Astonishing Fuel Economy (Gas Mileage)

I’ve been running a Ford Mondeo TDCI for a few years now. In many ways it’s a fantastic car. They are cheap to purchase as there’s plenty of them around and they don’t have the image cachet of a BMW or Audi.

For a family, it’s a capacious vehicle with plenty of space for passengers, along with a boot big enough for a few more bodies all of their luggage. With the rear seats folded down pretty much anything from Ikea or for trips to the local refuse site will fit.

The best bit though is it’s a surprisingly good driver’s car. Handling is a great balance between passenger comfort and ride quality, matched with superb dynamics and handling. It’s a car capable of 130mph, with swift acceleration and frugal fuel use (although admittedly not simultaneously!).

It’s this latter element that is amazing. In normal use, a mixture of spirited driving and more sensible commuting runs to work, the vehicle always returns > 50MPG.

Today, after almost two weeks of the most boring, careful driving it’s possible to imagine, I’ve just got 61MPG!

For a car that’s so large, has >150k miles on the clock and yet is still reasonably powerful (this is the 130PS version) that’s quite amazing and really raises questions about vehicles such as the Toyota Prius and other eco-focussed vehicles. Whilst these may have lower CO emissions, they rarely seem to display to the sort of economy figures I’d be expectingfrom a supposedly efficient vehicle. They are also nowhere near as much fun to drive.

It does raise the question as to why there aren’t more diesel hybrids, the lower emissions of almost any modern small diesel engine, coupled with their ease of driving that comes from the low RPM torque output would surely make a winning combination?

It seems that this is starting to happen, with Peugeot, Volvo and Mercedes amongst the first to produce such a combination.

All of this isn’t without it’s downsides to the driver though. Common rail diesel engines, as used in almost all modern diesels, are conceptually very simple bits of engineering. Fuel stored at mind-boggling high pressures in a reservoir (the common rail) is injected into the cylinders via the injectors. The high pressures and design of the injectors results in very efficient fuel burn owing to the efficient atomisation of the fuel.

These high pressures though take their toll. In the Mondeo, a CR system a few years old, the peak pressure is around 1600 bar, over 23000 psi! Compared to a petrol fuelled vehicle’s 50-100 psi this is a massive increase. Modern systems run at even higher fuel pressures.

The pumps to create this need fine tolerances and are under high stresses, as are the injectors whose job it is to meter precise quantities of fuel with great accuracy.

Whilst old low pressure diesel systems would pretty much run forever, failures in the newer systems are becoming more common. Injectors have shorter lives, due to both the stresses they are under and the increased number of operating cycles that results from the pilot injection techiques used to provide multiple fuel injections per combustion cycle, which results in quieter and smoother running.

Pumps are lubricated by the fuel passing through them and fuel lubricity changes as a result of  lower sulphur fuels need augmenting with additives  in order to prevent excessive pump wear. Even poor servicing technique, when changing a fuel filter for example, can result in latent wear that manifests itself as an expensive bill down the line.

Just about any failure on the common rail fuel system can result in £1000+ repair bills, with fuel pumps costing >£600 just for the parts, injectors costing >£250 each etc.

In addition the high torque surges present on modern diesel engines have to be managed to prevent both damage to the drivetrain and provide a good user experience. To this end dual-mass flywheels have become common. A part that would last the life of the vehicle is now failing with alarming regularity, and at very high cost to the end user.

The quality of the engineering therefore becomes critical, in a highly cost-centric market such as the automotive one, where for mainstream manufacturers they only need, or want, to last the length of the warranty period, there’s a fine balance to be had here.

Just one expensive bill wipes out a whole year’s fuel savings, rendering the benefits of the increased efficiency redundant. To the person that buys a new car regularly and disposes of it before the warranty expires, this isn’t an issue. But to the used car buyer this is a very real issue, and one that, having expoerienced it first hand, I will be giving very serious consideration to, when choosing my next car.


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