Fuel Useage Question

[hoopty5.0]

I've been calculating the advantages of moving from a 4.10 to a 3.31 gear. Assuming a 26" tall tire with 1:1 final drive ratio at a constant speed for simplicity, the engine would spin 19% less with the numerically lower gears.

For fuel useage, a slower spinning engine would have a shorter injector pulse width, so it's tough to use the same fuel consumption when calculating the different consumption rates. I.E an engine spinning at 2500 rpm will use less fuel than an engine at 1750 rpm, assuming similar load.

How would I go about figuring fuel consumption?



...yes, it's slow at work today.

 

[modulistic]

think about it like changing gears from 4th to 5th. in 4th your foot isnt into the pedal as deep but revs are higher. in fifth, the revs drop down, but you have to open the throttle a little more to keep the same speed.

at higher RPM, the throttle will not be open as far, so vaccum will be higher. when vac is high, the cylinders only partially fill and less fuel will need to be sprayed. at lower vaccum, the cylinders will fill more, so more fuel will need to be sprayed. at boost, the cylinders will be over-filled, so even more fuel will need to be sprayed.

at higher vaccum (higher rpm, same load), the pulse width will be shorter because there will be less fuel to spray per combustion cycle.

at lower vac, (lower RPM, throttle opened more) cylinders will be filled more, more fuel per stroke, longer PW.



max efficiency is at max torque rpom of the engine.

 

[hoopty5.0]

If pulse width decreases with RPM, wouldn't that negate the need for larger injectors? And going from 4th to 5th, the throttle closes, otherwise you'd keep accelerating, assuming you are trying to maintain a speed. Not sure I follow the logic there.

 

[modulistic]

at a particular RPM, the throttle can full open, full closed, or anywhere in between.

lets say 2000 rpm.

if the throttle is nearly closed, you arent drawing in much air, vacuum is high. pulse width is low, less fuel sprayed. you are spraying just enough fuel to keep speed constant

suddenly you snap the throttle wide open. you start drawing maximum air, the RPM hasnt changed a bit yet at this point, but you are drawing all the air your engine can breathe. pulse width increases to spray more fuel to match the increased amount of air coming in.


pulse width is not hard mapped to engine speed, pulse width varies depending on load.

 

[hoopty5.0]

That makes sense now. But it seems like you were trying to illustrate the point that numerically lower gears would be worse off on fuel economy, that's what threw me off.

 

[modulistic]

nah, I was just saying that just because your engine is turning slower doesnt mean that you will be automatically spraying less fuel.

take it to its aburd logical extreme. pretend you can shift into gear 5th, then 6th, then 7th ... then 30th gear. in that imaginary gear, pretend that one RPM will carry you 1 complete mile.

just pretend that it works.

in that one RPM, would you need to spray one tiny little Pulse of fuel, or would you need to spray closer to a cup of fuel to carry you that whole mile? and would that be efficient?

the answer is you would need an unusually large amount of fuel, and it would not be efficient.

 

[hoopty5.0]

Gotcha. So... to answer my initial question, would it be safe to assume that the difference in required fuel for each gear be negligible assuming the numerically lower gear might require more fuel?

The math showed 4,171.5 less engine revolutions per mile going to 3.31 from 4.10. Surely that has to account for less fuel assuming all other factors are held constant??

 

[modulistic]

thats a tough question, and its above my head to compute it. I do know that just because you turn down RPMs doesnt necessarily turn into better MPG. to test this, get in 5th gear - cruise at 800 RPMS and see how far you have to press the gas to maintain that RPM and speed.

every engine has a rpm where it is most efficient. That RPM is its peak torque RPM. That doesnt mean its most efficient to drive at peak torque RPM. Peak torque RPM might have you running at 100MPH, which would cause so much wind resistance that you would have the throttle open very far just to maintain that speed. What you need to do is figure out what speed you want to cruise at, what your max torque RPM is, and if you are comfortable cruising at that RPM. its all a balance.


the reason hybrids running internal combustion generators are so efficient is because you can tune the engine to be efficient at one particular RPM and stay at that RPM constantly - completely ignoring the rest of the power band. if the engine is just turning a generator to make electricity it works very well, you dont need low end torque to get the car moving or need to worry about power across a broad RPM range.

 

[hoopty5.0]

hmm. Well, I'm getting in the high teens mileage right now, but I know there is more in it. I was just hoping there was a way to tell what's happening mathematically without being able to data log.

It'd be cool to have the capability to datalog pulse width at different speeds with different rear gear ratios to see how it affects efficiency. Im sure you could get flow rates and eventually figure out fuel used over time.

My first serious 'mod' to this car will be a Megasquirt setup, which could do that I believe. I've wanted one for a couple of years now and it's going to happen this time. On top of that, I can tune target AFRs and definitely increase fuel economy in the cruise rpms. Won't be til Xmas bonus though

 

[toyman]

Then in addition to all these variables you have to introduce the characteristics associated with the formation and propagation of the flame kernel as the fuel ignites. That's why I suspect the rated fuel consumption numbers the manufacture provides and the real world numbers are quite different. I've done all the tuning (AEM-EMS) on mine and at cruise I run the AFR close to 16 at a steady load and rpm. In addition, on deceleration from speed with the throttle closed and rpm's above 1600 fuel is shut off.

 

[hoopty5.0]

Then in addition to all these variables you have to introduce the characteristics associated with the formation and propagation of the flame kernel as the fuel ignites. That's why I suspect the rated fuel consumption numbers the manufacture provides and the real world numbers are quite different. I've done all the tuning (AEM-EMS) on mine and at cruise I run the AFR close to 16 at a steady load and rpm. In addition, on deceleration from speed with the throttle closed and rpm's above 1600 fuel is shut off.

That's a topic above my pay grade, sir.

With regard to your tuning, what kind of mileage do you get?