Which Scenario Wears Diesel Engine Less?

[ISX]

I haven't been able to find anything on this topic and maybe it's because the wear is insignificant but I would still like to know, no matter how insignificant the wear, which scenario wears the engine less.

This is on a diesel engine..

On a 0F day, 3 identical diesels start up at the same time and after 30 seconds to get oil pressure and everything moving, diesel 1 takes off, slowly getting up to speed. Diesel 2 turns on the exhaust brake to put a load on the engine and sits there. Diesel 3 just idles.

As each diesel hits 100F coolant, how would they rank in wear over the period from the end of that initial 30 second wait until each hits 100F? As in, which one would wear the engine the least and which would wear the engine the most, and more importantly, why!? I have read about people saying idling causes twice the wear as driving it and vice versa and I'd like to see what you guys say about the issue.

I can see idling it for a long time being harder on it since the engine doesn't warm up as fast, so it would have a cold engine block being a huge mass of steel, yet the aluminum pistons would easily capture heat and expand but without the block being able to expand until later as it slowly heats up.

Starting and driving or idling with e brake puts a load on it which would expand the piston even more (since its even hotter) but the engine heats up quicker. So I still can't determine which is better.

As always, thanks for any and all help!

 

[jedishrfu]

wikipedia has an article on diesel performance that might help you figure this out:

http://en.wikipedia.org/wiki/Diesel_engine

 

[ISX]

That's about idling after engine is up to temp and starting it in the cold weather, not what happens in between those 2 times.

 

[cmb]

It's rather dependent on the engine, frankly. Tolerances, risk of bore wetting from the cold diesel spray, etc., not to mention whether you drive nice-'n'-slow or peg it like a race car from cold - driving style plays its part too.

But my considered opinion, having worked in automotive reliability for several years, is that if we're talking about a good modern diesel engine for a passenger car, already bedded in after a few 10,000 miles, then you get 1) least wear from the car driven off straight away, 2) more from the one left idling, and worst on 3) the one forced to heat up quick.

This is because in the case of (2) you have had the engine turning over for just as long as (1) but you've not yet gone anywhere! Total revs and engine time are higher for 2 than 1. The analysis is that simple! 3 is worst because it is like 2 but with side-load on the bore.

I have a small European diesel and I pull off immediately on startup. In fact, I typically have it in gear as I crank and engage the clutch as soon as the engine is running - on the first rotation, usually. I've done this for the 6 years I have had the car, and it's done 139,900 miles with no engine work at all during that time. It's still got the same clutch, even. How long does a car need to work faultlessly before you consider it 'negligible wear'?

I feel there are two other factors you are not taking into account. Firstly, modern diesels are so efficient that if you started a warm engine at 0C, let's say, then I'd expect the engine to actually get colder in the first few minutes. Diesels run 'open-throttle' and the throughput of air is actually a significant coolant to a diesel engine (unlike a petrol). The most efficient diesels take ages to warm up. Mine does +60mpg and takes 18 miles on the motorway to fully warm up. Leave it idling, it would simply never get up to working temperature. It's just too efficient.

The other thing is that any diesel less than 10 years old probably has a few exhaust treatments. If you idle one of those from cold, all you will likely do is contaminate the catalysts/particulate traps unduly.

So, when it is really cold, do wait a moment for the bores to warm as the tolerances are tight, and the heads too so that the cylinder gas is not unduly cooled before ignition. But you should be able to hear that pretty clear. When it is really too cold to drive immediately, the roughness in the engine will tell you the bore tolerance is a little tight. Just wait 'til it is past any period of lumpy idling and is idling as smoothly as you'd expect it too, then you're good to go.

 

[cmb]

I'll just add - one of the points on a diesel engine that has what I consider an 'unnecessary' wear opportunity is the turbocharger (which just about any diesel <10 years old now has). The problem is that when you first start the engine, the turbo has no oil pressure yet is driven to high rpm immediately.

There used to be a retrofit to military vehicles (it was a kit with its own NATO part number, I believe) to provide initial oil pressure to the turbo electrically to ensure that was lubricated before the engine started. But evidence appears to show (e.g. my car...and others...) that passenger car turbos these days seem to last the life of the vehicle OK without it.

But you should still care for your turbo - just in regards a start-up procedure, it should be evident not to rev the engine of a turbo car on start up. Similarly, when switching off, allow the engine to idle on for a bit if you've just left the motorway, so that the oil can cool the turbo and it can spool down to min rotation speed, before turning the engine off. It also helps even out the temperature on the head too, to avoid thermal fatigue in the head gasket.

