[OT] Engine braking

[Adam Wilson]

On Tue, 30 Jul 2013 12:02:46 -0700, Andrei Alexandrescu  
<SeeWebsiteForEmail at 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/