For those looking for that extra mile...

I designed and built a small PCB, basically a network of lowpass filters and an LED indicator, to allow me to observe fuel delivery in my Cavalier. The board attaches to a fuel injector and provides a voltage proportional to the injector's duty cycle to the DMM inside the car. The board keeps an average of the injector voltage so it remains fairly steady while the car coasts and climbs. I put the DMM in min/max mode and averaged the two to derive these numbers. Each test was a 15 mile run with the A/C off, the windows up and the cruise control on.

My car: 1998, 2200 (2.2L i4), 4spd Auto, stock body, 44psi in tires, cold air intake, 190lb payload.

MPH mV Eff
0 26 0 (idling)
0 34 0 (idling +A/C)
45 100 450
47 102 460
50 104 480
53 106 500 <- Peak Efficiency
55 116 470
65 164 396
75 226 332

The Efficiency index is calculated by: (MPH/mV)*1000 It's a relative figure to help compare efficiency at certain speeds. Higher numbers are better and represent more miles per gallon. For example, my car consumes 50% more fuel per mile at 75mph than at 53mph (500/332). By cruising at 53mph, I've made many 138 mile round trips to my university in Philadelphia with about 3.3 gallons of 87 octane fuel, yielding over 40mpg. The same trip used to consume over 4.5 gallons at 75mph, confirming my data.

I did this to pinpoint the sweetspot of my car. It's different for every make and model. The more aerodynamic the car, the higher in mph the sweetspot. Those with cars which get the best mileage at 80+ mph could care less about gas prices. Large vehicles have lower sweetspots, so when that Excursion blows past you at 80, you can bet he's going to be hurting at the pump.

The efficiency sweetspot of 53mph may vary (slightly) if you have a different engine or transmission. Since I could only test my own car, I can't give results for other drivetrain configurations or body types.

-On a purely highway commute, driving at 53mph instead of 75mph may yield over 180 more miles per tank.

-Every 3 and a half minutes spent idling with the A/C on wastes enough fuel to cover one mile at 53mph.

If you find this information useful, feel free to make it a sticky.

-Solid Snake

2002 Cavalier 2200 5spd

Also...

-The 2200 does not use any fuel while coasting downhill in top gear. The ECU cuts the injectors.

-It also does not use any more fuel while stopped in drive than it does stopped in neutral.

2002 Cavalier 2200 5spd

Oohh.. Interesting, as an electronics person myself, I'd like to see your pcb design. Just for curioucities sake and the posibility of mimiking your test on my 3spd 2200. Just a thought though, wouldnt you be able to calculate similar figures based on rpm and mpg youre getting? Granted the results would be more vague but should be fairly similar in the big picture. Wonder how the 3spd compares thought, with no overdrive and all.

I think what you did is pretty neat. If these were installed on cars from the factory people would start slowing down!

From my experience with a scanner is the injector pulse width changes very little except at very high throttle settings. The difference between say 55 and 60 is just the extra pulses for more RPM, faster you go more pulses per mile more fuel used.

Joe:
The number of pulses per mile (not per second) seems to be the same regardless of speed, given the same gear is used. If the car is in 4th the whole time, the injectors will fire the same amount of times but will stay open longer at higher speeds.

Qaz:
Topping the tank off and repeating the test, then seeing how many gallons the tank takes may give similar results, but I wanted to totally eliminate city miles from the test as they would skew results. Also, fuel consumption is the combination of throttle position, RPM and load. Reading the tach (if I had one) by itself wouldn't tell me anything useful. If fuel consumption follwed RPM directly, then our cars would get X amount of miles per gallon in Y gear regardless of speed. Also, I wanted to keep my foot out of the equation so I used cruise control. Automatics do downshift under cruise control if climbing, so RPM is not a constant. Even under cruise control, average injector voltage fluctuated wildly from 0 to 200mV at 50mph depending on the incline/decline of the road. This is why the lowpass filter was necessary. If your car has cruise control, you'll notice that the pedal moves depending on the incline of the road although RPM is constant.

BTW: Full out WOT acceleration near redline averaged around 950mV.

2002 Cavalier 2200 5spd

The schematic in its simplest form is a ~154Kohm resistor and a 1000uf capacitor in series (pay attention to polarity). I added several of these filters to get average and instant figures.

If you look under the black metal shield above the fuel rail, you can see the rightmost injector. The wire closest to your right is the positive terminal and the other is the negative terminal. I accessed the contacts by pushing sewing pins through the rubber seal of the connector along the wire.

Node 0 = Injector+, Resistor
Node 1 = Capacitor+, Resistor, DMM+
Node 2 = Injector-, Capacitor-, DMM-

Draw it out on paper, it will make more sense. Put a buddy in the passenger's seat to log numbers. I recommend my min/max method. If your meter doesn't log min/max, have the buddy do it. It's safer to have a passenger while logging since you can fully concentrate on the road. Make sure your DMM's impedence is very high or else results will be skewed. 1000uf will give you damped results and will absorb fluctuations. If you want instant numbers, use a much smaller capacitor, maybe 33uf.

Before you begin logging at highway speed, make sure the numbers are fluctuating in both directions. In other words, don't start logging min/max when first starting the engine and the numbers are only climbing, or obviously, your min will be zero and will drag down your results.

2002 Cavalier 2200 5spd