Is low specific output the secret of engine life?

[itguy08]

Originally Posted By: OVERKILL

You need to explain this further because at face value it doesn't make sense. The Cummins ISX, in top trim, is 600HP. That's a 15L engine. That's 40HP/L or a relatively low specific output.


I'd argue specific output is a poor metric. It measures HP/L which is generated from torque which is the value that we really care about. HP is derived from torque and RPM so it only exists because of torque. As Honda's early S2000 points out you can have a high HP engine that is "gutless" and needs really high RPM's because it has low torque. Is it stressed much? I'd say not as much as something pushing more torque at a lower RPM.

Sure, let's go with the Cummins ISX:
http://cumminsengines.com/isx15-heavy-duty-truck-2013?#ratings

ISX15 600: 600 HP, 2050 lb-ft.
40 hp/L, 136.6 lb-ft/L

Ecoboost 3.5: 365 HP, 420 lb-ft
104 hp/L, 120 lb-ft/L

Ecoboost 2.0: 240 HP, 270 lb-ft
120 hp/L, 135 lb-ft/L

Ford 4.6 3v: 315 HP, 325 lb-ft
68.47 hp/L, 70.65 lb-ft/L

Ford probably has the largest fleet of small displacement turbo engines in the industry. Dating to 2009 with the EB 3.5. When used as supplied (i.e.: not tuned) we don't see them with internal engine issues with any great regularity. Many of these are getting into the 100k+ range without major issues (one guy has 300k on his) so the design seems to be aging well. There have been turbo failures but that happens on all turbo engines. In stock form they don't seem to go boom that much and are aging well.

It's all in the design. You can have a low specific output engine that has poor durability due to design flaws (Vega and Beetle come to mind easily) or you can have a properly designed high specific output engine that has long durability. It's all up to the designer.

Maintenance also plays a large part. Take any of those "legendary" engines and neglect it and it will die early. Maintain it and it will live long. Same with a high S.O. engine.

 

[dblshock]

nice mild engine is easy on the transmission.

 

[jhellwig]

Look at the vehical mentioned with the exception of the German cars everything else is not the biggest .otor offered in said vehical. Look at the demographic that buys stuff like that. Old farts. Drive the car like an old [censored] and it will go far. Drive it like a punk kid who wants people to look at him and it will die young. While some motors are built better than others the driving style plays a huge roll in it.

And yes I have killed said low output engines when I was a dumb kid.

 

[KrisZ]

Originally Posted By: itguy08
I'd argue specific output is a poor metric. It measures HP/L which is generated from torque which is the value that we really care about. HP is derived from torque and RPM so it only exists because of torque. As Honda's early S2000 points out you can have a high HP engine that is "gutless" and needs really high RPM's because it has low torque. Is it stressed much? I'd say not as much as something pushing more torque at a lower RPM.


You could not be more wrong. RPM puts extreme loads on the reciprocating components. They quadruple with doubling the rotational speed. Also, the S2000 has one of the highest piston speeds. That engine's life would be measured is few hours if it had to haul a load at WOT up a steep grade.
That it why heavy duty applications have low revving engines. Some of the big ship engines spin at less than 500RPM at full load.

 

[Jarlaxle]

Originally Posted By: KrisZ
Originally Posted By: itguy08
I'd argue specific output is a poor metric. It measures HP/L which is generated from torque which is the value that we really care about. HP is derived from torque and RPM so it only exists because of torque. As Honda's early S2000 points out you can have a high HP engine that is "gutless" and needs really high RPM's because it has low torque. Is it stressed much? I'd say not as much as something pushing more torque at a lower RPM.


You could not be more wrong. RPM puts extreme loads on the reciprocating components. They quadruple with doubling the rotational speed. Also, the S2000 has one of the highest piston speeds. That engine's life would be measured is few hours if it had to haul a load at WOT up a steep grade.
That it why heavy duty applications have low revving engines. Some of the big ship engines spin at less than 500RPM at full load.


And yet...Hondas ran 3000+ RPM on the highway for decades,and still ran forever!

 

[itguy08]

Originally Posted By: KrisZ

You could not be more wrong. RPM puts extreme loads on the reciprocating components. They quadruple with doubling the rotational speed. Also, the S2000 has one of the highest piston speeds. That engine's life would be measured is few hours if it had to haul a load at WOT up a steep grade.


Because it's not designed to do it. Probably more to do with the cooling and oil systems than the engine design. I don't think the engine cares if it's accelerating a car or a load - it's the length of time at WOT and how you get that heat out and how you ensure the oil is cool enough, there is enough, etc.

