Which airplane?

wrote:
George Patterson wrote:
: You can get a mogas STC for the 152.

... from Petersen for 91 fuel, no?... isn't the 152 a high-compression
Lycoming O-235 at 110hp? 8.5:1 or 8.7:1 CR IIRC.

The EAA offers an STC for the O-235 and the Cessna 152. There's a note that the
engine "requires modification." Usually the only engine modification required
for an STC is replacement of the fuel pump, but I do not know if that's the case
with the 152.

Retrofitting an auto engine to an aircraft requires very careful packaging to get the
power/weight ratio comparable to an aircraft engine.

Right.

The main problem typically is associated with engine speeds. Auto engines are
usually horsepower rated at high speeds (typically around 5,000 rpm). The
propellors on most aircraft need to turn at no more than about 2,700 rpm. So,
you either need to add a gearbox to reduce shaft speed (which adds weight) or
you limit the engine to 2,700 rpm. Doing the latter means that the engine only
puts out about 60% of its rated horsepower, so you need a bigger engine, which
also adds weight.

In general, builders who need lots of power go with a gearbox. They usually have
larger planes and the weight penalty is relatively small. Running an engine at
high speed with a gearbox means that it's just not going to last as long as it
would when used for normal driving.

If you're going to run the engine at prop speeds, the best thing to do is to
change out the valve train. Go with the equivalent of a 3/4 race cam, replace
the valves with lighter ones, and replace the valve springs with lighter ones.
If carburetted, a main jet change may be in order, but modern car engines are
injected anyway. You may also have to tinker with the computer. All of the
changes reduce the life expectancy of the engine.

You also have to deal with the fact that most auto engines are water-cooled.
Your cooling system will take some expert design and will (guess what) add more
weight. I know of one pretty nice looking aircraft with a Subaru 4-banger in it
that sits on the ground a lot because it overheats easily.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

: ... from Petersen for 91 fuel, no?... isn't the 152 a high-compression
: Lycoming O-235 at 110hp? 8.5:1 or 8.7:1 CR IIRC.

: The EAA offers an STC for the O-235 and the Cessna 152. There's a note that the
: engine "requires modification." Usually the only engine modification required
: for an STC is replacement of the fuel pump, but I do not know if that's the case
: with the 152.

I thought that only Petersen had any high compression STCs. I know it's the
only one offered for the Cherokee (which also had a fuel pump requirement). Learn
something new everyday... I keep telling a flight instructor friend of mine that
instructs out of his own 152 that he should either put in higher compression pistons
("sparrowhawk" 125hp with 9.5:1 IIRC) if he's burning 100LL, or switch to mogas. He's
not buying the argument...

: Retrofitting an auto engine to an aircraft requires very careful packaging to get the
: power/weight ratio comparable to an aircraft engine.

: Right.

: The main problem typically is associated with engine speeds. Auto engines are
: usually horsepower rated at high speeds (typically around 5,000 rpm). The
: propellors on most aircraft need to turn at no more than about 2,700 rpm. So,
: you either need to add a gearbox to reduce shaft speed (which adds weight) or
: you limit the engine to 2,700 rpm. Doing the latter means that the engine only
: puts out about 60% of its rated horsepower, so you need a bigger engine, which
: also adds weight.

Yeah... good news/badnews. Less displacement, but more weight with the
gearbox. Tough to make the correct engineering compromises to match a traditional
aircraft engine. Also overlooked (at least some time ago) are the longitudinal
stresses and torsional vibration on the crankshaft of a direct-drive conversion. IIRC
lots of VW conversions snapped crankshafts when driven directly. Again... aircraft
engines were designed for that, auto engines not.

: You also have to deal with the fact that most auto engines are water-cooled.
: Your cooling system will take some expert design and will (guess what) add more
: weight. I know of one pretty nice looking aircraft with a Subaru 4-banger in it
: that sits on the ground a lot because it overheats easily.

Yes, but everything else being equal, that's a *good* thing IMO. Yes it adds
complexity, some weight, and another point of failure, but the decreased thermal
stresses are good all the way around. Much lower valve temps, CHT temps, tigher
clearances, no shock cooling, lower octane requirements, less heat-induced metal
fatigue, lower cooling drag, etc. I think if I were going to *build* my own aircraft,
I'd probably try to go with a Subaru with a turbonormalizer... but then again I still
drive my '85 GL Hatchback with a 1.8L boxer daily. It'd have to be a pretty small
plane to use that engine... the 2.5 or 3.3L would be better for a hauler.

-Cory

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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