I have tried shifting into neutral when coasting to a stop w/ my 2011 tdi wagen (automatic)which seems to register higher fuel economy on the display than leaving it in D. Does anyone have any thoughts about the bennefits or drawbacks of doing this? Would it be harder on the tranny? It really rolls a lot easier. I assume the brakes would get more wear.
tdi driving question
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It's not harder on the engine or transmission enough to make any difference.
Fuel savings depends on how much coasting you can do in gear vs. out of gear. Basically, coasting in gear with foot completely off the accelerator, you use no fuel because the engine is "winding down". However, as you noticed, the engine braking effect does slow you down. In neutral coasting the engine does use a little bit of fuel to keep idle but you can roll farther because there's no engine braking.
Fuel savings depends on how much coasting you can do in gear vs. out of gear. Basically, coasting in gear with foot completely off the accelerator, you use no fuel because the engine is "winding down". However, as you noticed, the engine braking effect does slow you down. In neutral coasting the engine does use a little bit of fuel to keep idle but you can roll farther because there's no engine braking.
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As Seatman mentions, you will theoretically burn more fuel by coasting to a stop in neutral than simply leaving it in gear.
All modern engines, gas or diesel, will shut off fuel flow through the injectors when you lift off the "gas" pedal while in gear. As a result, leaving the transmission in gear throughout your coast down should burn virtually no fuel whereas selecting neutral will require the engine to idle, which burns fuel.
In fact, my manual-transmission TDI has a clever little "recommended gear" indicator in the desplay between the tachometer and speedometer. It's a smart indicator, as it recommends changing gears based on instantaneous driving conditions. If I select neutral and coast down from, say, 60 MPH, the indicator will recommend downshifting to 5th, then 4th, then 3rd as I decelerate.
As for wear on the transmission, I doubt it's an issue. Your DSG will downshift automaticlaly as necessary as you slow in order to keep the engine revving at a sustainable RPM. So a coast down from 60-0 MPH might result in 6 downshifts in total. Seems like a lot, but like any automatic transmission, DSG or not, it was designed to do that, not to be manually manipulated into neutral every time you slow down.
If your'e seeing a measurable difference in average fuel economy by using this neutral technique, then I'm at a loss. How are you measuring economy? Instant MPGs on the display? Average over a few hundred miles on the same route using different techniques?
All modern engines, gas or diesel, will shut off fuel flow through the injectors when you lift off the "gas" pedal while in gear. As a result, leaving the transmission in gear throughout your coast down should burn virtually no fuel whereas selecting neutral will require the engine to idle, which burns fuel.
In fact, my manual-transmission TDI has a clever little "recommended gear" indicator in the desplay between the tachometer and speedometer. It's a smart indicator, as it recommends changing gears based on instantaneous driving conditions. If I select neutral and coast down from, say, 60 MPH, the indicator will recommend downshifting to 5th, then 4th, then 3rd as I decelerate.
As for wear on the transmission, I doubt it's an issue. Your DSG will downshift automaticlaly as necessary as you slow in order to keep the engine revving at a sustainable RPM. So a coast down from 60-0 MPH might result in 6 downshifts in total. Seems like a lot, but like any automatic transmission, DSG or not, it was designed to do that, not to be manually manipulated into neutral every time you slow down.
If your'e seeing a measurable difference in average fuel economy by using this neutral technique, then I'm at a loss. How are you measuring economy? Instant MPGs on the display? Average over a few hundred miles on the same route using different techniques?
As far as the mpg difference, I'm just going by the readout on the instant consumption display. When I decelerate in D w/ no foot on gas pedal the display shows --- or nothing; I assume this distance is simply ignored in the display and has no effect on the average, as soon as I press the accelerator again the mpg will appear. If its a hard acceleration it might show single digits at first; if I don't come to a full stop and can coast through an area w/ a light touch on the gas it will show as high as 200 which must be the max it goes to for a brief period. As long as I'm coasting in N, it reads 200 the entire time so I think this is incresing the average mpg readout. I just started trying this yesterday so I don't have any long term data but my average mpg per the display seems like its going up though this might be tricking the computer's calculations and not real world results. I know many variables affect the averages and I've only put 600 miles on this car so far so I'm still figuring some of it's features out, glad I found this site and I appreciate your input.
