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Fuel economy: why is 1.3 unleaded 'no better' than 1.9 diesel?

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 ksjs 06 Jan 2014
Happy New Year!

As part of a financial review I'm thinking about getting a new (to me) car. I currently drive a 1.9 diesel which achieves about 45mpg. Part of getting a new car would be to achieve fuel savings but when I look at smaller engine (used and therefore older) vehicles there appears to be no appreciable improvement in fuel economy i.e. a Ford Ka is quoted as also having 45 mpg. The situation is markedly different when considering recently manufactured (smaller) cars with efficient technologies which achieve an mpg of around 80! That's almost a 100% increase in efficiency or a 50% decrease in cost. Unfortunately they're beyond my budget.

My engine / car knowledge is limited but I had assumed a smaller engine meant less fuel consumption but this doesn't seem to be the case. Should I therefore expect no improvement in fuel efficiency based on a smaller engine alone?
 teflonpete 06 Jan 2014
In reply to ksjs:

If you're contemplating a smaller car, how about a smaller diesel? Something like a Citroen C3 1.4 HDi will return around 60 mpg, even cruising at brisk motorway speeds and the acceleration will knock a 1.3 petrol into the weeds. You can pick up 03 - 04 plates for less than £2k. Don't know what your budget is but something of that sort of size isn't a bad bet. I've got a 1.8 diesel focus estate now as I needed more room and that returns an average 50 mpg with quite a lot of motorway miles.
Removed User 06 Jan 2014
In reply to ksjs:
Well petrol is cheaper so it does in fact have higher fuel economy. Also you need to bear in mind tiny cars with little engines being flogged all the time fully loaded don't make for good fuel economy - somethiinig else to factor in.

Furthermore it should be of no suprise that old technologies are less fuel efficient.
Post edited at 09:59
 chris j 06 Jan 2014
In reply to ksjs:

> Happy New Year!

> The situation is markedly different when considering recently manufactured (smaller) cars with efficient technologies which achieve an mpg of around 80!

You do know those figures for brand new cars are pretty much unachievable in normal day to day driving as the tests are quite unrealistic.


> My engine / car knowledge is limited but I had assumed a smaller engine meant less fuel consumption but this doesn't seem to be the case. Should I therefore expect no improvement in fuel efficiency based on a smaller engine alone?

Basically smaller petrol is more efficient than larger petrol, smaller diesel is more efficient than larger diesel, small petrol is approximately as efficient as mid-size diesel...

How much mileage are you doing per year and what sort of driving (town/motorway)? It might well make sense to change cars but it also often seems that people are spending a few thousand getting a fuel efficient car to then save a couple of hundred punds a year in fuel costs.
 jkarran 06 Jan 2014
In reply to ksjs:

Fuel economy has as much to do with the whole car as it does the engine and fuel. Also cars designed to achieve good figures on the standard test cycle may be less capable in day to day use.

There are only so many small efficiency gains you can make with the engine, if you want to really save fuel you need to reduce the power required by reducing weight, drag and traveling slower.

Modern cars achieve high fuel efficiency numbers mostly through drag reduction and more efficient engines.

If you want an efficient 'old' car then one that's light, has EFI and comes near with a low CdA figure would be a good start http://en.wikipedia.org/wiki/Automobile_drag_coefficient

jk
 Mountain Llama 06 Jan 2014
In reply to ksjs: Have u looked at a skoda fabia 2nd hand? Good economy either petrol or diesel and cheap

 petellis 06 Jan 2014
In reply to ksjs:

If you really want to save fuel look at a car that was properly engineered to do so from the start. The Audi A2 is a good start, has all the features that JK talks about further down the thread and is ticking round to about 10 years old so getting affordable.

I'm guessing cost is putting you off but as little diesels go anything with the Fiat/vauxhall 1.3 diesel will be pretty efficient.

You might not be comparing apples with apples though. The original Ford Ka has a pretty old engine design compared to what I'm guessing is the VAG 1.9 diesel which has had pretty constant development.

