I can't seem to find a source that has a conclusive answer to the question, "Does the VP44 completely shut off fueling when truck is coasting in gear on a downgrade?"  It would seem to me that the VP44's high pressure solenoid valve would stop receiving signals under this condition if the APPS signal was at 0% (foot off throttle).  Does anyone know if this is true with certainty?

 

An example would be:  truck with a manual transmission coasting along on a down grade at 1700 rpm with foot off of throttle.  

 

- John

 

7 hours ago, wil440 said:

As ever John a good peice, but  doesn't a four stroke cycle take 2 revolutions to complete not one, so compression and power stroke would take 0.66 seconds for one revolution and exhaust and intake take the same, 180deg per stroke for a total of 720.  In your example one stroke would take 0.33 seconds ? 

 

Yes, the four stroke cycle engine does take two crankshaft revolutions to complete one cycle.  I believe I allowed for this in the first step of my calculation, "If a vehicle is decelerating with an engine speed of 1800 rpm, that would mean a camshaft speed of 900 rpm."  The rest of my calculations are based on the number of camshaft revolutions, not crankshaft revolutions, so I think my numbers are correct.  Maybe you can give me another double check.

 

 You wrote, "so compression and power stroke would take 0.66 seconds for one revolution and exhaust and intake take the same, 180deg per stroke for a total of 720.  In your example one stroke would take 0.33 seconds ?"  I am guessing just a typo error here - should be ".066" and ".033".

 

I also did one other test the same day that really supports your "engine compression holdback" theory.  As mentioned in my previous post, my testing was done in 3rd gear.  I did another test at the same location using 4th gear.  Here are the results:

 

  Truck stopped, engine off, 4th gear selected, park brake released, foot on brake pedal  -  upon releasing brake pedal truck would creep forward very slowly, slightly gaining sped, then in a few feet the truck would suddenly completely stop.  This event repeated itself in lurches endlessly.  In this scenario I believe the weight of the truck used gravity to push though engine friction to get the truck in motion and then the effect of engine compression definitely took over to repeatedly stop the truck.  While the truck was stopped, engine compression would bleed off, thus allowing for truck movement again continuously repeating the cycle.  I also think that under this condition whichever cylinder is in the power stroke would provide resistance as well,  This cylinder would start to pull a vacuum while truck is in motion and then bleed off when truck is stopped.  A person feels the same events when hand barring an engine over. 

 

In conclusion, I would agree that there can be significant engine compression braking, but at only extremely low engine rpm.  I believe that as soon as engine rpm increases, cylinder compression efficiency increases greatly and very rapidly, becoming very efficient at higher rpm's.  Consequently, the work being done by the compression stroke would be undone by the expansion of air in the following power stroke.  The operation of a compression release engine brake supports this concept.

 

I perused the internet as well and found sites supporting both of our theories.   Here is some information from Wiipedia - which everyone knows is Gospel (it's as good as "I saw it on TV.").

 

 Engine Braking 

 

Gasoline engines

The term "engine braking" refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released. This causes fuel injection to cease and the throttle valve to close almost completely, greatly restricting forced airflow from, for example, a turbocharger. The restriction causes a strong manifold vacuum which the cylinders have to work against, sapping much of the potential energy out of the system over time and producing the majority of the engine-braking effect.[1] This vacuum manifold effect can often be amplified by a down-shift, which induces a faster spinning drivechain to engage with the engine.

 

Engine braking is a viable method of controlling the speed at which a vehicle travels downhill. By shifting to a lower gear in a manual transmission, or selecting the "low" mode on an automatic transmission, engine braking reduces the need to repeatedly apply the foot brake, lowering the risk of the brakes overheating.[2]

While some of the braking force is produced due to friction in the drive train, this is negligible compared to the effect from the manifold vacuum caused by the air-flow restriction.

On an automatic transmission, engine braking often spontaneously increases the engine RPM, causing a sudden revving to occur even without applying the accelerator pedal.

 

Diesel engines

Diesel engines in personal cars provide little engine braking as they are not equipped with a throttle body and thus cannot draw a vacuum in the intake manifold.

In heavy vehicles the engine is often made to provide extra braking power to take some strain off the vehicle's regular brake system and to help avoid overheating the brakes. In its simplest form this consists of a butterfly valve that stops the exhaust flow. This is referred to as an exhaust brake and mostly found on older trucks. It has a limited effect, and more advanced systems as described below are near universal on newer heavy vehicles.

 

Compression release brake

A compression release brake (also known as a Jacobs brake or "jake brake"), is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere.

 

Normally, during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down. However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel.  Having lost the energy stored within the compressed air, there is no "spring back" so the engine must expend yet more energy pulling the piston back down again.   It is very effective, however, and creates immense amounts of braking force which significantly extends friction brake life – a 565 hp (421 kW) diesel engine can produce up to 600 hp (450 kW) of braking power at 2,100 RPM.[3]

 

Exhaust brake

An exhaust brake works by causing a restriction in the exhaust, much like the intake throttle causes in a gasoline engine. In simple terms, it works by increasing the back-pressure of the exhaust. Nearly all of these brakes are butterfly valves similar to a throttle valve, mounted downstream of the turbocharger if there is one.

