Headlines in the media regarding diesel cars are becoming rather familiar and much of it concerns emissions, which of course are quite closely related to fuel economy. The more fuel that you burn in your car, the more emissions you will inevitably create, although the relationships to the various types of gaseous emissions are somewhat complex.

It is rather disturbing that the press figures now being regularly quoted for noxious emissions, and the degree to which they exceed the EC test limits, are often not being related to the fuel consumptions achieved during these same tests. The independent bodies conducting these tests are, without doubt, testing cars on real roads, and in real driving conditions (as opposed to the static conditions of regulatory testing), but not according to any specific limits or precise test cycle, which doesn’t help when comparing one set of test results with another. No set of results is directly comparable with any other, due to the variability of road conditions, which is precisely why the, now somewhat devalued, “laboratory” tests on a dynamometer have always been used for regulatory purposes.

What is undeniable though is that the noxious emissions (primarily the oxides of nitrogen and particulates) do increase significantly as the performance potential of any car is exploited, out on the open road. The EC test cycles have long been known to be unrepresentative of real motoring and therefore unrepresentative of real driving emissions, which inevitably will increase at higher speeds and as you use higher rates of acceleration. There’s no measurable direct relationship between the noxious emissions and fuel economy though, which will vary with different cars, different drivers, and on different roads. But there’s no doubt that the less fuel you burn, the less of these most undesirable emissions you are likely to emit. So, the more economically that you drive, the less of them you will generate, which is an added incentive to drive economically; the closer you can get your fuel economy to those EU regulatory test figures, the more likely you are to similarly approach those “unattainable” emissions test limits.

We do know enough about the diesel fuel combustion process to know that high power demands of engines will produce proportionately more nitrogen oxides, and also that cold engines can also emit more of them – not necessarily because more are generated, but because the after-treatment systems to remove them don’t work as well on cold engines. Some of the recent scare stories about sky-high nitrogen oxide emissions in road tests have, in fact, been found to be down to testing with cold engines. Now we all have to start with a cold engine, but when such “real world” driving tests include an engine warm-up period that isn’t in the “soft” EU tests (when they are allowed to condition the cars at up to 30 degrees Celsius), then these tests are simply bound to produce higher emissions, and the shorter the test, the more distorted the results are likely to be. We have preached before about the benefits of keeping your car and engine as warm as you can overnight (garages are for cars, not old kitchen units and discarded trampolines!), in terms of engine wear, reliability, fuel economy, driveability, and good road vision, so let us all remember the emissions relationship too.

It’s something of a surprise that the oil companies and fuel additive suppliers haven’t jumped on the emissions bandwagon, by testing and (hopefully) proving that their products can help to keep engines clean and efficient, and thus help reduce urban air pollution. There are sometimes general claims to that effect made, but something more specifically relevant to the current diesel/nitrogen oxides situation would be particularly effective right now, one might think, although there’s no doubt in our minds whatsoever that good quality diesel fuels, with good additive packages, can benefit fuel emission levels, as well as fuel economy.

Victor Harman