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There is a Transfer case in 4wd vehicles that has different ratios for H and L gears. Hence the difference in speed.

Sharat, even in L it can gain speed depending on the engine / GB / Tyres / Surface etc etc.

There is no generic answer to these questions :D

Is that supposed to be enlightening? Why do you always resort to mockery?

I don't believe you understood my question.

I know Tanveer, that is my observation, I was asking why.

Anyway, looking at answers from Red Liner, Wolf and DirtyDan, I infer the following.

Looks like there is engine rpm Y beyond which the engine can't be forced to turn any faster from outside. After this point one has to press accelerator to rise the rpm. This is just a theory based on my observation. I could be wrong.

At higher gears, the difference between wheel speed and rpm is less. Therefore in an incline, the vehicle keeps gaining speed thanks momentum, until the engine rpm hits Y.

Example:

If the final ratio is 10, and Y is 1500rpm, the vehicle will stop gaining speed once the wheels are doing 150rpm. Until then it keeps gaining speed in the slope due to momentum.

Now let's look at a Low gear, where the final ratio is 50, the Y is still 1500rpm, the vehicle will stop gaining speed once the wheels are doing 30rpm. This is when we feel the engine braking.

In other words, the speed gain due to momentum happens in any gear, but it will stop gaining speed very quickly in lower gear, but takes longer to stop gaining speed in higher gears. In higher gears you might start using brakes before you reach this point.

Once you hit this point, if you don't have enough traction from the surface, the vehicle will start sliding. But if you have enough traction, you will continue in a constant pace without gaining any more speed.

This is why Jeeps in 1st low slide down steep slopes when there is not enough traction. But if the tyre has traction, the Jeep will come down in a constant speed without gaining speed. The engine braking will counter the momentum.

But is there a rpm Y beyond which the engine cannot be forced from the crankshaft side?

Sharat, Lets get this straight - I dont intend mocking you or anyone here.

And what I said is true if one understands how Gearboxes and transfer cases work.

Hi Samurai-san,

In my view, it is the torque multiplication that makes it happen. Lets saw the vehicle has a crawl ration of 30(this means that in 1st L 30 revolutions of the engine = 1 rev. of the wheel. ) for 1st L, 20 for 2nd L, 15 for 1st H, 10 for 3rd L etc.

So, while coming downhill, a force is experienced at the wheel, this force is experienced as torque at the hub-axle-diff.-prop-shaft-t.case-g.box-engine where the force is finally dispelled by the engine (with each cycle).

In 1st L, the engine will have to undergo 30 cycles for every revolution of the wheel. (thereby dispelling more energy and providing more resistance)

in 1st H, the engine will have to undergo 15 cycles for every revolution of the wheel. (thereby dispelling less energy and providing less resistance to the slope)

That is why in 1st H, the engine will revv. more w.r.to 1st L.

Further I don't think headers was trying to mock you. What he says is true regarding gradient. If you have a gradient steep enough (assuming you have enough traction), even the engine braking provided by 1st L will not be enough, your engine will revv. to the limit.

So you honestly believe I don't know the existence of transfer gear and it's function. In that case I guess no mockery was meant. :) BTW, I studied engineering in the 80s when we were forced to study mechanical/civil topics for two years, so I know how a gear works or differential works.

Ah! that is my question. What is that limit when the A-pedal is left alone?

No idea, that of course will depend on the engine - whether it's high-revving or not. Also type of injection (DI, IDI, CRDI) would play a major role. Also I believe some engines specifically meant commercial vehicles have a butterfly valve in the intake manifold to aid engine-braking.

The limit, what I called as rpm Y, can't be the idle rpm. It can't be redline either. It is somewhere in between, and obviously it will be different for different engine types.

But this is all conjecture. I want one of the engine techies to confirm or rip this apart. Where are you guys?

Question: If instead on 1st gear, you put second gear and go down the slope - will the engine RPM be higher?

It can't be the idle RPM, if it on a steep slope ; BUT it can be redline RPM on a very steep slope, and depending on lots of other factors.

agree:

I think it will be same, the mythical rpm Y. It is a conjecture, purely based on daily observation.

People who have visited my office know that there is a 100 meter long steep incline from the main road to the office gate. I always come down in this slope in No-ABC in various gears in GV as well as CJ340. It is a tar road, so traction is not a problem. I have noted that the vehicle doesn't gain speed after a point, which I call the Y rpm point.

I thought this would be possible only in the ECU-based engines.

did you try with the CJ340 in top gear (1:1 drive)...I think it should go faster than Y (but definitely not redline, I think you will run out of slope).

But if you run the same vehicle on a flat stretch, it will be at idle rpm right? So what is pushing the rpm higher on a decline? Gravity? So then gravity is pulling the vehicle down faster, then wheels spin faster than when on a flat stretch. If wheels spin faster, then the RPM goes up? So steeper the incline, faster the wheels spin, and more the RPM. The gear box and associated equipment only hold this power up to a certain extent (torque multiplication).

At some point of the rpm going up in a particular gear (say 1st), your gearbox will probably just snap unable to fight back at the engine power being held up.

No, the slope can make the engine reach that "Y" rpm, takes time though...

You are correct here, but irrespective of whether its an incline or not your highest gear in least final drive/ratio allows you to reach maximum speed. Highest gear is 4th in our case and the lowest ratio is the 4x2 then 4x4 4th gear then finally the might 4x4 1st gear, hence the crawling force in the last mentioned combination.

Example:

Correct

Correct. Engine braking + gear braking.

Correct. Well understood.

Correct.

The "Y" rpm that you are mentioning is indeed your redline, peak usable engine power in that particular gear. Always remember the torque produced at crank is always a constant at a certain RPM, its only the gearing that determines how slow or fast you want to accelerate. Higher gear = Higher speed and Lower gear = Lower speed. 4x2 = Higher speeds & 4x4 = Lower speeds (remember trasmission losses??!!)

If you understand the difference between Engine braking and Gear braking as mentioned by me in the below post, I certify that you have understood the logic behind your reasoning. :)

Generic Engine & gbox related questions, fire maadi, Id like to take a few...

Hi Wolf, isn't the rev-limit an artificial limit imposed using fuel (or ignition) cut-off ? Unless you have a mechanical governor (controlling an engine brake or something) shouldn't it be possible to rev any engine beyond the rev-limit 'from the crankshaft side' as Samurai has put it ?

There are different types of rev limiters, namely - Ignition contolled, Fuel controlled, Design controlled. The first 2 are controlled via a ECU or CDI unit however in the current context the max rpm limit is self imposed by virtue of the design of components. The xd4.9p engine that we are discussing I guess has a hydraulic tappet system (old gen), here the clearances are controlled by the building oil pressure. At a certain rpm the oil pressure builds so much that the tappets shut the valves almost completely, at this point there is no combustion resulting in no power output. This mechanically deliberates a design limitation causing an impromptu self imposed rev limiter...if one would like to call it that way.