St Lawrence Engine
Construction and Operation
This is a cross-section sketch of a St Lawrence 3 port, two-cycle
marine engine. Be aware of fundamental two-cycle construction.
Long bronze bearings support the rotating crankshaft. These are
supplied with grease from grease cups. While grease acts as a
lubricant, the grease also functions to seal the crankcase during
piston travel. In addition to the usual piston rings to seal the top
of the piston, a piston ring is installed at the bottom to seal
crankcase during piston travel.
Therefore crankcase of a two-cycle engine experiences a
negative pressure (or vacuum) that draws fuel/air mixture into the
crankcase when the piston moves in an upward direction.
At the next cycle, crankcase is pressurized when the piston
moves in a downward direction. This pushes air/fuel mixture into
combustion chamber via a transfer port.
During operation, air/ fuel mixture and exhaust gases are
sequenced by the upper and lower edges of the moving piston,
uncovering (opening) and covering (closing) three ports that are
built into cylinder side walls. Referring to the sketch, the three
ports are identified by RED LETTERS. Thus sequence is "piston"
One cycle occurs during one engine revolution. For example, when
operating at 600 RPM, one cycle occurs in 1/10 of one second.
Piston Controlled Operating Sequence
1 - POWER STROKE BEGINS
The air/fuel charge above the piston has been ignited by the spark plug.
Resulting chemical reaction produces a rapid rise in temperature that increases
combustion chamber pressure. This pressure pushes the piston down. Downward
movement is changed to crankshaft rotation by the connecting rod acting on the
crankshaft. Some of the energy in the rapidly expanding gases is transferred to
the large rotating flywheel.
2 - POWER STROKE CONTINUES
Piston speed is influenced by the load connected to the rotating crankshaft.
Engine is loaded by propeller rotating in water. If the engine is operated without
an attached load, or the load is too small as would occur with an under-size
propeller, piston speed will be too fast, possibly resulting in engine damage.
3 - POWER STROKE ENDS
Top edge of piston is about to uncover exhaust port thus ending the power
stroke. Continuing rotation before next power stroke is powered by the energy
contained in the rotating flywheel.
4 - EXHAUST PORT OPENS
Now the exhaust port is uncovered so exhaust gases begin to exit combustion
6 - COMPRESSION STARTS
First the bypass port is closed by top edge of piston, then exhaust port, thus sealing combustion chamber.
Flywheel continues to power crankshaft rotation. Upward piston movement starts to compress new charge.
Crankcase is sealed, thus upward moving piston causes a vacuum in the crankcase.
7 - COMPRESSION CONTINUES
Bottom edge of piston is about to uncover fuel intake port. Maximum crankcase vacuum has occurred.
St Lawrence Two-Cycle Marine Engines
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5 - FUEL TRANSFER OCCURS
Piston is at the bottom position. 180 degrees of crankshaft rotation from top
position has occurred.
By pass port is now fully uncovered by the top edge of the piston. When
combustion chamber pressure falls below crankcase pressure, fuel/air mixture
that was under pressure in the crankcase enters through the by-pass port into
the combustion chamber where it is deflected vertically by the piston deflector.
Thus incoming mixture flows up the back side of the cylinder, loops around the
cylinder head and pushes most of the burned gases out the exhaust port. This is
not a perfect process. Some of the new charge will always spill into the exhaust
pipe because exhaust port remains open as the piston starts to rise. Also some of
the exhaust gas will remain in the combustion chamber and dilute new charge.
8- FUEL TRANSFER OCCURS
Flywheel continues to power crankshaft rotation and upward piston movement. Fuel intake port is now
Because of crankcase vacuum, air is sucked through the carburetor where gasoline is pulled from the
carburetor bowl into the passing air stream where it mixes with air, then enters the crankcase. Carburetor
needle valve regulates the amount of gasoline that is pulled into the air stream.