Think Detroit Diesel 71 (Screaming Jimmy) - high RPM's for a Diesel and it will run at 2100-2500 RPM all day long and have a long service life. It's all design.

Quote:
That it why heavy duty applications have low revving engines. Some of the big ship engines spin at less than 500RPM at full load.


That's more to do to the size of them. To spin a piston you can stand on with a stroke measured in feet that's a lot of mass rotating. As well as getting the ounces of fuel for each explosion you can easily see why they run slow. Especially given a boating environment where a fire is a huge deal and a dead engine means you are stuck in the middle of an ocean.

It's all in design. How many 4 bangers are running around for 100k, 200k, 300k that run at 3-4k RPM on the highway. They last just as long as the old school V8 that loafs at 1500-2k RPM on the highway. All in design.

 

[KrisZ]

It's design for the application. 300k miles is nothing in commercial applications. You could have an engine that could pull a heavy load at 6k RPM, but that engine will be done for in a very short order when compared to a 15L diesel engine that spins at 1300RPM while pulling.

Design is one thing, but the application and physics involved is another. No design can overcome physical limitations.

 

[Jarlaxle]

The B-series Cummins runs right on the governor at highway speed in commercial use, and they still go 300,000+ miles. We have several...two run 2400RPM at 65, on a 2700RPM redline. Both run fine (and start right up even at sub-zero temps) with not quite 200K miles.

 

[OVERKILL]

Originally Posted By: itguy08


Think Detroit Diesel 71 (Screaming Jimmy) - high RPM's for a Diesel and it will run at 2100-2500 RPM all day long and have a long service life. It's all design.



but that's not really high RPM for a diesel, it is relatively high RPM for a OTR diesel (though I've seen many that high, I think our 7L ones are governed at around 2500) but something like a TDI or even the old Powerstroke spun significantly faster than that.

Low rotational speed/piston speed are indeed part of the reason for the longevity of OTR truck engines. And they are also designed to be run wide-open 24/7, unlike your typical car engine. That said, there were some automotive engines that are designed to be run at WOT all day, any of BMW's M-engines, the old Ford 300 I6 when repurposed for genset or pump duty...etc.

 

[OVERKILL]

Originally Posted By: itguy08


I'd argue specific output is a poor metric. It measures HP/L which is generated from torque which is the value that we really care about. HP is derived from torque and RPM so it only exists because of torque. As Honda's early S2000 points out you can have a high HP engine that is "gutless" and needs really high RPM's because it has low torque. Is it stressed much? I'd say not as much as something pushing more torque at a lower RPM.

Sure, let's go with the Cummins ISX:
http://cumminsengines.com/isx15-heavy-duty-truck-2013?#ratings

ISX15 600: 600 HP, 2050 lb-ft.
40 hp/L, 136.6 lb-ft/L

Ecoboost 3.5: 365 HP, 420 lb-ft
104 hp/L, 120 lb-ft/L

Ecoboost 2.0: 240 HP, 270 lb-ft
120 hp/L, 135 lb-ft/L

Ford 4.6 3v: 315 HP, 325 lb-ft
68.47 hp/L, 70.65 lb-ft/L

Ford probably has the largest fleet of small displacement turbo engines in the industry. Dating to 2009 with the EB 3.5. When used as supplied (i.e.: not tuned) we don't see them with internal engine issues with any great regularity. Many of these are getting into the 100k+ range without major issues (one guy has 300k on his) so the design seems to be aging well. There have been turbo failures but that happens on all turbo engines. In stock form they don't seem to go boom that much and are aging well.

It's all in the design. You can have a low specific output engine that has poor durability due to design flaws (Vega and Beetle come to mind easily) or you can have a properly designed high specific output engine that has long durability. It's all up to the designer.

Maintenance also plays a large part. Take any of those "legendary" engines and neglect it and it will die early. Maintain it and it will live long. Same with a high S.O. engine.


Specific output is a good metric for comparison here because it equalizes things. Torque simply looks at one component of what is required for power, which is the rate at which work can be done. A diesel generally makes more torque than a gasser of the same displacement. Using your above example, the Mercedes engine I mentioned earlier makes 152ft-lbs/L, or higher than anything in the list including the ISX.

Two engines making the same amount of power, by definition, will do the same amount work at the same rate. So our little torque-light S2000 mill can do the same amount of work at the same rate as a 220HP 7L kitty-CAT diesel. Now, will it last doing that? No. It will be spinning at the RPM required for it to make the most power, which is extremely high, and will require significant gear reduction to achieve the same torque to the axle to match what the lower speed diesel sees. All this means that its internal components are massively more stressed.

