Diesels & altitude

[Anonymous]

In 2 stroke diesels, some of the boost is used to scavenge the cylinders. At low rpm/low boost, the intake ports and exhaust valves need to be open long enough to do this. Would variable exhaust valve timing be more beneficial in achieving higher cylinder pressures at full power/high boost than a larger turbo? Do any 2 stroke diesels do this?

[jchnhtfd]

At the risk of starting another hare -- there is a seemingly unrelated issue involved in high altitude operations, which is one of the major reasons why mudchicken sees units under test: cooling. Without bothering with all the details, suffice it to say that a radiator of a given size can't cool an engine as effectively at high altitude as at sea level, all other things being equal, and most manufacturers worry that maybe, just maybe, at full power things may get too warm... Not usually a problem in automotive applications (automotive/truck radiators are moderately to hilariously oversize, in most applications) but very much a problem with a railway engine.

[kenneo]

QUOTE: Originally posted by jchnhtfd

At the risk of starting another hare -- there is a seemingly unrelated issue involved in high altitude operations, which is one of the major reasons why mudchicken sees units under test: cooling. Without bothering with all the details, suffice it to say that a radiator of a given size can't cool an engine as effectively at high altitude as at sea level, all other things being equal, and most manufacturers worry that maybe, just maybe, at full power things may get too warm... Not usually a problem in automotive applications (automotive/truck radiators are moderately to hilariously oversize, in most applications) but very much a problem with a railway engine.

That is the reason that SP had EMD make the Tunnel Motors. After two SD 40/45's transit a tunnel or snow/rock sheds in the Cascades or Sierras, the units behind the lead two start to overheat. Then they shut down and this causes all sorts of bad things to start happening. And this is happening at altitudes 1/2 as high as Rollins and Winter Park. Tunnel Motors have a larger radiator as well as air intakes down where the air is cooler.

[Leon Silverman]

To answer up829 question about variable valve timing; variable valve timing will not supplant the benefits of a larger turbo, which presumably would also provide a higher boost. Cylinder air has mass. This means it also has inertia and momentum. Increasing valve timing (duration and overlap) allows an engine to develop more power because it can breathe easier at high rpms. The increased valve timing gives the cylinder air more time to exit the cylinder by givng the air mass more time to get moving out of the cylinder ),overcoming its' inertia. The increased overlap (simultaneous opening of exhaust and intake valves) sucks more fresh air charge into the cylinder due to the siphoning effect of the fast moving exhaust gases. The momentum of the incoming air continues to add mass to the cylinder even after the piston has reached the bottom of its stroke. To take advantage of this, the intake valves are not closed until after the piston starts to move up again. This effect is usually not noticed until an engine reaches or exceeds about 3,000 rpm.
The downside of this effect is that when use a cam timing that develops high horsepower at high rpms, you have a relatively unresponsive engine at low speeds) off the line. Conversely, an engine set up to hit its' torque peak at low rpms will be very responsive in stop and go traffic but is weak passing cars at highway speeds.
Variable Valve timing gives you the best of both worlds . You wind up with an engine that is responsive at both high and low rpms.
Diesel engines utilize a heavy construction in order to withstand the high compression ratios. This heavy construction limits the maximum rpms that the engine can safely operate at. Diesel engines develop a lot of torque at low rpm because they are generally large displacement engines. Changing valve timing so that a diesel can operate at higher rpms is a waste of time unless you can also reduce the weight of the reciprocating masses (pistons, crankshafts, and connecting rods) to permit the higher rpms. Increasing turbo boost can increase torque output and horsepower without requiring the engine to operate at a higher rpm.
As I stated before, the horsepower boost from free breathing usually starts at around 3,000 rpm. Since locomotive diesels operate at a maximum of 1000 to1200 rpm, increased horsepower requires high boost pressures. Valve timing will not do it.

[Anonymous]

A rough rule of thumb for piston engines is a 7% loss in hp for every 1000 feet gain in altitude. For instance, your 300 hp normally aspirated truck drops down to about 200 hp at 5,000 feet. At 10,000 feet.... Well, it's gasping for air, right?