 

[ISX]

That's more of what I was looking for. I know the wear is very insignificant but there is some difference in wear and I was just trying to find out what wore the least. Mine has 350k miles on it and yeah it doesn't show a thing. Mine also takes just as long to warm up, but is a 6500lb truck and gets 27mpg.

As for idling, mine will get to 100F eventually, maybe 20-30 minutes. I don't have an exhaust brake but a lot of guys do and it loads the engine more so it warms up faster. I know that idling vs. driving is an obvious fuel waster, but I was talking about just getting the engine to 100F from 0F, aside from miles you aren't traveling or time being burned.

I am very curious about this side loading with an exhaust brake. Can you explain this more?

Thanks for helping me.

 

[Pkruse]

You pretty well already have your answer, but I'll offer a slightly different perspective.

Worst wear always happens with a cold start up, even when ambient is warm. Second worst wear happens when you load the engine. The wear/load relationship is anything but linear, so high loads are worse.

Normally highest loads are during accelerations. So if they all keep their accelerations low, especially until the engine is fully warmed up, then wear is very minimum.

Clearly, the worst wear case here is #3. It was put under a heavy load while the engine was still cool.

Not much difference between 1 and 2, since the critical sections of modern engines are designed to come up to temperature very quickly.

 

[ISX]

I guess I just don't see much difference between the exhaust brake and just driving off. We have pyrometers to measure exhaust gas temperature and when you start it up the pyrometer will be around 250F just idling. Idling it with the exhaust brake only raises it to 350F. Driving it when the engine is 0F shows a huge jump in temp, you might see 600F just to go 55mph, once the engine is warm that temp will be around 450F. The exhaust brake doesn't have much effect at low RPM/low flow periods, so how would it be wearing more when the pyrometer shows that driving off is much higher? Not trying to step on toes just trying to figure this out.

 

[Pkruse]

RPM is not an indicator of load. At any given RPM, you may have either a low load or a high load, or anything in between. This is even true at idle speeds. A modern diesel is not even firing on all cylinders at idle, but it will fire different cylinders on every revolution of the crank. As soon as you put that brake on, you have put a load onto the engine, even though RPM’s stay the same. Load equals wear.

Keep in mind that Jake brake causes the engine to transmit a great deal of power to the truck, in the reverse direction. Closing it puts a great deal of load on the engine, normally load that would otherwise go into the air brake system.

Think of the Jake brake as a device that splits the engine. It is partly an engine and partly an air compressor. The Jake brake essentially drives a huge volume of air through a small orifice. (Yes, it is more complicated than that, but for the purposes of this conversation we can model it that way.) That turns out to be a large load normally driven by the kinetic energy of a moving truck in deceleration, but in this case driven only by the engine.

 

[cmb]

A modern diesel is not even firing on all cylinders at idle, but it will fire different cylinders on every revolution of the crank.

I've never heard of cylinder shut down on diesels before, let alone at idle! This may be a specific application you are thinking of in large, >6 cyl applications, but I have never heard of it for conventional light road vehicles.

In light vehicles, the current state of the art for diesel injection, around 2000 bar, provides such fine fuel metering (typically up to a few pilot injections, then up to several power injections, for each and every stroke) that I can see no advantage at missing a cylinder out.

The only fuel cutoff in conventional light diesels to date that I am aware of is when you're 'off-throttle' and less than ~1,500 rpm. Under those circumstances, the fuel is essentially cut off, until the engine drops below the designated idle speed again.

 

[Pkruse]

Thanks for the insight, cmb. Nearly all my diesel experience is in large commercial and industrial engines. When we worked with smaller ones derived from the automobile and small truck industry, we really did not give them much of a thought because they are normally installed into equipment that the owner will scrap out before the engine wears out. That is why the common term among our mechanics was “throw away engine,” because it was normally still working perfectly well without an overhaul when the equipment was “thrown away.” This is also consistent with how most private vehicle owners treat them. When the car goes to the junk yard, the engine is most often still good. If it is not still good, that is probably the reason for sending the car to the junk yard rather than replacing or overhauling the engine.

When new fuel saving technology is introduced into the diesel market, it goes to Europe first because new technology is expensive. They pay so much for their fuel, that new technology becomes economically viable there first. Then it hits the US trucking market, because truckers buy a lot of fuel and are willing to pay the cost of new technology to save fuel. Then about 15 years later you see it introduced into the American private vehicle market, once the cost comes down.

