When/how to use a Jake brake

[Steve Ownby]

Originally Posted by Dr4Film

Well, after looking around the Cummins Engine web site I wasn't able to find anything about not using your Exhaust Brake too much for fear of cracking an Exhaust Manifold. However, I did find a very good article written by the Gale Banks Engineering department regarding the use of Exhaust Brakes and how they actually work. Here's a clip from that article but I have also listed the link below for people who want to read the entire article which is well worth the time. BTW, Gale Banks is one of the leading engineers who knows Cummins Engines inside and out. To understand how an exhaust brake works, we first need to understand how a diesel makes power – or more specifically, torque. After a diesel has ingested air on the intake stroke and compressed it on the compression stroke, fuel is injected directly into the cylinder. The heat of compression ignites the fuel, and combustion occurs releasing more heat to increase the pressure of the compressed air in the cylinder. This pressure then pushes the piston down on the power stroke to generate torque on the crankshaft. The more fuel that is burned, the more heat that is generated and the greater the pressure acting on the piston. And of course, more pressure means more torque. Now let’s look at the exhaust stroke for our diesel. Normally the piston just pushes the exhaust out past the open exhaust valve, and through the remaining exhaust conduits until the exhaust gases reach the atmosphere. When the driver releases the fuel throttle, almost no fuel is being injected into the cylinders, so there is very little more than the air that was initially ingested on the intake stroke to be expelled on the exhaust stroke. In fact, if we put a pressure sensor in the exhaust system, we’d find that the exhaust pressure is very close to zero during deceleration because the exhaust system can easily handle the exhaust flow. This means the pressure in the cylinder is also very close to zero on the exhaust stroke under these deceleration conditions, and zero pressure on the piston top during the exhaust stroke offers no resistance to the piston as it rises. Now let’s add an exhaust brake downstream from the turbocharger. An exhaust brake is basically a valve that can be closed in the exhaust system to restrict (but not totally close off) exhaust flow. This valve closes when the driver releases the fuel throttle. Under these conditions, the exhaust flow from the cylinders is bottlenecked and rapidly builds pressure in the exhaust system upstream from the exhaust brake. Depending on engine speed, this pressure can easily reach up to 60 PSI maximum working pressure. Maximum working pressure is limited as part of the design of an exhaust brake. In this example, that same 60 PSI also remains in the cylinder for the entire exhaust stroke (exhaust valve open) and exerts 60 PSI on the piston top to resist its upward movement. We can think of this as negative torque, slowing the engine for a braking effect. We might even think of this as just the opposite of the power stroke, and in effect, it is. Thus, you can see that simply restricting the exhaust flow can generate substantial braking. That’s what makes an exhaust brake so effective. Banks Power | How An Exhaust Brake Works So the important information that I came away with after reading the entire article is that when an Exhaust Brake has been activated there is hardly any combustion going on inside the engine as there's no fuel being delivered to the pistons. Therefore any HEAT factor from having the Exhaust Brake on is minimal. You cannot have POWER coming from the engine at the SAME time as Exhaust Compression. It's either one or the other but not BOTH at the same time. So I would venture to say that Cummins is correct in that excessive heat will crack your Exhaust Manifold. HOWEVER, it will most likely happen on the way up the mountain versus on the way down using your Exhaust Brake. So that's my story and I'm sticking to it but I am open to other opinions however list data not hearsay or I was told by a someone, etc. Dr4Film ----- Richard

Really excellent and informative post Richard. I remember a Cummins field engineer giving a seminar at a rally talking about the exhaust manifold glowing a deep red on a WOT climb with the turbo maxed out.

[DA_BUS]

I didn't have my chassis manual in the house - but I do have the actual Cummins engine manual available and this has good directions on the engine brake in that manual. Talks about grades, slippery roads, etc. Mine is a retarder....good luck and safe travels.

[Fred Cooper]

Originally Posted by jimbo2013

Ok looking at it; Cat 7.2 330hp

It says Jake like piston.

I have the same engine/transmission/exhaust brake/chassis as jimbo.

It is obvious that use of the engine brake is subject to a lot of personal opinion. I recognize that many believe it best to use it all of the time, but my preference is use it when coming down the mountain and use the service brakes for normal stops. I've put 60000 miles on my coach (now at 101,000) and the brake shoes look like they can easily go another 100000 miles. I do use the engine brake once or twice on each trip just to be sure it still works.

Some specifics for jimbo on his 330hp Cat/Allison/Freightliner combo:

Your cruise control will not work with the engine brake on unless someone has modified the circuit. Monaco changed to circuit to allow cruse control and engine brake use together in later years, but not the 2000 Holiday Rambler model.

Your cruise control will allow you to set fast idle by either pushing the button on the end of the turn signal stalk, or sliding the switch for resume.

When you turn on the exhaust brake and remove your foot from the go pedal, the transmission will display 2 but this means the transmission will try to shift down when it can do so without over reving the engine. The engine is governed to 2400 rpm when producing power, and 2600 rpm when used for engine braking. If it downshifts and speed increases so that 2600 rpm is exceeded, the transmission will upshift to protect the engine. (You can then get on the service brakes to get the speed down so the downshift can again occur.)

Enjoy your trip.

Fred

[GregOk80]

A Jake brake or Engine work almost the same. They use the engine to slow the vehicle down without the use of the service brakes. One word of caution from me it never NEVER use them in snow or icy conditions. The Jake or Engine brake will lock up the drive wheels and send you into a spin.