Diesel exhaust brakes

HickoryFarms · Nov 20, 2018

zing said:
Lots of threads asking about how to downshift in order to keep the exhaust brake on since it shuts off at 1100-1200 RPM (talking Cummins again here), but not a lot about how high they tend to rev. This guy commented that it seems to be max efficiency between 18-2800. Overall that seems to be about 500-600RPM less than where the Ford 6.7L seems to be working best at.
https://www.cumminsforum.com/forum/07-5-09-3rd-gen-6-7-liter-general-discussion/427361-6-7-exhaust-brake-how-do-i-know-its-working.html

When the torque converter unlocks, the exhaust brake disengages.

Raspy · Nov 22, 2018

HickoryFarms said:
Exhaust brakes require the torque converter to be locked on an automatic, otherwise there is no way to slow the drive train. The torque converter being locked is RPM based. The effectiveness of the exhaust brake itself will not be altered by the RPMs.

The RPM does very much affect the effectiveness of the exhaust brake. At about 1200 RPM it may be producing about 50 HP in retarding force. At 2400 RPM it will be producing about 140 HP in retarding force. Combine that with downshifting, where the engine then has better leverage against the truck and you have a brake that will skid the rear wheels in marginal traction conditions.

In the Aisin transmission, the torque converter locks as part of every shift. Not just based on RPM. The locking strategy in modern transmissions is far different than in older designs. It is done in such a way that you can hardly detect it and it holds the locked condition while towing. This makes the tranny develop far less heat and run much more efficiently. Locking is now just a normal part of shifting and occurs whenever the truck is moving. So, the TC is almost always locked when the truck is moving and in gear. This means too, that the brake is always available at any RPM over 1200.

Tow/Haul allows the brake to be used in a more aggressive fashion by keeping the RPM up (within reason) and downshifting early on down grades or when the service brake is applied. It can also be set, with a Cummins, to only activate when the brakes are applied. I've never seen mine go over 2400 RPM while automatically downshifting. Cummins engines do not have to be revved way up to do an enormous amount of work.

deerefan · Nov 22, 2018

Raspy said:
The RPM does very much affect the effectiveness of the exhaust brake. At about 1200 RPM it may be producing about 50 HP in retarding force. At 2400 RPM it will be producing about 140 HP in retarding force. Combine that with downshifting, where the engine then has better leverage against the truck and you have a brake that will skid the rear wheels in marginal traction conditions.

In the Aisin transmission, the torque converter locks as part of every shift. Not just based on RPM. The locking strategy in modern transmissions is far different than in older designs. It is done in such a way that you can hardly detect it and it holds the locked condition while towing. This makes the tranny develop far less heat and run much more efficiently. Locking is now just a normal part of shifting and occurs whenever the truck is moving. So, the TC is almost always locked when the truck is moving and in gear. This means too, that the brake is always available at any RPM over 1200.

Tow/Haul allows the brake to be used in a more aggressive fashion by keeping the RPM up (within reason) and downshifting early on down grades or when the service brake is applied. It can also be set, with a Cummins, to only activate when the brakes are applied. I've never seen mine go over 2400 RPM while automatically downshifting. Cummins engines do not have to be revved way up to do an enormous amount of work.

In my opinion, maybe that is why the Cummins has a long life-working at lower rpms, hence less wear/longer life.

Raspy · Nov 22, 2018

some zilch said:
an automatic and an exhaust brake :laughing:

never driven one in any size that worked

we jokingly called them "exhaust flappers"

Evidently, you haven'y driven a 4th Gen Ram. The exhaust brake works flawlessly and is a huge help in the mountains or when towing. It also works for just driving around empty. My truck has 140,000 miles on it and the original brake pads are only about 1/2 gone. The engine brake is one of the best things on the truck. It's pretty hard to argue that 140 HP of retarding force is not affective. And that power can be modulated to hold lightly on gradual hills, or to seriously slow down when stopping.

HickoryFarms · Nov 23, 2018

Raspy said:
The RPM does very much affect the effectiveness of the exhaust brake. At about 1200 RPM it may be producing about 50 HP in retarding force. At 2400 RPM it will be producing about 140 HP in retarding force. Combine that with downshifting, where the engine then has better leverage against the truck and you have a brake that will skid the rear wheels in marginal traction conditions.

In the Aisin transmission, the torque converter locks as part of every shift. Not just based on RPM. The locking strategy in modern transmissions is far different than in older designs. It is done in such a way that you can hardly detect it and it holds the locked condition while towing. This makes the tranny develop far less heat and run much more efficiently. Locking is now just a normal part of shifting and occurs whenever the truck is moving. So, the TC is almost always locked when the truck is moving and in gear. This means too, that the brake is always available at any RPM over 1200.

