High Load / Low RPM = Economy?

garciapiano

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Feb 12, 2018
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1997 Jetta TDI (1Z)
Hi there,

I'm of the opinion that the best fuel economy is to be had at the tallest gear possible at any given speed, to not lug the engine, thus maximizing mechanical work done during the combustion cycle. This effect is well-documented for naturally aspirated gasoline engines, but I'm not entirely sure if it applies to the TDI. Intuitively, high-load, low RPM is hard on the engine's internals, and there are some questions as to whether it is smart to do in a turbocharged engine.

Normally, when driving in low-load situations (such as on a flat city street) I make it a point to get into 4th or 5th if possible.... Typically I will shift in tandem with every 10 MPH gain. E.g., I'm going into 2nd at 10-15mph, 3rd around 25-30 mph and 4th around 35-40 mph and 5th around 45-50 mph. This pattern seems to line up nicely with the gear ratios of the trans. I accelerate smoothly and shift around 2300 RPM, effectively keeping me around 1800 RPM on average. I make it a point to rev out the engine hard every so often so it's not like I'm babying it.

Recently I've been experimenting with driving around in taller gears/lower RPMs just for the sake of fuel economy. I only do this with the oil at full operating temp to prevent any wear that would occur under the higher load. E.g. going into 4th at 30 MPH, and 5th at 40 MPH. I think the synchros are a little less happy to shift at these RPMS, but effectively I'm trying to model an eco-tuned automatic.

I have no way of metering actual live fuel consumption so I'm curious if anyone has any anecdotal or hard data they can provide regarding this. I have looked at the BSFC chart but really, once I'm at highway speeds I'm generally driving around where the BSFC peaks for the 1.9 TDI engine, in fuel/mph around 1900 RPM. In general, I tend not to exceed 70 MPH in the TDI, and drive around 65 MPH most of the time on the highway.
 

IndigoBlueWagon

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One caution: The 5th gear spline can be stripped without a lot of difficulty if you load up the transmission with torque at low revs.
 

Kravt

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Lexington, Ky
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2005 Passat
Intuition tells me that the difference you would see between the two shifting styles you suggest is fairly minimal, and not worth the additional stress on components. In my experience the most drastic change you can make for FE is incorporating the pulse and glide technique, but this will cause extra wear and tear on your drivetrain as well, major wear if not done precisely.
 

Kravt

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*the pulse and glide technique is also riskier than regular driving because of the added complexity
 

turbodieseldyke

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For any given speed, MPG is always better in the taller gear; but if you're accelerating, you don't want to take too long to reach cruising speed. Say you're accelerating up to 60mph, and you get 18mpg in 4th gear at a certain speed/accel, while 5th gives you 24mpg. However, if it takes you twice as long to get up to speed in 5th, then you are better off biting the bullet in 4th.

I have an accurate gauge that tells me instantaneous mpg, so I'm able to see the difference. I've just never bothered recording the data and determining the "best" way to accelerate. In reality, outside factors mostly determine your optimum solution, and those factors always change. E.g. traffic around you, red or green lights ahead, where you're going and how soon you need to get there, etc.

Even without looking at the chart, you can sense when you're hitting the higher rpms, and you're just burning extra fuel without gaining power. Time to shift.
 

garciapiano

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1997 Jetta TDI (1Z)
*the pulse and glide technique is also riskier than regular driving because of the added complexity
I'm pretty sure that pulse and glide would not work in a TDI. It works in the Toyota Prius due to its transmission decoupling setup that turns off the engine.
 

PassatLife

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I'm pretty sure that pulse and glide would not work in a TDI. It works in the Toyota Prius due to its transmission decoupling setup that turns off the engine.
It works if you have a manual...just accelerate in a certain gear and then coast in N...

Or you could EOC, engine off coasting
 

Kravt

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The P&G that can be done on our cars (or any conventional combustion drivetrain) is done a little different than the P&G done in Prius cars. It is done by accelerating quickly to gain enough momentum to get where you want to go, and then placing the car in neutral in order to glide. Perhaps the term Burn and Coast is more appropriate. In any case, some members here have seen FE increases over 15-20 percent when this method is used.
The principle of this improvement is actually very similar to your reasoning for shifting at lower RPM: engines produce power in a much more efficient way at high load. During acceleration, your engine has a relatively good fuel-used/acceleration-produced ratio as long as you are within the engine speeds that produce higher torque (~1700-3000). A conventional engine’s most inefficient power producing state is when at cruising speed with light throttle application. In this state, the power needed to push the car along is fairly little (low load), but the power needed to overcome the friction of engine internals is elevated by the higher RPM.
Basically, a driver using P&G keeps an engine in a higher load state for short bursts to achieve the same power as a driver maintaining a constant speed. While the idling in neutral between the bursts does use fuel, it is much less than the fuel required to keep engine speed high in constant speed cruising.
I’ll add that I don’t do this all the time, I just make an effort to accelerate fast and glide in neutral when it makes sense to. Wear and tear from doing this on the interstate is not worth it IMO. I never turn off my engine, and don’t advise that anyone else does.
 
