XM versus Xantia fuel economy

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Allanxantia
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XM versus Xantia fuel economy

Post by Allanxantia »

I have got the chance of an XM 2.1td estate manual. I would love one. How much heavier on fuel is it likely to be than my xantia 1.9D? I do a lot of motorway and the Xant is very low geared. It does however need a clutch and gearbox and hoping that it is not too bad a job
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Post by RichardW »

A 1.9D Xantia must be a slug... What does it do to the gallon? The XM would I imagine turn in at least 40 MPG, and it will be much much better to drive than the Xantia. XM spares are a bit on the pricey side though, it being so much less common. And as Peter noted in the other thread, the 2.1 is a bit of shoehorn job - it's reckoned to be the best of the XUD family though :lol:
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Post by Peter.N. »

Well, I can only speak of my own experience. I am on my fourth XM 2.1 td estate (manual) and they have all been very good on fuel, in fact for my purpose they are the best vehicle in the range for a combination of space, econmy and comfort, but of course fuel consumption ultimatly depends on where and how you drive.

I live in a rural area so most of my journeys are at least 10 miles. I generally drive at or within the speed limits and I dont thrash it. My average consumption is about 40 - 45 mpg but on parts of our trips to the North of Scotland I have managed in excess of 55 mpg. We returned early this morning from a few days in Norfolk, I filled up before we left, we covered about 730 miles and it is still showing a quarter full. 800 miles on a tankful (17 1/2 gall) is quite easily attainable and once last year I managed nearly 900. I think that with really frugal driving 1000 miles would be possible.

But all of that is with my driving, travelling when the roads are at their quietest, and generally in rural areas, because as I am retired I can choose my times and routes.

So that will give you an idea of , not necessaraly what you will get, but what is attainable. I do think however that the 2.1 td engine is the best in design and efficiency of the XUD range.
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Post by Peter.N. »

You just beat me Richard, must learn to type faster!
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Post by Kowalski »

The 2.1TD is marginally more thirsty than the 1.9TD and the 1.9TD is a little more thirsty than the 1.9D. The XM is a bit bigger and heavier than a Xantia, so you'll lose a little fuel economy there, the one you're looking at is an estate, estates aren't as aerodynamic as hatches so they use a little more fuel. Thats the downside covered.

The 2.1TD is 110 bhp vs 90 bhp for the 1.9TD and 70 for the 1.9D, so the XM is going to be faster than a Xantia, even if you go for a TD Xantia. XM estates are truly massive inside, they're as good as any Volvo estate (and better than the newer ones). They're also a bit more comfortable than a Xantia.

All in all the XM will be a better car but your fuel economy will fall, the official figures are something like 47mpg combined for a 1.9D Xantia to 40.1mpg for an XM estate. I think if you drive the XM carefully you'll not lose the full 7mpg, but if you thrash it you can lose far more ;)
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Post by Allanxantia »

To be honest my xantia is a second car runabout as I have an impreza which has a SERIOUS drink problem. The boot of my Xantia is full and heavy all the time as I store all my tools in it. My previous Omega estate sat very very low so the hydropneumatic citroens are great for me. The xantia is just a bit small. I also find that I have to drive it pretty hard to actually get anywhere! I am hoping the XM may be a little less stressed at this. I have looked for a xantia estate but they are not really that cheap to get an estate diesel. XMs being unloved seem to be ridiculously cheap. I get around 42mpg from the xantia, all stop start around town.
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Post by zzf00l »

In all honesty, I wish I had my old XM back! :cry:
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Post by Peter.N. »

I'm very pleased I still have mine. I have tried several other cars as potential replacements but not found any that are as comfortable,economical, roomy or quiet as the XM. The only thing that comes near it is the BMW 530d estate, but that only has about half as much room, the E class merc is about the same size, but they go rusty and are almost all auto's, and the Audi has a terrible ride.

