GO 4LO

How much boost can a Cummins handle? What is the weak link at this point?
If we assume low compression pistons, fire rings, 14mm studs, etc, I'd assume the 5.9 could handle 175-200 psi, right? What's holding us back at that point - firerings, stud material/dia?
Figured we'd get another technical discussion going, if we can get some of the more knowledgeable guys like Hohn and Don M to chime in...
Chris

MCummings

My truck held ~80 PSI for atleast 50,000 miles...

Lets see how long it holds up to 100-125?

I think cylinder walls/piston rings, and the ability to hold the intake valve closed....

Merrick

HOHN

The peak amount of boost the engine will handle is pretty much irrelevant.

Besides, you'll hurt your wallet as much as anything!

Seriously, though-- what matters is cylinder pressure and how is plays out over crankshaft rotation.

Basically, you want the highest BMEP (brake mean effective pressure) that you can get while maintaining the lowest possible PEAK pressure for a given BMEP.

A typical way to achieve the above is to lower compression ratio. Lowering compression also lowers expansion ratio (duh), which means that extremes in cylinder pressure are less. That means you have LESS pressure at the point of peak pressure, and MORE pressure at the point of least cylinder pressure.

With this approach, boost pressures can be raised substantially for a given amount of peak cylinder pressure, because we're compressing the air only 11 or 12 times, not 17 (like an ETH). Furthermore, this allows a greater MASS of air in the cylinder (volume is misleading when talking about air because it's both compressible and elastic). This greater mass of air should allow for the combustion of a greater amount of fuel.

Furthermore, we have made the engine less efficient in thermodynamic terms. In a pure sense, efficiency can be measure by the difference in peaks. Thus, if two engines both have peak pressures of 4000 psi, the one with the lower cylinder pressure at BDC is said to be more efficient. It has extracted more work from the available energy.

It isn't necessarily a bad thing that the engine is less thermodynamically efficient, because we have turbochargers that recover some of that lost energy. Less energy extracted from the burn means more is available to spin the turbo(s).

Unfortunately, the turbos are only recovering about 1/3rd of the energy that's dumped into the exhaust. Having that energy extracted in the cylinder is generally more efficient overall.

Consider the peak cylinder pressure a ceiling that you don't want to exceed (headgasket). You can raise the air flow and fueling rate until you approach that ceiling. As you get closer to it, you have to try and cheat a bit by getting more fuel and air in without getting closer to the ceiling. This means manipulating the compression ratio and upping boost well past 100psi.

Unfortunately, as you drop the compression ratio, hard starting becomes the least of your worries. With lower compression, intake and exhaust (breathing) efficiency suffers as well-- you'll get lower velocties in the the head ports, and the volumetric efficiency is lower.

End result-- lower compression kills bottom-end torque and makes the powerband much narrower.

So once we get to the ceiling (sealing? hehe) limit of the head, efforts to reduce peak pressure carry a high price tag. Efficiency falls off fast.

The sled pullers do this because 1) they hold RPM fairly constant, and 2) the engine isn't operated for long periods of time like a daily driver. When I say sled puller, I'm talking about trucks like the Scheid truck, not some wimpy Hollywood fictional truck from MGM studios J/K

Don't be too caught up in a number. Boost pressure is not the whole picture. If you have a higher-flowing head, you will see MORE airflow, but LESS boost! What matter is the MASS of air (lb/min) flowing, allowing a MASS of fuel to be burned. It's also of key importance HOW the burn occurs, as we want to make maximum use of the enthalpy (temperature and pressure) available to 1) turn the crankshaft, and 2) spin the turbo(s).

Sorry I rambled-- what was the question again?

DISCLAIMER-- what I write is very different that what Don writes. I can think and try to reason out loud while Don writes from tested experience and profession exertion day in and day out. He has less time than I do. I can only speak in generalities and principles-- Don speaks from specifics. Don has spent a lot of money and sweat on bombing these trucks-- he knows what works and what doesn't. I drive a warmed over rig with high hopes and not much money.

Bottom line-- I'd hesitate to put me in the same class with Don in any area-- excluding of course, my superior looks

J/K Don.

Justin

RowJ

Superior looks AND your vocabulary!..."Enthalpy".....Impressive!
Sent me to my dictionary.

RJ

GO 4LO

Thanks for the input, guys.

