The typical British company car has been powered by diesel for a decade or more. Fleet managers love oil-burners for their low running costs and high residuals, and drivers have also come to love them, not least for their fuel economy and tax-friendliness.
But might that dominance be coming to an end? Might the diesel even be in decline due to the appeal of petrol-electric hybrids and pure electric cars, stringent new emissions regulations and the resurgence of a rejuvenated petrol engine? That has been the rumour along motor alley for a couple of years now. And in the UK, at least, the theory appears to be backed by the stats.
Consider that in 2002, diesel-engined cars accounted for 23.5% of the UK's new car market. Three years later, this number had accelerated to 36.8% and by 2008, it had reached 43.5%. It was a rise as strong as a modern diesel's low-rev pulling power, and it didn't stop there - by 2011, diesels were claiming over one in every two new car sales, at 50.6%.
And then the market levelled off. Diesels took 50.8% last year, but in the first half of 2013 they dropped, falling to 49.0%. And no, it's not electric cars that are knocking them back; their share is a mere 1.5% so far this year, compared with the 1.3% they held at the halfway stage in 2012. Instead, it's petrol-engined cars whose sales have risen, by a couple of percentage points so far this year.
There are good reasons for this. Fuel prices are an obvious one, the average price of a litre of diesel being 139.4p, compared with petrol's 135p - 25p more per gallon.
Contrary to popular opinion, this price difference is not due to fuel duty, which is identical for both at 57.95p per litre, but to the demand for diesel and the oil industry's limited refining capacity for it.
The price gap between the two fuels almost disappeared during 2009, but re-emerged in winter 2010-11 and late 2011.
Whatever the reasons, the price difference between the fuels is small disincentive to buy diesel cars, especially as they tend to cost more than petrol models. The more robust engines needed for the higher pressures created within their cylinders, plus more complex fuel injection hardware are why.
Consider that a 115 bhp Ford Focus 1.6 TDCi Edge costs £17,895, compared with £16,305 for the 105 bhp 1.6 petrol Edge, for example.
Assuming you cover 12,000 miles annually, it will take 3.8 years to recoup the additional outlay, notwithstanding the fact that diesels hold their value better, perhaps to the tune of £1,500-plus at the end of that period.
But the broad consensus among car manufacturers and fleet buyers is that diesel only makes sense if you're covering more than 15,000 miles annually.
This is more true now as petrol engines have lately benefited from major improvements in fuel consumption, emissions and performance. Many petrol engines have been replaced with smaller capacity turbocharged units, as typified by Ford's Ecoboost range.
Its new three-cylinder, 1.0 litre turbo engine might seem too small to convincingly power a Focus, yet it produces 100 bhp, hits 62 mph in 12.5 seconds, produces competitively low CO2 emissions of just 109 g/km and returns an official combined fuel consumption figure of 58.9 mpg.
It's a more than decent drive too, the unusual beat of its smooth-spinning three cylinders is so well suppressed that many drivers will have no idea that it has an engine so small.
This downsizing trend is spreading right through the industry, making petrol engines decidedly more attractive. Although they currently cost more than their old-school equivalents, they're less expensive than a diesel of the same size, they're burnished by the allure of fresh technology, they're good to drive and usually turn in strikingly competitive CO2 numbers.
That's vital, because both vehicle excise duty and company car tax liabilities are based on this number.
Diesel cars could soon be going out of fashion
The price difference between a downsized petrol engine and a diesel can be expected to widen, too. Downsized petrols will become less expensive as their makers find ways to reduce cost, while diesels will get more expensive because the highly sophisticated fuel injection and exhaust-cleaning hardware required to get them through the imminent Euro 6 emission regulations are expensive.
So there are significant pressures on the diesel engine that seem to explain why its market share has levelled off.
But masked within the UK sales figures is another trend that is limiting diesel's market share. And that's the rise of city cars. These are the smallest mainstream cars on the market, typified by the Ford Ka, the Fiat 500 and the virtually identical Citroen C1/Peugeot 107/Toyota Aygo trio.
And, the ranks of the city car have recently been swelled further by another trio of clones in the shape of the excellent VW Up! and its Seat and Skoda equivalents.
The arrival of these three, a new Fiat Panda and very competitive offerings from Hyundai and Kia have all substantially boosted the size of this segment, which grew by a spectacular 202% between 2007 and 2012. The significance of this is that - Fiat and Ford apart - none is available with a diesel engine.
It might be a bit premature to be talking about the death of diesel, however. The view of Steve Williamson, diesel planning manager UK at market leader Ford, is that: 'We do not see any significant shift in the mix of fuels across Europe.
What we do see is that a combination of the latest technology of petrol engines, perhaps best demonstrated by the EcoBoost technology developed by Ford, along with some of the engineering challenges involved in meeting the upcoming emission standards for diesel engines, will stop the steady growth in diesel demand that we have seen over the past 20 years.'
Ford considers its success to be pretty robust, however.
Adds Williamson: 'The growth of diesel has been driven by customer acceptance. With improved performance and refinement, diesel has been embraced by the mainstream and we now have a generation of customers who believe that diesel is best.'
That belief is regularly revitalised too. 'Despite purchase price and fuel economy being apparently rational purchase considerations, it is clear that customers are just as open to their emotional influence in their choice of engine as they are on the way a vehicle looks,' he reckons.
'This is influenced by the fact that while you may buy a car once every three to four years, you buy fuel every two weeks - therefore, customers who place a high value on fuel economy will have their rational or emotional views about their vehicle/fuel choice reinforced on a regular basis.'
Ford also believes that the improved competitiveness of petrol engines will mean that they offer better value for most private buyers. But Williamson 'expects our diesel customers to remain a mix of higher mileage customers (including many fleet operators) who can realise an economic advantage and those other customers who continue to prefer the driving characteristics (primarily torque delivery) of a diesel and are prepared to pay a premium for it'.
That premium will also win buyers a better class of diesel engine in time, as demonstrated by Mazda's excellent new SkyActiv 2.2 diesel, whose radical low-compression design produces excellent fuel economy and a higher quality driving experience.
Toyota - now collaborating with BMW on diesel development - is expected to follow with a similar evolution, to which Mazda is already preparing an answer. Williamson believes there is 'still plenty of development remaining on current diesel engines to provide a future through to the next decade'.
So diesel is far from dead. Instead, it has found an equilibrium at today's fuel prices and with today's technology.
The good news for fleet managers and drivers alike is that the development of both kinds of engine is far from over, and that a whole phalanx of plug-in hybrids promising spectacular economy are coming along too, so the choice of what to drive will only get wider and more flexible - if at a price.