We seem to have reached something of a critical point in diesel fuel economy, with a good number of smaller diesel cars now offering EC combined figures of around 80mpg, corresponding to CO2 emissions of around 80 to 90g/km, although we know that this probably only equates to real life economy of maybe 60mpg plus. But where next, we might ask?

What prospects remain for further significant reductions from these sorts of figures, when the EC commitment is for the average emissions of all new cars sold to be reduced in phases down to a challenging 95g/km by 2020?

Manufacturers were ahead of target in 2012, with average new car emissions down to 132g/km, but seven years to reduce emissions by a further 35g/km is quite a fearsome challenge, with further reductions in NOx emissions also to be met. So what sort of technology and constructional changes might help achieve the 2020 target of 95g/km for carbon dioxide, equating to 77mpg in diesel terms?

We know that weight, aerodynamics, and power efficiency are the main factors involved, but a big factor will be downsizing, with many small and medium new cars already close to meeting the key 95g/km target figure.

A continuing move to buying smaller and lighter new cars in place of larger, heavier vehicles will undoubtedly make a big contribution to meeting these targets, and greater numbers of those will be low-emission hybrids and electric cars. So manufacturers will be continuing to reduce their weight and wind resistance, along with improving fuel efficiency.

They are already heavily into weight reduction, as we know from the significant reductions achieved by key new models, like the Volkswagen Golf and its close relations, the Peugeot 208, and Renault Clio, and there’s ample scope for further progress.

Aerodynamics and practicality tend to pull in opposite directions though, and the increasing popularity of SUV type cars suggests that there may be no great dividends in this area.

We’ve already seen much progress in saving, or recovering energy, using regenerative braking, stop/start systems, and reducing waste heat by achieving quicker engine warm-up. “Smart” computer-managed cooling systems offer further scope, and ways of harnessing exhaust gas heat energy may become viable, while LED lighting systems will also save energy over conventional technology.

But just how great is the threat of more frugal petrol cars, with smaller and more efficient turbocharged engines, which might displace diesel power as the fuel of choice? It’s significant, no doubt, particularly for city cars, but there’s a new generation of diesel engines already in the pipeline that will soon see diesel power fighting back.

Let’s not also forget that big cars like the Range Rover and Porsche Cayenne would be extinct by now, if refined and relatively economical diesel engines had not been developed to power them. But many big-volume diesel engines, whilst having been modified over the years to improve efficiency, are quite aged in basic design, and their replacements can’t be too far away.

Ground-breaking 1.5-litre three-cylinder 150bhp engines are due to appear soon in new range of MINIs and small BMWs. We foresee more smaller diesel engines of 1.4 to 1.6-litres, many probably also of three-cylinder construction, dragging fuel economy figures down towards real life figures of around 70mpg and perhaps even more.

There will be a bonus of more passenger space from the reduced engine bulk, while the use of more light alloys and composite plastics will allow chassis and body components to also shed weight, and we’re sure that smaller cars which hit figures below 80g/km of CO2 will be popular before very long.

Such speculation, based on sound reasoning, makes us feel sure that each new model will take us diesel drivers “The Extra Mile” and give us an even bigger “Extra Smile” whenever we fill up for many years more, and that such improvements mean it will be some good while yet before electric and fuel cell cars come to rule the roost.

So the diesel car you are driving is most unlikely to be your last, and the next one will undoubtedly cover an extra mile, or more, on every gallon of diesel.

Victor Harman