On 7/26/13 11:23 PM, Jordi Sayol wrote:
> On 27/07/13 01:25, Walter Bright wrote:
>>>> 2. Using the engine as a brake can cause unburned gas to wash
>>>> the oil off of the cylinder walls, resulting in excessive
>>>> wear.
>>>
>>> [citation needed]
>>
>> Mechanics at the dealer told me this. They had no reason to lie to
>> me.
>
> This absolutely true. About twenty years ago my friend's car broke
> down in a remote location. To bring the car to the nearest mechanic
> (2 or 3 kilometers), tied it to another car with a rope and used
> engine braking without ignition (engine was damaged) to prevent the
> spring effect. Result, pistons melted by excessive friction. This was
> due to the effect that Walter's mechanics clearly explained.

Thanks for this anecdote. It's at the very best circumstantial. (With the engine off, the oil pump wasn't even started!)

I've asked Walter for one credible source on the entire Internet documenting the case against engine braking. He was unable to produce one. Instead, he attempted to explain how an increase in hysteresis can cause additional wear on the engine (the parts not worn under forward use). However, this is what one poster in http://goo.gl/Ys099U had to say about that:

=================
Most of the time when you drive, you're putting a load (and causing wear) on what I'm going to call the "forward" face of each tooth on each gear in your drivetrain. The front of a tooth on the crankshaft pushes against the back of a tooth on the next gear in line, which pushes the next gear, etc. When you use "engine braking", all you are doing is engaging the teeth in the opposite direction, and putting force and wear on the faces that normally are just along for the ride.

Now, does that mean you're wearing your engine out faster? Marginally... but the parts you're wearing out would normally have to be replaced (if at all) because they'd worn out from the other side; you're wearing surfaces that would usually be thrown out with hardly any wear at all. To borrow a phrase from the medical field, your engine/transmission will die with that wear, not of it.
=================

Of course, that's just some guy on the Internet. That's why I am asking for a _credible_ source (e.g. expert mechanic, respected auto magazine etc) that explains why and how engine brake causes problems. I for one looked for a while without finding one. On the contrary, many vehicle manuals (I've seen Audi and Honda) advise using engine brake.


Andrei

On 7/30/2013 11:18 AM, Andrei Alexandrescu wrote:
> Thanks for this anecdote. It's at the very best circumstantial. (With the engine
> off, the oil pump wasn't even started!)

The oil pump is driven by the crankshaft, so if the engine is turning, the oil pump is. (There are some highly specialized race engines with an electric oil pump, but that is highly unlikely here.)

I was told by U-Haul that when towing a car long distance, you couldn't just put the manual transmission in neutral. You had to take the driveshaft out, because the transmission was designed to circulate the oil based on the front shaft turning, not the back shaft. It would sieze after a while if you only turned the back shaft.

> I've asked Walter for one credible source on the entire Internet documenting the
> case against engine braking. He was unable to produce one. Instead, he attempted
> to explain how an increase in hysteresis can cause additional wear on the engine
> (the parts not worn under forward use). However, this is what one poster in
> http://goo.gl/Ys099U had to say about that:
>
> =================
> Most of the time when you drive, you're putting a load (and causing wear) on
> what I'm going to call the "forward" face of each tooth on each gear in your
> drivetrain. The front of a tooth on the crankshaft pushes against the back of a
> tooth on the next gear in line, which pushes the next gear, etc. When you use
> "engine braking", all you are doing is engaging the teeth in the opposite
> direction, and putting force and wear on the faces that normally are just along
> for the ride.
>
> Now, does that mean you're wearing your engine out faster? Marginally... but the
> parts you're wearing out would normally have to be replaced (if at all) because
> they'd worn out from the other side; you're wearing surfaces that would usually
> be thrown out with hardly any wear at all. To borrow a phrase from the medical
> field, your engine/transmission will die with that wear, not of it.
> =================

I also pointed out the "hammering" effect of alternately forward driving then back driving the rotating parts, as the parts forcefully take up the slack of hysteresis.

I also pointed out the effect of unburned gas from backdriving washing oil off of the cylinder walls causing undue wear. This definitely happens with carbureted cars, but with modern fuel injection the fuel is shut off when backdriving.

On 7/30/13 11:35 AM, Walter Bright wrote:
> On 7/30/2013 11:18 AM, Andrei Alexandrescu wrote:
>> Thanks for this anecdote. It's at the very best circumstantial. (With
>> the engine
>> off, the oil pump wasn't even started!)
>
> The oil pump is driven by the crankshaft, so if the engine is turning,
> the oil pump is. (There are some highly specialized race engines with an
> electric oil pump, but that is highly unlikely here.)
>
> I was told by U-Haul that when towing a car long distance, you couldn't
> just put the manual transmission in neutral. You had to take the
> driveshaft out, because the transmission was designed to circulate the
> oil based on the front shaft turning, not the back shaft. It would sieze
> after a while if you only turned the back shaft.

