Please explain how my car uses NO fuel while in gear coasting downhill??

dsimmelink

Active member
Joined
Aug 13, 2008
Location
WA state
TDI
2009 Blue Graphite Metallic Jetta TDI Sedan
Hello,
This was taken from FuelAcademy.com's Tips page:

"Drive Downhill in Gear
Driving down hill in neutral (free wheeling) you will use some fuel. When you drive down hill in gear (the safest gear under the circumstances) in a modern day car you will use NO fuel whatsoever"

I was wondering if anybody could give me an explanation on how this is true? The good aerodynamics of my new Jetta (as opposed to my old pickup truck) allows it to actually increase speed on most downhill slopes I go down (when I put it in neutral) so I would think that is the best way to save gas while coasting.

So how does that method use some fuel (of course the engine has to run) while leaving the car in gear (and not stepping on the accelerator I would assume) use NO fuel???

I'm still trying to maximize my fuel economy so any knowledged info from this great community would be greatly appreciated.

Thanks!
 

02DslPwr

Veteran Member
Joined
Dec 20, 2004
Location
Leander Texas
TDI
2002 Jetta Wagon, TDI
They're right, you use no fuel while coasting downhill in gear. Reason being, the injectors are at 0% duty cycle (off). If you are in nuetral, the injectors have to keep the motor idling, so fuel useage is minimal but not zero.

When you coast in gear, you are using the momentum of the car to keep the motor spinning. Since the motor keeps spinning, and you arent giving it any throttle input, the injectors turn off. The ECU calculates that zero fuel is required to keep the motor from stalling. Sometimes you increase speed going downhill with no throttle because the rotating mass isnt more than the inertia of the car, so you speed up a bit.

Clear as mud?
 

Joe_Meehan

Top Post Dawg
Joined
Sep 3, 2005
Location
Ohio USA
TDI
NB TDI, 2002.5, Silver
It is true for at least most of our TDI's maybe all of them The car's computer looks around and says, why am I pumping fuel into an engine that does not need it and then shuts it off until it is needed. Not all cars are that smart.
 

je

Veteran Member
Joined
Jun 6, 2000
Location
Chesterton Shores, Ontario
TDI
-
It's like a fixed-gear bicycle. When you're rolling down a hill, you don't need to push on the pedal they just turn. Your speed might actually increase.
 

09tdiman

Veteran Member
Joined
Sep 15, 2008
Location
pittsburgh pa
TDI
none but getting a 09 soon (i relly want a 06 though!)
correct with 02dieselpower
think of a heavy truck or bus and going down hill they are just letting the motor "drag"
your not accelerating so no fuel is being put through the system TECHNICALLY
 

dsimmelink

Active member
Joined
Aug 13, 2008
Location
WA state
TDI
2009 Blue Graphite Metallic Jetta TDI Sedan
02DslPwr said:
....Clear as mud?
Excuse me while I wipe the mud off my shoulders. ;) Thanks so much for the great explanations! You guys rock!!
 

tttthumper

Veteran Member
Joined
Jan 9, 2008
Location
Pickering, Ontario
TDI
2006 Jetta TDI
I asked this in another thread and didn't get a full response.
Seeing GoFasters comment I'm wondering if this is really true then.

Does it use ZERO fuel? or minimal.

When I'm coasting in gear, the Scangauge (which is quite accurate on every other display stat), says I'm getting 0.0 LHK but also says that I am using .03-.05 L/Hour while coasting in gear.

30-50ml/hour = .5ml-.83ml / minute.
1tsp = 5ml.

I know this is such a small amount. But what is the truth.
Engine uses VERY small amount of fuel while coasting in gear, or ZERO.

When in neutral the range can be from .43-.59L/Hour (depending on operating temperature ) = 7.1-9.8 ml/minute

So idling or coasting in neutral you can use up to 2 tsps of diesel every minute
 

thebigarniedog

Master of the Obvious
Joined
Oct 14, 2007
Location
Fail Command (Central Ohio)
TDI
1998 Jetta tdi
About six months ago there was a thread of greater than five pages of comparing the fuel economy of coasting in gear versus neutral coasting. I bet you could find it using the search key ;) .
 

Thermo1223

Veteran Member
Joined
Apr 23, 2004
Location
Easton, PA
TDI
'00 Jetta 5M-'04 JW A5
I will say as early as '96 Toyota Tacomas used 0 while coasting in the gear.

The rpm for fuel to be reintroduced was 1200 for manual 1400 for auto.