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Here's my rule of thumb on the matter.
Highway speeds: if you have to touch the brakes, leave it in gear. Engine braking is more fuel efficient and will let you use less braking force. This is handy when you're on a decline and you want to let the momentum of the car keep you going but you're approaching traffic or you're already going as fast as you'd like to and the slope will cause you to go faster. However, if leaving it in gear will make you slow down so much that you'll have to press on the accelerator again, you're better off coasting in neutral the whole way.
Approaching a stop: Always leave it in gear, except for 1st gear (for manuals). It's most fuel efficient to row down the gears to a stop, but there's no point in trying to jam it into first. Yo're much better off shifting into first after you've come to a complete stop.
Highway speeds: if you have to touch the brakes, leave it in gear. Engine braking is more fuel efficient and will let you use less braking force. This is handy when you're on a decline and you want to let the momentum of the car keep you going but you're approaching traffic or you're already going as fast as you'd like to and the slope will cause you to go faster. However, if leaving it in gear will make you slow down so much that you'll have to press on the accelerator again, you're better off coasting in neutral the whole way.
Approaching a stop: Always leave it in gear, except for 1st gear (for manuals). It's most fuel efficient to row down the gears to a stop, but there's no point in trying to jam it into first. Yo're much better off shifting into first after you've come to a complete stop.
The instant consumption display goes to --- because you're not using any fuel. MPG = miles divided by gallons. What happens when you divide by zero? The distance is not ignored... it's added to the trip avg mpg.
Just leave it in gear. Besides, it can be dangerous in an emergency situation where you might need throttle input.
I apprecate the input. One thing I don't understand is the statement that fuel is shut off when you are braking or coasting w/ no throttle in D, while the engine is using more fuel in Neutral since it is idling. If the fuel supply was cut off, wouldn't the engine stall? It has to be using fuel to keep running doesn't it?
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With the transmission in D, your engine is always mechanically connected to the wheels through the transmission, and the ratio of that connection (engine speed to wheel speed) is determined by the gear selected at the time by the transmission.
So even with zero fuel flowing through the injectors and into the engine, the momentum of your car keeps you moving, the wheels turning, and thus the engine revving. The engine will only stall if the transmission remains in gear while you come to a stop because the engine will stop too.
The reason you feel a more pronounced deceleration when you lift off the throttle while in D (or in any gear) compared to neutral is this:
In this situation, because no fuel is being injected into the engine and no power is being made, the engine is being "driven" by the wheels, instead of the other way around. The engine is acting as a big, expensive air pump, sucking in air past the intake valves, compressing it to 16 times atmospheric pressure, and pushing it out the exhaust valves with no power-generating explosion of a fuel/air mixture in between to make power. This is "engine braking," in which the drag of the engine's mechanical (and aerodynamic) operation "brakes" your car, causing faster deceleration than selecting neutral, which disconnects the engine from the transmission (or more accurately, the transmission from the wheels).
One final thing: you probably notice this engine braking effect more with your DSG "automatic" transmission than with a traditional automatic because the DSG is more mechanically similar to a manual transmission that's automatically operated than a true auto trans. Your DSG has sets of helical gear cogs and clutch plates like a normal manual, while computers and electrohydraulic actuators take care of clutch-less gear changes. A traditional automatic has planetary gearsets connected to the engine through torque converter, which uses the movement of hydraulic fluid to transmit power. The result is that, even in drive, there's not much difference in deceleration in D versus in neutral because there's a certain amount of "slop" in the fluid-filled torque converter, though the fuel savings are still there.
That ended up being a pretty long post. But I hope it was helpful.
Well said Speed Racer, you sound like a mechanical engineer and I appreciate the explanation. I guess that is why my JSW TDI w/ dsg feels so different than my other car which is an automatic Nissan Altima. The Altima feels like it rolls much easier at slow speeds w/o touching the gas pedal, barely noticeable engine braking compared to the VW.
In D the Altima starts rolling as soon as you let off the brake but the VW just wants to sit there until you hit the throttle.
