Diesel vs Petrol

Sandrock

I've had several diesels and several petrols. I now have a diesel and a petrol.

A diesel version usually costs more than the petrol version if you are buying new. If buying second hand, diesels tend to hold their value slightly better.

Some diesels are extremely economical, some aren't all that great. It really depends on the car. My last diesel car was incredible - would average 45-50mpg for mixed driving, 60+mpg on a run and over 200 bhp. My current diesel, has slightly over half the power and only manages 35-40mpg average and 45-50mpg on a run.

Diesel fuel costs more than petrol, but only by 2-3 pence a litre.

Like for like, diesels usually cost more to service (roughly 20% more).

They are both as reliable as each other, petrol cars tend to have small low cost item failures more, whereas diesel cars tend to have fewer failures but cost more to replair due to the complex fuelling systems.

At the end of the day, you need to do the maths. I don't do huge miles but opted for a diesel as I was buying a used car and got a great deal - what swung it for me was the fact I do my own servicing and the fuel saving. As the car is a low value (~£2500) and just used to get me around during the winter, If I have a major failure I'd probably write off the car and sell it as scrap.

pitkin2020

Most diesels go further between services though compared with petrols, so whilst the parts may cost a little bit more most service schedules for diesels arent annual like petrols.

ahillsy

An increasing *potential* problem with diesels is if they come with a Diesel Particulate Filter (DPF) which more and more are now. This can mean it's not suitable for certain types of driving - bascially the DPF catches all the soot and instead of chucking it all out the exhaust, it captures it. It then has to go through a regeneration process to "burn off" all the soot at high temp - better for the environment. This process (may vary from make to make) requires high temps, which can only be achieved when driving at a reasonable speed for a period of time - so if you're doing a lot of short, stop-start journeys then you have the increased risk of the filter getting blocked. This can be very expensive (~£1K).

This would be my only reservation about going for a diesel over a petrol. This single factor alone, ruled out a diesel I was looking at, meaning I had to go for the petrol instead.

It's worth being aware, as not a lot of people are aware and car dealers aren't exactly upfront/forthcoming with this info.

molerat

Interestingly where I regularly hire cars from used to have a fleet of diesels and it was hard work to get a petrol from them. Now it is the reverse. The only explanation for this must be financial.

sebdangerfield

molerat wrote: »

Interestingly where I regularly hire cars from used to have a fleet of diesels and it was hard work to get a petrol from them. Now it is the reverse. The only explanation for this must be financial.

Exactly, the main reason hire companies used diesels more was because of public demand and resale. Because their customers wanted diesels to hire because of the market trend, thats what they hired out. Equally, because people's perseptions of diesels were so high because of clever marketing, high mpg etc they were worth more come the resale at the end of their life relative to the petrols.

I think this is changing slowly now.

Many people still try to argue that diesels are now more powerfull than petrol equivalents but very rarely use a real equivalent. For example, someone used the figures of the 320 petrol against the 325 diesel. Ultimately, a four cylinder non turbo against a 6 cylinder turbo. Hardly equivalent. BMW are still getting c240bhp from the diesel 6 cylinder lump against c270bhp from the petrol one. The gulf even bigger with the twin turbo ones. There really are very few genuine equivalent petrols and diesels where the diesels are more powerful.

People also use the frugillity argument but this is parodoxical when in the next breath they talk about how powerful it is. Surely if you buy a car to be quick and powerful then you're not buying it to be cheap to run? Similarly, if you buy a diesel you are buying it to be cheaper to run than the petrol and therefore performence isn't the most important deciding factor.

The biggest change between the two fuels in recent times are repair bills. Diesel engines used to be mildley more frugal than petrols but were so simple they would last forever. Now however, more technology has eaked more power, refinement and efficiency from the engines and as a result they are so complicated that they no longer last a lifetime and cost an enormous lot more should certain components fail. I've reported on here a few times of the 335d I owned for a while. I was hugely impressed with the technology and how far the engines have developed but it's still not a performence car and with a bill for nearly £4000 when the injectors failed I was less than impressed. People believe that they can have few benifits of diesels of old like reliability and longevity but with the modern refinements engineered into them. It's not just not possible. There is now far more to go wrong with a modern diesel engine and now we're into the 5-7 years after the surge in popularity of modern diesels we are seeing much more expensive and common failures.

