Inside the Two-Cycle Engine
February 1968 American Aircraft Modeler
while electric propulsion systems are gaining ground in the modeling
realm, 2- and 4-cylinder engines are still quite popular amongst
modelers. I have made a switchover totally to electric, but I sure
miss the sound and smell of the nitro engines. For those who still
use internal combustion engines, and for those who just want to
learn a little more about how these model engines work, this article
by Glenn Lee will be a very useful read.
Inside the Two-Cycle Engine
There's more to a miniature powerplant than meets
MANY test articles have been written about engines, but few have
explained many of the terms used. Almost all model engines used
today are two-cycle; that is the engine fires every revolution.
(See Fig. 1.) They are much simpler than four-cycle engines since
they have no oil pumps, timing gears, camshafts, or head valves.
They have their own problems, however, and this article will try
to explain some of the design differences in engines available today
and yesterday. Two-cycle engines: First, consider the methods of
getting a fresh charge of fuel and air into the cylinder and the
exhaust gases out. The four strokes use an entire revolution of
the crankshaft to pump the exhaust gases out and draw in a fresh
charge. This pumping process results in a large frictional loss
within the engine. In the two strokes, the charge is allowed in
and the exhaust out during the lower part of each revolution. This
results in a loss of power stroke and also mixes the fresh charge
with part of the just-fired charge.
The MVVS 2.5 c.c, glow engine features a rear exhaust, Schnuerle
porting, ball bearings and a crankshaft rotary valve intake.
A rear view of the MVVS 2.5 c.c. glow engine. Prototypes of
this engine have placed high in international speed contests.
A "double -piston" engine.
The experimental K&B 15RC glow engine built by Bill Wisniewski
and flown to first place in the 1964 World Championships. Bill
is famed for his "tuned pipe" used on speed engines.
Left-side of K&B 15RS with cylinder sleeve shows "lumps"
on case where three ports are located. Piston skirt notched
to match lower part of sleeve.
A 'side-port" engine. The air intake
usually enters at the rear of the case.
An O. S. 2.5 cc glow engine is an excellent low-priced powerplant.
Features cross scavenging with front rotary valve and bronze
A 1966 series K&B .29 high-performance engine has cross
scavenging, ball-bearing crankshaft, and a rear-disk rotary
valve intake. Cross scavenging is used in many engines today.
Rear-exhaust 2.5 cc engine built by the author using MOKI crankshaft,
Super Tigre piston and head, and homemade crankcase and sleeve
with Schnuerle porting.
Bypass covers are epoxied and bolted on this .40 cu. in. engine
built by the author. K&B .29 crankshaft and rotary valve
used; crankcase machined from bar stock.
The two-stroke engine also labors from "crankcase scavenging."
The crankcase is sealed, and the up and down travel of the piston
acts like a pump to give positive and negative pressure in the crankcase.
Valves are used to allow a fresh fuel-air charge to enter the case
when the piston is going up. The valve is then closed and the downward
travel of the piston compresses the charge. When the piston opens
the bypass port, this compressed charge is directed up into the
cylinder. As shown in Fig. 1., the exhaust port usually opens just
before the bypass port and allows the burned gases to get out with
a minimum of mixing with the fresh charge. This is "cylinder scavenging."
Several different methods of scavenging will be discussed later.
Figure 1: The two-cycle
a. Air intake is closed, piston is compressing the
b. Bottom of stroke. ports open. cylinder
scavenging tokes place.
c. Piston is drawing in
a fresh charge as it compresses the cylinder charge. Combustion
tokes place. air intake is almost closed again.
the crankcase acts as the fuel pump, lubricating oil must be mixed
with the fuel. The fuel is vaporized, and a layer of oil is left
on all surfaces. Most of the oil goes through the crankcase and
is burned in the cylinder or blown out the exhaust. This system
is simpler than any pressure oil system, and you get an oil change
every revolution. The cylinder walls are lubricated very well this
way, resulting in low friction and long life.
The oil mist
lubrication, however, does reduce the amount of fuel-air mixture
we can get into the cylinder. Bearings do not always get an adequate
supply of oil, and sleeve type bearings must be very loose. Scavenging:
Going back to cylinder scavenging, there are several different designs
regularly used as diagramed in Fig. 2. All of them use an air blast
coming through ports in the cylinder walls to force the exhaust
gases out. Some names given to these methods of scavenging are:
Uniflow, double-piston, cross, and loop. There are several variations
of loop scavenging: Schnuerle, Curtis, reverse loop, and laminar
The uniflow engine has the cylinder, and gases flow
only one way. The fresh charge coming in just above the piston pushes
the exhaust gases out the top. Some method must be used to open
the valve at the proper time, such as a cam and rocker arm. Many
years ago, Dooling experimented with an engine using a rotary valve
in the head driven by a gear train, but was troubled by seizing
problems due to high temperature exhaust gases.
