[OT] Engine braking

[Andrei Alexandrescu]

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