Best To Downshift, or Ride Brakes Downhill?

czeetah

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When cold.

House up hill. This question only applies to cold engine.

Its a warm up question.

To avoid riding brakes a lot I must drop to first gear (manual) and on way down hit 3500-4000 RPM. While in 2nd I might not literally RIDE the brakes -- its a lot lot lot of brake. Almost might as well be. Its a 10-20mph road and 2nd no brakes you QUICKLY speed up.

BUT with a cold engine this doesn't fit in line with "keep revs down until warm"

Its not a LONG downhill so I won't brake fade myself into a ditch. But its about a seven minute ride to drop 700-800 feet in elevation.

So ... Do I shred the brakes every day, or let the engine crank at high rpm when cold?
 

JaredC01

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Personally, I'd rather wear the brakes (which are a wear item in the first place) than the engine crank / rods / pistons with cold oil being the only thing keeping it lubricated.

Not sure of any long-term damage from the revving, but brakes are cheap in comparison to replacing an engine.
 

macoombi

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Depends on how long you're planning on keeping the car. If you're only going to keep it 5-8 years, let it rev. If you're going to run it into the ground ride the brakes and replace the pads often.
 

axnels2

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I live in Colorado and highways and high elevation passes here are covered with signs informing truckers to downshift and keep their brakes cool on steep descents ... When you need it, you must have your brakes in good shape and not overheated. Its a safety thing...
 

autoxerwgn

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Diesel engines generate little to no engine braking, due to the engines being fuel throttled and not air throttled. Downshifting to slow the car is near useless and will only increase wear and tear especially when done cold. This is the reason that large trucks use jake brakes and/or an exhaust brake to help slow the vehicle down.
 

axnels2

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autoxerwgn,

I don't know if I agree with you, based on my observations... I have an 09 JSW with DSG and when that thing down shifts I definitely feel very strong deceleration... When it misbehaves and jumps into too low of a gear I feel like I can fly through a windshield.
I am guessing big rigs needs need a jake brake to help, because they carry 20000+ lbs and engine by itself not strong enough for that weight...
 

nord

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All turned back to VW. Now a 2017 Hundai Tuscon. Not a single squalk in 10k miles.
I tend to believe that we're discussing a problem that doesn't exist. In this case there are certain unavoidable conditions which must be dealt with. Be aware of them and be considerate of the vehicle.

Brakes on our cars are (as stated) a wear item. They're rather inexpensive and they last a very long time. Be aware of them, but use them.

Engine braking? The setup of our TDI's is not exactly that of old time diesels. While engine braking may not be as good as that of a gasoline engine, it's still fairly robust. Combine it with the service brakes, don't allow revs to get out of hand, and all will be well.

Lubrication to a cold engine shouldn't be a worry. Synthetic oils tend to negate the issue. Again, I'd pay close attention to revs and attempt to perhaps not exceed 3k on your initial downhill cruise, but I wouldn't worry much past that. Your TDI will be just fine.
 

jjblbi

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lbi, nj
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I would pick a gear that kept the rpms at or below 2000 and use the brakes to modulate speed. When running down long grades I prefer to apply the brakes fairly hard, let off to cool and reapply them as needed.

Parts on a cold engine have not expanded to their normal sizes and tolerances are a little greater. Brakes are cheap.
 

whitedog

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Our Sprinter had cracked pistons. After a lot of investigation, I determined that the reason for the cracking was that the previous owner (or maybe our driver) would start the engine and drive it hard when still cold. What happens is the injectors would spray on each side of the piston pin boss area and since the piston pin was a heat sink, there was uneven heating on top of the piston. This uneven heating caused the top of the piston to develop a crack.

This is a very good example of why you shouldn't load a cold engine and has nothing to do with your scenario. When you are engine braking down that hill, no fuel is being injected (or very, very little depending on who you want to believe) therefore you won't be subjecting your engine to the uneven heating.

That being said, use your brakes.
 

40X40

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Our Sprinter had cracked pistons. After a lot of investigation, I determined that the reason for the cracking was that the previous owner (or maybe our driver) would start the engine and drive it hard when still cold. What happens is the injectors would spray on each side of the piston pin boss area and since the piston pin was a heat sink, there was uneven heating on top of the piston. This uneven heating caused the top of the piston to develop a crack.