 

Once again, you have forced me to "think" and consume more coffee...,

 

- John

 

 

 

On 4/30/2022 at 6:37 PM, Tractorman said:

Yes, the four stroke cycle engine does take two crankshaft revolutions to complete one cycle.  I believe I allowed for this in the first step of my calculation, "If a vehicle is decelerating with an engine speed of 1800 rpm, that would mean a camshaft speed of 900 rpm."  The rest of my calculations are based on the number of camshaft revolutions, not crankshaft revolutions, so I think my numbers are correct.  Maybe you can give me another double check.

I rechecked from crank speed not camshaft and the end result is as you say 0.033 seconds per revolution of the crank so 2 complete "strokes" or half of the cycle takes 0.033, the complete cycle taking 0.066 at 1800rpm (yes I forgot the nought after the point)

 

Your test in 4th gear as you say makes a compelling argument for diesel engine braking but I agree with what you say "low rpm only"  this low rpm was the basis of my very  first statement, diesels in low gear at idle have huge engine braking from my experience, it has to be for a couple of reasons as you say, number 1 is low gear working backwards driving the motor greatly magnifies any compression benefit, also any compression bleeding off so the downward force of expanding air after TDC on the "power stroke) is a net loss compared to the energy used to compress the air on the compression stroke,  and plain old engine friction, I would also say the act of compressing valve springs continually would also add to the energy needed to turn the engine over at idle in a low gear, I doubt valves on the way back up/closing adds anything.

 

Be interesting to see what slope 1st low would hold

 

At higher rpm I agree the Jake brake is the best for the reasons you say

 

Again John 5 or 6 cups of coffee   got to go work some caffeine off now

 

 

Neil, I have been thinking about how to put in words what I have enjoyed most about this conversation, but I think you nailed it in another thread you started this morning.  Referring to this thread, you said,

 

"As many would have noticed myself and @Tractorman have had what you would call a really interesting debate/aurgument in another thread, no crossed words, no fisticuffs, no nukes just different opinions, doesn't matter the outcome".

 

I could not have said it any better.  It is how you wrote that made this conversation worthwhile.  Anyone who reads this conversation can easily take away what they want without having to filter through irrelevant words or thoughts, regardless of opinion.  This is what I believe this forum is about - to put out the best information possible from forum members to help readers make informed decisions regarding maintenance and repairs of their Cummins powered trucks.

 

This has been worth hijacking my own thread.

 

- John

1 hour ago, Tractorman said:

This is what I believe this forum is about - to put out the best information possible from forum members to help readers make informed decisions regarding maintenance and repairs of their Cummins powered trucks.

100% agree John and this thread has been well worth the hijack (probably more me than you though that's done the hijacking)

 

As you said my friend each one of us waking up in the morning to a post by the other has resulted in strong coffee and for me thats been 3 or 4 cups before even thinking about a reply

1 hour ago, Tractorman said:

It is how you wrote that made this conversation worthwhile.

Funnily enough that's just what I think, your posts are fact, maths, science and a mind stretching read, while my posts are more based on seat of the pants what I see

 

One thing that did surprise me was the total lack of anyone else posting an opinion or for that matter posting anything at all on this thread but like you say that's more than likely made the debate easier to read.

Have a beer on me John as that was a perfect discussion     

Some of us kept reading but let you two discuss out the finer points without too many jumping in the middle of the mess. Thats one of the nice things you two can discuss out your different views and come to an understanding. Sometimes you gotta agree to disagree. 

28 minutes ago, wil440 said:

Have a beer on me John as that was a perfect discussion     

 

You made my day!

 

- John

1 hour ago, Mopar1973Man said:

Sometimes you gotta agree to disagree. 

I'm pretty sure the outcome was we agree on one side and we also agree on the other side

 

as in diesel engine braking is huge at idle in a low grear but pi~~ poor at higher rpm and from what I've purused on the internet this discussion won't go away

 

1 hour ago, Tractorman said:

You made my day!

2 hours ago, wil440 said:

One thing that did surprise me was the total lack of anyone else posting an opinion or for that matter posting anything at all on this thread 

@Tractormanand you were doing a fine job discussing this issue.   If I had injected may  into this conversation that's about what it would be worth.  Sometimes it's better to sit back, eat popcorn, and watch the show. 

 

That one of the great things about this forum:  top notch info, help, and exchange of views without the trolls.

 

 

The exhaust valve acts just like an expansion valve in an AC system. The gas laws say the temperature is directly proportional to the compression. That is why the exhaust temp drops QUICKLY when we reach the top of the hill and coast. The expansion of the compressed air creates a cooling effect. 
Diesel engines are compression ignition engines. Fuel is atomized for short bursts into super heated compressed air. 
At idle, the burst is measured in millisecond. Results =engine continues to run or idle coasting down the incline. 
If fuel is completely cutoff engine dies. 
The VP44 high pressure bursts to each cylinder are varied by the voltage received from the TPS which equates to load. 
At idle, the TPS minimizes/shortens the burst but the fuel pressure HAS to be high enough to pop the injector.