Exhaust brakes and downshifting

[Alluminati]

Please enlighten those of us who have never used exhaust brakes.

I blindly suppose the exhaust brake increases engine compression by restricting exhaust flow (closing exhaust valves?), which would reduce RPMs. I suppose that would only work if the transmission speed was locked to the engine. The result would be resistance to the drive axle.

Wouldn't this give the same effect as downshifting? The difference would be the exhaust brake slows the engine, while downshifting drives RPMs up. But the net result is the same. Please correct me if I'm wrong.

I have a 6-speed transmission which gives me 6 choices when descending a hill. How does one control speed when using exhaust brakes?

[dznf0g]

First off, exhaust brakes on the trucks we use here don't close the exhaust valves. "Jake brakes" on big trucks do that. On LD trucks it is a butterfly valve that closes off the exhaust pipe, just aft of the Y-pipe (on v8s).

Modern computer controlled exhaust brakes coordinate the downshifting with the valve for max deceleration effort for the given conditions. Because of the very high compression of a diesel engine, a closed off exhaust pipe offers a lot more resistance than a gas with lower compression and a throttle valve closed. That said, the latest iterations of gas engines and transmissions do a much better job of hill descent control than even a couple years ago, and are automatic when in TH mode (on GMs anyway). They're still not as resistant to rolling as a diesel though.

On a DMax, there is a switch for TH mode (of course) as well as an exhaust brake switch. There are different computer routines for having just the EB switch on and having both on. For the load of my 30'AS and gear, I found having both engaged was too much braking in the Rockies. I liked the feel and control of just having the EB switch on during descents.

[dznf0g]

Here's a pic of a typical EB:

https://sinisterdiesel.com/i-1651656...DmvxoCydfw_wcB

[Alluminati]

Thanks for the indepth answer.

So your truck is using both engine compression and downshifting to slow the truck?

Sounds very capable of holding back the speed of a 20,000# fiver.

Again, how do you choose your speed? By manually downshifting after the exhaust brake is engaged?

I’m aware that on average, diesel engines run at slightly more than double the compression of a gasoline engine.

I’m confused by all the crowing about how fantastic exhaust brakes are; almost to the point that I thought there was more to it than resistance to the drive axle. All I can figure is that those of us with gas engines don’t know how to downshift properly. My experience has been that proper downshifting on steep descents have been calm and easy. Beginning the descent at the right speed, and adjusting the gearing as the grade changes has presented no problems. My SUV is not designed to pull a 20,000# 5th wheel, and I don’t need as much braking power.

I don’t know how different it would be if I were not using TowHaul, since I never move the trailer without the TowHaul engaged.

[SeaLevel]

I've never had a diesel, but I was very impressed with how the grade braking works on my 2015 Silverado when we drove through the mountains this past summer. Even on 8-9% grades I only had to briefly touch the brakes a couple of times, and that was partly because I am used to braking and wanted to make sure they were still working.

[PaulnGina]

Here is another good example of an exhaust brake, and the one I plan on adding to our 1995 PowerStroke (will go with the higher-Dollar one). This site has some good illustrations and text.

http://www.usgear.cc/dcelerator.htm

[dznf0g]

I don't have a diesel. As I have said before, I am fortunate to have access to a variety of GM company vehicles as demos. I have towed with many variants of our powertrains and SUV/PUs over the past 5 years, in order to determine what I should buys wit mys own monies. I chose a 2015 1500, 6.2L, 8 speed, MaxTow Silverado. I did tow with 2 or 3 DMaxs. The "holdback" is better than any gas. But the automatic features of TH mode and grade braking on the gas models is pretty impressive. One still wants to start the descent at a low speed, regardless of gas or diesel, and let the respective systems do their jobs.

[Alumineer]

Originally Posted by Alluminati

All I can figure is that those of us with gas engines don’t know how to downshift properly.

I'm in a place where my daily commute involves a 2 mile descent on a 7 percent grade. I see lots of folks riding their brakes all the way down, and every couple of years there is a near disaster because some driver of a (diesel!) dump truck melts his brakes on the way down.

Like you, it seems apparent to me that many people have no clue about how to use engine braking to manage descents at a controlled speed. This is regardless of whether the vehicle is a sedan or an eighteen wheeler.

Gasoline engines have built-in 'engine braking' because when one closes the throttle, the engine turns into a large vacuum pump. It should not matter whether you close off the inlet or exhaust side... restricting the air flow is what provides the pumping resistance and therefore the braking effect.

Many diesel engines do not have any kind of throttle valve since it's not required for basic engine operation; hence the use of a 'jake brake' exhaust restrictor to regulate engine speed for braking.

In either case, the effectiveness of the engine braking is proportional to the engine RPM, which will be dependent on the transmission gearing and whether or not the torque converter (in an automatic trans) is locked. Like you, I've found the engine braking capability of my 2013 Tundra completely adequate under any of the towing conditions I've encountered.

[crispyboy]

The factory integrated exhaust brake on my 2009 truck uses a Holeset VGT HX351 (variable geometry turbo) http://www.myholsetturbo.com/vgt.html
There is a switch for the EB and the tow/haul so they can be used independently of each other. The EB is much more aggressive with the transmission switched to tow haul.
With the EB switched "on" and you let off the throttle the EB will activate so speed is controlled with the right foot. Service brakes are applied when coming to a stop or just a tap on really steep grades. When the cruise is set the truck will automatically downshift or upshift depending on the situation.
As the driver you can minimize the use of service brakes by anticipating the grade/traffic and do much of the controlling with your right foot.
I can also control the transmission shifting with a toggle switch so I can hold in a gear to limit upshifts if needed.

