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Old 3rd February 2010, 09:04   #31
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@Sankar i feel Jake brakes/exhaust brakes are secondary braking devices installed on heavy vehicles to compliment the primary brakes, as the inertia/momentum of these vehicles is very high, braking cannot be controlled only by regular brakes, please correct me if i am wrong.

Spike
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Old 3rd February 2010, 09:56   #32
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Originally Posted by khan_sultan View Post
. You sure..? I thought the diesel's are good at that....
So did I and Still do. But I realised something when we had a Palio diesel. If I took my foot off the acclerator it wouldn't slow down but the RPM remained constant for a while. That's when I realised that the engine management was controlling the revs. But it would eventuallly decrease that you could use the engine brake to fair effect. Can't say the same about a baleno.
One needs to notice what the engine rpm is in low gear going down slope. Most of the new engines are drive by wire and you will even notice that on the fuel pump. No acclerator cable on the palio diesel. The Marina is old fashioned so I enjoy the braking on that going down.

Spike also the exhaust brakes will prevent unnecessary wear and tear of the braking system on a truck. It can be used to slow down from a distance and also to go down slope slowly.

Last edited by deutscheafrikar : 3rd February 2010 at 10:00.
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Old 3rd February 2010, 10:09   #33
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Quote:
Originally Posted by SPIKE ARRESTOR View Post
@Sankar i feel Jake brakes/exhaust brakes are secondary braking devices installed on heavy vehicles to compliment the primary brakes, as the inertia/momentum of these vehicles is very high, braking cannot be controlled only by regular brakes, please correct me if i am wrong.

Spike
Jake brakes are secondary braking devices to aid engine braking in a Diesel engine. Because Diesel engine cannot engine brake? Its used in heavy vehicles because they need engine braking in slowing down.

Jake brake - Wikipedia, the free encyclopedia
"When the accelerator is released on moving vehicle, its forward momentum continues to turn the Diesel engine's crankshaft and compress air in the cylinders. As the piston passes through top dead center, the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning most of the energy expended in compression back to the crankshaft. Therefore the engine does not effectively aid in slowing the vehicle.

In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum, the balance coming from internal friction of the engine itself. Diesel engines, however, do not throttle air intake and hence do not provide engine braking from throttling losses.

A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder, thus preventing it from returning its energy back to the piston. Accordingly, engine drag increases and the vehicle speed is reduced."

Jacobs Vehicle Systems - How the Jake Brake Works
W/O Jakes Brake
"(2) Air is compressed by the engine piston. The energy required to compress this air is produced by the vehicle's driving wheels.

(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."

W Jakes Brake
"(2A) Air is compressed to approximately 500PSI by the engine piston. The energy required to compress this air is produced by the vehicle's driving wheels. Near top dead center, the Jacobs Engine Brake® opens the exhaust valves, venting the high pressure air and dissipating the stored energy through the exhaust system.

(3A) On the downward stroke, essentially no energy is returned to the piston (and to the driving wheels). There is a loss of energy. This loss is how the retarding work is done."


Speaking honestly i'm confused now! Everywhere it says that Diesels do not engine brake. Technically the reasons are valid enough. Then there are technically unverified instances saying that Diesels do engine brake.

Behram Sir, can you please help understand whats going on?

Last edited by Sankar : 3rd February 2010 at 10:19.
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Old 3rd February 2010, 11:29   #34
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Sankar,
Marveleous explanation. So i think most would now agree that Diesel's have more engine braking power than petrol. All these jake brakes and exhaust brakes are secondary devices to slow down heavier vehicle down a sleep incline.

Plus as spike said the primary motive of providing secondary braking system (via engine braking) through exhaust is that to reduce wear and tear on the primary braking system. In ghat sections you cannot crawl down in first gear all the while which is fainfully slow. Hence the exhaust braking / jake braking and all.

