[OT] Engine braking

[Adam Wilson]

On Tue, 30 Jul 2013 15:40:52 -0700, Walter Bright  
<newshound2 at digitalmars.com> wrote:

> On 7/30/2013 12:06 PM, Adam Wilson wrote:
>> 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 ... :-)
>
> How would you know if excessive wear was caused by engine braking or  
> not? Excessive wear can be caused by all kinds of things, like not  
> letting the engine warm up before driving it hard, or running long  
> between oil changes, shifting prematurely or too late, etc.
>

Personally, I wouldn't. :-) But my Dad studied metallurgy as a minor at UW  
and let's just say that he enjoys Metal Failure Analysis WAY more than one  
could charitably consider normal for a human.

Aviation Sidebar: His favorite metallurgy class was taught by an active  
duty Boeing incident response team member and his favorite story was his  
teacher dragging the failed main gear bogey of a 727 into the room and  
asking the class to figure out what went wrong.

If my dad says he has never seen that type of problem, he probably hasn't.  
Based on watching do failure analysis' on cars, my guess is that there  
would be telltales that clue him into what was happening. As far as the  
combustion chamber goes, when we rebuilt my 96 Accord it had some minor  
pitting from the repeated explosions but nothing else of note at at 225k.  
We resurfaced the barrels and moved on. And I'm a commensurate compression  
braker. :-)

Indeed, the other things you listed are quite evil on the internals of the  
engine. Particularly going too long between oil changes. But compression  
braking isn't on the list from an engineering standpoint. The components  
of the transmission and engine and much beefier than they strictly need to  
be. No manufacturer wants THAT recall at 5k per repair. Essentially, it's  
not any different than driving forward, you are just reversing the stress  
on components that were engineered to handle it moving forward. And in the  
case of automatics, the Torque Converter acts as a buffer between the  
engine and transmission at cruising speeds (varying by model) until it  
hits the lock-up.

>
>> 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.
>
> I'll have to add that my knowledge of these things is pre-1990. So are  
> the cars I work on :-)
>

Hehe, Emissions Control only really got complicated in the last 15 years  
or so. And most people drive cars newer than 15 years, unlike the Crazy  
Leader of D Who Shall Remain Nameless. ;-)

-- 
Adam Wilson
IRC: LightBender
Project Coordinator
The Horizon Project
http://www.thehorizonproject.org/