[dznf0g]

Originally Posted by crispyboy

The factory integrated exhaust brake on my 2009 truck uses a Holeset VGT HX351 (variable geometry turbo) http://www.myholsetturbo.com/vgt.html
There is a switch for the EB and the tow/haul so they can be used independently of each other. The EB is much more aggressive with the transmission switched to tow haul.
With the EB switched "on" and you let off the throttle the EB will activate so speed is controlled with the right foot. Service brakes are applied when coming to a stop or just a tap on really steep grades. When the cruise is set the truck will automatically downshift or upshift depending on the situation.
As the driver you can minimize the use of service brakes by anticipating the grade/traffic and do much of the controlling with your right foot.
I can also control the transmission shifting with a toggle switch so I can hold in a gear to limit upshifts if needed.

Ha, yeah...brain fart....forgot that changed with Variable vane turbos!

[Isuzusweet]

The most reliable form of braking........no wear as it is completely electromagnetic, so no pads to replace or wear items at all.

https://www.youtube.com/watch?v=5FMoUEmSsdE

Cheers
Tony

[russingr]

On my 2013 F250 6.7 diesel, the exhaust brake is automatic -- there is no manual control for it.

I love this feature when driving through the mountains in Colorado. As I ascend a mountain, I'm thinking about what speed I want to maintain on descent, and just before I crest the mountain, I set my cruise control at that speed (or a bit below). As I descend, it downshifts automatically, and maintains the speed I set for the most part, although if it's a particularly steep grade, it will sometimes go a bit faster than where the cruise control was set. However, on typical grades, I can often make it all the way down the mountain without ever touching the brakes, or using the brakes only rarely.

[MelGoddard]

I don't have a diesel, never did; but when decending a hill in the warmer weather, I turn on the air conditioner full max.
The compressor helps to keep the speed in check. ��

Trailer Trash with a Wing in a sling.��

[Cowbell]

On newer Dodge, I use both Tow/Haul mode and the EB. As Rich mentioned above, when going down a long decent the braking seems to be to much, with both the transmission and the EB engaged. I sometimes use just the EB which has excellent ability.
In commercial vehicles I've drivin in the past, you engage the EB by pulling on a handle. Pull further for more EB.
With the Dodge 2500, it has a two position switch. Hit the button once for "high" EB and then hit the button again for "automatic" EB. When in the "automatic" mode, you slow the truck to your desired speed with your pedal brake and the let go. The EB engages and feathers on and off to maintain that speed. It's a great system and a major reason to buy diesel, in my opinion.

[cliffcharb]

Diesel engines do not have a way to stop the introduction of air into the cylinders, unlike a gas engine which closes the butterfly when you let off of the accelerator pedal. Because of this there is no natural slowing of the rotation on a diesel. The exhaust brake acts as the induction butterfly, only opposite. "Jake" brakes generally change the timing of the valves to not build compression.

[dznf0g]

Originally Posted by cliffcharb

Diesel engines do not have a way to stop the introduction of air into the cylinders, unlike a gas engine which closes the butterfly when you let off of the accelerator pedal. Because of this there is no natural slowing of the rotation on a diesel. The exhaust brake acts as the induction butterfly, only opposite. "Jake" brakes generally change the timing of the valves to not build compression.

Actually, close but not quite.

https://en.wikipedia.org/wiki/Compre...e_engine_brake

[Alluminati]

So downshifting a diesel without EB will have little/no effect due to the lack of vacuum? Wow! Then I guess you'd need EB as a matter of survival any time you tow. I owned a diesel Jetta a few years ago, and have driven diesel busses for the church, so I know a little about diesel, but never was aware of this. ...Now I'm trying to remember when I ever needed to downshift the bus...

My Pruis will hold the speed down on a descent with the cruise control, but that's due to the principle of siphoning off excess inertia as a primary function of a hybrid. Kinda funny actually, since the engine noise rarely relates directly to speed. So you're cruising cross country, up hill and down. Then once in a great while, you're going down hill, paying it no attention, and the engine begins roaring because suddenly the generator that's sucking up all that inertia energy just can't keep up. But the Sequoia cruise will let the truck run downhill.

Sounds like at the end of the day, a diesel with EB is not better off than a gas engine with someone willing to downshift properly. Do I have this correct?

Do we need to have a class so all us gassers can learn how to descend hills safely?

[phann]

To add to Cowbell's comments, I find that with our RAM diesel, while in the higher gears, the EB will not cut in unless my speed is over approximately 45 mph. If I want to use the automatic EB at lower speeds (eg, on tight-winding, mountain roads), I shift down to 4th or 3rd, depending upon how fast I am going, and then the EB does kick in, further assisting in slowing the truck automatically and with minimal use of the brakes. I don't know how it all works, but it seems like the EB requires a minimum engine/transmission turnover rate, and/or exhaust volume, to be effective.

[phann]

PS, having driven both gas and diesel trucks, I definitely prefer having the exhaust braking of the diesel available. At downhill speeds over 45mph, the exhaust brake can slow the truck and trailer down, or maintain a downhill speed automatically, without having to downshift the tranny, and without having to touch the brakes.

[Larry C]

Originally Posted by MelGoddard

I don't have a diesel, never did; but when decending a hill in the warmer weather, I turn on the air conditioner full max.
The compressor helps to keep the speed in check. ��

Trailer Trash with a Wing in a sling.��

Are you serious? Just wondering.....

Larry