Common Rail Timing and Durability

[DIESELMAN]

That brings an interesting question... . What alloy are the pistons made from? Most casting alloys lose solidus by about 1060F and are liquidus ( melted ) by 1200 - 1210F.



Theres a lot of faith in thermodynamics that keeps them just cool enough not to melt even when everything is normal.



I didn't know there was any aluminum alloy or alluminum casting alloy that is greater than about 1240 - 1280 F melting point. Never thought about that part.

 

[radar doctor]

ELECTROMAGNETIC DEGUSSING AGENT

:{ Good grief :--) After reading all that stuff, I'm Afraid to START MY TRUCK Oo.

 

[jimnance]

I think the key is that the duration is the explosion is brief, after which fresh cool air is inhaled (sucked) into the cylinder providing needed cooling. Also, I understand the new high output engines have nozzles inside the engine which squirt oil on the underside of the piston. Again, this is a cooling aid.



Automotive gasoline engines have the same kind of heat to deal with. In fact, sometimes hotter - my EGT's in my aircraft engine are often 1550 F and uses Aluminum pistons.



Diesel engines have much higher cylinder PRESSURES which is why they tend to be heavier than thier gasoline cousins. Generally just about everything in the diesel engine needs to be stronger so the pressures don't crack or break something.

 

[Mathew Clausen]

Regarding exhaust temps - as I understand diesels have a longer combustion cycle, therefore the flame is in contact with the piston longer which allows more heat transfer to the piston compared with gas. Also - gas engines are sucking in fuel/air mixture which has more of a cooling effect from vaporizing the fuel than straight air in a diesel. Both of these combine to keep the piston cooler for the same exhaust temp or allow a higher temp before piston melting. I would think piston material melt point is very similar between diesel and gas (both aluminum).



An interesting side bar - all of the larger diesels in trucks (10 - 15L) use two piece pistons now. The upper part of the pison (bowl and ring lands) is STEEL, with the only aluminum part being the skirt.



Mathew

 

[rbattelle]

Bear in mind there's a huge difference between the EGT and the material temperature of your aluminum pistons. Rest assured your pistons are nowhere near the yield/plasticity point during normal operation.



As for the question about the effect of combustion chamber shape on diesels... what do you mean "vertex"? Did you mean to say "vortex"?



There is no shortage of experimetal diesels with novel combustion chamber arrangements. One of the best is a divided chamber, where combustion occurs in a chamber separated from the piston bore by an orifice. This type of engine allows much higher operating temperatures and pressures, and vastly increased resistance to detonation, plus enhanced mixing by the very high velocities experienced through the orifice. But they're very expensive to manufacture and modern open chamber designs (i. e. our trucks) have reached great new levels of efficiency and power.



To be honest, I'm not entirely sure what the advantages of a hemispherical chamber on a gas engine are. Seems to me it was done simply to make a production engine that more closely mimicks a spherical bomb (commonly used to test IC engine combustion characteristics). I suspect the lack of a "hemi" in a diesel may be due to strength issues (expensive to make a strong hemispherical chamber?).



From what I've read, it seems like using "squish" pistons is the best balance between cost and performance. A "squish" is really just a dish in the top of the piston which significantly increases turbulence strength during the compression stroke, which further increases mixing, and hence greatly improves power/efficiency over "traditional" flat-top pistons. I believe a similar effect can be achieved by a properly shaped upper combustion chamber.



As an aside, one way to improve diesel efficiency is to introduce hotter components such as glow plugs, which increase the evaporation rate and reduce delay angle (crank angle between start of injection and flame) as well as rate of pressure rise. Besides glow plugs, it's possible to achieve a similar effect with a stainless steel insert at the top of the piston (which becomes a nice hot surface for evaporation). I wonder if Cummins has tried this and decided it's not worth it?



Have I droned on long enough!? Sorry, I just love these sorts of engine discussions.

