When EGT's get too low

[GaryCarter]

Alan, while you are coasting, the engine is being driven by the rear wheels, no fuel required. And an injector control system that can switch the injector three times during one power stroke can certainly switch on and off fast enough to keep the engine from stalling when it falls back to idle rpm.

I've owned both the 7. 3 and 6. 0 Powerstrokes, and they've been shutting the injectors completely off on coast for a long time. Some links from the dark side:

Do injectors ever shut off completely?

Very quiet motor when letting off throttle!!!



I do not have any knowledge whether Cummins does this or not. If they don't, then they should.

Gary

 

[PSchwering]

Wow. Lots of interesting theory's flying around here. Some seem more reasonable than others. First of all, one of the long time disadvantages to a diesel engine is a lack of natural engine braking. This was until Clessie cummins invented the Jake brake. But back to the original problem. Anyone who has ever had a Spring & flyweight governed pump on a fuel pump test stand knows that when you set the throttle at idle, then spin the pump fast, it should not give any fuel. Now, I don't know about the newer electronic engines for fact, but if they were still injecting a small amount of fuel when driving down a hill, the engine braking would be significantly reduced from a spring & flyweight fuel control engine. every electronic truck I have driven has as good-if not better braking than a 12V truck. So, This leads me to say that they must go to Zero Fuel. Tough to wash down the cylinder walls without fuel being injected.



This is Just me thinking out loud. I will give it some more thought, & Surely there is some data out on the web. I will look & see if I can find anything. P.

 

[Devan Manis]

Alan, while you are coasting, the engine is being driven by the rear wheels, no fuel required. And an injector control system that can switch the injector three times during one power stroke can certainly switch on and off fast enough to keep the engine from stalling when it falls back to idle rpm.

I've owned both the 7. 3 and 6. 0 Powerstrokes, and they've been shutting the injectors completely off on coast for a long time. Some links from the dark side:

Do injectors ever shut off completely?

Very quiet motor when letting off throttle!!!



I do not have any knowledge whether Cummins does this or not. If they don't, then they should.

Gary

I completely agree with the above statement. Why inject fuel to keep an engine running when the wheels are keeping the rpm of the engine above the low end governor? Even if you were coasting down a hill and you have your engine at 3,200rpm(red line?) and you suddenly pushed in the clutch the computer would have PLENTY of time to react and keep the engine running at the low end governor,the same thing would happen if the engine was only 100rpm above the low end governor.

 

[PSchwering]

If there is no fuel injected, then the engine is off. These engines and the Navistar engines are not "off" when coasting. The pulse width might be narrow but it is not zero. Although minimal, there is still fuel being injected to keep the engine running. I'm not sure how Navistar can shut the fuel completely off. What restarts the engine when coasting at low speeds with the TC unlocked? The engine won't just freewheel until the truck comes to a stop and then start the injectors to make the engine run again.



Sounds like you might be confusing this with the fuel off type of governor system, also know as hit and miss. These trucks and I'm sure Navistar does it too, use changes in timing and fuel injection as the governor. Without a governor, an engine at idle would continue to slowly gain speed. By controlling timing and injection that problem doesn't exist. With the hit and miss type of governor, the fuel is actually shut off for a certain period to control engine speed.

Alan,



You are correct at slow speeds with the TC unlocked, The engine is turning at 800 RPM, burning diesel, just as if you were sitting still & Idleing. I think Most of this discussion centers around higher ground speeds, when the TC is locked, or folks With a Manual transmission. P.

 

[Alan Reagan]

Alan,



You are correct at slow speeds with the TC unlocked, The engine is turning at 800 RPM, burning diesel, just as if you were sitting still & Idleing. I think Most of this discussion centers around higher ground speeds, when the TC is locked, or folks With a Manual transmission. P.

Gotcha. With the electronic controls, it makes sense that it could be done. I looked at the Ford site discussions and I really don't notice that my engine gets noticeable quieter during coasting. I'll have to ponder this one.



