Sales of diesel-powered cars are falling, but does this mean petrol is really best?
Andrew Frankel Autocar
16 March 2019

The reason Porsche no longer offers diesel engines in any of its cars is that it was never a core power source for the brand, and now diesel is in decline it makes sense to focus on the petrol, hybrid and electric powertrains that are its future. At least that’s the official version. And I’m sure Porsche’s desire to distance itself from the Volkswagen Group’s latterly besmirched reputation for producing good, honest diesels had absolutely nothing to do with it. 

Whatever the truth, Porsche’s decision to turn its back on diesel is an enormous gamble, at least in Europe. Certainly key to Porsche’s thought process is that diesel makes no impact at all on its biggest markets outside the continent – such as the US, China and Japan. But now that Porsche is predominately an SUV manufacturer (between them, the Macan and Cayenne accounted for more than 60% of Porsche sales last year), and SUV sales in Europe are still predominately diesel, you can see that cutting yourself off from such customers would be tough, even for Porsche. 

But will it? What Porsche would like its diesel customers to do is simply accept their preferred fuel has had its day and, such are the advances in petrol technology, there is little to lose and much to gain by replacing, say, their old Macan Diesel S with a new 2.0-litre petrol Macan. Like this one. 

We felt a need to put this theory to the test, hence the presence here of another new SUV, but one powered by the black pump. I’d say a diesel-powered SUV is no more true to Alfa Romeo heritage than it is to Porsche’s, but Alfa is sticking by diesel, for now at least. 

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On paper, the diesel Stelvio and petrol Macan make an interesting pairing. Similar in price, performance and engine size, the diesel Alfa leads as expected with a healthy chunk of additional torque, the Macan with a commensurate power advantage. But before we put theory into practice, a thought or two about what’s actually being fought over here. The first thing to say is that were it not for the fallout from Dieselgate, I have no doubt at all Porsche would still be selling not only diesel Macans, but Cayennes and Panameras too. Of course they would: if your car is large and heavy, diesel offers a suite of advantages over petrol many would regard as insuperable. 

Most obviously they are more efficient, meaning they use less fuel – approximately 20% less or so, it is said, but my experience of big diesel burners is that their advantage over equivalent petrol cars can be greater still. In the case of these two and according to the latest WLTP measurements, the Alfa will return more than 40mpg, the Porsche less than 30mpg. 

And whatever you save in fuel, you also save in CO2. If you look at tailpipe CO2 emissions, you can see that they fall year after year until the moment people got spooked by Dieselgate. And they’ve been rising ever since. Coincidence? I think not. But there’s far more to it than that: I’d have the diesel Stelvio’s torque over the petrol Macan’s power because with heavy cars – and the Macan is properly heavy – torque is more useful. Peak power can only be developed once in the entire rev range, peak torque can be maintained from little more than idle to little less than peak power. And as it is torque that you feel when you accelerate, it would be hard to overstate its importance. 

Now consider that not only is the Stelvio more torquey than the Macan, it’s more than 200kg lighter too, so its crucial torque-to-weight ratio (probably the most important real-world determinant of performance potential despite the fact that hardly anyone uses it) is 209lb ft per tonne. The Macan? Just 146lb ft per tonne, a difference of more than 30%. And finally there’s range: the Stelvio has the potential to do more than 470 miles between fills even with its frankly pathetic 52-litre fuel tank. The Macan will take on board 65 litres of unleaded, but it will be a brave person who tries to stretch even that much fuel over 400 miles. Had the Stelvio the same-sized tank, it would get close to 600 miles. 

So it’s clear then? Porsche’s decision to bin diesel is akin to it taking a 12-bore shotgun and discharging both barrels into its feet. Or is there perhaps more going on here? 

