[OT] Engine braking

Adam Wilson flyboynw at gmail.com
Tue Jul 30 12:06:41 PDT 2013


On Tue, 30 Jul 2013 11:35:08 -0700, Walter Bright  
<newshound2 at 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/


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