Semi-hypothetical scenario time:

You are driving your car up a reasonably steep incline at a constant velocity. You can either drive in 3rd and keep the throttle about half-way in, or you can be in 4th with the throttle on the floor. Which way is more efficient/uses less gas?

From a physics perspecitve, the "work" that the engine is doing is exactly the same either way, just the negative of the work of gravity on the car down the hill. In 3rd your RPM is higher but the throttle is only part-way open, whereas is 4th, the RPM is lower but the throttle is all the way open. So I figure that the engine is getting about the same amount of air either way, but I am sure that there are a ton of small differences and losses in the engine system. Which way is actually more efficient in terms of gas?

I would guess that being in the higher gear is more efficient, since the small frictional forces of the pistons/crank will come into play at higher RPMS, perhaps others. Anyone want to weigh in?


[Modified by slyph777, 4:42 AM 12/14/2002]

 

IMO in the 3rd gear, cuz when you flored it on 4th the engine will start to build speed, now, if we are talking VTEC and on 3 gear you are on vtec and on 4th gear you're not on vtec, i'll say 4th, besides, if you think about it, on 3rd gear you TB is not fully open where on 4th gear the TB is fully open, so the injectors start putting more fuel in the system.

this might not be correct, i don't know much, but i tried!

 

I was thinking more along those times when you would be going a constant velocity in either gear. Perhaps 4th and 5th would have been a better example. I know that there are several large hills that I can just barely keep even in 5th with the throttle floored (stock EK).

Yes, in the higher gear the tb is open more, but the rpm is lower, so the engine will be combusting less, thus using less fuel.

I hadn't really thought of how VTEC would change things ... anyone?

 

it would depend upon the specific car in question, but generally speaking 3rd gear, due to mechanical advantage. The engine doesn't have to work as hard to maintain velocity, even though it is turning more RPM it doesn't need to generate as much power as it would in 4th. The small difference in RPM (which would concern frictional losses) is not enough to offset the extra amount of power you need to generate.

 

work = force * distance.
no matter what, youre going the same distance. but in 3rd gear, you are forcing the engine to move faster. technically (from the physics point of view) you are putting the same amount of work ACCORDING TO THE CAR, but there are other factors involved. if you stay in 4th gear, youll waste more gas. youve gotta figure out whats more valuable to you...to momentarily put stress on the engine, or waste a little more gas.

 

aren't you generating more power in 3rd (power = torque * RPM, roughly, and assuming the ideal flat torque curve)?

 

So, because of the gearing, the *engine* is doing less work in 4th, right (work = torque * angle)?

And what "other factors" are involved, and how are they involved?


[Modified by slyph777, 5:19 AM 12/14/2002]

 

you have to be more specific as your question, are you asking for fuel or what? or to work done by the engine? or by heat created, etc... you know what i mean.
Try setting one question as a goal, or two different questions.

EDIT: and as for efficiency you mean engine efficiency? if the car at 3rd gear is on peak torque then the engine would be at the top of the VE (volumetric efficiency) when at 4th gear the car wouldn't be at peak torque meaning it would be at some low % of the VE


[Modified by GZERO, 12:42 AM 12/14/2002]

 

in 4th gear vs 3rd, 4th will be at a higher load to keep the same speed as 3rd and
thus will consume more gas. more throttle= more load = more gas used

 

gawd ok.....

if you are in 4th, more work is done (on the *engine*) by the gasoline in order to keep the engine moving at a constant velocity. if you stay in third, you are putting more pressure to the piston, creating more work.
work also = pressure * area * distance
another factor is the transmission.
youre really making the situation more complicated than it is. what are you exactly trying to figure out here?

 

if this is your question, i believe 4th gear would be better over third...thats just what i've always hear - lower rpms = better gas milage when cuising. sorry, i dont have an explanation why though

 

4th gear would be more efficient.

The main reason is that you have the throttle open more, which means less pumping losses. This is one reason why bigger, more powerful engines are usually less gas efficient: they have the throttle more closed during regular driving. Also, if you've driven in a crx hf, you'll notice that you pretty much have the thing floored in 5th gear on the highway.

Another thing, there is more frictional losses at higher rpms.

You are right that theoretically the engne produces the same power whichever gear its in if the car goes at a constant set speed. Once you cut through all the torque, fuel, rpm, gear ratio formulas, it all comes down to the fact that if the wheels were seeing more power/ torque in one gear than another, acceleration would take place.

Also, the power input/ output of a transmission should equal (except for frictional losses, which are significant).



[Modified by Lsos, 9:13 AM 12/14/2002]

 

3rd would be more efficient because of less load on the motor. Less load means less heat loss into coolant, and less heat in the gearbox. It's all about energy losses. Think about a truck towing a car, you tow in a lower gear so you don't overheat your motor, tranny, or even rear end. The extra heat that is transfered into those parts of the car is wasted energy because it's not being transmitted to the ground.