And of course architecturally, the bigger your components, the slower the rotational speed generally is. That's why as diesel engine size increases, RPM at which maximum power is achieved decreases. This is not a requirement; you do not have to turn a big engine slow, but it benefits longevity to do so.

A fun example of two nary identical engines, and diesel engines at that, would be the 6L Powerstroke and the VT365. Trimmed out very differently due to application. The Powerstroke gave up low speed torque for a higher rev range and the ability to make more power (be faster) whilst the VT365 had a lower RPM ceiling but made more low RPM torque because its application didn't require it to be "fast", simply reliable (and we don't need to get into it not delivering well on that front, LOL!). You can look how the ISB is setup for a RAM vs a straight truck for a similar comparison.

 

[IndyIan]

Originally Posted By: OVERKILL
Originally Posted By: itguy08


Think Detroit Diesel 71 (Screaming Jimmy) - high RPM's for a Diesel and it will run at 2100-2500 RPM all day long and have a long service life. It's all design.



but that's not really high RPM for a diesel, it is relatively high RPM for a OTR diesel (though I've seen many that high, I think our 7L ones are governed at around 2500) but something like a TDI or even the old Powerstroke spun significantly faster than that.

Low rotational speed/piston speed are indeed part of the reason for the longevity of OTR truck engines. And they are also designed to be run wide-open 24/7, unlike your typical car engine. That said, there were some automotive engines that are designed to be run at WOT all day, any of BMW's M-engines, the old Ford 300 I6 when repurposed for genset or pump duty...etc.

I think the M engines have the cooling systems, both oil and coolant, to run WO longer than most car engines, but how would they do in marine duty? As well as a marine SBC? I suspect not just based on their complexity, but maybe some guy is out there running with twin M62s? Sure would sound nice anyways!

 

[OVERKILL]

Originally Posted By: IndyIan
Originally Posted By: OVERKILL
Originally Posted By: itguy08


Think Detroit Diesel 71 (Screaming Jimmy) - high RPM's for a Diesel and it will run at 2100-2500 RPM all day long and have a long service life. It's all design.



but that's not really high RPM for a diesel, it is relatively high RPM for a OTR diesel (though I've seen many that high, I think our 7L ones are governed at around 2500) but something like a TDI or even the old Powerstroke spun significantly faster than that.

Low rotational speed/piston speed are indeed part of the reason for the longevity of OTR truck engines. And they are also designed to be run wide-open 24/7, unlike your typical car engine. That said, there were some automotive engines that are designed to be run at WOT all day, any of BMW's M-engines, the old Ford 300 I6 when repurposed for genset or pump duty...etc.

I think the M engines have the cooling systems, both oil and coolant, to run WO longer than most car engines, but how would they do in marine duty? As well as a marine SBC? I suspect not just based on their complexity, but maybe some guy is out there running with twin M62s? Sure would sound nice anyways!


I think you are right, they wouldn't be as maintenance-friendly as your SBF or SBC in that application unless things like VANOS were eliminated making the engines less complex. I don't think durability would be an issue but I certainly think the complexity would be a hindrance and be cause for much more frequent service, something most people aren't keen on.

I think a pair of S62's in a big Donzi would sound bloody awesome!

 

[KrisZ]

Here are some interesting numbers.

The average 18 wheeler engine size is about 15L. Their maximum gross weight is 80,000lbs.
Even taking a conservative 50,000lbs loads, it puts 3300lbs per 1L of engine displacement. Maximum 80,000lbs puts that number at 5300lbs per 1L of engine displacement!
Now imagine a 1L engine pulling a 3000lbs to 5000lbs vehicle that would rev as any normal gasser would. It would not last too long in that kind of usage.

 

[itguy08]

Originally Posted By: KrisZ

Now imagine a 1L engine pulling a 3000lbs to 5000lbs vehicle that would rev as any normal gasser would. It would not last too long in that kind of usage.


You mean like the Ford Focus (1.0L Ecoboost, curb weight of 2,935 to 3,055 lbs), Fiesta (1.0L, 2,537 to 2,720 lbs)?
Or the Fusion (1.5L Ecoboost, 3,431 to 3,681 lbs)
Or the Chevy Cruze (1.4L, 2,635–2,866 lb)
Or the Escape (1.5L, 3,502 to 3,645 lbs)

Or the many European vehicles with small engines? It's all in the design!!!

 

[itguy08]

Originally Posted By: OVERKILL

Specific output is a good metric for comparison here because it equalizes things. Torque simply looks at one component of what is required for power, which is the rate at which work can be done.


