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bobgolden
Member Since: 16 Oct 2010
Location: South Yorkshire
Posts: 907
  
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| Fuel Economy - What Does It Mean ? |
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I have been wondering about my fuel economy for a while now - haven't we all 
My normal commute is 10 miles dual carriageway and M1 followed by a 5 mile crawl into the city. Usually takes 40 mins but is much worse on the return journey. During the winter, the trip was indicating around 22mpg average over a tankful. This has crept up slowly over the last month with an indicated 24mpg (actual was 21 when I did the maths).
Anyway, my question is whether the car burns fuel less efficiently or is it just that it doesn't get chance to convert the fuel to energy efficiently ? In other words is it simply clogging up the engine with unburnt fuel or just clogging up S Yorks with a thin coating of soot 
It's a MY11 D4 with almost 7k miles so it may be run in soon. Any views if sport mode may be more efficient ? 2010 MX-5 2.0 Sport Tech PRHT
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Gone Disco 4 HSE SD V6
Gone 2008 Jaguar XF 2.7TD Premium Luxury
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 23rd Mar 2012 9:31 pm |
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DSL
Keeper of the wheelie bin
Member Since: 11 May 2006
Location: Off again! :-)
Posts: 72790
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Consumption usually improves by 10 - 20% as things warm up compared with winter. Contrary to normal logic these cars do not run more efficiently in the cold.
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| 23rd Mar 2012 9:42 pm |
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Zapp79
Member Since: 25 Jan 2012
Location: Staffordshire
Posts: 129
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Sport mode will not be more fuel efficient, as it will hold gears for longer.
Not sure what you are asking - why does your 3 litre V 6 diesel engine burn a lot of fuel when shifting 2.5 tonnes of metal? Or, why were you using more fuel when it was in mid winter?
Don't forget, in colder weather, you will have the heating on, heated seats, heated screen etc.... all this uses electricity and puts added load onto the engine. More load = more diesel burnt.
Conventional thought is that engines do produce more power when fed with cold (denser) air..... but these high tech diesels are so efficient anyway that this doesn't make much difference and is insignificant when compared to the excesses of stop/start driving, air con pumps and a car full of high drain electrical stuff.
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| 23rd Mar 2012 9:47 pm |
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Hairy Dan
Member Since: 19 Jan 2011
Location: Co. Durham
Posts: 12319
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Also in the colder weather the Fuel Burning Heater will be used more and this sort of burns fuel Cheers Ian
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Club N.E.R.D.S
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| 23rd Mar 2012 10:07 pm |
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Zapp79
Member Since: 25 Jan 2012
Location: Staffordshire
Posts: 129
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I forgot about the FBH... it is ideal in that you can put it on timer for those times when you really need to burn a bit more fuel but can't make it to the vehicle.
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| 23rd Mar 2012 10:15 pm |
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APL
Member Since: 14 Sep 2011
Location: Europe
Posts: 1130
 
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Sadly, Discoverys are not ideally suited to short commuting journeys from a fuel economy point of view.
However, on a warm day, and with a warmed engine, and a long drive, you'd be surprised what economy figures you can achieve!
I actually managed to get mine to read an average of 6.9 litres/100km this week on a 600km round trip up to Northern Ireland........partially dual carriageway and motorway but mostly good N roads (A road equivalent). 2016 Discovery 4 Commercial
2019 Discovery 5 HSE
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| 23rd Mar 2012 11:01 pm |
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bobgolden
Member Since: 16 Oct 2010
Location: South Yorkshire
Posts: 907
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Thanks for the useful replies.
I guess what I'm curious about is whether I'm gunking up the engine with the low fuel economy or if the engine burns the fuel efficiently all the time so this doesn't happen. I usually take it for a, ahem, spirited run at weekends so (touch wood) no DPF issues.
On a decent run to LHR with a full car of luggage and keeping to around 70mph, I managed about 35mpg indicated - just over 30mpg actual. If I drive like Miss Daisy, it can easily show averages of 37mpg - but I think I only managed to do that for about 2-3 gallons worth ! 2010 MX-5 2.0 Sport Tech PRHT
2019 Suzuki Vitara AllGrip
Gone BMW X5 xDrive 30d M Sport (F15)
Gone Disco 4 HSE SD V6
Gone 2008 Jaguar XF 2.7TD Premium Luxury
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| 24th Mar 2012 1:36 pm |
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Zapp79
Member Since: 25 Jan 2012
Location: Staffordshire
Posts: 129
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These engines are full of electronics to make sure that they burn the optimum fuel/air mixture all the time. Sometimes this means that it runs a richer mixture, sometimes it will be a leaner mixture. Nothing the regular Joe can do to control that.
But what you can control is your driving habits. If all you do is short trips, using very little throttle and with the engine cold, then yes, you will be filling your DPF quickly and could be coking the engine. If left to carry on this could reduce fuel efficiency and even the life of the engine.
You don't have to drive like a boy racer though, it is sufficient to give it a good run every now and then - gets the engine nice and hot and burns off / blows the crud out the back. (The owners manual says you should do exactly this to empty out the DPF). Sounds like you got it covered with your weekend blast!
Ordinarily not a problem but it could give you some issues at MOT time (emissions test) if your exhaust system is full of crud. To help with this, I always go for a good drive a few days before an MOT, keeping the gears a bit lower than normal to keep the revs up. 
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| 24th Mar 2012 2:35 pm |
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MadJax
Member Since: 30 May 2011
Location: Adelaide
Posts: 120
 
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Might sound like a silly question, but what is a DPF and how would I know if I have one  Discovery 4 2.7 SE
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| 27th Mar 2012 6:27 am |
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buellbeast
Member Since: 30 Jul 2008
Location: Exile
Posts: 1885
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On a D4 if you have a DPF you'll have an extra exhaust box near the compressor underneath. Also if your chassis numbers eighth digit is a G then this denotes DPF fitted at factory. Club Forum Criminal !!!
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| 27th Mar 2012 6:55 am |
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