[headers]

Is wikipedia's theory ( as quoted by 4ePajero in the first post ) wrong?


I think that is very wrong!! no question - diesel has way more engine braking than a petrol... someone needs to vet that Wiki entry!!!

never forget - Wiki is people's opinions and not everything you read is true!!!

The Above was taken from

SA 4x4 Community Forum - The only forum for the offroad and 4x4 enthusiast - View Single Post - Engine braking - petrol and diesel engines.

[Sankar]

Originally Posted by star_aqua

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.

Yes i went through that forum before i posted the link here. But that link wasn't authoritative enough. If i got 100 results regarding engine braking, except that page everything else was against Diesel engine braking. Like the 4x4 community here, 4x4 community of 4x4community.co.za seems to agree with Diesel engine braking. Why only the 4x4 faction?

I have a good understanding regarding vehicles and engines, theoritically and practically. When it came to engine braking members of the 4x4 community here had a different experience than what should be normal in a Diesel engine.

Why people at Dieselstop forum seem to be after exhaust brakes saying their diesel trucks are not slowing down as good as a petrol truck? Why Jacobs Vehicles Systems have a tech page illustrating the lack of engine braking in diesel?

See, i'm also looking for answers. Except 4x4 community everyone thinks Petrol rules when it comes to engine braking. Best way would be to take a Petrol Jeep & Diesel Jeep (or Tata Safari Petrol&Diesel or Ambassador 1.8ISZ&2.0ISZ) to a hill station and drive down. Maybe even a Swift VDI against a VXI would do.

Coming to Swift, here's an interesting bit of info, This talk took place long time ago and i thought it would be appropriate to mention it here:
My cousin (in Chennai) bought a VDI and his dad (my uncle) has a VXI. So when we met my uncle he was telling us, amongst other things, about the car his son bought - mileage drivng, diesel petrol comparo etc. So i asked how the VDI felt compared to his car. He said the car is good, great mileage etc. But he told us that he found the VDI difficult to drive in traffic. I asked him why and was it due to the power surge (turbo kicking in)? He said when he lifts the foot off the throttle the car wasn't slowing down as good as his VXI, said he often had to use brakes to slow down, which his VXI didn't need in controlling the distance with vehicle in front in traffic conditions.



Anyway let the discussion continue


I cannot test this on my own now due to my limited mobility, but i do plan to test it on my own when i'm able to, with a Diesel Qualis and a Petrol Qualis http://www.team-bhp.com/forum/1571864-post19.html

[DirtyDan]

Originally Posted by robinson.s

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

My Invader has 4:88 ratio crown gears in the differentials, front and rear, instead of the of the usual 3:73 crown gears that come from the factory. Because of this it climbs the steep hills here, the Himalayas, like a goat. It really gives me some low end grunt. I am very pleased with the results of this install. So far. It also enhances engine braking since this gearing forces more engine revs per wheel revolution than the 3:73s.

The trade off is I lose some high speed ability. I think the motor starts sounding funky around 85-90 kph. But I will have to take it out and try it some more. I was being conservative and careful. Next time I will push it a bit further. The 4:88s were installed just a few days ago and I have not re-registered my funk meter. Because of the poor roads here we rarely get our buggys up to 65 kph let alone 85 so the 4:88s work great for me. You may want to try 4:27 crown gears for a not so radical compromise.

The install took a little experienced grinding of the case because the 4:88s are a little fatter than the 3:73s. Swami Mahindra in Chandigarh did the work for me. They also changed my speedometer gears to compensate for the new 4:88 gearing. What year is your Invader? If it is old enough it may not have the new 3:73s which are for enhancing fuel average but at the expense of low speed power.

[Sutripta]

Hi everybody,
Interesting discussion. I think I'll gatecrash. Been in the boondocks for last few days, else would have joined in earlier!

For me, like to like means vehicles of approx same weight, engines of roughly equivalent power, and geared for roughly equivalent top speed at or near its power peak, and rest of gearing to match. I don't think we can get vehicles which meet these criteria.

