My car is a 6-sp, first off.

I notice that if I'm going down a hill in gear, giving the car no gas, I still do not reach 99.0 MPG until a certain speed. The speed increases with lower gears/higher rpm.

My Eagle Talon has data acquisition, and I could see that if I was coasting in gear, the injector duty cycle was 0%, which makes sense. If you're coasting down a hill in gear, gravity is turning the engine. The duty cycle is not 0% when you're idling becuase you need to supply a minimal amount of fuel to keep the engine turning. If you let the car bog down even while coasting in gear it would begin to supply fuel again. It was pretty interesting to watch.

Does the Corvette still supply fuel to the engine when you're coasting along at 0% throttle in gear??

If not, why doesn't the DIC go to 99.0 MPG instantly? I would think that it would just look at duty cycle and fuel pressure and speed and calculate it directly off that, but it is definitely not the case. If you make a quick stop your instantenous fuel economy will not instantly show 0, it will go 1.0, 0.4, 0.2, 0.0, even as you've been stopped for a few seconds.

Anyway, how does it work? And also, what is the duty cycle of the injectors in gear coasting down a hill?

 

nobody?

 

Interesting question... I would like to see an explanation as well!

 

I asked this same MPG question a few weeks ago on here, and was told it calculates off the Injectors and time, some kind of algorithim... maybe someone can explain more.

 

I believe it does due to emissions regulations.

Bill

 

I think the display is just damped so that it is more easily read.

 

It calculates it (fuel used) using the signals sent to the fuel injectors, by adding up the fuel injected into the engine. Of course to get MPG it also has to look at the speed (MPH).

Here is some explanation of the fuel metering modes:

Fuel Metering Modes of Operation
Modes of Operation
The PCM looks at voltages from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called modes. The PCM controls all modes.

Starting Mode
With the ignition switch in the ON position (before engaging the starter), the PCM energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The PCM first checks speed density, then switches to the mass air flow (MAF) sensor. The PCM also uses the engine coolant temperature (ECT), throttle position (TP), and manifold absolute pressure (MAP) sensors to determine the proper air/fuel ratio for starting. The PCM controls the amount of fuel delivered in the starting mode by changing the pulse width of the injectors. This is done by pulsing the injectors for very short times.

Clear Flood Mode
If the engine floods, clear the engine by pushing the accelerator pedal down to the floor and then crank the engine. The PCM reduces the injector pulse width in order to increase the air to fuel ratio. The PCM holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the PCM returns to the starting mode.

Run Mode
The run mode has 2 conditions called Open Loop and Closed Loop. When the engine is first started, and engine speed is above a predetermined RPM, the system begins Open Loop operation. The PCM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP, and TP sensors. The system stays in Open Loop until meeting the following conditions:

Both HO2S have varying voltage output, showing that they are hot enough to operate properly. This depends upon the engine temperature.
The ECT sensor is above a specified temperature.
A specific amount of time has elapsed after starting the engine.
Specific values for the above conditions exist for each different engine, and are stored in the electrically erasable programmable read only memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the PCM calculates the air/fuel ratio (injector on-time) based on the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode
When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent possible hesitation, the PCM increases the pulse width to the injectors to provide extra fuel during acceleration. The PCM determines the amount of fuel required based upon the throttle position, the coolant temperature, the manifold air pressure, the mass air flow and the engine speed.

Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM looks at the corresponding changes in throttle position, manifold air pressure and mass air flow. The PCM shuts OFF fuel completely if the deceleration is very rapid, or for long periods (such as long closed throttle coast-down). The fuel shuts OFF in order to protect the catalytic converters.

Battery Voltage Correction Mode
When the battery voltage is low, the PCM compensates for the weak spark delivered by the ignition system in the following ways:

Increasing the amount of fuel delivered
Increasing the idle RPM
Increasing the ignition dwell time
Fuel Cutoff Mode
The PCM cuts off fuel from the fuel injectors when certain conditions are met. This fuel shut off mode protects the powertrain from damage and improves driveability. The control module disables the injectors under the following conditions:

The ignition is OFF (prevents engine run-on)
The ignition is ON but there is no ignition reference signal (prevents flooding or backfiring)
The engine speed is too high (above red line)
The vehicle speed is too high (above rated tire speed)
During an extended, high speed, closed throttle coast down (reduces emissions and increases engine braking)

 

Very interesting oldvetter, thanks!

So it looks like it does shut off the injectors while you're coasting in gear, at least if you're doing it for a while.

The weird thing is if that is the case the DIC should go to 99.0 MPG, even if there is an averaging effect that the instant fuel economy meter does. I have a lot of hills where I live so I do a lot of coasting (in gear) down them. On the quicker ones (55+ MPH) I will see 99.0 MPG, but there is a slower one (45 MPH) that I get probably 1.5 miles of coasting out of, and it is around 60-70 MPG, depending on how fast I am going. If I put the clutch in, MPG goes higher, even as speed remains the same and one would assume MPG should go down as it has to feed gas to keep the engine idling.

Just trying to figure out why the behavior is so odd. If it was purely measuring injector duty cycle data you wouldn't see this phenomenon.

 

inaccurately

 

You're forgetting that MPG is calculated using both fuel used and SPEED ....

Let's assume 3 scenarios ........

1) Engine is at idle and car is stopped (sitting at a red traffic light)

2) Engine is at idle and car is rolling down a hill in neutral at 20 MPH

3) Engine is at idle and car is rolling down a hill in neutral at 60 MPH

(Rolliing down hills in Neutral is a bad idea ... but these are examples).

And let's just say that at idle the engine uses 1 gallon of fuel per hour.

