UST008

June/July 2016 |

Unmanned Systems Technology

Multi-fuel, spark-ignited two-stroke

Single cylinder

42.6 x 35.0 mm = 49.89 cc

Naturally aspirated

Gasoline, Avgas or kerosene-based

fuel (Jet A, Jet A1, JP5, JP8 and so on)

Aluminium structure

Linerless, nickel silicon carbide-

coated bore

Two main bearings, roller and deep

groove ball

Steel crankshaft, one pin

Steel con rod

Reed valve

Light alloy piston; single ring

Electronic ignition, single (or twin) plug

Electronic, compressed-air assisted

direct injection

Engine management system

10.7:1 compression ratio

Maximum rpm, 6500

The N20 is an air-cooled single

cylinder two-stroke. Its cylinder with

integral head is die cast aluminium

and is attached to the crankcase via

four bolts.

The crankcase is machined from

billet aluminium, as is its endplate.

Three fasteners attach the endplate

to the main unit with an O-ring

seal. The endplate carries the billet

aluminium throttle body with a two-

bolt fixing.

Those same fasteners attach

the aluminium-and-foam air filter

assembly to the throttle body, again

with an O-ring seal. The throttle body

carries the butterfly-type throttle,

the servo that operates it and also a

throttle position sensor.

The reed valve body is a pressure-

moulded plastic component

sandwiched between the endplate

and the throttle body, clamped by

the latter’s attachment bolts. It is

wedge-shaped in cross-section such

that it presents a pair of reeds each

at 45

to the inlet face. Fibreglass

composite reeds are used.

Housed in a remote tank (actually

integrated with the fuel tank, in the

case of the ScanEagle), engine

lubricant is supplied via an electric

pump submerged in the tank at a

pressure in the 0.05-0.10 bar range.

The cantilevered crankshaft runs

in a roller bearing positioned in front

of its single web, and ahead of that

a deep-groove ball bearing; these

two main bearings sandwich the air

compressor drive lobe. Between the

lobe and the front main bearing is a

lip seal.

The main and big-end bearings are

all-metal while the carburised forged

steel crankshaft is machined on all

surfaces to very tight tolerances.

The crankshaft is formed in one

piece from the generator attachment

at the front to the crankpin at the

rear. Its cantilevered arrangement

allows it to have its roller and ball

main bearings slipped over from the

front, the single web running just

behind the former. A cap screws into

the end of the crankpin to retain the

big end.

A single axial bolt screws into the

nose of the crankshaft to retain the

generator rotor, to which in turn the

propeller is attached. The stator

is attached to the crankcase by

three bolts. The rotor is aluminium

and carries rare earth permanent

magnets, while the stator is steel with

copper coils.

The air compressor piston is

driven by an aluminium rod, the

lower end of which fits over the lobe

formed by the crankshaft with a

needle roller bearing between the

two, retained by the former.

This bearing is lubricated by the

oil mist in the crankcase.

The compressor rod is subject

to only a very short stroke and low

pressure compared with the main

engine con rod, and is isolated

from the heat of combustion. It

is connected to its piston by a

hardened steel pin that runs directly

in its small-end bore. The pin bore in

the piston is anodised for longevity.

The aluminium piston carries retainer

clips for the pin.

The air compressor assembly

is enclosed by a cap with O-ring

sealing and three bolts attaching it to

the crankcase. The air compressor

piston, which has a single ring of

undisclosed material, reciprocates

in a separate aluminium sleeve, atop

which is a steel disc that can lift to

exit compressed air – a coil spring

keeps it closed until the required

pressure has been generated. In

effect, this is a one-way check valve

with a high flow area.

Transfer ports in the cap that

closes the air compressor assembly

feed air from the crankcase to the

compression chamber. A further port

takes the compressed air through the

crankcase to a tube that in turn leads

up to the fuel rail assembly. The

compressed air is fed to the back of

the direct injector.

The fuel rail assembly consists

of a housing, the injector, a fuel

pressure regulator and the fuel and

compressed air supply lines. The

injector has a high-pressure Teflon

seal where it provides a combustion

seal to the engine cylinder.

An O-ring seal on the opposite end

seals the injector to the main fuel rail

assembly; the assembly housing is

formed of front and rear sections that

are machined from billet aluminium.

Two fasteners join the two halves,

Anatomy of the N20