Its not just the engine though, gearing, drag and weight all make a difference. Plus if its a diesel the fuel is more energy dense, and with a turbo you are adding a level of efficiency not available in a little naturally aspirated engine.
 ByEek 06 Jan 2014
In reply to Mountain Llama:

I was wondering when the Skoda would be rolled into the argument. (Other small diesel cars also fulfil the OPs brief)
 Fredt 06 Jan 2014
In reply to ksjs:

Just for info, I have a four year old Ford Focus 1.6 diesel, I do mostly motorway driving all over the country, and get between 55 and 60 mpg.
A couple days around town pulls it down to 50.
 Mountain Llama 06 Jan 2014
In reply to ByEek: the fabia fits the Bill, cheap and economical.

I drive a golf 2.0tdi, so the advice offered was given in good faith.

Regards Dave

OP ksjs 06 Jan 2014
In reply to ksjs: Some excellent posts - thanks people. Will have a proper look / reply later but for now a bit more info to inform discussion:

- mileage is under 10k pa (maybe 7k?)

- driving is a mix of urban, country roads and motorway. At a guess I'd say 20% was urban and / or slower country roads, 40% was decent country roads i.e. where you can do 50 - 60 but with occasional stops and 40% was motorway

- the vehicle rarely carries more than 1 person (me) and is nearly always unloaded except for 2 bouldering mats

- I recognise that ultimately the savings may be relatively small in the scheme of things (especially given my low annual mileage) but alongside reduced road tax we're maybe talking around £500 annual savings so, for me, worth serious consideration

- budget is £3,000 but ideally less

- there is something about a small vehicle that appeals simply for the fact that it is small. I can't explain this easily but it's something to do with perceived compactness, fitness for purpose and frugality. This has value / utility for me, I'm willing to pay for that and it makes me discount even small family vehicles in favour of the likes of Ka, Polo, Micra, Peugeot 1007 etc
 butteredfrog 06 Jan 2014
In reply to ksjs:
Factor in repair costs at your budget. Petrol engines are cheaper to repair and service in general. (Diesel engines cost more to repair, a replacment turbo is usually £1000). Also Insurance costs etc.

For example why spend £2000-£3000 on VW/Audi which is probably 10 yrs old with 90,000 on the clock, when with a bit of shopping around you could pick up a bulletproof 1.6 or 1.8 Astra G, for £800 - £1000: cheaper to insure, reliable and you still get 50mpg on a run, plus you save £1000 on the outlay.

Have a look what your local mechanics drive.

Cheers Adam

PS Ford Ka's rot dreadfully, Peugeot 1007 made out of biscuit tins, Micra not too bad, Suzuki Swift OK.
Post edited at 11:59
 jimtitt 06 Jan 2014
In reply to ksjs:

And your are measuring fuel economy in mpg but with two different fuels, diesel has about 16% more energy per unit volume than petrol.
 chris j 06 Jan 2014
In reply to ksjs:

Ok, so at 7000 miles per year you're looking at about £300 saving in fuel for diesel over petrol (http://www.fuel-economy.co.uk/calc.shtml ) assuming 35mpg for petrol at 130 ppl and 50mpg for diesel at 138 ppl. If you get a post 2006 vehicle then probably lower VED for the diesel based on CO2 output but you'd probably lose this back in higher servicing.

Diesel cars will probably have a higher purchase price as everyone thinks low mpg is the be all and end all so in your situation I'd get a small petrol car as you'll get something newer or nicer for the money.

Actually it's surprising how new you can get, 3-4 year old Fiat Pandas, Kia Picantos, Nissan Micras etc. Not my thing but if you want small...

http://www.autotrader.co.uk/search/used/cars/postcode/le24rw/radius/1500/fu...

 wintertree 06 Jan 2014
In reply to ksjs:
The best economy in non-hybrid cars comes from the common rail diesel engines (HDi, TDCI, CDTI etc. etc.) where the use of high pressure fuel injection into the cylinder at the peak of the compression stroke allows the timing of the combustion to be more optimised than in a conventional engine where both fuel and air are compressed together. Of course you pay for this in the increased 2nd hand cost, and if the high pressure fuel pump goes it'll cost you.

