Downhill motor brakeing

[PGreenleaf]

OK we all know gas engines have better motor braking then our diesels.

Reason being that the diesel has a wide open intake track, where the gasser has a throttle butterfly in the intake track. Right??

Two questions.

1 With a wide open intake the diesel can suck in more air on intake stroke, thus having more air in cylinder to compress on compresion stroke, then the gasser that has a closed butterfly that is restricting the air intake of the gasser. SO why doesn't the diesel have MORE motor braking???

And #2 question

2 Both engines are basecly designed the same,so if all it takes to give the gasser better motor braking is a $1. 00 butterfly in the intake track, why is it nessesary to spend hundreds on a exhaust brake for the diesel. ??? Why not just install a butterfly in the intake track like the gasser. ??????????????

 

[Hoefler]

Actually, it is the other way around. The gasser would take in less air because of the butterfly, a vacuum in the intake means less air to be drawn into the cylinder for compression. Gasser are also lower compression. While the diesel has an open intake, it can draw in as much air as possible and being higher compression, give more resistance or absorb more energy.

 

[PGreenleaf]

Actually, it is the other way around. The gasser would take in less air because of the butterfly, a vacuum in the intake means less air to be drawn into the cylinder for compression. Gasser are also lower compression. While the diesel has an open intake, it can draw in as much air as possible and being higher compression, give more resistance or absorb more energy.

Thats exacly what I said, So Hoefler, why is it the gasser has MORE motor braking

 

[ScottLewis]

It's not the air moving through that affects braking but the fact that the butterfly on the gasser intake (and on the diesel exhaust brake) keeps the air from flowing through that helps slowdown.



Dan

 

[PGreenleaf]

It's not the air moving through that affects braking but the fact that the butterfly on the gasser intake (and on the diesel exhaust brake) keeps the air from flowing through that helps slowdown.



Dan

OK Dan answer my second question

 

[betterthanstock]

The Diesel has more compression, but once the piston is past TDC (top dead center) the higher pressure pushes it right back down, like a spring, so it gets back most of the energy in compressing it in the first place.



A throttle plate will only produce about 14 psi worth of vacuum braking force, an exhaust brake is good for about 60 psi in our engines.



The noisy "jake" brakes you hear, dump the compressed air at the top (or near the top) of the cylinder stroke, out the tailpipe, by opening the exhaust valves, so the air does not push down on the piston. If you dump the air at 200 psi, that's a lot of braking force. Nobody make those for our little engines, and it would be lots more money (a new head, stronger valves, cams, actuators) than a simple exhaust plate.

Since they produce so much braking force, they usually come with a 2-4-6 cylinder switch, so they can apply as much as needed on that hill.



You can make your own exhaust butterfly plate, and install a little lever or pedal to actuate it, no electronics. Just don't go over 60 psi, or you may blow the manifold gasket, and/or damage the valves/ push rods, turbo bearings, etc, and also clutch plate retainers, or other transmission damage. Most exhaust brakes have holes drilled in the plate, to limit the maximum pressure at 3000 rpm to 60 psi.

 

[PGreenleaf]

The Diesel has more compression, but once the piston is past TDC (top dead center) the higher pressure pushes it right back down, like a spring, so it gets back most of the energy in compressing it in the first place.



A throttle plate will only produce about 14 psi worth of vacuum braking force, an exhaust brake is good for about 60 psi in our engines.



The noisy "jake" brakes you hear, dump the compressed air at the top (or near the top) of the cylinder stroke, out the tailpipe, by opening the exhaust valves, so the air does not push down on the piston. If you dump the air at 200 psi, that's a lot of braking force. Nobody make those for our little engines, and it would be lots more money (a new head, stronger valves, cams, actuators) than a simple exhaust plate.

Since they produce so much braking force, they usually come with a 2-4-6 cylinder switch, so they can apply as much as needed on that hill.



You can make your own exhaust butterfly plate, and install a little lever or pedal to actuate it, no electronics. Just don't go over 60 psi, or you may blow the manifold gasket, and/or damage the valves/ push rods, turbo bearings, etc, and also clutch plate retainers, or other transmission damage. Most exhaust brakes have holes drilled in the plate, to limit the maximum pressure at 3000 rpm to 60 psi.

OK what you say makes sence. Why cant we put a butterfly in the intake tube, like a gasser???

