On 15/11/2016 17:48, NY wrote:
"Clive George" wrote in message
...
On 15/11/2016 11:51, NY wrote:
Rotaries have always baffled me: how do you get your supply of fuel to
the cylinders without leakage where the stationary fuel tank feed meets
the rotating cylinder block. It's not like an electric motor where slip
rings or a commutator serve the equivalent purpose with electric
current: in the case of fuel, you need to prevent leakage.

You don't send fuel to the cylinders unless you have direct injection.
So a rotary diesel isn't going to work, but a carburetted petrol
engine as those old rotaries will be fine. The cylinders pull in
fuel/air. Since it's suck rather than blow for that mix, any leaks
will result in more air coming in rather than fuel leaking out, and
that's easy enough to cope with by making the mix a little richer.

Ah, of course. For some reason I visualising the carb on the rotary
part, but if it's on the static part, then a normally aspirated engine
would suck the mixture and hence any leakage would be inwards, and
air-only.

Do supercharged petrol engines compress the mixture, or do they compress
the air and then inject fuel into the stream between compressor and
cylinders? In other words, can you have non-fuel-injected carburettor
super/turbo-charged petrol engines? Presumably for a rotary engine, as
long as the supercharger is on the rotating part, it still sucks in
mixture (so leakage will be inwards) and then compresses it after the
leaky joint.

Yes, you can have super chargers and turbos on carburetted engines -
they predate widespread use of fuel injection by quite a long time.

You can have the carburettor on either side of the blower too (with a
super charger, not sure I'd want to try compressing a fuel/air mix with
a hot turbo).

It would be interesting trying to arrange a supercharger on the moving
engine. Probably theoretically possible, but not practical.

In either case, you've got the problem of not
being able to have a large (and therefore heavy) air reservoir to store
boost pressure for cases where the throttle is opened on a slow engine
and high boost is needed at a time when the engine can't yet generate
it.

Turbos and superchargers have been used for years (pre-ww2), and don't
use air reservoirs in the way you describe. Turbo lag is real, but the
get rounds for that aren't air reservoirs, and the lag isn't terribly
long anyway.

I hadn't realised that. I assumed that all charged engines had a
reservoir, and that it was in the exceptional case of very hard
acceleration from low engine speed (and hence low pressure) that the
reservoir became exhausted, and that this was what caused turbo lag.

I've learned something!

:-)

I'll let Peter Hill talk about the ways you reduce turbo lag - mostly
it's about either keeping the turbine spinning or just using a smaller
one which can accelerate faster.