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Old 2nd February 2010, 19:15   #16
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Originally Posted by Samurai View Post
Regarding engine braking, try this experiment. While driving down the slope in a road, press the A-pedal and accelerate, and then let go the A-pedal. Will the vehicle retard suddenly or continue in the same speed?
It'll lose RPM and slow down. But slowing down is not as rapid as a petrol motor, also if the vehicle is loaded it will slow down even lesser.

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Originally Posted by svsantosh View Post
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...
Its not about torque.
As you might know are air pumps.
Diesel engine is an air pump with no restriction in the passage of air. No vacuum (reason why there is a vacuum pump for servo brakes) and easy breathing.

Petrol engine is an air pump with a butterfly placed in its intake tract. When the butterfly is closed intake tract vacuum increases, making it difficult for the engine to breath.

This difficulty in breathing is what translate into engine braking.
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Old 2nd February 2010, 19:17   #17
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Guys,
Revability of the engine and the ability to get to redline depends on the charactrstics of the engine. Thats why may be safari revs up to 4500 rpm in 1st low in steep downhill. Depends upon how the engine is tuned. This would never happen in a old school diesel engines.
Which is that characterstic which makes it rev friendly and also hold engine braking iam clueless! May be its pump design, may be some thing to do with block design, may be flywheel weight etc etc

But in loose terms a petrol engine cannot match a diesel engine in terms of engine braking due to differences in the compression ratio. Petrols can rev higher than the highest reving diesel engine. Hence the resistance to rev without fuel supply will be higher on a diesel engine.
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Old 2nd February 2010, 19:25   #18
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This topic is too technical for me to discuss. I just know that XDP engine in my Jeep has very good engine braking. Why? That is beyond my knowledge.

I have also heard that turbo diesels have lot less engine braking compared to NA diesels.
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Old 2nd February 2010, 19:51   #19
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Default Engine braking in CRDe engines

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Originally Posted by Samurai View Post
Guys, Common rail diesel engines don't give engine braking like the old school engines. Something about ECU trying to act smart. So diesel Jeeps give very good engine braking while common rail Safari/Scorpio won't.
I can tell you the reason for this, CRDe engines have various types of sensors which give input to the ECU regarding the vehicles state,if you carefully observe the suspended pedal of a CRDe vehicle e.g. scorpio it has a clutch switch and a brake pedal switch, when the clutch pedal is pressed this gives a signal to the ECU that drive is getting disengaged so fuelling is corrected accordingly,similarly when brake pedal is depressed this signals the ECU to cut down the fuel supply hence the vehicle is aided in slowing down, all this is done to maintain good driveability, i apparently found this out in one of my test vehicle which had issues of power loss, on analysis i found that it had a faulty brake switch which was feeding wrong signals to the ECU.

@Vinod your explanation about the compression ratios is pretty much correct. Let me put in some technicalities here:- What does a flywheel do?- It is a store house of energy (here this energy is due to the rotation of the crankshaft (rotational mass)attached to the flywheel), it stores energy and gives it out when there is a shortage, this is one reason why the flywheel of a diesel engine is heavier than the flywheel of petrol engines of similar dimensions,this is pretty much to do with the torque characteristics of a diesel engine, the torque fluctuations in a diesel engine is greater when compared to a petrol engine, the greater this fluctuation the bigger the flywheel, so does your engine braking change. Remember engine braking is the resistance of the engine at different RPMs the heavier the flywheel the better is its resistance to motion. A simple physics here "A body continues to be in its state of rest or uniform motion unless driven by an external force" this is actually what happens here.

@Samurai- you may not have much technical info on this topic but your logic regarding engine braking is SPOT ON.

