more HP + less skinny pedal = better MPG, why not?

[Wicked_Sludge]

The two hypothetical engines are producing the SAME power (even though one may produce more peak power) because the vehicle is presumably cruising at the same speed.

this is something ive often tried to explain to myself but havnt quite been able to...

lets say we take 2 rangers: a 2.3 and 4.0. the trucks are identical other than their engines. same weight, same aerodynamics, same rolling resistance, etc. if you drive these 2 trucks down the freeway next to each other at the same speed, the 2.3 will get better mileage than the 4.0. why? it takes "X" amount of horsepower to move the the two chassis down the freeway at the same speed...so the engines should consume fuel at the same rate, right?

i realize there are certain efficiency factors here...but do they really account for the 60% or more difference in fuel economy?

 

[rangerbum]

this is something ive often tried to explain to myself but havnt quite been able to...

lets say we take 2 rangers: a 2.3 and 4.0. the trucks are identical other than their engines. same weight, same aerodynamics, same rolling resistance, etc. if you drive these 2 trucks down the freeway next to each other at the same speed, the 2.3 will get better mileage than the 4.0. why? it takes "X" amount of horsepower to move the the two chassis down the freeway at the same speed...so the engines should consume fuel at the same rate, right?

i realize there are certain efficiency factors here...but do they really account for the 60% or more difference in fuel economy?

It's a very realistic thought. The only explanation I can come up with is that the engine needs a certain amount of fuel to run, The lighter pistons, rods, crank, etc. are easier to turn when There are 2 less cylinders.

 

[bmxryder]

this is something ive often tried to explain to myself but havnt quite been able to...

lets say we take 2 rangers: a 2.3 and 4.0. the trucks are identical other than their engines. same weight, same aerodynamics, same rolling resistance, etc. if you drive these 2 trucks down the freeway next to each other at the same speed, the 2.3 will get better mileage than the 4.0. why? it takes "X" amount of horsepower to move the the two chassis down the freeway at the same speed...so the engines should consume fuel at the same rate, right?

i realize there are certain efficiency factors here...but do they really account for the 60% or more difference in fuel economy?

i think thats pretty simple and all in the size of the motor alone. there is literally more area to fill in the combustion chamber with gas on the 4.0 because it has to fill 1.7 more liters.

 

[Wicked_Sludge]

a 4.0 isnt using 4.0 liters of displacement at 1/4 throttle.

like simple surf said, gasoline contains a set amount of energy per gallon. so if it takes "X" amount of horsepower to move the two chassis, the engines should be flowing the same amount of air and fuel while cruising (the 4.0's throttle will be further closed than the 2.3's).

rangerbum, that sounds valid...but then you look at the newer hondas and chevys using active fuel management (that system that turns off one or more un-needed cylinders to save fuel), and with that its possible for a 315HP V-8 in a full-size truck to get 20MPG. thats the same or better than most 4.0 rangers and it has 2 more cylinders, more weight, and more aerodynamic drag to contend with.

kind of apples to oranges i suppose...but none the less...

 

[fastpakr]

Not really apples to oranges. Any engine is most efficient when operating close to torque peak at WOT. A 2.3 is closer to that range on the highway, where a 4.0's throttle is not as far open, and typically well below peak torque with stock gearing.

Also, if you set the cruise control at a consistent speed with both engines, the fuel efficiency difference is nowhere near 60%. I've managed 22mpg with a 4.0 on the highway, and about 27.5mpg with a 2.3 in a lighter RBV chassis. That's only a 20% difference, and does not account for differences in weight.

 

[inferno94]

The majority of the difference in mpg's (and why a turbo 200hp 4cyl engine is way more efficient than a 200hp NA v6) is the internal friction in the engine. With the v6 you have 50% more points of friction, they are in a v configuration that requires balance shafts and what fastpakr said, using an engine at peak tq at wot gets best efficiency for that engine. This would be why people say 4cyls and 3.0's need the 4.1:1 rear end for better mpg's, to purposefully keep revs up. So you see a smaller 4cyl engine would have to be of terrible design to compare in mpg to a v6 which would have to be an excellent design.

I think the new ecoboost idea is a wonderful thing that hasn't been brought about earlier simply for cost of construction reasons. Anyone who has driven a well built factory turbocharged vehicle should know they are plenty reliable, plenty powerful, and great on gas for their power level. the best part is the factory can change where the powerband is as simply as changing a turbo and some software.