In scenario 1 the MPG is 0 because .... duh .... we are covering 0 miles for every gallon burned

In scenario 2 the MPG is 20 because in an hour we will burn one gallon and have rolled 20 miles

In scenario 3 the MPG is 60 because in an hour we will burn one gallon and roll 60 miles.

Looking at MPG while the car is moving and the engine is disconnected from the wheels (clutch pedal on the floor or gearbox in neutral) is a meaningless number.

 

Why?
The computer still has speed info from the wheel sensors, and injector usage, so why is this number meaningless?

 

That is simply because you are rolling faster and it is consuming the same amount of fuel (little to no). The faster you are rolling, the farther you will get with it in the same fuel delivery mode

 

Black Z06, SDPC.

I'm not talking about coasting down a hill in neutral. I'm talking about doing it in gear, with 0% throttle. According to Oldvetter, this should put it in "deceleration mode" where the injectors are shut off. This means, if the injectors are truly off, as long as you are going 0.000000000001 MPH, you are getting infinite miles per gallon. The DIC does not reflect this, showing various MPG that is dependent on speed and rpm. Coast down a hill at 50 MPH in 3rd gear, 0% throttle, and again at 50 MPH in 5th gear, 0% throttle. You will get two different MPGs, neither of them 99.0. Finally, do it in neutral and you will get the highest MPG of them all. This leads me to believe that rpm is an input variable, and the "deceleration mode" is either not present or ignored. You should get the lowest MPG in neutral, and infinite while in gear.

So either the injectors are not shut off, or the DIC is doing something fishy.

What I'd really like to see is a list of input variables to see how the mileage is calculated.

 

Just a guess........
Maybe the injectors aren't usually shut off in decel, maintaining combustion to keep the cats hot so they'll work immediately when you get back on the throttle. An emissions thing?

 

You are obviously not entering said "deceleration mode." It seems to be 100% probability

 

Apparently you aren't reading the answers carefully ....

I showed you in post #10 above how your statement "You should get the lowest MPG in neutral" is incorrect .... in neutral and rolling the MPG will vary directly based on speed. If your speed is 0 (stopped), then you will see the display say 0.0 MPG .... if the car is moving with the gearbox in neutral the MPG displayed will change based on road speed.

You will only get "infinite" (99.9 displayed) ONLY if the DFCO (Deceleration Fuel Cut Off) parameters are met ....

From post #7 .....

Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM looks at the corresponding changes in throttle position, manifold air pressure and mass air flow. The PCM shuts OFF fuel completely if the deceleration is very rapid, or for long periods (such as long closed throttle coast-down). The fuel shuts OFF in order to protect the catalytic converters

The PCM will invoke DFCO when it wants to ... at that point you will see the instant mileage jump to 99.9 or whatever the highest display is. Apparently you have not reached the appropriate parameters for the PCM ... the PCM is still injecting fuel if you have an instant mileage reading lower than 99.9 ..... the service manual makes it clear that the average mileage is calculated by the IPC based on road speed and injector duty cycle .... there are no "secret" ingredients ..... the PCM has more important things to spend processor cycles on than screwing with the fuel mileage ... like injector and spark timing.

Note that it is the IPC that makes the calculation based on data from the PCM .... fuel consumption and vehicle speed are both reported by the PCM on the serial data network. The IPC uses the data for functions like the speedo, odometer, and mileage calculations. The data is reported on the network so all the cars computers that need it can read it ... such as the BCM uses the speed data to lock the vehicle doors on an M6 equipped car (personalization has AUTOLOCK set to ON) when the vehicle reaches 10 MPH. The SDM stores vehicle speed data (and other parameters) as part of its "black box" function.

Engine speed is related to fuel mileage, but is not part of the calculation. If I'm driving on a flat road at 60 MPH in 6th gear I'm burning less fuel than at 60 MPH in 3rd gear. That's why we have that "super-overdrive" 6th gear. The higher RPM use more fuel (the injectors fire more often at high RPM than at low RPM) and that is reflected in the fuel consumption the PCM reports. The RPM itself are not part of the calculation.

 

If you install larger fuel injectors, the DIC mpg reading will be off also.

 

I don't know how to say any more clearly that I am talking about coasting down a hill IN GEARwith 0% throttle. References to neutral are for comparison only. Again, I am talking about coasting IN GEAR.

REAL EXAMPLES from this morning, on a 5 minute downhill stretch that I travel every day to work.

50 MPH, 5th gear, 0% throttle: 65 MPG
50 MPH, 4th gear, 0% throttle: 50 MPG
50 MPH, neutral, 0% thottle: 80 MPG.

I understand the neutral case not being 99.0.

I do not understand the other two being non-infinite numbers, if the car is entering decel mode.

Please explain to me how in the preceding 3 scenarios, my assertation that the lowest MPG number SHOULD be in the neutral case, when if the car is in decel mode, you would get infinite mileage in the first two cases.

Thank you for your explanation on how the instant mileage works. I suppose I am just not entering decel mode. Now I'd like to know under what set of conditions you would enter it, as I would assume driving for miles and miles downhill qualifies as a "long period".

 

There are a whole bunch of parameters that the PCM uses to determine when it wants to invoke DFCO. They can be altered if you have a "tuner" package like EFILive or HPTuner.

Here's an example of people trying to alter the DFCO parameters to make it happen "sooner" ....

http://forum.efilive.com/archive/index.php/t-4785.html

As you can see .... DFCO isn't as "simple" as it sounds ..... including debate on whether or not you can get the PCM to ever report "zero" fuel usage.

 

Now that is some interesting info... thank you very much BlackZ06.

Quite different than my Talon... it would shut off the injectors immediately and only turn them back on once the engine started to bog.

Thanks everyone for your input.