Apols if I seem a bit grumpy

As a first step I recommended reading the owner manual. No owner has responded with the facts

Next, yes, of course any auto will manage the loads imposed upon it. This certainly does not mean it will manage them from a self-preservation perspective. I'm not aware of any auto designed for towing caravans and with a selectable towing mode.

As more and closely spaced ratios are added to an auto they tend to use them more often. That is the whole point of more ratios, to keep engine revs in a desired narrow range with a focus on economy. Autos are so smooth these days that I doubt the average driver would constantly know what gear they were in. Of greater obscurity to most is knowing whether the torque convertor is locked or unlocked. It takes familiarity and a keen and constant eye on the tacho to determine this status. Who both knows and bothers? I do. Driving at length with the convertor unlocked generates much greater transmission heat.

More engine torque does not necessarily translate to lugging it out in the highest gear. You are likely to find that an auto, even when not towing, will change down at any more than half throttle rather than use more of available torque. I suspect auto change design characteristics are dictated largely by economy targets.

Speaking of economy, Peter I don't believe those figures. They challenge the road test stats of many other brands . Even if your figures were obtained by careful and multiple tank-to-tank filling I would remain scratching my head.

I suspect, but feel free to chastise me, you are quoting from an inbuilt trip computer. My experience is these computers are both inconsistent and generous. How many manufacturers will choose accuracy over optimism when it comes to the all important and so visible economy stats? How many owners can be bothered keeping very careful and very accurate records of every single fuel fill since they bought their new car, maybe 7 years ago, like mine? It is a pain and takes dedication. This method produces the only economy stats I trust.

No issues with with the grumpiness sort of expected some response.

Firstly like to start my response outlining my qualifications, I am a licensed mechanic so understand the principles involved, admittedly been off the tools for sometime.

Yes, figures quoted are from trip computer. I have also double checked with official records, this is a fleet managed vehicle so I use the actual figures. What I can say is based on actual figures over a distance of 7200km I have an average fuel economy of 11.80 l/100km. This comprises about 2500km towing, 500 -700km highway driving and the rest short distance city driving.

One leg of the recent trip, all with the van on, was an actual average of 15.1 l/100km.

I am finding the Jeep with the 8 speed tends to hold the higher gears better than anything else I have towed with, this is largely due to the fact it has more torque and doesn't need to kick down to hold the speed. Most highway towing would see it sitting in 7th with the occasional drop into 6th and if cruising on flat road at 110kmh it would drop into 8th. The Jeep displays the gear it is in at all times.

Transmission temperature was also not an issue. I would monitor regularly, particularly after hill climbs, and it never exceeded 91 degrees, without the van it was in the high 80s.

Peter

This comments is not directed at anyone but could be of interesting.
Some people running with an exhaust thermocouple have posted on this forum that running a turbo diesel in its overdrive gear or its highest gear equates to about 80 degrees higher exhaust temperature than running in one gear lower so even though the vehicle will tow OK and the auto box is OK the turbo and exhaust valves may be running hotter and unless you have a thermocouple on the exhaust you will never never know.
This may eventually lead to engine problems at an earlier stage in its life only time will tell.

JR

Hi JR,

Not sure why that is, I would have thought an engine revving less and not under heavy load would result in lower temperatures. My Jeep at 100kmh is cruising along at about 1750 rpm in 8th, and doing it with ease if on the flat, dropping back to 6 would have it revving somewhere between 2500 and 3000. It is not that it is ever under heavy load while in overdrive as it drops back as soon as you give it some throttle any way.

I suspect some of these issue may be with manuals or when autos are driven in manual mode or it may be more of an issue with other tugs with less torque as you are always working them harder.

Around 1750 - 2000 rpm is the sweet spot for many diesels so I have always tried to keep it in that range.

Peter

Peter, thanks for the reply to my post. Missing still though is a response to my first point i.e. 'what does the owner manual advise'. Would you be so kind as to have a squiz at your manual as I can't imagine the topic of towing would not get a mention?

As far as exhaust temps are concerned I have no personal knowledge nor experience, but that won't stop me from theorising with basic physics.

What we are comparing is an engine powering (towing) a payload with the only variable being the transmission gear and consequent revs. You will agree that it doesn't matter which gear is used the engine must produce the same power to tow the payload in the same circumstances . I suggest therefore that the same amount of heat is generated in the exhaust system being directly related to engine power (the source of the heat). Hard to argue too much with that. If that is indeed the case then at lower engine revs less exhaust gas will be pumped through the system. If there is less volume of gas then the temp of it must be higher for the gas to contain the same amount of expended energy. So less revs (at the same engine power output) means higher exhaust gas temp.

To reiterate, this is only my physics based conjecture and perhaps we have an expert here who could clarify?

Hi OT,

Here are a couple of quotes from the owner's manual.


In my case I never experienced frequent shifting so I can see no need to assume I am smarter than the computer in determining the correct gear.


The key to this statement is reducing excessive shifting. As I was not experiencing excessive shifting and the transmission temperature was OK I can see no reason to manually change gears. The use of a lower gear for engine braking is another issue and I have used the paddles for this reason of couple of times, but personally prefer to use the brakes as it engages both the car and van brakes, heavy vehicle braking only can increase risk of sway.

And this last quote from the section on using the paddle shifters when towing

I believe this reference to heat would apply to coolant, transmission oil and exhaust temperatures. I don't agree with your logic about lower revs and exhaust temperature otherwise we all be overheating while pottering around town.

Peter

Peter, thanks for going to the trouble of quoting in detail from the manual. I am pleased to see how well Jeep have addressed the topic.

I reckon the excessive heat concern is only in the context of the auto. Toyota warn against prolonged use of higher revs in lower gears as a specific overheating risk to the auto. I'm mindful of this every time I climb a long steep hill flat out in 3rd at 3500 rpm.

Pottering around town won't generate much absolute exhaust heat as the average engine power output is relatively low. I was only addressing the scenario where the engine was working fairly hard towing a van and whether the gear ratio impacted exhaust gas temp. It is only under these extreme conditions that safe parameters may be breached.

If you do not agree with my physics (being the only evidence I could suggest) could you please help me out? As I said it was purely my conjecture prompted by JR's post. I too have seen many warnings about excessive exhaust gas temps when towing in too high a gear. I am quite interested in the fundamentals i.e. the physics of the issue. I don't have the ability to monitor my exhaust gas temps

Hi OT,

I did a bit of research on exhaust gas temp and think I now have a better understanding.

The issue is most likely related to high throttle/low speed driving eg holding a high gear with maximum throttle while climbing steep hill. What is happening in this scenario is the engine is being over fuelled and the excess fuel is still burning as it enters the exhaust.

You will find that most discussions around this are associated with the engine being chipped. This is logical as most modifications like this generally result in more fuel for a given throttle position.

In my case with the auto working correctly there was never a time when I had a condition of high throttle/high load/high gear/low speed. Other than very steep hills did I ever see a need to use much more than half throttle, holding the speed limit on flat would require just very light throttle. If I was trying to hold the speed on the bigger hills it would drop back as necessary and increase the revs, in this case most of the fuel would be burnt.

Peter

http://www.berrimadiesel.com/home/exhau ... -safe-egt/
http://www.dieselpowermag.com/features/ ... re_basics/
There's a lot on the net, about exhaust temp.

Hans