Point one is that regardless of its considerable theoretical advantage, the Stelvio holds nothing like the additional performance over the Porsche you might think. The quoted acceleration figures suggest there’s little to choose between them and that’s the way it appears on the road too. So what’s going on? Two things in my view: first, I suspect far less of the Porsche’s power is getting lost in translation between flywheel and Tarmac. One eye-opening decision Porsche made when turning a previous-generation Audi Q5 into the Macan was to ditch its inexpensive, off-the-peg torque converter automatic gearbox for its own seven-speed DSG transmission. But here you can see why. 

Second, despite having one less ratio, the Porsche gearbox is snappier than the Alfa’s and good at keeping the Macan’s motor percolating at the energetic speeds it needs to do its best work. 

But is it desirable for a heavy SUV to require judicious use of the right foot to deliver its performance? The answer is an emphatic ‘no’. These are not supercars, they are large, family cars that should be effortless to drive and there’s no question the Alfa not only provides stronger low- and mid-range shove but requires less effort to do so. If it could do it without the accompanying diesel clatter, it could close its case with confidence.

In the event, however, the engine’s coarseness perpetuates a narrative that says the Stelvio is just not sophisticated enough in execution to cut it alongside the long-time best car in its class. I’m not going to stray too far from the petrol/diesel debate that lies at the core of this story, but that rattle from under the bonnet sits alongside the rubbish materials in the cabin, the clunky navigation and at times irritating ergonomics as different colours in the same picture. It is as well for Alfa that the Stelvio looks so good and handles so well, for as a result it remains a car with genuine appeal for those happy to overlook such shortcomings. 

What we’re actually seeing here is the Macan beating the Stelvio despite and not because it is powered by petrol. The key difference between them is that the Porsche makes the most of petrol’s strength by having such a smooth engine and so responsive a gearbox, while the Alfa does not do enough to mitigate the drawbacks of diesel, principally its noise and harshness. 

Which rather begs another question: were the old Macan Diesel S still on sale, how would it fare against this petrol version? Well, it would cost more but, in my opinion, it would be worth it. It would be quicker, more responsive, still use less fuel, go further on a tank and, because it had six rather than four cylinders, would suffer few if any of the refinement issues that dog the Stelvio. 

There are wider inferences to draw here: although not everyone’s four-cylinder petrol engine is as smooth as the Macan’s, nor everyone’s four-cylinder diesel as challenging as the Stelvio’s, the truth is that diesel is inherently better suited to multi-cylinder applications. The smaller, lighter and cheaper a car is, the harder it is to make a case for diesel. At the other end of the spectrum, the V8 diesel is one of my favourite engine configurations, never bettered than when under the bonnet of the second-generation Porsche Panamera. What a shame it has been deleted, for it makes the V8 diesel also one of the most endangered configurations. 

In conclusion, I should point out that while the Macan would win this test if it were a straightforward comparison, I would maintain that the right diesel engine is inherently better suited to such a car than even the most fluently engineered petrol-powered equivalent. It is only because it does so well with such limited resources that the Porsche is able to overwhelm the Alfa here. That said, if running costs, range and low-down throttle response matter more than refinement, there is still a case for the Stelvio. 

So far as middling to large SUVs costing £40,000 and up are concerned, were all other things equal diesel would be the preferable power source and by a significant margin. But they’re not. So while diesel beats petrol, Porsche still beats Alfa. 

Read more

Opinion: Why we should buy new, cleaner-than-ever diesels​

Insight: Is it time to give up on the diesel engine?​

Porsche confirms it will no longer offer diesel engines​

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Comments
36

16 March 2019

I don't doubt that diesel is the better power source judged from the standpoint of efficiency, torque and CO2 emissions; the higher energy density of the fuel coupled with the superior thermal efficiency of the diesel engine make this a dead cert. 

But the real question, perhaps, is should we still be using fossil fuel at all to power these large, heavy, overpowered status symbols? it's a finite resource and whichever fuel is considered, burning it inevitably causes some pollution and CO2, neither of which are desirable for us or our planet.

Let's not forget that even the best combustion engines are much less than 50% efficient - and do we really need to take nearly two tonnes of metal with us everywhere we go?   These cars may be superb engineering achievements, but then so too was the Mallard steam locomotive.    

