New car - diesel or petrol?

Herzlos

If you're spending a lot more time in the car (as said it's at least an hour each way) you want to get something you like being in most of all.


You also don't want to rely on the manufacturers fuel economy figures, because you won't see them. Generally, the smaller the engine, the harder you need to work it and the bigger difference you'll see in fuel economy Vs the quoted. Have a look for the models in question, here: https://www.honestjohn.co.uk/real-mpg/


A 95bhp 1.0 (turbo?) is going to be awful on fuel if you're engaging the turbo, which you'll probably need to do to make any progress.
Is this it? https://www.honestjohn.co.uk/realmpg/volkswagen/polo-vi-2017/10-tsi
It's only gettting 42mpg in the real world (69% of the quoted figures). I don't see any figures for the 2017 Polo diesels to compare though.



I'd be inclined to go up a class, to a Focus and go for a slightly bigger diesel engine. You're doing enough miles that the increased fuel economy from diesels makes sense, and you'll find better in-gear acceleration which just makes the driving a bit more relaxed.


If it was me, I'd be looking at a 2-3 year old Focus 1.5 diesel, like this one: https://www.honestjohn.co.uk/realmpg/ford/focus-2014/15-tdci-105-econetic

AdrianC

Herzlos wrote: »

You also don't want to rely on the manufacturers fuel economy figures, because you won't see them. Generally, the smaller the engine, the harder you need to work it and the bigger difference you'll see in fuel economy Vs the quoted. Have a look for the models in question, here: https://www.honestjohn.co.uk/real-mpg/

Bearing in mind we're talking about a very newly-introduced model, there won't be much in the way of "real world" figures yet. WLTP figures are much closer to "reality" than the old NEDC were.

TheMoonandBack

I would definitely get petrol. For one reason, Diesel has been shown to be a real health hazard.
I drive a 1.4 tsi Skoda Octavia on a 75 mile round trip commute. It gets 43mpg real world. I have used a 1.0 polo loan car when mine has been in for servicing, and it was doing 50mpg on the trip computer which I estimate is 47mpg real world if the computer is the the same +3 mpg optimistic as my Octavia.
Previously I had a 1.9 tdi Octavia which did 49mpg but that was outweighed by dual mass flywheel, turbo and egr failures. I then ran a Toyota Auris hybrid which was great around town but the mpg plummets at 70mph where the petrol engine is just revving it’s nuts off.
Try the polo, it needs to be comfortable for you if you’re doing 25 k miles a year.

TheMoonandBack

Herzlos wrote: »

A 95bhp 1.0 (turbo?) is going to be awful on fuel if you're engaging the turbo, which you'll probably need to do to make any progress.
Is this it? https://www.honestjohn.co.uk/realmpg/volkswagen/polo-vi-2017/10-tsi
It's only gettting 42mpg in the real world (69% of the quoted figures). I don't see any figures for the 2017 Polo diesels to compare though.
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These engines develop their maximum torque from below 2000rpm, the turbo is working over the whole Rev range and doesn’t just kick in at high revs. I’ve driven these Polos and they are economical.
The 75bhp non turbo is also good.

AdrianC

TheMoonandBack wrote: »

I would definitely get petrol. For one reason, Diesel has been shown to be a real health hazard.

The current focus on diesel "health hazard" is around NOx - oxides of Nitrogen - emissions. The limits on those are FAR, FAR, FAR tighter than they used to be, so the difference between petrol and diesel is now trivial.

Post-2014 Euro 6 diesel emission limits are 0.08g/km of NOx. Petrol is 0.06g/km. The OP's 2008 petrol Fiesta will be Euro 4, 0.08g/km. By contrast, a 2000-2005 Euro 3 diesel would have a limit of 0.5g/km, a Euro3 petrol would be 0.15g. Pre-Euro 3, there was no direct limit on NOx.

The change from NEDC to WLTP testing last year doesn't change the limit, but changes how they're measured. This is why VAG have supply problems for diesels...

The forthcoming London ULEZ will restrict access to Euro4 petrol and Euro6 diesel, simply because they're pretty much the same thing as current limits.

TheMoonandBack

And to achieve euro 6 with diesel they have added dpf’s and or addblue systems which add in another layer of complexity. They have to burn diesel to regen the dpf on a regular basis causing extra emissions, and lowering mpg.

AdrianC

TheMoonandBack wrote: »

And to achieve euro 6 with diesel they have added dpf’s and or addblue systems which add in another layer of complexity.

WLTP Euro6 petrols are starting to include GPFs.

You want low emissions? There's going to be knock-on effects.

Simplest answer to improve localised urban emissions? Use public transport or walk.

jimjames

Herzlos wrote: »

You also don't want to rely on the manufacturers fuel economy figures, because you won't see them. Generally, the smaller the engine, the harder you need to work it and the bigger difference you'll see in fuel economy Vs the quoted. Have a look for the models in question, here: https://www.honestjohn.co.uk/real-mpg/

Completely agree. I've had a 1.25 petrol Fiesta and 1.9 TDI Golf. On paper the Fiesta is way more economical than the Golf. In reality the Golf is about 25% more economical on the same route.

Goudy

onomatopoeia99 wrote: »

My petrol car has a dual mass flywheel. When the clutch wore out the flywheel did not need to be replaced.

Diesels produce far greater torque than an equivalent sized petrol engine, typically around a third or more, more.

The torque spike this creates when changing gear is a lot greater, so the Dual Mass Flywheel takes a lot more hammering on the diesel.

They also produce this torque in a narrow rpm range and it usually comes in a big lump just as the gear needs changing, typically around 1600-1800 rpm point.

Along with less torque, petrol's tend to have a wider usable power spread and the driver isn't really stuck at changing when torque it at it's peak, this reduces the spike the DMF has to absorb.

They get fitted to petrol's to smooth out the changes rather than stop a massive spike being transmitted back through the crankshaft or into the gearbox.

Buy a Yaris Hybrid and you get no clutch or flywheel to worry about ever.

edde

Goudy wrote: »

The torque spike this creates when changing gear is a lot greater, so the Dual Mass Flywheel takes a lot more hammering on the diesel.

They also produce this torque in a narrow rpm range and it usually comes in a big lump just as the gear needs changing, typically around 1600-1800 rpm point.

Along with less torque, petrol's tend to have a wider usable power spread and the driver isn't really stuck at changing when torque it at it's peak, this reduces the spike the DMF has to absorb.

They get fitted to petrol's to smooth out the changes rather than stop a massive spike being transmitted back through the crankshaft or into the gearbox.

DMF do take more of a hammering but they are bigger than there petrol equivalents.

DMF's main purpose is to deal with the torsional loads the engines make. Modern diesel make loads of torque your right but the biggest issue is the shock loads they are usually worst at very low rpm not at max torque. People think a diesel with say 300lb of torque gives most load to the DMF at max torque (about 2k) but in reality the cranks can often give 5 times or more than that at certain rotational angles and often load the crank/gearbox/DMF in the opposite direction of motion at certain revs.

This is why why people try driving at low rpm and high throttle setting are usually the first to suffer. Go get the revs up and never drive under about 1.6k and never on full throttle till nearer 1.8k and the dmf has a much easier time.