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Old 7th February 2010, 17:04   #106
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Originally Posted by Sankar View Post
Headers, this is exactly what engine braking is
@Sankar : Well..well..you are referring to reducing speed through gears while i was referring to coming down a 70 degree climb in 4wd low without touching the brake using the engine ONLY.

The kind a inclines I talk about - am not sure you understand at the moment. Anyways..

The kind of engine braking you Talk about - yeah - Petrols are faster than diesels because of the design of petrol engines. But Modern CRDi engines are getting there - ALMOST
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Old 7th February 2010, 18:20   #107
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Sutripta a carb as you may know is a venturi and works off vacuum generated by the venturi to atomise fuel.

I don't see why in a FI system there would be need for a venturi effect.

This alone should give a carb engine a slightly higher degree of vacuum. I am not very sure this is so in actual. Just think it should be so on account of the venturi in a carb
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Old 7th February 2010, 18:26   #108
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The thread starter!

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Originally Posted by robinson.s View Post
Hi all,

I have just picked up a 4WD invader. The only issue I find in this vehicle is the crawling speed is high when it comes to heavy decline stretch.

Would be great if some one can help me understand, if I could add a additional low gear? Does the transfer case will have space for that? Or the next option would be to change the existing low gear. And what would be the ratio?

Thanks & Regards
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Originally Posted by headers View Post
@Sankar : Well..well..you are referring to reducing speed through gears while i was referring to coming down a 70 degree climb in 4wd low without touching the brake using the engine ONLY.
Quote:
Originally Posted by DKG View Post
Sutripta a carb as you may know is a venturi and works off vacuum generated by the venturi to atomise fuel.

I don't see why in a FI system there would be need for a venturi effect.

This alone should give a carb engine a slightly higher degree of vacuum. I am not very sure this is so in actual. Just think it should be so on account of the venturi in a carb

How we've got lost in this thread - While it may be educative for some - we've completely digressed from robinsons POV or question!
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Old 7th February 2010, 18:37   #109
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Originally Posted by headers View Post
@Sankar : Well..well..you are referring to reducing speed through gears while i was referring to coming down a 70 degree climb in 4wd low without touching the brake using the engine ONLY.

The kind a inclines I talk about - am not sure you understand at the moment. Anyways..

The kind of engine braking you Talk about - yeah - Petrols are faster than diesels because of the design of petrol engines. But Modern CRDi engines are getting there - ALMOST
Downshifting through the gear slowing down is possible. I was refering to slowing down (rate of slowing down) of the vehicle in the same gear as one lets off throttle. That gives the idea of actual engine braking that engine is doing.

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Originally Posted by star_aqua View Post
Sankar, i still could not able to see where did they have mentioned 'the diesel engines lack engine braking then petrol'. could you please copy and paste that statement here?
That site does not compare a Diesel and Petrol motor, so there is no comparison. There are other sites which compare Diesel and Petrol motors which tells you that Diesels lack engine braking compared to a petrol motor.

Jacobs site, amongst other sites, tells you why diesel lack engine braking.

(3) When the piston passes over top dead center and begins its downward stroke, the energy is returned to the piston (and to the driving wheels). Essentially no energy is absorbed and no net retarding work is done.

Diesel compresses the air in the cylinder, but most energy required to compress is returned. Petrol engine is in partial vacuum, piston has to work against the partial vacuum. Energy is used up driving the piston against the vacuum.

Still you have not told me about the modern Diesel engines you know that are using throttle plates in the inlet manifold. It shall be good info for all if you could share.

Those who have dynoed both petrol & diesel engines on a engine dyno will tell you which has more engine braking when the throttle is lifted off. Only have to see the rate of slowing down of both engines. No transmission, no wheels nothing just engines to compare. There are people here who have done that right?

Diesels lack engine braking when compared with petrol. 'Compared' is the keyword. Diesels have engine braking but its not as good as a petrol engine. Can't explain it further my friend. If Behram sir is willing he can make things clearer and better if he's an engine guy.

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Originally Posted by headers View Post
The thread starter!

How we've got lost in this thread - While it may be educative for some - we've completely digressed from robinsons POV or question!
Maybe all the engine braking posts can be made as a separate Engine Braking thread.

Last edited by Sankar : 7th February 2010 at 18:43.
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Old 7th February 2010, 18:55   #110
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I was refering to slowing down (rate of slowing down) of the vehicle in the same gear as one lets off throttle. That gives the idea of actual engine braking that engine is doing.
Then its obviously Petrol which is more aggressive than the diesels ones.

Quote:
Diesels lack engine braking when compared with petrol. 'Compared' is the keyword. Diesels have engine braking but its not as good as a petrol engine. Can't explain it further my friend. If Behram sir is willing he can make things clearer and better if he's an engine guy.

