[Sankar]

Eventhough there isn't clear answer on this thread, more than half the world agrees on the fact that Diesels have lesser engine braking than Petrol.

[headers]

Originally Posted by Sankar

Eventhough there isn't clear answer on this thread, more than half the world agrees on the fact that Diesels have lesser engine braking than Petrol.

I'm not sure I understood you Sankar: What is half the world? The people contributing here?

[Sankar]

Originally Posted by headers

I'm not sure I understood you Sankar: What is half the world? The people contributing here?

Rest of the world apart from tbhp, mabe not half but more like 3/4th
Reason is, i was not able to find a single bit of knowledge on the net where its authoritatively mentioned that Diesel engine have engine braking as good as a Petrol engine. Everywhere whats mentioned is the lack of Diesel engine braking (without the aids like jake/exhaust brakes).

Same principles that i've learnt over the years, regarding engine braking & engine behaviour, is accentuated everywhere; lack of throttle valve, lack of vacuum, return of energy, lack of engine braking.

Some engines behave strange, why?

[headers]

Originally Posted by Sankar

Rest of the world apart from tbhp, mabe not half but more like 3/4th

ROFL:

Reason is, i was not able to find a single bit of knowledge on the net where its authoritatively mentioned that Diesel engine have engine braking as good as a Petrol engine. Everywhere whats mentioned is the lack of Diesel engine braking (without the aids like jake/exhaust brakes).

You cant compare an orange and an apple, my friend.

Some engines behave strange, why?

Because, their designers would have had a strange dream!

[JeepCaptain]

Originally Posted by ex670c

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.

Hi Arka,

Thanks for the detailed explanation with different Jeeps and GB & TC combination.

Can you please explain crawl speed and crawl ratio for

CJ3B with XDP4.90 mated to T90 Gearbox. (dont know the differentials are replaced ) But this is what Robinson drives now and he is trying to compare with his newly acquired Invader 4WD with stock differential and DI engine with NGT520GB?? ( I guess)

And when he says engine braking, I guess, manuvering steep short inclines with engine braking in 4x4 mode mostly on slush/mud or rocky terrain without pressing the brakes. Because the moment you depress the brakes on above terrains, your Jeep becomes a slide. Robinson hope we are talking the same thing here.

Shibu

[tsk1979]

Well I am talking about this thing.
2 cars, same weight, same gear ratios. One diesel, one petrol.
Which of those will roll down an incline faster in first gear, given no throttle input, and incline steep enough to forcible spin engine to rpm much greater than idle speed, and the surface is very grippy, with absolutely no wheel spin or sliding

[headers]

Originally Posted by tsk1979

Well I am talking about this thing.
2 cars, same weight, same gear ratios. One diesel, one petrol.
Which of those will roll down an incline faster in first gear, given no throttle input, and incline steep enough to forcible spin engine to rpm much greater than idle speed, and the surface is very grippy, with absolutely no wheel spin or sliding

If 2 cars have identical ECU mapping, similar flywheels, similar engine timing, and SAME gear ratios both will be identical.

[Sankar]

Originally Posted by tsk1979

Well I am talking about this thing.
2 cars, same weight, same gear ratios. One diesel, one petrol.
Which of those will roll down an incline faster in first gear, given no throttle input, and incline steep enough to forcible spin engine to rpm much greater than idle speed, and the surface is very grippy, with absolutely no wheel spin or sliding

Originally Posted by headers

If 2 cars have identical ECU mapping, similar flywheels, similar engine timing, and SAME gear ratios both will be identical.

In such an ideal condition Petrol vehicle would have better engine braking.


I think the definition of engine braking has been lost in the 6 page long discussion.

Engine Braking: "Engine braking is the act of using the retarding forces within an engine to slow a vehicle down, as opposed to using an external braking mechanism."

[vinod_nookala]

Originally Posted by Sankar

I think the definition of engine braking has been lost in the 6 page long discussion.

. I am still not able to get answer for my basic question-

Why is that my practical experience speaks different to what has been stated here?

I have driven a petrol jeep with similar gearing as that of a diesel jeep and even similar diff ratio and similar weight. Diesel jeep had better engine braking than the petrol one be it in 4wd low or otherwise! This comparision would be apt- like comparing apple to apple, not like comparing a gypsy to a jeep or a tata safari crdi to a mpfi petrol cars etc etc

Guys iam still looking for a convincing answer!

DKG-You had B275 engine on your jeep earlier to petrol with 4.27 diff ratio and now you have silky smooth hurricane petrol with 5.38 ratio. Even though you had a higher diff ratio which engine had better engine braking?

Edit: Arka can also put his driving experience here as he had a Diesel engine in his 3b in the past!

Robinson.s-Thanks for initiating such good topic but by what you have written about your driving jeep cherokee having an extra low gear for speed off road events iam clueless. Can you please explain about this gear a bit please?


p.s- I must agree this thread has been educative to me on various fronts. Thanks guys

[headers]

Originally Posted by vinod_nookala

. I am still not able to get answer for my basic question-

Why is that my practical experience speaks different to what has been stated here?

I have driven a petrol jeep with similar gearing as that of a diesel jeep and even similar diff ratio and similar weight. Diesel jeep had better engine braking than the petrol one be it in 4wd low or otherwise!

