less fuel efficient, but they will also be poorer at stopping you.

Re: The effect of rain on fuel consumption : Tue Mar 06, 2012 12:00 am  

I switched from a 9mpg 130mph Subaru to a 60mpg 84mph Citroen for my daily commute . It was like having a huge pay rise . Just glad I no longer have the Subaru or the V12 XJS . Worst car I ever had for fuel consumption was an Escort that did about 3 mpg . It was rather quick though . Back in the olden days ( after the dinosaurs but before mobile phones ) power meant loadsa petrol thrown into the engine . Nowadays you can buy a 0-60 in sub 5 seconds DIESEL car that will return 40mpg .

Re: The effect of rain on fuel consumption : Tue Mar 06, 2012 4:59 am  

Wet weather actually helps 2-stroke engines. They run slightly leaner in it, meaning lower fuel consumption with a greater bang, meaning better power output. It's not a huge difference, but you're aware of it.



Oh, and what's a traffic jam?

Re: The effect of rain on fuel consumption : Tue Mar 06, 2012 5:19 am  

Scooter Nik wrote:

Wet weather actually helps 2-stroke engines. They run slightly leaner in it, meaning lower fuel consumption with a greater bang, meaning better power output. It's not a huge difference, but you're aware of it.

Moist air should improve fuel consumption of any engine, simply due to the fact that it carries more oxygen

Re: The effect of rain on fuel consumption : Tue Mar 06, 2012 11:35 am  

cod'ead wrote: Moist air should improve fuel consumption of any engine, simply due to the fact that it carries more oxygen I am not sure that is true. Don't forget that the water vapour is displacing air for any given volume and hence the oxygen content is lower. http://www.theweatherprediction.com/habyhints/260/ The oxygen levels in the displacing water vapour droplets would have to higher than 21% and that's not the case. http://www.engineeringtoolbox.com/oxyge ... d_841.html Given that a gram mole of a gas occupies 22.4 litres at STP and that oxygen is 21% of air. I think that there is 32/22.4 x 21% = O.30g/l of Oxygen in dry air at STP which equates to 300mg/l, much higher than that in water. I could be wrong.

cod'ead wrote: Moist air should improve fuel consumption of any engine, simply due to the fact that it carries more oxygen I am not sure that is true. Don't forget that the water vapour is displacing air for any given volume and hence the oxygen content is lower. http://www.theweatherprediction.com/habyhints/260/ The oxygen levels in the displacing water vapour droplets would have to higher than 21% and that's not the case. http://www.engineeringtoolbox.com/oxyge ... d_841.html Given that a gram mole of a gas occupies 22.4 litres at STP and that oxygen is 21% of air. I think that there is 32/22.4 x 21% = O.30g/l of Oxygen in dry air at STP which equates to 300mg/l, much higher than that in water. I could be wrong.

Re: The effect of rain on fuel consumption : Tue Mar 06, 2012 12:40 pm  

Stand-Offish wrote: I am not sure that is true. Don't forget that the water vapour is displacing air for any given volume and hence the oxygen content is lower. http://www.theweatherprediction.com/habyhints/260/ The oxygen levels in the displacing water vapour droplets would have to higher than 21% and that's not the case. http://www.engineeringtoolbox.com/oxyge ... d_841.html Given that a gram mole of a gas occupies 22.4 litres at STP and that oxygen is 21% of air. I think that there is 32/22.4 x 21% = O.30g/l of Oxygen in dry air at STP which equates to 300mg/l, much higher than that in water. I could be wrong. However, I'm not sure it's right. My even more scientific calculation is that usually, after it's been raining, the air can be incredibly fresh and satisfing, which I'd always sort of put down to all the invigorated plants including grass and trees going to town with their photosynthesis, and ramping up oxygen output; making oxygen while the rain shines, sort of thing. I know that in good rainy conditions most plants can go absolutely berserk, just like they can close down in a drought. So to me, any calculation needs to start with this: despite the fact that the AVERAGE oxygen content of all air may well be 21%, what is the actual local oxygen percentage in a prolonged spell of good rainy growing weather? I'd bet it would be above average.

Stand-Offish wrote: I am not sure that is true. Don't forget that the water vapour is displacing air for any given volume and hence the oxygen content is lower. http://www.theweatherprediction.com/habyhints/260/ The oxygen levels in the displacing water vapour droplets would have to higher than 21% and that's not the case. http://www.engineeringtoolbox.com/oxyge ... d_841.html Given that a gram mole of a gas occupies 22.4 litres at STP and that oxygen is 21% of air. I think that there is 32/22.4 x 21% = O.30g/l of Oxygen in dry air at STP which equates to 300mg/l, much higher than that in water. I could be wrong. However, I'm not sure it's right. My even more scientific calculation is that usually, after it's been raining, the air can be incredibly fresh and satisfing, which I'd always sort of put down to all the invigorated plants including grass and trees going to town with their photosynthesis, and ramping up oxygen output; making oxygen while the rain shines, sort of thing. I know that in good rainy conditions most plants can go absolutely berserk, just like they can close down in a drought. So to me, any calculation needs to start with this: despite the fact that the AVERAGE oxygen content of all air may well be 21%, what is the actual local oxygen percentage in a prolonged spell of good rainy growing weather? I'd bet it would be above average.