16 March 2019

Yes, electric is the future - no doubt. Once the technolgy is more mature, the take up will increase. Until then, for the vast majority - it will remain an expensive compromise.

16 March 2019
LP in Brighton wrote:

I don't doubt that diesel is the better power source judged from the standpoint of efficiency, torque and CO2 emissions; the higher energy density of the fuel coupled with the superior thermal efficiency of the diesel engine make this a dead cert. 

But the real question, perhaps, is should we still be using fossil fuel at all to power these large, heavy, overpowered status symbols? it's a finite resource and whichever fuel is considered, burning it inevitably causes some pollution and CO2, neither of which are desirable for us or our planet.

Let's not forget that even the best combustion engines are much less than 50% efficient - and do we really need to take nearly two tonnes of metal with us everywhere we go?   These cars may be superb engineering achievements, but then so too was the Mallard steam locomotive.    

Electric is the future and manufacturers are moving there, but it cant be done overnight and dont forget that when we do get there, making electricity will STILL "cause some pollution and CO2", less, definately and that can only be a good thing, but making it will still cause pollution and CO2. Why is it that so many people think once almost all cars are electric they will cause absolutely no pollution at all ? This is total madness as it will be 50 years+ at least before we re able to fully harness the amount of energy the sun gives us for free.

XXXX just went POP.

17 March 2019
typos1 wrote:

LP in Brighton wrote:

I don't doubt that diesel is the better power source judged from the standpoint of efficiency, torque and CO2 emissions; the higher energy density of the fuel coupled with the superior thermal efficiency of the diesel engine make this a dead cert. 

But the real question, perhaps, is should we still be using fossil fuel at all to power these large, heavy, overpowered status symbols? it's a finite resource and whichever fuel is considered, burning it inevitably causes some pollution and CO2, neither of which are desirable for us or our planet.

Let's not forget that even the best combustion engines are much less than 50% efficient - and do we really need to take nearly two tonnes of metal with us everywhere we go?   These cars may be superb engineering achievements, but then so too was the Mallard steam locomotive.    

Electric is the future and manufacturers are moving there, but it cant be done overnight and dont forget that when we do get there, making electricity will STILL "cause some pollution and CO2", less, definately and that can only be a good thing, but making it will still cause pollution and CO2. Why is it that so many people think once almost all cars are electric they will cause absolutely no pollution at all ? This is total madness as it will be 50 years+ at least before we re able to fully harness the amount of energy the sun gives us for free.

 

You have a point.... but surely even the dimmest consumer out there is going to cotton onto the fact that EVs produce CO2, so long as the UK Government continues to frighten off the French, Chinese, Toshiba and Hitachi from building nuclear reactors. ? 20% of UK electricity is still coal-fired: which is your point, of course :-)

Surely, whilst we grapple with this conundrum, Autocar and other 'influencers' ought to be putting the boot into SUVs which (beyond a Renault Captur-size of vehicle) are utterly ludicrous answers to a question that nobody (with any mechanical literacy) ever asked ?

A sign of a decadent society whose actions don't give a toss about future generations ? The jury is leaning to the affirmative, I think.

BertoniBertone

289

17 March 2019

Its not really a case of 'frightening off' the French & Chinese from building our reactors....its more a case that they commence construction and then try to hold us to ransome with ludicrous pr kw prices.

We should never allow something as important as our energy supplies to be in the hands of such shisters! They are not to be trusted.

18 March 2019
BertoniBertone wrote:

typos1 wrote:

LP in Brighton wrote:

I don't doubt that diesel is the better power source judged from the standpoint of efficiency, torque and CO2 emissions; the higher energy density of the fuel coupled with the superior thermal efficiency of the diesel engine make this a dead cert. 

But the real question, perhaps, is should we still be using fossil fuel at all to power these large, heavy, overpowered status symbols? it's a finite resource and whichever fuel is considered, burning it inevitably causes some pollution and CO2, neither of which are desirable for us or our planet.