My understanding is that the efficiency gains are down to producing multiple injection/combustion events within a single stroke - up to 5 or 6 in some engines now. This is easier to achieve in a diesel engine where you "simply" inject high pressure fuel into the cylinder and it goes boom - harder to achieve in a petrol car, although the fiat twin air is moving in that direction, albeit in regulating the availability of air instead of fuel.
Post edited at 12:57
 Blue Straggler 06 Jan 2014
In reply to ksjs:

Suzuki Cappuccino?


Only half-joking
In reply to ksjs:

My previous car was an Octavia 1.9tdi diesel estate -- it gave me excellent economy and reliability for 9 years.

I was in a fortunate position where I could buy a new model about 18 months ago but this time opted for the 1.4 tsi Petrol. I don't do a big mileage these days, and it would have taken me something like 9 years to recoup the extra cost ( about £1200 IIRC) of the diesel engine with the additional mpg it would offer. As the 1.4 petrol has a chaincam, and no dpf filters to clog, I've also managed to avoid some additional potential servicing costs as the car gets older.
OP ksjs 06 Jan 2014
In reply to jimtitt:

So diesel gives 16% more energy per unit i.e. if 1 litre of unleaded took my car 100 miles, 1 litre of diesel would take the same vehicle (assuming the engine could handle both fuel types) 116 miles?

The small difference in pump price (3-5p per litre) is therefore significantly offset by increased energy density?

I can't believe I didn't know this! Thanks.
OP ksjs 06 Jan 2014
In reply to chris j:

> Ok, so at 7000 miles per year you're looking at about £300 saving in fuel for diesel over petrol (http://www.fuel-economy.co.uk/calc.shtml ) assuming 35mpg for petrol at 130 ppl and 50mpg for diesel at 138 ppl. If you get a post 2006 vehicle then probably lower VED for the diesel based on CO2 output but you'd probably lose this back in higher servicing.

I GUARANTEE I do 45mpg in a 1.9 diesel but there you go. Anyway, here you're saying better economy in a diesel is offset by the higher servicing costs for diesels?

> Diesel cars will probably have a higher purchase price as everyone thinks low mpg is the be all and end all so in your situation I'd get a small petrol car as you'll get something newer or nicer for the money.

Good point.

> Actually it's surprising how new you can get, 3-4 year old Fiat Pandas, Kia Picantos, Nissan Micras etc. Not my thing but if you want small...


Thanks for that.
OP ksjs 06 Jan 2014
In reply to chris j:

> You do know those figures for brand new cars are pretty much unachievable in normal day to day driving as the tests are quite unrealistic.

I thought they weren't that off, after all official figures for my vehicle seem about right?

> Basically smaller petrol is more efficient than larger petrol, smaller diesel is more efficient than larger diesel, small petrol is approximately as efficient as mid-size diesel...

This is useful - it might make mid-size diesels more of an option for me.

> How much mileage are you doing per year and what sort of driving (town/motorway)? It might well make sense to change cars but it also often seems that people are spending a few thousand getting a fuel efficient car to then save a couple of hundred punds a year in fuel costs.

I think it's more than this i.e. there's a road tax saving too. Also my current vehicle is about 13 years old which may mean disproportionate future servicing costs.

OP ksjs 06 Jan 2014
In reply to Fredt: I drove a Focus in a previous life and thought it was one of the smoothest, most responsive and nicest to drive cars I'd been in. Before deciding on it I test drove an Audi A2 and VW Golf, neither came close for me.
 jkarran 06 Jan 2014
In reply to ksjs:

> I think it's more than this i.e. there's a road tax saving too. Also my current vehicle is about 13 years old which may mean disproportionate future servicing costs.

Unless there's a lot of value left in it (collectible?) if it isn't actually costing you extra to service/fix and the objective is purely to save money then I'd be tempted to keep running it for now. Pump your tyres up, drive it a little slower, drive it a little less. It's a known risk operationally with no capital outlay required, a new car is an unknown risk that costs you and will only ever yield a small fuel/tax saving given your modest annual mileage.

jk
johnj 06 Jan 2014
In reply to wintertree:

> The best economy in non-hybrid cars comes from the common rail diesel engines (HDi, TDCI, CDTI etc. etc.) where the use of high pressure fuel injection into the cylinder at the peak of the compression stroke allows the timing of the combustion to be more optimised than in a conventional engine where both fuel and air are compressed together. Of course you pay for this in the increased 2nd hand cost, and if the high pressure fuel pump goes it'll cost you.

