Re: Aircraft emissions techie question

Date:         17 Dec 99 01:57:45 
From:         MJ <rmjones@cyberhighway.net>
Organization: CyberHighway
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Jack Pease wrote:
> Don't suppose anyone can help me with a technie question.
>
> NOx emissions rise with engine power. NOx is saved if a pilot selects 85%
> thrust on take off rather than 100% thrust, and presumably save fuel, so why
> isn't this done all the time if the engines have enough in reserve???

Safety is the primary concern.  Assume an engine is lost at or just
above V1 (the speed at which you either abort or continue a take-off if
an engine is lost).  If your remaining engine(s) is only at "85%" (your
value) you must take the added step of throttling up the remaining
engine(s) to 100%.  While the new generation of engines spool up much
faster than previous, it still takes time.  This could mean the
difference between the situation being a non-event or a marginal event.

Further, there are still a significant number of birds out there that
are short on thrust, and eat up an awful lot of real estate in any case.

Based on what I know of NOx production from gas turbines in general, I
also suspect that the higher NOx production over a shorter period of
time (assuming a throttle-back after reaching a safe altitude) would
roughly equal the total NOx produced by a low-power departure.  At
least, the difference would not be great.

Then, you have the added CO emissions likely brought on by reduced
firing temps of a lower-power departure.  In a low-power departure, you
would be trading NOx for CO.

Add in the need to clear the runway as quickly as is reasonably possible
to allow for more ops (ten seconds or so per departure would really add
up over hundreds, or thousands, of operations per day), plus the added
time aircraft would sit idling, doing no useful work, but still emitting
to some degree) and you begin to see why full-power departures are
GENERALLY desireable.

> Does
> it reduce ambient noise if a plane shoots down the runway on 100% power then
> thrusts down once over the airport boundary, or is it better to be 85%
> thurst all the way but lower on the airport boundary?

Yes, in terms of lessening the impact for the most people, it makes
sense to depart at maximum performance, and then throttle back when
safe.

> Modern engines produce more NOx than old ones, but why do the authorities
> claim that PM10 emissions are dropping hugely when it appears there are not
> emission factors for this pollutant?

I don't completely understand your question.  Are you saying that PM10
is not tested for from aircraft engines?  In any case, yes PM10
production from aircraft engines is much less than w/ previous
generations.  Remember what a B-707 looked like on departure?  You don't
see that plume of particulate matter (including PM10) like you used to.
That is a result of much better combustion efficiency, of course.  Which
is why NOx produced per-engine is up, of course.

> Why is SO2 considered irrelevent when if you plug in SO2 into the UK
> national emissions database (http://www.aeat.co.uk/netcen/airqual/naei/ ) do
> you get two dirty great red splodges over Gatwick and Heathrow? Where is
> this SO2 coming from, given that avgas is a light distallate??

Somebody else would be in a better position to answer that one (I think
I know, but am just not certain enough!)

To further discuss your concern re: NOx production, dry-low NOx
combuston systems suitable for use in aero engines (read: relaible
enough) are probably not too many years away.  The bugs are being worked
out in aero-derivatives used in industrial applications (like power
generation and pipeline compression).  While the teething pains have
been considerable with the first generation of products, the second
generation designs are much more reliable and operate satisfactorily
over a wide range of power settings.  Not quite there yet for use in
aero engines, but getting close.  Between the risks of higher emissions
levels (primarily NOx) or catastophic 'combustor rumble' I'll take the
higher emissions (for now) anyday!

> IS there anybody out there who can give an independent and authoritative
> answer on these questions?

Don't know about the authoritative part (just applying what I know as a
pilot, mechanical engineer and power-plant guy) but definitely
independent!

Cheers,

MJ