I hope i have summurised things above. :-)
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Old 3rd February 2010, 11:31   #35
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I beg to disagree here. I think diesels do not have the same engine braking as petrols
Link here
Quote:
Diesel engines do not maintain a throttle vacuum as they do not have an intake throttle. The fuel itself is the throttle, and thus diesel engines are not subject to the same engine braking effects as gasoline engines are.
However, there are several other mechanisms which diesel engines use that could also technically fall under the term engine braking
Engine braking - Wikipedia, the free encyclopedia
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Old 3rd February 2010, 11:46   #36
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I have driven petrol cars before and now am driving Swift D for last 50K kms. IMO Diesels have less engine braking as compared to petrols due to huge torquey engines they tend to pull the car faster and faster even without accelerating.
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Old 3rd February 2010, 12:03   #37
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This is crazy, half the guys are saying diesels have more engine braking and rest are saying petrol has more engine braking. This is not a matter of opinion, we need real facts here.

I have a 2.0L petrol vehicle and a 2.1L diesel vehicle. The petrol vehicle has no engine braking at all, while the diesel vehicle has massive engine braking. But it is not as simple as that. My petrol vehicle is modern with ECU engine management, while the diesel vehicle has IDI engine with no electronics of any sort.

We need more classification:
1) Petrol with carb
2) Petrol with MPFI
3) Diesel NA
4) Diesel with Turbo
5) Diesel with common rail

I think each of these behave differently with regards to engine braking, please suggest if I have left out anything.
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Old 3rd February 2010, 12:04   #38
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Quote:
Originally Posted by vinod_nookala View Post
Sankar,
Marveleous explanation. So i think most would now agree that Diesel's have more engine braking power than petrol. All these jake brakes and exhaust brakes are secondary devices to slow down heavier vehicle down a sleep incline.

Plus as spike said the primary motive of providing secondary braking system (via engine braking) through exhaust is that to reduce wear and tear on the primary braking system. In ghat sections you cannot crawl down in first gear all the while which is fainfully slow. Hence the exhaust braking / jake braking and all.
Thanks Vinod, explanation was in the source, i just highlighted it. But it explains why Diesels have less engine braking compared to a Petrol engine.

Yep Jake and exhaust brakes are to induce engine braking in Diesel engine to complement the primary brakes.

Exhaust brakes are different from Jake brake in the sense that it blocks the flow of exhaust gases to achieve pumping loss and induce engine brake.

With Jake/Exhaust brake Diesels have more engine braking.

Quote:
Originally Posted by tsk1979 View Post
I beg to disagree here. I think diesels do not have the same engine braking as petrols
Link here

Engine braking - Wikipedia, the free encyclopedia
Tanveer and i seem to be on the same page
Yep Diesels do not have engine braking like petrol. This is what i too understand, unless we are all mistaking engine braking for something else which i do not know.

This is why I requested Behram sir to help clear the situation.

Quote:
Originally Posted by Samurai View Post
This is crazy, half the guys are saying diesels have more engine braking and rest are saying petrol has more engine braking. This is not a matter of opinion, we need real facts here.

I have a 2.0L petrol vehicle and a 2.1L diesel vehicle. The petrol vehicle has no engine braking at all, while the diesel vehicle has massive engine braking. But it is not as simple as that. My petrol vehicle is modern with ECU engine management, while the diesel vehicle has IDI engine with no electronics of any sort.

We need more classification:
1) Petrol with carb
2) Petrol with MPFI
3) Diesel NA
4) Diesel with Turbo
5) Diesel with common rail

I think each of these behave differently with regards to engine braking, please suggest if I have left out anything.
We need facts.

You're right Samurai san.

Also a Petrol vehicle with AC on have less engine braking than a Petrol vehicle with AC off. Why? Because when AC is turned on ECU/AC circuit in carb increases the idle RPM.

In petrol vehicle engine braking related to only one thing idle RPM. If the ECU/Carb does not allow the idle RPM to fall there is no engine braking.