 

[Big MAK]

Other then during cold weather glow plugs don't improve combustion efficiency; they're only to pre-heat the combustion chamber -- serving the same purpose as a grid heater except the grid heater pre-heats the air before it goes into the combustion chamber (grid heater = better design for same effect). Once the block/chamber is at operating temperatures they both have no purpose.



I would think normal operating temperature is as hot as you would want to get -- let's assume melting the engine isn't an issue -- there'd still be an upper limit in temperature; i. e. some point at which the fuel detonates w/o compression. I would think that would be too hard to control; it's too variable. Fuel additives, altitude, etc. Way easier to regulate air/fuel then expansion volume.

 

[phoekema]

another major difference

Along with all the other differences between diesel and gas engines that have been mentioned, there is one major one. In a gas engine fuel enters the engine under a vacuum so the injectors need relatively low fuel pressure to work. Fuel is injected in a diesel, however, at the peak (or very close to it) compression pressure and therefore the injection process requires extremely high pressures in comparison to a gasoline engine.

I don't have a degree in this area, but I marvel that it is even possible to combine such high pressures and precision re: timing and amount of fuel delivered. Ol' Rudolph Diesel must have had himself one powerful squirt gun when he made the first diesel engine! I wonder how many 'experts' told him "that will never work".

 

[rbattelle]

Although glow plugs are not generally operated once the engine is up to temperature, the presence of a hot surface within the combustion chamber (i. e. necessarily hotter than the majority of the chamber) during normal operation will increase efficiency and durability by reducing delay angle and rate of pressure rise (primarily because the rate of evaporation of the fuel spray is increased).



If we ran higher-quality fuels less prone to detonation you could run a set of glow plugs constantly and see great benefit.



Are there any engines out there that run with glow plugs operating all the time?

 

[DIESELMAN]

Well thats one more thing the PSD&Cheby has in common with toys..... Glow Plugs, same as an . 049 Airplane Engine, same technology, same sound, we all outgrew toys though didn't we?



BTW, I think the highest metal temp in a diesel combustion chamber is only about 600F sustained at 1100 EGT. ( annular area around the piston face midway from the center to the edge, but who cares, right Oil splash or 'J' jets cool this area from underneath) Even the valve edges and faces are cooler.

 

[tgbol]

LADYJANE

The piston on most diesels have a cup or depression for the fuel to spray into and form the cumbustion chamber with a flat head surface. although a hemispherical head will breath better the squish area is too big for a diesel. Also the lower rpm of diesels and the use of turbos overcome the lack of air flow in the head design. The greatest advantage of the 24 valve head is that the injector is centerec in the combustion chamber, not the flow of 4 valves.

 

[ladyjaine]

Squish Pistons

OK, I can understand how a dish in the piston would increase efficiency of combustion. It more closely approaches a sphere, a very efficient shape. Is the dish in the piston aligned with the injector? That is to say, are the injectors centered at the top? I should think so. I understood that gasser's injectors didn't have to be centered at the top, because of the mixture of fuel and air that the cylinder was compressing. If my 'picture it' is working right, a diesel injector should produce a circular shaped spray that covers the area of the piston head. More like a shower than a faucet, but very high pressure.



Jean

 

[rbattelle]

The dish covers a relatively large portion of the piston top, but it's purpose is not to bring the chamber dimensions closer to that of a sphere. Rather, including a squish (or dish) increases the turbulence intensity near the top of the stroke, which improves mixing and evaporation.



It's important to remember there that in a diesel we're dealing with relatively large "droplets" of fuel; gassers typically have much better atomization. Combustion will occur around the edges of the droplets where fuel/air ratio reaches the correct value. So the faster you can evaporate those big drops into tiny ones the more efficient you'll be. High levels of turbulence are a great way of helping things along.