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PassatLife

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Personally I've found that in the city downshifting when coming to lights and having the injectors go into overrun fuel cut nets bigger gains in mpg than P&G does. I saw a 5mpg gain from doing this over P&G. I have my IQ set pretty low which from my understanding results in higher fuel consumption at idle or in this case coasting in N. For long highway trips I tend to EOC big hills or straight empty sections, vs coasting in neutral. You can bump start the car to reduce wear and tear on your starter. I would just advise to keep an eye on your battery level if you do this often, and keep track of how many brake pumps you have left
 

turbodieseldyke

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I always thought EOC referred to injectors-off, not ignition-off. Meaning, ignition-on, engine-turning, zero-fuel-injected. That definition comes from my mpg gauge, which tracks "EOC" by counting miles driven with zero fuel consumed.

Usually you want the ECU to do your fuel-cutoff for you (by downshifting, or coasting downhill in 5th), but I also remember seeing some guys on a hypermiler forum with an Injector-Killer switch wired to the gear shifter. They'd shift to neutral, press the button to starve the engine, then slip back into gear to bump start when desired. Could never wrap my head around "why", just to save pennies burned during idle (and also saving wear-and-tear on the ign switch, which is how normal people would turn an engine off). Some people just have a stick-toitiveness that I lack.
 

garciapiano

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I always thought EOC referred to injectors-off, not ignition-off. Meaning, ignition-on, engine-turning, zero-fuel-injected. That definition comes from my mpg gauge, which tracks "EOC" by counting miles driven with zero fuel consumed.

Usually you want the ECU to do your fuel-cutoff for you (by downshifting, or coasting downhill in 5th), but I also remember seeing some guys on a hypermiler forum with an Injector-Killer switch wired to the gear shifter. They'd shift to neutral, press the button to starve the engine, then slip back into gear to bump start when desired. Could never wrap my head around "why", just to save pennies burned during idle (and also saving wear-and-tear on the ign switch, which is how normal people would turn an engine off). Some people just have a stick-toitiveness that I lack.
Idle burns far more than you’d expect. But I promise you the cost savings are offset by the additional wear on the drivetrain when bump starting at speed. Add to that the loss of power steering and you have potentially a dangerous situation. I’ve been told that bump starting TDIs is generally a no-no.
 

IndigoBlueWagon

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I read years ago that a 1.9 TDI burns about 13 oz of fuel an hour at idle. Really not that much.
 

PassatLife

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Idle burns far more than you’d expect. But I promise you the cost savings are offset by the additional wear on the drivetrain when bump starting at speed. Add to that the loss of power steering and you have potentially a dangerous situation. I’ve been told that bump starting TDIs is generally a no-no.
There is additional wear on the clutch and flywheel but its negligible. It would be the same as rev matching from neutral into whatever gear you want. The only time I could think of where it would be detrimental is if you didn't rev match...but then that's user error not a fundamental error. Bump starting a TDI is the same as any other car, as long as you do it correctly and smoothly there shouldn't be any issue. Just make sure you keep your key to ON so you dont cause steering lock

I read years ago that a 1.9 TDI burns about 13 oz of fuel an hour at idle. Really not that much.
I feel like with all the different setups people have this really varies from car to car. I'm running DLC 1019s with my iq set to around 2 and ac running. My car definitely uses quite a bit of fuel at idle from my various tests, which consequently makes P&G not as effective as injector overrun or EOCing
 

ToddA1

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I read years ago that a 1.9 TDI burns about 13 oz of fuel an hour at idle. Really not that much.

I remember something along those lines. I also remember all the people who screamed I was wasting fuel with my remote starters, to warm the car up. Some of these were also running Frost Heaters, for hours, to preheat their cars...

-Todd
 

Steve Addy

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97 Mk3
I remember something along those lines. I also remember all the people who screamed I was wasting fuel with my remote starters, to warm the car up. Some of these were also running Frost Heaters, for hours, to preheat their cars...

-Todd
Yeah, what's with the running the frost heater for 2 hr before starting? I would think an appropriately sized heater would be sufficient at no more than an hour. I have a neighbor who remote starts his plugged in Colorado duramax sometimes 3 times (20 min max each cycle before it shuts off automatically) before he ever comes out of the house!

I honestly don't know why he bought the thing, he doesn't go anywhere far enough that would justify a diesel and to boot it's weighted down with a large Tepui, that's bolted to a basket that's clamped to a Yakima quad bar setup over a vinyl tonneau cover with associated capped pvc pipes and what not strapped on for good measure. He used to take the Tepui off in the winter but he eventually stopped doing that because the chore was too heavy and I can't help him with it.
 

ToddA1

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I’d think they were running the Frost Heaters longer than 2 hours, if expected to do what it claimed. FH used a 750w Zero Start. When I cobbled mine together, I went with a 1500w heater and needed to run it around 2-3 hours to get sufficient heat, at start up.

Once the coolant circulated, the temp needle would drop significantly, as did the heat, but it was still better than starting dead cold.

-Todd
 

Steve Addy

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I’d think they were running the Frost Heaters longer than 2 hours, if expected to do what it claimed. FH used a 750w Zero Start. When I cobbled mine together, I went with a 1500w heater and needed to run it around 2-3 hours to get sufficient heat, at start up.

Once the coolant circulated, the temp needle would drop significantly, as did the heat, but it was still better than starting dead cold.

-Todd
Todd is that with passive or active circulation? I have a hard time believing that these heaters can actually generate some circulation without a pump but I guess it's possible.

Steve
 

Steve Addy

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Iowa
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97 Mk3
Passive... purely convection. It definitely works.

-Todd
Maybe I'm not plugging the one in on the Dakota for long enough. I think it's a 1000 watt unit. I have another, a 1500 watt but I was going to put that on the 6.5TD.

Steve
 
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