Throttle Position During Acceleration and its effect on FE

[BEN_EJ8]

I A-B-A tested this, and it looks like my civic has a powerful lust for gasoline when I lower the shift point and give it 40-50% throttle. Ive been driving like an old lady for a while, accelerating so slow I get passed within 5 seconds of the light turning green sometimes, averaging 37 mpg while shifting at 2500. Then, after reading this thread, I tried giving what I think is a little less than half throttle while accelerating from a stop, and shifting at only 2200. Not good at all; for 3 weeks I got 33 mpg on every fillup; atleast its consistent

Now this past week I went back to my old acceleration, and since its been raining every single day for 2 weeks I have been coasting less, but still managed to get my best mpg to date, a whopping 6 mpg more than lowering my shiftpoint/raising throttle open a little. Im mad that its been raining every day; I couldve very easily surpassed my goal, but instead Im stuck less than 1 mpg away with winter weather setting in any day now.

[JanGeo]

BSFC - ok how do you figure out where you are operating on the graph . . . by the fuel consumption in gph? We don't have torque measuring instruments in our cars.

[omgwtfbyobbq]

Originally Posted by JanGeo

BSFC - ok how do you figure out where you are operating on the graph . . . by the fuel consumption in gph? We don't have torque measuring instruments in our cars.

Sure we do, our right feet, er, foot? At least on FI cars that use a TB. When a drive by wire system is in use even throttle is out of the driver's hands, er, feet. So in that case it would be hard to tell w/o a tps reading. But as long as I have a tach in a TB/FI car, I know that when I floor it, it's sucking in as much air as possible, and the torque at some rpm is proportional to how much air I let in the engine via the gas pedal.

Something I've noticed when driving cars with drive by wire is that throttle tip in seems excessive. The same amount of gas pedal movement produces much more torque in a drive by wire car compared to a TB car, so I think that quite a bit of ECU programming has gone into minimizing pumping losses this way by almost always accelerating in nice BSFC regions, so that manufacturers can further increase the mpg the average driver gets.

Back to TB/FI, we can assume that minimum BSFC happens where the sum of friction and pumping losses is minimized, so at some lower rpm for friction, higher throttle area fr pumping losses.

[JanGeo]

Yeah but the graph curve for a certain BSFC is only available at a certain range of RPM you have to really know the fuel consumption - torque is all relative to gearing and incline and full throttle is not where you want to be in most cases according to the graphs. I guess I will go by the acceleration vs pedal input and stay in the area where the acceleration is the best for the least pedal used - the crisp zone - before I enter the bog zone when I give it too much pedal.

[omgwtfbyobbq]

The thing with full throttle is that even though the mixture's richened on purpose, it's still better than minimal throttle by a long shot. For instance in the first graph I posted, at full throttle fuel consumption only becomes worse than 1/3rd throttle at 4k rpm. A confound is that going at full throttle for any extended period of time will have you going faster than at minimal throttle so unless you have exactly the same average speed, you're wasting energy more efficiently. ; ) At exactly the same average speed, 1/2-full throttle should have the engine operating about twice as efficeintly when accelerating, which is why drive by wire cars are so peppy, they are almost always going at some significant portion of throttle to mimize BSFC.Like you said, going by acceleration compared to pedal on a drive by wire system is best since hopefully, the ECU has already picked a good throttle/rpm range for efficiency so you can just concentrate on driving slowly.

[JanGeo]

I have a feeling with my VVT-i engine with the variable intake valve timing they already compensate for the pumping losses at low throttle by closing the intake valve after BTC allowing some of the pumping losses to be recovered on the compression stroke by closing the intake on the upstroke of the compression stroke allowing the vacuum to pull the piston up. Will have to find the graph for my engine a Scion xB.

[omgwtfbyobbq]

I think what you're refering to is the atkinson cycle, which initially draws in more air then it needs, then leaves the intake valve open after the throttle plate has closed in order to have some of the air pushed back in the manifold and reduce the difference in pressure/pumping losses.

iirc a vvti engine doesn't stay open to change the amount of air/pressure after the intake valve has closed, which is what's responsible for a significant portion of low load pumping losses. it just opens earlier, almost during the exhaust stroke, to let more air in at a lower rpm as well as allow for some measure of internal egr. it allows for nicer low end torque/power w/o sacrificing high end torque/power. Here's an animation of the process.

It's definitely better than a normal engine because it virtually eliminates an entire section of low load operation when accelerating by pulling in more air than the normal engine can, but it's still not as good as the atkinson cycle which can minimize pumping losses at any engine load/speed, like cruising on the freeway.

[Compaq888]

Accelarating too much is bad. I winded out 4th till 35mph and my street went from 42mpg to 38mpg.

The 5th gear is so econonomical that you can press the hell out of the gas pedal and it will still get better mileage than the other gears.

So I'm thinking accelarate half throttle in all gears and shift into 5th at still accelarate half throttle to your desired speed and then cruise at that speed in 5th.

[BEN_EJ8]

Originally Posted by Compaq888

Accelarating too much is bad. I winded out 4th till 35mph and my street went from 42mpg to 38mpg.

I wind 4th out til 40-45 in my civic while accelerating and get 5-6 better mpg than when shifting at 35. My car seems to defy the normal results when I drive for FE

[Compaq888]

Originally Posted by BEN_EJ8

I wind 4th out til 40-45 in my civic while accelerating and get 5-6 better mpg than when shifting at 35. My car seems to defy the normal results when I drive for FE

Your car is averaging 37mpg. Mine average 39 because I like to accelarate like I'm racing somebody. I won't see another sub 40mpg tank again. I tried to wind out 4th longer and look what it did.(lost a couple mpg on the street) 5th is the most economical gear, doesn't matter if your're going WOT in 5th.

My new plan is to shift into second at <10mph, into 3rd at 15mph, into 4th at 20mph and into 5th at 27-28mph and then just use 5th half throttle till I get to my desired speed.

Your car is an ex, which has a totally different tranny and IM. Which changes everything. What's good for your car might be bad for mine.