This is a very good example of why you shouldn't load a cold engine and has nothing to do with your scenario. When you are engine braking down that hill, no fuel is being injected (or very, very little depending on who you want to believe) therefore you won't be subjecting your engine to the uneven heating.

That being said, use your brakes.


+1^

What Doggie said.

Use the appropriate gear(s) AND use your brakes.

Bill
 

fxk

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Diesel engines generate little to no engine braking, due to the engines being fuel throttled and not air throttled. Downshifting to slow the car is near useless and will only increase wear and tear especially when done cold. This is the reason that large trucks use jake brakes and/or an exhaust brake to help slow the vehicle down.
^^^ This ^^^

This is why the big rigs have a device called a "JAKE BRAKE" *Jacobs Brake.
http://en.wikipedia.org/wiki/Compression_release_engine_brake
 

40X40

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Having driven class 8 rigs with Jake brakes as well as owning 3 TDIs, I can tell you that there is a definite difference between the two.

Use the appropriate gear(s) AND your brakes.

I try to drive like I have a clue, hoping that the other drivers will notice and try to do the same.... Yeah, that will happen.... LOL.

Bill
 

atc98002

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I too live on a fairly steep hill. I use a combination of engine braking (3rd gear, no higher than 2500RPM) and wheel brakes. The speed limit on the street is 30 MPH, and the engine braking is enough to keep me below 35 until I hit the last portion of the slope, then I hit the brakes. My MFI generally shows over 80 MPG at the bottom :).

But I agree that I wouldn't do it any more extreme than this, and keep the RPMs from getting too high.
 

passsattdi

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Going downhill, the turbo fins should be retracted and spinning as slow as at idle. Granted the engine being driven hard especially with our blowtorch effect, is an issue. As have been stated, the synthetic oil should flow. I certainly would try to keep the rev's down but I certainly use downhill breaking. Does cool down the turbo so not as much waiting for cool down.
 

50pascals

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Brakes are cheap and easy - kill 'em.

On long downhills (like mountain roads) do not ride your brake the whole way. Downshift to help some, and go on and off the brakes as needed to maintain the desired speed + / - 5 mph.

Ridng your brakes down a long grade will create a lot of heat for not a lot of braking.
 

fxk

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Diesels under no/low fuel situations, such as engine braking, compress the air in the cylinders regardless, and after TDC the compressed air alone (no fuel) transfers its energy back to the crank. The air-spring effect. Perfectly efficient? Heck no. But engine braking is much reduced compared to the gasser counterpart.

An air-throttled engine (gas) compresses far less air (next to none) when the throttle is closed, and therefore has nothing to give back to the crank, therefore increasing engine braking. No air, no spring. In some EFI engines, no fuel, either.

The de-activation of cylinders, first seen in the gasser Cadillac 8-6-4 engine basically turns off the gas injectors to those individual cylinders, thereby saving fuel. The engine doesn't operate at much of a loss here for the same reason a diesel doesn't have much engine braking - the air-spring effect in those part-throttled cylinders.

Should you use diesel engine braking during a descent? Of course. Just be aware that the braking will be far less than one would expect - especially if they have just come from a gasser, or the other car is a gasser.

Anything to help slow the car without needing to ride the brakes - hot brakes don't work well, and with old fluid in the system, the absorbed water could boil, and the brake pedal would go all the way to the floor - no brakes at all (fluid doesn't compress much, but gasses compress quite easily).

Funny, I had forgotten this, and was surprised with my new JSW TDI going down a decent grade when I dropped two gears and did not slow down. Revs went up. Huh? Then I remembered - Oh yeah. Diesel engine braking ain't much.

