The p7100 is positive displacement right? so why delevery valves and injectors?

[T. Baker]

Help me out here. The p7100 pump should pump a certain amount out of its plunger based on rack position. Since diesel fuel will not compress and I asume there is very little leakage past the plunger after the ports are closed then it should not matter what delivery valves and injectors you have because the same amount of fuel will come out each stroke of the plunger. Every one talks about the power improvement they get from injectors and DVs but the pump plungers regulate how much fuel gets to the engine!! What am I missing?

Thank you,

Treven.

 

[Rammin On]

The P7100 provides metered fuel from the plunger and barrel assy to a constant volume delivery valve ... . tis not a positive displacment pump.

 

[T. Baker]

"Delivery Valves-- purpose and design. All port and-helix metering fuel-injection pumps (with the exception of Detroit Diesel and Catapillar engines) using a unit-injector injection system employ a delivery valve. Its purpose is to accelerate the beginging of injection, to ensure quick ending of injection, and to maintain residual-line pressure about 30 percent lower than the injector opening pressure. " I just reread the chapter on Bosh pumps in my Diesel engine text book from when I was in diesel school. The delivery valve does not control the volume of fuel delivered! It is to make injection start and stop precisely and to stop dribble at the end etc. It is the effective stroke of the plunger (controled by rack position) that controls the amount of fuel delived to the combustion chamber. The only thing I can think of is that if the delivery valves and injectors are too small that the pressure goes so high that some fuel leaks around the plunger and barrel so that less fuel ends up being delivered but that seems highly unlikely because of the precision fitting of barel and plunger assembly.

Treven.

 

[Strick-9]

#1: Diesel fuel DOES compress under 15,000 psi.



#2: The delivery valve is a restriction during flow, as is the injector nozzle, and injection lines. Opening up these restrictions will allow the fuel to get into the cylinder quicker and cause cylinder pressures to rise quicker and make more power. Power is a function of a lot more variables than just fuel quantity.



For maximum power we would like to keep cylinder pressures as low as possible until TDC. Then we would like a HUGE pressure spike and maintain that pressure for the entire stroke of the piston. The pressure spike is great for making power, but hard on internal components. So every engine is a compromise of power versus longevity. By lessening the restriction of fuel flow people are trading a little longevity (by more intense pressure spikes) to make more power. The timing of the fuel delivery is more important than the quantity delivered.



There is a lot more to it, but it's more fun to figure it out yourself.



-Chris

 

[T. Baker]

Chris, I guessing from what you are saying that for power levels up to 300 hp and up to 3k rpm that stock delivery valves and injectors will work fine? I assume that adjusting the timing will put the pressure peak where I want it and I will just have to live with the stock rise and fall of the pressure curve. I guess when I made this post I was thinking of my needs wich are not more than 300 hp and 3k rpm. I can see that things will change with higher rpm and huge fuel flows. by the way did any of the broken turbo you had make it past the intercooler and into the engine? I hope not!!! I love reading about your diesel blood and guts storys. Before I got my Cummins I had had some intresting failures of my nissan engines because of some serious modifications and some faulty parts etc. Turbo diesels are a hell of a fun hobby though and nessary drivers for us.

 

[Rammin On]

T. Baker ... . I never said the delivery valve controled the volume ... the volume or quantity of fuel is dependant on the effective stroke of the plunger ... and this varies as the fuel rack turns the pump plunger ... the helix on the plunger opens the port to change the delivery point ... . the delivery valve is a constant volume valve - to be precise - it's the little piston part that fits in the valve holder ... there is alot involved with the injection cycle.



Keith

 

[T. Baker]

My point is that just because you bought 370s and 191 DVs You are not nessasarly pumping more fuel to the engine than somone with small DVs and injectors. I would like to have a better understanding of how DVs and injectors increase fuel to the cylinders. We all seem to agree that the plungers and helix and rack position control the amount of fuel. From what I understand about hydrolics the DVs and the injectors WILL FLOW what the plungers put out or somthing is going to break. It also would seem to me that the profile of the pump cam is the biggest factor in the curve of fuel flow. There is some compression of the fuel aparently and some flex in the interior volume of the fuel lines that could affect things some especially at high rpm and extreemly high presure. I am trying to get some new understanding of the injection cycle so I apreciate the responses to my thoughts and questions.

Treven.

 

[Rammin On]

Comparing the size of the retraction piston part of the 167 dv's and the 191 dv's ... . the 191 dv's actually allow a larger volume of fuel to accumulate in the chamber before injection. The retraction piston part of the dv is right below the valve seat. I wish I could post a picture of this ... would be a better explaination of what the valve looks like ... .



Keith

 

[T. Baker]

Keith, I have been looking at some of the pictures of delevery valves that I have. It looks like once the piston lifts off it's seat and fuel begins to flow that the delivery valve is doing nothing until the presure begins to drop then it slams shut. It looks like fuel just flows through the chamber above the piston and on down the injector line. Since the chamber is full of fuel all the time I would think that fuel does not "acumulate there" it just flows through that space. I guess that the size of that chamber will affect the shock wave characateristcs of the fuel down the injector line just like plenum chambers on intake manifolds. In the '40s Herculese made some diesel engins wich had injector lines different lenghths and they ran a little uneven. That was before they relized all injector lines should be the same length to make injection timing exactly the same. My understanding of the delivery valve is that it would flow endlessly if that was what the pump did. From the description I have read the DV is basically a sophisticated check valve. If anyone knows of decent reading material on this subject please let me know.

Treven.

