Exhaust Brake?

[sti-nkr]

So basically an exhaust butterfly that is manually, or electronically, actuated. With a smaller orifice designed to keep pressure between it and the turbo at or below 60psi?

Yeah, bringing it back from the dead. I'm curious about one of these. Had one on my 07 dodge and loved it.

 

[Kohburn]

a strong throttle plate int he exhaust with a turbo charger external wastegate plumbed in to bypass the throttle plate. the spring rate of the wastegate will determine the max backpressure of the brake.

simple and safe. then you just have to decide if you want it to be electronic, pneumatic, or cable actuated.

I am planning to use a servo actuated dump valve triggered by a micro switch.

 

[JasonS]

In a gasoline (ignition) type engine, the throttle resists the "sucktion" of air into the combustion chamber, thus creating an "engine brake" effect. The diesel is always wide open (=has no throttle) and there is very little resisting the piston movement.

I don't agree. In my experience, a vacuum pump has much lower loss when operating at a high vacuum. It isn't pumping anything.

To think of another way... Think of the diesel engine compressing the air like compressing a spring. The engine rotates compressing and releasing, compressing and releasing, etc. If you were to entirely block off the intake of a gas engine, you have the same thing but it is atmospheric pressure on the back side of the piston pushing the piston up.

 

[Kohburn]

I don't agree. In my experience, a vacuum pump has much lower loss when operating at a high vacuum. It isn't pumping anything..

it isn't operating inside of a vacuum, it is removing the molecules to create a vacuum with every stroke of the piston when the trottle plate is closed. That process requires a lot of power.

you really have to think of it as pressure differential on the inlet and outlet of the pump.

with a gas engine with a trottle closed you will have a theoretical absolute zero pressure on the inlet and ~14 psi on the outlet. so a delta of ~14psi

with a deisel engine with no throttle plate to close you have 14 on the inlet and 14 on the outlet. there is no resistance, ie a delta of zero.

now if the deisel has an exhaust brake set to 30 psi, you have an inlet pressure of 14 and an exhaust of 30 resulting in a delta of 16psi. bump that up to 50psi setting on the exhaust brake and you have a delta of 35psi over twice the braking energy as an inlet throttle.


(this example of course ignores the displacement, rpm, volumetric efficiency, and compression ratio of the engines and how that stroke generates braking force as that is a seperate issue, with high efficiency the compressed air acts more as an air spring on the piston which creates less braking force than the internal friction of the engine)

 

[jkelly66]

My head is buzzing from all the technical jargen on this thread. But my opinion of engine brakes is that they can be a life saver at times but wont fix stupid when negotiating a steep grade the best brake is common sense and being ever vigilant on whats around you and always do the posted speed or under. I don't care how many hot dog cowboy truckers pass me going down a grade.
I prefer the good old hydraulic engine brake (Jakob's Brake) as a veteran of the pacific northwest and running down mountains like the Cabbage or Donner in all sorts of weather at 80,000 gross it has saved my brakes and my narrow butt many times. My experience has been if you are not taking care to not apply to much pressure or fan your brakes it doesn't take long for them to heat up to the point where they are almost useless.
Not sure I will ever need a engine brake on my M35 although I have often thought of making myself a brake pressure gauge that I could view in my cab.

 

[jkelly66]

One more note about engine brakes be careful using them on slick surfaces they can cause you to brake traction really quick.

 

[mudguppy]

... Yeah, bringing it back from the dead. I'm curious about one of these. Had one on my 07 dodge and loved it.

if this is the factory 'exhaust brake' you are refering to, then it's a completely different animal. the one on your CTD utilizes the VGT turbine housing to restict exhaust flow through the turbo.

 

[juddspaintballs]

I'm just tossing this one out there, because I haven't researched it a lot, but the new fire engines my employer just bought are equipped with jake brakes and a nifty device called a Telma retarder. Between the two, you can almost stop the truck without touching the brakes. The Telma retarder is an opposing electro-magnet on the driveline, and it works REALLY well. I wonder if we could fit one of those on a deuce?

Telma Retarders

 

[mudguppy]

that's pretty interesting - how is it actuated (button on dash, microswitch on brake pedal, ...)?

depending on it's size, it'd have to be installed right at the transfercase output in order to leave enough shaft length for the large amount of axle movement at the front tandem.

i wonder how much power it takes...? very cool.

 

[juddspaintballs]

It's computer controlled, so you can have it activated by brake pedal, or whenever your throttle is off. I think the intended control is activated when the throttle is off, but with a switch to deactivate it once you come to a complete stop (so it doesn't burn itself up). From what I've read, it can draw up to 160A, but for the short amount of time it's drawing that much, your batteries should be able to handle the load.

 

[JasonS]

it isn't operating inside of a vacuum, it is removing the molecules to create a vacuum with every stroke of the piston when the trottle plate is closed. That process requires a lot of power.