Can't fight physics. 186,000 mi/sec isn't just a good idea. It's the law!

frank
 

Rico567

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This is one of those areas that doesn't have an either / or answer. It depends. In the case of the OP's specific scenario, I would most certainly be using my brakes. OTOH, there are definitely cases for employing judicious gear selection. We routinely visit family in Chattanooga, and each time transition Monteagle Mountain about 40 miles West of Chattanooga on I-24 and its four mile 6% grade. (BTW, this stretch of I-24 was actually reconstructed due to problems trucks had with this grade) In either direction, I simply select 5th gear on the DSG (your gear may vary on the manual or etc., of course) when the car starts to go too fast in 6th, and 5th gear controls the car's descent very well -with a tap on the brakes now and then- until the bottom of the grade is reached.
So- there is no simple answer to this question, although in many cases the economic ("cheap brakes") argument makes sense.
 

whitedog

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An air-throttled engine (gas) compresses far less air (next to none) when the throttle is closed, and therefore has nothing to give back to the crank, therefore increasing engine braking. No air, no spring. In some EFI engines, no fuel, either.
This part makes sense to me finally. I can see that the engine is also fighting to suck air past the closed throttle plate making it brake harder.

The thing that I have always had trouble with is that a diesel starter will be a bigger, higher horsepower motor than for a comparable gasser engine so from that, I concluded that a diesel engine must have more braking.

Adding in the above, I can certainly understand it.

Now, there are folks that will say, "But you can feel the different braking effects in this car vs. that car." But how can you compare apples to apples here? A similar sized engine (2.0 or 1.8T) will have different power, so is that comparable? I don't know? And an engine with comparable power, will have different displacement, or different power curves. So how does that effect engine braking? I don't know.

I think this is my biggest hurdle in fully understanding why a gasser has more engine braking than a diesel. I over analyze things.
 

fxk

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Vast wilderness between DC and Baltimore
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2014 Sportwagen TDI
This part makes sense to me finally. I can see that the engine is also fighting to suck air past the closed throttle plate making it brake harder.

The thing that I have always had trouble with is that a diesel starter will be a bigger, higher horsepower motor than for a comparable gasser engine so from that, I concluded that a diesel engine must have more braking.

Adding in the above, I can certainly understand it.

Now, there are folks that will say, "But you can feel the different braking effects in this car vs. that car." But how can you compare apples to apples here? A similar sized engine (2.0 or 1.8T) will have different power, so is that comparable? I don't know? And an engine with comparable power, will have different displacement, or different power curves. So how does that effect engine braking? I don't know.

I think this is my biggest hurdle in fully understanding why a gasser has more engine braking than a diesel. I over analyze things.
I just love to see the lightbulb go on. Thanks for sharing!!!

Yep. The high vacuum, low volume of compressed air, compression ratios all part of the equation.
I had a similar epiphany. My reasoning was the high compression ratio and the energy needed for that compression (like the bigger, heavier starter in your example) would be the determining factor. It wasn't until something I read pointed out the air-spring effect. Bingo!

Ever look at a compression gauge as the engine turns throttle open vs throttle closed?

Just for the sake of argument, even before the 2014 spec "hybrid" engines in the F1 cars, they had the means to dial in differing amounts of engine braking - one of the driver/track parameters available to the chief mechanic - probably delaying the exhaust valve closing to reduce the volume of air in the cylinders, leaving the throttle plates open further with less or no fuel.
Who knows the sorcery used in these engines!

So engine braking is dependent on lots of engine and transmission variables.

Dropping a gear in one car vs another car at the same speed will most likely feel different car to car. If the ratios on the tranny are a little different on either the gear you're coming from, or the gear you're going to. Add in weight of the car (how well the driver heel-and-toe the pedals)

frank
 
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fxk

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Which leads me to another question.
The TDI engines have some sort of waste-gate, no?
During closed fuel (closed throttle) deceleration (engine braking), does the waste gate open to dump the pressure from the turbo?
frank
 

nord

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Compression ratio of a diesel is higher than a gas engine. It follows that air is squeezed tighter in order to heat it enough to cause ignition. By virtue of this fact the diesel demands a more robust battery and starter at minimum.

Engine braking is somewhat of a misunderstood phenomena between the two types. Your diesel, absent of a throttling flap being active, still has to compress intake air. Thus, you do have engine braking... Compression braking.

Gasoline engines are (as noted) throttle-controlled. Close the throttle and the plate blocks intake air. Thus, a vacuum is created as the pistons attempt to pull in air. In other words the engine is working against a vacuum rather than compression. So actually vacuum braking in any real sense.