Figure 2: Types of scavenging.
d. Original Schnuerle,
g. Schnuerle with a boost
h. Reverse loop,
Cross scavenging is the type used in many engines sold today,
such as McCoy, K&B, Fox, and O.S. Cross-flow engines have the
bypass ports located on one side of the cylinder and the exhaust
ports on the opposite side. The piston has a deflector baffle on
the top to deflect the incoming charge up into the top of the cylinder.
The cross-flow engines have several minor disadvantages.
The piston baffle is directly in the combustion chamber, and disturbs
uniform combustion. It also overheats and distorts the piston. In
fact, the baffle on speed engines usually gets melted away by ultra-high
nitro content fuels. There is also a tendency toward a loss of the
fresh charge out the exhaust port, with a resulting loss of power
A modification of the cross-flow design is
the laminar-flow engine, notably the Super Tigre with its patented
bypass port system. In this design, the top edges of the bypass
ports are beveled with a double angle such that the air does not
break away from the cylinder wall. The fresh charge flows around
the bevel and is directed up into the cylinder. The piston top can
now be flat, resulting in a uniform combustion chamber, and heat
distortions on the piston are minimized.
The loop scavenged
engines have the bypass ports located to direct the fresh charge
against the wall opposite the exhaust port. The charge then loops
up into the cylinder, forcing the exhaust gases down the opposite
side and out.
The original Schnuerle system used four ports.
The bypass ports were on opposite sides of the cylinder and the
exhaust ports were between them. The charge came through the bypass
ports, met in the center, and then traveled to the top of the cylinder.
The exhaust gases were forced down the sides. Several brands of
engines have been built using this system - well-known engines,
Better results were obtained using a single exhaust
port with the bypass ports located on each side of the exhaust and
directed toward the opposite cylinder wall. The latest MVVS engine
uses this system. Bill Wisniewski's engines with which he won the
last two World Championships used Schnuerle porting with a small
additional port opposite the exhaust port. This is called a boost
port, uses a laminar flow bevel at the top edge, and directs a charge
from under the piston up the cylinder wall. The newest Cox Mark
II also uses a similar system.
One additional benefit from
the Schnuerle porting is improved idling characteristics for RC
or other throttle operation. In a cross scavenged engine, the wet
fuel charge is directed at the glow plug. At rich settings and low
rpm this wet charge puts out the glow plug and the engine stops.
With loop scavenging the charge loops past the filament, and the
engines can run at very rich, slow settings without stopping.
The Curtis scavenging uses multiple ports. The ones opposite
the exhaust have laminar flow top edges while the others direct
the flow away from the exhaust similar to the Schnuerle system.
The reverse-loop scavenging has two bypass ports that direct
the fresh charge just above the exhaust port. The charge loops up
that cylinder wall, down the side opposite the exhaust, and across
the top of the piston. A slight variation to this is the swirl scavenging
where one port is directed above the exhaust port and one is directed
against the wall opposite the exhaust port. This results in a rotating
charge giving high turbulence in the cylinder.
engine shown in Fig. 3 has two cylinders with a common combustion
chamber and two crankshafts geared together. The inlet ports are
located in one end of the cylinder and exhaust ports in the other.
By properly phasing the pistons, the exhaust ports can open before
the inlet ports, and also close before the inlet ports. The exhaust
gases can be cleared with a minimum loss of fresh charge. Complications
are the gears and the double height of the engine. Vibration is
Porting: The biggest
problem of scavenging two-cycle engines is to separate the exhaust
residue and the incoming fresh charge. In most engines, the exhaust
port opens slightly ahead of the bypass port. The rapid rush of
the exhaust gases from the cylinder can cause the pressure in the
cylinder to drop below atmospheric, and the resulting vacuum can
draw part of the exhaust back into the cylinder.
exhaust port opens too soon, part of the incoming fresh charge can
be lost out the exhaust. The pressure in the cylinder is very high
during combustion, and very little time is required to let these
gases out when the exhaust port opens. The crankcase pressure, however,
is very low, on the order of six pounds per square inch. This low
pressure cannot force the fresh charge into the cylinder very fast,
so the bypass ports must be raised or widened to improve performance.
