2WD/4HI/4LO and fuel economy

[TJMexico]

This is really just an academic or rhetorical question, but I'm curious as to what the general consensus would be.

Now, I'm asking in my context, but the idea applies universally. I drove up to a friend's ranch today, the same route that I posted photos from a while back. It's a rough road, all gravel, big rocks and bedrock, and almost all ups and downs. I finally did the whole run today in 4LO. Didn't shift out of low range until I got back to the main road (which really is not much better than the road to my friend's place). Previously I would shift between 4LO and 4HI, depending mostly on the slant of the road. Really, low range is most useful on those long and steep downhill runs where high range or 2WD would mean either careening downhill at a dangerous pace or burning up my brake pads.

It felt like the Jeep ran a lot better using 4LO all the way as opposed to shifting back in high range periodically. What I'm wondering about is fuel consumption. I see that the tachometer stays about in the same range as if I were juicing it high range, maybe even sometimes a little lower. Given this fact, I'm figuring that my fuel consumption in low range should not be appreciably higher than it would be in high, and might even be better as the engine is not having to work as hard to pull me up hills. Even on the downhill runs the revs don't get way up as I crawl down.

What do you all think?

(I cannot really go by how much fuel I put into the tank when I fill her up as I'm siphoning out of gasoline jugs due to the fact that I am in a little village up in the mountains with no gasoline pumps; ergo, I really can't get a consistent full point. And I know that fuel consumption is really not at the top of most of our lists of priorities, but as I said, this is an academic or rhetorical question.)

 

[andyj]

The engine revs a lot more for the same distance in 4L than in 4H, so fuel consumption is higher. My TJ guzzles 13.5l per 100km in 4H, about 18l when sand driving in 4L and 23l in 4L in the Victoria High Country which is mainly very steep, rocky terrain.

 

[GoldenGorilla]

There is a tradeoff if you are lugging below a range around the powerband to a higher rpm where the engine is more efficient, but my experience is these things drink alot more fuel in 4Lo.
I can get around 15.5mpg these days on highway driving. Recently did a drive that was mostly highway with a short 20 miles or so offroad and maybe 6 miles of that downhill in 4Lo. Fuel economy was in the mid 13s on that tank and only a little was in 4Lo.

 

[ColoJeep]

In my opinion , you answered your own question. Since your Jeep is an automatic , it is working the engine more in high range by slipping the torque convertor
more creating more heat . Heat equals fuel . The engine spinning faster in low range "could " use a little more fuel , but in YOUR scenario I believe 4L is much easier on the Jeep and fuel usage . Sounds like you already use your head , and drive for the conditions . Many don't today and we end up with the "send it " mentality.

 

[Steel City 06]

If you compared 4Hi to 4Lo at the same engine speed and same road speed (using different transmission gears to make up the gearing difference), 4Hi is more efficient simply because the driveline losses are significantly more due to the much higher rotational speed of the transmission and transfer case internals. Frictional power losses scale linearly with rotational velocity, and fluid drag losses scale with the cube of rotational speed. (You can measure this loss if you tested this by adding a temperature sensor to the transmission. The transmission will be much warmer in the 4Low case.)

4Low does also provide significant additional driveline inertia that helps smooth out small sharp changes in speed, which is often why it feels smoother even with the same overall gearing.

Also interestingly, theoretically at highway speeds in a straight line, our TJs are ever so slightly more efficient in 4Hi than in 2Hi. Since the TJ has no front axle hubs, the front driveline is always spinning, meaning there is no additional driveline loss from running in 4Hi than in 2Hi. However, due to the static slip effects that tires experience under normal road conditions, when you split the load among four tires rather than two, you halve the load, and since both load and slip are linearly decreased, the energy (and also power) loss at each tire is reduced by 75%. Since you have twice the number of tires under linear loads, that means you lose only 50% as much energy to static tire slip in 4Hi as you would 2Hi.

The caveat to that is that in curves below a certain radius, the difference in the distance of paths traveled between front and rear tires can negate that effect and cause the tires to be more heavily loaded and slip more in 4Hi than in 2Hi.

That also does not apply to vehicles that have front axle disconnects like the YJ or JK+ Wranglers, or most any modern 4WD vehicle. The driveline losses from having the front axle and driveshaft spin significantly exceed the losses from only having two versus four tires under load.

 

[astjp2]

there is a formula for PLANK, and BMEP and it has to do with loading of the engine and fuel economy, lugging an engine can use more fuel than higher rpm's. BMEP is brake mean affective pressure and I dont remember the PLANK, but I remember that rpm and load affects economy.

