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Driving Emotion

Real life fuel economy changes with aero alterations

by Julian Edgar

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Back in July I wrote an opinion piece in which I suggested that new cars were going nowhere in aerodynamic development, especially with regards to decreasing aero drag. (See Car Aerodynamics Have Stalled.)

One of the emails we received after the article appeared suggested that aerodynamic improvements weren’t all that relevant because (unlike the aircraft cited in the article) few people travelled at high speed, aero features would add weight, the car would look ugly, and it might lose aerodynamic stability. The latter three points I think are simply answered by the actual engineering techniques used to reduce aero drag - a slippery tail-end of a car doesn’t need to cost any more to produce than a blunt end, etc.

However, what about travelling speeds?

Specifically, the writer said: High speed travelling accounts for maybe 10% of the average urbanite's travelling. So your 20% [drag] reduction may knock a whole 2% off their total fuel bill.

Of course the writer is quite correct if you’re in a car that spends much of its time stuck in traffic jams, negotiating urban car-parks or accelerating to 60 km/h before slowing for a red light, only to repeat the process many times.

But it seems to me that more Australians now drive on freeways than ever before. In Sydney, Melbourne, Brisbane and Perth freeways are widely used, with many having been built in the last few decades. (Even Adelaide has the SE freeway and the newish southern main road.) I’d go as far as to suggest that a high proportion of local commuters would now use freeways as part of their daily trip. And of course, the longer the daily commute, the more relevant the fuel bill becomes.

But what differences do aerodynamics make to the fuel economy of a commute that includes 100 and 110 km/h travel? The other day I had an easy way of finding out.

I needed to travel from my home just inland of the Gold Coast in Queensland to Redcliffe, to Brisbane’s north. The distance is around 100 kilometres and the whole distance is effectively done on freeways or main highways. I was making the trip to pick up a piece of mechanical equipment that I’d bought which I figured would fit into the boot of my 6 cylinder EF Falcon – although probably with the boot lid up a little.

So I got onto the freeway and reset the fuel computer. I then selected fifth gear and stayed in that gear nearly all the way. Sure, there were some down-changes for a toll booth, and later in the trip, some traffic lights, but it was otherwise about a fuel economical trip as it’s possible to get. The Falcon, a surprisingly fuel-efficient car for its size, achieved 7.5 litres/100 km for the trip. (And when in the past I’ve checked the fuel consumption readout against filling the tank, the fuel computer has actually shown itself to be a little pessimistic!)

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I reached Redcliffe, loaded the scroll saw into the boot and then closed the lid as far as possible. As expected, the boot-lid wouldn’t shut – the gap was about 200mm. Because the Falcon is an aero-efficient design, you’d expect attached flow across the boot lid, so the raised boot lid would have the result of increasing the size of the wake, perhaps by a little less than would occur if the airflow separated at the end of the roof.

Given that the route is quite flat and the changes in speed minor, the extra mass of the scroll saw would have made very little difference to the fuel consumption (rolling drag goes up only very slowly with weight). There was also little wind and the return route was completed driving in the same way as the initial journey.

So, was there a measurable change in fuel consumption? You bet! The average fuel consumption with the boot lid propped open a little was 8.6 litres/100 km, an increase of no less than 15 per cent!

Of course, the point of our letter writer remains valid – if I drove the Falc around a traffic-choked city with the boot open, the change in fuel consumption would be immeasurably small. But if I commuted to work down that same freeway, any reasonable aero improvement would clearly make itself felt at the fuel pump...

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