OK, I've pondered. I read an interesting article on Diesel particulate measurements where Sandia Labs has developed a way to measure them with a laser system. While I read several articles stating that diesels "shut off" during coasting, the Sandia Labs article stated that Diesel particulates during deceleration measured 8 ppb versus 11 ppb at idle. In their tests, there is some type of combustion or their would be nothing but clean air.



http://www.sandia.gov/news-center/news-releases/2004/comb-research-auto/particulates.html



The article states that the diesel injection goes on and off intermittently during descent, primarily to keep the catalyst temperature up.



With all that said, it's interesting that the EGT temps of the original post run around 200 degrees. At idle the temps on my 01 would run about 240-260. So is the 200 degree temp a result of compression only if there is no fueling? I'm inclined to think that the fueling is minimal or intermittent, possibly, but not completely off. I sent an email to Cummins to see what their response is.

 

[PSchwering]

Interesting. I am inclined to believe that some fuel might be injected to keep the catalyst hot. If I were trying to do that - I would probably inject the fuel charge late, so as to provide heat for the catalyst, but not lose what engine braking ability the engine does have. By injecting the fuel late, the emissions per fuel delivered could be high, so based on your numbers, 8pbb during decel, Vs 11 pbb at idle, you can't say that you are burning 22% less fuel at decel than at idle It is possibly considerably less.



Heat of compression could not give you 200 degree EGT's - You have good heat on the compression stroke, But most of the heat you produce is absorbed back as the pressure is released on the "Power" stroke. The other side of that, is with all of the internal engine components being very hot, it would take a lot of coasting to get egt's down to Ambient, or close to it.



My 94, with the Spring & Weight governor - no Catalyst - EGT's only get down to about 200 going down the hills I have gone down, but In Southern Indiana, we just don't get to drive down hill for miles on end, Like the folks originating this post do. I would be interested in what EGT's get down to on a 12 Valve going down similar hills. With a 12V, you don't have multiple fueling events, variable timing, or anything like that to deal with.



Just some more random thoughts! P.

 

[Alan Reagan]

And here is the reply I received from Cummins:



"Summary: Coasting fuel consumption



Solution:

Thanks for your Email message.



Alan, there is a small amount of fuel flowing to the injectors during coasting, to lubricate the injectors to avoid plunger scoring or seizing inside the injector. Also some pumps need fuel flow for cooling and lubrication. "



This is the way I have always understood it to be. That accounts for EGTs at around 200 and the fact that there is not a significant change in engine sound. Now, if the Powerstroke is shutting off fuel completely as indicated by the guys on their website, that may account for some of their injector problems.

 

[rbattelle]

And here is the reply I received from Cummins:



"Summary: Coasting fuel consumption



Solution:

Thanks for your Email message.



Alan, there is a small amount of fuel flowing to the injectors during coasting, to lubricate the injectors to avoid plunger scoring or seizing inside the injector. Also some pumps need fuel flow for cooling and lubrication. "



This is the way I have always understood it to be. That accounts for EGTs at around 200 and the fact that there is not a significant change in engine sound. Now, if the Powerstroke is shutting off fuel completely as indicated by the guys on their website, that may account for some of their injector problems.

Good info... thanks Alan.



-Ryan

 

[Devan Manis]

I gladly stand corrected.

 

[Alan Reagan]

I wasn't thinking too much about the pump lubrication before, but I would hate to know that when I was coasting, the pump was just pressurizing fuel with none going through it for cooling and lubrication. If Navistar is truly shutting off the fuel, then they must have a bypass system that allows fuel to flow back to the tank. Reading their web posts, that might account for the strange knocking noise when the engine restarts while coasting. I don't see how they could avoid cavitation in the pump when switching from bypass (with no pressure) to injector pressurization. Although electronic valves are fast, they are not instantaneous.