A turbo CAN negate this, if it has a gate on it that normally liimits the pressure boost under normal altitude operation. At high altitude the turbo would then be used at 100% of its capacity to keep things humming along. If that 4400 hp locomotive normally has 30% of the turbo output bypassed, then at roughly 5,000 feet the bypass would close down and you'd still get 4400 hp out of it. At 7,000 feet you'd get maybe a 15% reduction in power.

I personally don't know how the locomotives have their turbos set up. More turbo can give you better performance at altitude, but costs more and takes up more space.

Mark in Utah

[Anonymous]

Of course it would be impossible to make a 9 bearinf V-8, and I am well aware that locomotive V-s use "common" or paired mains. I was merely pointing out that the crankshaftin the IH v-8, as manufactured, would not accept a greater amount of turbo boost without failure. Therefore, the Cummins equipped Dodge would be easier to set to overcome altitude loss of natural air density.

[Anonymous]

In trucking we had computers on our engines that understood what the "outside" was like and understood what needed to be done to create the power.

When I came thru Eisenhower (over 12,000 feet) in colorado I did not detect any "driveability" issues with the Cummins I had under my hood. Even though I was suffering from oxygen problems being that high up from sea level. (Dont smoke like I did for many years)

I have flown in private planes at 13,000 feet and can attest to the loss in performance both at the wing and engine. Never mind the threat of sleep that kills. (Hypoxia? -spelling)

There is always a limit somewhere at a point above land on the earth that a desiel will fail to run.

This remains true for all "Airbreathing" engines and people.

Large desiels are my personal favorites when it gets VERY cold. You need to keep it lit and the fuel warm. As long you have warm fuel and able to keep it lit then you will survive.

Since my trucking days in the rockies are over, I am quite content to remain near sea level. On a recent flight on southwest I could tell the cabin pressure was "Higher" in altitude than what I am accustomed to being at.

[Anonymous]

QUOTE: Originally posted by jchnhtfd

At the risk of starting another hare -- there is a seemingly unrelated issue involved in high altitude operations, which is one of the major reasons why mudchicken sees units under test: cooling. Without bothering with all the details, suffice it to say that a radiator of a given size can't cool an engine as effectively at high altitude as at sea level, all other things being equal, and most manufacturers worry that maybe, just maybe, at full power things may get too warm... Not usually a problem in automotive applications (automotive/truck radiators are moderately to hilariously oversize, in most applications) but very much a problem with a railway engine.

They aint big enough. You will learn this when you try to cross the dead valley to the western areas of Nevada in summer. My problem with older cars and some trucks was actually radiators that were too small and did not contain enough evaporative area to get the heat out.

There are two kinds of cooling. RAM air cooling is helpful when a vehicle is at speed the pressure of the air flowing into the vehicle's front and the "pass thru"

The other form of cooling is simple radiating. This happens when you are in hot traffic stuck idling at gridlock stop and go. All vehicles have a limit.

Some of the aircraft engines such as the Pratt and Whitney are marvelous in not needing radiators other than what air cooling already provides. Although I venture that they operate in atmosphere that is quite freezing and that helps alot.

[adrianspeeder]

QUOTE: Originally posted by jwalpacific

Of course it would be impossible to make a 9 bearinf V-8, and I am well aware that locomotive V-s use "common" or paired mains. I was merely pointing out that the crankshaftin the IH v-8, as manufactured, would not accept a greater amount of turbo boost without failure. Therefore, the Cummins equipped Dodge would be easier to set to overcome altitude loss of natural air density.

I sniff a cornbinder hater. Perhaps he hasn't heard of every frikin powerstroker like me that mod the heck out of our fords and get waaaay over stock boost and never snapped a rod. It will take a lot more than just high boost to brake something on any engine. Lots of air is useless if there ain't enough fuel to use it. Next on my list is propane injection to get that ford up to the modded cummins slayer level. My V8 will whooop your I6... [:D][:D][:D]

And what is with this anti intercooler stuff? The increased heat from compressed air will offset increased boost. The only thing an intercooler hurts is turbo lag time. That can be solved with a heavier foot when reving before dumping a clutch, or a higher stall torque converter for the slushbox fans.