The exhaust port must open before the bypass port, so it
must be raised along with the bypass port. The portion of the wall
given to porting must be subtracted from the working stroke. So,
the height of the ports must be matched to the rpm range at which
the engine will be run and also to the burning rate of the fuel
used. Racing engines using high nitro content fuels have very high,
wide ports, while stunt or sport engines have much lower ports.
Port opening periods are usually noted as so many degrees
of crankshaft rotation. This is "exhaust timing" and "bypassing
timing." For speed engines the best exhaust timing has been found
to be near 140 degrees, which means that the piston starts to uncover
the exhaust opening when the crankshaft is 70 degrees from bottom
dead center and closes when the crankshaft is 70 degrees past bottom
dead center. Bypass timing varies from 120 to 130 degrees. The Super
Tigre engines have symmetrical timing, the exhaust and bypass open
simultaneously. The high pressure of the exhaust gases holds the
fresh charge in the crankcase until the majority of the exhaust
has gone out the exhaust port and pressure in the cylinder has been
reduced below that of the crankcase. This gives the same effect
as opening the exhaust port before the bypass yet allows a higher,
larger bypass port to be used.
improve scavenging in the cylinder, the main factors are time and
the amount of fresh charge that you can get in. At 24,000 rpm, the
bypass port is open for less than 1/1000 of a second. This is not
enough time to allow a fresh charge to travel from the lower part
of the crankcase all the way up into the cylinder. The top of the
bypass chamber in the crankcase must be large enough to store a
charge until the piston opens the port, letting the charge into
the cylinder quite quickly.
Crankcase passages must be as
large as possible to allow unrestricted flow of gases. On the other
hand, this reduces crankcase pumping efficiency and can be detrimental
to high speed performance. It has been found that the best solution
to this problem is to "pack" the crankcase as much as possible,
yet leave a large chamber right next to the bypass ports. Some engines,
notably the Dooling, have transfer passages cut through the wall
of the piston to allow the charge to travel into short, curved bypass
passages. This also allows fresh charges to cool the inside of the
piston a little better.
One of the greatest improvements
in engine design in the last years has been the metallurgy of the
sleeve-piston combination. The leaded steel sleeve and hardened
cast iron piston is hard to beat, although chrome plating is still
being experimentally used. A chrome plated sleeve is almost a necessity
for top performance from a ringed, aluminum piston engine since
friction is very high between aluminum and steel.
Pistons: Pistons, whether iron or aluminum, must
be as stiff as possible to minimize warping and heat distortion.
The best pistons have annular rings inside just above or below the
wrist pin holes which aid in keeping them round. This greatly increases
the cost of manufacture, but is usually necessary for high performance.
Even with a properly designed and manufactured engine, proper
break-in of the sleeve and piston is required. Many attempts have
been made to minimize or eliminate break-in running, but few methods
are successful. For best performance, both the cylinder and the
piston should be as round as possible and have the proper clearance
to start with. Lapping the piston in its sleeve with some kind of
abrasive compound usually results in a ruined engine since softer
parts of the sleeve get cut deeper than harder areas. Also, the
harder piston will force the abrasive into the soft metal of the
sleeve; it does not get washed out, and will most likely cut too
much clearance during the first runs.
Heating of the piston
is not uniform during running, since intensely hot combustion gases
heat the top causing it to expand more than the rest of the piston.
The metal near the top of a lapped piston must be worn away to allow
for this expansion before peak performance can be reached and maximum
nitro fuel can be used. This metal worn away amounts to several
thousandths of an inch off the diameter. Some of it can be ground
away before running, but it is easy to grind too much unless you
really know what you are doing. The piston also develops a bulge
on the hotter exhaust side which must be worn away. Larger engines
have used asymmetrical "earn-turned" pistons where this metal was
ground away before assembly. It again is very difficult to grind
the proper amount from a piston of the size we use.
two stroke engines run very hot, and air cooling is usually uneven
and inadequate. The main cooling is from the fresh air and fuel
coming into the crankcase. Most high performance engines use a
"hanger" type cylinder sleeve supported only by the lip at the top.
The aluminum crankcase expands more than the sleeve, and even though
it may expand unevenly, it does not squeeze the sleeve out of shape.
Warped cases or warped sleeves are usually the greatest detriments
to engine performance.
The importance of proper break-in
cannot be overemphasized. Engines on the bench should be run at
or slightly above the rpm that they will operate at in the air.
