SAAB advice: lug the engine (from 1982)

[1sttruck]

I have a Cummins 5.9 liter, inline 6, 'standard output' in my truck. I think it's a common style of design for a medium/heavy duty engine, using a long stroke, high compression (16 to 17 to 1 in mine), a turbo, high pressure common rail fuel injection, and a massive, stiff block. The engine weighs over 900 lbs. The peak torque of 460 ft lbs (600 in the new models) is at around 1600 rpm, and it produces 235 max horsepower (300 or so in the new ones) horsepower at around 2700 to 2800 rpm. Redline is something like 3000 rpm. Horsepower ratings are typically low for diesels, and are in fact misleading due to the common practice of reporting peak HP. Looking at say 350 hp engines, one could be talking about a drag bike, a hot 2 liter gas engine, a medium sized gas engine in a truck or car, or a large diesel producing 1500 to 2500 ft lbs of torque. If you're trying to get 80k lbs of loaded truck moving, the low rpm torque ends up producing decent amounts of hp at a low rpm, at least compared to a peaky, smaller displacement engine that can end up producing lots more peak hp, but little power at low rpms.

The high torque seems to be a product of the high compression, and the nice long stroke which provides ample opportunity to get as much fuel injected as possible. For high HP in a smaller displacement you'd want more cylinders, and a larger bore with a shorter stroke to get the rpms up. For maximum fuel economy I'd think that a smaller displacement turbodiesel runnning at higher loads would work better than a lugging a larger one, but one would need more gears for handling heavier loads.

 

[Bret Chase]

HP is simply (TQ X RPM)/ 5250... if the rpms are low, tq has to be much higher to get the same amount of power.

 

[Doug Hillary]

Hi,
1sttruck - you said;
"...the low rpm torque ends up producing decent amounts of hp at a low rpm,..."

This statement is unfortunately typical of the very common misunderstanding of both. Much depends on the management technologies used on the engine too of course

"Power" (hp,kW and etc) of course equals speed (the speed at which the work is done at) and torque (the "twisting" effect) (lbs/ft, Nm and etc.) equals pulling ability or the effort

Bret Chase - sorry but the formula is maybe a little wrong

Regards
Doug

 

[bulwnkl]

Well actually, the formula is correct. Given ANY two engines with identical HP curves, appropriate transmissions, and operating speed ranges of (let's say) 1200-1800 rpm and 2400-3600 rpm, you will not be able to tell the difference in operation of the two engines apart from exhaust note and gauge readings. This is because torque and speed are freely exchangeable barring a little extra friction loss in the higher-speed unit. The transmission's function is to make this exchange.

Remember that 10,000 ft-lbs of torque at 0 rpm isn't accomplishing anything at all (except maybe breaking axles), but just 2 ft-lbs at 1/1,000 rpm is getting something done.

May I assume that no one has any of the type of comparative numbers I was asking about above? Again, I'm not suggesting that a powertrain design difference/change could not take advantage of a different operating protocol or vice versa. Rather, I'm looking for a practical comparison of the two operating protocols in a given OTR truck powertrain.

 

[bulwnkl]

G-Man, you're right. A diesel's design results in much greater force which is translated into much greater torque. It also requires much greater strength to keep from grenading, which generally requires much greater mass, which generally restricts the engine to much lower speeds.

However, the cycle efficiency declines in a diesel-cycle engine because of injector duration. In order to get more fuel in to the combustion chamber, the injector must stay open longer. Each additional increment of fuel is therefore injected later and later in the power stroke, where it is incrementally less beneficial due to the declining cylinder pressures and increasing rate of pressure drop. There are various design factors which can (or could) mitigate this effect, which is why I was asking about any given drivetrain rather than comparing 1960s technology to 2000s technology.

 

[Audi Junkie]

I will try the 1-3-5 shifts. Been doing 1-2-4-5. I tell the kids, 4th gear is your friend... 20mph to 70. When I short-shifted my lil cousin's car, he was like WHAT? WHY? This is the kid who does 70mph in a 35 and burns gas like crazy. Less shifting and lower rpm w/ deep throttle does seem like it saves gas.