Last edited by Sankar : 3rd February 2010 at 12:13.
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Old 3rd February 2010, 12:18   #39
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Just a wild guess.

Old Diesel engines - IDI/DI non aspirated will come down slowly on a slope because of the way the combustion happens in these engines.

The rpms are dependent upon the air fuel mixture being present in the cylinder at the time of combustion. (Here is another difference in the way a petrol engine works). And in a diesel engine the air/fuel mixture is a hetergenous mix at any given time. So the engine will only do least amount of work (bhp) at idling rpms holding back the drivetrain.

The characteristics of diesel may play a part here too that is it will not gain rpm easily.

This goes out of the window when a turbo or common rail is added as now the engine breathes much better in the earlier case and is electronically controlled in the second. Not letting the simple characteristics of a simple diesel engine kick in.

Will a Thar or a Scorpio crawl down an incline like a CJ340/CL340, 550/540 will? I dont think so.

Basically the reluctance of a diesel engine to rev (properties of diesel, compression based engine) is what helps engine braking in the old gen diesel engines.

Last edited by Spitfire : 3rd February 2010 at 12:26.
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Old 3rd February 2010, 12:20   #40
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Quote:
Originally Posted by Sankar View Post

Tanveer and i seem to be on the same page
Yep Diesels do not have engine braking like petrol. This is what i too understand, unless we are all mistaking engine braking for something else which i do not know.
That page needs more citation. I will try to post the exact details of each type of engine brake later. have to leave now.
jake brakes or exhaust brakes are not used in diesel engine due to the reason they are weak, they are used due to reason the trucks weigh more than 10 tonnes.
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Old 3rd February 2010, 12:26   #41
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Quote:
Originally Posted by Sankar View Post
Speaking honestly i'm confused now! Everywhere it says that Diesels do not engine brake. Technically the reasons are valid enough. Then there are technically unverified instances saying that Diesels do engine brake.
Quote:
Originally Posted by vinod_nookala View Post
Sankar,
Marveleous explanation. So i think most would now agree that Diesel's have more engine braking power than petrol.
Vinod - I think Sankar's looong post was to prove diesels DONT have engine braking. I think you read it otherwise. But as the swordsman says....

Quote:
Originally Posted by Samurai View Post
This is crazy
I have PM'd Behram, before this thread confuses the wit out of me!!
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Old 3rd February 2010, 12:26   #42
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Default Crawl Speed and its Factors

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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
Quote:
Originally Posted by svsantosh View Post
Sharat, I am thinking a petrol engine will retard more (loose RPM quicker) than a diesel. Why dont we bring torque into the discussion. Does the inherent nature of diesels having higher Torque makes it lazy to loose RPMs' and thus inhibit engine braking; a petrol has lot lesser torque to shed...
Hi Robinson,

The Slowest speed possible on a vehicle with out using brakes is know as the crawl speed.

Crawl Speed = (Engine Speed at Peak Torque Rpm/ Crawl Ratio) X (Circumference of Tyre in Meters)

Gears have 2 effects on the engine and wheels.

1) Slows the engine speed to the wheels

2) Increases the engine torque to the wheels.

Crawl Ratio = 1st Gear X T-Case Ratio X Diff-Ratio

for an Invader with NGT520 & 3.73:1 Diff-Ratio (29" Tyres circumference = 2.31mts)

Crawl Ratio = 4.3:1 X 2.46:1 X 3.73:1 = 34:1

Crawl Speed = 1800/34 X 2.31 = 122 meters per minute or 2 meters per second.

For an MM550XD with KMT90 & 4.88:1 Diff-Ratio (31" Tyres circumference = 2.47mts)

Crawl Ratio = 3.986:1 X 2.46:1 X 4.88:1 = 47.8:1

Crawl Speed = 2000/47 X 2.47 = 105 meters per minute or 1.75 meters per second.

