[OT] Engine braking

[Walter Bright]

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.