SAAB advice: lug the engine (from 1982)

[JHZR2]

This was posted by "Dave 37" on saabcentral.com

Its old info, but I find it interesting. Dont GM V8 cars skip gears for extra MPG?

Here is the post:

I read an article from the 1982 issue of Saab Soundings, the magazine published by Saab Scania of America which suggests trying the 1-3-5 method of shifting to save 10% fuel.

The story has a picture of Olle Grandlund, noted as "in charge of engine testing at Trollhattan".

They recommend driving a manual transmission car to 1500-2000 rpm then shifting from 1st to 3rd, flooring the accelerator, and repeat the same procedure jumping from 3rd gear to 5th gear.

They explain that this reducing the pumping loss. With a small throttle opening, the engine has to work to pump air in. This wasted energy is reduced when the throttle is wide open. It says that you can save up to 10% in fuel this way.

The current 9-3 owner's manual cautions not to drive with a wide open throttle during the break-in period of (?) 3000 mi (or was it the first 1200 mi).

Opinions on lugging an engine?

 

[Gary Allan]

Well, I don't agree with the WOT (not that it's invalid) ..but short shifting and "keeping just ahead of the engine" with the gas pedal yielded stellar MPG over common "gas and go" techiques. My 320i routinely yielded 24-25 mpg driving "normally" (for a BMW) ..driven like I had an egg between my foot and the gas pedal ..I got 30 ..but it ran like crap the next day. My mother's 2.8 Citation (carbed) could yield 35 on my 300+ mile runs and my Chevette could yield over 40. 55 was the max speed.


Car & Driver did an article on this during that era, IIRC. It confirmed that the technique yielded better MPG in urban driving.

 

[Pablo]

There is some logic to keeping the revs at the lower end of the power band. But truly lugging an engine is not good for the engine in the long run - (jump) - ultimately being worse for the environment than an unlikely 10% fuel savings.

It's kind of funny when you think of this technique and today's relatively sophisticated ECU's and AT's.

 

[G-MAN]

Remember BMW's "Eta" program from the early 80s? It was based on the same principle. They took their standard 2.8 inline six, put in a milder cam and lowered the valve spring rates. This yielded a low rpm torque peak and a very flat torque curve. It cut HP down to something like 120 and the redline was 4500 RPM. These 2.8e equipped cars had taller gearing and the engine was designed to run at lower RPMs and greater throttle openings than the typical 2.8i. BMW claimed a significant MPG benefit over the standard 2.8 engine.

If you see any old 3 or 5 series Bimmers running around with 328e or 528e on the trunk, they have the Eta engine in them.

 

[bulwnkl]

That technique works very well for a great many gasoline-powered vehicles. The lugging is only a problem if you have insufficient oil pressure or flow at that lower rpm. If not, lug away. There's nothing inherently "bad" about an Otto-cycle engine running at one particular rpm range over another. To suggest there is is just foolishness. Again, the "bad" aspect of lugging has everything to do with the lubrication system and, to a much lesser degree, the cooling system.

The owner's manual for our Neon (and every other '90s or newer car I've owned) lists suggested shift speeds. There's a chart for normal acceleration and one for rapid acceleration (don't recall exact wording). The "normal" shift chart has you shifting at speeds which most anyone I know would consider "lugging." You're driving with your foot nearly to the floor when you shift into 4th at 29mph just to get moderate acceleration. If you have a hill to climb, you're lucky to hold your speed. I don't even like the sound of the engine that well at that speed. However, it gets demonstrably better economy when driven that way.

 

[Pablo]

Maybe we need to define lugging - but it is not foolishness to drive a car within the designed torque/power band and is more economical to take advantage of the low end. G-man gives an excellent example of designing the engine driveline for the user to do this - vs. telling Jane Doe to floor it here, but not there, etc.

I agree that the worst aspect has to do with the oiling system - especially those that seem to not build oil pressure with sufficient flow at lower rpms.

 

[XS650]

quote:

---

Originally posted by JHZR2:
This was posted by "Dave 37" on saabcentral.com

Its old info, but I find it interesting. Dont GM V8 cars skip gears for extra MPG?

---

My 96 Corvette with 6 speed skipped from 1st to 4th at light throttle acceleration, which was lot of normal city driving. I disabled the skip shift.

Several other GM cars with 6 speed manual transmission also did that.

