Fuel Economy Testing Part One
by David Finlay (12 May 03)
How do you measure the distance a car will travel on a given amount of fuel? The makers of a particularly useless television programme once tried to do this by taking three cars, putting in a gallon of petrol, setting the tripmeters to zero and then driving them through city traffic until they spluttered to a halt.
At first sight this seems like the obvious method, but a few seconds of logical thought would have shown that it's a completely potty idea. Never mind that the cars were known to be about to break down, endangering other road users. Never mind that the last few drops of fuel would not be picked up, so the entire gallon would not be used. Never mind that running a car dry is, second only to putting a grenade inside it, the quickest and easiest way of destroying a catalytic convertor.
Even ignoring all that, the programme makers could not possibly have made a sensible comparison with the official consumption figures of the three cars (which is what they were trying to do). You can't average, say, 30mpg on a given piece of road and then assume that 30mpg is the normal rate at which your car uses fuel.
A second journey on exactly the same road could produce a completely different figure according to variations in traffic conditions, driving style, wind direction, tyre pressures, the number of passengers, the amount of luggage and all the possible combinations of lights, windscreen wipers, radios, satellite navigation systems that may be switched on at the time.
Official economy figures can't be determined this way. They are necessarily arbitrary, and we've had enough correspondence from CARkeys readers lately to realise that it's time to look into how they are calculated.
The first surprising fact in this feature (and there are several more to follow) is that the amount of fuel going into the engine isn't measured at all. The tests used to be carried out this way, but it's quite a tricky process in these days of injection engines because some of the fuel that leaves the tank returns to it without going into the engine. The fuel sent and the fuel returned both have to be metered very accurately, and European economy tests are based on the conceptually more complicated but in reality simpler method of exhaust gas analysis.
It's a bit like emissions testing. In fact, it is emissions testing. Very few people outside the motor industry have any idea how this works, so let's take a quick look.
Emissions tests are conducted in specially-built labs inside car factories. The cars being tested (which make up about 1% of total production) sit on rolling roads - basically sets of rollers which are spun by the driven wheels. The test drivers then have to drive along a virtual "route" determined by European Union legislation. The route is about seven miles long, takes twenty minutes to complete, and is divided into two parts called Urban and Extra Urban.
The test drivers follow a speed trace - accelerate from rest to 80km/h, then back off to 35km/h, and so on - and they have to be incredibly accurate. Errors of up to 2km/h are ignored, but the total permitted time for greater discrepancies is one second in the whole twenty minutes. Any more than that and the test has to be abandoned and restarted, to the great annoyance of everyone else in the lab. These testers are among the unsung heroes of the driving world.
In establishing the speed trace, the authorities had to allow for all the types of vehicle that would have to be put through it, including very slow ones. This leads to the amazing statistic that the time allowed for the 0-100km/h (0-62mph) part of the test is FIFTY SECONDS! Clearly, high-performance cars are not extended at all - the tricky bit is trying to make sure they don't accelerate more quickly - and it might seem that this immediately invalidates the test. On the other hand, there is a balance involved in the fact that the test doesn't include the equivalent of, for example, long periods of very light-throttle motorway cruising, when real-world fuel economy is much better than it tends to be in the lab.
At the end of the test, four bags of gas are taken for sample. Two are filled with the exhaust produced by the car during the Urban and Extra Urban periods of the test. The other two are filled with air taken from the lab (and therefore sucked in by the car's induction system) during each period; these may differ from each other by minute amounts, and they are required for comparison because there may be more of a certain gas floating around in the lab than the car itself produces. It's the difference between the ambient air and the exhaust emission that determines figures, not simply the emission itself.
For economy testing purposes, the important gases are carbon monoxide (CO) and carbon dioxide (CO2). There is a maximum legal amount of CO that can be produced, and although the CO2 figure is chosen by the manufacturer, the test must show that the car emits that quantity of it.
Modern cars produce around a hundred thousand times as much CO2 as CO, though that's not a reflection of what the engine does. It's down to the catalytic convertor, one of whose main purposes is to convert CO into CO2 (the former is poisonous and the latter isn't, whatever its effects on the atmosphere may be). The discrepancy in the amounts of each gas produced means that CO2 is therefore by far the most important element in determining fuel economy figures.
Very complex computer programs churn out these figures for the Urban and Extra Urban parts of the test. The Combined fuel economy rating is an average of the two (CO2 emissions, incidentally, are always expressed as a combination).
As mentioned at the beginning of this article, real-life economy figures can only relate to the exact circumstances of a particular journey. Official figures, for all the compromises involved, are at least conducted in identical conditions for all cars, and give some sort of comparison between similar models. That's why CARkeys always publishes the official figures rather than conducting its own tests - there is simply nothing that any magazine could realistically do to give a sensible alternative reading.
But there's a lot more to the subject than we've covered here. How, for example, can a rolling road replicate the aerodynamic and friction resistance found in normal motoring? Does a car's fuel economy change as it gets older? And - in response to several e-mails we've had from readers lately - why do some cars seem to achieve their Combined economy figures with ease while others get nowhere near them.
Those questions are answered in the concluding part of this feature.