Generic Fuel Injection Question

[kc5f]

I was driving my CVT Versa this morning without cruise on and got to wondering...

On a level stretch with the rpm at 2500 I was running about 74mph. Going up a long, low hill and keeping it at 2500 the car stayed at about 70mph. Going down a long, low hill at 2500 it stayed at about 78mph.

It seems safe to assume that at the same rpm going faster would mean I'd get farther on the same amount of fuel (higher mileage), and likewise at the same rpm going slower would mean lower mileage.

What I was wondering was whether, at the same rpm, when the engine is working harder to go up a hill is the fuel injection system putting the same amount of gas into each cylinder for each spark as when on level road, or does it add more gas for more "oomph".

Related, is there more slip in the transmission when accelerating or going uphill so that the same rpm gives less speed than on level ground? And I wonder which has more effect - the loss in mileage due to increased use of gas or due to slip in the transmission?

Just wondering...

[Alphahawk]

I can't answer that question as I am looking into that very same thing. The crusie does a great job of keeping the speed up and the rpm about the same on a long low sloped hill......much better than I can do without cruise. I don't know about the injectors or fuel use. I do know that using cruise traveling at 60mph I get far better gas mileage than at traveling 55mph without using cruise. I am going to test using the cruise on my 55mph trip today and will let you know. I have been making a 200 mile roundtrip about 3 times a week on a somewhat hilly hi-way but never used the cruise and would get between 35 and 37 mpg. After taking a 200 mile trip on interstate yesterday....using cruise all the way......on similar road conditions and drivng at 60mph I got 41.8mpg I always thought cruise was your enemy on hilly roads....and it may be on very steep hills but I am not sure on low sloping hills. I stayed up late last night reading lots of articles about CVT. I am still not at the base I want to be as to how this interacts with the fuel consumption. I do know that when coasting down a hill with a CVT in gear it gives much more braking to a car than a regular automatic. This CVT is a strange bird to most of us but in my opinion and from all I have read it is a great tool for saving gas.

[lain]

too early for questions like this...ill come back to this thread some time this afternoonish.

[Red Devil]

Isnt it that every 5 mph over 60 you lose X% of fuel efficiency? So going faster over 60 would equal less mpg regardless of engine rpm's right?

[Alphahawk]

That may be true but on my old Ford Explorer SportTrac my mpg was much better at 65mph than 55mph. For those that may question that I don't know why but I owned that truck 8 years and had over 260,000 miles on it when I sold it. I did an mpg check on every tank of gas that went in that truck. This CVT is a whole new ball game. It seems to me that it is great on the open road for mpg but not so in city driving. I don't do much city driving and am glad I don't. My trip today was a 200 mile one with the cruise set at 55mph and I got 35mpg. But I really cannot compare to the trip yesterday as on this one the hills are short but steep. That must account for the lower mpg. But as on guy on the forum has already stated "I need to quit be so anal about MPG". Although it does make for good disscussion.

[kc5f]

Well, my original question really had nothing to do with mileage at different speeds. I understand that wind resistance, etc, will affect my mileage.

What I really wanted to know was at any given rpm, if more gas is injected into the cylinder per firing if you're going uphill or accelerating than if you were coasting or going downhill at the same rpm. My gut says yes, and that the CVT also compensates for however much power is available net of carrying the car up the hill or adding in the extra energy going downhill.

Just a thought question...

[hiimjered]

Someone who knows more, please correct me if I'm wrong, but:

Short answer: yes the amount of fuel injected can vary depending on work being done.

Long answer:The amount of fuel the injectors release is based on a combination of information coming from the MAF sensor and the upstream O2 sensor. Basically the amount of fuel injected is dependant on the amount of oxygen entering the cylinder.

So fuel usage should be consistent for a given amount of throttle. When you open up the throttle, more air is released into the engine, sending a little more into each cylinder. The injectors pump fuel to match the added oxygen and a little more power is produced, spinning the engine a little harder and producing more power. If the amount of resistance increases equally, the RPMs will remain the same, but the fuel usage will rise.

Still that increase is unlikely to be nearly as significant as the increase in usage as RPMs increase. The amount of fuel used at a given RPM speed is unlikely to vary more than 25% while the amount of fuel used when spinning at 3k instead of 2k is naturally a 50% increase.

RPMs should be a really good gauge of fuel usage (although if you are really curious, you could get a scanguage 2 but overall, the harder you press the throttle the more fuel you use.

Once again, I'm not an engineer, so I could be a little off.

[Alphahawk]

I did a search on your question....way over my head......but if you Google it you will find many answers to that question. Some articles say less gas goes into the cylinder coasting in gear ...says the ECU is programmed that way. If you are in neutral......which tells the car it is in a no load idle position.......which uses more gas. Need an engineer for this one.

[Alphahawk]

You are right about the Scan gauge. After all the articles I just finished reading one can get confused with rpm and gas usage.....especially with the CVT. I do not want to spend the money on one but having been in the missile field for 30 years now my curiosity is up and the only way to find these things out is with the proper test equipment and from what I read the Scan gauge 2 is as good as I will be able to afford and tell me some real time info.

[Vahagn23]

been traveling trhough the I40 and its motly doing 80s here and the fuel efficiency is not as great, I did 90-100 to pass a truck and I can see the car drinking fuel like water. When I do 60 its good. My RPM at 80 is 3k

[kc5f]

Thanks hiimjered and Alphahawk - You really answered what I was wondering. You made me think about one other thing when you mentioned the scangage. Does it get fuel consumption numbers from the computer based on cylinder firings, or does it actually know from the fuel injectors when extra/less gas is used per firing?

(Sorry for all the technical questions - I really don't have interest in lowering my car or putting in LED's over the cupholders, but I'm interested in the innards!)

[Alphahawk]

It will give you many readings such as fuel being used....or I think what you are looking for ......how far are the injectors open in a percentage format such as....Throttlle Open 20%. You can read about it at the ScanGuage 2 page. There are now more features...than just a few months ago.....and the price has dropped a little.

[kc5f]

I'd checked out the website, but just looked again. The do list "fuel rate" as one of the gauge options, and I was just wondering if anyone knew if this was calculated based on cylinder firings or actually using some calculation of that and the injection rate. I know some things like "distance to empty" are estimates, and was wondering how accurate this one is.

[hiimjered]

The fuel rate factor is one of the ones determined by your car's PCM. The PCM determines fuel rate by calculating the amount of fuel that will pass through the injectors based on the expected fuel pressure and the current injector duration. Basically it is a calculation of the fuel going through the injectors.

[kc5f]

Thanks, again! I knew there'd be somebody at this site who'd have answers to my strange questions!