And Ryan, based on the Sandia Labratory tests, coasting could produce lower consumption than idle over the same period. However, I believe I had read that the higher RPMs during coasting essentially bring the consumption back to what is experienced at idle. I can't find that now so I can't prove that one.



Back to the original question... ... . The reduced fuel combined with operating engine temperature should not cause any wash down problem in the cylinders.

 

[Alan Reagan]

now, what about a p-pump?

As in the P700? They still deliver a minimal amount of fuel during coasting. Fuel is what cools P700. It is lubed by the oil pump. Shut off the fuel and leave the pump turning and it will muss it up.

 

[Devan Manis]

Are you asking if they do or saying they do? If they don't the fuel will still flow through the pump because the lift pump is still running pushing fuel past the OF valve.

 

[Alan Reagan]

Are you asking if they do or saying they do? If they don't the fuel will still flow through the pump because the lift pump is still running pushing fuel past the OF valve.

I'm thinking the valve is before the injector line outputs of the pump, isn't it? If it goes through the OF valve, it doesn't make it to the injectors and the same problem occurs with scoring and seizing as a result. It's minimal fuel flow but it's there.

 

[Devan Manis]

I have no clue never really looked at a stock OF valve.

 

[TKingsbury]

The injectors on 12v engines are just pressure reliefs. If there is no pressure to the injector nothing happens, therfore, no scoring. If it had cam driven injectors, I could see wanting fuel flow because the injectors would be churning along with the engine.

Travis. .

 

[Alan Reagan]

The injectors on 12v engines are just pressure reliefs. If there is no pressure to the injector nothing happens, therfore, no scoring. If it had cam driven injectors, I could see wanting fuel flow because the injectors would be churning along with the engine.

Travis. .

You are correct.



Now to further examine the P7100, I called A&M Turbo in Macon, GA and asked them if the fuel shuts off. The answer, like the one from Cummins is no. The P7100 maintains a small amount of fuel flow to the injectors. The way it was explained was that even though the engine is coasting, the pump still applies fuel to maintain an engine load.



I asked about the Ford's and he said they do, in fact shut down the fuel completely and turn the engine into an air compressor for braking down to 20 mph.



He also told me that the new Cummins Common Rail uses four injector control pulses (I thought it was three). However, the CR system used allows for up to 13 injector pulse events. In the future, they may decide to completely shut down but for now, they flow a small amount to maintain a load.



I sure hope this helps someone.

 

[PSchwering]

I am going to have to do some more checking on this. The p7100 is a simple Port & Helix metered pump. I have never seen a P7100 on a test stand, but I have seen other P&H Bosch pumps, & They delivered Zero Fuel at high speeds, when the fuel control was set at idle. I suppose the p7100 may have some type of mechanism to stop the fuel rack from reaching zero delivery, but it would have to move out of the way to alow shutdown..... P.

 

[Alan Reagan]

I guess the only way that this could be verified one way or the other is to do the tests that Sandi Labs is doing where they are measuring exhuast particles. The amount of fuel is so little that it is effectively shut off. But I remember with my 98 12V, the EGT still stayed around 200 when coasting.



Could it be that fuel might still be added to maintain a heat load on the engine as the mechanic suggests? Also, the engine braking on my 98 was pretty much non existent. Of course the TC would unlock when decelerating. I don't know. Unless someone here has the ability to actually measure the exhaust while coasting we may never know.

 

[PSchwering]

I took an injection systems course in college, so I went to dig out my Bosch textbook, to research this. It is Not with the rest of my books. I did find my class Notes, but they did not help much. I suppose unless someone knows differently, we are going to have leave this one at "Very little, if any" fuel is delivered coasting down a hill with a p7100 pump. Sound reasonable?

P.

 

[Allnew2Me]

It seems that nobody really knows for sure if the fuel is totally off when in decel, but my original question I think has been answered. If indeed any fuel is present in the cylinders it is not enough to cause a washdown effect so it seems. i won't worry about coasting down grades anymore!