 

[XS650]

quote:

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Originally posted by bulwnkl:
Well actually, the formula is correct.

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It would be correcter if it said 5252.113122....

 

[Doug Hillary]

Hi,
bulwnkl - you said;
"May I assume that no one has any of the type of comparative numbers I was asking about above? Again, I'm not suggesting that a powertrain design difference/change could not take advantage of a different operating protocol or vice versa. Rather, I'm looking for a practical comparison of the two operating protocols in a given OTR truck powertrain."

Can you give me some examples of the "comparative numbers" you are looking for please? And please define the "practical comparison" you seek - thanks

I may be missing the plot.............

Bret & bulwnkl - please give some examples of "the formula"

Regards
Doug

 

[Shannow]

Doug,
the formula is for ftlbs, RPM, and hp. 5252 is the constant linkin gthe old units.

If they used Nm, radians per second, and Watts, it would be so much simpler.

As to your assertion that drivers influence consumption, my better half is always 3MPG down on my mileage, even when I'm in the passenger seat.

 

[mechtech]

Skipping gears should do no harm, and have a very probably benefit of fuel economy[gas engines].
When the throttle is opened more [in a higher gear at the same speed], there is less manifold vacuum. Vacuum is produced with engine power, so you are saving that amount of energy. Also, you are simply running at less RPMs.
There will be an offseting factor in that your ignition timing advance will be less [whether programmed or vacuum], and make it less efficient.
At least, you'll save some synchro wear.
I like HP and ft. lbs - you can keep Nm, Watts, and soccer.

 

[bulwnkl]

quote:

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Originally posted by Shannow:
Doug,
the formula is for ftlbs, RPM, and hp. 5252 is the constant linkin gthe old units.

If they used Nm, radians per second, and Watts, it would be so much simpler.

---

True, but why discard so well-established a system when my calculator doesn't care?

 

[bulwnkl]

quote:

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Originally posted by Doug Hillary:
Can you give me some examples of the "comparative numbers" you are looking for please? And please define the "practical comparison" you seek - thanks

---

quote:

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My question relates to what you've seen with any given engine/transmission combination. Any design series, doesn't matter. How does "lugging" that combination versus up-shifting it at rated speed impact its fuel efficiency?

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What I'm saying is:

Go out and choose a powertrain from your fleet. Doesn't matter which, just choose one. Now, with that powertrain, operate it in, let's say, a 1200-1600 rpm range and collect fuel economy data. Then, operate that same powertrain in, let's say, a 1500-1900 rpm range and collect fuel economy data. Now compare the two sets of data. What do you see? On an older series engine perhaps the comparative ranges would be more like 1500-1900 vs. 1800-2200.

This comparison could be a difficult one given lubrication and cooling system designs/limitations, but that is the kind of comparison I'm trying to ask about.

Better explanation?

 

[John K]

For cars I have always believed lugging was bad, and that it produces extreme stresses on the rods, crankshaft, and bearings and others. Trucks may be a different matter, but in the old days it was not uncommon to throw a rod going up a hill by someone who refused to downshift. In reading about Porsche 911's, they even caution about some engines during the 1990s that are very sensitive to driving with rpms too low. We may be comparing apples to oranges, but I hate to hear engines under lugging conditions.

 

[John K]

I remember my grandfather driving a 46 DeSoto with 4 speed semi-auto (or 2 speed with hi-lo, whatever the h it was) He would go from 1 to 4. Worse was a guy I worked with delivering groceries in a 3 spd column shift Dodge panel truck. He'd get it rolling in 1st and drop it into 3rd, then reach for the choke while complaining how badly it ran. It bucked, knocked and generally sounded terrible. He had no clue. He also worked for the fire dept, don't know if he ever drove the fire trucks or not. Maybe he picked it up from driving diesels? I can imagine the younger set on here wondering what I am talking about.... 1946! The Dodge was in the late 1950s. Column shift? Manual choke and throtle???

 

[Pablo]

"drop it into 3rd, then reach for the choke"

Why?