Vinod - practical experience speaks and you still want to get answers - you ought to go back to elementary physics. IMO this thread is just meandering here and there without focus

[DKG]

Originally Posted by vinod_nookala

DKG-You had B275 engine on your jeep earlier to petrol with 4.27 diff ratio and now you have silky smooth hurricane petrol with 5.38 ratio. Even though you had a higher diff ratio which engine had better engine braking?

Very interesting discussion. Just read through it from start. Here's what I can say in the hope it answers questions from both sides

a. The very term engine braking refers to use of vacuum within an engine to retard.

On that score we have to concede to Sankar who so patiently has been harping this accurate techical fact all along.

As Sankar has repeatedly said by virtue of the design of a petrol engine and its intake system where there is the double effect of a venturi and a closed throttle valve creating an engine vacuum inherently yes a petrol engine enjoys engine braking naturally when you lift off the throttle.

b. All drivetrains experience some frictional lossses. However these frictional losses are not of much consequence to induce engine braking

c. The compression ratio, flyhweel weight, gearbox ratios and final drive ration all affect the overall retardation of a vehicle during deceleration. This is not and should not be confused with the term engine braking

Engine braking purely refers to retardation on account of vacuum. So techically and in actuality a petrol engined vehicle will have superior engine braking.

A lot of guys are engaging in this lively discussion based on their personal driving experiences and their actual experiences are contrary to what has been stated by Sankar correctly. Their experience in my understanding is referring to retardation on account of other factors other than engine braking.

So while we must concede the discussion to Sankar on grounds that technically when we are talking about engine braking its referring to primary use of vacuum there are other elements which aid retardation.

In most diesels the intake of air is not controlled by a throttle valve. The power strokes are controlled more by the quantum of fuel injected into the combustion chamber.Now on idle a typical diesel engine will not have any vacuum. Only lesser fuel injected.

So diesel engines do not "engine brake" (considering the term refers to use of vacuum to retard)

Having said this lets discuss real life experiences.

1.Compare a low compression engine to a high compression engine. Its obvious you need more power to put a high compression engine through its compression stroke.

So on account of compression the engine provides greater resistance which can retard a vehicle bring driven by wheels (downhill scenario)

So diesels have a natural resistance advantage to petrols here.

2. Heavier flywheel. This has a upside and a downside in terms of retardation. On a short downhill a heavier mass will have more inertial mass retarding the effect of wheels attempting to turn the engine. BUT this is shortlived. In no time does the flywheel gather energy as it rotates and this actually has the opposite effect and makes the engine keep revving.

Again Sankar rightly observed that a low mass flywheel will retard better over a longer downhill slope compared to a high mass flywheel. Advantage to petrols as they naturally don't require heavier flywheels

3. Gearing and final drives

As a layman speaking a smaller gear driving a larger gear will have torque multiplication when driving and torque reduction when driven. So its harder to turn a 5.38 axle based drivetrain with the wheels as compared to a 4.27 axle. ie More resistance encountered by wheels when attempting to reverse turn a 5.38

Same applies for gearbox rations. What multiplies torque when driving will work against the wheels when being driven

So a car retards more when driven by a lower ratio drivetrain.

So in summary guys, Sankar is right when it comes to the term engine braking. It refers ONLY to use of vacuum to retard a vehcle. Petrols inherently enjoy more manifold/cylinder vacuum while diesels enjoy none (most cases) and hence if the discussion is purely referring to the term engine braking a petrol always brakes better on account of engine braking.

But diesels have advantages on account of compression but not enough to surpass that of vacuum. Which is why diesel engines employ the aid of jake brakes or exhaust brakes to create vacuum within to retard a vehicle.

To answer your question Vinod if I am coming downhill from Ooty which do I think will retard more efficiently, my previous diesel config or the petrol. I think the petrol

[vinod_nookala]

Originally Posted by DKG

So in summary guys, Sankar is right when it comes to the term engine braking. It refers ONLY to use of vacuum to retard a vehcle. Petrols inherently enjoy more manifold/cylinder vacuum while diesels enjoy none (most cases) and hence if the discussion is purely referring to the term engine braking a petrol always brakes better on account of engine braking.

But diesels have advantages on account of compression but not enough to surpass that of vacuum. Which is why diesel engines employ the aid of jake brakes or exhaust brakes to create vacuum within to retard a vehicle.

To answer your question Vinod if I am coming downhill from Ooty which do I think will retard more efficiently, my previous diesel config or the petrol. I think the petrol

Very effectively put DKG, thanks!! Now tell me when are we going to ooty?

[headers]

Originally Posted by DKG

To answer your question Vinod if I am coming downhill from Ooty which do I think will retard more efficiently, my previous diesel config or the petrol. I think the petrol

DKG Dada, I salute you

Originally Posted by vinod_nookala

Very effectively put DKG, thanks!! Now tell me when are we going to ooty?

Vinumyfriend: I believe otherwise in 4wd, but in normal cars what he says seems true!

[Sankar]

Originally Posted by DKG

Very interesting discussion. Just read through it from start. Here's what I can say in the hope it answers questions from both sides...

Thanks for explaning this further DKG!

[star_aqua]

Originally Posted by Sankar

Thanks for explaning this further DKG!

Sankar, usually members here in 4x4 section, post their own experience rather than going through wikipedia. now check out the same link which you posted earlier. Engine braking - petrol and diesel engines. - SA 4x4 Community Forum - The only forum for the offroad and 4x4 enthusiast

most of the points are valid in that thread and it seems to be concluding.