dump valves?????????????????

J

jasio1978

Tuner
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devon
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chrysler 300c
I'm confused dot com

If you want to avoid turbo lag, why dump the pressure at every opportunity. maybe im looking at it the wrong way, but what is the reason and how does it work. cheers.
 
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It dumps the pressure that builds up AFTER the turbo , when the throttle is closed. This is done to avoid the air going back through the turbo, often making it spin the other way and damaging it. Also , air forcing its way back through the turbo creates more turbo lag, so you might notice a turbo lag reduction when fitting dump valves.

Some valve types release the pressure back into the intake manifold before the turbo, others such as the blow off valve release it straight into the atmosphere.
 
Turboshaft said:
my fault , i didn't notice this is the diesel chat.
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DVs are not especially benenficial with throttled engines either. Variable geometry turbos are becoming ever more commonplace. Unless you really do need to change up at 10,000rpm with your right foot hard down they have little value in road cars.
 
so as far as my car goes, its a non starter. i wasnt gonna do it but did wonder if there was aplus or minus side to it, and thought id ask what they were all about as i just joined the site and it was puzzling me. thank you for the replies... now i know and it all makes sense. i did see a pug 306hdi the other day hissing away on the gear change, but thats the only one ive seen.... cheers all
 
I've seen loads of diesels with dump valves.
As already said a Dump Valve relieves pressure around the turbo allowing it to carry on spinning so when you change and jump back on the go pedal the turbo is ready to boost more or less straight away.
 
well in theory the turbo lag is lower if the pressure difference between the pressure of the air going into the intake and the pressure of the compressed air (ie before and after the turbo) is smaller. So theoretically, in order to have 0 turbo lag you need higher pressure before the turbo and lower pressure after , but of course this is not practical.

This concept is similar to the thermal efficiency of an engine which is related to the temperature difference of the cold and hot sources (it's actually a ratio)

To reiterate, the higher the pressure after the turbo , the longer it will take for the turbo to spool up. So from this point of view, dump valves are beneficial.
 
Turboshaft said:
well in theory the turbo lag is lower if the pressure difference between the pressure of the air going into the intake and the pressure of the compressed air (ie before and after the turbo) is smaller. So theoretically, in order to have 0 turbo lag you need higher pressure before the turbo and lower pressure after , but of course this is not practical.

This concept is similar to the thermal efficiency of an engine which is related to the temperature difference of the cold and hot sources (it's actually a ratio)

To reiterate, the higher the pressure after the turbo , the longer it will take for the turbo to spool up. So from this point of view, dump valves are beneficial.
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Yes, but how do you plan to increase the pressure downstream of the turbo's intake system without a throttle in the engine to restrict gas flow?

Also turbo lag and spool up are not the same. There is the inertia of the turbo shaft itself. This is finite and measurable simply because both it and the air flowing through have mass. If the turbo shaft assembly were massless then rotational inertia would not be an issue; therefore turbo latency would not exist.

Q E not D
 
You are right , i should have referred to it as spool up time exclusively instead of turbo lag. Indeed , spool up time is not the same as turbo lag, but it's one of the reasons for it.

As for how to increase pressure without a throttle plate, i thought that the rate at which pressure is relieved into the cylinders was too low, but at a second thought that's just improbable.

Btw is there an overlap in the opening of the intake and exhaust valves in a diesel engine?
 
Interesting thread,I have some experience with turbochargers, but this experience stops at around the late 80's technology.
About the only method that I know to reduce turbo lag is to use a small fast spooling turbo. The more you free up exhaust gas flow, intercooler piping drag, etc will also help . If you have a lot of time, and can solve all the space and piping issues, you could use a small turbo to spool up a big one, I have done this once and it works.
 
Turboshaft said:
You are right , i should have referred to it as spool up time exclusively instead of turbo lag. Indeed , spool up time is not the same as turbo lag, but it's one of the reasons for it.

As for how to increase pressure without a throttle plate, i thought that the rate at which pressure is relieved into the cylinders was too low, but at a second thought that's just improbable.

Btw is there an overlap in the opening of the intake and exhaust valves in a diesel engine?
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There is some overlap much the same as there is in 4 stroke petrol units.
 
I wont pretend i know as much as you 2, i do understand what your saying tho, so thanks for the education.

As for valve overlap, is this not what they refer to as scavenging, and do not all engines rely on it. or am i still confussed??????????

thanks for all the posts.
 
All engines rely upon scavenging to a degree. The problem is that its efficiency depends upon lots of factors. You cannot make an engine scavenge equally at all RPMs. Variable length intake tracts go some way to assisting this.

Combustion chamber design also plays a big part.

2 strokes are the things to study for this: These engines would not operate at all without the scavenging effect.
 
A DV is still really required on a road car but generally they have diviter valves so you don't get the big "woosh",

Diesels apparently don't need them, but I have seen (well heard) diesels with induction kits the air wooshing back though the filter the wrong way when they are off the gas, like a wastegate flutter and I guess that can't be good for them, so maybe a DV couldn't hurt,
the car I heard it on was an alfa 156 2.4 jtd
 
Like i said you are probably right but from what I can gather it should help stop the boost going back though the air filter and back though the turbo which isn't good
 
jarrus said:
Like i said you are probably right but from what I can gather it should help stop the boost going back though the air filter and back though the turbo which isn't good
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It cannot go back through the air filter because there's no throttle in a diesel engine.

DIESEL ENGINES DO NOT HAVE THROTTLES :amazed::amazed::amazed::amazed:
 
HDi fun said:
It cannot go back through the air filter because there's no throttle in a diesel engine.