 

[freedom_in_4low]

there is a formula for PLANK, and BMEP and it has to do with loading of the engine and fuel economy, lugging an engine can use more fuel than higher rpm's. BMEP is brake mean affective pressure and I dont remember the PLANK, but I remember that rpm and load affects economy.

In the form I'm used to, PLANK refers to the relation of HP to MEP.

hp = MEP * (Lan)/33000

L= stroke (in feet)
a = bore area (in square inches)
N = engine rpm
n = 2N * num of cylinders/number of strokes per cycle

 

[freedom_in_4low]

If you compared 4Hi to 4Lo at the same engine speed and same road speed (using different transmission gears to make up the gearing difference), 4Hi is more efficient simply because the driveline losses are significantly more due to the much higher rotational speed of the transmission and transfer case internals. Frictional power losses scale linearly with rotational velocity, and fluid drag losses scale with the cube of rotational speed. (You can measure this loss if you tested this by adding a temperature sensor to the transmission. The transmission will be much warmer in the 4Low case.)

4Low does also provide significant additional driveline inertia that helps smooth out small sharp changes in speed, which is often why it feels smoother even with the same overall gearing.

Also interestingly, theoretically at highway speeds in a straight line, our TJs are ever so slightly more efficient in 4Hi than in 2Hi. Since the TJ has no front axle hubs, the front driveline is always spinning, meaning there is no additional driveline loss from running in 4Hi than in 2Hi. However, due to the static slip effects that tires experience under normal road conditions, when you split the load among four tires rather than two, you halve the load, and since both load and slip are linearly decreased, the energy (and also power) loss at each tire is reduced by 75%. Since you have twice the number of tires under linear loads, that means you lose only 50% as much energy to static tire slip in 4Hi as you would 2Hi.

The caveat to that is that in curves below a certain radius, the difference in the distance of paths traveled between front and rear tires can negate that effect and cause the tires to be more heavily loaded and slip more in 4Hi than in 2Hi.

That also does not apply to vehicles that have front axle disconnects like the YJ or JK+ Wranglers, or most any modern 4WD vehicle. The driveline losses from having the front axle and driveshaft spin significantly exceed the losses from only having two versus four tires under load.

One thing I would add is that Low range adds an extra gear reduction stage, which will be about a 3% loss.

 

[TJMexico]

There is a tradeoff if you are lugging below a range around the powerband to a higher rpm where the engine is more efficient, but my experience is these things drink alot more fuel in 4Lo.
I can get around 15.5mpg these days on highway driving. Recently did a drive that was mostly highway with a short 20 miles or so offroad and maybe 6 miles of that downhill in 4Lo. Fuel economy was in the mid 13s on that tank and only a little was in 4Lo.

Where abouts is the powerband for the stock 6 cyl 4.0l ?

 

[TJMexico]

In my opinion , you answered your own question. Since your Jeep is an automatic , it is working the engine more in high range by slipping the torque convertor
more creating more heat . Heat equals fuel . The engine spinning faster in low range "could " use a little more fuel , but in YOUR scenario I believe 4L is much easier on the Jeep and fuel usage . Sounds like you already use your head , and drive for the conditions . Many don't today and we end up with the "send it " mentality.

That is precisely what I am trying to do, use my head and adjust my driving for the conditions.

As some pointed out, low range definitely would burn more fuel under normal driving conditions. My conditions are anything but "normal."

A few weeks ago (as I reported in a post) the tranny way overheated going up a long rugged mountain road in high range (2wd I think). Your comment about the torque converter generating more heat hits home.

As I said in my OP, it's not about how much fuel I am actually burning. I'm trying to figure out how this effects the overall performance of the drive train and the longevity of the machine.

Thanks for your insightful input.

 

[TJMexico]

First, I probably misstated in my OP. While I was thinking about fuel consumption, what really concerns me is the wear and heat generation on the drivetrain in general. Several folks have gone ahead and addressed that aspect of matter. Thank you for the insight!

Now, to further that line of thought ...

If you compared 4Hi to 4Lo at the same engine speed and same road speed (using different transmission gears to make up the gearing difference), 4Hi is more efficient simply because the driveline losses are significantly more due to the much higher rotational speed of the transmission and transfer case internals. Frictional power losses scale linearly with rotational velocity, and fluid drag losses scale with the cube of rotational speed. (You can measure this loss if you tested this by adding a temperature sensor to the transmission. The transmission will be much warmer in the 4Low case.)

...

Addressing that last statement (highlighted in bold and italics):

Are you saying that a hotter transmission will always be the result in 4lo, regardless of driving conditions?