From my understanding - and the thread starters question - the mad declines one does in offroading [> 60 degrees over 40 ft] the diesel engines are able to hold the vehicle in 4wd - 1 low and inch down. This would be vouched by many who came to the recent Bangalore Annual OTR [and did the rocky downhill - lunchbreak area]


I'm not sure a CJ3B with a 3 speed GB and Hurricane engine would be able to match that!!
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Old 7th February 2010, 19:27   #111
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Originally Posted by SPIKE ARRESTOR View Post
@star aqua it seems like you keep a good eye on M&Ms marketing strategies, even i didn't notice about the active surge damper mentioned by you, the GB has a speedo sensor fitted in it (don't remember the ratio though either it will be 17/5 or 19/6), this gives feedback to the ECU and input to the instrument cluster which displays the speed, for 4wd vehicles the speedo sensor is mounted in the transfer case.
Spike, remember 'Active Surge Damper' is the new generation name for 'Spike Arrestor' . Are you sure it is only the speedo info? Thats the basic info required, but when M&M says 'advanced' there should be some more info sent from GB.

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Originally Posted by headers View Post

How we've got lost in this thread - While it may be educative for some - we've completely digressed from robinsons POV or question!
Yeah! i think the solution was offered by DD already and other cheaper solution would be to run on tires with smaller dia with the cost of GC. so for Robinson, no cheaper solution is available at the moment.

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Originally Posted by Sankar View Post
Downshifting through the gear slowing down is possible. I was refering to slowing down (rate of slowing down) of the vehicle in the same gear as one lets off throttle. That gives the idea of actual engine braking that engine is doing.



That site does not compare a Diesel and Petrol motor, so there is no comparison. There are other sites which compare Diesel and Petrol motors which tells you that Diesels lack engine braking compared to a petrol motor.

Jacobs site, amongst other sites, tells you why diesel lack engine braking.

(3) When the piston passes over top dead center and begins its downward stroke, the energy is returned to the piston (and to the driving wheels). Essentially no energy is absorbed and no net retarding work is done.

Diesel compresses the air in the cylinder, but most energy required to compress is returned. Petrol engine is in partial vacuum, piston has to work against the partial vacuum. Energy is used up driving the piston against the vacuum.

Still you have not told me about the modern Diesel engines you know that are using throttle plates in the inlet manifold. It shall be good info for all if you could share.

Those who have dynoed both petrol & diesel engines on a engine dyno will tell you which has more engine braking when the throttle is lifted off. Only have to see the rate of slowing down of both engines. No transmission, no wheels nothing just engines to compare. There are people here who have done that right?

Diesels lack engine braking when compared with petrol. 'Compared' is the keyword. Diesels have engine braking but its not as good as a petrol engine. Can't explain it further my friend. If Behram sir is willing he can make things clearer and better if he's an engine guy.



Maybe all the engine braking posts can be made as a separate Engine Braking thread.
Sankar, they have just explained how their jake brake works. 'no net retarding work is done' it doesn't mean the diesel engine has no engine braking or less than petrol. if you believe that the energy returned will keep the engine rev further, then do this on a diesel engine.. observe what happens when you turn the ignition keys off. and the same thing on petrol. check out which stall first and also why the diesel shudders.

say if X amount of energy is required to compress a diesel per cycle and Y amount for petrol. do u agree here X>Y? now if X is returned back, it cannot be utilized for next cycle. it will be almost lost when it reaches BDC in the previous cycle. now again you need to apply X amount of energy for the next cycle. where do you get this X amount of energy?? which should be always greater then Y.
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Old 7th February 2010, 21:44   #112
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Quote:
Originally Posted by DKG View Post
Sutripta a carb as you may know is a venturi and works off vacuum generated by the venturi to atomise fuel.

I don't see why in a FI system there would be need for a venturi effect.

This alone should give a carb engine a slightly higher degree of vacuum. I am not very sure this is so in actual. Just think it should be so on account of the venturi in a carb
Hi,
I think we are talking at tangents to each other!
A FI system does not need a venturi.
In a carb, the venturi is sized to cause the least pressure drop at maximum power, consistent with light load drivability. The pressure drop is most where the venturi is most constricted (application of Bernoulli's principle) but the dp across the venturi should not be much. With the throttle plate closed, the amount of air passing through the engine is very low. The pressure drop across the venturi in such circumstances will be close to negligible. As such, there should be no difference between an FI and carb engine for our discussion.

Actually my basic problem is with the premise that one gets better engine braking with inlet throttled off.

