Pulse and glide experiment

[jim-frank]

Vehicle: 1993 Nissan Sentra SE, 1.6 liter, 2 door, 5 speed manual tranny
Aero mods: front airdam, about 4.5 inches above roadway
Engine mods: K&N low restriction air filter, hot air intake system
MPG monitoring: Sound card and MPG.java program on laptop computer.

I finally got the sound card interface calibrated to my satisfaction and discovered some interesting things about how the fuel injection is programmed for this car. At around 60-70 mph, there is an increase in fuel economy at about 75% throttle, peaking at 50-51 mpg at 67 mph. Holding a steady speed at 65 gives about 42 mpg, steady speed at 55 gives about 45 mpg.

This is an ideal situation for pulse and glide operation. I had a long stretch of highway to myself, with a posted speed of 65 mph. I decided to accelerate at about 75% throttle in fifth gear to 69 mph, then coast to 60 mph. I chose those limits because the Colorado State Patrol will rarely give a ticket for 4mph over the limit, and the average speed was 65 mph. With a little practice, I could coast mainly down the slightly rolling hills and through curves, and accelerate uphill and through straight sections.

The mpg during acceleration varied from 41 to 52 mpg, then shot up to 250-300 mpg during coast phase. The average mpg I achieved over a 30 mile stretch was 74.54!

After passing through a town, I had another 30 minute section of 65 mph speeds, but this time I had to use the limits of 65-69 mph, because of following traffic. I decided that if I was speeding slightly, and kept the average speed above 65, the person behind me wouldn't have too much cause to complain. Using these parameters netted an average mileage of 65.2 mpg.

The overall economy of a 186 mile stretch, including 12 miles over an 11,000 foot pass, was 57.5 mpg! That's 6mpg over my best oong trip average so far, and really illustrates the benefits of the P&G technique. It also indicates to me that the fuel injection computer can probably be greatly optimized on this car for economy at highway speeds.

I'm thinking about maybe some O2 sensor spoofing, to see if I can lean the mix at very low throttle openings.

Anyway, I just wanted to post these results. Driving style really trumps any tweaks I have made so far...

[Daox]

Very nice results.

What is this sound card interface you speak of?

[Tango Charlie]

Very Cool! P&G is a lot of fun through any kind of rolling or hilly terrain. The excellent numbers make it all the more fun!
What is the 'MPG.java' program you refer to?

[jim-frank]

diy mpg gauge - Page 3 - GasSavers.org - Helping You Save at the Pump

That's where I ran across the most comprehensive information.

I've attached the zipped file. It's a java application that uses a soundcard input to monitor injector pulsewidth and vehicle speed.

'java -jar Mpg.zip' is the command line that you must use to get the proggy to work. I put the zipped file on the desktop (win2K) and wrote a batch file called mpg.bat that contains the line above. The program will write a file called mpg.properties that contains the scaling factors to get correct vehicle speed and mileage figures.

You have to attach a resistor and clamping diode set to two places on the engine: the vehicle speed sensor, and the switched side of one of the fuel injectors. These inputs go to the right and left channel of a soundcard line input. The thread at gassavers.org has plenty of detail on how to approach this project.

I found the best way was to set the speed sensor scaling first, by comparing the speedometer with the program's reported speed. I used a GPS receiver actually, as it is more accurate than my speedo in the car.

Then keep the program running while you take a nice long drive- maybe half a tank of gas or more. Calculate the gas mileage in the usual way, then compare to the program, and tweak the scale factor.

[blownb310]

This is fantastic information. I'm a Nissan Sentra owner [and former Nissan tech] and have never seen that kind of mileage from a B13 Sentra. What else is done to the car besides your driving techniques listed here?

Mike

[jim-frank]

The tires are inflated to 45 psi, I have a K&N air filter, I've removed the stock "Y" shaped airbox intake horn, so there's just a hole going into the airbox, and I've put a hot air intake from the exhaust manifold over to the intake box. There's also a front air dam that is just tall enough to reach below the dangly bits under the car, so that the airflow is diverted from *some* interference.

Keep in mind that this mileage is on the open highway, with a fully warmed up engine, one passenger, no AC.

The latest tank is 36mpg, that's around town from home to work and such. I tried to P&G as much as possible within rush 'hour' traffic. (Rush minute, in my little home town... )

I still want to build a 555 timer circuit that will make a 1.5 volt variable frequency/duty cycle square wave that I can add to the Oxy sensor's output, to convince the ECU that the engine is running a little richer than it really is.

The engine has about 160,000 miles on it, so it's well broken in by now...

[rmay635703]

This is great news, I saw the soundcard thing a LONG time ago but have never checked up on what came of it.

Maybe my 93 suburban could get a mileage guage with this approach, then I could determine why I have gotten up to around 23mpg when I have the thing filled to the ceiling and have a genny off the hitch and only get around 21mpg empty most of the time (my father gets better mileage half empty than me and drives it much harder, perhaps he is inadvertantly pulse and gliding as he never holds a speed on the highway)

[blownb310]

Jim, any new tweaks or changes you can share with us?

Have you ever considered replacing your standard 3.89 final drive gears with the 3.65 set from the '91 4-speed E model trans?

Mike

[jim-frank]

Mike, I hadn't seriously considered it. Unless the rear end gives me trouble, I'll probably defer tweaking until there's an inescapable reason to do so...

I did build the O2 sensor override circuit, but it didn't have any discernable effect. The engine computer is apparently smart enough to ignore it when I start messing around. If I recall correctly, it *did* cause the engine to stall the first couple of times I tried it, but the ECU ignored it after that.

It's hard to measure any effects that aren't swamped by driving style. I get anywhere from 45-65 mpg on the highway now, just by P&G, keeping rpm's down, and coasting with the clutch out and the tranny in fifth gear.

The fuel injection will shut off entirely after about three-five seconds if the car is pushing the engine.

I've considered using a potentiometer to shift the DC voltage from the hot wire sensor in the airstream into the MAF. That would be an easy way to lean the mixture. However, I don't want to do that until I can also install an exhaust heat sensor that has a low enough thermal mass so that I can see hazardous increases in temperature in just a couple of seconds.

The car has almost 170,000 miles on it now. It leaks oil and power steering fluid, but other than that, is running great.

[blownb310]

Originally Posted by jim-frank

Mike, I hadn't seriously considered it. Unless the rear end gives me trouble, I'll probably defer tweaking until there's an inescapable reason to do so...

I think I'd do the swap if I had a GA16DE powered B13 like yours. I wish there was a final drive swap for my B13 SE-R, which uses a 4.16 final drive [or even better just a taller 5th gear].

Originally Posted by jim-frank

The car has almost 170,000 miles on it now. It leaks oil and power steering fluid, but other than that, is running great.

It will run great for the foreseeable future. I was a Nissan tech for years and if maintained properly the GA16DE's last forever. The oil leak[s] are usually just the front crank seal and/or the oil pressure sensor. Both are not that hard [or expensive] to fix.

Mike