So, if anyone can tell me of a car that is equal in all respects to the 2.1 td manual XM estate, please tell me! :cry:
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Post by davehughes »

I have a 1.9TD Xantia. I bought it because my new job requires a 150mile round trip each day on country lane and motorway. This costs me 10 pounds a day in diesel which is what I was spending on diesel in a brand new golf tdi. I am very impressed that a 12 year old car with the same cc as a brand new golf acheives this consumption. Also you said that a 1.9D must be like a slug, a week after I bought the Xantia, the turbo pipe came undone on the pressurised side on a motorway and the difference was incredible (having no turbo) the car lurched when it happened and lost power and the lack of acceleration was also noticable. If you are going for a turbo diesel it's worth losing a bit of fuel economy for the acceleration. Especially as you do lots of motorway driving like me.
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Post by citroenxm »

Hi all

I wouldn't say the 2.1td is more thirsty. I think its a matter of power to weight ratio. The Xantia is a little to heavy for the 1.9td engine, so it has to work a little harder to move the car. The 110bhp of the 2.1 obviously suits the XM weight a bit better, as they do give a better consumption over the Xantia! Strange but true!
You cant really look at official fuel figures they will never be true

I fing 47mpg hard for a n/a Xantia, as the unit has to work even harder to move the car. I had a Turbo D Xantia, and stuggled 40mpg. Before that my BX TD was at the 50's mpg.

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Kowalski wrote:The 2.1TD is marginally more thirsty than the 1.9TD and the 1.9TD is a little more thirsty than the 1.9D. The XM is a bit bigger and heavier than a Xantia, so you'll lose a little fuel economy there, the one you're looking at is an estate, estates aren't as aerodynamic as hatches so they use a little more fuel. Thats the downside covered.

The 2.1TD is 110 bhp vs 90 bhp for the 1.9TD and 70 for the 1.9D, so the XM is going to be faster than a Xantia, even if you go for a TD Xantia. XM estates are truly massive inside, they're as good as any Volvo estate (and better than the newer ones). They're also a bit more comfortable than a Xantia.

All in all the XM will be a better car but your fuel economy will fall, the official figures are something like 47mpg combined for a 1.9D Xantia to 40.1mpg for an XM estate. I think if you drive the XM carefully you'll not lose the full 7mpg, but if you thrash it you can lose far more ;)
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Post by Peter.N. »

What citroenxm says is very true. Fuel consumption on a diesel has more to do with power to weight ratio than engine size. A diesel engine is much more efficient at small throttle openings because its running on a much weaker mixture, so the more lightly stressed it is the better the consumption will be. That's one reason that modern diesels are much more fuel efficient than the much smaller engines of 20 or so years ago. Also, the improvement in power to wieght ratio means they can be much longer geared, giving a further improvement by a decrease in friction losses. The reason more powerful engines show a higher fuel consumption on paper, is because if you have the extra power you are inclined to use it, drive it with the same acceleration and speed of a smaller engine and it will give you a better consumption.
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Post by Kowalski »

Peter.N. wrote:What citroenxm says is very true. Fuel consumption on a diesel has more to do with power to weight ratio than engine size. A diesel engine is much more efficient at small throttle openings because its running on a much weaker mixture, so the more lightly stressed it is the better the consumption will be. That's one reason that modern diesels are much more fuel efficient than the much smaller engines of 20 or so years ago. Also, the improvement in power to wieght ratio means they can be much longer geared, giving a further improvement by a decrease in friction losses. The reason more powerful engines show a higher fuel consumption on paper, is because if you have the extra power you are inclined to use it, drive it with the same acceleration and speed of a smaller engine and it will give you a better consumption.
We've had various arguments about where diesels give best economy....

To recap, they give best economy when used at as close to full throttle as possible. At small throttle levels they give absolutely terrible fuel efficency.

I managed to find some graphs that plot the specific fuel consumption of a few different diesel engines (that is lbs of fuel per horsepower hour). The graphs I have show torque (i.e. throttle opening) vs specific fuel consumption and have various plot lines on them for various different rpms. The graph goes almost vertically upwards at the left had end, thats small throttle economy for you. At the right hand end they rise slowly, except for the turbo diesel engines. The turbo diesel engines economy goes off badly as the turbo comes in, badly so for non-intercooled engines.

