Go Back   Team-BHP > Buckle Up > 4x4 & Off-Roading > 4x4 Technical


Reply
 
Thread Tools Search this Thread
Old 6th July 2010, 19:12   #151
Senior - BHPian
 
SPIKE ARRESTOR's Avatar
 
Join Date: Nov 2009
Location: Deutschland
Posts: 2,653
Thanked: 679 Times
Default

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

Spike
Attached Thumbnails
Unable to achieve engine braking-gear-selection.jpg  

SPIKE ARRESTOR is offline   Reply With Quote
Old 6th July 2010, 19:21   #152
Team-BHP Support
 
Samurai's Avatar
 
Join Date: Jan 2005
Location: B'lore-Manipal
Posts: 22,042
Thanked: 13,488 Times
Default

Quote:
Originally Posted by Red Liner View Post
But if you run the same vehicle on a flat stretch, it will be at idle rpm right?
Right.

Quote:
Originally Posted by Red Liner View Post
So what is pushing the rpm higher on a decline? Gravity?
Right.

Quote:
Originally Posted by Red Liner View Post
So then gravity is pulling the vehicle down faster, then wheels spin faster than when on a flat stretch.
Right.

Quote:
Originally Posted by Red Liner View Post
If wheels spin faster, then the RPM goes up? So steeper the incline, faster the wheels spin, and more the RPM.
Right, but only up to the Y rpm.

Quote:
Originally Posted by Red Liner View Post
The gear box and associated equipment only hold this power up to a certain extent (torque multiplication).
Why will gearbox hold up any power, other than transmission losses? It only multiplies and transfers to the other side.

Quote:
Originally Posted by Red Liner View Post
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.
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.

Unable to achieve engine braking-slope-003.jpg

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.

Quote:
Originally Posted by im_srini View Post
Hi Wolf, isn't the rev-limit an artificial limit imposed using fuel (or ignition) cut-off ? Unless you have a mechanical governor or something shouldn't it be possible to rev an engine beyond the rev-limit 'from the crankshaft side' as Samurai has put it ?
I don't know the mechanicals involved here, but I too believe it can't be the redline.

Quote:
Originally Posted by The Wolf View Post
The "Y" rpm that you are mentioning is indeed your redline, peak usable engine power in that particular gear.
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.

Quote:
Originally Posted by The Wolf View Post
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.
I won't accept the certificate unless I am sure I understood.
Samurai is offline   Reply With Quote
Old 6th July 2010, 19:35   #153
Senior - BHPian
 
The Wolf's Avatar
 
Join Date: Jan 2005
Location: Bangalore
Posts: 1,116
Thanked: 58 Times
Default

Quote:
Originally Posted by Samurai View Post
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.
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 Theoretically the gear should max out.

Quote:
Originally Posted by Samurai View Post
I won't accept the certificate unless I am sure I understood.
You already have sir, this is as simple as you aren't thinking it is

Last edited by The Wolf : 6th July 2010 at 19:37.
The Wolf is offline   Reply With Quote
Old 6th July 2010, 21:22   #154
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,439
Thanked: 1,929 Times
Default

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
Sutripta is offline   Reply With Quote
Old 6th July 2010, 22:15   #155
Team-BHP Support
 
Jaggu's Avatar
 
Join Date: May 2004
Location: Bangalore
Posts: 17,490
Thanked: 6,768 Times
Default

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.
Jaggu is offline   Reply With Quote
Old 6th July 2010, 22:40   #156
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,439
Thanked: 1,929 Times
Default

Quote:
Originally Posted by Jaggu View Post
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
Sutripta is offline   Reply With Quote
Old 6th July 2010, 22:42   #157
Team-BHP Support
 
Samurai's Avatar
 
Join Date: Jan 2005
Location: B'lore-Manipal
Posts: 22,042
Thanked: 13,488 Times
Default

Quote:
Originally Posted by Sutripta View Post
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).
Are you saying there is nothing like engine braking?
Samurai is offline   Reply With Quote
Old 6th July 2010, 22:51   #158
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,439
Thanked: 1,929 Times
Default

Quote:
Originally Posted by Samurai View Post
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.

