Fuel display and a/c - observations of my 2013 Suzuki Swift

[BLSTIC]

*This copy and paste is from my post on a Swift specific forum, so the fuel consumption display part probably doesn't apply to any US market cars. But you aren't all from the US and it's interesting read nonetheless. Also the part regarding A/C effect on injector cut-off *has* been confirmed on several other late model MT vehicles and I think it deserves a mention under the a/c dot point on the hypermiling page*

Being an engineer at heart I like to get the most out of my machines, be that performance, economy, comfort, sound quality, whatever. So with great interest I began to watch my fuel consumption display.

First up, how the numbers appear:

I suggest you set the display to l/100km (or whatever the equivalent Volume/Distance is in your region, not Distance per Volume) for maximum technical benefit from the display as, at least in metric form, it is limited to a max value of 30. If set to km/l and you cruise at 60km/h with the a/c off you can actually max this out with a tailwind, so it's of limited use, but 30l/100km is very hard to get.

You'll notice that the instantaneous display takes *some time* to settle on a number. This is because it's not really instantaneous, or even a timed rolling average... It's an average of the last *displayed* value and the current *actual* usage. Refreshed every second or so.

(Last display + Current actual) / 2 = New Display

This is based on observations that when coasting down when the injectors would normally be cut (2000rpm and zero throttle, engine warmed up) the display drops by exactly 50% per refresh (plus rounding errors, which always go to the high side) regardless of change in speed. A similar thing happens when going from injector cut to steady state cruise (i.e. setting cruise control with zero throttle), where the *change* seems to drop by 50% per refresh.

Also, the display seems to be some 20% or so optimistic (somewhat consistent though, so at least it's of some use for predicting fuel usage).

But I promised A/C data... Oh yeah...

Through observing this with the a/c switched on (in the tropics it almost never disengages, I mean it's 33 degrees C with 95% humidity half the time, it will never win) injector cut is set at 1500rpm. Which coincides with 60km/h (give or take a small margin).

However with the a/c switched OFF the injector cut is lowered to a surprisingly low 1100rpm, and the car loses much of its engine braking. The side effect of this is that you can coast for a very long time without using fuel.

One example is within 500m of my house there starts a 500m 'straight' with some traffic calming chicanes in it, with an elevation drop of 3m over 40 at the start of it. If I'm doing 65km/h at the bottom of the hill I can coast under cut-off until I have to stop for the stop sign at the end of the straight. Through well timed gear changes I can get down to 10km/h without the display reading more than 0.1l/100km. And it does this within 60 seconds of a cold start...

The AVERAGE fuel consumption display in my car gets reset every tank. Last tank it displayed 5.5l/100km (6.79 measured via fill-up and odometer), that was with the a/c running all the time. This tank I have used the a/c very sparingly (typically only when the car was parked and is being driven in full sun, or it's raining and there's a safety issue with screens fogging up) and it's displaying 4.5. If the ratio remains the same, that's a creditable 5.55l/100km in full urban conditions...

Thoughts/observations/etc welcome

Ben