I suppose the VW's turbo does not really engage until you press the accelerator which takes some getting used to when switching from 1 car to the other, though at speed the response in the VW seems instant whereas it barely wants to move when in D w/ no foot on the gas on level or uphill grade. I guess that is just the difference in the engineering of the two.
In D the Altima starts rolling as soon as you let off the brake but the VW just wants to sit there until you hit the throttle.
I suppose the VW's turbo does not really engage until you press the accelerator which takes some getting used to when switching from 1 car to the other, though at speed the response in the VW seems instant whereas it barely wants to move when in D w/ no foot on the gas on level or uphill grade. I guess that is just the difference in the engineering of the two.
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My take on this is it does not matter.. Let's say that out of the total operating cycle of the vehicle, you are actually slowing down 10% of the time.. Even be conservative and say it's 20% of the duty cycle. Then the difference between fuel consumed idling or shut off while slowing down in gear, won't even show up on the radar screen. .Ignore the instantaneous fuel economy on the MFI.. Check it tank fill up by tank fill up.. You won't see any difference. Just drive and enjoy the car.
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I'm not a mechanical engineer or anything. Just a copywriter with a life-long interest in all things automotive and a thick streak of car geek
Turbospool1 is right, it probably makes a negligible difference in certain situations. If your commute involves a quick drive to the highway entrance ramp, 30 minutes at highway speed, and a quick drive from the exit ramp to your desination, you spend so little time actually decelerating that you won't see a consistent difference in mileage since other variables, like temperature and traffic, will make a much bigger difference.
But it can make a potentially larger difference if you spend a lot more time decelerating. Here in Chicago, I feel like I spend most of my drive slowing down. That's an exaggeration, of course, but with so much accelerating and decelerating between stoplights, I do notice that it's easier to keep my average MPG above 30 if I do a bit of planning and keep the car in gear while rolling up to a stoplight or departing Lake Shore Drive exit ramps. It's a small difference, to be sure, but it's there. And since you'll never get BETTER mileage by shifting to neutral in your DSG, there's surely no point in doing so, as it's not a normal procedure while driving an automatic.
As for the other differences you feel between your Altima and the VW, it's also due to the difference between a DSG and traditional automatic transmission, not the turbo. Your Altima has either a 4 speed automatic or a CVT, depending on how old it is, but either way, power is transmitted from the engine to the transmission though a torque converter. Basically, a torque converter uses the spinning motion of the engine's flywheel to push hydraulic fluid onto an impeller, which is connected to the transmission. There is no direct mechanical connection between the engine and the transmission while standing still in D. This allows the engine to stay running without the wheels turning. When you accelerate, a surge of hydraulic fluid flows through the torque converter and applies torque to the transmission, turning the wheels. But even while standing still, there is still some fluid flow through the converter, so when you lift off the brake, the car "creeps" forward. This is a very simplified explanation, but you get the idea.
Since the DSG is more similar to a clutch-operated manual transmission, the transmission must completely disengage from the engine when the car is stationary. The clutch doesn't engage until you command acceleration through the pedal, so there's no "creep" like in a regular automatic, and there's a slight hesitation at first press of the throttle as the transmission attempts to smoothly engage the clutch to get the car rolling. The turbo definately takes a moment to spool up, and this turbo lag certainly magnifies the delay. But in my experience, turbocharged cars with regular automatic transmissions suffer from the same delay as well.
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I wish this would be implemented in Golf TDI's MDI...
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The DSG in my 2011 Jetta does indeed "creep" when you let off the pedal. You can feel the clutch smoothly engage when you let off the brake pedal. It also keeps the car from rolling backward on a hill. You can let off the brake and the car won't roll until you apply the gas. Very cool. The DSG is amazing! It's like the best of both worlds. Hands free operation, economy, and performance.
Another thing I've noticed with my DSG is that it downshifts sequentially through the gears when slowing so if you get back on the throttle, you are in the right gear. Very cool.
The only time I've had an issues with it was when I was intentionally trying to "fool" it. I was rolling along at idle in first gear, gave it some throttle, then immediately let off again. I could feel it herk and jerk like a real manual transmission would do in that circumstance if you didn't push in the clutch. It stopped with I tapped the brake and the clutch disengaged.
I think they must have made some significant changes to the DSG for 2011, because mine is wonderful.
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