Tow/Haul allows the brake to be used in a more aggressive fashion by keeping the RPM up (within reason) and downshifting early on down grades or when the service brake is applied. It can also be set, with a Cummins, to only activate when the brakes are applied. I've never seen mine go over 2400 RPM while automatically downshifting. Cummins engines do not have to be revved way up to do an enormous amount of work.

“Retarding force,” aka exhaust back pressure, aka drive pressure. What are you getting these drive pressure readings with? I’ve never seen 140 pounds of drive pressure, even with compound turbos on my 2014. And the as69rc torque converter locks from 2nd gear on.

Raspy · Nov 23, 2018

HickoryFarms said:
“Retarding force,” aka exhaust back pressure, aka drive pressure. What are you getting these drive pressure readings with? I’ve never seen 140 pounds of drive pressure, even with compound turbos on my 2014. And the as69rc torque converter locks from 2nd gear on.

The Ram Cummins engine brake is rated in horsepower, not back pressure. This can be described as "retarding force". It's not rated in back pressure or drive pressure. It says right on the gauge, on the dash, how many horsepower the brake is producing in retarding force. This is not the same thing as turbo boost. The Cummins turbo has a sliding collar that closes off the exhaust for braking. Boost is produced by engine exhaust, under power, spinning up the turbo. Braking force is produced by closing off the exhaust and letting the engine act as a pump that produces much higher cylinder pressure on the exhaust stroke when there is no fuel being injected. This makes the engine work against a very restrictive exhaust. It is also used at idle when warming up in cold weather to make the engine work harder while idling, to warm up faster. Every upshift, 2nd through 6th includes locking the torque converter. This means the brake can be engaged anytime the RPM is over 1200, in any gear but 1st. In second it is so powerful they warn against using it in anything but perfect traction and I've had mine skid the rear tires on wet pavement. Modern trannys not only have a newer torque converter (TC) lockup strategy for efficiency, as compared to years ago in the 48RE, but it's also to accommodate the engine brake in a very seamless way since the TC must be locked for the brake to work properly. Older Chrysler trannys, anything before the last generation of the 48RE, were not rated for engine brakes because of the reverse thrust on the main shaft it produced. They simply had a bronze thrust bearing instead of the later roller thrust bearing. Besides that, they had no interface to the engine brake unless you grounded the lockup clutch terminal on the main transmission plug (his may have been part of the brake controller in aftermarket engine brakes, I don't know). This grounding was done by a bunch of guys, me included, who added a switch to manually lock the TC. We called it the "mystery switch".

HickoryFarms · Nov 23, 2018

Raspy said:
The Ram Cummins engine brake is rated in horsepower, not back pressure. This can be described as "retarding force". It's not rated in back pressure or drive pressure. It says right on the gauge, on the dash, how many horsepower the brake is producing in retarding force. This is not the same thing as turbo boost. The Cummins turbo has a sliding collar that closes off the exhaust for braking. Boost is produced by engine exhaust, under power, spinning up the turbo. Braking force is produced by closing off the exhaust and letting the engine act as a pump that produces much higher cylinder pressure on the exhaust stroke when there is no fuel being injected. This makes the engine work against a very restrictive exhaust. It is also used at idle when warming up in cold weather to make the engine work harder while idling, to warm up faster. Every upshift, 2nd through 6th includes locking the torque converter. This means the brake can be engaged anytime the RPM is over 1200, in any gear but 1st. In second it is so powerful they warn against using it in anything but perfect traction and I've had mine skid the rear tires on wet pavement. Modern trannys not only have a newer torque converter (TC) lockup strategy for efficiency, as compared to years ago in the 48RE, but it's also to accommodate the engine brake in a very seamless way since the TC must be locked for the brake to work properly. Older Chrysler trannys, anything before the last generation of the 48RE, were not rated for engine brakes because of the reverse thrust on the main shaft it produced. They simply had a bronze thrust bearing instead of the later roller thrust bearing. Besides that, they had no interface to the engine brake unless you grounded the lockup clutch terminal on the main transmission plug (his may have been part of the brake controller in aftermarket engine brakes, I don't know). This grounding was done by a bunch of guys, me included, who added a switch to manually lock the TC. We called it the "mystery switch".

Watch a drive pressure sensor and compare it to the “retarding HP gauge” that the EVIC provides. It’s is the same value. The factory gauge is reading exhaust back pressure. Nobody warns about using the exhaust brake in second gear, the manual clearly states to not use the exhaust brake in ANY adverse conditions. The reason previous generations did not have an exhaust brake is simple, because the did not have a variable geometry turbo charger. Oh, and the variable geometry turbo (VGT) that cummins uses does not have a sliding “collar,” that would be a VNT charger. The VGT charger that cummins uses has veins, not a collar. Why did previous generation ram trucks not have a VGT turbo? Well that’s simple, because they did not have emissions equipment. The VGT charger was introduced as part of the emissions equipment. Again, “retarding horsepower” is exhaust back pressure.