But a factor not usually considered is gear ratios. Gasoline engines turn faster and have a wider rpm range than a diesel. At least a wider normal or safe operating range. The real test for engine braking is actually a matter of the total resistance able to be exerted by a given engine over a given distance under given circumstances while keeping the engine in a safe operating envelope.

The general consensus is that a gasoline engine will win in this department. Both types, though, offer engine braking. They just do it a bit differently.
 

Lincoln

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I'd go down the hill in 2nd with moderate braking. Shouldn't damage engine at moderate revs, and shouldn't overheat brakes with moderate braking. Safe both ways.

Were your hill short and steep, I'd use brakes alone--they're cheap and easy to replace and won't overheat in a short distance, whereas revs could get higher than I'd be comfortable with.
 

Salsaman06

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Compression ratio of a diesel is higher than a gas engine. It follows that air is squeezed tighter in order to heat it enough to cause ignition. By virtue of this fact the diesel demands a more robust battery and starter at minimum.

Engine braking is somewhat of a misunderstood phenomena between the two types. Your diesel, absent of a throttling flap being active, still has to compress intake air. Thus, you do have engine braking... Compression braking.....

I tend to believe that we're discussing a problem that doesn't exist. In this case there are certain unavoidable conditions which must be dealt with. Be aware of them and be considerate of the vehicle.

Brakes on our cars are (as stated) a wear item. They're rather inexpensive and they last a very long time. Be aware of them, but use them.

Engine braking? The setup of our TDI's is not exactly that of old time diesels. While engine braking may not be as good as that of a gasoline engine, it's still fairly robust. Combine it with the service brakes, don't allow revs to get out of hand, and all will be well.

Lubrication to a cold engine shouldn't be a worry. Synthetic oils tend to negate the issue. Again, I'd pay close attention to revs and attempt to perhaps not exceed 3k on your initial downhill cruise, but I wouldn't worry much past that. Your TDI will be just fine.
Yes, I agree, there IS engine braking. And czeetah, I agree with nord about using both the engine and the brakes.


A little science lesson:
With regard to engine braking , the underlying physics - specifically the law of conservation of energy that says you can't create or destroy energy, only convert it from one form to another would seem to support the "air spring" theory IF and only if the *total* energy going into the compression stroke were being returned in the compressed air expanding against the piston driving it back down. But the reality is that the energy exerted to compress the air is not completely returned to push the piston back down. Some of that energy is transferred to the engine block and heads as thermal energy. The rest stays within the compressed air molecules driving the piston back down as the air expands. BUT, because some of the energy was transferred to the engine block and heads, there is less energy applied to pushing the piston back down than it took to push it up and compress the air. Thus the overall result is a braking affect by the compression stroke. There are other things like the timing of the exhaust valve opening that "bleed off" some the compression stroke energy and contribute to the braking effect. Also what is being overlooked are things like the amount of energy needed to just rotate the engine itself (more energy is required to rotate the engine faster), the friction of the moving parts, accessories like the alternator, ac compressor etc all contribute to the overall engine "braking" effect.


Your brakes are a simple energy conversion and transferrence device. They convert the mechanical energy of the moving car (via the rotating disk) to thermal energy (via friction) and then transfer the thermal energy into the surrounding air. The faster your brakes can accomplish this process the quicker it can slow down the vehicle. Brakes fade when discs or drums have converted as much mechanical energy to thermal energy as they can to the point before the pads and shoes lose their effectiveness. The way to avoid this is to transfer that thermal energy to the surrounding air as quickly as possible (I.e. cool the brakes as fast as possible).
 
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nayr

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My TDI engine brakes alot better than my gasser of the same displacement.. I tow alot and do alot of engine braking and imho its as good as a gasser of nearly twice the displacement.. Ive attributed it to the higher compression.

I can drive through rocky mountain national park @ ~25mph in 2nd gear with a trailer in tow and set cruise control and never need to down shift or touch my brakes... there's some pretty agressive grades in both directions and its all slow.. might be higher RPM than I'd like but it beats racking through gears over and over and always using the brakes.

imho engine braking down one side of the mountain is no more wear and tear than driving up the other side.. except perhaps to the throw out bearing, those seem to be alot more worn since I moved to Colorado, but that could be the traffic :x

Nothing like driving down pikes peak in heavy traffic to come to the mandatory brake check station half way down and have all 50 minivans and suv's infront of you forced to wait for a cool down and open the rest of the highway up for a high speed decent :)

in the OP's case I'd get a frost heater and engine brake down the mountains without worry.
 
