[eugene188]

So I just got my Bluetooth ODBII unit today and took the car for a drive, in the city and on the highways. I used the Android program "torque" to connect to the ODBII to get information. I set the app to display the instantaneous and average fuel consumption ratings. I was quite surprised to say the least!

I found that on the highway if I was going at 100km/h (via cruise control) that the instantaneous reading would have a median of about 6.4L/100km. HOWEVER, it fluctuated between 10L/100km and 3.5L/100km depending on if I was going up a *slight* incline or decline. I checked and the RPMs stayed the same, as the inclines were not large enough to make it kick down a gear to keep the speed.

I also found that If I accelerated from 100 to 110 slowly without making the RPMs jump up, the instantaneous fuel consumption would jump to 11L/100km before settling back down to around 7.4L/100km. I thought it would increase from 6.4 to 7.4...not jump up and then go back down.

I was always under the impression that fuel consumption was mostly dependent on the RPM (in addition to wind, aerodynamics, etc.). Now having installed the ODBII unit, it seems like the computer will decide on its own how much gasoline it will inject at the SAME RPM depending on the road. This could be because I put it in cruise control. I think I will try manually controlling the speed which will be much harder to hold it constant.

[Phil №❶]

Well, you are measuring instantaneous fuel consumption, I would expect those results. Best way to think of what you describe is imagine your'e the rider of a pushbike being asked to do the same thing. I've done it, the effort to climb an incline to maintain speed is immediate and difficult.

To ask the car to actually accelerate even on flat ground requires the same amount of energy + the additional energy to propel the WHOLE WEIGHT of the vehicle at whatever Delta V you select. Your cruise control is working fine and probably can do a better job on flat ground than you can. Not the same situation on hills though as it can't anticipate an oncoming incline, in other words, it reacts to inclines, not anticipates, anticipation driving is cheaper.

There are some smarter memebers who may be able to explain much better. 

[eugene188]

I fully understand the bicycle concept.

I just had no idea that the computer could also tell when the car was on an incline. In older cruise control cars, it was the case that when a car started going up a hill, it would keep the RPM the same as the speed decreased (because it was going up the hill). Then when there was a big enough gap, it would automatically kick down a gear, or open the throttle. Either way, the RPMs increased.

Now it seems that it computer will inject more gasoline while keeping the RPMs constant and not need to kick the RPMs up to generate the power needed to keep the speed, (unless the incline is very steep). It definitely makes for a smoother ride when you don't suddenly have the RPMs jump up.

When I first got this car, I noticed how the cruise control was different in dealing with hills compared to our 2009 Sonata.

I fully understand the bicycle concept.

I just had no idea that the computer could also tell when the car was on an incline. In older cruise control cars, it was the case that when a car started going up a hill, it would keep the RPM the same as the speed decreased (because it was going up the hill). Then when there was a big enough gap, it would automatically kick down a gear, or open the throttle. Either way, the RPMs increased.

Now it seems that it computer will inject more gasoline while keeping the RPMs constant and not need to kick the RPMs up to generate the power needed to keep the speed, (unless the incline is very steep). It definitely makes for a smoother ride when you don't suddenly have the RPMs jump up.

When I first got this car, I noticed how the cruise control was different in dealing with hills compared to our 2009 Sonata.

I want to jump in here... no disrespect but your understanding of how your car works is ... well just all wrong.

Disregarding any slip in the torque converter (should there be one) there is a fixed relationship between the engine speed and the car speed. One cannot change without the other unless changing gear of course and then there is a new fixed (but different) relationship.

The CC just attempts to maintain a given speed. It has no idea what forces act on the car.

[komaterpillar]

 

[eugene188]

I fully understand the bicycle concept.

I just had no idea that the computer could also tell when the car was on an incline. In older cruise control cars, it was the case that when a car started going up a hill, it would keep the RPM the same as the speed decreased (because it was going up the hill). Then when there was a big enough gap, it would automatically kick down a gear, or open the throttle. Either way, the RPMs increased.

Now it seems that it computer will inject more gasoline while keeping the RPMs constant and not need to kick the RPMs up to generate the power needed to keep the speed, (unless the incline is very steep). It definitely makes for a smoother ride when you don't suddenly have the RPMs jump up.

When I first got this car, I noticed how the cruise control was different in dealing with hills compared to our 2009 Sonata.

I want to jump in here... no disrespect but your understanding of how your car works is ... well just all wrong.

Disregarding any slip in the torque converter (should there be one) there is a fixed relationship between the engine speed and the car speed. One cannot change without the other unless changing gear of course and then there is a new fixed (but different) relationship.

The CC just attempts to maintain a given speed. It has no idea what forces act on the car.

this is why i am surprised that the fuel consumptions change while the RPM stay the same!

I always thought speed was directly related to RPM. However, this ODBII unit is saying otherwise. This is why i'm confused!

I took a course in Engines a long time ago, this the ODBII is making it seem like magic rite now. Unless it is improperly calibrated.

How do you guys explain that at the SAME speed, RPM and gear ratio that the fuel consumption can go from about 3L/100km to 10L/100km?

[Asterix]

 

How do you guys explain that at the SAME speed, RPM and gear ratio that the fuel consumption can go from about 3L/100km to 10L/100km?