What you say here I saw 15-20 years ago in the large engines. I remember that Mercedes introduced it about that time in passenger sedans in Europe, but American trucks had been using it for several years by then. When questioned as to when they will export it to the US, his response was a simple, “Never. Their fuel is so cheap that the customer is not willing to pay 12-15,000 dollars extra to pay for this level of fuel economy.” Shortly after that, I rented a full sized Renault mini-van in Spain, about the size of a modern Caravan. It had this same technology, and got 45-50 mpg for the two months I drove it around southern Spain.

 

[cmb]

I am very curious about this side loading with an exhaust brake. Can you explain this more?

It was a comment that generally applies to a reciprocating piston under load. When the piston has a pressure on the crown, it will be thrust sideways against the bore because it is hinging on the crank (which is not central to the load vector). This is just large-body mechanics for standard pistons, nothing complicated if you draw out the picture, but it tends to be a bit more critical for small diesels for the following reasons - a) the piston rings (that are responsible for the compliance of the seal in the bore) end up under more sideways compression, and they are already under higher pressures on the chamber side, so they get a good load, and the metallurgy and tribology tend to be a notch above petrol engines to withstand the diesel engine environment, b) when cold [the subject of the question] not all the injected diesel may burn and instead may settle on the piston bore, washing the oil off the lining, thus reducing lubrication on the return stroke, and c) the return stroke also ends up with higher loading, cf a petrol engine, on account of the higher compression pressures.

I mention small bore diesels specifically because once you drop below around 500cc per cylinder then the spray pattern ends up having a limited space to operate in. So a smaller bore leads to a higher risk of bore-washing, thus elevated wear. To some extent, the advent of piezo >1500 bar injectors a few years back mitigates this and smaller cylinders can now be made more reliably, but, for example, when the VAG (Volkswagen/Audi/Skoda/Seat/Ford Galaxy) 4cyl 1896cc common rail (less so with the later 'PD') was in general use here in Europe it developed a reputation for developing oval shaped bores after around 70,000 miles. This is widely cited in industry press as being due to this wall-wetting issue. In this case, then, you can see that it is beneficial to get the engine hot quicker than slower to get to a point where unburnt diesel is less likely to occur, so I would tend to suspect warming it up by idling would tend to encourage 'ovalling' of the bores more so than just driving away.

As for the effect of the brake loading, the thing is this type of device tends to need a lot of power, and lots of power at low, idling, rpm means lots of torque, which means higher piston pressures thus more side loading. Sounds a perfect recipe to wear out your engine quicker than any other means to me!

One last point, there are other engine cycles that have less cylinder loading, for example the Atkinson cycle as used in the Prius. In this engine, the modified cranking arrangement can offset side loading from one part of the engine cycle to another. In general, engines might be [are] designed to minimise side-load wear behaviour during the most arduous moments of a pistons cycle, but the best way of reducing side load, all else being equal, is to use lower loading at higher rpm [rather than high loading at lower rpm], thus, again, favouring driving off. Eventually, large side load wear will ultimately end with 'piston slap', where the piston is translating across the cylinder during each cycle as well as up and down! In a diesel, I suspect it is common not to hear the early onset of piston slap [that would be easily detectable in a petrol car] due to the harsh diesel combustion note, so I doubt if users of diesel cars can really tell if their engine is beginning to show signs of such bore wear, until the compression plummets and the EGR system begins to bung up on a routine basis (indicating high crank pressures from blow-by gases).

I hope all that is useful info for you.

 

[Pkruse]

CMT: Thanks for a fresh insight for what to me is an old concern. The resulting piston slap causes the large engines all sorts of problems, resulting in what we called "liner rot." Cavitation resulting from the vibrations on the water side of the liners cause the steel to rot away. Then you leak coolant into the combustion chamber.

 

[ISX]

Dang you guys are really informing! I do have one more thing to throw into the mix.

Pkruse mentions cylinders not firing and although this might not be what he meant, it is something that I know about. On the dodge cummins 24 valve VP44 engines (1998.5-2002), they have a 3 cylinder high idle mode for the coldest times. What this does is it kills 3 cylinders (this engine is an inline 6) and runs off the other 3, the same 3 every time it engages. It apparently only engages when engine coolant temp plummets to under 13F, maybe lower. It also fuels the 3 cylinders hard enough to maintain a 1200RPM idle speed. After coolant gets over a preset point (maybe 140F), it turns off. The idea is that by having it idle higher than the stock 800RPM along with having to drive 3 dead cylinders, drives engine loading up enough to allow it to warm up faster. Now it sounds horrendous when it is doing this, but where would that fall under the realm of wear?