Without torque there would be no horsepower so I'd say torque is the most important metric. HP just means how fast it can get done. You could run your car on a B&S 3.5HP lawn mower engine. You'd go very slow but it could be done - you'd need the right gears.

Quote:
Two engines making the same amount of power, by definition, will do the same amount work at the same rate. So our little torque-light S2000 mill can do the same amount of work at the same rate as a 220HP 7L kitty-CAT diesel. Now, will it last doing that? No. It will be spinning at the RPM required for it to make the most power, which is extremely high, and will require significant gear reduction to achieve the same torque to the axle to match what the lower speed diesel sees. All this means that its internal components are massively more stressed.


As long as the S2000 stays within its design parameters it will last just as long at, say 5K RPM and max output as it would at 3K RPM and max output. IIRC what happens is the cooling systems usually can't keep up and that's what holds engines back. Flog them like going up a hill at WOT and you'll quickly overheat because there is not enough cooling capacity. The engine doesn't care as long as it is designed properly, and that includes the cooling, oiling, and internal components.

The S2000 was not designed for long term, high load applications. That doesn't mean someone couldn't design a similar engine that is.

Quote:
A fun example of two nary identical engines, and diesel engines at that, would be the 6L Powerstroke and the VT365. Trimmed out very differently due to application. The Powerstroke gave up low speed torque for a higher rev range and the ability to make more power (be faster) whilst the VT365 had a lower RPM ceiling but made more low RPM torque because its application didn't require it to be "fast", simply reliable (and we don't need to get into it not delivering well on that front, LOL!).


Yeah the 6L was not reliable in any application... Even Navistar didn't get them running right in their trucks!

But compare the 7.3L - a reliable workhorse in both the F-series as well as the International/Navistar applications. And tuned differently, IIRC.

It's all in the design.

 

[KrisZ]

Originally Posted By: itguy08
You mean like the Ford Focus (1.0L Ecoboost, curb weight of 2,935 to 3,055 lbs), Fiesta (1.0L, 2,537 to 2,720 lbs)?
Or the Fusion (1.5L Ecoboost, 3,431 to 3,681 lbs)
Or the Chevy Cruze (1.4L, 2,635–2,866 lb)
Or the Escape (1.5L, 3,502 to 3,645 lbs)

Or the many European vehicles with small engines? It's all in the design!!!


You keep saying and you are right, however you do not fully understand the full meaning of that phrase. The cars you mentioned have a life expectancy of 150k miles not 1 million miles. The 18 wheelers have a life expectance of 1 million miles and it is part of the design to have a low revving/low stress engine to accomplish this goal as economically as possible.

The way you mention it, is as if those heavy duty engine designers were somehow missing something or knew less than the light duty engine designers.

 

[OVERKILL]

Originally Posted By: itguy08


Without torque there would be no horsepower so I'd say torque is the most important metric.


Yes, but without RPM we wouldn't have horsepower either, since it requires movement, whilst you can apply torque and perform no work

Originally Posted By: itguy08
HP just means how fast it can get done. You could run your car on a B&S 3.5HP lawn mower engine. You'd go very slow but it could be done - you'd need the right gears.


Exactly, which is why things are sized differently depending on application. Just like a 9,000RPM 4-popper spun to the moon would make a poor choice for an 18 wheeler (and wouldn't last spinning 9K all the time) while a 15L diesel spinning 2K would be a poor choice for a track car because it would need an insane number of gears due to its narrow RPM range.

Originally Posted By: itguy08
As long as the S2000 stays within its design parameters it will last just as long at, say 5K RPM and max output as it would at 3K RPM and max output. IIRC what happens is the cooling systems usually can't keep up and that's what holds engines back. Flog them like going up a hill at WOT and you'll quickly overheat because there is not enough cooling capacity. The engine doesn't care as long as it is designed properly, and that includes the cooling, oiling, and internal components.


I don't believe that to be the case. The piston/rings/walls only have a certain number (an insanely high number but still) of strokes in them. The rate of wear directly correlates with RPM; an engine spinning 9,000RPM is wearing the cylinders faster than one spinning 2,000RPM. Certainly tweaking technology and choice of materials plays a role here but ultimately high cylinder pressure (high load) and high rate of scrub at that high load will wear the bores faster than that same load but at a far lower rate. The bearing area, assuming always hydrodynamic and ignoring fatigue are not affected by this but anything that is boundary will have wear that follows frequency.

Originally Posted By: itguy08
The S2000 was not designed for long term, high load applications. That doesn't mean someone couldn't design a similar engine that is.


