A question on Fuel Economy

My 3.0i does around 40-42Mpg at a constant 60mph. I can provide a slightly more scientific response, in order of significance...

2 extra cylinders and 50-100% extra displacement is extra surface area in which the engine can loose heat via absorbtion. ~70-80% of burned fuel is lost as heat energy so thefore the Z4 can loose more.

A Naturally Aspirated Petrol engine requests power according to its throttle position. When youre just cruising using a small amount of available power, a small amount of throttle is also required (3.0l Z4). Throttling the engine causes a large amount of pumping loss as the engine is trying to 'breathe' against a mostly closed plate. In a smaller engine (Focus), it will be working harder, and that plate will be more open allowing it to breathe easier. Engines are most efficient at around 80% available power - the Focus will be MUCH closer to this magic figure.

Z4 has bigger tyres, incurring much higher rolling resistance (less effect at higher speeds as air resistance increases)

Z4 has a prop shaft powering rear wheels meaning power needs to be transferred transversely, incurring a slight energy loss also.
 
My Focus ST returned about 23/24 on a good day, 19/20 most of the time. On two separate trips to Scotland in my 3.0, 500 miles there, 500 miles back plus a 300 mile proper hoon in the middle, I got 32/33 with a computed average speed of 77mph...
 
It depends how you drive the car if your smooth with your right foot it makes a huge difference, depends on your route, how far etc etc

But sounds like you need to have the car looked at..

Ive averaged 26.6mpg in the Z4M over the last 3.5k. Saying that the A69 from Carlisle to Newc in rush hour the other week sat behind some car at 50mph the whole way and got 38mpg out of the M….

Doing 80 instead of 70 uses 15-20% more fuel.
 
I've found the 3.0si very sensitive to throttle use - a steady light throttle is needed to get anywhere near 30+mpg - as soon as you start to use the performance it dips immediately and significantly. For a long transit it is useful to have the ability to reach those high mpg though it does takes some discipline!

:driving: :thumbsup:
 
indizee said:
Hadies said:
I completely understand that I'm comparing a big engine to a small one, and when I put my foot down my car is capable of showing a LOT more fuel through the engine and accelerating the car much quicker.
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It's because of this. You're comparing a big engine to a small one. Bigger cylinders, more of them, more power.

It's like asking why isn't the fuel consumption the same at idle?
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:rofl: budfox is just winding you up. im having a hoot reading this :D
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It's not a wind-up at all. It's a serious question.

Petrol has a calorific value. We explode it and it releases a certain amount of energy that forces pistons to move, developing the power in the engine. My original question, asked why my car would use quite a lot more fuel than a Focus. They weigh they same, are probably not much different aerodynamically, and therefore should need about the same amount of power to move them at a constant speed.

Now I know there's a question of engine efficiency (wasted fuel, friction on parts etc.) but overall I see little difference between moving a Focus and moving a Z4 at the same speed.

It looks like the next poster in the thread has explained some of this. I shall take a look.
 
EdButler said:
My 3.0i does around 40-42Mpg at a constant 60mph. I can provide a slightly more scientific response, in order of significance...

2 extra cylinders and 50-100% extra displacement is extra surface area in which the engine can loose heat via absorbtion. ~70-80% of burned fuel is lost as heat energy so thefore the Z4 can loose more.

A Naturally Aspirated Petrol engine requests power according to its throttle position. When youre just cruising using a small amount of available power, a small amount of throttle is also required (3.0l Z4). Throttling the engine causes a large amount of pumping loss as the engine is trying to 'breathe' against a mostly closed plate. In a smaller engine (Focus), it will be working harder, and that plate will be more open allowing it to breathe easier. Engines are most efficient at around 80% available power - the Focus will be MUCH closer to this magic figure.

Z4 has bigger tyres, incurring much higher rolling resistance (less effect at higher speeds as air resistance increases)

Z4 has a prop shaft powering rear wheels meaning power needs to be transferred transversely, incurring a slight energy loss also.
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Now THAT is the sort of answer I was looking for. Very interesting that so much fuel is wasted in generating heat. (I got the rolling resistance thing, and that was one of my original assumptions. I was wondering where else the extra fuel went).

I'm glad I've had an answer or two, and also that people can perhaps see that it wasn't such a stupid question.

Cheers all.
 
Lots of people are freaking me out. ive got a 2003 3.0i. Ive only done one long trip. I got ~32 mpg (taking the mileage and divide by the fuel added after). The dash was reading ~31 mpg if i remember correctly. No Cruise cntrl. How are you measuring you MPG?

For combined, i get like 23 mpg. These sound kind of low compared to the numbers other people are throwing out there.
 
JohnnyC said:
For combined, i get like 23 mpg. These sound kind of low compared to the numbers other people are throwing out there.
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Don't forget a US gallon is smaller, by approximately 20%, than than UK Imperial gallon

The U.S. liquid gallon and the U.S. dry gallon are different units defined by different means. The U.S. liquid gallon is defined as 231 cubic inches and equates to approximately 3.785 litres. One imperial gallon is equivalent to approximately 1.2 U.S. liquid gallons.
 
Drove back from Duxford to Loughborough at a steady 60 today and the computer told me I got 57MPG. I reset it before setting off. Never driven it quite that slowly before but nice to know it can be economical if I want it to :thumbsup: I guess dropping those "all important" 2 cylinders was a good move afterall
 
stlymch said:
Drove back from Duxford to Loughborough at a steady 60 today and the computer told me I got 57MPG. I reset it before setting off. Never driven it quite that slowly before but nice to know it can be economical if I want it to :thumbsup: I guess dropping those "all important" 2 cylinders was a good move afterall
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I drove back from Duxford also today (live in Suffolk) and as the weather was so good I thought make the most of it with the roof down and cruise home at 60 mph almost all the way and my trip computer showed a figure go 31.7 mpg which earlier in the day in the convoy was down to 18.5 (just to catch up with the lead pack)
 
At consatant Motorway bimble at 55-65 mph Our MR does 32-34 mpg. A 3.0 should be bettr that that.
 
Just a word of caution, don't take the mfi display information as gospel, always do full tank to tank full measurement. The on board display can be grossly inaccurate, for example, if you have dirty air filter, air mass meters are also often knackered after 20K miles, the system uses a heated wire, which is cooled by increasing airflow changing resistance. When the wire gets coated in oxidant crud, it outputs spurious values, affecting the MPG displayed. The older the car/the higher the mileage, the less reliable the value displayed, in general.
 
A V12 of the same capacity will use more fuel than a straight 6. Same applies for a straight 6 versus 4 pot.

If you have 2 extra cylinders firing for each revolution of the crank, it figures that you will burn more fuel
 
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