So that invalidates the anecdote.

> I also pointed out the "hammering" effect of alternately forward driving
> then back driving the rotating parts, as the parts forcefully take up
> the slack of hysteresis.

I guess any brisk adjustment of throttle would be unadvisable, one direction or another (i.e. releasing the clutch with a large difference in rotation). Back driving, however, happens as soon as one just lifts the foot off the pedal - the inertia of the car pushes on the engine.

> I also pointed out the effect of unburned gas from backdriving washing
> oil off of the cylinder walls causing undue wear. This definitely
> happens with carbureted cars, but with modern fuel injection the fuel is
> shut off when backdriving.

That's my understanding as well. With fuel injection, essentially backdriving is rolling on zero gas consumption while preserving some mechanical energy - aweee-sooome.


Andrei

On Tue, 30 Jul 2013 11:35:08 -0700, Walter Bright <newshound2@digitalmars.com> wrote:

> On 7/30/2013 11:18 AM, Andrei Alexandrescu wrote:
>> Thanks for this anecdote. It's at the very best circumstantial. (With the engine
>> off, the oil pump wasn't even started!)
>
> The oil pump is driven by the crankshaft, so if the engine is turning, the oil pump is. (There are some highly specialized race engines with an electric oil pump, but that is highly unlikely here.)
>
> I was told by U-Haul that when towing a car long distance, you couldn't just put the manual transmission in neutral. You had to take the driveshaft out, because the transmission was designed to circulate the oil based on the front shaft turning, not the back shaft. It would sieze after a while if you only turned the back shaft.
>

That depends entirely on your specific car and how you want to tow it. Four-down towing is the preferred method and since all four wheels are touching the ground all you need to do is make sure that the transmission is self lubricating. For example, my wife's Manual 2002 Honda CR-V is ideal for towing even though it's AWD because both the transmission and rear differential are self-lubricating. You have to change the fluid more often, 40k instead of 120k, but that's about it. There are whole websites devoted to which cars are best for this and how to do it in the RV world. :-)

>> I've asked Walter for one credible source on the entire Internet documenting the
>> case against engine braking. He was unable to produce one. Instead, he attempted
>> to explain how an increase in hysteresis can cause additional wear on the engine
>> (the parts not worn under forward use). However, this is what one poster in
>> http://goo.gl/Ys099U had to say about that:
>>
>> =================
>> Most of the time when you drive, you're putting a load (and causing wear) on
>> what I'm going to call the "forward" face of each tooth on each gear in your
>> drivetrain. The front of a tooth on the crankshaft pushes against the back of a
>> tooth on the next gear in line, which pushes the next gear, etc. When you use
>> "engine braking", all you are doing is engaging the teeth in the opposite
>> direction, and putting force and wear on the faces that normally are just along
>> for the ride.
>>
>> Now, does that mean you're wearing your engine out faster? Marginally... but the
>> parts you're wearing out would normally have to be replaced (if at all) because
>> they'd worn out from the other side; you're wearing surfaces that would usually
>> be thrown out with hardly any wear at all. To borrow a phrase from the medical
>> field, your engine/transmission will die with that wear, not of it.
>> =================
>
> I also pointed out the "hammering" effect of alternately forward driving then back driving the rotating parts, as the parts forcefully take up the slack of hysteresis.
>
> I also pointed out the effect of unburned gas from backdriving washing oil off of the cylinder walls causing undue wear. This definitely happens with carbureted cars, but with modern fuel injection the fuel is shut off when backdriving.
>

My dad has been an ASE Master Technician for my entire life and teaches Emissions Certification classes for our state. What I am about to say is based stuff I've picked up from him.

I would go one step further and point out that in modern vehicles, those made after the EPA catalytic converter and air quality mandates of the early 80's, that any oil in the combustion chamber is a Very Bad Thing. Unburned hydrocarbons are highly destructive to catalytic converters and oil never burns completely during combustion. In fact we rebuilt the engine on my 1996 Honda Accord in 2010 precisely because it was starting to burn oil. And indeed, a year later the catalytic converter failed anyway due to the excessive strain placed on it by the partially burned oil that was forced through it prior to the rebuild.