> My understanding is that the efficiency gains are down to producing multiple injection/combustion events within a single stroke - up to 5 or 6 in some engines now. This is easier to achieve in a diesel engine where you "simply" inject high pressure fuel into the cylinder and it goes boom - harder to achieve in a petrol car, although the fiat twin air is moving in that direction, albeit in regulating the availability of air instead of fuel.


Common rail plays a part, but a big part of the efficacy of modern diesel motors is turbocharger technology, all modern turbos have nozzle assemblies around the turbine or hot end which gives a big range in A/R ratio, this gives a lot of room for economy or boost/power, the blade shapes of the compressor also is always been modified with OEM development

Although a turbocharger isn't free energy, it makes the system more efficient as it recycles losses, more further developments will be super conductor heat exchanges on exhaust system to convert heat to power thus reducing the amount of drive belts sucking hp out of the motor, and hydrogen/hydroxyl fuel cells.
OP ksjs 06 Jan 2014
In reply to ksjs: Not so easy when you start to take account of all variables / factors:

- the nett saving for a relatively small annual mileage i.e. not huge
- driving styles and how this relates to real life mpg and therefore cost calculations
- reduced or no road tax for low emission vehicles
- increased diesel servicing costs versus unleaded
- yet the relative simplicity and therefore lower cost of maintaining an older vehicle i.e. less electronics
- but the likelihood for higher servicing costs associated with an old vehicle i.e. sticking with what I've got
- increased confidence and comfort driving a newer and 'better' vehicle (I have to say however that my current vehicle [Citroen Berlingo] seems to be a bit of a trojan, it's never given a problem, apart from a perished head gasket, failed to start or broken down; hard to knock it really / part with it
- my desire for something smaller and arguably more suitable

More thinking required. Thanks everyone for the help.
OP ksjs 06 Jan 2014
In reply to jkarran:

> It's a known risk operationally with no capital outlay required, a new car is an unknown risk that costs you and will only ever yield a small fuel/tax saving given your modest annual mileage.

This may be the deciding factor, it's been so good. Not quite collectible just yet!

I can't however escape the idea that it's just too big for what I need / want and my partner also has a van anyway. Also if I save £500 pa over 4 years (in reduced tax and increased fuel efficiency) and get £500 from selling it then I've paid for a new (used) car costing* say £2,500 over 4 years.

At the same time I also get the value from a newer, smaller and more comfortable vehicle.

*Financing isn't a cost.

 balmybaldwin 06 Jan 2014
In reply to ksjs:
> (In reply to chris j)
> In reply to chris j:


> You do know those figures for brand new cars are pretty much unachievable in normal day to day driving as the tests are quite unrealistic.


>
> I thought they weren't that off, after all official figures for my vehicle seem about right?
>

there's been quite a lot in the press about this... it's not that all manufacturers do it, but some have been "testing" the cars in unrealistic states - All electonics turned off (e.g. aircon, stabilisation systems), taping up panel gaps etc. (Not to mention these are done on long vitually straight testing grounds with no hills, no other traffic etc
johnj 06 Jan 2014
In reply to balmybaldwin:

It's the way the motor industry works, the only thing what hasn't really improved in the last 25 years is fuel efficiency, really by now modern cars should be returning about 150 miles per gallon, but no they're just crawling up past the 50 mark for an average, and all the while we get more and more electrics which achieve the sum total of nothing apart from been able to go quicker whilst paying less attention......

And breath
 chris j 06 Jan 2014
In reply to ksjs:

> I thought they weren't that off, after all official figures for my vehicle seem about right?

