The purpose of this discussion is to provide a user-friendly tutorial on the topic of the bystarter, an essential and often misunderstood device on most conventionally-aspirated (carbureted) scooter engines. It is not applicable to engines equipped with fuel injection. The photographs and illustrations contained herein are intended to provide a generalized concept of bystarter architecture and function and, as such, do not depict any particular make/model. Additionally, most are not to scale.
Background:
Before getting into an explanation of what a bystarter is and how it works, it might be a good idea to present a little info about fuel and conventionally-aspirated (carbureted) internal combustion engines. A good starting point is what's called "stoichiometric air/fuel ratio". In layman's terms, this is the ratio of air-to-fuel at which the mixture burns the most efficiently. For gasoline this value is approximately 14.7 to 1, meaning 14.7 parts air to 1 part gasoline. Now, this is all well and good once the normal operating temperature of an engine has been achieved. However, things change when we're talking about a "cold" engine. In that scenario, a "richer" mixture is required, and can be achieved in one of two ways: restricting the volume of air, or increasing the volume of fuel.
The "less air" approach was traditionally used for automobiles and motorcycles. It involved decreasing ("choking") the flow of air through the carburetor by means of a movable plate called a choke (or butterfly) built into the intake side of the carburetor. Controlled either manually or automatically, this plate would be closed for cold starts and then opened as the engine warmed up. Most basic lawn and garden equipment such as mowers, weed trimmers, and chainsaws still use this approach, with the choke plate operated by a manual lever.
The "more fuel" approach is where the bystarter, in conjunction with an extra passage in the carburetor, comes in. Rather than decreasing airflow, the "enrichment circuit" formed by these two components automatically and temporarily increases the amount of available fuel.
Bystarters are also referred to as "auto-bystarters", "auto-chokes" and sometimes just simply "chokes". The latter two terms are dead wrong as the device doesn't achieve its intended purpose by "choking" anything, and the former two seem to be based upon bastardized or faulty English. So I'll use the term "enricher" from this point forward.
The Ins and Outs:
The simplest parts of the enrichment circuit are the inlet port and the outlet. These provide the endpoints for a flow of air to draw extra fuel into the engine. The following two photographs depict their typical appearance. The first photo also shows the typical location and appearance of the bystarter (enricher) itself. It is usually black, held in place by some sort of removable clamping bracket, and connected to a pair of wires.
"Cold" Engine Mode:
The following illustration depicts the major components of the enricher, namely:
- Heater
- Expandable Material
- Plunger
- Needle
- Jet
In "cold" engine mode, these components are in the following states:
- Once the engine has been started, alternating current from the stator starts flowing into the heater. How quickly the heater rises to its
highest temperature is a function of its resistance. For now, we'll assume that it is as cool as the ambient air surrounding it.
- The (thermally) expandable material below the heater is also initially the same temperature as the ambient air. At this temperature, it is in a contracted state.
- The plunger, which is spring-loaded to counteract the enlargement of the expandable material, is retracted.
- The needle, which is connected to the plunger via a shaft and spring mechanism, is also retracted.
- Due to the retracted state of the needle, the jet, which allows access to the fuel supply, is fully unblocked.
As the engine is started, the vacuum created by the intake stroke of the piston draws air through the outlet and inlet port. This flow of air, in turn, draws fuel up through the jet. Once the fuel enters the inlet port-to-outlet stream, it atomizes and forms a combustible mixture for use by the engine. This supply is in addition to the fuel/air mixture provide by the carburetor's normal idle circuit, and that's what the whole "enrichment" thing is about.
"Transitional" Mode:
In between the point where the engine is cold and the point where it is at normal operating temperature, the enricher "transitions" as the temperature of its heater rises. The expandable material responds to the change in temperature by gradually expanding and exerting downward force on the plunger. This force is transferred to the needle, gradually extending it such that it begins to block the jet and reduce the air flow/fuel supply in the enrichment circuit.
"Hot" Mode:
The following illustration depicts the state of the enricher components once the engine has achieved normal operating temperature:
- The heater, now fully warmed up, has caused the expandable material to achieve its maximum length.
- The expandable material now exerts maximum pressure on the plunger.
- The plunger transfers this force to the needle which is now fully extended.
- This results in the complete blockage of the jet, cutting off the supply of fuel and air through the enrichment circuit.
Failure Modes:
1. If there is a clog in the inlet port, the outlet, or both, no air can be drawn through the enrichment circuit. As a consequence, no additional air/fuel mixture can be supplied to the engine. This will result in a hard starting condition for a cold engine.
2. If there is a clog in the jet, or somewhere in the path leading to the jet, then only extra air will be supplied to the engine while the enricher is in "cold" mode. This will result in an even harder starting condition for a cold engine since it is not just being deprived of the additional air/fuel mixture, but is actually being supplied with a greater volume of (cold) air. This effectively "thins" out the mixture supplied by the normal idle circuit, making it even more difficult to get it to fire.
3. If a fault in the heater, expandable material, plunger, or needle causes the enricher to get "stuck" in "hot" mode (air/fuel flow totally blocked), the effect will essentially be the same as that described in the clogged inlet port/ clogged outlet scenario.
4. If a fault in the heater, expandable material, plunger, or needle causes the enricher to get "stuck" in "cold" mode (full air/fuel flow through the enrichment circuit), then the engine will be constantly running in a rich mixture state. While cold starts will be easily achieved, the maintenance of an over-rich mixture (long after the point where it is actually beneficial to starting) can lead to spark plug fouling, backfiring, and increased carbon deposits in the combustion chamber and on the valves.
Final Point:
In my opinion, and only my opinion, the primary shortcoming of the enricher is that it does not rely upon any feedback mechanism to determine when the engine is actually warm enough to run reliably without enrichment. Once the enricher itself has reached "hot" mode then, as far as it is concerned, the engine is good to go without any additional "help". Unfortunately, there may be rare times/circumstances when such is not the case.
In any case, there you are. If you have any questions, or have spotted any errors in my explanation, don't hesitate to contact me. I'll do my best to set the situation right.
Again, nice job.
I guess anything that keeps me off the streets is probably in the best interest of society.
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