My dad actually recommended engine braking (the correct term is "compression braking" btw, Thanks Dad!) as a way to reduce wear on the brakes. The google poster is correct in this statement that all you're doing is putting strain on parts that aren't used that way much, unless you reverse a lot. We see cars ranging from the early 80's on up, including carbureted, and we've NEVER once seen a car with a transmission or engine that died because of compression braking. Given our sample size of somewhere over 10,000 ... :-)

The automotive industry has spent obscene amounts of money getting the absolute cleanest burn they can to meet CAFE standards, and the very first thing they did was get the oil out of the combustion chamber. I'll also say that based on my dad's experience's with the Emissions class that even competent techs are having a VERY difficult time understanding this stuff, the chemistry involved is Ph.D stuff, and now ignition system are getting they way too. My dad has often lamented that working on cars is now more about understanding the computer control systems than it is the mechanics of it. Your average dealer tech probably has no clue what they are talking about since they have no reason to invest in learning this stuff. They don't see the car again after the warranty runs out and these systems rarely fail in five years. At least that's been my dad's experience with them.

-- 
Adam Wilson
IRC: LightBender
Project Coordinator
The Horizon Project
http://www.thehorizonproject.org/

On Tue, 30 Jul 2013 12:02:46 -0700, Andrei Alexandrescu <SeeWebsiteForEmail@erdani.org> wrote:

> On 7/30/13 11:35 AM, Walter Bright wrote:
>> On 7/30/2013 11:18 AM, Andrei Alexandrescu wrote:
>>> Thanks for this anecdote. It's at the very best circumstantial. (With
>>> the engine
>>> off, the oil pump wasn't even started!)
>>
>> The oil pump is driven by the crankshaft, so if the engine is turning,
>> the oil pump is. (There are some highly specialized race engines with an
>> electric oil pump, but that is highly unlikely here.)
>>
>> I was told by U-Haul that when towing a car long distance, you couldn't
>> just put the manual transmission in neutral. You had to take the
>> driveshaft out, because the transmission was designed to circulate the
>> oil based on the front shaft turning, not the back shaft. It would sieze
>> after a while if you only turned the back shaft.
>
> So that invalidates the anecdote.
>
>> I also pointed out the "hammering" effect of alternately forward driving
>> then back driving the rotating parts, as the parts forcefully take up
>> the slack of hysteresis.
>
> I guess any brisk adjustment of throttle would be unadvisable, one direction or another (i.e. releasing the clutch with a large difference in rotation). Back driving, however, happens as soon as one just lifts the foot off the pedal - the inertia of the car pushes on the engine.
>
>> I also pointed out the effect of unburned gas from backdriving washing
>> oil off of the cylinder walls causing undue wear. This definitely
>> happens with carbureted cars, but with modern fuel injection the fuel is
>> shut off when backdriving.
>
> That's my understanding as well. With fuel injection, essentially backdriving is rolling on zero gas consumption while preserving some mechanical energy - aweee-sooome.
>
>
> Andrei

Back driving ("compression braking" in the automotive world) is indeed a recommend procedure in modern cars. My dad (ASE Master Tech) recommends it as a way to save wear on the brakes and is as you've noted, quite an efficient use of energy. Heck, it's one of the first things he taught me how to do when I was learning how to drive.

Toyota took it one step further and built a capability into the Prius where the electric driveline reverses it's polarity and uses motors to slow down the car while simultaneously recharging the battery as the car slows down instead of using the brakes. It's called regenerative braking. Needless to say, we don't do brakes very often on Prius'.

-- 
Adam Wilson
IRC: LightBender
Project Coordinator
The Horizon Project
http://www.thehorizonproject.org/

On 30/07/13 21:02, Andrei Alexandrescu wrote:
> On 7/30/13 11:35 AM, Walter Bright wrote:
>> On 7/30/2013 11:18 AM, Andrei Alexandrescu wrote:
>>> Thanks for this anecdote. It's at the very best circumstantial. (With
>>> the engine
>>> off, the oil pump wasn't even started!)
>>
>> The oil pump is driven by the crankshaft, so if the engine is turning, the oil pump is. (There are some highly specialized race engines with an electric oil pump, but that is highly unlikely here.)
>>
>> I was told by U-Haul that when towing a car long distance, you couldn't just put the manual transmission in neutral. You had to take the driveshaft out, because the transmission was designed to circulate the oil based on the front shaft turning, not the back shaft. It would sieze after a while if you only turned the back shaft.
> 
> So that invalidates the anecdote.

I feel guilty...

> 
>> I also pointed out the "hammering" effect of alternately forward driving then back driving the rotating parts, as the parts forcefully take up the slack of hysteresis.
> 
> I guess any brisk adjustment of throttle would be unadvisable, one direction or another (i.e. releasing the clutch with a large difference in rotation). Back driving, however, happens as soon as one just lifts the foot off the pedal - the inertia of the car pushes on the engine.
> 
>> I also pointed out the effect of unburned gas from backdriving washing oil off of the cylinder walls causing undue wear. This definitely happens with carbureted cars, but with modern fuel injection the fuel is shut off when backdriving.
> 
> That's my understanding as well. With fuel injection, essentially backdriving is rolling on zero gas consumption while preserving some mechanical energy - aweee-sooome.
> 
> 
> Andrei
> 

-- 
Jordi Sayol

On Tue, Jul 30, 2013 at 12:16:11PM -0700, Adam Wilson wrote: [...]
> Toyota took it one step further and built a capability into the Prius where the electric driveline reverses it's polarity and uses motors to slow down the car while simultaneously recharging the battery as the car slows down instead of using the brakes. It's called regenerative braking. Needless to say, we don't do brakes very often on Prius'.
[...]