 

[Vaughn MacKenzie]

Operating EGTs

There are a couple reasons I can think of why max "safe" sustained EGTs are lower on a diesel than a gasser:



At compression, the pressure is dramatically higher, party due to the high compression, party due to the full gulp of air on each cycle. The peak cylinder temperature certainly is MUCH higher as a result. And then we haven't even added boost yet. This isn't necessarily reflected in your EGT gauge because once the gases expand, the temperature drops. The temp spike, though brief, is far higher than that of a gasser. At 30lbs of boost, cylinder pressures and probably temps are roughly 4 times higher than a gasser at WOT.



If you start pushing the piston material to the point it begins to soften, it's gonna let go a lot sooner than a gasser due to the incredible cylinder pressures. So we need an additional margin of safety here.



Rudolf Diesel when he was starting to build diesels had one explode on him. If I'm not mistaken, the early diesels did not have direct injection (technology & means were not available). I don't know how fuel was introduced to the cylinder back then, it may have been injected directly to the cylinder during the intake stroke and then combustion initiated by spark, running lower compression ratios (just a guess).



I had an overhead valve lawnmower once which I was able to run on pure diesel. It was not possible to start it on diesel, I had to repeat several cycles of putting gas down the carb until it had enough heat to run on the diesel. Smoked like a banshee until it got warm, but incredibly once it warmed up it was totally smoke-free and had plenty of power to mow the lawn. Combustion was noticeably sharper, more vibration. If I encountered deep grass, it rattled badly and smoked when lugged down. Normal grass, it purred right along. Smelled great! When I shut it off, it took it about 30 seconds to slow down and stop due to self-ignition. The spark plug was contributing to the process when I was mowing the grass though.



Vaughn

 

[Big MAK]

We're not worthy

Define "Diesel Enthusiast"



- someone who runs their lawnmower on diesel; apparently just to see if they can.



Cool!



Oo.

 

[Ray Torresdal]

N14 milage

Jimnance: Since when do you see N14's getting 7+ mpg? I've been driving an older N14 part time, grain hauling with standard height grain trailer, loaded one direction, empty the other, and 7 mpg is something the guys with the big Detroits see, not me!!! Granted, since it has been cold, there has been more idle time, but the owner assured me that my milage was par for that engine, and typically 1/2 to 1 mpg less than the Detroits. I would like to see comparative test data on the big rig engines, milage under similar conditions, MTBF, etc.



Ray

 

[jimnance]

We use a card lock fuel service, and this give me a very accurate accounting of fuel economy on each unit since the driver must enter the odometer reading when fueling.



I know we are averaging 6. 7 to 6. 8 consistantly. Most of our trips are local averaging less than 150 miles with about six stops per trip. Our loads are light only about 21,000 lbs, but bulky. It's flatbed work, and when the wind blows it pulls like 45K.



My buddy with over 100 trucks in his fleet averages about 7. 0 MPG, but you are correct. I was thinking he was running Cummins, but he is running Detroit's.

 

[DiTrani]

EGT

It's my understanding that EGT measured in the exhaust is not a direct measure of temperature in the cylinder. The combustion process is ongoing when the exhaust valves open and the mixture is burning as it exits the cylinder. I think peak temperatures actually occur as the gases expand into the exhaust manifold/header.



So 1300* measured pre-turbo doesn't necessarily mean that your pistons are seeing that same heat.

 

[rbattelle]

cditrani,



The process you describe sounds more like a 2-stroke than a 4-stroke engine. I believe the vast majority of burning is completed by the time the cylinder is exhausting, partially due to the extra-long stroke.

 

[DiTrani]

While the phenomenon is more fundamental in a 2-stroke, it happens in 4 strokes as well.

 

[HBarlow]

Diesel Technology

I'm writing to say Thank You to those of you who have contributed to this discussion. It is generous of you to provide this free classroom training to "students" like me. I hope discussions like this one continue.



I've been sitting here on the sidelines soaking it all up. Each of you have contributed to my modest knowledge and understanding and I appreciate it.



Harvey