[OT] Engine braking

[Andrei Alexandrescu]

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