I live just down the road from a bus garage (South Croydon) and catch a
bus pretty much every day. I quite often see (and sometimes get on)
buses which have just come from the garage that are giving off a pretty
large amount of blue smokey exhaust. There is usually enough smoke that
the air behind the bus is turned hazy for a short while afterwards, and
that the smoke also gets into the passenger compartment. These are
double-decker buses that are no more than a couple of years old. Afraid
I haven't paid any attention to what model of bus they are.

What I was wondering was whether these buses have some sort of engine
damage, or whether they need to warm up properly. I suspect the latter
because the times it's been worst, the bus has been cold so presumably
making first trip of the day, and also because it seems to have got
better by the end of my journey (around 10 mins later).

Can anyone confirm my suspicion, and if so, are the bus companies
"supposed" to let the buses warm up before sending them out?

Apples wrote:
I live just down the road from a bus garage (South Croydon) and catch
a
bus pretty much every day. I quite often see (and sometimes get on)
buses which have just come from the garage that are giving off a
pretty
large amount of blue smokey exhaust. There is usually enough smoke
that
the air behind the bus is turned hazy for a short while afterwards,
and
that the smoke also gets into the passenger compartment. These are
double-decker buses that are no more than a couple of years old.
Afraid
I haven't paid any attention to what model of bus they are.

What I was wondering was whether these buses have some sort of engine
damage, or whether they need to warm up properly. I suspect the
latter
because the times it's been worst, the bus has been cold so
presumably
making first trip of the day, and also because it seems to have got
better by the end of my journey (around 10 mins later).

Can anyone confirm my suspicion, and if so, are the bus companies
"supposed" to let the buses warm up before sending them out?

I am no expert but as a former diesel car owner, a diesel will always
smoke a bit on start up as you are pumping unburnt fuel into the
exhaust until the engine fires up. I guess in cold weather this will
condense in the exhaust system then evapourate out over time. But 10
minutes seems excessive. My car would smoke a bit for the first few
hundred yards and that was all. A bigger diesel may behave differently.
Kevin

wrote:
Apples wrote:
I quite often see (and sometimes get on)
buses which have just come from the garage that are giving off a
pretty large amount of blue smokey exhaust...
What I was wondering was whether these buses have some sort of engine
damage, or whether they need to warm up properly.

I am no expert but as a former diesel car owner, a diesel will always
smoke a bit on start up as you are pumping unburnt fuel into the
exhaust until the engine fires up. I guess in cold weather this will
condense in the exhaust system then evapourate out over time. But 10
minutes seems excessive. My car would smoke a bit for the first few
hundred yards and that was all. A bigger diesel may behave differently.

Warm up is the main reason. For the comparison with cars, the only thing
cars and buses have in common is that the engine works the same way yet
that's about it. AFAIK the biggest diesel in cars was the Mercedes 300D
(6 cylinder 3.0 litres) the most common are about 2 litres now. The most
common bus engine will be around 10 to 12 litres (the smallest about 7
litres going up to 14 in coaches). These things do behave a little
different, especially when cold.

Yet there could be another reason: additional heating system (like
Webasto or Eberspächer). These tend to smoke quite a bit too.
As for that I've recently seen a car smoking like hell from the engine
compartment (wheel housing to be precise) but it was certainly not from
the engine itself. From what I know it looked more like heating.

Warm up is the main reason. For the comparison with cars, the only thing
cars and buses have in common is that the engine works the same way yet
that's about it. AFAIK the biggest diesel in cars was the Mercedes 300D (6
cylinder 3.0 litres) the most common are about 2 litres now. The most
common bus engine will be around 10 to 12 litres (the smallest about 7
litres going up to 14 in coaches). These things do behave a little
different, especially when cold.

Yet there could be another reason: additional heating system (like Webasto
or Eberspächer). These tend to smoke quite a bit too.
As for that I've recently seen a car smoking like hell from the engine
compartment (wheel housing to be precise) but it was certainly not from
the engine itself. From what I know it looked more like heating.

