[balenoed_]

Originally Posted by CrAzY dRiVeR

...such slow speeds!

Ah! There you go. May be it was below the ideal rpm range and hence optimum vaccum wasn't generated inside the booster cylinder to brake and stop the car.
(Technically speaking in Kia/Hyundai terms. )

[ChoosetoCruze]

Originally Posted by CrAzY dRiVeR

Saw something crazy yesterday, which could quite possibly have been a braking issue with the Seltos.

Speed could have been just about 30 kmph only - I just didn't expect a crash! Felt like the Seltos didn't slow down by much even when swerving.

A good read on braking distances - https://www.rac.co.uk/drive/advice/l...ing-distances/

I would think from 30kmph to a standing stop would take less than 2-3 seconds in a normal car with the driver paying full attention on the road?

[audioholic]

Originally Posted by balenoed_

Ah! There you go. May be it was below the ideal rpm range and hence optimum vaccum wasn't generated inside the booster cylinder to brake and stop the car.
(Technically speaking in Kia/Hyundai terms. )

No you got it wrong As per Kia terms the driver hasn't gently pressed the brake pedal gradually and slammed it. Or in that short moment of time the driver has pressed multiple times so ABS has assumed after the first braking that the job is done. Thats as per the logic of the technician which was mentioned earlier in this thread.

Originally Posted by ChoosetoCruze

I would think from 30kmph to a standing stop would take less than 2-3 seconds in a normal car with the driver paying full attention on the road?

From 30kmph you can come to standstill almost instantly if you slam the brakes hard. Thats the typical speeds within residential areas in Germany and the reason I am told is that in these areas in case of an emergency the driver can stop almost immediately without worrying about braking distance. So it should be less than a second IMO.

[SPIKE ARRESTOR]

Originally Posted by Sutripta

The petrols - any of these turbo petrols? What is the vacuum source for these turbo petrols?

Sutripta

Haven't seen this particular model, my educated guess would be - Alternator.

Originally Posted by ashis89

I tried this today on my diesel manual Seltos. Stopped at a signal which was at a very mild incline.
Brake system running out of vacuum doesn't seem to be the problem, at least in the diesel.

Trying to better understand the problem and the failure mode. Could you please post a pic of the engine bay with brake booster and its connections clearly visible?

Similarly we will need a pic for the petrol variant(s) to get a better understanding of the system.

Originally Posted by audioholic

From 30kmph you can come to standstill almost instantly if you slam the brakes hard. Thats the typical speeds within residential areas in Germany and the reason I am told is that in these areas in case of an emergency the driver can stop almost immediately without worrying about braking distance.

Correct, in fact this is part of the driving test in Germany, called as "Gefahrbremsung in geschlossener Ortschaften" , panic braking in residential areas.

Spike

[Jeroen]

Originally Posted by SPIKE ARRESTOR

Haven't seen this particular model, my educated guess would be - Alternator.

Not quite sure I understand your suggestion here. Alternators produce electricity, not vacuum.

Originally Posted by Sutripta

The petrols - any of these turbo petrols? What is the vacuum source for these turbo petrols?

Just to answer your question lets do a little historical exploration of vacuum being used for brake boosting and see what is what.

I have taken a few images from my classic cars. The main reason for that is that on modern cars, it is almost impossible to see any details under the hood. Usually there is just some massive plastic engine cover staring at you. But underneath some general principle reign!

The most basic, carburator petrol engine. This is my Mercedes W123, petrol 2.0L carburator engine



One is the vacuum line connected to the brake booster, the big black thing

Two is the vacuum line connected to the inlet manifold by means of a banjo bolt.

If you follow the vacuum line you will also see the check valve. The check valve ensure vacuum is maintained in the brake booster, even if the engine is not running. (As explained in quite a few earlier post by several members, when you pump the brakes with the engine switched off, the remaining vacuum is used up quickly.

Here is my Alfa Romeo Spider with its 2.0L fuel injection engine

One is again the vacuum line connection on the brake booster.

Two is where the vacuum line is connected to the inlet manifold. A little difficult to see, but it also has a build in check valve.

Three is the throttle valve. So the vacuum is picked up down stream of the throttle valve.



Next, if we take my Jaguar XJR, V8 4.0L petrol engine with supercharged. No images, because there is too much plastic in the way. But again the vacuum is created just down stream of the throttle plate.

In very general terms for petrol turbo engines; they tend to have sufficient vacuum just behind the throttle plate. Only when the turbo starts to really spool up does that change. Obviously, they will have check valves too. In quite a few cases you might see some vacuum cannisters for extra volume of vacuum as well. Plainly put, petrol turbo engines are not that much different from NA petrol engine when it comes to how they create necessary vacuum.

The problem with the turbo is that when you floor the throttle your vacuum will disappear all together (not so in the NA petrol engine). But then again you don’t stomp on the brake whilst you are putting the pedal to the metal! As soon as you come of the pedal, the throttle valve will close and you have a massive vacuum. Whilst you have the hammer down the check valve (and maybe some vacuum booster / canister) will ensure the brakes work immediately when you need them.