The advancement of gas engine use occurred as a result of understanding the advantage of compression prior to ignition. This occurred in 1876 in
Germany by Nikolaus Otto. Otto's engine used a four-stroke cycle where combustion occurred once during two engine revolutions. Even now the
four-stroke cycle is referred to as the Otto cycle. In 1890 a Scottish engineer, Sir Dugald Clerk, introduced an engine where combustion occurred each
engine revolution. This is referred to as the two-stroke cycle. Clerk's engine used a separate chamber for taking in the combustible charge and
transferring it to the power cylinder. In 1892, Frederic Caswell Cock, an Englishman, patented the two-cycle method where combustible charge entered
the crankcase before transfer to the combustion chamber via an integral transfer port.
Typical two cylinder St
Bore 3 1/4", Stroke 3/1/2' x 2
Flywheel is 14" diameter
6 HP @ 850 RPM
Typical single cylinder St
Bore 3 1/4", Stroke 3 1/2'"
Flywheel is 12" diameter
3 HP @ 850 RPM
Two-cycle engines use few moving parts compared to many parts
used for construction of four-cycle engines where sequence is
controlled by valves opening and closing by the action of a
rotating cam shaft. The simple two-cycle construction is one
reason for their popularity early 1900. Two-cycle engines are
dependable and easy to repair.
From about 1900 to 1920, many organizations manufactured two
cycle marine engines. By 1950 most small marine engines were
outboard style. Now because of environmental concern, almost all
small marine engines use the four stroke cycle.
St Lawrence Engine Company manufactured two-cycle marine
engines from 1905 to about 1949 at a facility located in Canada at
Brockville, Ontario. Only minor improvements occurred during the
period, notably changing to aluminium pistons about 1935.
Gasoline Engine Combustion Principles
Gasoline is a hydrocarbon, a compound of Hydrogen(H), and Carbon(C). When gasoline is present as a vapour it will mix with air (that is
already a vapour). When mixture of air and gasoline vapour is approximately 12 - 16 parts air to 1 part gasoline, the mixture is combustible,
thus will burn if ignited. Combustion starts when the mixture is exposed to temperature above its self ignition temperature, about 495
About 21 % of air is oxygen. When gasoline combusts, oxygen molecules in air combine with hydrogen molecules in gasoline to form water;
and with carbon molecules to form carbon monoxide, then carbon dioxide. A lot of heat is produced as a consequence of both reactions.
Water and Carbon Dioxide are waste products that result from the combustion process. These are exhausted to atmosphere. It is heat
that is produced during combustion that produces engine power.
In a gas engine, combustion is started at ignition by heat that occurs in the spark plug gap. Temperature
is a measure of the quantity of heat. In the spark gap temperature is many times more than 495F.
The combustion process that starts at the spark plug creates a flame. Spark at spark plug occurs only briefly
to start combustion. The high flame temperature continues the combustion process until either the oxygen is
used up or the gasoline vapour is used up. If as a result of good engine management, both are used up at the
same time then the maximum possible heat has been extracted and maximum power attained.
If the gasoline vapour /air mix is rich in gasoline, but is within a combustible range, some of the carbon will be oxidized only to Carbon
Monoxide as there is insufficient oxygen present to continue oxidation to Carbon Dioxide. The heat liberated from oxidizing only to Carbon
Monoxide is much less, only 30% compared to oxidizing to Carbon Dioxide. It is important to avoid a rich mixture as engine output is less
because less heat is produced. Carbon monoxide is a very poisonous when inhaled, thus for both reasons, rich mixture must be avoided.
The production of heat, as a result of combustion, is the fundamental gas engine event. When combustion occurs inside the enclosed
combustion chamber, the resulting heat causes rapid pressure rise. Because piston blocks-off bottom of the combustion chamber and
because piston can move vertically, pressure in the combustion chamber drives the piston downward. The mechanical arrangement of
piston, connecting rod and crankshaft convert downward piston movement to rotary motion.This results in propeller rotation.
Gertrude a "go slow boat"
Powered by a single cylinder 3HP St Lawrence two-cycle marine engine.
Speed at 850 RPM is 5.5 Knots.
Gasoline consumption is less than 2 litres per hour (1/2 USgal per hour).
Owner designed/built Kitchen Rudder provides neutral, reverse and stern thrust.
Construction, Operation, Combustion
Principles, Historical Snippet
To repeat video,drag play bar to start position then click Play
3 HP St Lawrence Engine, circa 1949
3 1/4" Bore x 3 1/2" Stroke, 850 RPM