I got a Prius when renting a car once, and it was incredibly fuel-efficient. I had it for a week, and drove it all over the place including from LAX all the way to Irvine and back, and the tank was still half full by the time I returned the car. I found that when braking, the dashboard display shows that the battery is recharging. It even has a diagram to show you how efficient your driving is, and from experimentation, I found that gradual acceleration/braking resulted in the highest efficiency (keeps the battery bar near the top) -- probably because it was maximally postponing fuel consumption and using regenerative braking instead of the brake pads.

My only regret was paying for the fuel plan (full tank of gas), because I underestimated the car's efficiency, when I could've just let them fill up half the tank at the end for a lower total price instead.


T

-- 
If you think you are too small to make a difference, try sleeping in a closed room with a mosquito. -- Jan van Steenbergen

On 7/30/13 1:01 PM, H. S. Teoh wrote:
> My only regret was paying for the fuel plan (full tank of gas), because
> I underestimated the car's efficiency, when I could've just let them
> fill up half the tank at the end for a lower total price instead.

Yah, never do that. Whoever came up with that idea was a marketing genius. Must have made a bunch of extra $ to the rental companies.

Andrei

On 7/30/2013 12:06 PM, Adam Wilson wrote:
> My dad has been an ASE Master Technician for my entire life and teaches
> Emissions Certification classes for our state. What I am about to say is based
> stuff I've picked up from him.
>
> I would go one step further and point out that in modern vehicles, those made
> after the EPA catalytic converter and air quality mandates of the early 80's,
> that any oil in the combustion chamber is a Very Bad Thing. Unburned
> hydrocarbons are highly destructive to catalytic converters and oil never burns
> completely during combustion. In fact we rebuilt the engine on my 1996 Honda
> Accord in 2010 precisely because it was starting to burn oil. And indeed, a year
> later the catalytic converter failed anyway due to the excessive strain placed
> on it by the partially burned oil that was forced through it prior to the rebuild.
>
> My dad actually recommended engine braking (the correct term is "compression
> braking" btw, Thanks Dad!) as a way to reduce wear on the brakes. The google
> poster is correct in this statement that all you're doing is putting strain on
> parts that aren't used that way much, unless you reverse a lot. We see cars
> ranging from the early 80's on up, including carbureted, and we've NEVER once
> seen a car with a transmission or engine that died because of compression
> braking. Given our sample size of somewhere over 10,000 ... :-)

How would you know if excessive wear was caused by engine braking or not? Excessive wear can be caused by all kinds of things, like not letting the engine warm up before driving it hard, or running long between oil changes, shifting prematurely or too late, etc.


> The automotive industry has spent obscene amounts of money getting the absolute
> cleanest burn they can to meet CAFE standards, and the very first thing they did
> was get the oil out of the combustion chamber. I'll also say that based on my
> dad's experience's with the Emissions class that even competent techs are having
> a VERY difficult time understanding this stuff, the chemistry involved is Ph.D
> stuff, and now ignition system are getting they way too. My dad has often
> lamented that working on cars is now more about understanding the computer
> control systems than it is the mechanics of it. Your average dealer tech
> probably has no clue what they are talking about since they have no reason to
> invest in learning this stuff. They don't see the car again after the warranty
> runs out and these systems rarely fail in five years. At least that's been my
> dad's experience with them.

I'll have to add that my knowledge of these things is pre-1990. So are the cars I work on :-)

On 7/30/2013 12:16 PM, Adam Wilson wrote:
> Back driving ("compression braking" in the automotive world) is indeed a
> recommend procedure in modern cars. My dad (ASE Master Tech) recommends it as a
> way to save wear on the brakes and is as you've noted, quite an efficient use of
> energy. Heck, it's one of the first things he taught me how to do when I was
> learning how to drive.
>
> Toyota took it one step further and built a capability into the Prius where the
> electric driveline reverses it's polarity and uses motors to slow down the car
> while simultaneously recharging the battery as the car slows down instead of
> using the brakes. It's called regenerative braking. Needless to say, we don't do
> brakes very often on Prius'.

If the engine *is designed for it*, that's a different story entirely. The engines I work on were not designed for it.