 

"....which means less pumping losses" pumping in reference to what? if you are talking about gas... MORE gas would need to be pumped in because there is more airflow for the WOT.

"Also, if you've driven in a crx hf, you'll notice that you pretty much have the thing floored in 5th gear on the highway." the only reason why fifth gear is more efficient on the highway is because it runs at a lower gear ratio. but if you floor it in 5th and the car never accelerates because of lack of power (like going up a long steep hill...lol) you will see better gas milage in 4th gear because it will have more power at a more "relaxed state" of the engine.

Basically Efficiency all comes down to:
- having enough power to accelerate or hold a constant speed
- keeping the engine in a relaxed state i.e. lower RPM/what gear
- also how much you are on the throttle

In short if you need to accelerate and you arent getting enought power downshift and use less throttle than what you would need to accelerate at the same rate in a higher gear for the greatest fuel economy. As for going a constant speed the same rules apply if you, for some reason cant hold a certain speed in the current gear then downshift, but it is ideal for you to be in the highest gear to get the lowest RPM = less air (unless of course you have to use WOT)

 

I agree with jdmDXrex. The engine is most fuel efficient under load at its peak VE, otherwise, the peak lb-ft rpm. So if you're driving uphill, and you're closer to the highest VE in 3rd gear, then 3rd it is. But if in 3rd you're going over, and 4th is closer, then 4th it is.

But if you're highway driving on flat roads only, then since there is only a low charge applied to the engine, you'll want to drive in the taller gears for fuel efficiency. Ususally, engineers gear most road cars (at least not sports purposed cars) to be the most fuel efficient at the whereabouts of 55mph, maybe 60 in some cases.

Anyways, there's an easy way to know if you're most fuel efficient going uphill. If you start feeling that in 4th or 5th gear you're giving more throttle than the response you should be getting back, it's time to go down one gear. Repeat if necessary.


[Modified by sauceman77, 1:45 PM 12/14/2002]

 

I am really looking for fuel consumed, since I assume that that is a reasonable way to compare the "efficiency" of the engine in 2 different scenarios, all other things being equal. Am I wrong in this assumption?

 

You're not totally wrong, but not totally right either. There are more variables than that. There's the gearing of the tranny that will influence, and also the weight of the cars amongst others. Take the very fuel efficient 1.3 litre hybrid insight engine and put it in an accord, and it won't be so fuel efficient anymore, see?

 

No more gas would need to pumped. In both scenarios the engine produces more or less the same power, which more or less indicates how much air is flowing. You are at more open throttle in 4th, but lower rpms. Another thing that closed throttle does is it lowers the dynamic compression ratio...a lot.

I maintain that WOT is the most fuel efficient state for an engine. Engineers strive to be able to run an engine at WOT.

I agree with this. But, think about what happens to VE when you put a restriction in the airflow, such as a closed throttle.


[Modified by Lsos, 5:06 PM 12/14/2002]

 

you may be right with this but as far as downshifting i was talking more about only needing to if you dont have enough power(torque) in this case to get up the hill at a constant speed then it would be better to downshift.

lets say you are going up the hill in 4th at 2500 rpms at ~ 40mph now lets say that when you are going up the hill you want to accelerate to 50 mph before you get to the top of the hill. (if you have a B Series this probably wont apply since the motor put out more hp and tq but for the purposes lets just say if you have one then you are just going up a REALLY steep hill... hehehe)

Wouldnt you get better more efficient gas milage if you downshifted to 3rd and sped up to 50 and then shifted into 4th at a higher rpm(once again more hp/tq than before - easier power) before the top of the hill???
--- OR ---
Would you think that it would be more fuel efficient to go WOT in 4th the whole time and barely be able to accelerate to 50 since (remember WOT) your rpm are so low that you barely even have enough power to merely maintain your speed much less accerate up the hill???

i would think that in this case, the first one would be the ideal choice

 

Well, I guess I really don't know how to answer this one. I'm only talking about gas mileage going at a constant speed, not with acceleration.

If the aim is to accelerate and you can't do it in 4th, then obviously you need to downshift. If the aim is gas mileage over a distance, the gas mileage is going to be worse in 3rd but not even because of my WOT theories and all that, but simply because you're producing more power in 3rd. If you want to accelerate really fast then reduce the gear even futher, or hit the nitrous bottle...either way you're going to use more gas. In 4th gear, it may seem like the engine's dying and it can't do it....and this might very well be the case, but nevertheless it is running at peak efficieny.

Anyways, all I'm saying is that an engine is most efficient wil the throttle open all the way (max VE, lowest pumping losses) and at low rpms...within design limits. Under these conditions it will have the least losses, and put the most power to the ground per unit fuel burned. Whether this is practical or not is a different story.