Hohn,
Bill Gotthelf has helped me out w/ explanations on DTR sometimes, and he gave me an approximate limit of 130 psi using the 14mm studs and fire-rings.
Assuming this was with 17:1 compression, I believe (if I calculated correctly) that with 14:1 comp, I could go to 161 psi without raising cylinder pressure over the previous example. Anything lower than 14, I'm starting to get a little leary of (don't want to have to start her on ether every time! haahaa
Would you say the 14:1 is a reasonable limit for a vehicle that is driven some on the streets (not a dd, though), or would you go higher/lower?
At this boost limit, is it still the fire rings / gasket that is the problem? Could this be solved with more clamp load on the rings, or different ring design (doubled concentric rings maybe)?
Couldn't we use a gasket / stud setup that was just barely weaker than the head itself, or does head deflection become an issue before failure? Either with larger studs, different stud material, different rings, copper / mls gasket, thicker head casting to prevent deflection, etc.??

Thanks again for the input!

Chris

HOHN

Bill's estimate of 130 psi seems good enough to me. I really don't have enough experience with it to be able to give you a good number.

Your calculations are misleading you. It's not just a simple matter of 17*130=2210, so 2210/14=157 ish psi.

It's completely non-linear as to how the cylinder pressure game works. Moreover, we're not really that concerned with the pressure in the cylinder BEFORE the fuel is injected, it's AFTER-- where pressure goes from 3000-4000psi up to 10,000 plus (or higher, these are just WAGs). (note: WAG= Wild Guess)

Keep in mind the big picture as to how this works. If we are taking in a certain amount of air at, say 20psia (to keep the numbers easy), then we compress it 17:1 in an ETH engine, what's the cylinder pressure when it's compressed? 20*17=340psi, right??


WRONG!!! What happens to the temperature? We know that when we compress the air, it gets hotter. Hot air wants to expand-- i.e. it's less dense.

So we have hot air trying to expand while at the SAME TIME the mean old piston is squishing it into smaller and smaller space. The end results is instead of having 340 psi in the cylinder, we end up with a MUCH higher value. You could calculate it using ideal gas law, and the number would be much higher than 340psi! MAybe 2 or 3 times that.

Now, we have this high-pressure, hot air held in a tight volume and we inject fuel into the mix. We've just increased both the cylinder pressure and the temperature by a HUGE amount! But if this injection occurs right at TDC, then the increase in cylinder pressure is happening at the same time that the piston is moving down. So our rapidly expanding gases have a place to go!

By now, you can see where we're going-- TIMING! If you advance the timing significantly, we are injecting fuel (and causing rapidly expanding gases) while the piston is still rising, reducing the room in the cylinder.

Hmm-- the piston is compressing flaming gases that are doing all the can to expand, but have no room! Nowhere to go! CAN YOU SAY HUGE PEAK CYLINDER PRESSURE???

Sorry to digress. All that to say that you can't just drop compression and think you'll magically be able to calculate how much boost you can now run "safely". We have to account for the temperature, pressure, volume, density, and mass of what's going on in the cylinder relative to the piston. ABOVE ALL, WE HAVE TO ACCOUNT FOR THE HUGE EFFECT OF TIMING.

Timing has more of an influence on peak cylinder pressures than anything else. You can advance timing excessively and LOSE hp, while also creating a MUCH HIGHER PEAK cylinder pressure.

Now let me address your questions specifically, Chris:

1) I'd think that 14:1 is much to low for a diesel engine that wants to see ANY street action. You just lose so much from lowering the compression! The engine's performance and enjoyability just dies when you gut the compression. You'll regret going below, say, 15:1. Remember, here-- the game isn't necessarily about compression ratio-- it's about peak cylinder pressure (the spike just after TDC). As such, compression ratio is only a part of the picture.

2) Would I go higher, or lower? Well, what did Cummins do when the ETH came out? THEY RAISED THE COMPRESSION RATIO. ETC= 16.4:1 ETH=17:1..... Also note that it's a very mild increase in compression. Other diesel engines run higher compression ratios. TDIs are 19.5:1. Non-turbocharged diesels run ratios well into the 20s.
In general, a higher compression ratio is always a good thing at anything less than WFO. You'll also hit a point of diminishing returns. Increasing compression 2 points from 14 to 16 is a lot more significant than increasing it those same 2 points from 20 to 22.
Personally, I wouldn't touch the CR on a rig that sees any kind of street action, unless you are having chronic cylinder sealing issues-- and in that case, I'd re-examine the timing I was running (both "paper" timing and real world timing, influenced by injector size and fuel cetane).

At the boost limit, the first problem is clamp load. The head wants to move around like a sheetmetal screw on a vibrating table!

Second, is lateral head movement. If the gasket can move WITH the head as it walks around on the block, then it's going to hold much better.