For my MM540XD with KMT90 & 5.38:1 Diff-Ratio (31" Tyres circumference = 2.47mts)

Crawl Ratio = 3.986:1 X 2.46:1 X 5.38:1 = 52:1

Crawl Speed = 2000/52 X 2.47 = 94.9 meters per minute or 1.58 meters per second.

For a CJ340 with KMT90 & 5.38:1 Diff-Ratio (29" Tyres circumference = 2.31mts)

Crawl Ratio = 3.986:1 X 2.46:1 X 5.38:1 = 52:1

Crawl Speed = 2000/52 X 2.31 = 88.8 meters per minute or 1.48 meters per second

For a CJ500D with KMT90 & 4.27:1 Diff-Ratio (28" Tyres Circumference = 2.23mts)

Crawl Ratio = 3.986:1 X 2.46:1 X 4.27:1 = 41.8:1

Crawl Speed = 1500/41.8 X 2.23 = 90 meters per minute or 1.33 meters per second.

Notice the difference in the last 4 examples? The effect of the Differential Ratio, Tyres, and Engine Peak Torque RPM on Crawl Speed!!!!!!

The easiest method to reduce the Crawl Ratio is to Change the Diff-Ratio.

For the Invader you can go for 47/11 or 43/10 in both cases you will have to change the differential carrier, spider gears and Bearings.

Why do some engines rev up while descending on idle, it is because the wheels load up the engine and acts like a load governor and the engine revs up to balance the load.

However sometimes its an indication of a weak clutch cover assembly/pressure plate if you are familiar with the vehicle only then can you confirm that.

If 2 engines with the same (nearly similar) displacement i.e 2.1L then the petrol will always generate more torque.

XD4.9P 2112cc = 12Kgm @ 2000rpm
F4-134 2187cc = 16Kgm @ 2000rpm

However a TCIC crde diesel with the same displacement will generate much more torque.

M-Hawk 2180cc = 26Kgm @ 1800rpm

Regards,

Arka

PS - Max Torque at the wheels is peak engine torque X Crawl Ratio.
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Old 3rd February 2010, 12:29   #43
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Many offroaders are going by there "offroad experience" of lower gear.
However, on a 10-20 foot slope, you can't tell which has more braking.
Based on my experience of driving the Indica/Safari/Santro and Getz down long slopes(many kms) I have seen that even in the old indica, in first gear the engine would slowly start accelerating close to redline in 1st gear if the descent was very steep.
Ditto with the safari. I have to keep tapping the brakes.
However on similar mountain roads, the getz and the santro could do very well in 2nd gear, with engine in midrange, and less frequent tapping of brakes required.
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Old 3rd February 2010, 12:32   #44
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Originally Posted by star_aqua View Post
That page needs more citation. I will try to post the exact details of each type of engine brake later. have to leave now.
jake brakes or exhaust brakes are not used in diesel engine due to the reason they are weak, they are used due to reason the trucks weigh more than 10 tonnes.
Page we agree upon is this: Diesels lack engine braking like a Petrol engine.

Agree that trucks weigh more and needs additional braking. Jake brake and Exhaust brake was brought into the loop to illustrate Diesels lack engine braking.

Any info regarding engine braking would be helpful.
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Old 3rd February 2010, 12:39   #45
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Originally Posted by Sankar View Post
Thanks Vinod, explanation was in the source, i just highlighted it. But it explains why Diesels have less engine braking compared to a Petrol engine.
My diesel jeep and petrol Gypsy driving experience says otherwise. Theory says something different. Why?

With Crde'ed engine ECU plays a big role in engine rpm management. Thats why may be new age engines are faster. SPike has given explanation to it.

Di BSII turbo also has an ECU if iam not wrong. Is that showing up on higher rpm?

p.s Arka only you can give such brilliant crawl ratio explanations!!

Last edited by khan_sultan : 3rd February 2010 at 15:56.
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