Sorry, but this is wrong. We have numerous types of roughing and high vacuum pumps at my job. Some of them allow monitoring of instantaneous power. Power drops with pressure. Maintaining a vacuum takes very little power. There are no pumping losses if you are not pumping anything. Whatever power it takes to creat a vacuum (downward stroke) is put back into the system when the piston starts back on the upward stroke.

 

[Hammer]

Whatever power it takes to creat a vacuum (downward stroke) is put back into the system when the piston starts back on the upward stroke.

That works until you figure in the valves opening and closing.

 

[JasonS]

That works until you figure in the valves opening and closing.

No, it still works. The intake valve opens as the piston is on the downward stroke, the chamber is at a pressure less than atmosphere (it takes work to to pressurize ambient atmosphere), the intake closes, the piston moves on the upward stroke which is aided by higher ambient atmospheric pressure.

 

[Hammer]

Yeah, but vacuum is lost once the exhaust valve opens.

I can see where you are working with a vacuum pump where you would maintain the vacuum in the cylinder for each cycle of the piston.
But this doesn't apply when the exhaust valve opens and lets pressure back into the cylinder. Once the exhaust valve opens, you loose the balance in the system, and the vacuum doesn't 'help lift' the piston.
Sorry if my wording isn't up to par, I am pretty sick right now and I feel like my head is in a cloudy mist....

And one other thing, a lot of engines will take that vacuum and apply it to the crankcase. So, the crankcase in a gas engine is rarely at atmospheric pressure.

Or maybe I am just missing the point on what you are saying.....

 

[Kohburn]

No, it still works. The intake valve opens as the piston is on the downward stroke, the chamber is at a pressure less than atmosphere (it takes work to to pressurize ambient atmosphere), the intake closes, the piston moves on the upward stroke which is aided by higher ambient atmospheric pressure.

have you ever seen the graph of valve height throughout the 4 strokes?

Cam profile, timing, and overlap play a huge role in the behavior of an engine that is not under power. This does not translate to pumps designed specificly for generating vacuums which are usually multi stage pumps.

your experience is anecdotal but does not apply.

 

[JasonS]

have you ever seen the graph of valve height throughout the 4 strokes?

Cam profile, timing, and overlap play a huge role in the behavior of an engine that is not under power. This does not translate to pumps designed specificly for generating vacuums which are usually multi stage pumps.

your experience is anecdotal but does not apply.

To a first approximation engines are air pumps and, with the butterflies closed, are vacuum pumps. Obvioulsy, a closer look complicates the analysis. My point is simply that there are no real pumping losses for an engine or pump which is not pumping anything.

 

[Hammer]

My point is simply that there are no real pumping losses for an engine or pump which is not pumping anything.

That is a fairly true statement.
Problem is, the exhaust valve lets air back into the cylinder, thus starting the entire vacuum pump affect again.
Then when the intake valve opens in it's next cycle, it (having approx 24" vacuum in the intake,) will equalize with the current volume of air in the cylinder (from the exhaust valve letting it in.) Then on the intake stroke, it is pulling a vacuum again.

On your vacuum pumps, there are one way valves to keep from letting air in, different valve timings, etc. I am NOT an expert on those. But I know that while the engine is an air pump, it doesn't have the same characteristics as a dedicated vacuum pump.

 

[JasonS]

That is a fairly true statement.
Problem is, the exhaust valve lets air back into the cylinder, thus starting the entire vacuum pump affect again.
Then when the intake valve opens in it's next cycle, it (having approx 24" vacuum in the intake,) will equalize with the current volume of air in the cylinder (from the exhaust valve letting it in.) Then on the intake stroke, it is pulling a vacuum again.

On your vacuum pumps, there are one way valves to keep from letting air in, different valve timings, etc. I am NOT an expert on those. But I know that while the engine is an air pump, it doesn't have the same characteristics as a dedicated vacuum pump.

This will be my last post, I promise.

I agree that a vacuum pump is not EXACTLY the same as an engine. The vacuum pumps that we use do not have any one way valves (turbo, diffusion, cryo, roughing, etc).

Let's look at the case where the intake is open, exhaust is closed, and the piston is on the downstroke. The pressure in the chamber is 15psi less than atmospheric. There is then 15psi pushing on the bottom of the piston (crankshaft side). Yes, it takes energy to do this but once the intake valve closes, the 15psi on the back side of the piston pushes back with an equal force on the upstroke. It is a reversible process.

 

[jwaller]

remember that the jake on the cummins motors turns OFF the IP when the Jake comes on thereby lowering the exhaust temps and keeping from overheating the exhaust valves and turbo.

 

[dmetalmiki]

Nudged my memory

I just realised (yet) another standard fitting on my (old) Kraz 255B.. (locking difs air brakes Centifugal engine oil filter and..) AN exhaust brake! sound great trundling down hills or aproaching a rally site!..not that the 15 1/2 ltr V8 diesel doesn't sound awsome aniway..can't even begin to think why so many folk think eastern block military stuff is...inferior..it certainly isn't