Coming to my experiences, the CJ500D with the B275s had significantly superior engine braking compared to the F134. And this despite the power advantage of the F134. Similar experience with Ambassadors, both Isuzu powered.

The jeeps I have driven in the hills (Pankhabari, Peshok, Singla innumerable times).

Unfortunately, haven't driven the Isuzu petrol in the hills, but in plains driving, perception is the same:- the diesel has better engine braking.

It is true that the Scorpio tends to run away when coming down these same roads, but that I had always put down to it being heavier, and turbocharged. And maybe the ECU has something to do with it.

For me, my own experience, and what I know of engines/ vehicles are in sync.

Regards
Sutripta

[DirtyDan]

Gentlemen, we are not solving the thread's stated problem. Even if we all agreed on this engine braking issue, we have not directly addressed his query. OMG! I've contracted moderators disease! Headers, let me borrow your horse pistol.

[Sutripta]

Originally Posted by DirtyDan

Headers, let me borrow your horse pistol.

Sure you didn't mean horse whip? ! Isn't that what the moderators here carry?

Sutripta

[headers]

Originally Posted by Sankar

He said when he lifts the foot off the throttle the car wasn't slowing down as good as his VXI, said he often had to use brakes to slow down, which his VXI didn't need in controlling the distance with vehicle in front in traffic conditions.

That is true for most diesel cars. They do NOT slow down as well as petrol cars during A pedal lift off!

Originally Posted by DirtyDan

Headers, let me borrow your horse pistol.

oh DD, why me

Originally Posted by Sutripta

Sure you didn't mean horse whip? ! Isn't that what the moderators here carry?

Sutripta

You guys are getting suggestive!!

[DKG]

I was just wondering about F1 cars. They seem to slow down as soon as you lift off. The engines there may have low mass flywheels which is why it blips so quickly and dies so quick too. The engine braking in those engines must be phenomenal

I am also inclined to think the carb engines have better engine braking vis-a-vis fuel injected units. The double effect of a venturi along with a closed throttle possibly generates more vacuum

Programmed fuel injection is controlling fuel flow more efficiently and may not rely as much on throttle opening to enhance or retard engine speed as done in a carb.

Further to enhance fuel economy the ECU may actually inhibit vacuum to allow the vehicle to roll rather than retard when you lift off.

Originally Posted by vinod_nookala

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

Honestly we need to be there this monsoon ! Lets hope Beejay can put something together

[Sutripta]

Hi,
My comments in bold

Originally Posted by DKG

I was just wondering about F1 cars. They seem to slow down as soon as you lift off. The engines there may have low mass flywheels which is why it blips so quickly and dies so quick too. The engine braking in those engines must be phenomenal

My take: Result of power to inertia ratio

I am also inclined to think the carb engines have better engine braking vis-a-vis fuel injected units. The double effect of a venturi along with a closed throttle possibly generates more vacuum

Absolutely don't follow what you are saying.

Programmed fuel injection is controlling fuel flow more efficiently and may not rely as much on throttle opening to enhance or retard engine speed as done in a carb.

AFAIK, fuel flow is controlled by measuring the quantity of air going through the engine, measured by a mass flow sensor, or deriving it from manifold air pressure. And further tweaked by things like temperature, readings from the lambda sensor etc. The throttle controls the amount of air passing through.
In a carb system also, the throttle controls the amount of air passing through the system. It is the job of the carb to see to it that that air always has the right amount of fuel. On the overrun, with throttle closed, in a carb, slightly more fuel than what is used for idle will pass through. This is too lean to burn properly, and will not contribute to power. For the purposes of our discussion, I don't think it makes any difference whether the petrol engine is FI, or runs a carb.

Further to enhance fuel economy the ECU may actually inhibit vacuum to allow the vehicle to roll rather than retard when you lift off.

If we see the engine on the overrun as a badly designed airpump, I think closing the inlet would have exactly the opposite effect:- absorb less power.