Now we come a crossroads-- are the o-rings (i'll lump fire rings together with O-rings) there to seal the cylinder, or to help prevent the head from "walking" on the block? Well, both, kinda. It's hard to seal a block that's walking, no?

Let's break it down by asking the basic questions: what seals the head? What causes that seal to fail? What forces are acting on the point of sealing?

Clamp load is the first thing that seals the head. But we'll soon get to the point where cylinder pressure is enough to overcome that clamping force. The head lifts, the seal is lost, and we've blown a HG.

So how can we take a given amount of clamp load and make the most of it? An o-ring focuses this clamp load at a point. It's the difference between 40 pounds of sand in a bucket vs 40pounds of pressure on a needle-- which one will penetrate? By focusing the clamp load at a point--the o-ring-- you get a seal that has traction against the head and block-- it digs in and forms an effective barrier to pressure. So an O-ring makes much better use of available clamping force. This usually means that the head would have to lift FARTHER to blow the seal than it would with just a regular HG. Since the clamping load is basically just a huge spring, it takes MORE pressure to lift it FARTHER.
I'd imagine that the o-rings also offer some enhanced lateral stability for the head.

I don't see how a doubled concentric ring would help that much. It WOULD seem to give more lateral stability to the head (holding it in place side to side). But if the head lifts enough to overcome one O-ring, it's lifted enough to overcome both, I'd think.

** Up to the limit of block/head distortion, more clamp load is ALWAYS a good thing**

I don't like 14mm studs because with a huge amount of torque on them, they can distort the block enough to warrant remachining the whole thing.

I can't tell you if head deflection (by that, I take you to mean actual flexing of the head) is an issue before failure.

They've successfully used a "fractured cap" design to prevent cap walk on connecting rods. It would be nice if you could design an interference-type locating means for the head. The main problem IMHO is that we are asking the studs to locate and clamp the head. Maybe you could BOMB the existing head locating dowels to make the head more laterally stable? Maybe a freeze-fit dowel (hit the dowel pin with liquid nitrogen, then fit it in the block)?

For my HP needs, the upgraded clamping available from the Don's ARP studs will hold just fine. If it doesn't, I'll look at MLS/ARP combo. I don't like O-rings if I don't need them (fire rings are worse) because I feel that the deck surface of the block and the head are more vulnerable to cracking with them installed.

jlh

GO 4LO

Sorry - forgot about this thread! Thanks for all the info, Hohn. Couple questions for you:
1. Could you explain the 'fractured cap' design a little more?
2. Would it be possible to use something deeper than an o-ring? Like the dowel pin you're talking about, but make it a 1/2" tall cylinder fit into 1/4" deep grooves. Then the head would have to lift 1/4" before failure?
3. Since you add more fuel at higher boost, would that added fuel spray (and water injection) drop the air temps much in the cylinder?
Thanks!
Chris

HOHN

Fractured caps are made differently than traditional connecting rods. A regular rod has two pieces machined to fit together, and their mating surfaces are smooth. Because the mating surfaces are smooth, they have little friction with each other. The rod bolts/capscrews have to provide enough clamping to prevent cap walk using this very low level of friction. Moreover, the interference fit of the capscrews is what locates the cap on the rod, so alignment is determined by the tolerance of the bolt and its hole.

This is where the ARP wave-loc rod bolts were so revolutionary. Not only were they less susceptible to stress risers on the bolt's shanks, the wavy sides made for a much tighter fit of the bolt in the cap.

With a "fractured cap", the rod bolts have a much easier job to do.

Factured cap rods are made differently. Instead of starting with two pieces of metal and machining them to fit, you start with a one-piece rod. Then, it is cooled to a VERY low temperature (say -350F) using a refrigerated gas like liquid nitrogen. Then this cooled rod is subjected to a sudden force, like being put in a metal press brake or chiseling at a point.

The rod shatters because it's so cold, but it does so right along the line where the impact was! Thus, the engineers can make a rod that fractures into two pieces, and THE TWO PIECES MATCH PERFECTLY.

Since they match so well, the cap naturally wants to be aligned on the rod. Now there's an interference fit giving the rod cap lateral stability so it won't walk around. Since the rod doesn't need as much clamping force now, the maker can use a smaller rod bolt/screw, which allows them to make the rod stronger.

As for head sealing, I'm sure that a deeper grooved ring could be used, but most blocks and heads won't take it that well. The deck surfaces are critical for the strength and stability of the head and block.

The ultimate solution to head sealing is for the head and block to be one piece of metal with no joint. That would give you great sealing and stability, but the valve jobs would suck!!