From least wear to most, I think I can now say:
1. Drive off like a sunday driver
2. Idle engine
3. Idle with exhaust brake

I guess 3 cyl idle would be a 4th item but I don't know where it would rank.

 

[Ranger Mike]

From your original post..three engined identical. It is not stated but I assume they are all just assembled and not yet broke in..this is an initial start up and the question is which will wear least.
The idle engine, next the dry rev engine and last is the drive off.
if this is not the case then please correct me.

Upon start up OF BRAND NEW engine there are a lot of things that happen. I address two areas..piston rings and cam shaft.
all other components are pretty much the same in the three scenarios, i.r. rod and crank bearings are going to wear the same regardless of load once oil pressure comes up.

On initial fire up we have to look at parts where shear can happen. Pistons have ring groves and rings and here is where we run into a good break in vs bad break in scenario.
New parts and no run in and require break into seat the rings properly. This is compounded by having a very high compression ration that means fuel is blowing past the rings until they heat up enough to properly seal in the cylinder, Engine blocks have round cylinders and pistons are manufactured as oval or egg shaped as a matter of strength of design. Anyway , until the ring can do its job we have fuel blow by polluting the oil. Just check the dip stick a few days after a new oil change. This diluted oil wil not lubricate as well as non diluted oil. When we REV the engine our load it this increases the blow by UNTIL the piston / ring combination seals. Revving also increase shear characteristics on t he edge of the piston rings and cylinder walls. Not to get too much into metallurgy but we have micropscopic interaction happening as we are seating the rings to the cylinder for long term durability. Automotive engine builders went to plateau honing to quicken this break in process a few years back. We also have significant piston rock in the cylinder bore UNTIL the piston heats up. More metal to metal interaction. Now we are not talking about a lot of material being removed but it is measureable.
The cam shaft requires significant lubrication upon start up and the best break-in procedure on a new camshaft is to start the engine and go to fast idle to make sure the lobes get plenty of oil for the first 20 minutes. Again this is an area where we have mucho metal to metal contacts..shear ..and we need a lot of oil to assist break in.

So for my part, i'll bet on the idling engine for MINIMUM material removal of the three. The Engine with highest load at start would have most wear.

the following was found on link
http://www.eng.wayne.edu/page.php?id=759SYNOPSIS

The goal of this research is to develop a new technique to determine the in-situ wear in the liner at the top ring reversal point (TRRP) in an engine under actual running conditions without the need to disassemble the engine. The wear probe is then utilized to measure the wear and surface roughness at the TRRP of the cylinder liner by using a 3-D roughness tracer (laser stylus). A 3-D surface topography of a pre-designated area on the wear probe is obtained before and after a wear test is made. The volume of the material above a common reference plane is calculated using a numerical integration algorithm. The decrease in the volume after the wear test is measured, and the average nominal wear depth, which is the ratio of wear volume over apparent worn area, is calculated over the whole area. The surface texture of the wear probe is examined by using a Scanning Electron Microscope and an optical microscope. The wear mechanisms are analyzed. The wear particles from the engine oil are studied by using ferrographic technology. The research is conducted on a single-cylinder, four-stroke, air cooled, S.I. engine. The wear test is repeated after specified intervals of time over the break-in period. In addition to wear measurements, the total instantaneous engine frictional torque (IFT) is measured.

The following conclusions are achieved, based on this research:

The wear occurred in the highest rate at the beginning of the break-in period. The wear rate then decreased sharply during the first hour. The wear reached its low steady rate after three and a half hours of engine break-in. Accordingly, the wear particles reached a high concentration in the oil after the first hour. Change the oil after one hour would reduce the wear in an engine.

The engine friction IFT and surface roughness Ra are highest at the beginning of the break-in period, and decreased at a lower rate than the wear rate. About half of the total change in mean IFT and Ra during the break-in period occurred after the wear reached its low steady rate.
The total power loss in friction (dwf/dt) consists of two parts, i.e. a transient part (dwt/dt) which has its highest value at the beginning and diminishes at the end of the break-in, and a steady part (dws/dt) which continues after the break-in period.
The piston ring assembly (PRA) friction and surface roughness was found to be linearly correlated during break-in. The reason is that the decrease in surface roughness changes the lubrication from mixed to hydrodynamic regime and results in a drop in friction.
A change in wear-rate was not associated with a corresponded change in friction. PRA friction and wear are not well correlated during the break-in period.
Correlations have been developed for wear-rate, surface roughness, engine frictional torque, and energy lost to overcome friction during and after the break-in period.
The primary wear mechanisms of the cylinder liner on TRRP were found to be abrasion, plastic deformation and fatigue.
Abrasion was found to be the main wear mechanism to cause the liner wear during engine break-in. Folded material along the honing marks and graphite flakes on surface, produced during the honing process, broke off the surface and became wear debris to cause further abrasive wear.