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jrm

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play around with the NMS, you will notice that engine breaking will increase when the brake lights are on- this is due to the ecm opening up the throttle valves and bringing back some of the classic diesel compression breaking (Google voice spelling) vs the nice coasting feature you have at no throttle without your foot on the brake. These cars are smart and use both intake and exhaust throttles to change compression breaking for gasser like coasting without the brake being pressed vs great compression breaking as soon as the brake lights are lit up. DSG cars will also downshift pretty aggressively with the brake lights lit up- iv seen a downshift to 3rd at 45MPH more than once
 

jhinsc

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I started playing around with DSG, downshifting manually when I need to slow down and noted the instant MFD would indicate "200" mpg meaning it's still burning fuel, vs "---", meaning fuel is cutoff when I use brakes alone. Braking when coming down a hill to maintain speed works well because the DSG will downshift and I can let off on the brakes and it will hold the gear until I step on the accelerator pedal again.
 

NSTDI

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If diesels have higher compression than gas engine (say diesels are 16:1 or higher, most gas say 10:1, numbers aren't important, just saying diesels have a significantly higher compression ratio in the cylinder), you would think they have higher engine braking, all other things like displacement, gear ratios, weight, etc. being the same. So say a 2.0L gas vs a 2.0L TDI in a Golf or Jetta.

From what I am reading here, diesels dump their compression from their cylinders when decelerating faster than a gas? So they have lower compression in the cylinders when decelerating and the gas is higher?

Don
 

fxk

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<snip>

From what I am reading here, diesels dump their compression from their cylinders when decelerating faster than a gas? So they have lower compression in the cylinders when decelerating and the gas is higher?

Don
No, diesels don't "dump" compression at all. They use it. The diesel squeezes a full cylinder of air very tightly using crank energy up until top dead center. Then the crank rotates and starts the piston down. The compressed air in the cylinder expands similar to if it had fuel/air explosion - but at obviously much less power, and transfers the majority of the energy back into the crank. Yes, some energy was lost during compression to internal friction, heat conversion, blowby, etc, but the majority of the energy is still in the compressed air. Hence the air-spring. The expansion will continue until bottom dead center or until the valves open. Remember, nearly all diesels do not have an air-throttled intake tract.

Gassers work exactly the same under Wide Open Throttle. Try coasting down a hill with the ignition off - and see whether there is more engine braking with the gas pedal down or up (providing you don't have throttle-by-wire). Open throttle will have less engine braking. It's compressing more air, but it is not fighting vacuum behind the throttle plate, and uses the air-spring effect. Close the throttle, and there's more engine braking.

The vacuum created by the closed throttle will offer a large amount of resistance to the engine (braking) and the partial charge in the cylinder will compress easily, and will not spring back with any force, further slowing the rotation of the crank.

Do diesels have engine braking? Yes. Do gassers have engine braking? Yes. All other things equal (weight, rpm, valve timing, etc.) the gasser will out engine-brake a diesel every time. Any time one uses engine braking, just watch the tach, and keep the revs reasonable.

Of course, if the TDI has clogged intake runners, it will have more engine braking than a clean TDI engine. That's another story, with the carbon deposits effectively air-throttling the engine.

Read up on the Jacobs Brake (Jake Brakes or compression brakes) if you do not believe me.

The compression-brake principle is to open the exhaust valve or supplementary valve to dump the air compressed in the cylinder just as it hits TDC. The compression has slowed the engine a little, and after the valve opens, the air-spring is gone, and now the piston travels down under the vacuum of closed valves, creating a new phase of engine braking.

The byproduct of the release of the compressed air suddenly is the expansion of the air released and what causes the extra noise under braking. That noise is quite substantial, and should indicate the amount of power lost purposefully. Also why there are signs on the outskirts of town telling truckers that use of engine braking or "Jake Brakes" use prohibited.

frank
 
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