Regards
Sutripta

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

Last edited by Sutripta : 6th July 2010 at 22:54. Reason: Added PS.
Sutripta is offline   Reply With Quote
Old 6th July 2010, 22:58   #159
Team-BHP Support
 
Samurai's Avatar
 
Join Date: Jan 2005
Location: B'lore-Manipal
Posts: 22,042
Thanked: 13,488 Times
Default

Quote:
Originally Posted by Sutripta View Post
Fail to see the reasoning behind (what I've said in reaching) this conclusion.
Because you said that the rpm keeps rising as external power is fed. You mentioned no opposing force.

Quote:
Originally Posted by Sutripta View Post
PS and OT. Just noticed moderators' posts dont have edit buttons!
You mean you can see edit button on other's posts?
Samurai is offline   Reply With Quote
Old 6th July 2010, 23:08   #160
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,439
Thanked: 1,929 Times
Default

Quote:
Originally Posted by Samurai View Post
Because you said that the rpm keeps rising as external power is fed. You mentioned no opposing force.

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.


Quote:
You mean you can see edit button on other's posts?
You're right, actually no. Saw your post sandwiched between mine, both of which had the edit button glaring at me.

Regards
Sutripta
Sutripta is offline   Reply With Quote
Old 6th July 2010, 23:19   #161
Team-BHP Support
 
Samurai's Avatar
 
Join Date: Jan 2005
Location: B'lore-Manipal
Posts: 22,042
Thanked: 13,488 Times
Default

Quote:
Originally Posted by Sutripta View Post
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.
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.
Samurai is offline   Reply With Quote
Old 6th July 2010, 23:34   #162
Senior - BHPian
 
The Wolf's Avatar
 
Join Date: Jan 2005
Location: Bangalore
Posts: 1,116
Thanked: 58 Times
Default

Quote:
Originally Posted by Samurai View Post
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...

Last edited by The Wolf : 6th July 2010 at 23:37.
The Wolf is offline   Reply With Quote
Old 6th July 2010, 23:51   #163
Distinguished - BHPian
 
Join Date: Jul 2009
Location: Kolkata
Posts: 3,439
Thanked: 1,929 Times
Default

Quote:
Originally Posted by Samurai View Post
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.
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!
Sutripta is offline   Reply With Quote
Old 7th July 2010, 06:44   #164
Senior - BHPian
 
headers's Avatar
 
Join Date: May 2006
Location: Greater Chennai
Posts: 4,548
Thanked: 424 Times
Default

Quote:
Originally Posted by The Wolf View Post
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.
Quote:
Originally Posted by headers View Post
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
Quote:
Originally Posted by Samurai View Post
Is that supposed to be enlightening? Why do you always resort to mockery?
@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!
headers is offline   Reply With Quote
Old 7th July 2010, 07:55   #165
Team-BHP Support
 
Samurai's Avatar
 
Join Date: Jan 2005
Location: B'lore-Manipal
Posts: 22,042
Thanked: 13,488 Times
Default

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.
Samurai is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Which has more Engine Braking? A petrol or a diesel engine? anilisanil Technical Stuff 3 16th September 2011 14:47
Autocar India & Ford Fiesta achieve 33 kpl? windsurfer The Indian Car Scene 58 22nd August 2011 20:44
How to achieve Max FE in an Opel Corsa? raj_7703 Technical Stuff 22 2nd July 2010 01:17
Max kmpl , and how to achieve it TURBOSAM Technical Stuff 200 6th June 2010 11:28
How to achieve better engine sound? raj_7703 Modifications & Accessories 7 6th November 2009 19:10


All times are GMT +5.5. The time now is 16:42.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks