of 27
Current View
Harry’s Engine
Topics Num. 6
Carburetors
By Harry Higley
1
Harry’s Engine Topics Number Six
Carburetors
THE OHLSSON 60
What’s a spark ignition Ohlsson 60 doing on the
cover of a Topics about carburetors? The answer:
that particular engine had a feature for operating at
two speeds. Notice the two sets of pickup posts (1),
which attach to the points inside the timer housing.
The upper points were timed so the spark occurred
as it should near top dead center, which let the
engine operate at its normal high speed. For low
speed the flyer switched to the lower points and the
spark occurred earlier; the rising piston would fight
the already ignited expanding gasses. Predictably,
the engine would run lousy and slow down. At least
it should have worked that way.
Running a spark ignition engine in the era before
glow plugs took fortitude and perseverance. Even
given these two qualities
a modeler did not always
experience success. Beside the points, spark ignition
required a coil, condenser, spark plug, batteries and
wiring. To add to these complications, the fuel
contained seventy weight motor oil that sprayed over
the ingnition system, often fouling it.
Complicating this even more, the flyer switched
between high and low speed points by sending
electrical impulses through the control lines to a relay
in the model. The insula
ted control lines had an
enormous resistance and the electricity that survived
the trip up the lines barely actuated the relay and
sometimes it didn’t. I think a company called Deco
made this two speed electrical hardware.
But, an Ohlsson 60 exactly like the one on the cover
had its day at the 1950
Dallas National Model
Airplane Championships. The United States Navy
sponsored the Control Line
Navy Carrier event for
the first time at that contest. A modeler of some note,
S Calhoun Smith, won the event with a beautiful
Ohlsson 60 powered Douglass Skyraider of his own
design. As a twelve year old I read about it in
Air
Trails
, which had an article about Smith’s win and
featured plans for the Skyraider. The Navy Carrier
Event created a market for throttled engines.
SOME EARLY THROTTLES
The first commercial attempt at a throttled glow
engine came from K&B. They offered a 19 equipped
with two needle valves. For high speed only the
lower needle supplied fuel. For low speed both
needle valves fed fuel to the engine, which richened
the fuel mixture and slowed the engine down. It
would flood and die if both needle valves were left
open for very long. Let’s call this phenomenon “fuel
loading”. The flyer frequently stopped the flow to the
upper valve to let the engine clear the excess fuel [a
technique we’ll call “burping”].
Fox supplied many different engines with provisions
in the block for two needle valves, though I don’t
think he ever offered engines equipped with two
valves. A recent mockup of a Fox 19 with two valves
appears in (2 and 3). Fuel for the lower needle valve
went directly from the tank to the engine by way of
the upper fuel line (3). Fuel for the upper needle
valve went through an on/off switch. The flyer would
have a spring or rubber band loaded third line
connected to the switch lever. He would pull the
third line to open the switch and let the third line go
slack to close it. If all the lines went slack during low
speed, the spring loading would shut off the upper
needle valve, which would allow the engine to speed
up. That tensioned the lines allowing the flyer to
regain control.
Switching the upper needle valve on and off could
also be accomplished using electricity sent up the
lines to a solenoid or relay that operated a vent. With
the vent closed fuel would flow to the upper valve.
With the vent open the upper valve would suck only
air through the vent.