Re: air - fuel ratio

"z" <gzuckier@xxxxxxxxxxxxxx> wrote in message
news:e7a73155-1e9e-4bd6-8955-464cab4a30b8@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On May 8, 1:50 pm, N8N <njna...@xxxxxxxxxxx> wrote:
2: Unthrottled operation. In the Diesel cycle load is controlled by
fuel
alone. This reduces gas pumping losses incurred during the
intake/exhaust strokes. In future Diesels this may fade somewhat as
throttling is being used to facilitate achieving the EGR levels needed
to meet oxides of nitrogen regulations.

So having an excess of air, compared with the minimum needed to fully burn
the fuel, is actually a bad thing for a diesel, is it, as regards the
production of NOx?

What he said. I'm not sure how much 2) contributes, but sort of as an
aside, this is the reason why you see sewer-pipe sized exhaust pipes
on Diesel vehicles - they are essentially pumping the full
displacement of the engine (or more, if supercharged) of air through
the engine every two revolutions, unlike a gasoline engine where this
only happens at WOT.

I'd not thought about it like that, till now, but you're right. At a typical
cruising speed of maybe 70 mph, that's about 2500 rpm so for a 2-litre
engine that's 2500/2 * 2 = 2500 litres of air per minute or 2500/60 = 42
litres of air per second. That's quite a flow rate through a pipe that's
maybe 8 cm in diameter.

Even at idling speed, there's a real draught from the exhaust pipe on my
car!


How much does the effect of the turbo vary with engine speed and therefore
exhaust gas flow? Presumably the faster the engine is going, the higher the
inlet pressure of air that the turbo delivers to the inlet-stroke
cylinder(s) and so the greater the amount of air and therefore the greater
the amount of fuel that can be burned efficiently before you get partial
combustion.


and why they can sit and idle all day on a teaspoon of fuel.

Yes, which is why at fairgrounds etc you often see little generators on
sideshows and stands with the engine just ticking over and a tiny fuel tank
on top. Interesting to hear how little the engine note changes as extra
electrical load is applied, unlike petrol generators which are running much
faster (to get the required torque) but which slow down as more current is
drawn.


.

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