 

[Pkruse]

Of all those participating in this discussion, my brain seems to be the only one stuck in the world of over the road trucks. Clearly, there are differences and distinctions between them and small personal vehicles. With modern computer controlled fuel injection systems, the engine design engineer has many options, and may very well employ different options at different portions of the duty cycle. What ISX says is the most logical and perhaps common solution chosen by design engineers for the warm up portion of the duty cycle for engines being designed today. But a different solutions meets design objectives much better for the part of the duty cycle in which the trucker is idling his engine all night to power the A/C while he sleeps at a truck stop.

I must apologize to the rest of you who spend so much brain effort on the smaller diesels in cars and light trucks. My job for three decades was to minimize the total cost of ownership for fleet vehicles. These small engines have never been worth my effort to think much about because regardless of how they are treated they will almost always outlast the equipment they are installed in: So I would invest my mental efforts in other directions where I had a higher potential of saving my employer money.

 

[ISX]

So now we know what happens during break in period. Thanks for that.

Pkruse, your information is very good. I have been around big and small diesels and they aren't that different.

 

[cmb]

On initial fire up we have to look at parts where shear can happen. Pistons have ring groves and rings and here is where we run into a good break in vs bad break in scenario.
New parts and no run in and require break into seat the rings properly. This is compounded by having a very high compression ration that means fuel is blowing past the rings until they heat up enough to properly seal in the cylinder,

I think you might find you have that backwards. Surely the reason a cold engine is more difficult to turn over is because there is less clearance in the bore until it warms through, not more. Blow-by gases are higher during the hotter parts of the usage cycle, not the colder parts. And, besides, we're talking diesel - no fuel is carried in blow-by gases.

Sorry, I also disagree with your answer to the op question. The question was to do with the wear rate whilst rising to a working temperature. If you leave a diesel idling from cold then it will take for ages to get to temperature, whereas an engine that is doing work will heat up quickly. An idling engine may well have a tenth of the wear rate of an engine being driven, but if it takes 11 times as long to heat through then the wear is greater (plus you've done no miles yet, and still have to drive those miles too!) The engine driving a brake is a bigger load than driving gently, and it also happens at lower speed thus much higher torque so I think has to be the worst.

 

[cmb]

On the dodge cummins 24 valve VP44 engines (1998.5-2002), they have a 3 cylinder high idle mode for the coldest times. What this does is it kills 3 cylinders (this engine is an inline 6) and runs off the other 3, the same 3 every time it engages. It apparently only engages when engine coolant temp plummets to under 13F, maybe lower. It also fuels the 3 cylinders hard enough to maintain a 1200RPM idle speed. After coolant gets over a preset point (maybe 140F), it turns off. The idea is that by having it idle higher than the stock 800RPM along with having to drive 3 dead cylinders, drives engine loading up enough to allow it to warm up faster. Now it sounds horrendous when it is doing this, but where would that fall under the realm of wear?

I suspect that this may not be done either for the explicit reason of 'warming the engine up', nor 'to reduce wear'. I rather think this is for fuelling reasons. When the [big] cylinders are cold there must be some minimum amount of fuel which is enough to 'feed' the heat losses (into the cylinder) and still have enough left over to power the piston stroke to drive the engine. The thing is, it may be that this minimum fuelling would run the engine too fast were all cylinders operating at the same time, so they just cut back some of the cylinders.

I don't know, that's just a guess. But it sounds to me more like a solution to cold-temp fuelling - a bit like the equivalent to if you were to run a choke on a carburettor'd car - you're driving the strokes with a richer fuelling than you usually need to overcome the thermal losses, but for the diesel which is effectively 'wide open throttle' all the time, that'd just idle the engine too fast.

Look at it this way - as the engine goes through one rotation, it requires a certain number of Joules energy in, irrespective of the energy [in any cylinder] out. Whether you are compressing the air in 6 cylinders when 3 fire, or compressing 6 cylinders when 6 are firing - you're still compressing 6 cylinders - and that takes a given amount of energy. So whether you burn fuel and convert that much energy in 3 cylinders, or 6, you still have to generate power from those strokes enough to overcome the compression cycles and the resistance of the engine. The only benefit I can see to running 3 cyls to achieve that is where there are too many losses per combustion stroke to get full burn of the fuel - maybe, even, it is it ensure better combustion in those 3 cyls so as to avoid the wall wetting issue I mentioned.