The simplest method if you have a manual is to just switch the ignition off. No change in engine feel equals no fuel.
 

dsimmelink

Active member
Joined
Aug 13, 2008
Location
WA state
TDI
2009 Blue Graphite Metallic Jetta TDI Sedan
Thermo1223 said:
The simplest method if you have a manual is to just switch the ignition off. No change in engine feel equals no fuel.
I haven't tried it with my car but isn't this a VERY dangerous thing to do? I remember an old car of mine would lose break and steering function when I turned the ignition off.
 

whitedog

Veteran Member
Joined
Jul 12, 2004
Location
Bend, Oregon
TDI
2004 Jetta that I fill by myself
dsimmelink said:
I haven't tried it with my car but isn't this a VERY dangerous thing to do? I remember an old car of mine would lose break and steering function when I turned the ignition off.
It's just for a quick test, planned carefully for location and time.
 

TdiRacing

Vendor
Joined
Aug 19, 2003
Location
Baltimore, MD
TDI
2006 Jetta TDI Cup
So how does the engine run if it uses no fuel? Go pop an injector and see what you get. No fuel? Wrong. You may be seeing less fuel yes, but there is always fuel flowing when the engine is idling, etc...this notion is so misunderstood by most people.

When you are coasting at idle it yields the least amount of fuel being consumed.

If it is in gear, you are at more rpm's given the same injected fuel per stroke, so you use more fuel than in idle. Make sense. 903 rpm vs...whatever-- but more than 903 will use more fuel. This is the key point to understand. When you are in a situation to use gravity to your advantage, idling will yield the lowest consumption. Rolling down the same hill in gear at say 3000 rpms will use 3x the fuel. Make sense?
 

whitedog

Veteran Member
Joined
Jul 12, 2004
Location
Bend, Oregon
TDI
2004 Jetta that I fill by myself
Since gravity is turning the engine, why wouldn't the computer just shut off all fuel delivery? It doesn't need any fuel to keep turning.
 

tditom

Top Post Dawg
Joined
Sep 5, 2001
Location
san antonio & austin
TDI
formerly: 2001 Golf GL, '97 Passat (RIP) '98 NB, '05 B5 sedan
TdiRacing said:
So how does the engine run if it uses no fuel? Go pop an injector and see what you get. No fuel? Wrong. You may be seeing less fuel yes, but there is always fuel flowing when the engine is idling, etc...this notion is so misunderstood by most people.

When you are coasting at idle it yields the least amount of fuel being consumed.

If it is in gear, you are at more rpm's given the same injected fuel per stroke, so you use more fuel than in idle. Make sense. 903 rpm vs...whatever-- but more than 903 will use more fuel. This is the key point to understand. When you are in a situation to use gravity to your advantage, idling will yield the lowest consumption. Rolling down the same hill in gear at say 3000 rpms will use 3x the fuel. Make sense?
no. it makes no sense. see the hundred other threads on this subject and a technical explanation from DB on how the pump works.

engine in gear, engine above 903 rpm, no go pedal=no fuel consumption at that point.

a point to be considered, though, is that disengaging the engine from the wheels will allow you to coast longer, and may result in less fuel consumption compared to engine braking for that particular situation.
 

DPM

Top Post Dawg
Joined
Mar 16, 2001
Location
Newtownards, N. Ireland
TDI
2019 Rav4 AWD Hybrid, Citroen C4 BlueHDI
If the QA collar is driven to the zero IQ position, there can be no injection. It's one of the engine's redundant shutoff systems, after all...
 

Bob_Fout

Oil Wanker
Joined
Sep 5, 2004
Location
Indiana
TDI
2003 Jetta - Alaska Green (sold) / 2015 GTI 2.0T
Those with EGT gauges can see EGT plummet when in overrun conditions vs. temp at idle. Mine's gotten in the low 200s *F during extended downhill high-rev coasting.
 

TdiRacing

Vendor
Joined
Aug 19, 2003
Location
Baltimore, MD
TDI
2006 Jetta TDI Cup
Listen. This is just ridiculous. For every revolution of the engine it consumes SOME fuel, NOT ZERO. If it was zero it would not run. Don't read into this crap. What makes a diesel run? Just air? Come on people. Every stroke of each piston consumes fuel.

The only way to conserve it is to keep the rpm's down, for example, on hills depress clutch and idle. You will use more brakes, but that is how it is. All a trade off.
 

whitedog

Veteran Member
Joined
Jul 12, 2004
Location
Bend, Oregon
TDI
2004 Jetta that I fill by myself
What I'm saying is that the engine does not need to produce power in an over run condition. Since it does not need to produce power, it does not need to consume fuel.
 

Bob_Fout

Oil Wanker
Joined
Sep 5, 2004
Location
Indiana
TDI
2003 Jetta - Alaska Green (sold) / 2015 GTI 2.0T
Sir Isaac Newton is TURNING in his grave over this thread.

