Author Topic: New owners.  (Read 7524 times)

Jocko

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Re: New owners.
« Reply #15 on: June 19, 2018, 01:18:17 PM »
If possible I always drive at the speed limit (ish) road conditions permitting. Every car I have owned or driven diesel or petrol powered has used less fuel on a motorway or long journey than when used in urban conditions.
I find my Mk 1 Jazz is the opposite. I can get better mpg driving in town, and on country roads, than I can on motorway driving. On the motorway/dual carriageway I get better mpg at 60 mph than I do at 70 mph.

andruec

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Re: New owners.
« Reply #16 on: June 19, 2018, 01:41:20 PM »
If possible I always drive at the speed limit (ish) road conditions permitting. Every car I have owned or driven diesel or petrol powered has used less fuel on a motorway or long journey than when used in urban conditions.
Oh definitely. Measured pump to pump I'm currently hitting around 54mpg on my 80% rural, 20% urban commute. I think my Mk2 used to be around 51mpg on more or less the same route.

So there is improved consumption but it's not a huge amount. It might be that my tendency to kick the engine out of Atkinson mode when leaving roundabouts means I'm not seeing all the benefits it has to offer but I don't like dithering around.
« Last Edit: June 19, 2018, 01:53:01 PM by andruec »

andruec

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Re: New owners.
« Reply #17 on: June 19, 2018, 01:51:27 PM »
I disagree. An engine is lightly loaded cruising at motorway speeds. It is acceleration, shifting a lump of metal and its contents up to speed that uses fuel. My MT 1.3 engine ran at about 2,500 revs at 60 mph and 3,000 revs at 70 mph. My 1.5 CVT engine runs at about 2,000 revs at 60 mph and 2,500 revs at 70 mph although this varies with gradient.. Cars use less fuel on motorways because there is no acceleration from a standstill and the engine is lightly loaded.
They use less than they would around town, true. However drag squares as speed doubles so there is roughly 30% more drag at 70mph than at 60mph. At 80mph the car is having to fight 60% more drag than it would at 60mph. And at least at 60mph the Jazz can operate its engine in the efficient Atkinson cycle mode.

Skyrider

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Re: New owners.
« Reply #18 on: June 19, 2018, 02:47:50 PM »
My 1.5 uses a little more fuel than my 1.3 did but it is worth every penny!
« Last Edit: June 19, 2018, 03:47:06 PM by Skyrider »

Skyrider

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Re: New owners.
« Reply #19 on: June 20, 2018, 09:00:17 AM »
I disagree. An engine is lightly loaded cruising at motorway speeds. It is acceleration, shifting a lump of metal and its contents up to speed that uses fuel. My MT 1.3 engine ran at about 2,500 revs at 60 mph and 3,000 revs at 70 mph. My 1.5 CVT engine runs at about 2,000 revs at 60 mph and 2,500 revs at 70 mph although this varies with gradient.. Cars use less fuel on motorways because there is no acceleration from a standstill and the engine is lightly loaded.
They use less than they would around town, true. However drag squares as speed doubles so there is roughly 30% more drag at 70mph than at 60mph. At 80mph the car is having to fight 60% more drag than it would at 60mph. And at least at 60mph the Jazz can operate its engine in the efficient Atkinson cycle mode.

So assuming the engine I using almost 100% of its power to cruise at 110 mph (which it can) what proportion of its power will it be using cruising at 60 mph? Would it be relatively lightly loaded at this speed or even 70 mph compared to 110 mph or accelerating from 0 to 60 on a wide open throttle?

You have to think about an engines capabilities, not how people use it. Most drivers never heavily load their engine even though it is designed to withstand it.
« Last Edit: June 20, 2018, 09:06:58 AM by Skyrider »

Jocko

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Re: New owners.
« Reply #20 on: June 20, 2018, 09:08:20 AM »
It would require something like a quarter of the power to overcome drag at 60 mph as against 110 mph. it would still incur similar friction losses and such, irrespective of the speed (slightly less at lower speed).

Skyrider

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Re: New owners.
« Reply #21 on: June 20, 2018, 09:25:59 AM »
It would require something like a quarter of the power to overcome drag at 60 mph as against 110 mph. it would still incur similar friction losses and such, irrespective of the speed (slightly less at lower speed).

Exactly, at 60 / 70 mph cruising it is lightly loaded.

barcam

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Re: New owners.
« Reply #22 on: June 20, 2018, 02:45:26 PM »
My 1.5 uses a little more fuel than my 1.3 did but it is worth every penny!

