Claim of economy in early upshifts

[shams13]

people say that mercedes is designed to shift-up early to give better economy but one thing confuses me. Driving the car at such a low speed like 45KM/h at 4th, tough the RPM are around 2000 but to pull the car it needs almost half the throttle pressed or more and the economy needle goes all the way to red, so I think early shift is not making good economy.

[Duke2.6]

Best engine thermal efficiency is high load (low manifold vacuum for lower pumping loss) at less than WOT (doesn't activate full load enrichment), and low engine revs (minimum engine internal friction).

With a manual this means short shifting (2000-2500) and using about 3/4 throttle to accelerate.

OEs play some games with auto trans shift points to maximize EPA numbers, which would generally mean early upshifts at less than WOT.

Duke

[shams13]

couldn't understand.

[JimSmith]

I believe the points Duke 2.6 were making is that engine efficiency is maximized when the losses are lowest. The items he cited that contribute to losses are intake manifold vacuum and internal friction. Lower rpm operation reduces internal friction, and low intake manifold vacuum reduces the work the engine has to do to get air in the cylinders (called pumping losses - the intake cycle is one part of this loss mechanism, and the exhaust cycle is the other part).

The economy indicator needle on the dash is a vacuum gage that measures the intake manifold vacuum and presents the reading on scale of relative operating economy. That vacuum is created by the engine sucking air past the intake throttle plate. Intake vaccuum is a very helpful parameter to measure to understand how well the engine is running or tuned. Unfortunately, the indicator for "economy" in the car is very simplistic, and merely intended to be a visual aid to let people, who can't make the connection between the throttle position with their right foot and economy, know when they are deep into the pedal they are likely using more fuel than they need to use. The gage has no common sense and only measures vacuum, so it cannot consider the actual driving conditions (are you accelerating, going up a hill, have a heavy load, etc). It should be used as an "indicator" and not a gage that measures actual economy of operation.

Making the vacuum takes energy away from the rear wheels, so you have to burn more fuel which reduces your mileage. As odd as it may seem it is not trivial. The object of low resistance, high flow air cleaners is to reduce this effect once the throttle is open fairly far, and they are often cited as being capable of making significant (measureable) fuel economy gains. Technically the engine operates with the fewest pumping losses with a throttle position open as far as it will go. So, better operating efficiency is achieved with the throttle as far open as it will go without triggering any fuel enrichment features to achieve maximum power, such as are used when you are at wide open throttle (WOT).

Consequently with an automatic transmission, to achieve the highest operating efficiency (best fuel economy) the manufacturer programs the transmission shift points to occur at an rpm with lower friction losses (a lower rpm than where you achieve full power) and where you will accelerate between shifts with the throttle open to near the threshold of turning on the fuel enrichment feature needed for delivering high power.

For most of the automobiles we drive, at steady state, flat and level road conditions at speeds to keep up with all the speeding traffic of today, you will be able to drive in high gear and have your economy needle pegged on the "economy" side of the indicator, or at least out of the red zone. If this is not the case, you may have an engine tuning parameter that is not correct. I hope this helps. Jim

[shams13]

Thanks Jim for such a nice explanation. OK i will give you a scenario and with that you can assist me.
I am going at say 70KM/h at 4th and there is a round about ahead, to cross it obviously i have to reduce speed and make it almost 40-45KM/h, after crossing the round about and get to a plain road i have two options now;

1) full throttle to shift down to 3rd and then release the pedal so it is pressed 3/4 (ECONOMY NEEDLE IS AT ECONOMY SIDE)

2) keep the throttle 1/2 and don't shift down when on road, but to accelerate from 40KM/h in 4th, the economy needle goes into red

ok now forget about econ needle, which method you prefer???

[Duke2.6]

If you open the throttle half way at low revs - say 2000 - manifold vacuum goes to essentially zero so pumping loss is minimized, but you do not activate WOT enrichment, and the engine will not downshift, so internal engine friction remains low. This is the most efficient acceleration mode, but acceleration will be less than WOT or enough throttle to cause a downshift.

Another way to understand engine efficiency is gearing. Say your engine is geared for 3000 at 60 and achieves a certain fuel economy on a level road. If we regear it for 2000 at 60, it will have to produce the same power to cruise on a level road. This means the throttle will have to be opened more which will reduce pumping loss because of reduced vacuum, and the engine would not be dissipating as much energy as internal friction. The result is that the engine is operating at higher thermal efficiency and fuel consumption will drop about 10 percent.

The downside is that the engine will have less potential power at the lower revs, so hill climbing may require a downshift.

One reason why modern cars achieve better fuel economy than older cars is that the modern cars typically have more transmission ratios and taller cruising gears. The Corvette is a good example. Despite a 400 HP engine its sixth gear yields about 1500 at 60, and it achieves 28 MPG on the EPA highway cycle.

BTW, MB removed the "economy gage" because it was misunderstood by owners. Using manifold vacuum as a surrogate for economy is ONLY valid for level road cruising, not acceleration or hill climbing.

If you cruise at progressively higher speeds on a level road, you will notice that as cruise speed is increased manifold vacuum is less and the needle approaches the red zone. The power to maintain speed increases with the CUBE of speed, and most people understand intuitively that higher cruise speed reduces fuel economy.

Duke

[manny]

My 1985 Mustang GT ( 5 speed ) had an upshift light.
You could'nt shift gears fast enough to keep the light out.
The benefits of short-shifting is a well proven concept.
This can also be observed on some of the newer automatic transmissions, when selecting " economy " or " sport " mode.

[Duke2.6]

One of the "issues" with the EPA emission certification test, which is used to determine EPA fuel economy numbers is that all manual transmission cars must shift at the same speed, so a Ferrari Enzo has to shift 1-2, 2-3, etc. at the same speed as a Honda Civic.

It was VW that convinced the EPA to allow the operator to shift per the on-dash shift light, which resulted in better economy numbers, and some other manufacturers followed suit.

Corvettes, going back at least to the first C5 have a "skip shift" feature that automatically selects fourth on the 1-2 shift if shift speed and throttle opening are in a certain range, which corresponds to the EPA driving cycle. It's a PIA in normal driving, so most owners learn to drive around it by shifting 1-2 early or late or buy a shorting plug to disable it.

Duke

[mctwin2kman]

Thank god the MB does not have that idiot light! I hate them things. I will shift when I feel like it and do not need a light to tell me!

[manny]

Originally Posted by mctwin2kman

I will shift when I feel like it and do not need a light to tell me!

That's why I disconnected mine on the Mustang after about 2 weeks.
Found it even more annoying than my mother-inlaw.

[nglitz]

What the light is telling you is that you're revving unnecessarily for the amount you're pressing on the throttle. IOW, "If you're not going to accelerate any faster than this, you might as well shift".

MY '84 Fiero had a shift light. It did take a while to get used to it & realize what it was telling me. "Accelerate harder, or shift, dummy". IIRC at WOT it only lit at redline.

Not that a Fiero could accelerate all that hard anyway.

[mctwin2kman]

Originally Posted by nglitz

What the light is telling you is that you're revving unnecessarily for the amount you're pressing on the throttle. IOW, "If you're not going to accelerate any faster than this, you might as well shift".

MY '84 Fiero had a shift light. It did take a while to get used to it & realize what it was telling me. "Accelerate harder, or shift, dummy". IIRC at WOT it only lit at redline.

Not that a Fiero could accelerate all that hard anyway.

Yes but sometimes I do not need to be in 5th or 6th gear at 30MPH like it suggests!