My old 2002 1.8 TDCi Focus was pretty much spot on as well whereas my 2010 Skoda is about 20% off what it should be. Not sure if the test changed in the last 10 years but basically the engine load during the test is so low that if the engine has a turbo it won't ever spool up and the engine is much more economical. Most modern diesels and many small petrols have turbos, QED. Hence the many car reviews saying that when they drove their small engined car in a vaguely spirited manner they got 35 mpg rather than the 70 mpg test result. The Fiat Multiair is apparently a fine example of this.

If you look at the what car or honest john websites i think they show what people get in the real world, or there is a website called fuelly.com which does the same.
In reply to ksjs:

I recently bought a Peugot 1.4 Diesel '54 plate 206 SW (Estate) for 2.5k and have been very pleased with it so far, it achieves a very good mpg. For example, I drove from Sheffield to South Devon, to North Wales and back to Sheffield over xmas on around a tank of fuel (around £80). Devon to NW was exactly half a tank, or approx £40 and I was at 70 pretty much the whole way.

It's also got a reasonable-sized boot too. One thing that is worth factoring in is that Diesel's last longer than petrols so are also a financial benefit.

Apparently (according to my dad) the French are also into not getting rid of cars, so French cars are supposed to last well.

HTH Dunc
In reply to ksjs:
Hate to get boring, but it's down to the higher thermal efficiency of compression ignition vs spark ignition engines. The basic result is it takes less heat to produce the same crankshaft power, so requires less fuel when compared against petrol mpg.
Ever wondered why it takes the passenger compartment heater longer to get up to a reasonable temperature in a diesel car on a cold morning compared to a petrol engined car?
 johncook 06 Jan 2014
In reply to Duncan Campbell - UKC:

Bought a really cheap peugeot 306 TDi (13 years old, 250,000miles £300) had minor repairs done so after two years total outgoings leave me with a standing price of about £750 (this includes a pair of cam belts and a full service evry year whether it needs one or not, and two MOT's)
Boing OCD (and a tight Yorkshireman) I record my mileages. Overall I am getting 56 mpg. On a run to Chamonix I got there from Sheffield on one well full tank, approx 74mpg. Aimed for 70mph on motorways and speed limit on all other roads. Overall on that trip I got over 60mpg, and that included town driving and mountain passes.
I can be a 'spirited' driver, but owing to being broke I now mostly tend to drive for economy. You can save a lot of money by getting into top gear and staying there. I take roundabouts at the speed limit if they are clear, and will also try to straighten them out. As the car corners really well I can stay in a high gear round most corners in the Peak (so long as there is decent visibility and no trundlers!)and can do Lanberis pass in top and still be in the power band and stay legal. Too low revs uses fuel as inefficiently as too high revs. try to anticipate traffic lights so that I never have to use first, and rarely use second. (Again sunject to wallies texting when the lights change!!)
Fuel economy is in the hands of the driver. The manufacturers have specialist 'economical' drivers for the testing, and other drivers for when they want to show the car in normal situations.
Tyre pressures can also make a big difference to economy.
 Marcus Tierney 06 Jan 2014
In reply to ksjs:

I have only skimmed through your replies but I have a diesel for other reasons not just economy, I like the way they drive. Just more relaxing and if you do put your foot down occasionally I don't feel guilty for thrashing it. Its also a nice feeling when i fill up and i have done 600 miles on a tank knowing i can still do 0-60 in a little over 8secs. I have tried the new ecoboost and Tsi engines and they are very good and economical...but thrashy. The newer the engines in both petrol and diesel are getting so much better. The 1.6 VAG engine in my wives car is sweet, economical and still fun and relaxing to drive.
 rug 06 Jan 2014
In reply to Duncan Campbell - UKC:

> One thing that is worth factoring in is that Diesel's last longer than petrols so are also a financial benefit.

I'm not sure this is as much a factor as it used to be. My current car is petrol engined, and currently around 185k miles on the clock. I think it's going to be rust/bearings/whatever that kill the car, rather than engine failure.

The Berlingo (OP's car) is a very draggy shape, so driving it a little slower will have a marked difference on mpg. Drove one to the alps a couple of years ago, and we were getting 60+ mpg by driving at a steady 65 mph. A little slower, a lot cheaper ...

Rug


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