 

[Doug Hillary]

Hi,
SAAB-Scania, Volvo, Mack and DB initiated the lugging concept during the 1970s

Pablo - in the International Trucking Industry where we have spent the last several decades asking/teaching our Drivers to allow lugging, the definition we use is:

Lugging = when the engine will not accellerate the vehicle at full throttle

Our Drivers have been taught to use engine torque rather than engine speed (progressive & skip shifting is a requirement)and to monitor;

a) oil temperature
b) oil pressure
c) engine temperature
d) pyro reading

If all three of a,b,c) are holding and not rising or at/near the alarm level the engine is NOT under extreme stress. The upper level reading on the pyro will usually mean a downshift is required

The requirement to retain Drivers came about from about 1975 with the introduction of engines that could develop maximum torque around 1100rpm against the previous standards set for decades around 1800rpm
Later, ECM's changed this again. Have we been successful Internationally? - marginally so over a period of 30 years. The best moves have been to build in progressive & skip shifting and torque based gearshifts into the ECM, somewhat isolating the Driver's input in the process

The only problem encountered was high torque/low driveline speed related vibrations (mainly in double OD boxes) which caused gearbox failures. An interim "fix" was to use a synthetic gearbox lubricant

Personally, I have always skip shifted my cars whenever possible (to avoid excessive shearing

)

Regards

Doug

 

[Pablo]

Cool - seems like a good way to blow up a GM.....I didn't just type that, did I

PS I skip shift as well.

 

[Doug Hillary]

Hi Pablo,
I hope they did'nt fire you - blowing up a GM!!!

Actually the 2cycle GM 92TTA (I think it was called the "Fuel Squeezer") and a special version of the N14 Cummins were the first of the popular US engines to enable lugging.
I drove both the GM & Cummins prototypes in 1975 in and around New York and around rural New Jersey. They had 9 speed boxes against the "norm" of 15 speeds
They were both "logs" unless you quickly learned how to drive them properly using the lower number of gears and their spread to full effect.
Understanding the technology and the intent was half the battle - it was a case of forgetting the exhaust noise (many Drivers shifted gears via that) and concentrating on the rev counter and anticipating the road ahead
They were developed further after input and with some added refinements
I spent many happy hours with both GM's and Cummins Engineers on this - and with Industry Fleet Engineers too
The Euro Manufacturers - and especially Daimler-Benz - were way in front with engine development at this stage

On release to the Industry, Driver "acceptance" was very poor indeed. Drivers who would rev 350hp GM8V71s out to 3200 for max power (with max torque @ 1800) could not cope with a quieter 8V92TTA with max power/revs at 2100 and max torque at 1600 and only 9 speeds to play with

Non acceptance amongst older Drivers still exists today!!

Regards
Doug

 

[eljefino]

I accellerate at mid-throttle, upshifting when I find myself accellerating "too fast".

When I find myself needing power, I usually hold 3rd gear (at mid-throttle) until cruising speed then drop straight into 5th and level off my accelleration. Redline in 3rd is well above legal freeway speeds.

Sometimes this puts me in 4th at 20, and 5th at 32 mph. But the cars never complain (Saturn SOHCs).

 

[Jonny Z]

My car is drive-by-wire. Would the computer even allow a WOT when I am moving 30 MPH in 5th with my right foot to the floor?
The throttle plate only takes my foot as a suggestion I was told, and it has a mind of its own...
BTW commenting on the BMW Eta program, doesn't variable cam timing get the best of both? Mild profile at low RPM for TQ and sharp profile at high for HP.
Sometimes I wonder how much of these old tips apply to today's super smart cars.

 

[brianl703]

I tried the 1-3-5 skip shifting on the drive home from work, it seemed like it might have made a little bit of a fuel economy improvement(I have a Scanguage). I normally shift 1-2-3-4-5 at around 2200RPM.

 

[brianl703]

quote:

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Originally posted by Jonny Z:
BTW commenting on the BMW Eta program, doesn't variable cam timing get the best of both? Mild profile at low RPM for TQ and sharp profile at high for HP.

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Even split-port induction can do it..Ford got 170HP out of a 2.5L engine which produces 75% of peak torque at 1500RPM.

 

[G-MAN]

quote:

---

Originally posted by Jonny Z:
BTW commenting on the BMW Eta program, doesn't variable cam timing get the best of both? Mild profile at low RPM for TQ and sharp profile at high for HP.

---

BMW's Eta Program was a long time before electronically controlled valve timing became feasible. BMW actually touted the Eta concept as their alternative to going diesel. Of course, a decade later BMW was making some the best (and most powerful) I6 and V8 diesel passenger cars sold in Europe, so go figure.

 

[55Test]

I almost always skip 4th gear in my 88 Celica. I find with normal acceleration on flat ground if traffic conditions permit, 5th gear is good to go at 55-60 km/h . 3rd gear shifted at 2200 rpms or so gives me about 60 km/h so I usually shift to 5th at that speed unless I need more acceleration.

 

[bulwnkl]

Doug H:

How do you find "lugging" compares with historically more "traditional" driving in diesel fleets in terms of fuel economy? I'm talking within the same engine design: Big Cam IV vs. Big Cam IV as opposed to a Big Cam vs. "regular" Cummins.

