So I'm getting into paramotoring and it requires me to have more mechanical knowledge of two-stroke engines because the rigs are pretty rudimentary and a lot of repairs and adjustments would need to be done in the field.
I think I've pieced together some things but I want people to fill in the gaps or make some corrections.
Everything is pertaining to simple two-stroke motors with Walbro / membrane carburetors.
-- Foundation --
An engine takes a certain ratio of fuel + air, applies a spark to it, and ignites it to produce heat, an expansion of gas, and some waste products like CO2, H2O, and other hydrocarbons from incomplete combustion.
This up-down motion is mechanically turned into continuous rotational motion through the crankshaft + flywheel.
-- Starting the Engine --
Once an engine is going, the up/down of the combustion cycle is enough to keep it going on its own. As long as fuel, air, and a spark are applied at the right times, it will keep on going on its own.
The problem is getting this cycle to start in the first place.
- Pull Start -
In a pull-start system, some fuel is initially injected into the combustion chamber (priming), and the user pulls on a line that rotates the flywheel. The flywheel rotates, spins the crankshaft, and starts the piston's up/down cycle as well as the sparks. The piston compresses the fuel (from priming) and air. The rotation also activates a generator that creates a spark at the right time. If everything goes right, combustion happens and sends the motor into its self-sustaining cycle.
Starting can fail if:
Too much fuel in the chamber from priming - won't combust
Too little fuel in the chamber from not priming enough - won't combust
Spark plug is worn out
Generator isn't working
- Electric Start -
The same thing needs to happen - something needs to turn that flywheel initially.
A battery is used to turn a second, smaller electric motor, called a Starter Motor. The starter motor's crankshaft is attached to the flywheel or crankshaft of the gasoline engine. The user presses a button which runs the starter motor which turns the primed gasoline motor which starts the combustion cycle. Is the starter motor then disengaged? Or does it then do double-duty to recharge the battery?
So electric starts add extra weight and complexity - a battery, a *second* motor, and all the connection points between the starter motor and the gasoline motor. A pull start is basically a rope and some smart connections to the flywheel.
Starting can fail if:
Too much fuel in the chamber from priming - won't combust
Too little fuel in the chamber from not priming enough - won't combust
Spark plug is worn out
Spark plug generator isn't working
Battery too dead
Electric motor isn't working
-- Walbro / Membrane Carburetor --
A carburetor sends a specific ratio of atomized fuel and air into the combustion chamber. It has an internal chamber that stores a bit of fuel from the fuel line, and it has a mechanism for transferring the right ratio of air and fuel into the combustion chamber.
A float-type carburetor uses a floating object to keep its internal fuel storage chamber at a certain level, but these do not work in all physical orientations, which is where the Walbro Carburetor steps in.
The Walbro Carb uses a rubber-trampoline-like membrane that goes up and down to keep the internal fuel chamber at the right fuel level. It then uses a second membrane to pump the right amount of fuel into the combustion chamber. The interaction of the engine piston trying to suck in new fuel + air and some one-way valves that allow pressure to build inside the carb, are what activate the flexing of the membranes.
-- Adjusting RPMs and air/fuel mixture --
It should be noted that adjusting the air/fuel mixture is actually only adjusting the amount of *fuel* getting into the engine. You don't really do anything like turn a screw to adjust how much air gets into the combustion chamber.
The RPMs at idle is adjusted through a screw that determines how closed or open the throttle valve will be at idle.
The Low Speed Fuel Adjustment determines how much fuel gets to the combustion chamber at idle. It's simply a needle valve that either constricts or widens an opening that the fuel needs to pass through on its way to getting sucked into the combustion chamber.
The High Speed Fuel Adjustment determines how much fuel gets to the combustion chamber at any speed BUT idle. It's simply a needle valve that either constricts or widens an opening that the fuel needs to pass through on its way to getting sucked into the combustion chamber.
A Walbro Carb can fail if the rubber gaskets or the membranes get old. Or if the one-way valves malfunction.
-- Fuel Mixtures --
A rich fuel mixture means more fuel than the engine's ideal mix. Way too much fuel and combustion will stop. A little rich means it'll still run, but there will be unburnt and partially-burnt fuel (more pollution?).
A lean fuel mixture means less fuel than the engine's ideal mix. Too little fuel and the engine also doesn't run. A bit lean means ALL the fuel gets combusted, moreso than if it was a bit rich. This leads to HIGHER engine temperatures and shorter engine life. Yes, paradoxically (at least to me), too little fuel can lead to higher temperatures because of more complete combustion of the fuel that is there.
