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Although not directly related to engine braking, this pic may be of some importance here, as to why gear shifting is required.

Spike

Right.

Right.

Right.

Right, but only up to the Y rpm.

Why will gearbox hold up any power, other than transmission losses? It only multiplies and transfers to the other side.

Now, this might happen if you downshift when you are at a higher speed. The non-syncho 1st gear won't allow you anyway. If you started from rolling speed in 1st, then I don't think you will ever reach a speed purely due to gravity that can snap the gearbox. Once the momentum becomes equal to the opposing force from the engine, the speed will remain constant. This I believe happens in rpm Y.

BTW, this is the slope to my office gate.



Here, when I come down in:

1st H gear of CJ340, I achieve constant speed within 5 meters (Y rpm)
2nd H gear of CJ340, I achieve constant speed within 15-20 meters (Y rpm)
3rd H gear of CJ340, I run out of slope before reaching constant speed ( < Y rmp). But I don't need to brake to control the speed.

1st H gear of GV, I achieve constant speed within 15-20 meters (Y rpm)
2nd H gear of GV, I run out of slope before reaching constant speed ( < Y rmp). I also need to brake to control the speed.

I don't know the mechanicals involved here, but I too believe it can't be the redline.

Now this is what I doubt. If you downshift from higher gear to lower gear without reducing speed, then I agree we might see the redlining scenario. But if you start in a gear down the slope, without any A-pedal input, I doubt you can reach redline rpm. The speed will stabilize much before that, at the Y rpm point.

I won't accept the certificate unless I am sure I understood.:D

Sharath, normally the engine braking is high enough not to let you reach the top wheel speed/max crank rpm allowed. Yes you are right, it wouldnt be possible in most cases without throttle input. I dont know what will happen if its a 30deg decline of 1000mts and the wheels are not losing traction :D Theoretically the gear should max out.

You already have sir, this is as simple as you aren't thinking it is ;)

Hi,
No such thing as a "Y" rpm. You feed in more external power, and the rpm will go up. You keep on feeding in more power, and it will cross the redline (assuming non-destruction/ failure). However, it is unlikely to happen if the external power source is the vehicle running down a slope.

For clarity of thought, think that your vehicle is being towed - in gear, on flat ground. Your speed (= engine rpm) will depend on the power being fed in by the towing vehicle. Assume that the towing vehicle is powerful enough, and you are running gumballs.

In general, vehicle will stabilise at a speed where power supplied = power dissipated. Holds irrespective of whether its going up a slope, down a slope, or running on flat ground.

Regards
Sutripta

In diesels without ECU i think the pump holds the key, at idle pump limits the fuel feed and the engine is restricted to this RPM? Just a theory.

Yes, diesel pumps have built in governing systems, mechanical or hydraulic. The ECU also does the same job, through electronics/ software.

But atmost, these can totally cut off fuel supply. When the power source is external, this lack of fuel will be irrelevant. The engine will turn (dissipating power, acting as a brake) because of the external power source. Incidentally, this is what happens everytime on the overrun.

Regards
Sutripta

Are you saying there is nothing like engine braking?

Of course there is engine braking!

Fail to see the reasoning behind (what I've said in reaching) this conclusion. stupid:

Regards
Sutripta

PS and OT. Just noticed moderators' posts dont have edit buttons!

Because you said that the rpm keeps rising as external power is fed. You mentioned no opposing force.

You mean you can see edit button on other's posts?:)

The opposing force is the engine acting as (an inefficient) pump. More power fed in than what is being dissipated = rise in rpm till dissipation equals power fed in.


You're right, actually no. Saw your post sandwiched between mine, both of which had the edit button glaring at me.

Regards
Sutripta

Let's focus on this. So in case of running down the slope, it is no danger of redlining. When does the rpm stop rising then? I know it stops based on my regular observation.

As I mentioned below the rpm will stop rising in the case where the descending speed due to gravity is less than engine+gear braking strength. Given an extremely steep & long descend there will be a point where the gravity will supersede engine+gear braking strength but that point is seldom scene in real life as just before that point the tires wouldve largely lost their traction coz our playing field is normally not tarmac or more resistive surfaces.

Wish we had this conversation in person, it wouldve been so much easier for me to explain with my hands, pen, table, chair, god knows what all...:D

Off hand, I'd say that the slopes necessary to redline an engine in the lower gears would cause you to break traction long before that. (Though I'd like to see what an overloaded Nano(spacious= easy to overload) would do coming down an extended steep slope.) Which is why in the thought experiment I said you're running gumballs. There is no theoretical upper rpm limit when externally driven.

The engine dissipates a certain amount of energy per cycle. There is also velocity component (because of friction ) so energy lost per cycle also increases with rpm.

The energy you are inputting to the system (your vehicle) = vertical drop.
The energy you are dissipating = the usual (tyres, aerodynamic drag etc) + engine being made to turn over.

For a given slope, the vehicle will pick up speed till these two equal. And then it will remain at that speed.

In a lower gear, for a given ground speed, engine will be turning over faster= more cycles/ unit time = dissipating energy faster = dissipating more power. This will cause equilibrium to be attained at a lower speed.

If Spike can give the power absorbtion graphs for a common engine, maybe we can try our hands at working out the equilibrium speed for a given slope/ gear combination.

Regards
Sutripta

PS. Further discussions tomorrow night. More interested now in seeing how a ball behaves on flat surfaces!

@The_Wolf: +1 to that:

It is much easier to explain in person rather than on the keyboard.

And Samurai - Honestly there are so many variables for each vehicle that one needs to appreciate to understand the engine braking.

It is near impossible to explore all options

A silly gear change or even a bad/erratic fuel supply will have a different effect on Engine Braking!

So the conclusion is there is a Y rpm, but it is not constant, it can vary based on many parameters. I'll be travelling whole day, won't be able to respond till tonight.