Why no new diesel radials?

On-Condition wrote:
Just wondering, exactly why are radial diesels not part of the
diesel revival? You can't beat radials on airplanes for smooth
reliability. I once saw a cracked-off jug pounding through the
cowling as the engine was still running (the single Otter had
just landed).

Guiberson: 4 stroke 9 cylinder radial, 2 valves per cylinder, 300HP.

Packard: 4 stroke 9 cylinder 1 valve per cylinder (Neato) 225HP.

Zoche is a ported 2 stroke, if I recall it was supposed to be available
in 2 4 and 8 cylinders.

There is an V4 in development in the US, a piston-opposed 2 cyclinder 2
stroke 100HP engine in the UK that is apparently shipping, and 150HP
engine (I think in france?) that is also shipping and I believe is
either certified or approaching certification in the 172. (Anybody know
whether the ICAO agreements make that a certifiable install in the
USA?)

One thing I always wondered about the old radial diesels was how prone
these engines were to hydraulic lock? I mean you can take a plug out of
a gas radial and drain oil, but at the lower compression ratio you
should just have to walk a couple blades through once in a while to
prevent it. At 15 to 1 compression ratio, I would think diesel radials
would be much more susceptable to overpressuring the jugs. Looking at
the zoche engine this seems doubly odd, since there are no valves to
let excess engine oil out of the lower cylinders. Anybody know how they
deal with this?

I've read quite a bit of conflicting information about diesel radials.
One account said that the packard had good high altitude performance
because the nature of a diesel is to always have a lean air/fuel ratio,
so it it would richen the mixture into a better ratio as it climbed.
I've read in usenet other accounts that said that the high altitude
performance sucked. Guess we'll just have to take one out and fly it to
see :-)

The packard was 1 valve per jug, (reasonable since it is just air, not
fuel) ran on stove oil. The arrangement has a donut flange on the top
of the cylinders and uses ram air to clear the exhaust gasses. The side
effect though is that your exhaust is not funneled, so the pilot and
pax of a single engine bird end up sucking diesel exhaust. Obviously
hanging the engines under the wings fixes that, but in the thirties
that was easier said than done.

Then you have fuel considerations. Stove oil and summer diesel can gel
at altitude, requiring some fairly complex added equipment to correct
the problem, which I am guessing is why all the new ones are designed
to run on Jet-A. Somebody else mentioned a priming issue as well in a
previous thread. In essence, if you stop the engine from fuel
starvation, the fuel injectors suck air out of the cylinders instead of
sucking fuel out of the fuel line, so you can't restart. (Am I
understanding that problem correctly?) old diesels have hand priming
pumps to fix this. Look under the hood of Mercedes 240D for example, it
looks like a little accordion on the drivers side of the engine. I
believe common rail injection solves this problem though.

So the system infrustructure for using a diesel is a bit more complex
even though you don't need magnetos. Complex = weight. Though in
general I expect if it had ever become a reasonably standard practice
the kinks would have been worked out a long time ago. Actually it
probably would have been become more practical as the number of jugs
grew because of the increase in mechanical efficiency native to the
radial configuration. (Can you imagine an 18 cylinder diesel WASP sized
engine?) But now there are turbines for that kind of power, so theres
no reason to do it.

Weight is an issue with smaller engines yes. The packard made 2/3's of
the rated power of a comparable gas engine of the era at the same
weight. (And that is an educated guess, I don't feel like looking it
up, but it is probably generous.) Higher pressure in the jugs requires
more steel to be reasonably reliable. No way to get around that. Like
all things diesel, you have to accept some derated performance if you
want the benefits.

I read an article about a PT6A that was tested with B100 mixed into the
fuel. If I remember they noted no significant change in performance up
to about 20% blend or so. Though I have to wonder, since the PT6A is
sometimes configured to derate its max power whether this was accurate.
(Am I correct there? Isn't the one in the Caravan derated to 600HP from
1000?)Also the test was relatively short compared to the actual
lifespan of turbine so the results though positive, were not conclusive
in my humble interpretation of the report. But it was a while ago that
I read that.

So that about sizes up my understanding of the current situation

What I'd like to see someone work out a new turbine design with an
injector and hot section suitable for running on SVO or B100. Stick
that on a couple of 747's, diesel trains, and cruise liners and you
save thousands of tons of net-CO2 emissions a year.

Anybody is welcomed to jump in and correct me where I'm wrong. I'm sure
some of this in misinformation. This is usenet after all.

-Matt