(points for those who figure out the hidden meaning and the play on words in that sentence)
 

tditom

Top Post Dawg
Joined
Sep 5, 2001
Location
san antonio & austin
TDI
formerly: 2001 Golf GL, '97 Passat (RIP) '98 NB, '05 B5 sedan
too lazy to search? here ya go (pay particular attention to the function of the "control collar"):
Inside the TDI injection pump there is a "Cam Plate" that is turned by the pumps timing belt pulley. The cam plate is also turning a small "Plunger", these two assemblies rotate in unison. The rotating plunger is mounted to a stationary plate or "Roller Ring" that has four rollers that provide the contours for the spinning cam plate to follow and cause the plunger to extend in and out on. The cam plate contours are what determines how far the plunger slides in and out of the fuel compression chamber and the size of the contours are what can vary from year to year effectively determining how hight the fuel pressures can reach. Since the roller ring assembly (fig 2) is "mostly" stationary the compression of the small amounts of fuel can occur at exactly the same time. The caviate to this is that the roller ring assembly can be rotated slightly clockwise or counter clockwise effectively adjusting when the compression of the fuel actually occurs without having to integrate a complex system for injection timing.



The looming question is how do you control the quantity of fuel ie speed of the motor? The above describes how you effectively create fuel pressure and adjust timing but the next key is the "Fuel Collar" (Fig 7). The fuel collar is a simple ring that the plunger slides through during each and every injection stroke. In the plunger there is an elongated hole (figure 5, top) that when uncovered allows all remaining fuel in the compression chamber to be vented back into the internal body of the pump effectively ending the fuel delivery for that cylinder. This means that all unused fuel is simply vented internally without having to flow to and from each injector. The injectors ONLY recieve what is needed and the fuel collar is what directly determines engine speed and power output.



So internally you have an oscilating and rotating plunger that aligns with the four or in some cases 5 or even 6 outlet ports depending on the pump layout. In the case of the VW ALH TDI, with every complete horizontal stroke of the plunger, the plunger itself rotates 90 degrees. This means that it goes from start point to full extension and back as it rotates a complete 90 degrees of rotation. With one full 360 degrees of rotation the plunger will actuate 4 complete injection events while varying timing and injection quantity on each and every stroke.



The collar is controlled by an electrical servo that receives its control signal from the cars ECU. The computer looks at all engine sensors and determines when and how much fuel to inject.

On each injector there are fuel return lines. Since the insides of the injectors are not perfect there is sometimes some fuel that is leaked internally. Rather than allowing that fuel to remain in the injector Bosch allows for a return line to return that fuel back to the cars fuel filter where it is recirculated, filtered and eventually burned in the engine.

The pump can be described as having 2 seperate pumps. The first is often described as a "Low Stage Vane type pump" This provides "Head Pressure" to make the fuel push its way into the plunger chamber fast enough not to allow cavitation of the plunger. The pressure of the pump is around around 100 psi. When the plunger compresses the fuel the pressures can reach as high as 19,000 psi at the fuel nozzle.



Hope that helps.

DB
someone else can explain the details for the PD
 

whitedog

Veteran Member
Joined
Jul 12, 2004
Location
Bend, Oregon
TDI
2004 Jetta that I fill by myself
The PD is easier. The injectors are spill, pump, spill. This means that there is fuel flow through the injector until the solenoid activates and stops the spill, causing the rise in pressure and eventually injection.

If the computer sees that it doesn't need fuel to run, it just doesn't energize the solenoids.
 

Drivbiwire

Zehntes Jahr der Veteran
Joined
Oct 13, 1998
Location
Boise, Idaho
TDI
2013 Passat TDI, Newmar Ventana 8.3L ISC 3945, 2016 E250 BT, 2000 Jetta TDI
Picture a wheel rolling down a hill. Weight and gravity pull the wheel resulting in a state of disipation.

If a TDI is not being commanded to accelerate and the engine rpms are being sustained while "Requested Load = 0" fueling is reduced to 0 mg/stroke in a coasting or even a state of acceleration (as a function of a steep downward ramp).

As stated MANY MANY MANY times the condition is offically called "OVERRUN".

It's a fact, a TDI when coasting CAN in fact use ZERO FUEL since rpms are being sustained by the coasting condition or the movement of the car down an incline which is sustaining rpms above idle or the "requested Load value".

DB
 
Last edited:

LarryLymb

Veteran Member
Joined
Apr 13, 2002
Location
Lehigh Acres, FL
TDI
Jetta TDI Wagon 2003 Mojave Beige, 5 speed
For Bob Fout who stated that "in situations where the ground is flat...". In those cases you are using up energy (fuel) that got you to the speed when you started coasting in gear. It is much better to plan ahead than to use the engine to slow down for that reason.
 
Top