Anorak fact: Quite a few EGR valves operated on engine (manifold) vacuum too!

The above is just very general. The actual implementation of how the vacuum is generated on modern petrol cars differs substantially. And you might come across some that do have a vacuum pump too!

Jeroen

[Sutripta]

Originally Posted by Jeroen

Not quite sure I understand your suggestion here. Alternators produce electricity, not vacuum.

Normally the vacuum pumps are an add on to the alternator.

Just to answer your question lets do a little historical exploration of vacuum being used for brake boosting and see what is what.
...
The above is just very general.

Wonder about the necessity of such a loooong piece to essentially say 'just beyond a (suction side) throttle plate'.

But in a modern throttle by wire, GDI, and complex emission rules, the old simple 'must be' solutions do not necessarily apply. And so it is always good to hedge ones bets. Like

The actual implementation of how the vacuum is generated on modern petrol cars differs substantially. And you might come across some that do have a vacuum pump too!

and wait for confirmation. Which might vary from implementation to implementation.

Sutripta

[a4anurag]

Originally Posted by Sutripta

The petrols - any of these turbo petrols? What is the vacuum source for these turbo petrols?

Originally Posted by srini1785

I dont see how the Piston vacuum generation theory works with tubo inlet (positive pressure in inlet).

Originally Posted by SPIKE ARRESTOR

Haven't seen this particular model, my educated guess would be - Alternator.

Haven't got a chance to check the 1.4 T-GDI engines on the Creta/Seltos but quick search on Boodmo for the 1.0L BoosterJet engine and 1.0L EcoBoost, reveals a separate vaccum pump.

[Motard_Blr]

Originally Posted by srini1785

In that case would a vacuum reservoir with sufficient capacity connected in series with the brake booster line solve the problem?.

Attachment 2071698



I will wait till somone who has a first hand knowledge of the issue replies but in case the issue also ocurs with a turbo petrol , i dont see how the Piston vacuum generation theory works with tubo inlet (positive pressure in inlet).

I think a vacuum reservoir would help by increasing the amount of vacuum available, if that is indeed the problem. But I believe that most vacuum boosters already have some amount of spare capacity.

There would be no difference in turbo petrol engines because the vacuum is between the throttle and the cylinder head, whereas the turbo is upstream of the throttle.

Edit: @a4anurag just posted that the turbo baleno and the turbo ecosport both have vacuum pumps. This must mean that the vacuum in the manifold is insufficient or inconsistent to be used for the brake booster.

Most, if not all, petrol cars these days are equipped with throttle-by-wire. So there is no direct link between the accelerator pedal and the throttle valve. When the foot is removed from the pedal the throttle doesn't close immediately with these throttles. In an effort to avoid jerks during deceleration, the throttle closes gradually. This means that vacuum build up inside the manifold is gradual and this may be the reason why the brakes don't seem to work at that moment.

Jason Fenske of Engineering Explained has a video on electronically-actuated throttles, that's worth a watch.

[Jeroen]

Originally Posted by Sutripta

Wonder about the necessity of such a loooong piece to essentially say 'just beyond a (suction side) throttle plate'.

I thought I would add a bit of context for our members. I was reading a survey the other day; 65.2% of those calling themselves petrol head could not point out a brake booster under the hood.

I figured you were smart enough to distill your specific part. Was I wrong?

Originally Posted by Sutripta

But in a modern throttle by wire, GDI, and complex emission rules, the old simple 'must be' solutions do not necessarily apply. And so it is always good to hedge ones bets. Like

and wait for confirmation. Which might vary from implementation to implementation.

I am not a betting man, but I would be the first one to question the limitation of my experience and insights. To my defence: I think it is far more easier to talk about general principle on older cars than on new cars. The variations on a same theme seem to be much greater on new cars, than old ones. E.g. one NA carbureted car is, technically, very similar to the next one. Whereas these days every new car seems to have endless varieties on any number of technical solutions.

It is probably me, slowly dementing and getting stuck in the past.

Originally Posted by a4anurag

Haven't got a chance to check the 1.4 T-GDI engines on the Creta/Seltos but quick search on Boodmo for the 1.0L BoosterJet engine and 1.0L EcoBoost, reveals a separate vaccum pump.

I can’t really see because of all the plastic installed, but it appears as if our 1.5 Ecoboost Ford Focus has something bolted onto the side of the engine too. Could this be a vacuum pump?

Who ever thought of the name eco boost? Can we find that guy and shoot him, please!

Originally Posted by Motard_Blr

I think a vacuum reservoir would help by increasing the amount of vacuum available, if that is indeed the problem. But I believe that most vacuum boosters already have some amount of spare capacity.

True, the brake booster itself is if you like a reservoir.

Originally Posted by Motard_Blr

There would be no difference in turbo petrol engines because the vacuum is between the throttle and the cylinder head, whereas the turbo is upstream of the throttle..

Correct. No matter what, when the throttle valve closed you are going to get a vacuum.Whether that is enough/sufficient from a brake boosting point of view is a different matter,

Originally Posted by Motard_Blr

Edit: @a4anurag just posted that the turbo baleno and the turbo ecosport both have vacuum pumps. This must mean that the vacuum in the manifold is insufficient or inconsistent to be used for the brake booster.