I second that, I love the extra power of the 1.5 engine for filtering into traffic lanes on the motorway and pulling away on roundabouts. Not as economical as the 1.3 but pretty good for a petrol engine.

andruec

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Re: New owners.
« Reply #23 on: June 20, 2018, 04:30:41 PM »
I disagree. An engine is lightly loaded cruising at motorway speeds. It is acceleration, shifting a lump of metal and its contents up to speed that uses fuel. My MT 1.3 engine ran at about 2,500 revs at 60 mph and 3,000 revs at 70 mph. My 1.5 CVT engine runs at about 2,000 revs at 60 mph and 2,500 revs at 70 mph although this varies with gradient.. Cars use less fuel on motorways because there is no acceleration from a standstill and the engine is lightly loaded.
They use less than they would around town, true. However drag squares as speed doubles so there is roughly 30% more drag at 70mph than at 60mph. At 80mph the car is having to fight 60% more drag than it would at 60mph. And at least at 60mph the Jazz can operate its engine in the efficient Atkinson cycle mode.

So assuming the engine I using almost 100% of its power to cruise at 110 mph (which it can) what proportion of its power will it be using cruising at 60 mph? Would it be relatively lightly loaded at this speed or even 70 mph compared to 110 mph or accelerating from 0 to 60 on a wide open throttle?

You have to think about an engines capabilities, not how people use it. Most drivers never heavily load their engine even though it is designed to withstand it.
I think you're over complicating this and possibly just failed to understand my original post. A summary of what I wrote was that in order of 'least fuel consumed per mile' travelled we have:

* Steady 60mph.
* Steady 70mph.
* Urban driving.

You're not going to find anyone that would disagree with that statement. The extent to which those three differ in 'fuel consumed per mile driven' will vary by vehicle but very few vehicles (probably none at all) will break that pattern. I honestly don't know why you're trying to argue with me because you seem to have made up something to object to that I never posted.

At no point have I suggested that motorway driving at any steady normal speed is less efficient than urban driving. All I've ever said is that 70mph uses more fuel than 60mph.
« Last Edit: June 20, 2018, 04:41:14 PM by andruec »

Skyrider

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Re: New owners.
« Reply #24 on: June 20, 2018, 05:24:09 PM »
I must have picked up on your statement,

"At normal motorway speeds the engine has to do a lot of work and that means it has to consume a lot of fuel."

Which I do not agree with.

We all use cars, they use fuel, how much only matters to the driver, or not in my case. As long as my car is in the ballpark for normal use I am happy, if not I will get it fixed. To date my car has varied between 37 mpg and 45 mpg depending on use. I have not done a long journey yet but will be doing an 800 mile round trip in a couple of weeks that will give an indication of motorway consumption.

Jocko

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Re: New owners.
« Reply #25 on: June 20, 2018, 06:00:49 PM »
You're not going to find anyone that would disagree with that statement.
I would disagree with that. My car gives better urban mpg than it does at 70 mph. 60 gives about the same as urban.

Downsizer

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Re: New owners.
« Reply #26 on: June 20, 2018, 07:16:15 PM »
You're not going to find anyone that would disagree with that statement.
I would disagree with that. My car gives better urban mpg than it does at 70 mph. 60 gives about the same as urban.
Clearly, urban driving varies enormously from time to time and place to place.  The official urban test cycle is the only reproducible benchmark.

Skyrider

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Re: New owners.
« Reply #27 on: June 20, 2018, 07:23:45 PM »
 I know where Jocko lives, his local urban is nothing like big city urban. It would be many drivers idea of driving bliss. :-)

Edit -

When my London resident BiL is here he jokes that if we see half a dozen cars at once we think it is a traffic jam. It is not that good but you get the drift.
« Last Edit: June 20, 2018, 07:33:46 PM by Skyrider »

Jocko

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Re: New owners.
« Reply #28 on: June 20, 2018, 09:13:13 PM »
I know where Jocko lives, his local urban is nothing like big city urban.
I can also get good mpg driving in Edinburgh. I think it is because almost the entire city is 20 mph zones Just stick it in 4th and tootle about.

Skyrider

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Re: New owners.
« Reply #29 on: June 20, 2018, 09:26:41 PM »
I was on the Edinburgh city bypass this afternoon, speed limit 70 mph,  actual speed about 10 mph! The KY postcode areas are usually quiet for traffic.

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