Colojeep's comment makes sense to me based on my limited experience with this drivetrain:

In my opinion , you answered your own question. Since your Jeep is an automatic , it is working the engine more in high range by slipping the torque convertor more creating more heat. Heat equals fuel . The engine spinning faster in low range "could " use a little more fuel , but in YOUR scenario I believe 4L is much easier on the Jeep and fuel usage .

As I reported in a recent post after a trip into the mountains (see here), I made a long, rough and slow climb in high range (may have been 2wd, not sure) and the tranny way overheated to the point of scaring me. I though had burned it up for a minute. Running this entire last trip (roads as rough or rougher and some steeper but not as long) in low range all the way seemed to be a lot easier on the drivetain altogether.

I really need a temp sensor in the transmission but getting the pan down here is not an option for now. A transmssion shop can install one for me, but I am not confident in their soldering technique.

ETA: Also, in the above linked thread, Moglocker reported the following:

I have a temp gauge,and have noticed when I’m in 4low the trans temps cool way down(I guess because you are using the t-case)but you definitely need to have your buddy weld in an auto meter bung and get a gauge on the thing.

An analogy I previously used was my mountain bike. My legs are the engine, the front derailleur is the transmission and the rear derailleur is the transfer case.

The low gear ratio that the rear derailleur is in makes the work load on my legs and the front derailleur much easier at a lower speeds and on worse or steep roads. Likewise, based on my limited experience with this drivetrain, it seems that the load on everything front of the transfer case is reduced if the transfer is in low range under extreme driving conditions.

Obviously, on a long, straight and smooth stretch of pavement I'll have the bicycle in high gear and my legs will be happily pumping out revolutions, the front derailleur humming along happily and the chain doing its job. When I hit a steep uphill stretch or a rocky trail, though, the load on my legs in high gear becomes much harder and my front derailleur feels the stain if my rear derailleur is in a high gear position. Maybe the analogy is not a good one, but it seems to make sense to me.

Again, my driving conditions are not normal. They, actually, are pretty much what the machine is designed for. La Negrita seldom sees pavement and most of the surfaces she runs over are very rough and often very steep.

I'll include the elevation-over-distance graph and the speed-over-time graph from that trip up the big mountain when my tranny overheated. The overheat built up to around the 50km mark and I was in high range the whole run up that hill. Probably at about 110 minutes or somewhere after 45km is when I realized that my transmission was hot.

 

[Steel City 06]

First, I probably misstated in my OP. While I was thinking about fuel consumption, what really concerns me is the wear and heat generation on the drivetrain in general. Several folks have gone ahead and addressed that aspect of matter. Thank you for the insight!

Now, to further that line of thought ...

Addressing that last statement (highlighted in bold and italics):

Are you saying that a hotter transmission will always be the result in 4lo, regardless of driving conditions?

Colojeep's comment makes sense to me based on my limited experience with this drivetrain:


As I reported in a recent post after a trip into the mountains (see here), I made a long, rough and slow climb in high range (may have been 2wd, not sure) and the tranny way overheated to the point of scaring me. I though had burned it up for a minute. Running this entire last trip (roads as rough or rougher and some steeper but not as long) in low range all the way seemed to be a lot easier on the drivetain altogether.

I really need a temp sensor in the transmission but getting the pan down here is not an option for now. A transmssion shop can install one for me, but I am not confident in their soldering technique.

ETA: Also, in the above linked thread, Moglocker reported the following:


An analogy I previously used was my mountain bike. My legs are the engine, the front derailleur is the transmission and the rear derailleur is the transfer case.

The low gear ratio that the rear derailleur is in makes the work load on my legs and the front derailleur much easier at a lower speeds and on worse or steep roads. Likewise, based on my limited experience with this drivetrain, it seems that the load on everything front of the transfer case is reduced if the transfer is in low range under extreme driving conditions.

Obviously, on a long, straight and smooth stretch of pavement I'll have the bicycle in high gear and my legs will be happily pumping out revolutions, the front derailleur humming along happily and the chain doing its job. When I hit a steep uphill stretch or a rocky trail, though, the load on my legs in high gear becomes much harder and my front derailleur feels the stain if my rear derailleur is in a high gear position. Maybe the analogy is not a good one, but it seems to make sense to me.

Again, my driving conditions are not normal. They, actually, are pretty much what the machine is designed for. La Negrita seldom sees pavement and most of the surfaces she runs over are very rough and often very steep.

I'll include the elevation-over-distance graph and the speed-over-time graph from that trip up the big mountain when my tranny overheated. The overheat built up to around the 50km mark and I was in high range the whole run up that hill. Probably at about 110 minutes or somewhere after 45km is when I realized that my transmission was hot.