A smaller diameter, lower pitch prop allows the engine to be run
at operating rpm with a rich needle valve setting. The excess fuel
mixture keeps the engine cool and lubricated to prevent tight parts
One other aspect of proper break-in has to
do with the instability of some piston materials. Hardened cast
iron is unstable and will actually grow in dimension when it is
heat cycled. This growth can be as much as .001" per inch of diameter.
As an engine is run, the piston is heated and cooled during every
stroke, resulting in a slow growth. This growth, however slight,
must be worn away, and the engine is not broken in until the piston
has stabilized. The time required for this varies according to the
heat treatment and the alloy and can be several hours of high rpm
Head Design: Various head shapes
are shown in Fig. 4. The classic domed piston and hemispherical
or matched combustion chamber has almost totally been replaced by
fiat top pistons and "squish band" heads. The squish band is a circular
band that fits very close to the piston at top dead center, and
"squishes" the trapped charge into a central combustion chamber.
The diameter of the chamber is usually about 65% of the bore diameter,
and the depth is varied to give the correct compression ratio. A
variation of the squish band head is the "trench" head, where the
combustion chamber is a trench milled straight across, leaving wide
squish flats on each side.
If the squish
band is too close to the piston, a hydraulic lock can occur. That
is, part of the fuel charge cannot get squished out of the way in
time and is trapped. Extreme compression ratios result in the squish
areas, and the result is erratic running and broken conrods. One
way to relieve this problem is to give the squish band a slight
angle relative to the piston; three degrees seems to work.
Squish band heads do have an effect on the allowable nitro content
of racing fuels. Nitro contents as high as 70 and 80 % have been
used without detonation.
Many other head shapes have
been tried, such as the trumpet head in the "Rattler" engines, but
compression ratio seems to make a bigger difference than head shape.
Compression ratios as high as 18 to one have been used, but few
glow plugs will stand up to such punishment. The compression ratio
must be matched not only to your fuel, but to the weather as well.
Test running and test flying is the only way to find the proper
Air Intake: So far,
we have talked about cylinder-piston combinations and head shapes,
but we must also have an efficient means of getting fuel and air
into the engine.
The simplest method of air induction
is the "side port" system as shown in Fig. 5 where the intake port
is uncovered by the piston skirt when the piston nears the top of
its stroke. A pipe leads from the needle valve to the port, and
when the port is opened by the piston skirt, the vacuum in the crankcase
draws in the fresh charge. Many model engines have been built this
way, but better results are obtained by rotary valves.
Some engines have been built using reed valves. These are simply
a one-way valve formed by flat, thin, spring steel or beryllium
copper reeds. When the piston goes up, negative pressure opens the
reed allowing the fresh charge to come in, and when the piston starts
down the reed closes. Disadvantages to this system are that the
intake timing cannot be controlled, and the engine can also run
in either direction.
The best, yet most complicated
and most expensive system is the rotary valve. There are several
types, but all of them use a rotary shaft or disk to open and close
the air intake hole at the proper time. The simplest rotary valve
is the hole through the crankshaft that valves the fuel and air
into the crankcase through a port in the main bearing. One advantage
to this system is the oil mist cooling of the crankshaft and bearings.
The disadvantage is that the crankcase compression cannot be very
high with the large hole in the crankshaft. Oversize bearings must
also be used.
The rear rotary valve is a disk or drum
that is rotated by the crankpin. A large segment of the disk is
cut away to allow passage of fuel and air, and opens and closes
the intake port as it is rotated. Different manufacturers use different
intake timing, but usually the valve opens after the crankshaft
has rotated 35 to 45 degrees past bottom dead center and closes
near 45 degrees after top dead center.
Much has been
written on hop-up procedures where techniques are stressed on polishing
all air passages, but this can sometimes give a reduction of performance.
Oil from the fuel will stick to a highly polished surface while
it can be swept away from a rougher surface. If it sticks and piles
up in the bypass, the result will be a smaller passage for air flow.
Any gain from polishing is usually from the removal of metal, giving
a larger passage.
I hope this article has explained
a few of the principles of two-cycle engines without antagonizing
anyone. I have purposely neglected such things as superchargers
or tuned systems. Most engine designs are many years old, but there
should be room for more development of the basic systems. Titanium
pistons, for example, do not work, but bushed titanium conrods are
already being used. Such metals as beryllium and single crystal
iron must be tried for pistons or sleeves. Much experimental work
Posted June 18, 2011