It'd be good to get a definitive reason why a truck manuf would say to do this.

 

[Jasongreat]

I suspect that this may not be done either for the explicit reason of 'warming the engine up', nor 'to reduce wear'. I rather think this is for fuelling reasons. When the [big] cylinders are cold there must be some minimum amount of fuel which is enough to 'feed' the heat losses (into the cylinder) and still have enough left over to power the piston stroke to drive the engine. The thing is, it may be that this minimum fuelling would run the engine too fast were all cylinders operating at the same time, so they just cut back some of the cylinders.

I don't know, that's just a guess. But it sounds to me more like a solution to cold-temp fuelling - a bit like the equivalent to if you were to run a choke on a carburettor'd car - you're driving the strokes with a richer fuelling than you usually need to overcome the thermal losses, but for the diesel which is effectively 'wide open throttle' all the time, that'd just idle the engine too fast.

Look at it this way - as the engine goes through one rotation, it requires a certain number of Joules energy in, irrespective of the energy [in any cylinder] out. Whether you are compressing the air in 6 cylinders when 3 fire, or compressing 6 cylinders when 6 are firing - you're still compressing 6 cylinders - and that takes a given amount of energy. So whether you burn fuel and convert that much energy in 3 cylinders, or 6, you still have to generate power from those strokes enough to overcome the compression cycles and the resistance of the engine. The only benefit I can see to running 3 cyls to achieve that is where there are too many losses per combustion stroke to get full burn of the fuel - maybe, even, it is it ensure better combustion in those 3 cyls so as to avoid the wall wetting issue I mentioned.

It'd be good to get a definitive reason why a truck manuf would say to do this.

I have a 99 dodge cummins and I can tell you they will hardly warmup at all until you start putting a load to it. Hence the idea cummins came up with, use the compression of three cylinders to put a load on the engine.

The stock dodge program has this option disabled, it takes either a trip to the dealer or an aftermarket programmer to enable it. Mine will kick in just about any time the temp drops to 20 or below as long as the engine isn't warm. It is nice to come out of the house have the windshield defrosted and the engine close to operating temp and raring to go.

 

[cmb]

I have a 99 dodge cummins and I can tell you they will hardly warmup at all until you start putting a load to it.

Exactly.

Hence the idea cummins came up with, use the compression of three cylinders to put a load on the engine.

I'd like to see a technical explanation from Cummins on what 'this idea' actually is. I mean, if it is simply cutting fuelling to those 3 cylinders, then it will actually slightly reduce the total loading on the engine. An idling engine already has the load of all the cylinders to pump around, whether it is running on one or all cylinders. The total compressive loads for a rotation are the same!

The only way for it to increase the load is if there is an additional valve actuating mechanism which opens the exhaust valve of the unfuelled cylinder between T- and BDC. I just don't think they'd go to such lengths to build in such a mechanism just to run on 3 cyl out of 6 for a rather spurious reason as this (which seems likely to accelerate wear significantly).

 

[ISX]

With a scangauge you can see engine load and at idle the EGT is at 250F and load is 0%. With 3 cylinder thing activated, EGT is now 600F and engine load is 15%. It does idle up to 1200RPM while in 3 cylinder mode, probably to smooth it out some (sounds awful). You can see it here at 2:10 it kicks into 3 cylinder mode after revving slowly up to 1200RPM on all 6.

With just high idle on all 6 (1200RPM) engine has 0% load and EGT is 300F. So there is twice the heat coming out of the exhaust manifold by kicking off 3 cylinders.

 

[Jasongreat]

Exactly.



I'd like to see a technical explanation from Cummins on what 'this idea' actually is. I mean, if it is simply cutting fuelling to those 3 cylinders, then it will actually slightly reduce the total loading on the engine. An idling engine already has the load of all the cylinders to pump around, whether it is running on one or all cylinders. The total compressive loads for a rotation are the same!

The only way for it to increase the load is if there is an additional valve actuating mechanism which opens the exhaust valve of the unfuelled cylinder between T- and BDC. I just don't think they'd go to such lengths to build in such a mechanism just to run on 3 cyl out of 6 for a rather spurious reason as this (which seems likely to accelerate wear significantly).