 

[Strick-9]

If you're not scared. Remove your delivery valves and drive the truck around. How do you think it would run?



I've done it. No harm done, but the way the truck runs will surprise you.



Yes, DV's are check valves. Let's just assume removing the delivery valves would decrease power by about 150-200 hp. It would also idle lower. Less fuel volume over the entire range. But why?



Once the helix opens the port in the barrel to vent injection pressure down to supply pressure, the delivery valve is preventing high pressure fuel in the injection lines from expanding backwards into the barrel.



So basically, fuel is still being delivered through the injectors long after the port has opened in the barrel as long as you have the check valves in place.



When the check valves are removed, fuel is only being delivered while the barrel is pushing fuel and as soon as the port opens on the helix, the fuel pressure all the way to the injecotor drops in pressure and fuel delivery is stopped. Thus the huge loss in power.



If this is true, how important is camshaft profile for fuel delivery?

The fuel is being compressed and held by the delivery valve not pushed through the injector by the shape of the cam lobe.



But cam lobes can be machined for MUCH more fill time and allow a lot more heavy fueling at higher rpm (2800+).



Like I said, there's a lot more to it, but it isn't near as much fun reading about it as it is doing it. When you get the crazy idea to remove your delivery valves and you're sitting in the cockpit with your hand on the key trying to convince your hand to start the motor. And you shrug your shoulders and wince as the starter begins to rotate, now THAT'S excitement.



T. Baker, you don't need any reading material, you just need to start turning more wrenches.





No, seriously, I've got three books from Bosch on pump stuff and none of it is that useful in my opinion a big waste of money. If you find some good info, let me know. I'm always looking for more.



-Chris

 

[T. Baker]

Ok so the DVs are glorified check valves that keep the fuel from backing up and dumping through the spill port on the plunger. I understand they are nessasary. I don't understand how the 191s do a better job of checking return flow. Does the piston displace fuel when the spring pushes it back down and deliver the amount that it displaces to the injector? I really need a good picture of the DVs. Anybody out there have an old DV I could disect?

Treven.

 

[Rammin On]

Trevan ... .



I have a good pic of the DV ... will try to get it to you ... I'm sending you a PM ... . easier to splain this on the phone ...



Keith

 

[drees1]

Compressable fuel

Srick-9,



You are right on the money! I will add a couple things to help. Fuel is very compressable, more so than water. That is why all lines and must be the same length, and the volume in the system for each cylinder exactly the same.



T. Baker,



Consider the fuel like air, compressable, and it will all come together. The delivery valves serve several functions as mentioned. Smaller ones may "check" the back flow a little more precisely, but are an orfice in the system, and the positive displacement plunger is compressing fuel to a degree around the valve. More noticeable when bigger than designed delivery is needed.



Also consider that timing is set at the start of injection. Added fuel displacement in the pump comes in later in the cycle and at less efficient compression after tdc. So bigger injectors and delivery valves get the fuel in sooner (because it is compressable) and make more power than the same fuel quantity put in later in the stroke. Also, very important, reduced cam stress in the pump pushing more fuel through big injectors than more fuel through small injectors. Overfueling small injectors has limiting returns, poorer efficiency and high pump stress. The existing designs are very high pressures for emissions, and will make great power and economy by less compression and getting it in sooner. Spray atomization is not as good but still plenty good to see gains in both. One last thing, all things the same larger injectors will produce more fuel flow with the same pump setting, as less pump displacement wasted compressing fuel. In the lab we tested engines with very high cam profiles and could not achieve any better results because small injectors only increasd pressure and compression in the fuel and did not flow much more. A 25% larger pump volume only flowed 3% more fuel with out an injector change. Engine was a CAT 3406 prototype at the time. Hope this helps.



Doug Rees.

 

[M Barnett]

Doug Rees; I notice in your signature you don't have the 191 valves in your truck. I installed them in my truck to get rid of the stumble and if I remember right the ones in my truck were marked 151. My truck runs great now but it smokes a lot under heavy throttle. I am wondering if the 191s aren't too big for my set up. Its not a race truck.

Michael

 

[drees1]

M. Barnett,



I was speaking in general terms as far as how changes might effect things. I have much diesel experience, but only about 8 months on the Dodge. But I do see you have a pretty hefty torque plate, injectors, and now del. valves. Are you running stock turbo? Even with good boost on the inlet side, excess exhaust pressure in the manifold due to small turbo will hang up the air flow. Bigger turbo, more air, and you should clean up. What is your EGT when smoking? One of the experienced could chime in here and help explain why the smoke with your combination. This board is great. How else would we figure all this out??



My sig is correct, but I am seriously thinking my next upgrade should be HX40 and 370's, for same concern as yours. Any one want to help?



Doug Rees

 

[M Barnett]

Thanks Doug: Yes that is the problem now, I can get 1300 going though the gears. I have a 14 wastegated turbo housing on the way now and will see what that does to get the temps and smoke down. It really runs great and doesn't smoke with normal driving. I am one of those old farts that doesn't want to blow up my truck, but boy, is it fun to drive!

Michael

 

[Cummins Corvette]

What a cool thread! Keep it up guys; I'm lovin this! There is another thread in the P & A forum about a guy selling custom race pumps with 13mm plungers/barrels. I don't know much about the 13mm plungers, but if the maximum stroke (determined by the length of hexix and cam lift) is the same between the 12 and 13mm, then the 13mm should be able to displace 17% more fuel. That isn't accounting for the restriction of the injectors and delivery valves though. What do ya'all think?