Quote:
Originally Posted by Sankar View Post
When the piston passes over top dead center and begins its downward stroke, the energy is returned to the piston (and to the driving wheels). Essentially no energy is absorbed and no net retarding work is done.
Sir, I have huge problems with this statement. This would be true if air were a very high quality steel spring. It is not. And esp. in an indirect injection diesel, there is significant pumping losses as air is pushed into the combustion chamber.

Quote:
Diesel compresses the air in the cylinder, but most energy required to compress is returned. Petrol engine is in partial vacuum, piston has to work against the partial vacuum. Energy is used up driving the piston against the vacuum.
Let us accept your initial premise for arguments sake. Now the piston will be doing work on the inlet stroke. But this work will be returned on the compression stroke (air as perfect spring). We are back to square one.

Quote:
Diesels lack engine braking when compared with petrol. 'Compared' is the keyword. Diesels have engine braking but its not as good as a petrol engine. Can't explain it further my friend. If Behram sir is willing he can make things clearer and better if he's an engine guy.
Actually this runs counter to everything I "know" of engines, and so am more than a little interested. But don't think this is the thread to get into esoteric details.

Two random thoughts.

a) GM (and later some other companies too) had introduced a cylinder cutoff system in their lineup. IIRC, the system was developed by Eaton. Does anyone remember whether the inlet was held open, or closed, when the cylinder was deactivated?

b) If anyone has access to a 4 or 6 cylinder engine running twin carbs, one can test out the alternatives very simply. Anyone out here willing to try?

Regards
Sutripta
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Old 8th February 2010, 08:29   #113
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Quote:
Originally Posted by Sutripta View Post
Sir, I have huge problems with this statement. This would be true if air were a very high quality steel spring. It is not. And esp. in an indirect injection diesel, there is significant pumping losses as air is pushed into the combustion chamber.

>>Sir, air suspension in a bus? Airsprings? Air suspension in a Merc S Class? If still not convinced i have posted a simple syringe experiment before.

>>IDI is a closed chamber during powerstroke, right? Compressed air = air spring. Doesn't matter if the IDI has a precombustion chamber.

Let us accept your initial premise for arguments sake. Now the piston will be doing work on the inlet stroke. But this work will be returned on the compression stroke (air as perfect spring). We are back to square one.


Actually this runs counter to everything I "know" of engines, and so am more than a little interested. But don't think this is the thread to get into esoteric details.

>>Becauseprobably his explaining would end this discussion. Mine just keeps on getting elastic.


Regards
Sutripta
Quote:
Originally Posted by star_aqua View Post
if you believe that the energy returned will keep the engine rev further, then do this on a diesel engine.. observe what happens when you turn the ignition keys off. and the same thing on petrol. check out which stall first and also why the diesel shudders.

>>Petrol. Even without the engine off, especially when the foot is completely off the throttle pedal.

say if X amount of energy is required to compress a diesel per cycle and Y amount for petrol. do u agree here X>Y? now if X is returned back, it cannot be utilized for next cycle. it will be almost lost when it reaches BDC in the previous cycle. now again you need to apply X amount of energy for the next cycle. where do you get this X amount of energy?? which should be always greater then Y.
Agreed. But where is this energy returned to? Flywheel? What a flywheel does is it stores the energy.
So in the next cycle energy to turn the engine over is provided by wheels+energy stored in the flywheel.

Quote:
Originally Posted by headers View Post
Then its obviously Petrol which is more aggressive than the diesels ones.
Yes sir. I rest my case.
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Old 8th February 2010, 09:26   #114
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Quote:
Originally Posted by Sutripta View Post
A FI system does not need a venturi.
This is exactly what I implied in my earlier post that a FI does not have a venturi whereas a carb engine does. This adds to the vacuum generation in a carb engine and hence I guess there just might be an added advantage in a carb when it comes to vacuum generation. As to whether the venturi in a carb system has a considerable/negligible effect is something I don't know for a fact.

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Originally Posted by Sutripta View Post
Actually my basic problem is with the premise that one gets better engine braking with inlet throttled off.
What exactly is your problem with that premise? Its so obvious that engine braking has everything to do with the inlet blocked off !!

How else is vacuum generated if not for the inlet blocked off when you let off the throttle?

Last edited by DKG : 8th February 2010 at 09:46.
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Old 8th February 2010, 12:22   #115
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Originally Posted by Sankar View Post
Agreed. But where is this energy returned to? Flywheel? What a flywheel does is it stores the energy.
So in the next cycle energy to turn the engine over is provided by wheels+energy stored in the flywheel.
Now you agreed that diesel engine needs more energy to turn it and petrol needs less. and you narrowed down to flywheel. hope you are not comparing the petrol with the slow speed diesel engines(1or 2cyl) used in generators. We are comparing hi speed diesel engines with petrol engine. and remember petrol engines too have flywheel. and they weigh appropriately for both.
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Old 8th February 2010, 12:53   #116
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Default Throttle Plates

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Originally Posted by Sankar View Post
Still you have not told me about the modern Diesel engines you know that are using throttle plates in the inlet manifold. It shall be good info for all if you could share.
Hi Sankar,

OM616
Nissan SD25
DCM Toyota Engines (INDIA)
Mitsubishi Canter Engines (INDIA)

These are some engines I have seen with a butterfly vane on the inlet manifold.

