Thinking out loud on marine injectors IP fuel delivery again ....

[Hubert]

I just read Jim's article on marine injector differences.

Interesting that at lower rpm you can have full fuel delivery rate at mm^3/1000 strokes but at higher rpm the max rates are reduced. I have thought this might be due to peak injector line pressure and associated differences in injector flow rates.

Wonder if some of it is also due or has bearing because of any effect of pumping losses vs pump speed???? Looking at the plungers and rotor clearances I imagine there is some slippage/leakage or pumping loss due to fuel viscosity and the speed of the cam plunger pumping action. Such that at lower speed there is some appreciable loss due to blow past the plungers then at higher rpm you actually get more flow due to the plungers squishing the fuel faster and less time for slippage past them.

Interesting too that the fuel temp is measured and intuitively the viscosity might be factored in for any of this slippage???? Interesting that the PCM totally controls cranking/starting and idle fuel metering and the DS pumps don't have hard cranking when warm and old vs DB pumps.

I can see where if the orifice is bigger the injection line peak pressure might be lower and this would lessen any tendancy for this leakage/slippage/pumping loss and you could get more through put of the injector even though the fuel rate calibration is the same or setup is the same.

What are some ya'll thoughts?

[More Power]

There are fuel curves built into the mechanical pumps. I was told back in the late 1990s that the marine pumps used a little different fuel curve than those used in an on-road application.

Regarding decreasing fuel at higher rpms, some of it (in a mech pump) may be due to the fuel curve and some is likely due to the inability of the fuel to flow through all of the small internal passages inside the pump and then adequately charge the plungers at higher rpms.

I also suspect that the thermodynamics of diesel fuel combustion is such that at higher rpms it become increasingly less efficient at burning the fuel. Diesel fuel burns slower than gas. I've noticed through the years that every light-duty diesel engine I've looked at all produce their best fuel economy at ~1800-rpm. It doesn't matter whether we're discussing a 6.2, a 6.5 a Duramax, a 5.9L Cummins or a Powerstroke. Even the Jeep CRD is factory geared to produce ~1800-rpm at 65-mph, which just happens to be the ideal sweetspot for fuel economy.

I contend that diesel fuel is not like gasoline, in that a gas engine can be spun to 10K+ (a motorcycle engine can run to 13-14K) and it continues to make power. A diesel, not so much.

Just musings....

Jim

[Hubert]

I agree mostly with your thoughts of diesel vs gas and the associated sweet spot at ~1800 being more effecient for diesel.

But I think that is more by design than thermodynamics of fuel. Yes diesel burns slower than gas in the context of a recipricating piston engine but I would think that is more a function of air to fuel mix ratio, compression ratio, and stroke etc. Fuel flame propagation speed is determined by several things one of which is the temperature of combustion or maybe differently said the load on the engine. With a gas engine since air and fuel are mixed at a certain ratio I tend to think its more or less has to spin faster when more fuel is burned for NA engines so you can pump through more air. But diesel RPM is much more independant of fuel rates.

Big trucks with longer stroke and bigger bore turn even slower than small Diesels because they are set up for being torque monsters etc also. And although its different animal entirely a turbine spins really fast and burns something very similar to diesel fuel.

I think you are right on with speed of pump rate and differences in resistance to flow in short time with the total resistance to flow through all the path. I'd like to say more but time doesn't permit.

[Hubert]

Here are my thoughts....

Just kinda guessing this is how they might design it or what has evolved??? Its a bit of trade off through the rpm / power band and limited number of gears. Auto's (pickups etc) generally operate at idle then between 1400? and 2600? during acceleration then designed and or tuned for cruise to be some lower middle ground that can hold speed with hills and somewhat varying load. 1800 rpm seems to be a good choice for today's powertrains and speed limits so most automakers tune for that speed. Over the years this has seemed to trend down. It seems they have evolved to that from mpg, emissions, and learning what has worked well over the years.

Having a rapid "explosion" of diesel at near tdc just stresses the bottom end and builds more wasted heat due to position of crank throw. I have heard truckers say high cetane fuel actually feels a tad weaker ( in older trucks) due to this (but it does help in starting and emissions). Those big longer stroke low rpm engines like a slower burn to be feel like they have more torque.

OK so that's the target 1800 ish rpm. I would guess the injection atomization and temperature of combustion are the 2 major factors in flame front speed. The amount of heat and rate of heat in the cylinder would then also be very dependant on rate of injection (not counting the amount of injection).

What apparently peninsular and/or bosch found was with the larger orifice it probably? increased the rate of injection penetration and associated flame front / heat rate in the cylinder which improved upper rpm performance (due to mechanical timing difference of higer rpm). This is exactly what a marine application likes. A truck however doesn't necessarily like this.

One thing for old one squirt diesel injection that works well in conjunction is when you hold back a diesel engine and increase fuel the combustion temperature rises and thus flame propagation speed increases. I can see depending on how you set up, load up, and drive an engine say for instance heath's high rpm racing 6.5 it can be tuned for better higher rpm power vs one set up a powerband at 1800 rpm. Its still somewhat of a trade off in torque curve peak but I bet some interesting interactions of the mechanics of pumping etc that Heath saw some merit in for his strategy (since the amount of fuel seems to be the limiting factor for more power when going all out) .

[Hubert]

I don't have any scoop on Heath's speed truck. I just read the articles and I imagine there is some more to it. He did use hi-flow injectors but don't remember mention of any DS pump mods but his programming. He was able to run up the RPM's ( and probably increased fuelrate too at higher RPM ???).

