wrote in message
...
On Mar 22, 9:33 pm, "Dale Scroggins"
wrote:

SNIP

SNIP
2. No ignition system needed.

Not to be a ball-buster, but the injection systems for the new
generation of diesels is incredibly fancy. From a risk analysis
standpoint, it is hard to say whether this system in place of magnetos
is a gain or a loss.

Fuel control systems for new generation turbine engines are also incredibly
fancy. They have more redundancy than automotive systems, and some have
mechanical back-up, but from recent news reports, aren't utterly reliable.
But they seem to be tolerably reliable.

Subaru appears to be using common rail and electronic injection control, but
it's likely their engine could be adapted to a more basic mechanical system
if desired. Subaru probably doesn't build the injection components, but
purchases them from a specialty manufacturer. The VW TDI engine, for
example, uses electronic injection control when installed in a Jetta, but
when sold as a stationary engine, uses a purely mechanical injection system
provided by Bosch.

Even mechanical diesel injection systems have failure modes, of course. So
do gasoline injection systems and carburetors. None of them are utterly
reliable, but all seem to be tolerably reliable.

Leaving fuel control concerns aside for now, however, the fact remains that
diesels do not require electrical ignition systems. A gas engine depends
upon air, compression, fuel control, and ignition. The diesel depends upon
air, compression, and fuel control.

Aircraft ignition systems are high cost, high maintenance, and can interfere
with other aircraft systems. I've maintained, troubleshot, and repaired
these systems for thirty years. I would not miss them if they fell from
favor. During the same period, I've maintained, troubleshot, and repaired
aircraft fuel control systems from carburetors to FADEC. Diesel electronic
fuel control systems are not any more complex and need not be any less
reliable than these systems.


3. Turbocharging a diesel is a win-win proposition. EGT is lower, so
turbo
system parts last as long as the engine, usually. Turbocharging improves
both performance and fuel efficiency (not always true for gassers).

True, but boost levels in diesels inversely effect their reputation
for reliability.

As do boost levels in gasoline engines. However, turbo-normalization does
not usually impact reliability, if sufficient engine cooling is available at
altitude. Turbodiesels normally see about 10" of boost, and operate
reliably regardless of ambient pressure, as long as the turbocharger and
cooling system can cope.

SNIP

8. I can make biodiesel for 67 cents a gallon. You or I can buy
off-road
diesel or heating oil for much less than either auto gas, avgas or jet A.

Biodiesel gels at a higher temp than does winter diesel, and be damned
sure you have no rubber in the fuel system. And the 67 cents is
quoted for recycled oil. Not my first pick at altitude.

I fly mostly in the southern US in the spring, summer, and fall. If temps
at altitude (or on the ground) were below safe limits for biodiesel, I would
choose another fuel. However, much like turbine engines do, diesels return
large quantities of warm fuel back to the fuel tanks in flight. Fuel temps
rarely drop to ambient temperature. I agree, however, that it isn't a risk
worth taking.

I've made fuel from waste oil, but my current supply is virgin oil that was
rejected as a food ingredient. Eight thousand gallons of it. It should
last a while.

9. For a given trip, less fuel is needed, both in volume and weight.
25%
to 35%, depending on the trip profile. If the diesel is turbocharged and
gasser is not, the diesel's ability to cruise significantly higher may
result in a bigger efficiency spread.

This is dependent on tuning the boost map, and whether the blower is
big enough. Off the shelf this may or may not be true. I have not read
flight tests for any new European diesels which have been impressive
in high altitude performance. I doubt this is because of any
fundemental flaws, but rather performance tuning.

Agreed. Cooling is also an issue. Even with liquid cooling, thin air at
altitude may limit the installation's ability to shed waste heat.
Turbocharged gas engines face the same problem. However, several
manufacturers have produced piston aircraft capable of sustained cruising
in the lower Flight Levels. Diesels theoretically produce a bit less waste
heat than gasoline engines. It would be interesting to experiment with one.

SNIP

I dig your enthusiasm. Please post if you hear of any new diesel/
biodiesel aviation projects. I would be interested in reading any
related results. I believe that the new generation of diesels are the
long term solution for keeping GA operating costs down. But it will
take a while.

-Matt

The SmartCar aluminum diesel engine and the Toyota Yaris engine (Europe,
aluminum diesel) might be good candidates for small aircraft. However, this
Subaru seems to be an ideal candidate since its power and weight specs fit
so many available airframes. PRSUs are also available. I suspect we will
see these engines installed fairly quickly in European experimentals.

DS