Fuel consumption - your opinions please.

bxbodger · Post by **bxbodger** » 15 Jun 2005, 22:23

<blockquote id="quote">quote:<hr height="1" noshade id="quote">, but I'm just using common sense that dictates to me that you will use a lot more fuel to maintain your speed at 30 in 5th because you will need more throttle action! <hr height="1" noshade id="quote"></blockquote id="quote">
But your revs will be lower, less bangs per minute......
So the AA say big throttle and low revs, and the IAM say light throttle, my brain hurts.....with my N/A diesel, it doesnt seem to matter how I drive it,fast, slow, loaded, empty,consumption stays more or less exactly the same with only a 1 or 2 mpg difference between fills..... unless its got a roofbox on, however, in which case consumption plummets.

PowerLee · Post by **PowerLee** » 15 Jun 2005, 22:24

I always thought it was best to keep the engine rpm between 2K & 4K rpm when driving.
I wont go past 2500rpm untill the water temp is above 70 deg C, Then I try & keep between 2K & 4K rpm, I certainly wouldn't use 5th gear at any time below 50mph & its normally 60mph before I select 5th.
I can get an easy 30 to 35 mpg out of my 2 litre 8 valve petrol 405 & that includes some hard driving & stop start town work.

fastandfurryous · Post by **fastandfurryous** » 15 Jun 2005, 22:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by PowerLee

I always thought it was best to keep the engine rpm between 2K & 4K rpm when driving.
<hr height="1" noshade id="quote"></blockquote id="quote">
Which is of course perfectly in keeping with the high torque areas of most car road engines, which is why this is indeed ideal.
Obviously there are exceptions: My Landrover diesel tops out at 2600rpm, whereas a scooby-doo turbo does bugger-all below about 3500, but for the average petrol engine on the road 2k - 4k is ideal. An XUD will probably prefer something like 1500-3500, but then the peak torque for all engines is different.

fastandfurryous · Post by **fastandfurryous** » 15 Jun 2005, 23:02

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by TomH
but I'm just using common sense that dictates to me that you will use a lot more fuel to maintain your speed at 30 in 5th because you will need more throttle action!
<hr height="1" noshade id="quote"></blockquote id="quote">
There are 2 cosiderations here. The power needed to keep the car travelling at 30mph, and the power needed to overcome pumping losses in the engine. That combined power is generated in your combustion engine.
In 5th gear, you will indeed incurr less pumping losses than in 3rd, as engine speeds will be slower. Unfortunately you are so far away from peak specific fuel consumption at this point, that it takes a fair bit of fuel to generate the power needed.
In 3rd gear, the pumping losses are indeed greater and so you have to generate (a litte) more power, but since you are now much closer to the peak specific fuel consumtion of the engine, this power is generated much more efficiently.
The difference between these 2 situations could be as much as DOUBLE the fuel consumption for being that far from the peak specific fuel consumtion point of the engine. A good diesel engine can achieve (If I remember correctly) 215g/KWH at it's peak torque point, but can be as bad as 400g/KWH when it's being worked at the wrong speed.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">
At motorway speeds or at 30 in 3rd you are at or nearer the efficient level of peak torque, so the car will maintain speed with only light throttle acion...no?
<hr height="1" noshade id="quote"></blockquote id="quote">
Yes, actually! (and that was a far faster explanation than mine)[:D]

Kowalski · Post by **Kowalski** » 16 Jun 2005, 02:20

30 mph in 3rd needs my accelarator to be pressed down significantly, where as in 5th it barely needs to be touched, (this is irrelevant on a diesel because the engine has a governer and the accelarator is requesting a governed speed).
Going back to what I said earlier, "engines are more efficient at full load than at part load" I'm talking about torque output here rather than power, i.e. full throttle at low revs produces the same power as part throttle at higher revs but does it more efficiently. The difference between full and part throttle efficiency is much greater than the difference between being at peak torque (i.e. peak efficiency) and somewhere else in the rev range. Kubota publishes the best graphs for showing this, the specific fuel consumption of an engine doesn't vary very much over its rev range at full throttle.
When you are in a higher gear, the engine has to produce more torque to produce the same power as it would at a higher speed, so it gets closer to its "full load" status and therefore is more efficient.

FrenchLeave · Post by **FrenchLeave** » 16 Jun 2005, 02:28

Just sticking my fourpennyworth in. The power required from your engine is dependent upon three things: engine pumping losses, rolling resistance and drag. Rolling resistance (tyres, bearings, meshing gears etc.) is virtually linear with respect to speed, I suspect pumping losses are probably the same. But drag increases as the square of the speed, which means that the power required to overcome drag increases as the cube of the speed! So when considering the fuel consumpton of your car you have to take into account not just specific fuel consumption but also vehicle speed and gearing.
I'm not sure where that leaves us except I have a mental picture of a power required and power available curve.

fastandfurryous · Post by **fastandfurryous** » 16 Jun 2005, 02:57

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Kowalski

The difference between full and part throttle efficiency is much greater than the difference between being at peak torque (i.e. peak efficiency) and somewhere else in the rev range.<hr height="1" noshade id="quote"></blockquote id="quote">
Sorry, Wrong. Get a fuel map for any road diesel engine and you will see that your statement holds very little truth. Not just a single curve for specific fuel consumption, but a complete fuel map.