 

[snake]

A more interesting comparison would be to compare a 3.0 to a 4.0. Most 4.0's get what the 3.0 gets, (my 4.0 will often beat a 3.0) and both have 6 injectors to feed with a closer weight to boot.

Discuss:

 

[Wicked_Sludge]

3.0s still average higher economy than 4.0s. there are exceptions to every rule...meaning there are 3.0s that get crappy mileage and there are 4.0s that get great mileage. but on the whole the 3.0s do better.

 

[bmxryder]

a 4.0 isnt using 4.0 liters of displacement at 1/4 throttle.

like simple surf said, gasoline contains a set amount of energy per gallon. so if it takes "X" amount of horsepower to move the two chassis, the engines should be flowing the same amount of air and fuel while cruising (the 4.0's throttle will be further closed than the 2.3's).

QUOTE]

i dont see how what i said didnt make sense.

so take this out of proportion. suppossed the 4.0's pistons were the size of a trains pistons. it is just literally going to take more fuel to fill the combustion chamber to make the explosion inside.

as for the throttle being closed more in the 4.0 i would say they would be about equal since when your holding one speed your only fighting wind and friction. since the vehicles are the exact same then both aroedynamics and friction would be the same (not counting the friction within the motors). it just comes down to the 2.9 needing less fuel to run.

 

[Wicked_Sludge]

what you said did make sense for full throttle application. the 4.0 has larger chambers, but it isnt filling them all the way under part throttle cruise. it takes 4.0 liters for the 4.0 to make 160HP. it isnt making 160HP while cruising down the freeway...its making closer to..say 30-40HP (whatever it takes to overcome wind and rolling resistance). so its only using a fraction of its displacement (the cylinders arent full by the time the intake valve closes).

the same goes with the 2.3. if it took 2.3 liters of displacement to go 60MPH. you would have the throttle planted and you would be turning the 2.3's peak HP RPM, but you would by just holding steady at 60 MPH. since the 2.3 can accelerate beyond 60MPH, it must take considerably less than 2.3 liters of displacement (and 1/14th that displacement in fuel) to do 60MPH.

i guess the only explaination is the extra friction points inside the engine. but that still doesnt explain why active fuel management works.

BTW, fastpakr, i was assuming 18MPG for the 4.0 and 30MPG for the 2.3, thats where i got my 60% figure. obviously those figures will vary. but even 20% difference is a larger difference. its just weird to think that the 4.0 is at least 20% less efficient than the 2.3 even just driving steadily down the freeway..

 

[Mutant Pony]

Your just thinking too much into it. less cylinders-less fuel, solved.

Actually, I firmly believe that a 5.0 in a Ranger could get better fuel mileage than a 4.0. The problem is that when people do 5.0 swaps is that they do it for the extra power. Obtaining that extra power requires more fuel.

 

[rangerbum]

a 4.0 isnt using 4.0 liters of displacement at 1/4 throttle.

like simple surf said, gasoline contains a set amount of energy per gallon. so if it takes "X" amount of horsepower to move the two chassis, the engines should be flowing the same amount of air and fuel while cruising (the 4.0's throttle will be further closed than the 2.3's).

rangerbum, that sounds valid...but then you look at the newer hondas and chevys using active fuel management (that system that turns off one or more un-needed cylinders to save fuel), and with that its possible for a 315HP V-8 in a full-size truck to get 20MPG. thats the same or better than most 4.0 rangers and it has 2 more cylinders, more weight, and more aerodynamic drag to contend with.

kind of apples to oranges i suppose...but none the less...

I think it might still apply, if I'm understanding Active Fuel Management correctly, it shuts down from using 8 cylinders, to using either 6 or 4. By cutting the fuel off to those cylinders, displacement is reduced. But i could very likely be wrong on what Active fuel management is, as I am no professional.

 

[kerry]

It seems to me that when the engine management computer is being told to increase performance, it dials timing, fuel injectors, etc. such that plenty of fuel is on hand to prevent detonation and provide the extra umph ( provides more than the stochiometric air/fuel ratio). Under less demanding conditions, the computer optomizes efficiency by providing no more than the stoichometric amount. This will make a huge difference in fuel economy. The larger the displacement, the more difference it will make.