 

[Bret Chase]

Originally posted by 1sttruck:


Well, lets compare an 11 liter Cummins ISM 280, an inline 6 that produces a measly 280 hp at 2100 rpm, but also produces 1150 lb-ft of torque at 1200 rpm, and the Infiniti G35 that also produces 280 hp. The G35 does 280 hp at 6000 rpm and 270 lb ft of torque at 4800 rpm. [/QUOTE

Now if we ignore engine longevity for a moment... if you simply put a 4:1 gear reduction at the flywheel of the G35 engine, you've got 1080 ft/lbs at 1200 rpm output (4800rpm input).. not all that different than the 1150ft/lbs of the Cummins.
Geared in this mannor, the G35 would do just fine.

-Bret

on a side note, when did 280HP become measly? my first vehicle was a '78 F150 w/ a 112HP 300 I6.

 

[XS650]

quote:

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Originally posted by bulwnkl:
G-Man, you're right. A diesel's design results in much greater force which is translated into much greater torque.

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Diesels put out more torque than gas engine because diesels are generally turbocharged and have more displacement than a gas engine.

If you compare apples vs apples, both gas and diesel naturally aspirated 4-strokes, a gas engine puts out more torque per cubic inch than a diesel.

Most any decent modern gas engine, high speed or low speed engine, puts out about 1 lb-ft/in^3.

A naturally aspirated 4 stroke doesel is doing good to develop 0.8 lb-ft/in^3.

The reason fo that is that both engine worl by burning hydrocarbon fuels to expand air and move the pistons. A gas engine uses essentially 100% of the oxygen in the air, a diesel is doing good to use 80% of the oxygen. If they try to use more, they lay down a smoke screen but don't make much more power.

 

[1sttruck]

"Now if we ignore engine longevity for a moment... if you simply put a 4:1 gear reduction at the flywheel of the G35 engine, you've got 1080 ft/lbs at 1200 rpm output (4800rpm input).. not all that different than the 1150ft/lbs of the Cummins.
Geared in this mannor, the G35 would do just fine."

The G35 evidently has a 6600 rpm redline, so geared 4:1 it would have a hp peak at 1500 rpm and a redline at 1650 rpm. The top speed of the G35 is limited 142 mph, so 4:1 gets you to 35 mph, maxed out.

 

[John K]

Pablo -

Because it ran so bad he thought the mixture needed changing I guess. I was young, not yet driving. I rode around in that truck with 3 or 4 different drivers, and none of the others did anything like it. He recognized it sounded terrible, he just didn't relate it to the way he drove it!

 

[1sttruck]

"1sttruck - you said;
"...the low rpm torque ends up producing decent amounts of hp at a low rpm,..."

This statement is unfortunately typical of the very common misunderstanding of both. Much depends on the management technologies used on the engine too of course"

Well, lets compare an 11 liter Cummins ISM 280, an inline 6 that produces a measly 280 hp at 2100 rpm, but also produces 1150 lb-ft of torque at 1200 rpm, and the Infiniti G35 that also produces 280 hp. The G35 does 280 hp at 6000 rpm and 270 lb ft of torque at 4800 rpm. I don't have hp/torque curves, but I'm guessing that for hauling a decent load, say a 30k lb trailer, the Cummins will be better suited with the 1150 lbs ft or torque at 1200 rpm than the G35 will with 270 lbs ft of torque at 4800 rpm. With enough rpms, low enough gearing, and a manual tranny in order to minimize driveline loss, the G35 could get the load moving, but it would be at low speeds on flat areas without some sort of fancy gearbox/splitter. I know that the 460 lbs ft of torque at 1600 rpm in my truck will work as some have done so for shorter distances, but it's almost 2x the rated max capacity for the 1 ton version and 2.5X the 3/4 ton (the main difference between the 1 ton and 3/4 ton seems to be springs and dual rear wheels, as most everything else is the same).

A friend has an old 20k lb rated GM flatbed, with a splitter for the axle. It uses the 350 gas engine. He says that you're always maxing out the revs, it won't do much over 50 mph, and he's on the 2nd engine at 80k miles, but it does work.