Let's not forget that even the best combustion engines are much less than 50% efficient - and do we really need to take nearly two tonnes of metal with us everywhere we go?   These cars may be superb engineering achievements, but then so too was the Mallard steam locomotive.    

Electric is the future and manufacturers are moving there, but it cant be done overnight and dont forget that when we do get there, making electricity will STILL "cause some pollution and CO2", less, definately and that can only be a good thing, but making it will still cause pollution and CO2. Why is it that so many people think once almost all cars are electric they will cause absolutely no pollution at all ? This is total madness as it will be 50 years+ at least before we re able to fully harness the amount of energy the sun gives us for free.

 

You have a point.... but surely even the dimmest consumer out there is going to cotton onto the fact that EVs produce CO2, so long as the UK Government continues to frighten off the French, Chinese, Toshiba and Hitachi from building nuclear reactors. ? 20% of UK electricity is still coal-fired: which is your point, of course :-)

..

Gotta correct, coal is less than 5% and will be 0% before 2025.

Wind Power is near to 20% though and you don't have to be dim to know that

typos1 - Just can’t respect opinion

16 March 2019

I'd love to meet the person who'd be willing to splurge £45k on a 4-pot oil burning Alfa.

How much is that going to be worth in 3 years? £15k max?

16 March 2019
gloc wrote:

I'd love to meet the person who'd be willing to splurge £45k on a 4-pot oil burning Alfa.

How much is that going to be worth in 3 years? £15k max?

No need to consult the crystal ball, just look at the GFV under a PCP.  GFV won't tell you what the trade price will be, but it will tell you the minimum someone is willing to give you for the car. According to Alfa's price configuration, if you take average annual mileage, I've seen a GFV for:

£44090 diesel Alfa - £20761

The exact same car with a petrol engine costs £1500 more, so GFV of

£45590 petrol Alfa  - £22207

In other words using the exact same criteria, the GFV of diesel  -v- petrol is the same.

Your assumption that diesel values will plummet is factless and based on specualtion - real world figures suggest otherwise.

16 March 2019
scotty5 wrote:

gloc wrote:

I'd love to meet the person who'd be willing to splurge £45k on a 4-pot oil burning Alfa.

How much is that going to be worth in 3 years? £15k max?

No need to consult the crystal ball, just look at the GFV under a PCP.  GFV won't tell you what the trade price will be, but it will tell you the minimum someone is willing to give you for the car. According to Alfa's price configuration, if you take average annual mileage, I've seen a GFV for:

£44090 diesel Alfa - £20761

The exact same car with a petrol engine costs £1500 more, so GFV of

£45590 petrol Alfa  - £22207

In other words using the exact same criteria, the GFV of diesel  -v- petrol is the same.

Your assumption that diesel values will plummet is factless and based on specualtion - real world figures suggest otherwise.

Firstly those aren't 'real world' figures, that's how much Alfa will charge you to keep the car after 3 years, not how much you'd be able to sell the car for. A TWO year old £45k Diesel Stelvio will only fetch £20500 on WBAC, god knows what a 3 yr old would fetch. Everyone (well most people) knows that manufacturers inflate these GFV to try to prop up residuals.

Secondly, you bizarrely made the assumption that I was making a petrol vs. diesel point. I was pointing out the insanity of buying a new Alfa regardless of fuel type-would have thought that obvious enough ;-)

 

16 March 2019

The problem with diesel was never 2.0 TDi engines doing 18k + miles per year but Honest John readers who never met a car related issues that they could understand for themselves buying diesels and clocking up 6000 miles a year and then having a melt down when the DPF clogged up 'for no reason'.

I'd guess Porsche has good handle in how it's customers drive and are fairly confident that the 18k a year driver is a small enough constituency to not cause major issues.

On the economy question, a 3.3 Kia Sorrento run over 2 weeks on the west coast was doing around 18mpg (UK) in traffic and 33-40 on the open road. I estimated it would add about 33% to my fuel bill if I was switching from a diesel to petrol Sorrento. The car was 5 up plus luggage for much of the driving time.

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