That is my thinking too.

Originally Posted by Motard_Blr

Most, if not all, petrol cars these days are equipped with throttle-by-wire. So there is no direct link between the accelerator pedal and the throttle valve. When the foot is removed from the pedal the throttle doesn't close immediately with these throttles. In an effort to avoid jerks during deceleration, the throttle closes gradually. This means that vacuum build up inside the manifold is gradual and this may be the reason why the brakes don't seem to work at that moment.

Well, I don’t think so really. The gradual closing you are mentioning still takes place in a matter of a less than 0,1 of second, 0,2 seconds at best! So gradual is really a relative term. Mazda is advertising here in the Netherlands; their unique selling point is that there is a 0,2 second delay in hitting the throttle and the engine actually responding. Nobody, and I mean nobody, has been able to explain why you would want that, but that is what Mazda believes will sell more cars.

Sorry, all in Dutch:

https://www.youtube.com/watch?v=pXQlLficugM

Jeroen

[Sutripta]

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[srini1785]

Originally Posted by Jeroen

I thought I would add a bit of context for our members. I was reading a survey the other day; 65.2% of those calling themselves petrol head could not point out a brake booster under the hood.

Count me on the 65.2%.

Originally Posted by Sutripta

Normally the vacuum pumps are an add on to the alternator.

Is it ?. i did not know that. Do i take it that its on the same belt or is it a single composite unit?.

I wonder if the models under discussion have a Vacuum switch?. If not i would be sorely disappointed. When you can add a sensor to measure the butt pressure on the driver seat, why can't you add one to check if there is sufficient vacuum and that too to operate a critical part like a brake?.

The general circuit for vacuum pumps is something like this:


Switch looks like this :


Switch goes off, your car is on collision course.

PS : All images taken off internet.

[Gansan]

Originally Posted by Jeroen

65.2% of those calling themselves petrol head could not point out a brake booster under the hood.

I was one of them, until I had to replace mine last year. The brake fluid level in the reservoir was was reducing slowly every few weeks. Inspection of the wheels and brakes did not reveal any fluid leak. Then they diagnosed the booster to be the problem and replaced it, after which the problem stopped.

But I did not experience any braking problem, was worried that suddenly all the brake fluid will vanish and I will have no brakes. It would be scary to experience something like what is described by the OP in this thread. Irrespective of how much electronics and other intricate parts are involved in the braking of latest cars. If I stomp on the brake, it should stop. Period. The brakes should be idiot proof.

My car has a basic braking system. The manual says even if Servo assistance is lost the brakes will still work, we just have to press harder. Thankfully I never had to experience it so far.

If I was in that car with wooden brakes with a toll plaza fast approaching, my reaction would have been to downshift quickly but sequentially through the gears till first gear, and then pull the hand brake lever. The engine and gear box be damned!

[The Mechanic]

Originally Posted by srini1785

Is it ?. i did not know that. Do i take it that its on the same belt or is it a single composite unit?.

I

Single composite unit with the vacuum pump attached behind the alternator on the same shaft. A common example is 2.5 diesel engine in Bolero.

The alternator vacuum pump assembly looks like this:-

https://boodmo.com/catalog/part-alternator-1387740/

Another common system I have seen in diesel engines is vacuum pump attached to and driven by cam shaft. Common example will be the FIAT 1.3 MJD Engine. It looks like a cap on the gear box side of engine attached to the cam shaft.

https://www.99rpm.com/fiat/vacuum-pu...mjd-punto.html

[sridhar-v]

Slightly off topic, but possibly relevant:-

Early to mid 1990's. I was driving my business partner's Tata Sierra to Mumbai, with some family members. It was one of the first 50 cars sold. (old school diesel engine with a glow plug and all that!).
When climbing down the infamous Bhor Ghat the brakes went all wooden. I immediately downshifted to first gear and then literally stood on the brake pedal. Due to the slope the diesel pump started sucking air and the engine sputtered and coughed like mad. Quickly managed to bring the car to a halt. I just sat for some time and then started the engine.
I diagnosed the issue as a booster leak and lack of baffling in the fuel tank.
We completed the journey and later on TELCO (as Tata Motors was known then) R&D engineers at the plant confirmed my diagnosis. They replaced a leaking hose. But nothing could be done about the lack of baffles in the tank.
So we made it a point to keep the tank full.

My point is that booster vacuum failure can happen anytime. So it is better to drive a wee bit slower than the road and traffic warrants.

[Jeroen]

Originally Posted by srini1785

I wonder if the models under discussion have a Vacuum switch?. If not i would be sorely disappointed.

Switch goes off, your car is on collision course.
.

What does the switch do? I suspect it just closes when no power/engine shut down? So it ensures whatever vacuum is present is kept, even if the engine is not running.

In which case it does exactly the same as a check valve. Not sure why they made this more complicated than needed? So there is probably more to it.

Maybe it is needed on a turbo engine as the vacuum in the manifold varies wildly depending on the turbo/throttle setting. So it closes if you hit the throttle perhaps?

Jeroen