I ask because lugging allows an Otto-cycle engine to operate at peak cycle-efficiency, but lugging forces a "traditional" (i.e. non-pulse-injection) Diesel-cycle engine into its lowest cycle efficiency.

Just curious.

 

[Doug Hillary]

Hi,
the "standard" research on fuel efficiency in Trucking was done in the early 1970s by Daimler Benz. I was fortunate to be a part of this
Much was contributed by the SAAB-Scania and Mack joint venture too

The end result was a generalised move to "high torque" rise engines. High "torque rise" means that as revs (and power) die back, torque increases. At one stage the "sweet point" was determined as about a 500rpm loss to reach maximum torque. Electronic engine management (as pioneered by Detroit Diesel's DDEC system) enabled the maximum torque band to be flattened so that the engine may actually develop maximum torque at a point say from 900 to 1450rpm (and a maximum operational engine speed of 1650rpm). This required the retraining of Drivers again - now the power/torque/revs/gearing equation is gradually being overtaken by electrically controlled air/electric shifts taken from the ECM via a multitude of sensors. Again, thanks to Daimler Benz and due to research done in the 1970s

To briefly answer your question, the average on road fuel economy of today's heavy diesel is at least 2mpg better than the Big Cam 1,2,3 and etc
Firstly,if one of the older engines (say a Big Cam 3)was driven as todays engines can be they would probably overheat and etc.
Secondly, it is true though that two Drivers operating that same vehicle can get enormously different fuel economy. The best result is usually achieved by the Driver who is smooth and can anticipate his progress. They will usually use lower revs when accellerating, skip shift and "hold on" longer when climbing, taking two gears initially rather than one - and etc!
They usually get better component life too!

Concerning todays better fuel efficiency, much of this is due to the ability to minimise gear downshifts (by lugging longer) when climbing for instance, which reduce road speed, and cause ever increasing fuel use. Progressive upshifts by using maximum torque say after gear 3 or 4 and skip shifting are but some of the user reality in decreasing fuel use

One reason why it was difficult to lug earlier diesels was their technology. Poor radiator capacity and coolant flow, low oil pressure/flow and high oil temperature, restrictions to air flow and some driveline factors (angle, structure, lubricants and etc) are just some of these

Most modern on-road diesels now operate mostly at about 80% of their maximum engine load factor. Sometimes this may be at full throttle for hours on end!
Newer ECMs with full vehicle management (such as in the Mercedes "Actros" or the latest MAN heavies) keep the engine in a narrow efficiency band via precise component management - the Driver becomes more like the "just a steerer" of Trucking mytholgy

Engine & driveline durability now make the "million miles without touching" dream an attainable reality - most of this has been via computer determined technologies and better metallurgy

What has killed fuel economy in Heavy Trucks more than any other thing is the NEED for emission controls

Unfortunately this subject is simply too complex to cover much further on here due to the enormous variables involved but I hope this helps a bit

Regards
Doug

 

[bulwnkl]

Sort of. I'm completely familiar with torque rise. Farm tractors of American design (Case-IH and John Deere; European designs like the current Massey-Fergusons do not operate this way) are horsepower rated at 2000rpm. Used to be 2200 or so, but now 2000. This is also commonly referred to as "rated speed" and is the rpm at which the tractor will run at full "throttle" under load (the computer limits rpm to 2000 in all but road gear). However, the torque rise is so large that peak horsepower is actually generated (in my Case-IH MX270, for instance) at ~1750-1800rpm. This has many positive impacts both psychologically and practically for the tractor and its owner.

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?

I ask because the cycle efficiency of a Diesel-cycle engine decreases as engine rpm declines and injector duration increases. This is the opposite of an Otto-cycle engine, whose cycle efficiency increases as engine rpm decreases and throttle opening increases.

Ultimately, I'm pointing out the fact that lugging should be vastly more beneficial in terms of fuel economy to a gasoline engine than a diesel (it should actually hurt a diesel's economy) if all else is held equal. However, since there is a lot more to a powertrain than simply the cycle efficiency of the engine, I'm asking about practical application of the two operating styles in any given powertrain combination in an OTR fleet. I already have good experience/information regarding farm tractors, but their load patterns are quite different and much higher than OTR trucks.

[ August 03, 2005, 11:50 AM: Message edited by: bulwnkl ]

 

[G-MAN]

One thing to keep in mind is that the "low rpm/higher throttle opening" objective for gas engines has essentially always been the norm for diesel engines. Compared to gasoline engines, diesel engines always generate max torque at lower rpms. And diesel engines don't have a throttle, so from idle to max rpms they are always running unthrottled, i.e., at WOT. (This is why diesel engines have almost zero mainfold vacuum across the rpm range and aux vacuum pumps have to be used to power vacuum controlled accessories.)