Regards
Sutripta

[JeepCaptain]

Originally Posted by DirtyDan

My Invader has 4:88 ratio crown gears in the differentials, front and rear, instead of the of the usual 3:73 crown gears that come from the factory. Because of this it climbs the steep hills here, the Himalayas, like a goat. It really gives me some low end grunt. I am very pleased with the results of this install. So far. It also enhances engine braking since this gearing forces more engine revs per wheel revolution than the 3:73s.

The trade off is I lose some high speed ability. I think the motor starts sounding funky around 85-90 kph. But I will have to take it out and try it some more. I was being conservative and careful. Next time I will push it a bit further. The 4:88s were installed just a few days ago and I have not re-registered my funk meter. Because of the poor roads here we rarely get our buggys up to 65 kph let alone 85 so the 4:88s work great for me. You may want to try 4:27 crown gears for a not so radical compromise.

The install took a little experienced grinding of the case because the 4:88s are a little fatter than the 3:73s. Swami Mahindra in Chandigarh did the work for me. They also changed my speedometer gears to compensate for the new 4:88 gearing. What year is your Invader? If it is old enough it may not have the new 3:73s which are for enhancing fuel average but at the expense of low speed power.

hmmm. Thanks DirtyDan for breaking the suspense. I was wondering who has the 4:88 gearing Invader in our forum ever since 4x4addict mentioned that in the beginning of this thread.

DD, I guess he has a 2004 model invader.

Robinson, I guess you have the answer now and couple of choices aswell .
If you want to have balance between top speed and offroading,go for 4:27 differentials like 4x4addict did. Else if you are ok with top speed of 85-90 as DD says and want to have maximum fun in OTR's as you do with your CJ3B now go for the 4:88.

Originally Posted by SPIKE ARRESTOR

adding a bit to that, there is a transfer case with 2.48 ratio in Low mode

Spike

Spike, This is news to me. Is it from M&M? More info please.

Shibu

[star_aqua]

Originally Posted by Sankar

Like the 4x4 community here, 4x4 community of 4x4community.co.za seems to agree with Diesel engine braking. Why only the 4x4 faction?

I have a good understanding regarding vehicles and engines, theoritically and practically. When it came to engine braking members of the 4x4 community here had a different experience than what should be normal in a Diesel engine.

Welcome to 4x4 section .
This is the section where people do spend most of their time on understanding the vehicle, engine and the drivetrain, not through theories but practically.

Originally Posted by Sankar

Why Jacobs Vehicles Systems have a tech page illustrating the lack of engine braking in diesel?

I did not find where did they have mentioned. and remember they have to market their product also .

but please go through this what i found from their page:

FAQ10: Can you explain why Jacobs Engine Brakes on certain engines are more powerful than on others?
ANSWER: Jacobs Engine Brakes are designed for specific engine applications, and retarding performance will vary depending on several factors of the design. One thing that is common to all engine brakes is that retarding horsepower increases as engine RPM increases. Optimum performance of the engine brake is achieved near rated engine RPM. In general, the same things that affect positive power influence retarding power. The most important factors affecting retarding performance are engine displacement, compression ratio, turbo type, level of boost, and the timing of the valve-opening event. Lets look in detail at how these factors affect the brake performance. In it's simplest description, the Jacobs Engine Brake converts a diesel engine into an air compressor. All other factors being equal, the larger the engine displacement, the more powerful an air compressor it can make, and the higher the retarding performance. Compression ratios of diesel engines are typically around 15:1. At the top of the piston stroke the air will occupy 1/15th the original volume, and be at around 500 psi. It takes power to compress air to high pressures; the engine crankshaft supplies this power. Higher compression ratios produce higher cylinder pressures, and absorb more power in doing so. If the turbo is able to provide more boost, the starting volume of air will be at a higher initial pressure. This will result in a much higher final pressure. The higher the final pressure, the more power is absorbed.

and here in below explanation it doesn't mean that petrol engines have more engine braking and they doesn't require an aid for engine braking.