Kowalski · Post by **Kowalski** » 16 Jun 2005, 03:21

The energy in fuel that goes into your engine comes out of it again in useful (mechanical drive) and useless (waste heat) energy. At idle, an engine can produce no useful energy, i.e. 0% efficiency.
Once you open the throttle slightly, you are getting a small amount of useful work out of the engine but you are still using the basic amount of energy it takes to make the engine idle, i.e. at small loads the useless energy is significantly larger than the useful energy. The bigger the load the more significant the useful energy becomes compared to the waste energy.
http://www.kubotaengine.com/curves/V3300-T-E.htm
Have a look at the specific fuel consumption curve, you'll notice it varies by less than 10% over the complete rev range.
http://www.kubotaengine.com/kubotaperformance.htm
There are a load of graphs for other engines showing the same sort of pattern.

fastandfurryous · Post by **fastandfurryous** » 16 Jun 2005, 04:13

As I said earlier, in different words, the energy required to make the engine idle (ie the pumping losses) are far smaller than the difference in effeciency between being at the best specific fuel consumption RPM and not.
The graph that kubota show for specific fuel consumption is averaged. It is not a full fuel map for the engine. You are also looking at an engine that has a very limited RPM range.
You will also notice that the curve is rising at 2600rpm. Translated on to a road engine that has a rev range from 1000-5000 rpm rather than 1400-2600 as that one does, the variance in specific fuel consumption at full load is far greater (over 50% in most cases, and possibly as high as 100% variance on some poorly designed engines).
The only thing that the curve you have highlighted shows is that specific fuel consumption does indeed increase at engine speeds below peak torque, so getting back to the original point, doing 30 in 5th is at a worse specific fuel consumption than doing 30 in 3rd, especially as 30mph in 5th is probably about 1000rpm, and so WAY off the best Specific fuel consumption speed.
Also, putting on a different hat: as an IAM driver, doing 30mph in 5th is downright stupid, as it gives you very little control. You have no power available compared to being in 3rd, and engine braking is severely limited. You should be in 3rd, which would be about 2000rpm. Much safer.

Kowalski · Post by **Kowalski** » 16 Jun 2005, 05:20

What is relavent about those graphs is that they show specific fuel consumption figures for 1000rpm and 2000rpm on a diesel engine. Why are these two RPM figures significant? They're significant because at 30 mph in 3rd and 5th the engine is doing about 2000rpm and 1000rpm respectively. The variation is less than 10% over that range.
I'd agree that if you were using full throttle 30mph in 3rd would be more efficient (it would also be using much more fuel), but generally when I'm cruising at 30mph I don't need anything like full throttle, very little throttle in fact. This is an extremely light load and because its a very light load the pumping losses are large compared to the useful work being done. At peak efficiency a typical car diesel engine can be 40% efficient and this is nothing like peak efficiency.
I don't depend on 3rd gear in a 30 zone for my safety.

fastandfurryous · Post by **fastandfurryous** » 16 Jun 2005, 05:50

I am fed up with having these arguments with you when you clearly don't have a full understanding of what you are talking about, and seem to make the same point over and over again using different words.
Go away, find a FULL FUEL MAP for a diesel engine, understand it, and then realise the errors of your arguments.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Kowalski

I don't depend on 3rd gear in a 30 zone for my safety.
<hr height="1" noshade id="quote"></blockquote id="quote">
You disagree with the IAM and the recomendations of police drivers then? well, you're entitled to that I guess.

Kowalski · Post by **Kowalski** » 16 Jun 2005, 15:22

I'm still to be convinced. Where would one obtain "a FULL FUEL MAP for a diesel engine"?
The IAM's advice isn't completely consistent they are the ones who give the “brakes to slow – gears to go" advice aren't they?

Homer · Post by **Homer** » 16 Jun 2005, 22:29

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Kowalski

The IAM's advice isn't completely consistent they are the ones who give the “brakes to slow – gears to go" advice aren't they?
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Yes, correctly.
If you would like to discuss driving technique then you are more than welcome at the www.safespeed.org forums. IAM "best practice" is a bit of a current topic over there.

Paulmi16 · Post by **Paulmi16** » 17 Jun 2005, 14:23

If the dealer is saying its perfectly normal, then tell them you want another one for week to see if that does the same mpg or not.
Paul.

rbruce1314 · Post by **rbruce1314** » 17 Jun 2005, 17:02

Getting back to basics keep reminding yourself that EVERY time you apply the brakes you are wasting the fuel that got you up to that speed.