Adrianspeeder

[Anonymous]

QUOTE: Originally posted by Leon Silverman

To answer up829 question about variable valve timing; variable valve timing will not supplant the benefits of a larger turbo, which presumably would also provide a higher boost. Cylinder air has mass. This means it also has inertia and momentum. Increasing valve timing (duration and overlap) allows an engine to develop more power because it can breathe easier at high rpms. The increased valve timing gives the cylinder air more time to exit the cylinder by givng the air mass more time to get moving out of the cylinder ),overcoming its' inertia. The increased overlap (simultaneous opening of exhaust and intake valves) sucks more fresh air charge into the cylinder due to the siphoning effect of the fast moving exhaust gases. The momentum of the incoming air continues to add mass to the cylinder even after the piston has reached the bottom of its stroke. To take advantage of this, the intake valves are not closed until after the piston starts to move up again. This effect is usually not noticed until an engine reaches or exceeds about 3,000 rpm.
The downside of this effect is that when use a cam timing that develops high horsepower at high rpms, you have a relatively unresponsive engine at low speeds) off the line. Conversely, an engine set up to hit its' torque peak at low rpms will be very responsive in stop and go traffic but is weak passing cars at highway speeds.
Variable Valve timing gives you the best of both worlds . You wind up with an engine that is responsive at both high and low rpms.
Diesel engines utilize a heavy construction in order to withstand the high compression ratios. This heavy construction limits the maximum rpms that the engine can safely operate at. Diesel engines develop a lot of torque at low rpm because they are generally large displacement engines. Changing valve timing so that a diesel can operate at higher rpms is a waste of time unless you can also reduce the weight of the reciprocating masses (pistons, crankshafts, and connecting rods) to permit the higher rpms. Increasing turbo boost can increase torque output and horsepower without requiring the engine to operate at a higher rpm.
As I stated before, the horsepower boost from free breathing usually starts at around 3,000 rpm. Since locomotive diesels operate at a maximum of 1000 to1200 rpm, increased horsepower requires high boost pressures. Valve timing will not do it.

I was thinking more along the lines that the increased boost in Run 8 might decrease the scavenge time beyond the reduction provided by the modest increase in RPM. This would depend on the difference in boost between Run 1 and Run 8 and whether these engines are already optimized for Run 8 or compromised in order to allow cold startup. If I understand these engines correctly, the incoming charge is not compressed in the crankcase like a 2 stroke gasoline engine, instead a blower or turbo-supercharger is required to scavenge the cylinders.

[broncoman]

Does anyone know the reasoning that EMD decided to use a gear driven/exhaust overdriven turbo instead of just adding a turbo to the existing supercharger like. Great story by Mr. Strack.
On a diesel, do you get an increase in HP with increases in compression ratios. If so had any railroads experimented with this for units who spent time mainly in higher areas to compensate?

Dave

[Overmod]

bronc, there are some very good accounts on the Web about the EMD overrunning turbo.

Basically, there just isn't enough exhaust volume at the lower runs to produce spin on a turbo of 'economical' best size for full power. You might get around this with multiple small turbos, but that's not particularly economical either in terms of equipment or maintenance. What EMD did was recognize that you'd get better boost off a mechanical drive at low rpm, phasing over to turbo boost at high rpm, and eliminate essentially 100% of the turbo lag problem at the same time.

If there's a problem with EMD's implementation, it's in the original design of the overrunning clutch. Randy and some of the other wise diesel heads on this forum can tell you how they go bad, and probably how they could best be improved.

One point about the Roots blower as fitted to 567s (and 645s a la GP38) is that they're positive-displacement blowers -- developing air movement and pressure even at comparatively low speed, and increasing in proportion to driving rpm -- as opposed to things like centrifugal compressors (which most turbos effectively are) which require relatively high rpm to make effective boost. Note that a gear-driven turbo is an intermediate stage in this respect, which allows you to get rid of separate scavenge blowers if you want without sacrificing the advantages of turbo boost at high engine output...