Hope this clarifies

Spike

Last edited by SPIKE ARRESTOR : 2nd February 2010 at 19:53.
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Old 2nd February 2010, 20:04   #20
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@robinson: The diff ratio for the Invader is 3.73. This was done to improve milage and keep CMVR off Mr. Behram's back. Another member on this Forum had changed it to 4.88. I am using 4.27 which is the ratio on the DI engine without the Turbo. You loose your top speed a little but drivability improves. The other option maybe to find the lower gear that Spike suggested.
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Old 2nd February 2010, 20:25   #21
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I renamed the thread to match the discussion.

Now invader has turbo DI engine, that means it won't have much engine braking?

Spike, I understood the flywheel part. But how does flywheel aid engine braking is still puzzling to me.
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Old 2nd February 2010, 20:39   #22
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Heavy flywheel does not aid engine braking, it keeps it momentum. Lighter flywheel aids engine braking, it loses its stored energy faster. This you can notice in petrol engines too - especially when someone shifts from regular flywheels to lighter flywheels.

Simple term - to have engine braking there should be pumping losses. This is why an exhaust brake is installed on heavy vehicles. To create pumping loss and engine braking.

Revvability of engines etc does not have anything to do with engine braking directly.

So if the ECU cuts fuel when clutch is pressed or foot taken off the throttle then it should aid engine braking. Not otherwise.
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Old 2nd February 2010, 20:44   #23
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Quote:
Originally Posted by Samurai View Post
Spike, I understood the flywheel part. But how does flywheel aid engine braking is still puzzling to me.
Heavier fly wheels need more force to turn them. Hence when going downhill they offer more resistance to turning thereby holding the vehicle. I guess with CRDe/ECU controlled engines, you need to press the brakes a little or just enough pressure to get the ECU to cut down fuel supply to aid with engine braking.
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Old 2nd February 2010, 21:06   #24
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Quote:
Originally Posted by 4x4addict View Post
Heavier fly wheels need more force to turn them. Hence when going downhill they offer more resistance to turning thereby holding the vehicle.
Resistance to turning is provided by the engine, actually once a heavy flywheel is set in motion it keeps going for a long time.

Heavy flywheel = less engine braking

Damn these are all basics!

Pumping loss = engine braking
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Old 2nd February 2010, 21:20   #25
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mmmm, all this theory translated into practical info means this to me:

A Jeep can do this: Come down a slope without a driver -- EFFORTLESSLY



A stock petrol gypsy: NEVER..... It will just hurtle down and crash

Rest all is mumbo jumbo in off-roading speak.
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Old 2nd February 2010, 21:30   #26
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Guys Guys, i am afraid we are on 2 different pages here. Sankar and Tanveer is saying about a "Long" Descend - ie something like climbing down from Ooty thru Kallatti. Try your Jeep's engine braking here, you will notice that RPM will go crazy after some time, hence increasing your Vehicles Speed, and remember you are on 2WD here. For preventing this, Trucks has Exhaust Brakes. Now Samurai, K_S and other OTR guys are saying about an engine braking on incline which is not that long. The Pearl Valley Rock which Happy Wheel is mentioning is a very Steep BUT Very short as well. Now imagine you are on 4WD Low also, and all four of your wheels has grip, which will further contribute to your engine braking.

PS: I tried to climb down with Engine braking of my Jeep while coming back from ootty, and after some time i realized that RPM is going crazy and speed is increasing. I did the same thing with my Alto which was much more effective

Hope I did not messed-up further.

Thanks
--Sree--

Last edited by sreerajunnithan : 2nd February 2010 at 21:32.
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Old 2nd February 2010, 21:47   #27
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Yep on two pages i guess, Sreeraj Like you said 4WD low makes lot of difference!!

A stock petrol engine will stall when the throttle is let off. This can be tried in any car, put it in first gear and go down the hill and close the throttle it will gradually slow down and stall.

