Which would use less fuel?

[BLINGMW]

Just curious on what you guys think. These numbers are made up using a gear/speed calculator, hypothetical, let's say 4cyl, NA, 5spd, OBD1 car if it matters, maybe good enough for demonstration purposes. Do you think the type of car would radically change the answer?

I'm going 45 MPH. I want to accelerate to 65 in exactly 10 seconds. Which is most efficient? Why? Assume each of these take exactly 10 seconds:

1) 5th, WOT, RPM will go from 2000->2900

2) 4th, maybe ~85% throttle, RPM from 2500->3600

3) 3rd, light throttle, RPM from 3400->4900

I would think staying in 5th would be best, since WOT is known to be more efficient, but the fact that WOT means a richer burn kind of leaves it an unknown for me. So maybe staying just shy of WOT gives you the best efficiency? I'm not sure!! How will I sleep at night?

[Mike]

i vote #2, but that's just my gut.

[ScottyB]

i say it also depends alot on the power curve...but i'd second Mike and say #2

[Apoc]

Wouldn't the option with the least amount of throttle be the most efficient because you're not dumping as much fuel into the engine?

edit: I think the example is flawed/misleading because 3rd gear wouldn't be light throttle, it would be a light throttle increase. You'd still need more throttle to start with to do the 45mph stated to begin with. So my answer is it really depends on the gearing of the car and how much throttle required to maintain 45 in each gear.

[Andy]

WOT is the most efficient, 5th gear would be the lowest RPM so I vote 5th gear.

[CaptainHenreh]

WOT is the most efficient, 5th gear would be the lowest RPM so I vote 5th gear.

+1 Fuel is added by throttle position, RPM, and load. (otherwise you wouldn't need a MAF/MAP, just a TPS) WOT=Max volumetric efficiency.

You want low load, low RPM, as much throttle as possible.

We've had this discussion on MM before, I think. The consensus, IIRC, was RPM's as low as possible, as light a load as possible, with the throttle as wide as possible, without lugging the engine.

[Beej]

Hmm.

Why is it a given that WOT is the most efficient? Seems to me that would assume that it's not an ideal fuel map.

The details of engine tuning are beyond my knowledge scope...but I am curious about this.

[Mike]

without lugging the engine.

and i think that is where we struggle with this question. i come in from a honda viewpoint... WOT at 2k rpm in 5th is going to be "lugging the engine." if we do the same in say a vette or something equally ballsy, we'll scoot along just fine.

SO, i think this question greatly depends on the actual car.

[CaptainHenreh]

Hmm.

Why is it a given that WOT is the most efficient? Seems to me that would assume that it's not an ideal fuel map.

Well, depending on the injection map, you're right...it might not be. But without an intricate knowledge of your own car's fuel map you have to assume that a low load and WOT would be the most fuel "efficient", due to simple fluid mechanics.

But I am not an engineer, I only read engineering books. So anything I post, unless referenced or specifically labeled otherwise, is just a SWAG.

Although I have read, many times, that the most efficient way to drive is WOT to 2500rpm, then shift.

[PDenbigh]

I voted #3. I watch the instant MPG gauge in the E36 all the time, and have found that I get the best MPG at light load. A high rpm/light load does better than a low rpm/heavy load.

I'm making the following assumptions here:

Since you said 5th, WOT = 3rd, light throttle, I'm guessing you are going about 35mph ish?

[Mike]

you're also assuming that the bmw gauge is correct. i think the bimmerworld guys said they were a joke and can only be counted on as rough estimates as sustained speed/throttle.

[PDenbigh]

I agree Mike, and I doubt it's very accurate. But, my above claims were made with it at sustained speeds/throttle. So, whereas the exact numbers might not be right, the position is enoguh to make some conclusion.

The only variable I change is the load. I keep the throttle position and speed the same. In such an experiemnt, when going up hill at 70mph in 5th vs 4th, most often 4th reviels the higher MPG.

YMMV (ha ha ha)

[Mike]

hrug: we need an engineer in here

[BLINGMW]

We've had this discussion on MM before, I think. The consensus, IIRC, was RPM's as low as possible, as light a load as possible, with the throttle as wide as possible, without lugging the engine.

We did. But at the time I didn't think of the fact that the car ignores the O2 sensor and uses a richer fuel map at WOT. So it makes me wonder if something just shy of WOT would actually be better.

I think the example is flawed/misleading because 3rd gear wouldn't be light throttle, it would be a light throttle increase.

It would be "not much throttle", or "less than you'd have to give it in 4th or 5th". Light(er) throttle? Whatever it is, just the correct amount to get the car to accelerate from 45 to 65 in 10 sec. Which in 3rd, is not much.

i say it also depends alot on the power curve...but i'd second Mike and say #2

It probably matters, I'm sure. By how much? I don't know. Would it help if I said this engine has near flat torque from 2k to 5k? There are plenty of 4cyl engines that are like that.

i come in from a honda viewpoint... WOT at 2k rpm in 5th is going to be "lugging the engine."