Regards,

Arka
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Old 8th February 2010, 14:15   #117
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My 2 cents.

Considering engine braking in Diesel vs Petrol, diesel has more braking provided we take the basic diesel engine and not all the funny/cool/smooth things added to it. Simply take the effort needed to keep the engine rotating (after all, thats all the effort that gets converted to braking). But once you add DI, CRDI and a ECU to the picture, it all comes down to the ECU programmer and the sensors that they are depending on to determine the the Engine state. Two different makes of Diesels engines do not behave the same. There are diesels where we have a throttle body (air intake determines RPM) and then there are others that fuel controlled throttles (fuel pump determines the RPM).

Please correct any of my wrong assumptions or conclusions. Thanks.
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Old 8th February 2010, 15:03   #118
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Gentlemen most of you seem to grossly underestimate the power of vacuum. The compression or frictional forces are no match to the retardation that vacuum affords. A diesel engine that does not have vacuum being generated will never match the retarding force of vacuum in a petrol.

If the higher compression were that adequate there would be no need for the Jake brake or the exhaust brake.

Vacuum in application can lift tons. Its pretty deadly

Its the "power" of vacuum in application that has also led to slipper clutches in superbikes as the retarding forces of vacuum when you downshift can actually lock the rear wheel unsettling the rider. Now I think they are using alternative techniques to effectively manage this vacuum to gain just the right quantum of retardation during down shifts.

Last edited by DKG : 8th February 2010 at 15:04.
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Old 8th February 2010, 15:51   #119
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Originally Posted by star_aqua View Post
Now you agreed that diesel engine needs more energy to turn it and petrol needs less. and you narrowed down to flywheel. hope you are not comparing the petrol with the slow speed diesel engines(1or 2cyl) used in generators. We are comparing hi speed diesel engines with petrol engine. and remember petrol engines too have flywheel. and they weigh appropriately for both.
If we are talking about two engines of near identical capacity, Diesel engine would have a much heavier flywheel than its Petrol counterpart.

See, i mentioned flywheel ok i accept. But i did not narrow down to it. Flywheels were mentioned to make it clear where that energy is gonna get stored when piston gets pushed down by that air spring. But Petrol engine is in partial vacuum you got to accept that fact, so everytime the piston is moving up & down the energy is being used up, its not being returned because there is very little air there to compress and there is no air spring.

OT:
In a Petrol engine with bad worn out valve guide the engine smokes when its idling, but as the throttle gets opened the smoking becomes less. This is due to engine oil being sucked in through the valve guide in the closed throttle partial vacuum phase (not applicable for engines with stem seals), once the vacuum is relieved by opening the throttle, engine sucks in air instead of oil. I had this issue on my CI Bullet with a bad valve guide but good ring seal.

Quote:
Originally Posted by ex670c View Post
Hi Sankar,

OM616
Nissan SD25
DCM Toyota Engines (INDIA)
Mitsubishi Canter Engines (INDIA)

These are some engines I have seen with a butterfly vane on the inlet manifold.

Regards,

Arka
Thanks for this info Arka
Do you know what controls the actuation of the butterfly valve in these engines and what it does?

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Originally Posted by DKG View Post
Gentlemen most of you seem to grossly underestimate the power of vacuum. The compression or frictional forces are no match to the retardation that vacuum affords. A diesel engine that does not have vacuum being generated will never match the retarding force of vacuum in a petrol.

If the higher compression were that adequate there would be no need for the Jake brake or the exhaust brake.

Vacuum in application can lift tons. Its pretty deadly

Its the "power" of vacuum in application that has also led to slipper clutches in superbikes as the retarding forces of vacuum when you downshift can actually lock the rear wheel unsettling the rider. Now I think they are using alternative techniques to effectively manage this vacuum to gain just the right quantum of retardation during down shifts.
Vacuum rules

Last edited by Sankar : 8th February 2010 at 16:08.
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Old 8th February 2010, 17:03   #120
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Guess its a quite lot of information flowing on. If I just have to understand in simple words;
The current Gear ratio in my jeep(invader) is {Low 2.46 and the axle ratio is 4.27}. What would be the best ratio for low gear and axle to get a breaking of 1.5 mts per/scnd.
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