I guess Khan Sultan's Gypsy with crawler gear will be able to do this!
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Old 2nd February 2010, 22:12   #28
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Originally Posted by Sankar View Post
A stock petrol engine will stall when the throttle is let off. This can be tried in any car, put it in first gear and go down the hill and close the throttle it will gradually slow down and stall.
May be it did in carb petrol engines. I go down the hill everyday in my Petrol GV, it keeps on accelerating unless I brake. Only in 1st gear it doesn't hurtle down, but it doesn't stall either for lack of A-pedal.
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Old 3rd February 2010, 03:49   #29
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Quote:
Originally Posted by sreerajunnithan View Post
Guys Guys, i am afraid we are on 2 different pages here. Sankar and Tanveer is saying about a "Long" Descend - ie something like climbing down from Ooty thru Kallatti. Try your Jeep's engine braking here, you will notice that RPM will go crazy after some time, hence increasing your Vehicles Speed, and remember you are on 2WD here. For preventing this, Trucks has Exhaust Brakes. Now Samurai, K_S and other OTR guys are saying about an engine braking on incline which is not that long. The Pearl Valley Rock which Happy Wheel is mentioning is a very Steep BUT Very short as well. Now imagine you are on 4WD Low also, and all four of your wheels has grip, which will further contribute to your engine braking.

PS: I tried to climb down with Engine braking of my Jeep while coming back from ootty, and after some time i realized that RPM is going crazy and speed is increasing. I did the same thing with my Alto which was much more effective

Hope I did not messed-up further.

Thanks
--Sree--
not just two different pages, many different pages here. guys let me try my level best here.. the diesels have very good engine braking and the petrols doesn't. its all due to the vast difference in the compression ratio for diesel and petrol. diesels will usually have more than 20:1 and petrols usually less than 10:1.

why do people like petrol engines rather than diesel in cars earlier?? the petrols were more comfortable and there were no surges/jerks in powertrain when ever you release the accelerator(reason: compression ratio). very less or no vibrations (reason: compression ratio). try push starting a diesel car and the petrol car. you will see the difference.

now why do modern diesels doesn't have this engine braking?? its all because of the technology that was put into modern diesel to make it simply behave like a petrol. those surges/jerks are suppressed through the surge dampner, a comfort feature in the ECU which makes us to feel that the engine allways keeps pulling and doesn't offer any back force and yes its according what Spike has posted. when your intention is to slow down, you will surely press the brakes and there is a sudden cut off in the fuel untill it reaches idle rpm.

engine braking is used only on trucks and hence they are equiped with a button which on activated, disables all those comfort functions and works normally as our JEEP engines

and DI engines have less compression ratio than the IDI.

There latest rotary pump controls the surge with the help of governors in it. Check out the driving behaviour on a XD3P and older XDP. you can make out the difference. its all the pump cotrol that matters here.
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Old 3rd February 2010, 08:28   #30
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Quote:
Originally Posted by Samurai View Post
May be it did in carb petrol engines. I go down the hill everyday in my Petrol GV, it keeps on accelerating unless I brake. Only in 1st gear it doesn't hurtle down, but it doesn't stall either for lack of A-pedal.
Yes you're right MPFI engines keep the revs high.


Here is some info i found while googling this morning: Manifold vacuum - Wikipedia, the free encyclopedia
"Since low-pressure is only created on the over-run (such as when descending hills with a closed throttle), not over a wide range of situations as in a petrol engine, a vacuum tank is fitted."
Ok so Diesels engine brake on the over-run.

Here they say some Diesels have engine braking and some don't: Please explain diesel engine braking? - Diesel Forum - TheDieselStop.com
Some of their Diesels engine brake and some don't.

This is the most interesting one. We have other people wondering about the same thing since and they started a thread few days ahead of us: Engine braking - petrol and diesel engines. - SA 4x4 Community Forum - The only forum for the offroad and 4x4 enthusiast
Very good info there. Diesels do engine brake.


In that case why have they invented Jake brakes and exhaust brakes for heavy haulers? (Jake brakes were invented in the old tech non crdi Diesel days). Interesting thing about Jake brake is that it brakes by letting out the compression, keeping the exhaust valves open. Why not let the compression do the braking?

Last edited by Sankar : 3rd February 2010 at 08:30.
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