I'm not sure how YOU define lugging an engine, but from what I understand it's a point where the engine is missing, hesitating under load. Even at 1k, full throttle, flat surface, in my wife's Accord for example, it will pull itself up (eventually), and doesn't miss/hesistate. So is that "lugging the engine"? I would not think so. Let's assume the engine we're talking about isn't a 9k RPM big bike engine/built race motor and actually makes decent torque at 2k. A mid-nineties commuter car. Camry. Accord. 240SX? Miata? Something with a basic 4cyl. Assume it doesn't have variable valve timing, and a 6k redline just to simplify things.

*edit* Mike, I just took a look at some dyno plots of S2000s. Many start between 2 and 2.5k, where the torque is around 125ft/lbs, and is near flat from there to 6. So I kind of doubt that 2k for your car is really "lugging it" either? :dunno:

[CaptainHenreh]

I think maybe we should look at this a different way.

Maybe we should take an inventory of all the FI inputs that determine the duty cycle (is that the right term?) of an injector, and go from there. I mean obviously throttle position, revs, oxygen sensor output, timing advance, mafs output, etc all affect how much fuel is put into the engine.

Bring on the science! Where's Sentra Mike when you need him?

[Mike]

i come in from a honda viewpoint... WOT at 2k rpm in 5th is going to be "lugging the engine."

I'm not sure how YOU define lugging an engine, but from what I understand it's a point where the engine is missing, hesitating under load. Even at 1k, full throttle, flat surface, in my wife's Accord for example, it will pull itself up (eventually), and doesn't miss/hesistate. So is that "lugging the engine"? I would not think so. Let's assume the engine we're talking about isn't a 9k RPM big bike engine/built race motor and actually makes decent torque at 2k. A mid-nineties commuter car. Camry. Accord. 240SX? Miata? Something with a basic 4cyl. Assume it doesn't have variable valve timing, and a 6k redline just to simplify things.

*edit* Mike, I just took a look at some dyno plots of S2000s. Many start between 2 and 2.5k, where the torque is around 125ft/lbs, and is near flat from there to 6. So I kind of doubt that 2k for your car is really "lugging it" either? :dunno:

yes, the car is being "lugged" at WOT at 2k rpms in 5th gear. trust me, i drive it every day. the civic, on the other hand, pulls fine from idle in just about any gear. the s2k feels like it's going to fall apart if you're not in the right gear when you floor it.

[BLINGMW]

the s2k feels like it's going to fall apart if you're not in the right gear when you floor it.

OK, sounds like we define it the same way then, interesting!

I voted #3. I watch the instant MPG gauge in the E36 all the time, and have found that I get the best MPG at light load.

See, that goes against how I understand it "should" work... don't know what to say about that!

[D_Eclipse9916]

Theres good discussion here, but I still dont get why you guys are saying WOT is optimal efficiency. Maybe im missing something, but at WOT your looking at less efficiency then at cruising or idle. WOT has to be less efficient as some of the reason of your car being "rich" at WOT is to help keep the combustion chamber cool.

I may be wrong. To give an example of fuel efficiency I Will give a very simplistic version of what "could" be a way to figure out which one is most "efficient" or which one uses the least amount of fuel, two completely different things.

Lets say we are dealing with one "unit" of gasoline as our starting point to not make it complicated.
At lets say WOT at 2000 rpms. .90 Injector duty, so 90% of a fuel unit that could possibly be pushed through an injector (keeping injector sizes the same between runs) and at 2000 rpms.

So.... 2000x.90x(the amount of time it takes to accelerate from your base speed to targeted speed)

Then lets say.... 4000 rpms x .30 injector duty cycle x amount of time it takes to accelerate at that input.


Now you say, what if the car is running rich or lean causing more or less horsepower? Thats why I used an acceleration time input. So you ignore all other inputs (thats why this is a very simplified equation)

Now I am not a physics major and there are tons of variables within this equation, but I would like some CONSTRUCTIVE not destructive input.

PS- to make this real fun, realize that a turbo doesnt spool at 2000rpm at full throttle but then does as it accelerates through 2500-3000rpm so you not only have a variable injector duty but there is an inverse relation as more fuel enters with less time to accelerate.

[CaptainHenreh]

The reason people (such as myself) are pointing to WOT is that the throttle plate is a restriction in the intake stream, and as long as the throttle is not completely open, the engine "wastes" some of it's energy in creating a vacuum, rather than turning freely.

A diesel, for example, doesn't have a throttle plate and is throttled strictly by fuel.

[D_Eclipse9916]

Well Rex, this is good for discussion, how bout the waste of energy pushing out more exhaust from the cylinders at WOT, could that possibly offset the throttle plate?