Raspy · Nov 23, 2018

HickoryFarms said:
Oh, and the variable geometry turbo (VGT) that cummins uses does not have a sliding “collar,” that would be a VNT charger. The VGT charger that cummins uses has veins, not a collar. Why did previous generation ram trucks not have a VGT turbo? Well that’s simple, because they did not have emissions equipment. The VGT charger was introduced as part of the emissions equipment. Again, “retarding horsepower” is exhaust back pressure.

Retarding horsepower is exhaust back pressure. There are two kinds of back pressure. One is caused by the engine working hard under load while burning a lot of fuel and spinning the turbo. The other is cause by closing off the exhaust to make the engine pump up back pressure while not using any fuel. One causes the turbo to spin up and create more intake manifold pressure, the other simply makes the engine hold back by increasing the exhaust manifold pressure. When braking, Ram calls the retarding force, "horsepower" and they show it in the evic.
That is not my word for it and it's not "drive pressure spinning the turbo, it's back pressure caused by the VGT sliding ring closing off the exhaust. This ring moves stationary vanes in and out of a matching plate to narrow or widen the exhaust pathway. That process changes the restriction and velocity of the exhaust gasses to tune the turbo to the horsepower being produced and to hasten spool-up at lower RPM. When moved beyond it's normal range, the ring acts as a brake by closing the exhaust path. Perhaps you should do a bit more research as Cummins calls this piece a "sliding ring". And they call this turbo a VGT. The vanes don't change angle, they slide in and out of matching holes to change their effectiveness.

Quote:
VGT EXHAUST BRAKE
The VGT Exhaust Brake works in conjunction with the engine and transmission to provide an integrated braking system to help slow the vehicle. This is commonly referred to as exhaust braking. Braking power is achieved by modulating the sliding nozzle ring to restrict the flow of exhaust gasses from the engine, this will create high back pressure on the engine. The high back pressure creates a high level of resistance to the motion of the pistons within the engine and this resistance is used to reduce engine speed and thus vehicle.

The exhaust brake feature will only function when the Exhaust Brake Switch to the ON position. With the switch in the ON Position and the vehicle moving faster then 5 MPH; the exhaust brake will automatically operate when pressure is removed from the accelerator pedal allowing the ECM to see 0% throttle and 0% fuel delivery.

HickoryFarms · Nov 23, 2018

Raspy said:
Retarding horsepower is exhaust back pressure. There are two kinds of back pressure. One is caused by the engine working hard under load while burning a lot of fuel and spinning the turbo. The other is cause by closing off the exhaust to make the engine pump up back pressure while not using any fuel. One causes the turbo to spin up and create more intake manifold pressure, the other simply makes the engine hold back by increasing the exhaust manifold pressure. When braking, Ram calls the retarding force, "horsepower" and they show it in the evic.
That is not my word for it and it's not "drive pressure spinning the turbo, it's back pressure caused by the VGT sliding ring closing off the exhaust. This ring moves stationary vanes in and out of a matching plate to narrow or widen the exhaust pathway. That process changes the restriction and velocity of the exhaust gasses to tune the turbo to the horsepower being produced and to hasten spool-up at lower RPM. When moved beyond it's normal range, the ring acts as a brake by closing the exhaust path. Perhaps you should do a bit more research as Cummins calls this piece a "sliding ring". And they call this turbo a VGT. The vanes don't change angle, they slide in and out of matching holes to change their effectiveness.

Quote:
VGT EXHAUST BRAKE
The VGT Exhaust Brake works in conjunction with the engine and transmission to provide an integrated braking system to help slow the vehicle. This is commonly referred to as exhaust braking. Braking power is achieved by modulating the sliding nozzle ring to restrict the flow of exhaust gasses from the engine, this will create high back pressure on the engine. The high back pressure creates a high level of resistance to the motion of the pistons within the engine and this resistance is used to reduce engine speed and thus vehicle.

The exhaust brake feature will only function when the Exhaust Brake Switch to the ON position. With the switch in the ON Position and the vehicle moving faster then 5 MPH; the exhaust brake will automatically operate when pressure is removed from the accelerator pedal allowing the ECM to see 0% throttle and 0% fuel delivery.

Correct, exhaust back pressure, which is what I previously said it was, is drive pressure. Exhaust back pressure is drive pressure. Drive pressure (exhaust back pressure) is the pressure measured in the exhaust manifold between the exhaust valve and the turbo. Drive pressure (exhaust back pressure) IS what drives a turbocharger. Have you ever taken apart a VGT turbo?

Raspy · Nov 24, 2018

Hick,

I suggest you take up your argument with Cummins. They will be delighted to learn how their own turbos are designed and what they should be called.