Boost no boost/diesel 101

[DPKetchum]

Hey guys explain why with no load turbos don't boost?? In a heated discussion with someone. Why no boost and no engine vacuum? Thanks. DP

 

[Greenleaf]

Well,,,,,, That's a deep subject.

Turbo can't spin fast enough with engine at idle speed be it fast or slow idle. Have to have engine under a load of some sorts. Heat and fuel. Not so much the speed of engine, rather the heat/fuel. Will produce most boost psi when at max fuel, gov. wide open, not WOT.



No vac. cause there is no valve aka throttle body/carb to limit the flow of air into the intake/combustion chamber.





What do ya think? Confused?



Scott

 

[cerberusiam]

Turbos spin in relationship to the amount of exhaust gas expansion. The amount of expansion is in direct relationship to the amount of fuel being burned. Under a 'no load' situation there is only enough fuel to maintain rpms at a constant. A diesel takes a full charge of air on every intake so there is essentially no boost if the engine is taking all the volume produced by the turbo which is spinning in relationship to exhaust flow.



Boost occurs when more fuel than is needed to maintain rpms is injected and the engine tries to accelerate creating a load condition. Just remember 'in order to make boost you need to burn fuel, the more fuel you burn in a combustion sequence the more boost is made'. This is a rather simplified explanation that does not take into account air restrictions, exhaust restrictions, turbo map, etc, but it captures the idea.



No vacuum is because there are essentialy no air metering devices like carb or throttle body and the engine takes a full charge on every intake cycle.



Clear as mud. Right?

 

[Greenleaf]

the engine takes a full charge on every intake cycle.

Oo. Oo.



Yah Baby!

 

[DPKetchum]

Ok at 2200 rpm under a no load condition is the turbo spinning as fast as it would at 2200 rpm under a load?

 

[hdm48]

Turbin speed is NOT proportional to engine speed. It is controled by exhaust pressure. Which is controled by air expansion due to the excelerated oxidation of diesel fuel (AKA-burning).

You can reach full boost at low RPMs or high as long as you don't hit your de-fueling point. The new electric engines will not let you do this for emission issues.

With a manual trans you can reach full boost as soon as the RPMs come up high enough for the pump to make full pressure.

To simplify it further, the turbo is ran by expanding exhaust gasses, no gasses-no boost.

Now to cut off the next question. Yes, big rigs going down a hill using the Jake Brake create boost. This is because fuel is added when on the Jake. This is how they can make up to 73% engine rated power backwards.

Now don't you wish you could use a Jake instead of those funky little exhaust brakes?

OOPPS, goin' off topic.



Dave

 

[cerberusiam]

Yeah, yeah, what Dave said.



The question to the answer is not rpm but how much fuel is being delivered and creating exhaust flow.



Dave, I understand the operation of the funky little exhaust brakes. Got a quick and dirty explanation on the operation of a Jake? Oh, and a little more info on the '73% of engine rated power backwards' would surely enhance the thread.

 

[BushWakr - RIP]

All you gotta do is see Dave's truck and you'll quickly understand the 73% power backwards theory... .



Pastor Bob...

Our Lady Of Blessed Acceleration Church.

 

[hdm48]

In simple terms, a Jake brake modifies the exhaust valve timing so the engine has a second compression stroke instead of an exhaust stroke (not completely though). Then it also modifies the fuel timing to give it fuel during this added stroke. The braking power is determined by the duration of valve timing and the amount of fuel added. How's that?

OR, you could add a hydraulic retarder to your flywheel and get 90%. Retarders run off engine oil pressure.



Pastor Bob, are you saying I drive faster backwards than I do forward? Careful, you may be a member of the clergy, but I gotta brand new stick, with ergonomicly designed, EPA approved, OSHA certified hand grip.



And it's "black, with a RED stripe"





Dave

 

[BCaldwell]

Here I thought dave was going to say it was white with red stripes Bill

 

[dpuckett]

Originally posted by DPKetchum

Hey guys explain why with no load turbos don't boost?? In a heated discussion with someone. Why no boost and no engine vacuum? Thanks. DP

1. Turbo will make boost under no load, full throttle; had a 93 with a H1C-12 that would make 9 psi if I floored it sitting there. But, hard as this may be for gasser guys to understand, diesels are incredibly efficient- when tuned properly, they use no more fuel than is needed for the load. SOmething to do with governor flyweights and centrifugal force allowing the plunger to plunge farther (correct me if I'm wrong). Turbo speed is directly proportional to engine load, like has been said before, just not in so scientific terms. (use that to make gasser dude appear more uninformed. ) That is why turbos are more efficient than blowers and superchargers- they rob no power from the engine to make boost. And that is why they make little or no boost at idle (unless you have BOMBed "slightly. ")

2. Hopefully, there is never any engine vacuum. Back to diesel efficiency and free breathing. Again, already been covered. Good luck in your argument. Sounds like you may lose, even though you're right.

Daniel

 

[formula]

Re: Re: Boost no boost/diesel 101

Originally posted by dpuckett

That is why turbos are more efficient than blowers and superchargers- they rob no power from the engine to make boost.

While the efficiency, specifically the adiabatic efficiency of a turbocharger is greater than a "blower" by NO means does that mean that it does not require energy to drive it. It is the exhaust gas energy (drive pressure) created by the engine that drives the turbocharger. The lesser efficiencies of blowers are due to the drag of the mechanical drive system and the method used to compress the air charge (roots, screw, planetary driven centrifugal). Also, the no-load full speed boost seen with a H1C-12 is due to the small turbine housing creating higher exhaust gas velocity over the turbine blades.



Cheers,

Sean