FAQ6: Do you have any engine brakes or exhaust brakes for Gasoline engines?
ANSWER: Because of the low back pressure limits of gasoline engines, the performance of a gasoline exhaust brake would not be substantial. Because the air intake in a gasoline engine is throttled and the compression ratio is much lower than in a diesel, the performance of a gasoline engine brake would also be insignificant. Because of this we do not offer any retarders for gasoline engines. However, Jacobs is actively seeking the commercialization of its Variable Valve Actuation System (VVA) for gasoline engines. The benefits of increased fuel economy (throttling with intake valves instead of a butterfly), miller cycle valve timing, EGR, and even compression release braking are just some of the reasons VVA would be an attractive option for gasoline engines in the light truck and passenger car markets.

source for FAQs: Jacobs Vehicle Systems - Frequently Asked Questions

i hope it clarifies your doubts.

At first place you need to understand that these engine brakes are only used for commercial vehicles and not for passenger cars until unless you carry few tonne of luggage in it.

since commercial vehicles are load carriers and they are equipped with diesel engines and to have better control, they prefer engine brakes and it doesn't mean that the diesel engines lack engine braking but its not sufficient for the amount of load they carry.

I have the experience of driving a bus as well as few trucks. and a license to drive heavy duty vehicles. hence the comments are from my own experience and also from my professional experience.

do our indian trucks have jake brakes(decompression valves) in it?? ask any bus/truck drivers would they like to have a petrol engine in it.

[siddartha]

yes, i agree that "PURE" engine braking by definition has everything to do with the vacuum.

but from a practical point of view i feel i cannot fully agree with the part that - effect of drive train is negligible as wiki puts it and also others here.



so somebody please explain , so if during engine braking drive train friction is negligible why do we downshift while coming down a slope?


isnt it the drive train that causes the increase in the engine rpm , so isnt that giving the pistons more number of strokes to use the vacuum effectively ? so isnt the more number of strokes of the pistons makin a difference in retarding the vehicle?

[Sankar]

Originally Posted by headers

That is true for most diesel cars. They do NOT slow down as well as petrol cars during A pedal lift off!

Headers, this is exactly what engine braking is

Originally Posted by star_aqua

Welcome to 4x4 section .
This is the section where people do spend most of their time on understanding the vehicle, engine and the drivetrain, not through theories but practically.

A good foundation in theory & fundamentals is essential for understanding what happens in the field practically

Originally Posted by star_aqua

I did not find where did they have mentioned. and remember they have to market their product also .

Jacobs Vehicle Systems - How the Jake Brake Works

[star_aqua]

Originally Posted by siddartha

yes, i agree that "PURE" engine braking by definition has everything to do with the vacuum.

but from a practical point of view i feel i cannot fully agree with the part that - effect of drive train is negligible as wiki puts it and also others here.



so somebody please explain , so if during engine braking drive train friction is negligible why do we downshift while coming down a slope?


isnt it the drive train that causes the increase in the engine rpm , so isnt that giving the pistons more number of strokes to use the vacuum effectively ? so isnt the more number of strokes of the pistons makin a difference in retarding the vehicle?

Sidda, engine braking is usually done through downshifting the gears. when the engine is in overrun there will be no combustion happening in the engine and its a dead engine. if you want to slow down more then you downshift. say if the engine is offering x amount of retarding force to the wheels in 4th gear(assume 1:1), when you downshift the x is multiplied with the 3rd gear ratio. that is why you feel a jerk when downshift and release the clutch. this was more in diesels and people used to like petrol because of this. but eventually more efforts were put into pump control to mask all these diesel characteristics which was making uncomfortable in driving. Look at the new bolero, how M&M is marketing. In the features of new bolero they have mentioned ''Active Surge Damper'' and a word 'advanced' added to it because the GB sends the feedback to the ECM.

I hope Spike can give more info on this.

Originally Posted by Sankar

Jacobs Vehicle Systems - How the Jake Brake Works

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?

[SPIKE ARRESTOR]

@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