Am I kind of on the right track here?
I think I've pieced together some things but I want people to fill in the gaps or make some corrections.
Everything is pertaining to simple two-stroke motors with Walbro / membrane carburetors.
-- Foundation --
An engine takes a certain ratio of fuel + air, applies a spark to it, and ignites it to produce heat, an expansion of gas, and some waste products like CO2, H2O, and other hydrocarbons from incomplete combustion.
This up-down motion is mechanically turned into continuous rotational motion through the crankshaft + flywheel.
-- Starting the Engine --
Once an engine is going, the up/down of the combustion cycle is enough to keep it going on its own. As long as fuel, air, and a spark are applied at the right times, it will keep on going on its own.
The problem is getting this cycle to start in the first place.
- Pull Start -
In a pull-start system, some fuel is initially injected into the combustion chamber (priming), and the user pulls on a line that rotates the flywheel. The flywheel rotates, spins the crankshaft, and starts the piston's up/down cycle as well as the sparks. The piston compresses the fuel (from priming) and air. The rotation also activates a generator that creates a spark at the right time. If everything goes right, combustion happens and sends the motor into its self-sustaining cycle.
Starting can fail if:
Too much fuel in the chamber from priming - won't combust
Too little fuel in the chamber from not priming enough - won't combust
Spark plug is worn out
Generator isn't working
- Electric Start -
The same thing needs to happen - something needs to turn that flywheel initially.
A battery is used to turn a second, smaller electric motor, called a Starter Motor. The starter motor's crankshaft is attached to the flywheel or crankshaft of the gasoline engine. The user presses a button which runs the starter motor which turns the primed gasoline motor which starts the combustion cycle. Is the starter motor then disengaged? Or does it then do double-duty to recharge the battery?
So electric starts add extra weight and complexity - a battery, a *second* motor, and all the connection points between the starter motor and the gasoline motor. A pull start is basically a rope and some smart connections to the flywheel.
Starting can fail if:
Too much fuel in the chamber from priming - won't combust
Too little fuel in the chamber from not priming enough - won't combust
Spark plug is worn out
Spark plug generator isn't working
Battery too dead
Electric motor isn't working
-- Walbro / Membrane Carburetor --
A carburetor sends a specific ratio of atomized fuel and air into the combustion chamber. It has an internal chamber that stores a bit of fuel from the fuel line, and it has a mechanism for transferring the right ratio of air and fuel into the combustion chamber.
A float-type carburetor uses a floating object to keep its internal fuel storage chamber at a certain level, but these do not work in all physical orientations, which is where the Walbro Carburetor steps in.
The Walbro Carb uses a rubber-trampoline-like membrane that goes up and down to keep the internal fuel chamber at the right fuel level. It then uses a second membrane to pump the right amount of fuel into the combustion chamber. The interaction of the engine piston trying to suck in new fuel + air and some one-way valves that allow pressure to build inside the carb, are what activate the flexing of the membranes.
-- Adjusting RPMs and air/fuel mixture --
It should be noted that adjusting the air/fuel mixture is actually only adjusting the amount of *fuel* getting into the engine. You don't really do anything like turn a screw to adjust how much air gets into the combustion chamber.
The RPMs at idle is adjusted through a screw that determines how closed or open the throttle valve will be at idle.
The Low Speed Fuel Adjustment determines how much fuel gets to the combustion chamber at idle. It's simply a needle valve that either constricts or widens an opening that the fuel needs to pass through on its way to getting sucked into the combustion chamber.
The High Speed Fuel Adjustment determines how much fuel gets to the combustion chamber at any speed BUT idle. It's simply a needle valve that either constricts or widens an opening that the fuel needs to pass through on its way to getting sucked into the combustion chamber.
A Walbro Carb can fail if the rubber gaskets or the membranes get old. Or if the one-way valves malfunction.
-- Fuel Mixtures --
A rich fuel mixture means more fuel than the engine's ideal mix. Way too much fuel and combustion will stop. A little rich means it'll still run, but there will be unburnt and partially-burnt fuel (more pollution?).
A lean fuel mixture means less fuel than the engine's ideal mix. Too little fuel and the engine also doesn't run. A bit lean means ALL the fuel gets combusted, moreso than if it was a bit rich. This leads to HIGHER engine temperatures and shorter engine life. Yes, paradoxically (at least to me), too little fuel can lead to higher temperatures because of more complete combustion of the fuel that is there.
Am I kind of on the right track here?
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