DIESEL ENGINES DO NOT HAVE THROTTLES :amazed::amazed::amazed::amazed:
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There would still be an amount of pressure build up like a petrol turbo so would see a little benefit from it.
But for any noticable gains you'd have to be running some serious boost.
 
i will agree with Hdi . as there is no throttle there is no where for the pressure build up to be really and more when you come off the go pedal,other than through the valves or wastegate, add to this most diesels now utilise VNT turbos the boost can be controlled better.

there is quite often wastegate flutter on diesels
 
Well yeah thats what I heard, wastegate flutter, that still can't be good can it? so maybe a dv can cure this, purely from a safety point of view, maybe
 
Wastegate flutter is unavoidable to prevent overboost. If you fit a DV you'll just replace it with DV flutter I suppose. Which is likely to be a lot louder.
 
Look. Whatever you've heard or seen, the bottom line is diesels have no need or use for dump valves.

Also, dump valves and wastegates are completely unrelated. If you think they are, you have some very basic reading to do on understanding how turbochargers, wastegates and the like work.
 
MasterAuron said:
Look. Whatever you've heard or seen, the bottom line is diesels have no need or use for dump valves.

Also, dump valves and wastegates are completely unrelated. If you think they are, you have some very basic reading to do on understanding how turbochargers, wastegates and the like work.
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Thankyou.
 
Man, i love this site. as i said i have a good knowledge, i dont know it all by any means, and you lot are helping my knowledge grow.

it all makes sense to me tho..:amazed:
 
petescustompipes said:
whats scavenging?
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Scavenging is using the escaping exhaust gases velocity to create a slight vacuum in the combustion chamber/cylinder so that it helps to draw in a better intake charge of air and for that charge to arrive faster. Or more accurately, for the intake gas velocity to be higher. Thus increasing turbulence which in turn helps air/fuel mixing and ignition.

It's for this reason that the exhaust and intake valve open periods overlap between the exhaust and intake phases.
 
Turboshaft said:
it also helps trapped exhaust gasses in the cylinder mix with the air/fuel mixture
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That's pointless - exhaust gases need to go south, whether via a turbine or not. You don't want them remaining in the combustion chambers.

Diesel engines do not have throttles - there's nothing to dump.
 
HDi fun said:
That's pointless - exhaust gases need to go south, whether via a turbine or not. You don't want them remaining in the combustion chambers.

Diesel engines do not have throttles - there's nothing to dump.
Click to expand...


there will always be a small quantity of exhaust gases in the cylinder that won't flush no matter how you tune your exhaust manifold/ports and the best you can do is to make sure that they mix properly with the air/fuel otherwise you won't get a good combustion.
 
Turboshaft said:
there will always be a small quantity of exhaust gases in the cylinder that won't flush no matter how you tune your exhaust manifold/ports and the best you can do is to make sure that they mix properly with the air/fuel otherwise you won't get a good combustion.
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I know that. But what has it got to do with dump valves?
 
I was thinking that as well,

but the only reason I would fit a dump valve on a diesel is to stop the wastegate flutter which really isn't healthy for a turbo, and some owners on turbo diesels report that the excess pressure reverses though the air filter, not good as well, I think this is due to the fact the the engines demand for air has decreased, I know technically that diesels always run lean unless you've got the hammer down but if you shove too much air in the engine will stall, same applies on a diesel engine if you add too much nitrous oxide, it just stalls and you can just start it up again,

i'd just use one to protect my turbo if i was running some "unholy" boost because all the extra has to be consumed by the engine as there is no throttle but it doesn't need it, hense a dump valve might have some purpose on a turbo diesel, I just got to make sure that if i have one it doesn't scare old people....
 
I'm not aware of any engine makers who fit DVs to diesels. That's because there is no throttle and therefore nothing to dump.

Air goes into engine, gets compressed, expands again and then goes out the exhaust side.

Tell me this: What would trigger the DV?
 
No one does, that's my point, you have to tap into the brake vacuum line and use a solenoid and a switch on the fuel pump arm or tap into the drive by wire throttle pedal and when you let off the pedal it dumps the pressure
 
And if you did trigger it what's it going to dump?

Sorry but I cannot see the purpose of this with a diesel engine which breathes freely regardless of pedal position.
 
So. From what your saying is i dont need a dv. Cool, i wont waste the rhino.
Back to scavenging. I was always under the immpression that the idea of valve overlap, was more geared towards getting the exhaust out of the pot, then getting extra fuel air mix in.
Would it not be the same amount of mix going in, just that it was combusting better, due to no co, which dosnt burn.
 
jasio1978 said:
Would it not be the same amount of mix going in, just that it was combusting better, due to no co, which dosnt burn.
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That's exactly my point , i didn't state that there will be extra air/fuel in, I was trying to explain that the overlap also causes a swirl which helps the residual co mix with the air/fuel and combust better.

To reiterate, the overlap forces the exhaust gas out, but a small fraction of it goes back into the pot (because of resonance/pulse effects i was told) and gets mixed into the air/fuel mixture. The better they mix , the better they burn.

Sorry again for not making myself clear , English is not my native language.
 
jasio1978 said:
So. From what your saying is i dont need a dv. Cool, i wont waste the rhino.
Back to scavenging. I was always under the immpression that the idea of valve overlap, was more geared towards getting the exhaust out of the pot, then getting extra fuel air mix in.
Would it not be the same amount of mix going in, just that it was combusting better, due to no co, which dosnt burn.
Click to expand...

You don't need a DV. Valve overlap is the norm, scavenging is totally normal with four stroke engines. It's simply a matter of the degree as to how much overlap is designed in.

VVT systems adjust the valve timing in real time simply to make the best of what is ultimately a compromise.

As of yet the automotive industry hasn't designed and built the ideal internal combustion engine,
 
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