turbo owners: caution in transport

[Smokin Diesel]

Ok full explanation, 2 stroke supercharged diesels and turbo diesels have useful valve overlap, the two stroke for scavenging and the turbo for exhaust valve cooling and scavenging. But look at a diesel exhaust manifold, there is no tuning. This goes extra for the turbo diesel, the turbo destroys any exhaust harmonics.

Average normally aspirated diesels don't have a real lot of functional overlap. What I mean by functional overlap is the valve lift is small during overlap. This is because under full load there is residual pressure in the cylinder and it would blow back out the intake if it was open very much.

Ken

Two stroke diesels don't have intake valves, just exhaust. So valve overlap doesn't apply at all in these engines.

And the low valve lift in a 4-cycle diesel is because of the fact that the pistons in a diesel engine just about contact the head at TDC (to get the high compression ratio). There simply isn't room for the valves to be open much during the time they can overlap. Not a turbo/non-turbo thing, just a diesel thing. Nonetheless, they all have about the same degrees of overlap.

 

[Volfandt]

The "overlap" in a two cycle diesel comes when the intake port is open at the same time as the exhaust valve. It helps scavange the cylinder of burnt fuel as already been posted.....

This phenomenon was probably more prevalant in the older 2-cycle diesels w/vertical exhaust stacks but I personally won't totally rule it out in modern 4-cycle diesels, contrary to some opinions mentioned in this thread.
Theres no overwelming evidence in either camp.
Never say never.......

Howstuffworks "The Diesel Two-Stroke Cycle"

 

[Hvytrkmech]

thanks for all the input in this discussion, an example of the forum medium at it's best holding opposing viewpoints. but we need to move from the empirical and physics observations to reality and experience. who has actually experienced this phenomenon? maybe someone should post the question on some kind of diesel truckers forum.....in the meanwhile, I'll be sure the driver covers the stack of my brand new M8540 turbo as he delivers it to my farm tomorrow. and let the skeptics continue to ridicule.....or should I invite you all to a tractor pull at my place and settle it that way bb

In my mere 27 years of being a diesel mechanic (class 6-8), and being A heavy tow operator for the same amount of time all for the same company, I can personally testify to the effects of pin wheeling. Not only is it an absolute problem and HUGE contributor to premature or actual turbocharger failure it is an expensive lesson for a unexperienced driver to learn. We don't have a turn over of employees but some of the junior guy's learned the hard way, like when they went out onto the highway to pick up billy big rigger super trucker with the wide open straight pipes, we split the unit grab the tractor from the rear and head to the destination, he decides its only 20 miles so he does not bag the exhaust, too bad for him it cost him $1,200 bucks for a re manned turbo. I told him in the past but he did not want to listen, guess what he does on every tractor tow now? It is an sop at our company, we bought several cases of the orange plastic bags your Sunday paper comes in, slide one bag over the exhaust and duct tape around the bag, then nothing sticks to anything. As a matter of fact ALL class 8 engine manufactures have notices in regards to this very issue. Now, not all circumstances are the same as my previous example (obviously), A truck with an unmodified exhaust system (baffled muffler) will present less of a risk that a truck with a straight pipe exhaust, BUT the risk is only slightly less. Others have mentioned about valves being opened/ closed etc..., more valves tend to be opened or partially opened at the same time with the engine off, and since all exhaust manifolds are wide open bridges it allows enough positive pressure to charge the system for the ENTIRE time the vehicle is in motion thus causing the pinwheeling, and if not immediately causing turbocharger failure, causing it to fail VERY SHORTLY after engine re operation.

All the non believers and naysayers, be advised your turbo is on borrowed time.

Not knocking anyone or disrespecting anyone, just pointing out facts, not fiction or opinion.


Tim

 

[slowzuki]

Sorry yes, the porting overlap is the same thing as valve overlap.

We will have to do some testing here some time, send a miniature camera down the exhaust and tow some turbos!

 

[Smokin Diesel]

The "overlap" in a two cycle diesel comes when the intake port is open at the same time as the exhaust valve. It helps scavange the cylinder of burnt fuel as already been posted.....

Semantics. I took it literally as valve overlap, not timing overlap.

 

[Wayne County Hose]

I asked one of my customers about this. They are by far the largest excavating company in the area. They have 30 excavators if this says anything about the amount of equipment they have and how much they move them around. I know they have at least 3 low-boys that haul equipment around. The shop foreman told me that they duct tape the exhaust shut, otherwise the turbo spins in reverse with no oil and destroys them. They used to blow turbos in transport and since they cover the exhaust they don't blow turbos anymore.

 

[have_blue]

The shop foreman told me that they duct tape the exhaust shut, otherwise the turbo spins in reverse with no oil and destroys them.

The turbo probably won't even spin fast enough to get warm, much less damage the bearings. I bet an 80mph air flow over the exhaust pipe won't even spool it up to 2000rpm... not even close to the 75,000-120,000rpm that turbos normally run.

I would be a little bit concerned about sand or grit getting into the turbo bearings and engine valves, but not overly.

 

[2Botas]

The turbo probably won't even spin fast enough to get warm, much less damage the bearings. I bet an 80mph air flow over the exhaust pipe won't even spool it up to 2000rpm... not even close to the 75,000-120,000rpm that turbos normally run.

I would be a little bit concerned about sand or grit getting into the turbo bearings and engine valves, but not overly.

I think the issue is that most turbos dont have a ball bearing center section. Most use bushings and forced oil. The forced oil is essentially the bearing. It creates a barrier between the bushing and the shaft. So, if the engine is not running, the steel shaft runs directly on the brass bushing a scores both surfaces. 2000rpms would ruin the turbo almost instantly. even 25 rpms would cause alot of damage over an extended period of time.

 

[SPYDERLK]

I think the issue is that most turbos dont have a ball bearing center section. Most use bushings and forced oil. The forced oil is essentially the bearing. It creates a barrier between the bushing and the shaft. So, if the engine is not running, the steel shaft runs directly on the brass bushing a scores both surfaces. 2000rpms would ruin the turbo almost instantly. even 25 rpms would cause alot of damage over an extended period of time.

There is residual oil even tho it is not flowing to carry away bearing heat. Oiled brass on steel is pretty happy. The question would be does the turbo spin at all. If so, is it fast enuf to cause it to run dry and forceful enuf that it doesnt stop as it dries. I dont think 25 rpm qualifys. Bushings with residual oil work for long periods with light sideload in electric motors. We are talking similar rpm as conjecture, but without load.
larry

 

[ray66v]

There is residual oil even tho it is not flowing to carry away bearing heat. Oiled brass on steel is pretty happy. The question would be does the turbo spin at all. If so, is it fast enuf to cause it to run dry and forceful enuf that it doesnt stop as it dries. I dont think 25 rpm qualifys. Bushings with residual oil work for long periods with light sideload in electric motors. We are talking similar rpm as conjecture, but without load.
larry

Right, residual oil is capable of protecting the system from damage. it is not going to dry up or bun off due to any amount of pinwheeling.

As an example of this, have you ever heard of the oil being drained from an engine and it being run until it fails? I have a friend that goes to bike week at Daytona every year. He has seen them put motorcycles on a stage and drain all the oil, then run them WIDE OPEN