So taking that example, can you think of any engine that spins to a similar RPM, with a similar power density designed for the purpose of longevity? I don't believe one exists. The relationship between component selection for robustness (heavy) runs contrary for that required for high RPM power (lightness). The best that can be hoped for is a compromise using titanium or some other exotic metal to achieve relatively good robustness at a minimum weight penalty. Rods stretch, bolts stretch, higher piston speed engines stretch rods more. This is why the aluminum rods used in high performance race cars have a stretch and life limit on them. They are necessary for the RPM and power target but do not have the longevity to work in something designed for high mileage. And then you have to think of the life of components like valve springs which have to be heavier (and wear out sooner) in high RPM applications as well.


Originally Posted By: itguy08
Yeah the 6L was not reliable in any application... Even Navistar didn't get them running right in their trucks!

But compare the 7.3L - a reliable workhorse in both the F-series as well as the International/Navistar applications. And tuned differently, IIRC.

It's all in the design.


Yes, the DT444 is another example. It spun higher in the Ford lineup (like the 6L, but not as high) due to the target audience. It was also tuned somewhat like the 6L with more power, less torque in the trucks and lower power, lower RPM, but higher torque in the straight frames and busses.

I would posit that if you were to perform a study contrasting the life expectancy between the DT444 and the 7.3L Powerstroke you would find that while both have extremely long lives that on average the DT444 will last longer due to the lower RPM it would see during its life.

 

[itguy08]

Originally Posted By: KrisZ

You keep saying and you are right, however you do not fully understand the full meaning of that phrase. The cars you mentioned have a life expectancy of 150k miles not 1 million miles. The 18 wheelers have a life expectance of 1 million miles and it is part of the design to have a low revving/low stress engine to accomplish this goal as economically as possible.

The way you mention it, is as if those heavy duty engine designers were somehow missing something or knew less than the light duty engine designers.


You do realize few 18 wheeler engines go 1 million miles before a rebuild right? They are designed to be rebuilt easily and relatively cheaply.

http://www.akmicorp.com/aftermarket-trends-over-the-years/

Quote:


In 1982 an alternator would last an average of 132,000 miles, clutches an average of 171,000 miles and a Class 8 engine would be overhauled at around 276,000 miles.
Today, an alternator would last an average of 278,000 miles, clutches an average of 369,000 miles and a Class 8 engines would be overhauled at 680,000 miles.
Technology has increased the life of heavy duty truck parts, so they do not need to be replaced as often. Most truck owners today may never replace a clutch or overhaul an engine.


These guys say one 700k plan on a rebuild: http://www.smart-trucking.com/buying-a-used-big-rig.html

Not to mention the big diesels are designed for their mileage. Cars are not. You could put a million mile engine in a car but there's no money or incentive in it. A million miles in a passenger car would be 66 years at the US average of 15k miles per year! In a big rig which may see 15k a week it makes sense.

It's all DESIGN!

 

[KrisZ]

Originally Posted By: itguy08
It's all DESIGN!


You have no clue.

Why don't they design a 6 liter engine that would rev to 6k RPM for the 18 wheelers then?
Low RPM = low inertia forces, low cycle fatigue = longer component life. It's part of choosing a DESIGN. You cannot out design these limitations. Mitigate to some extent, but not eliminate.

 

[RoadRaceJosh]

Originally Posted By: mjoekingz28
276hp from a 4.6 sounds like high output.


I believe the OP is blinded from today's engines that use turbos, high(er) compression, Variable valve timing and probably alot of other (cheats) that IMO do nothing for a street car and are more for the racetrack (or police pursuit type motoring).


Cheats? Let us do some comparing. I used to own a 1974 Dodge D100 pickup. 3.7 liters, pushrods, sub 8:1 compression, about 100 hp and around 17 MPG highway. That same engine package was offered through 1987. Now I have a relatively low tech, by 2015 standards, Nissan Frontier which has port injection, 9.5:1 compression ratio, 4 valves/cylinder and variable intake valve timing. None of this is racy or esoteric and it makes 152hp and gets 24 MPG highway.

4 valves per cylinder was noteworthy, outside of exotics, in a passenger car 30 years ago with release of the Toyota 4AGE. Variable valve timing was of note about 30 years ago as well with Nissan's use on the VG30E and Honda's 1989 release of VTEC. Continuously variable valve timing has been around for over 20 years. Variable valve timing has been used primarily to broaden torque curves rather than boost high speed power. Case in point, the TU (technical update) BMW M50 engine.

M50B25 141 kW (189 hp) @ 5900 245 N·m (181 lb·ft) @ 4700 1990
M50B25TU 141 kW (189 hp) @ 5900 245 N·m (181 lb·ft) @ 4200 1993