View attachment 642701
View attachment 642702

When comparing like conditions in 4Hi and 4Low, in typical use cases the transmission will be warmer in 4Low. That said it will be well within normal temperatures, and not really anything to worry about.

My assumption was based on a manual transmission, an auto might actually run cooler in 4Low due to the effects of the torque converter.

For a manual transmission, the transmission will still be cooler in 4Low than it would be at highway speeds, so no need to worry about the heat.

For an auto, generally the heat won't be an issue in 4Hi or 4Low except in very high demand cases. In the case of very slow speeds (less than 10 mph overall) with high load, an auto will probably run cooler in 4Low than in 4Hi.

Most automatic drivers that do lots of rock crawling will install a secondary transmission cooler in series with the heat exchanger in the radiator to help dissipate extra heat.

 

[GoldenGorilla]

Where abouts is the powerband for the stock 6 cyl 4.0l ?

It is usually technically regarded as the range from peak torque to peak horsepower - 3200-4550 rpm on the TJ 4.0L generally. At 2500 rpm you've gotten most of the way to peak torque so functionally, 2500-4550 is a decent way to think of it. If you're lugging it just means you are applying more throttle, thus fuel at a lower RPM where it isn't making power efficiently. It is more efficient to cruise at a lower rpm when power demand is low. I'm not saying you always want to be in the power band.

 

[TJMexico]

When comparing like conditions in 4Hi and 4Low, in typical use cases the transmission will be warmer in 4Low. That said it will be well within normal temperatures, and not really anything to worry about.

My assumption was based on a manual transmission, an auto might actually run cooler in 4Low due to the effects of the torque converter.

For a manual transmission, the transmission will still be cooler in 4Low than it would be at highway speeds, so no need to worry about the heat.

For an auto, generally the heat won't be an issue in 4Hi or 4Low except in very high demand cases. In the case of very slow speeds (less than 10 mph overall) with high load, an auto will probably run cooler in 4Low than in 4Hi.

Most automatic drivers that do lots of rock crawling will install a secondary transmission cooler in series with the heat exchanger in the radiator to help dissipate extra heat.

Yeah, I meant to specify 42RLE in my OP but forgot, I think. My bad. Thanks for the input.

I will have a look at that second inline heat exchanger. If you know off the top of your head of a website that might have details, send it this way please. I'll be looking meanwhile, anyway.

ETA: What about installing an electric fan on top of my existing cooler. I have have a transmission oil cooler on top (in front) of the radiator. Would putting a fan on that be as helpful? (Though summer temps here can get well over 110F.)

I'm looking for info on installing a secondary transmission cooler in series but have not found anything yet. Would that be on in front of the radiator like the existing one?

I did see this line:

The design and placement of such a secondary heat exchanger must account for fluid pressure, thermal efficiency, and compatibility with the existing cooling system. For instance, heat exchangers rated for over 300 PSI are rare, making high-pressure-rated units like the FSD billet aluminum heat exchanger a preferred but costly option.

 

[TJMexico]

It is usually technically regarded as the range from peak torque to peak horsepower - 3200-4550 rpm on the TJ 4.0L generally. At 2500 rpm you've gotten most of the way to peak torque so functionally, 2500-4550 is a decent way to think of it. If you're lugging it just means you are applying more throttle, thus fuel at a lower RPM where it isn't making power efficiently. It is more efficient to cruise at a lower rpm when power demand is low. I'm not saying you always want to be in the power band.

OK, so bare with me as I ask some ignorant questions.

Given this, does this mean that if I am crawling down a steep hill or grunting up a rough trail, I´d be better off letting the engine run somewhere above 2500 rpm? Even if it means keeping the tranny in 2nd gear?

I needing to balance peak fuel efficiency (not to save a few pennies on gasoline, but to let the engine run as cleanly as possible) with taking care of my transmission and not forcing it to overwork.

I need this Jeep to last as long as possible and the little things add up.

 

[TheBoogieman]

I can honestly say that the one thing I've never thought about while driving in 4LO is my gas mileage. A good auto trans cooler would be my only concern.

 

[TJMexico]

I can honestly say that the one thing I've never thought about while driving in 4LO is my gas mileage. A good auto trans cooler would be my only concern.

Yeah. As I said, fuel consumption is not really my concern; rather, what the rate of fuel consumption along with transmission temperature is trying to tell me.

My Jeep has a cooler installed, but I don't know if I need to look for something bigger or - as Steel City 06 said - look into a secondary inline device. Or, as I said, maybe install an electric fan over the existing cooler.