Let's stay with the bike. 

If you ride the bike at x speed on flat road you rotate the pedals at xx rev' s. If you want to keep the same speed in the same gear when coming to an incline, you will need to maintain the same revs, but you need to produce more power to keep the speed.

Same thing happens in the car.

When coming to the incline, the ecu adjust the ignition from e.g 8 º before TDC (Top Dead Center) to e.g 15 º enhance the fuel injection accordingly, but as the car is still travelling at the same speed, it maintains the same rev's.

Hope this makes sense....

.. i am surprised that the fuel consumptions change while the RPM stay the same!

Fuel consumption is, for the purpose of this discussion, completely unrelated to engine RPM and it is this sticking point that has led to your misunderstanding.

Fuel consumption is related only to the work done by the engine to maintain the demanded speed for the prevailing conditions, incline decline etc.

You have agreed it takes more effort to ride a bike up hill. Take that analogy to mean the engine must produce more power to drive up hill also. More power means more fuel must be burned to provide the energy.

Simply put, consumption will be directly proportional to the pressure you apply with your right foot! If you push firmer to go up a hill the RPM will only rise if the extra power produced is greater than that required to maintain current speed. If RPM rises so will speed - in direct proportion. i.e., RPM and car speed are in lock-step.

[Phil №❶]

Another thing to consider here is if you think in old technology, that is, carby engines there were 2 mechanisms at play.

Firstly, a small pump that sprayed fuel into the inlet manifold to provide instantaneous power and a rich mixture.

Secondly the vacuum of the engine the drew fuel at a much reduced rate to maintain the engine load and that was it. If the load on the engine varied either to incline or accelerate, YOU had to make a physical adjustment to the carby settings, using the throttle which would cause a variation in instantaneous fuel consumption.

In our fuel injected cars the amount of fuel is metered according to many variables, air temp, pressure engine temp, throttle position and of course cruise control. At every power stroke a new rate of fuel is calculated according to those settings in order to maintain a happy compromise between performance and economy. The instantaneous will vary because of this. 

[eugene188]

In our fuel injected cars the amount of fuel is metered according to many variables, air temp, pressure engine temp, throttle position and of course cruise control. At every power stroke a new rate of fuel is calculated according to those settings in order to maintain a happy compromise between performance and economy. The instantaneous will vary because of this. 

This is what I was getting at! From the ODB readings, I get that the computer is able to change the amount of fuel that it sprays, INDEPENDENT of the RPM of the engine. This was what surprised me.

I was thinking old school and thought that RPM was the only thing that governed fuel consumption.

In our older cars, it seemed the only way to keep up a fixed speed going up a hill was to increase RPM, but now it seems that is not necessarily the case anymore.

I haven't actively read about car technologies since high school and never even got into the nitty gritty before then. Looks like we are well past the stage of using different lobes on a camshaft (similar to VTEC) and gotten much closer to drive-by-wire where the engine can always get the best timing for intake or exhaust.

[Phil №❶]

It would not be possible to achieve the efficiences and pollution reductions, without this technology, trouble is, we can't service our cars at home anymore. 

[Keith]

And its OBDII not ODBII surely?
Small point but worth making.

Or did I miss something?

I was thinking old school and thought that RPM was the only thing that governed fuel consumption.

This was never old school. Instead it was always a nonsense as I've tried to explain.

In our older cars, it seemed the only way to keep up a fixed speed going up a hill was to increase RPM, but now it seems that is not necessarily the case anymore.

NO, NO, NO.  I can't help if you don't read what I write... I give up!!!!

[komaterpillar]

In our older cars, it seemed the only way to keep up a fixed speed going up a hill was to increase RPM, but now it seems that is not necessarily the case anymore.

NO, NO, NO.  I can't help if you don't read what I write... I give up!!!!

 

[komaterpillar]

This is what I was getting at! From the ODB readings, I get that the computer is able to change the amount of fuel that it sprays, INDEPENDENT of the RPM of the engine. This was what surprised me.

I was thinking old school and thought that RPM was the only thing that governed fuel consumption.

any efi engine can alter the amount of fuel injected irrespective of rpm. injector pulse length is governed by a number of fuel maps, one fuel map will use throttle position and manifold absolute pressure to work out a injector pulse length, another map will then take rpm and work out the number of pulses per second.

the ecu will then use other inputs like intake air temp, coolant temp, 02 sensor signal, knock sensor signal, to make adjustments to the above maps.

so there is almost an infinite number of fuel rates for a given rpm and therefore an infinite number of power outputs for the same rpm.

your rpm and speed stay the same (because they are directly proportional) when going up a hill because your cc opens the throttle plate wich changes the tps sensor signal, wich changes the map sensor signal, wich inturn tells the ecu to pick a different fuel rate from the parametres governed by the fuel maps available to it. the ecu then uses inputs from all of the afore mentioned sensors to fine tune the pulse length and frequency.

bare in mind the fuel rate is recalculated once for every rotation of the crank, so at 4000rpm that is 66 times a second! 
 
clear as mud?   

[Asterix]

clear as mud?   

Sooo simple.... 

I knew we could count on you, Kom