 

[100 Proof]

OK what you say makes sence. Why cant we put a butterfly in the intake tube, like a gasser???

As stated above: "A throttle plate will only produce about 14 psi worth of vacuum braking force, an exhaust brake is good for about 60 psi in our engines. " You can do it, but it would not be nearly as effective. I'm not so sure it would be much less expensive as a retrofit. Hope this helps...



100 Proof

 

[PGreenleaf]

As stated above: "A throttle plate will only produce about 14 psi worth of vacuum braking force, an exhaust brake is good for about 60 psi in our engines. " You can do it, but it would not be nearly as effective. I'm not so sure it would be much less expensive as a retrofit. Hope this helps...



100 Proof

I realize the exhaust brake would be more effective, but wouldnt a simple butterfly in the intake put the diesel on equal par with the gasser???

 

[jelag]

Diesel engines control the speed of the engine by adding fuel to an already high volume of air... and when the fuel is injected the fuel explodes..... with out all this hot air in the cylinders... . your engine with a butterfly. . there wouldn't be enough air... nor heat for the fuel to explode and nothing would happen... The high pressure, hot gases (air) in the chamber is what makes the fuel explode upon injection. . there is no ignition sourse..... thus... more fuel, more power and rpm... .



Your gas engine uses a mixture of air and fuel..... air is controlled by a butterfly... and fuel is controlled by the injection system... . air and fuel in the proper ratio must be present in the combustion chamber when the spark occurs for proper ignition... to increase rpm you must do both... open the throttle plate. . and add fuel to the proper ratio to make more HP... and a faster running engine...



Two complete technologies... that don't interchange... .



I can go into more detail if you need it.....



When you go down hill and turn on an exhaust brake... your putting a plug in the exhaust stream... . and allowing up to 60 psi of back pressure... this back pressure puts gases in the combustion chamber when they are not usually there... . this means that the piston will come up on the exhaust stroke under compression... . thus absorbing energy... . this energy absorbed is about 90% of what the engine can make. Thus its ability to retard HP... this by the way is much greater than the ability of the gasoline engine when the throttle plate is closed and the pistions are going up and down under a vacuum... .

 

[PGreenleaf]

Jelag, Your perfectly correct on what you say, and I understand the difference between a gasser and diesel on how they run.

The question is about motor braking. Why cant a butterfly be installed in the manifold of a diesel and make it as good as a gasser for motor braking??

You would only use it coming down the mountain, when you want power you'd have to open it up. In other words it would have to be open and the diesel would operate normally. But when you started down that long grade you could shut it , same as a gasser .

I guess my thinking is this setup could be made for a lot less money then a exhaust brake?? Maybe not. ??

 

[cerberusiam]

But when you started down that long grade you could shut it , same as a gasser

Then you would get little or no engine braking. Without that incoming air charge you cannot develop the 60 psi of back pressure on the pistons to provide the energy absorbtion needed for engine braking.



Remember, the same exhaust flow that is used to capture 1/3 of the energy created by combustion can be used in reverse to provide a retarding effect. You have to maintain air flow to promote combustion to get exhaust flow to create back pressure.

 

[HPetrat]

With the amount of air that most diesels will move through them, installing an air damper/shut-off on the intake side also runs the risk of "vacuuming" (sucking oil past the rings and seals) of the engine, especially on a long downhill grade.

In other words, you could load the cylinders with engine oil and fuel, that when you release the intake air damper and allow the engine to breath again, it could run "wild. "



I have seen this occurr on marine diesels were intake air shut-offs were accidentilly tripped at wide open throttle, the shut-offs reset and the engines immediately restarted only to here the rpm's hit high notes that really make one pucker.



HP

 

[PGreenleaf]

With the amount of air that most diesels will move through them, installing an air damper/shut-off on the intake side also runs the risk of "vacuuming" (sucking oil past the rings and seals) of the engine, especially on a long downhill grade.

In other words, you could load the cylinders with engine oil and fuel, that when you release the intake air damper and allow the engine to breath again, it could run "wild. "



I have seen this occurr on marine diesels were intake air shut-offs were accidentilly tripped at wide open throttle, the shut-offs reset and the engines immediately restarted only to here the rpm's hit high notes that really make one pucker.



HP

OK that makes sence, not a good idea, dont need no runaway diesels

 

[klenger]

That is very interesting. I learned something today.