For me, the most laughable 'benefit' to running a diesel are the green credentials. A few years ago the government decided to tax our cars relative to the emmissions they spew out. They had a plethora of horrible chemicals to choose from like corbon monoxide, carbon dioxide, differing horrible hydrocarbons and nitrogen oxides. The government chose to use carbon dioxide as the yard stick and it just happens that thats pretty much the only gas diesels cars produce less of than diesels. Diesels really aren't greener. They're far far more polluting, for this reason diesel particulate filter technologys are being produced for when another re-think happens and cars are measured by the other gasses they produce. Diesel particulate filters.......... now weren't we talking about them the other day!?

I know I come accross as hating of diesels but I'm not. I'm not a fan because I prefer performence cars and diesels, even the powerful ones, are always the more economical choice and therfore not focussed enough for me. However, because of diesel technology we now have improvements to petrol engines like fsi and better turbo scolling so I'm massivley gratefull of the technology they've provided, I just think that people should get ralisting about their expectations of what diesels can do.

missile

sebdangerfield wrote: »

There really are very few genuine equivalent petrols and diesels where the diesels are more powerful.
I'm not a fan because I prefer performence cars and diesels, even the powerful ones, are always the more economical choice and therfore not focussed enough for me. what diesels can do.

Does this one have enough performance for you? :rotfl:

lara400

utterly fantastic post sebdangerfield!

I know next to nothing about cars and I really, really need to buy a car this year. I have always had petrol and assumed it is cheaper than diesel due to the prices of oil between them. Having pretty much read though the posting in this forum I now seem to understand diesel does more mpg than a petrol (I do less than 6000 miles a year). So, maybe I will go for diesel - 5 year old car btw!

Only problem after reading your post (and others) is the expense of fixing faulty things.....might put me off..

thanks,
Lara400

sebdangerfield

missile wrote: »

Does this one have enough performance for you? :rotfl:

Hmmmm, do I need to define equivalent?

Audi R10TDI;
5.5 Litre twin turbo V12 with c670 bhp

The cars it raced against....

Bently Speed8, Audi R8
3.6 Litre and 4 Litre twin turbo V8s with c670bhp

Also, the point Audi made when they held the publicity event (which did wonders for their modern range of A4 diesel sales (read into that what you will) regarding entering a diesel into the race was they were happy to have a deficiet of power over the other (petrol) competitors (in 2006, the tables are closer now) because they would make time up in less fuel stops and a proprtionaly higer rev limit compared to the peak power, allowing for less gear changes and better reliability over the 24 hours.

pitkin2020

You can't compare the diesel engine against an equivelent petrol engine on a like for like basis. For the engines to achieve the same BHP the diesel will have to higher capacity. A 2.0 petrol compared with a 2.0 diesel will usually have more BHP but the diesel generally will have more torque.
Just because you choose diesel over petrol for the MPG benefits doesn't mean you don't want performance aswell, but then it depends on HOW MUCH performance you want. If your main interest in doing 180 up the motorway and get from 0-60 in under 3.5 seconds then fuel economy really shouldn't matter to you.
For the average driver the diesels are powerful enough with the added bonus of better MPG, even when you give it some welly.

missile

sebdangerfield wrote: »

Hmmmm, .

Hmmm, it did win three times :T

Maybe not what you would call a performance car:rolleyes: the Audi A3 2.0 TDI makes less power but more torque than A3 2.0TFSI. I would suggest torque is as important as bhp. On a journey from Lands End to John O'groats which would get there first?

I am not suggesting diesel is better than petrol. IMHO the choice is not as simple nor as clear cut as you are suggesting. PS: I currently own a diesel AND a petrol car.