I don't understand what you're asking in your third question.....

jlh

ramtd02

just so you know...there is a truck here in edmonton that is running 11.40@129mph, and he is running 142 psi of boost on the track....this engine has been going for 2 years with no rebuilds....how ever he does lower it too 100 psi when he is on the street.

GO 4LO

Thanks, Hohn,
I understand what you're saying about the fractured caps now. In my third question, I'm referring back to where you were talking about the drastic rise in internal cylinder temps during compression of the intake air. Will the injection of additional fuel cause more or less heat to build in the gas as it's compressed? The fuel will no doubt be cooler than the air, right, but it will also be basically incompressible. In other words, will running excess fuel make cylinder pressure better or worse?
Also, water injection, as I understand it, helps absorb some of the heat generated during combustion (right, or no?). If we assume that more water is injected as fuel and boost are increased (and at a rate that keeps egt's constant), will this keep cylinder pressures constant, or will those remain high due to the incompressibility of the water?
Thanks again.
Chris

GO 4LO

Wow. Any idea what turbo setup and fuel system he's running?
Chris

HOHN

Chris-
The moment of injection causes a very brief cooling of the air as the the fuel vaporizes. Just as sweat evaporates (vaporizes) and cools you, so with the compressed intake air. But this is only for a nanosecond!
Fuel is injected as a fine mist which then vaporizes and combusts. Diesel fuel does not burn well as a liquid, but when vaporized-- LOOK OUT!
So, initial injection cools the cylinder a bit, but the combustion that immediately follows cranks up the temp. The more fuel, the more it's initially cooled (takes more heat to vaporize the fuel), but then the burn that follows is also bigger (assuming you have enough air).

Compressibility of the fuel is a non-issue. The volume of fuel injected compared to the available space is tiny. Also, the fuel vaporizes so fast that you really don't have a liquid to compress-- it's a vapor (more like a gas).

The answer to running excess fuel depends on how much excess. As fueling is increased (with the amount of air held constant) you will hit a point where you don't have enough Oxygen to burn it all. Then it will start to smoke.

But you can increase fueling past this point and still make more power. So you see a lot of big hp trucks making a lot of smoke. Not very fuel efficient, but the power is there.

Now if you increase fueling PAST the point where power is increasing, you've basically "put the fire out". Here, you're injecting so much fuel that the cylinder temp drops so far that it won't support combustion. Since there isn't enough heat to BURN the fuel, just OXIDIZE it, you will see HUGE amounts of smoke!

One reason the big HP pullers can run so much fuel is that they are running cylinder temps (and EGT) so extreme that the engine is helping to keep enough heat to burn the fuel. Hot pistons, head, exhaust valves, etc all help to keep in enough heat to light the fire.

Think about how a CTD is sluggish and unresponsive on a cold start in cold weather. Diesels run best (and most efficiently) when they're running on the hot side of liveable.

As for the water injection--

Water injection doesn't really "absorb" much of the heat of combustion. Besides, that heat is what makes power-- do you really want it dissipated??
Water injection cools the intake air because it takes heat to vaporize the water. Because it's injected as a fine mist (sound familiar?), it takes heat away from the intake air (like sweat).

Water injection increases the MASS FLOW going through the engine. Because more mass is flowing though it, each given unit of mass can carry less thermal energy and keep the total constant. It's like having 100 bags of sand w/ 5lbs in each, instead of just one big 500lb bag.

The increase in mass also gives it a kind of "thermal inertia". In other words, it will both cool off and heat up more slowly. The same amount of heat that will boil a drop of water (like a drop on a hot skillet) will barely raise the temp of a gallon of water at all.

Finally, in the high temp of combustion, the water molecule can separate into hydrogen and oxygen gasses. With more O2 available, the fuel will burn faster, so EGT will be less by a little bit.

I can't really say what happens with cylinder pressures, as that depends on where the timing is set, how much fuel, air, and water are all being thrown in there. Generally, the water will take more time to heat up, so your pre-injection peak temp will be lower.

The water's incompressibility is a non-issue because you're not injecting that much. You'd have to get really sick with the water to approach hydraulic lock.

Justin

HOHN

I'm guessing some nasty RV275s and an EZ

broken_axle

I'm not much for brains but I kept mine at 110 PSI for 80,000 miles. But my coppers gave out on the head shorty there after. Like I said I'm not much for brains. I can tell you that it can dig into the walet fast! I don't know what went wrong but Cummins thought it was funny . This time I plan to listen to the old guys and know when to back off. It's all fun and games till some one gets an eye put out

ramtd02

He's got a b-1 bomber and a big brother.....and hes also running mach 6 injectors. with the ez drag comp and tst pm3 stacked.