How can removing fuel from a cylinder make it less of a load, even slightly? I understand that an idling engine already has loads present in all cylinders, however as soon as you add fuel the cylinder makes net power from the combustion of the fuel/air mixture, minus the amount of load. Take away the fuel and there is no energy produced only used(load) as the piston compresses the air in the cylinder.

 

[cmb]

How can removing fuel from a cylinder make it less of a load, even slightly?

Consider the compression 'energy sink' elements of an engine rotation separately from the ignition 'energy source'.

If you have an engine rotating - whether it is running on one, two, 6, or even being cranked over from an external source - is the 'energy sink' [the compression] different between the cases? Do you some how not need to create compression in a cylinder if it is being fuelled? ... clearly, each 'compression' is a compression!

Similarly, the other total loads on the engine (bearing, piston, camshaft, etc., frictional losses) are essentially independent of whether combustion is going on in the cylinders.

So to cause an engine to rotate, idling, at a given speed requires a certain amount of power to oppose this load. The load is the same!

The reason fuelling off saves a little fuel (viz., reduces the load) is because if you have combustion going on in a cylinder then it will lose heat to the block/head/exhaust/etc.. Therefore, the return stroke will not fully realize all the thermal heat of the combustion. (We know this - engines are far from 100% efficient!). Whereas, if the cylinder is cold and all you have is compression on the gas in the cylinder and no combustion (it gets hot, but not as hot as a combustion cycles) so the thermal losses are lower (Newton's law of cooling, and all that). Now, reduce the number of opportunities for thermal loss/reduce the surfaces into which thermal losses occur, and, bingo, more useful thermal energy goes into opposing the loads instead of heating the engine up!

It is not very much more efficient. But cylinder shut-down is used as a means to save fuel for motors in the 'cruise' condition. I used to have it on a '77 Cadillac Seville, many years ago. I don't think it saved an ounce of fuel, given I was driving on UK's stop-start motorways, I think any slight benefit was ruined by it. But cylinder shut-down is a method used to save fuel and it cannot both be an extra load AND save fuel!

Just think about the 'usual' load on an engine - those 3 cylinders are always a compressive load to the engine, whether they are being used to burn fuel/generate power, or not.

 

[Ranger Mike]

CMB states..

I think you might find you have that backwards. Surely the reason a cold engine is more difficult to turn over is because there is less clearance in the bore until it warms through, not more. Blow-by gases are higher during the hotter parts of the usage cycle, not the colder parts. And, besides, we're talking diesel - no fuel is carried in blow-by gases.

you have manged to bring a huge smile to my face...I need not post anymore regarding this matter...

 

[cmb]

Yes, sorry, I retract that. (I can't delete it from the orignal post, now.) Thinking about it a second time, the piston is more likely to thermally expand than is the bore (it's got no cooling! Doh!). I was basing my initial comment just on considering the frictional load resistance in the engine (but that's not just down to piston rings).

Apologies.

One thing is certaintly true - in the first few strokes then the blow by is much higher. Until the cylinder bore is wet with oil it does not seal properly. That much is certainly true. My ageing 240,000 mile Citroen had such poor compression that I used to pour neat oil stright into the intake manifold of a morning to wet the cylinders, and also reduce the volume in the cylinder. Sometimes I put too much in and it would hydralock! I'd have to wait a while for it to seep out past the worn-out rings. But once it was going, the dynamic pressure and wetted bores provided enough compression to keep it going.

{...and, no doubt, the reducing clearance as it warmed up. ;) - hat-tip}

 

[Ranger Mike]

CMB..no apologies necessary. this forum is a very wonderful tool that assists in the Learning process and we all benefit from the discussion...you are a friend!

 

[Jasongreat]

Just think about the 'usual' load on an engine - those 3 cylinders are always a compressive load to the engine, whether they are being used to burn fuel/generate power, or not.

Those three cylinders are always a load on an engine, no argument here. Since an engine is a machine that converts energy into mechanical force or motion. Once we remove fuel from three cyl. we have a three cyl engine. Which has a greater load, a three cylinder engine with six compressive loads or a six cylinder engine with six compressive loads? The cummins warms up faster because for three cyl to turn six it takes more fuel, it also has to spin the engine at 1200 rpm instead of 850 to keep a kind of smooth idle which takes some additional fuel, more fuel, more heat.

 

[ISX]

So the ultimate wear scenario is 3 cylinder high idle and the exhaust brake because of all the load on the 3 cylinders, causing those 3 to wear a lot, plus the exhaust stroke side load wear from the exhaust brake resistance, right?

