It seems axiomatic that improved aerodynamics will
result in better fuel economy - and that smoothing the underside of a vehicle is
one way of gaining that increased slipperiness. However, we found this is simply
not always the case...
The car in question was a Honda Insight.
Already boasting excellent aerodynamics (a drag
coefficient of just 0.25 and a small frontal area), the hybrid Honda has been
successfully modified to improve its already world-beating fuel economy.
We’ve altered the amount of part-throttle Exhaust
Gas Recirculation (EGR) that occurs (this reduces pumping losses - see Tweaking the EGR, Part 1 and Tweaking the EGR, Part 1); altered the way in which
the ECU monitors throttle position (so allowing the engine management to stay
longer in its ultra-lean ‘lean cruise’ mode – see Giving the Insight a Good Driver); and changed the intake to
give a slight positive pressure through part of the intake system (see We Have a Record!). In addition we’ve fitted
dashboard LEDs that show when the air/fuel ratio is ultra lean or very rich, and
another LED that indicates the action of the EGR valve (see Monitoring Factory Oxygen Sensors, Part 2). These LEDs allow the driver
to slightly alter driving behaviour, so improving fuel economy.
In the past we’ve also tried aerodynamic vortex
generators across the trailing edge of the hatchback, something that measurably
worsened fuel economy (see Blowing the Vortex, Part 4).
So what about trialling some more aero changes?
Inspection under the car shows that while the Honda runs more underbody
panelling than many cars, the rear half of the underside still looks rather
poor. The torsion beam axle is fully exposed, and there are panel gaps between
the fuel tank, spare wheel well and axle location.
On the other hand, the front underbody panels are
smoother and more continuous – although gaps still exist.
Trial Rear Undertray
Using ‘Corflute’ lightweight sign material, held
in place with cable ties and heavy duty duct tape, a rear undertray was formed
that totally enclosed the rear axle, leaving exposed only the springs and lower
parts of the dampers.
For safety reasons, the undertray did not enclose
the exhaust pipe or rear muffler.
In addition, two ramp-shaped fairings were
...and then positioned in front of the rear
The rear underside of the car then looked like
this. Installing the fairings and undertray took about three hours of work with
scissors, tape and Corflute signs.
The easiest way of assessing whether or not a real
world reduction in drag has occurred is to measure open-road fuel economy. If
the drag has been lessened by an amount that results in a measurable improvement
in fuel economy, clearly the modification has been successful.
Testing was undertaken on a freeway. A
64-kilometre loop was driven, half heading in one direction and the other half
in the other direction. The car was driven at 105 km/h for about half the route
and at 100 km/h for the other half. The same driving style was used for each
test and the traffic was such that very similar runs could be made.
So, how to assess the fuel economy? The Insight’s
trip fuel economy readout was used; importantly, this reads in litres/100km to
only one decimal place. This means that to try to gain a feel for very small
changes, some estimates needed to be made of the missing second decimal
The first test was conducted with the rear
undertray and fairings in place. This resulted in a displayed fuel economy of
2.9 litres/100km, but importantly, as the car exited the freeway and slowed to a
stop, this dropped to 2.8 litres/100km. Therefore, the fuel consumption with the
undertray and fairings in place was very close to the 2.8/2.9 changeover point –
say around 2.84 litres/100km.
The next test was made with the undertray in place
but the fairings removed. This resulted in a measured 2.9 litres/100km but this
time the figure stayed unchanging as the car came to a halt. Clearly then, the
fuel consumption in this test was a little higher. Driving off in urban
conditions (ie in conditions that give poorer fuel economy) and watching how
long it took for the display to change to 3.0 litres/100km gave an indication of
the second decimal place (ie if the consumption had actually been 2.94, then it
would very quickly ratchet up to a displayed 3.0 once the car moved off again.)
But the figure was slow to increase – I gave it a guesstimate of about 2.90
The final test was with the underside of the car
standard – both the fairings and the undertray were removed. Again after the 64
kilometres, the display showed 2.9 litres/100km; this time it was even slower to
increase once thirstier driving was undertaken. Guesstimate? Say, 2.88
So, going on both the displayed data (ie to one
decimal place) and the data guessed on the basis of how long it took the display
to increase once thirstier driving was undertaken, the results look like
Displayed Fuel Consumption
Estimated Second Decimal Place
Rear undertray and fairings
So over the three tests we’re talking an estimated
variation of 0.06 litres/100km, or 2 per cent. Even driving carefully over the
same route, I’d expect at least this variation in fuel economy - just by driving
differences. Therefore, I think that the impact of the aero mods was negligible.
I also could not pick any differences in noise or
A couple of important points.
Obviously, it would be nice to have fuel
consumption actually displayed to two decimal places, rather than having to
estimate the final decimal place. However, after watching the display for many
thousands of kilometres, I know it’s possible to get a good feel as to whether a
displayed 2.9 litres/100km is near to changing to 3.0 litres/100km – or to 2.8.
In the testing described here, the terrain and driving conditions also meant the fuel
consumption improved as each drive progressed – so another way of getting a feel
for that second decimal place was to observe whether the number had only just
changed to 2.9 litres/100, or had done so much earlier in the drive. I am
confident that the second decimal place guesstimates are in the right
To be worthy of pursuing further, I think the aero
mods should have delivered a clear improvement, one that would cause say a 5 per
cent change in fuel economy. A change of this magnitude would have been directly
observable on the fuel consumption display.
Finally, testing conducted over thousands of
kilometres may show clearer results. However, if an improvement isn’t going to
be shown in the sort of testing that was actually undertaken, I don’t think it’s
worthwhile doing much more extensive testing to try to find gains that probably