[broncoman]

Overmod,

I probably didn't word my question right. I agree with what you said. My question was more towards why EMD didn't just keep the roots charger and add a turbo instead of designing a new turbo that would incorporate a way to produce that boost mechanically until the exhaust was such to run it as a normal. In my profession I have seen 16v-149 gensets that have a turbo blowing through the roots blower. I was assuming this is how the marine engines do it also. Most of the time I am learning to repair 6v and 8v 53s and 92s. I was curious as to why EMD didn't go this route and instead designed a turbo to do both jobs. It seems a better deal in that you have one item doing two jobs. From what I have read on other posts the turbo assembly is only good for about 5 years. I didn't know if this was a result from it having to do both jobs or was this due to environment?
So back to the question stated better; why did EMD go this way instead of Detroit Diesels way?

Randy?

[rvos1979]

QUOTE: Originally posted by adrianspeeder

QUOTE: Originally posted by jwalpacific

Of course it would be impossible to make a 9 bearinf V-8, and I am well aware that locomotive V-s use "common" or paired mains. I was merely pointing out that the crankshaftin the IH v-8, as manufactured, would not accept a greater amount of turbo boost without failure. Therefore, the Cummins equipped Dodge would be easier to set to overcome altitude loss of natural air density.

I sniff a cornbinder hater. Perhaps he hasn't heard of every frikin powerstroker like me that mod the heck out of our fords and get waaaay over stock boost and never snapped a rod. It will take a lot more than just high boost to brake something on any engine. Lots of air is useless if there ain't enough fuel to use it. Next on my list is propane injection to get that ford up to the modded cummins slayer level. My V8 will whooop your I6... [:D][:D][:D]

And what is with this anti intercooler stuff? The increased heat from compressed air will offset increased boost. The only thing an intercooler hurts is turbo lag time. That can be solved with a heavier foot when reving before dumping a clutch, or a higher stall torque converter for the slushbox fans.

Adrianspeeder

Adrian,

I've got nothing against IH diesels, I'm partial to the DT466 myself, I'm just not real thrilled with the HEUI setup in the DT444E (7.3 Powerstroke). I can't say how the six liter engines are, though, never played with them yet (I'm VERY interested in that variable geometry turbo, though, the turbo kit from Banks sounds like it will eliminate lag).

I was digging through my old Mopar magazines, got some numbers for you to shoot for. This was done on a chassis dyno, a guy at the Carlisle Nationals in PA made 763HP and 1470ft/lb torque with his '01 Cummins ISB.

I'm reminded of an older Ram ad:

"It's what you get when you cross a diesel locomotive with a really nice sofa."

Randy

[Overmod]

bronco, I'd always thought that the principal reason for the elimination of the Roots blowers on the turbo EMD engines was system cost. But remember that a Roots, being a positive-displacement blower with very fine internal clearance between the rotors, is only going to pass a given volume per minute, with that volume being determined entirely by what the drive gears -- and the crank rpm -- are providing. Meanwhile, the turbo boost on these big, slow engines is very peaky, with proportionally higher amounts coming on only in the very highest run settings. (Randy Stahl can tell you much better than I can exactly where the boost comes up to meaningful numbers). The point here is that the only effect the turbo would have when feeding through the Roots would be to pressurize the air, increasing its density, at the entrance to the blower -- and I think this would produce more back-pressure than necessary, not exactly the thing you want for good clean scavenging on a two-stroke diesel of this design, precisely at a time when what you most want is no restrictions in the intake tract...

Personally, I think 763/1470 out of a built 6BT is conservative. Last I looked, even the for-publication numbers of some of the Enterprise engines were north of 1100 nominal hp, and the more successful competitors in pulls aren't saying... This is NOT a power rating you're likely to be able to get out of a 6.0 or Duramax without re-inventing the grenade... among other things, aluminum heads no matter how you try to cool them won't take the EGTs as well. I've been waiting several years now to see a PowerStroke (or Duramax) version of that 240-mph Cummins-powered Dakota (or whatever the small-body pickup was) -- surely there are some 200-mph-plus Ford diesels running around out there (probably in the offroad-racing world), but I haven't yet seen any...

Not that you can't make pretty good power out of the 7.3, though. Diesel Dynamics put one in an Excursion for one of the aftermarket-equipment folks... I don't remember who, but adrianspeeder probably knows... and even with all the silly bars and roof racks and stuff increasing the curb weight, the thing ran something like a 12.7 quarter at 127mph (I invite someone with the exact figures to provide them). I think that's reasonable performance by anyone's standards...