> > Dan Elliott wrote:
> >
> > I do a lot of freeway driving and I am wondering if I am
> > being hard on my car by driving it at 80mph? It doesn't knock
> > or sound bad. Just thinking it might be more prudent somehow
> > to go slower.
> Frederick Moir wrote:
>
> Only 80? It's not even breathing hard yet!
I'm guessing when you say you have a "300DT" you mean a 300D
turbo and not "300TD" which means it's a wagon. Mercedes never
used that designation.
Anyway, whether it's a wagon or not, it won't be harmed by
making it run hard. As Fred says, "only 80 MPH" (130 km/h).
Diesel engines, especially those with pre-chambers like your
car has, will suffer with various problems _unless_ the
combustion chamber regularly gets hot enough to burn off
accumulating carbon.
To get the combustion chamber hot enough to burn off carbon,
the engine need to be at or near max power and at or near max
RPM for at least a couple minutes. With normally aspirated
engines, this isn't so hard - but with the turbo engines it
can be hard to do while maintaining lawful speeds.
The tricky bit is that getting the engine to "bump" against
the governor isn't by itself good enough. A diesel cycle
engine, in any state except maximum power, has an excess of
air. This extra air cools the combustion chamber, so to
effectively burn off the carbon we need to run the engine with
no extra air - that is, supply enough fuel to consume all the
oxygen in the air that's in the chamber. On a correctly
adjusted engine, this state occurs anytime the accelerator
(fuel feed) pedal is against the floor AND the engine is _not_
bumping the governor.
So, you put your foot to the floor - but long before "a few
minutes" has passed, the car is well exceeding lawful speeds
in most places. A big hill can help. There are a couple
hills on the interstate where I often drive. If traffic is
appropriate I will slow down a bit, shift the transmission
into 3rd, put my foot to the floor and climb the hill. The
hills are not really long enough or steep enough. That is,
I'm to the top of the hill in a minute or so, and typically
the engine is also bumping the governor.
Side note: The governor is monitoring engine speed. To keep
the engine from overspeeding, the governor reduces the
fuel being delivered to the engine. You don't feel
it, but the effect is the same as if it were lifting
your foot off the floor.
So, running your car at lawful speeds on any interstate in the
USA is not bad for it. In fact, that alone won't make it work
hard enough to keep it clean. Loafing around town _is_ bad
for it.
-- Philip
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