Why do traditional two stroke engines not have an Auto-Advance/Retard mechanism like a four stroke engine?
Two stroke motors tend to have a statically set ignition point (generally at about 22 degrees BTDC) whereas a four stroke motor has an auto advance/retard mechanism with a static timing typically about 12 degrees BTDC. On a four stroke engine the advance unit is designed to allow for the burn time of the fuel/air mixture, which is relatvely constant, so that the spark must occur earlier the more the revs increase. Why is the same not true on a two stroke motor?
Answers:
Its not strictly true that 2 stroke engines do not have ignition point adjust. I work with large marine 2 strokes (Sulzers and B&W's) burning heavy fuel. Some of these engines are as big as a house 12 cylinders with 900mm bore 3000mm stroke. They are mostly all fitted with VIT this means variable ignition timing. This is used to increase the MEP (Mean Effective Pressure) to the maximum possible when the engine is running at lower than maximum rated power. This would be used to maximise fuel efficiency during slow speed passage.
Outboard motors do have an advance( by linkage)
You probably is not right. The advance retard system can be adjusted on the cam in a four stroke engine to vary the amount of fuel intate and if the firing is advanced then we will get a better power stroke close TDC.
For two stroke the firing again can be retarded or advanced on the dynamo cam. Only thing is you cannot control the fuel intake because there is no valve for two stroke. Same amount is pumped in all the time. Statically you can change the fuel flow into the cylinder in 2 stroke when you increase the inlet vent.
For both advance retard system can be incorporated.
Its probably due to the fact that advancing the spark would mean that its the intake valve that is just about be covered during the cycle, this limits the time period that the spark can ignight. In a 4-stroke the valve area is minimal and is poppet type, which is self-sealing, therefore you can get away with having the spark moments before the valve is shut. But in a two-stroke the valve area is so much bigger that if you adjust the timing the spark may ignight the mixture whilst too much intake valve is showing, resulting in the flame path engulfing the oil sump area where the fuel and air mix, this could blow the engine.
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Answers:
Its not strictly true that 2 stroke engines do not have ignition point adjust. I work with large marine 2 strokes (Sulzers and B&W's) burning heavy fuel. Some of these engines are as big as a house 12 cylinders with 900mm bore 3000mm stroke. They are mostly all fitted with VIT this means variable ignition timing. This is used to increase the MEP (Mean Effective Pressure) to the maximum possible when the engine is running at lower than maximum rated power. This would be used to maximise fuel efficiency during slow speed passage.
Outboard motors do have an advance( by linkage)
You probably is not right. The advance retard system can be adjusted on the cam in a four stroke engine to vary the amount of fuel intate and if the firing is advanced then we will get a better power stroke close TDC.
For two stroke the firing again can be retarded or advanced on the dynamo cam. Only thing is you cannot control the fuel intake because there is no valve for two stroke. Same amount is pumped in all the time. Statically you can change the fuel flow into the cylinder in 2 stroke when you increase the inlet vent.
For both advance retard system can be incorporated.
Its probably due to the fact that advancing the spark would mean that its the intake valve that is just about be covered during the cycle, this limits the time period that the spark can ignight. In a 4-stroke the valve area is minimal and is poppet type, which is self-sealing, therefore you can get away with having the spark moments before the valve is shut. But in a two-stroke the valve area is so much bigger that if you adjust the timing the spark may ignight the mixture whilst too much intake valve is showing, resulting in the flame path engulfing the oil sump area where the fuel and air mix, this could blow the engine.
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