 

[Jasongreat]

So the ultimate wear scenario is 3 cylinder high idle and the exhaust brake because of all the load on the 3 cylinders, causing those 3 to wear a lot, plus the exhaust stroke side load wear from the exhaust brake resistance, right?

It doesn't use the same three cylinders, iirc. It rotates the three cylinders being used to spread it out over all six cylinders evenly. The years of dodge with this feature do not have exhaust brakes as OE. The quicker an engine gets to operating temp should help reduce wear, I would think , and since without the 3 cyl option one would have to drive off to get the engine to warm up. It seems to me the load on 3cyl rotating six cylinders at 1200 rpm, would still wear signifigantly less, than with the load of keeping six cylinders rotating while moving 8000 lbs down the road.

 

[cmb]

It still strikes me that this makes more sense as a fuelling option. Like a comment above says, the exhaust temp is hotter. Note; hotter exhaust doesn't necessarily mean there is more heat (as in thermal energy) passing through the exhaust manifold, because there are less combustion gases flowing.

My guess for what you end up with is; a) a more enriched combustion in those cylinders that are fuelled (guaranteeing better cold weather combustion), and b) a lower throughput of combustion gases, thus hotter exhaust (because you still have to generate the same power to idle the engine at the same speed, you're just doing it with less exhaust gas volume), thus maximising transfer of exhaust thermal energy to the exhaust side of the engine - thus faster warm-up.

So there is extra fuel going into each cylinder, but not necessarily the engine as a whole. In fact, I think the total fuel going in would probably be less (if we were talking about equivalent idle speeds) because the volumetric efficiency of the fuelled cylinders would be equivalently higher with more fuel [higher combustion temperature] being fed into each.

I do feel the 'extra-load-on-the-engine' argument for why it is used to be illogical, because you either have enough power generated that matches the idling loads, or you don't in which case the idle speed changes. The idling loads are much the same, same pumping losses, same compressive loads, same friction loads for the idling speed. What extra loads can there be when you don't fuel some of those cylinders?

 

[Jasongreat]

I do feel the 'extra-load-on-the-engine' argument for why it is used to be illogical, because you either have enough power generated that matches the idling loads, or you don't in which case the idle speed changes. The idling loads are much the same, same pumping losses, same compressive loads, same friction loads for the idling speed. What extra loads can there be when you don't fuel some of those cylinders?

No need to 'feel' anything. Let's try this, fill a five gallon bucket with water, use both hands to lift over your head, now try it again using only one hand, which warms you up quicker? Why, the total load is the same?

Here is a clip from a dodge technical service bulletin discussing the new high idle option: Symptom/Condition:

Extended idle operation, especially in cold weather, can lead to stuck valves and bent push-tubes due to insufficient cylinder heat. This allows varnishes/oils to condense on the exhaust valve stems, leading to stuck valves, and damaged valve train components. A new software feature, enabled or disabled through the DRB III (See instructions under Repair Procedure), reduces the chance of valve sticking and improves cab heat warm-up time.

 

[cmb]

No need to 'feel' anything. Let's try this, fill a five gallon bucket with water, use both hands to lift over your head, now try it again using only one hand, which warms you up quicker? Why, the total load is the same?

I'm with you on that analogy. It's a 'different way' to convert the same amount of energy to deliver the same 'useful' work [albeit idling], but because all the [same] work is done with a half of the resources [hands, or cylinders], those particular resources end up with all the waste heat - viz twice as much. I'm getting pretty confident here that the reason this works out for an engine is because you have a half of the flowing gases, so less heat goes out the tailpipe meaning more must stay in the engine.

 

[Jasongreat]

I'm with you on that analogy. It's a 'different way' to convert the same amount of energy to deliver the same 'useful' work [albeit idling], but because all the [same] work is done with a half of the resources [hands, or cylinders], those particular resources end up with all the waste heat - viz twice as much. I'm getting pretty confident here that the reason this works out for an engine is because you have a half of the flowing gases, so less heat goes out the tailpipe meaning more must stay in the engine.

Would we have half the flowing gasses though? Doesnt it take a certain amount of fuel and air to produce a certain amount of power? Let's say it takes 10hp to rotate the engine, it is always going to take 10 hp to turn the engine, be it through 6 cyl or 3. So the gasses would stay the same, i would think, they would just all be coming from 3 cyl instead of six and on top of that, wouldn't the 3 dead cyl's also be flowing air, adding even more airflow?