So, according to the information I have, you've got to use full throttle without using the turbo, also for a specific power output, its generally more economical to get it at a lower rpm, unless the turbo gets involved.

Diesel engines are more economical than 20 years ago because they've changed from indirect injection to direct which is inherently more economical. They've also got smaller for the same power output too.

The part of my driving that dominates my fuel economy is part throttle cruise, a smaller engine would be more economical for me for that part of my driving, but not for the accelarating up to speed.
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Post by Peter.N. »

Kowalski

I would be interested to know where you got your figures from because your conclusions are just the opposite to what I have found in practise. Diesels are inherently thermally ineficient at full or near full throttle operation, that's why an equivelent size engine needs a larger radiator than its petrol counterpart, at small low load throttle openings they are extremely thermally efficient which is demonstrated by the fact that some diesel cars need an auxilliary heater. This doesn't apply just to modern engines. In about 1960 I fitted a Perkins 4/99 engine to a Vauxhall Cresta, admittedly it had less power available than the straight six petrol it replaced, but, in normal driving the same ammount of energy was required but the heat output from the engine was insufficient to run the heater at normal temperature, in fact, when the heater was turned on the water temperature would almost immediatly start to drop. So what is the relevance of this? well, when fuel is burned it produces energy in the form of heat and power, so, the less heat the more power. Petrol engines produce much hotter exhaust gasses, which is waste energy, one of the factors in the improved economy of diesels, on the other hand, diesels give more of thier heat energy to the cooling system, hence the afformentioned larger raditor, but only at full or near full throttle.

Check the fuel consumption figures of diesel cars on test against that of their petrol counterparts, I think you will find, almost without exception that the figures obtained on part throttle operation, i,e, constant speed are much better than the compareable petrol version. Whereas when petrol v diesel tests are carried out at full throttle i,e, performance testing, the diesels fuel consumption is often little better than the petrol.
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Post by Kowalski »

Off hand, the only graph I can link to is this one. I'll have to see if I can dig out some better graphs, good graphs are rather like hens teeth ;)

http://www.deltahawkengines.com/perfor01.shtml

This isn't ideal because its a 2 stroke aircraft diesel engine, with a turbo and no intercooling but in any case, as you can see the curves are for the most point 'U' shaped. What is nice about this graph is that for a specific horse power output you can look up which is the most efficient speed to get it at.

There are a few interesting things to note. For the 1500 rpm line, the economy doesn't drop off as you open the throttle more. This would seem to indicate that the turbo can't provide a lot of boost at low rpms. If you look at the leftmost two lines, the trend they indicate is that extremely small throttle openings that specific fuel consumption heads off towards infinity i.e. towards 0% efficiency at very small loads.

The reason that diesels need a bigger radiator than petrols is down to compression ratio and combustion temperatures, these are higher on diesels. This means that more heat is recovered from burning the fuel but also more heat is transmitted into the block and head and needs removing. Adding a turbo here makes things worse.

Modern diesel engines have turbos where as the majority of petrol cars dont'. When you put a turbo on an engine you give it the ability to drink more fuel, atmospheric pressure is about 15psi, if you add 15psi of boost the car will be able to drink twice as much fuel (in theory). When you use a lot of turbo boost, the intake temperature goes up too (even with an intercooler) which doesn't help your efficiency.

The thing about the technical details and the nice graphs is that they don't account for how the cars are driven. They don't take into account traffic and roadworks, so in theory you may know how to get best efficiency out of your car but in practice you end up in the wrong gear, or you accelarate then the traffic stops.....
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Post by Rostami »

As far as i understand, diese engines are more efficient than petrol counterparts because of a higher compression ratio that leads to a more stechiometric combustion. Having a higher binary at lower engine speeds than petrol engines, the engine´s losses are inherently lower, since friction losses are porportional to engine speed.
My experience also shows that the 1.9TD and 2.1TD are very economical when driven slowly and without strong accelerations, becoming rather thursty at high speeds, given that they have only 8 or 12 valves.
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