Using an air-fuel ratio guage to improve driving

[BLSTIC]

Once upon a time I owned a Legacy GT wagon. The heavier version of a turbo AWD car. Not the best, but it had two other things going for it. It was manual, and free.

My first 'mod' to the car was a combination of a boost controller, boost guage, and an air-fue ratio meter (10LED version that monitored the factory sensor). Together they cost me a whopping $75AU new... But to the point, the O2 sensor kit...

Once installed, the $15 sensor reading kit clearly showed four things vital to fuel economy...

1) When the car was warmed up enough for closed loop
2) When the closed loop was actually operating
3) When the inector cut-off was operating
4) When load was too great for closed loop.

Now I get to explain the virtues of each number above and how I extracted the most from it...

1) When the car was warm enough for closed loop

Simply put, the O2 sensor warms up extremely quicky on this car. Typically with two minutes of driving I can see the sensor voltage start to flicker and cycle. 30 seconds after that, it was moving at full speed. Noticing this I was able to adjust my warm-up cycle. The new cycle went start the car first, then put the seat belt on, push the buttons for various defrosters, and drive. All up a 10 second operation before I was driving normally. No special treatment was given, and this got the car in closed loop faster. Driving softly could easily cost me a minute of closed loop operation. It also got my heater working faster. In NZ, that is a GOOD THING...

2) and 4) When the closed loop was operating and when load was too much.

There are a few things to note about this. On the legacy closed loop is determined by throttle position and RPM. Half throttle up to 2500rpm tapering off to zero at 4000rpm. There is no correlation to the airflow meter or manifold pressure.

Before the guage, my normal driving style was to use light throttle as much as possible, being quite slow in the process. I would also change up at around 2500rpm. But then I got a glimpse of my first BSFC graph that referenced manifold pressure AND rpm. This revealed that the highest efficiency is achieved at the highest load. But added twist is that BSFC is influenced heavily by the mixture. So my mantra became "Highest open loop load you can manage". This was when I got my O2 guage.

So after getting the guage I noted that at up to around 3500rpm closed loop was available at zero vacuum and boost. So that's how I drove. If the car wasn't idling or cruising, the throttle was opened enough to show 0psi on my boost guage. Up-shifting was around 2500rpm. Initially it was 3000rpm, but I found that would end up out-accelerating other traffic.

It was worth noting that with the boost controller wound up (to give 14 psi at WOT) I was able to get a significant 4psi under closed loop at around 2800rpm. With it set to 4psi, I could hardly get on boost at all. Although having the wastegate shut under cruise probably didn't help much, climbing a hill the extra boost without going into open loop would have...

3) Injector cut-off

To get the injector cut-off three conditions must be met. 1) Engine at operating temperature, 2) *ZERO* throttle, and 3) Above 2000rpm.

Unfortunately the injector cut-off was not easy to get. Because of the nature of the hills and the gearing, I could only get it in top gear above 80km/h, and changing down resulted in the car slowing too much. However I did develop the habit of using no throttle at all when coasting downhill, which probably saved me about $50 over the year I had the car...

The good bits: Results.

After purchasing the car, and sorting out the dying coils, I was able to get 300km out of a 50L tank around town. Not a huge amount, but given that the car weighed in at 1500kg dry, is AWD, had 270,000km on its 2.0L engine, and could still make mincemeat of boy-racers, 6km/l around town is acceptable. After adjusting driving style, with the help of the guage I was able to get 330km, with a best of 350km. A 10% improvement.

It's worth noting that my 10% improvement came without slowing the car down at all or holding up traffic. In fact I was now accelerating noticably faster... If I chose to change up at higher revs and coast longer, I probably could have stretched that to 15%... Highway economy went from 380km to 400km for 40L (all of my highway trips at the time were to work and back, a fixed distance. I couldn't use the full 50L capacity without ending up 30km away from the nearest bowser).

Also note that none of those tanks were particularly gentle. I went cruising with mates every friday night, and that involved many laps and some full throtte. And if I cam aross someone holding me up on the highway, I didn't stuff around (typical overtaking manuver hit 140km/h). And maximum boost went from 8psi to 14 at the same time that I started driving to the air-fuel ratio meter. There were no hidden factors that helped me over that period...

There you go, a hoons guide to hypermiling...

(oh, and the dancing display made by the meter when in closed loop made for a good conversation piece by passengers, not the least because it was an exposed circuit board mounted by my boost guage)