The engines in modern buses now such as the Volvo B7TL or Dennis Trident or
even the Mercedes Citaro bendibuses tend to be quite small at 7 litres, a
move away from larger engines. The mini buses such as Darts have even
smaller engines such as 5.9 litre for the older ones, to 3.9 litre for the
newest Dart SLF. But yes they can be a bit smoke on start up. Once running
however I dare any car driver to floor his (diesel) car against its
rev-limiter and not be scared of either the engine falling to bits or large
amounts of smoke coming out. Mind you my coach won't rev fully in neutral
against the rev-limiter. Are cars protected in the same way now?

David B wrote:


The engines in modern buses now such as the Volvo B7TL or Dennis Trident or
even the Mercedes Citaro bendibuses tend to be quite small at 7 litres, a
move away from larger engines.

The Citaro G with only 7 litres, I doubt that. The standard and the L
both have the same 12 litre engine, only one is limited to 250hp and the
other develops 350hp.

The mini buses such as Darts have even
smaller engines such as 5.9 litre for the older ones, to 3.9 litre for the
newest Dart SLF.

The mini and midi buses are somewhat different ;-)

But yes they can be a bit smoke on start up. Once running
however I dare any car driver to floor his (diesel) car against its
rev-limiter and not be scared of either the engine falling to bits or large
amounts of smoke coming out.

But then that's going to be a deep black cloud - soot. It is said that
one _should_ do this every now and then, just to clean the exhaust system.

Mind you my coach won't rev fully in neutral
against the rev-limiter. Are cars protected in the same way now?

Isn't every diesel? You can only kill it with the wrong gear
downhill/slowing down. Everything other is a myth.

"Guy Perry" wrote in message
y.telekom.at...
David B wrote:


But yes they can be a bit smoke on start up. Once running however I dare
any car driver to floor his (diesel) car against its rev-limiter and not
be scared of either the engine falling to bits or large amounts of smoke
coming out.

I've only hit the rev limiter once in a diesel-engined car, and that was
years ago when I was pulling out into traffic in a situation where there was
very little gap between cars and I'd been waiting five minutes for someone
to take pity on me and let me in. The sensation of hitting the rev limiter
was not nice: rather than simply preventing the throttle opening any wider,
it actually cut the power for a fraction of a second, then reapplied it and
so on repeatedly, rather like ABS applies and releases the brakes
intermittently. For the second or so before I'd changed up to second, the
car behind was lost in a cloud of black smoke ;-)

Normally there's no need to floor the accelerator: diesels produce maximum
power and torque well below the maximum engine speed (eg at around 2000 rpm
whereas the limit is around 5000 rpm) so if you go much beyond this speed,
the car accelerates less well than if you keep changing up when the engine
reaches about 2500 rpm. But in the heat of the moment when you want to get
out of a tight situation, you tend to forget to change gear!

When I drove buses a few years ago on the routemaster/leyland titans etc
warming up made no difference.

if u left them ticking over to warm up the saloons before going out the
exhaust would bellow out big white clouds of the stuff for a while.

now days most moden buses use a newer green enviro friendly fuel i
believe...
"Martin Underwood" wrote in message
...
"Guy Perry" wrote in message
y.telekom.at...
David B wrote:


But yes they can be a bit smoke on start up. Once running however I dare
any car driver to floor his (diesel) car against its rev-limiter and not
be scared of either the engine falling to bits or large amounts of smoke
coming out.

I've only hit the rev limiter once in a diesel-engined car, and that was
years ago when I was pulling out into traffic in a situation where there
was very little gap between cars and I'd been waiting five minutes for
someone to take pity on me and let me in. The sensation of hitting the rev
limiter was not nice: rather than simply preventing the throttle opening
any wider, it actually cut the power for a fraction of a second, then
reapplied it and so on repeatedly, rather like ABS applies and releases
the brakes intermittently. For the second or so before I'd changed up to
second, the car behind was lost in a cloud of black smoke ;-)

Normally there's no need to floor the accelerator: diesels produce maximum
power and torque well below the maximum engine speed (eg at around 2000
rpm whereas the limit is around 5000 rpm) so if you go much beyond this
speed, the car accelerates less well than if you keep changing up when the
engine reaches about 2500 rpm. But in the heat of the moment when you want
to get out of a tight situation, you tend to forget to change gear!