Interesting that the "feed the beast" mod seems to help (at max fuelrates) actual IP fuel delivery. IIRC I think it helps mostly to balance the output (some cylinders drop in output). I think it helps this plunger charge or fill when max fuelrates are pumped through to the injectors. And the unbalanced output is due to the plungers lack of fill, starving, or having to pull fuel??? IIRC Heath did use a hi flow lift pump and its a recommended mod when reprogrammed and maxing fuelrates out.

I saw a clip of Heath's truck at Bonneville somewhere and it was low smoke. Some other Duramax racer was belching black smoke. Wonder if Heath rolled into it easier due to setup, programming, and strategy for the setup ???

[SmithvilleD]

The longer stroke making bigger torque at lower rpms & tapering off earlier as rpms build probably relates the speed of combustion limiting factor as Jim mentions.

Peak piston speed (mid-stroke, @90 degree crank angle) increases as stroke (crank throw) increases. If the speed of combustion is, in fact, becomes a power limiting factor as rpms get towards the top end - it makes sense that it would limit efficiency/power somewhat lower in the rpm range on a longer stroke engine w/ faster (relatively) peak piston speed/rpm.

[SmithvilleD]

Can't recall the peak rpm from memory, but the Banks Duramax drag truck spins to higher rpms than common in diesels. I've read some of the mods to make that possible, such as using stronger, far more expensive, & lighter valvetrain components to avoid valve float while keeping valve spring rates workable.

Might well be logical that the much higher pressure injection systems resulting in finer sprays, droplets, could allow combustion to proceed faster. Also, combustion rate tends to increase as cylinder pressures increase; believe that's the reason injection timing tables lessen timing advance as the engine load levels increase. Figure that's to keep the peak cyl pressure happening at the right time relative to TDC/crank angle.

If combustion happens faster at high cyl pressures, keeping the inj timing/advance as early as it is under low load conditions, at high load levels the combustion would start at the same time, but hit peak pressures quicker/earlier in the crank rotation/piston relative to TDC, risking significant pressure build before TDC, essentially developing negative torque production, much less the risk of excessive cyl press peaks & potential head gasket sealing issues.

[More Power]

Can't recall the peak rpm from memory, but the Banks Duramax drag truck spins to higher rpms than common in diesels. I've read some of the mods to make that possible, such as using stronger, far more expensive, & lighter valvetrain components to avoid valve float while keeping valve spring rates workable.

As I recall, the Banks Duramax powered S10 drag truck spins into the 5K+ range. There is value is having higher RPMs available for a drag or Bonny truck because it extends the gearing for higher top speeds.

Jim

[SmithvilleD]

Plus simply more hp/power pulses per unit time. Until torque tapers off to the extent the additional rate of power pulses can't overcome that each power pulse isn't as strong.

When watching gasser race engine builders evolve, you can sort of see that "lightbulb" go on, when they begin to grasp the airflow limitation reasons why the torque curve always falls off at some point as rpms rise. It's simply the % cylinder fill is going down as rpms rise. Shorter & shorter time interval that the valves are off their seats & air can move.

[93GMCSierra]

I read that a race only cummins 5.9L I6 had max rpm in the 6800 range.

[tnt1197]

1) Turbine engines run with continuous injection, and the T53L series(mil- hueys & cobras) had an effective CR of 8.5-1 at the entrance to the combustion section. Power was regulated with a "trim" screw IAW charts in the maintenance test flight manual. (retired as a huey mechanic)
2) Spark timing is advance curved by rpm and, in the fly by wire motorcyle ECU's, both the spark and fuel events are varied according to load, throttle position, and RPM.
3) When I was a young military mechanic(engineers and field artillery), we attempted to do some "performance enhancements" on Continental multifuels (2&1/2T & 5T trucks) and GMC 8V72Ts (self propelled howitzer). By using injectors from "stationary" engines;ie generators, we could increase fuel delivery which within "reason" gave a satisfactory performance gain. However since most of us subscribed to the "more is better" school, we put the biggest rated injectors that supply could ship us. At least in the 8V72s, they tended to run just like an old street 2-stroke- Blubber and smoke until the port timing was flowing right and then hit like a bomb went off. I used to have a picture of a M109 SP howitzer with the first 4 roadwheels off of the ground on asphalt.
4) All of our diesel ground equipment has been converted to NATO spec JP-8 since prior to the first Gulf War along with all aircraft from JP-4 to JP8.

[Hubert]

Points taken and I agree at some point torque will fall off with diesel (and gas too) because air fill (in combination with diesel burn rate for diesel). But I think it can be higher than 1800 rpm appreciably and the 1800 rpm peak was "tuned/designed" for on road use for the 6.5. And in general for other on road engines. It sure sounds like Peninsular pushed to upwards of 2600-2800 rpm and with more tweaking it might could be pushed higher.?

OK, the turbine was way bad comparison but was thinking if Diesel/JP-8 could burn fast enough for that application it could be fast enought for a recipricating engine at higher RPM. I was half thinking of how fast after burners would have to burn ( but forgot to think about the super high temps too.

I still kind of think of a diesel's rpm as being the resultant of fuel burn and torque/power vs a gasser (NA) that needs rpm to be able to burn appreciably more gas. Comparing 1000-3400 rpm vs 1000-5500? rpm for avg engines.

I can see rpm range extending gears especially in drag racing but would think Bonneville would be more of a tune of top 2-3 ??? gear ratios (which I think Heath may still be doing some). As long as bottom of next gear is appreciably higher than top of previous gear and assuming torque and HP climp appreciably the vehicle will continue to accelerate. Right ???