I see beatiful scale models of IC, but seldom see the ignition coils, condenser and DC source, they are hidden away under the stand.
How does the scaling work?
In a full size engine we need about 80mJ for each spark.
Size of electric transfomer/coil/generators is prportional to power and inversely to frequency.
Frequency:
Models runs at the same or higher RPM than full size.
The frequency work on both sides of the equation it increase the power and reduce the size for the same power so it is rougly a wash.
Spark Energy:
I am guessing here so please correct me.
Igniting the fuel in a model should take the same energy per spark that a full size. The voltage is the same since the compression ratio and the fuel type are the same. I do not know about the current though.
Do model spark plug require less energy to ignite the charge in a model IC engine?
Assuming it does then the coil size should be rougly equal to a full size and consume the same current from the battery.
Can some of you set me straight on the basic physic of scaling the ingnition components?
Thanks
Question about the scaling of ignition components.
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tornitore45
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- Location: USA Texas, Austin
Question about the scaling of ignition components.
Mauro Gaetano
in Austin TX
in Austin TX
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Lew Hartswick
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- Joined: Sat Jan 04, 2003 10:45 am
- Location: Albuquerque NM
To scale inductors, you have to scale power or frequency.
A small coil can produce less power, or must operate at a higher frequency.
Some form of C-D ignition would allow decreasing the size of the coil as low as the voltage requirements allow.
Gap controls voltage required (along with compression). Voltage and current are teh power required. Current is what provides the heat to "light" the charge. The power is normally far above the "ignition power", both due to inefficiency, and the need to be "certain" it lights.
A small gap and similar current WOULD reduce power somewhat, but not THAT much.
So some form of electronic driver to produce a high frequency current is beeded to get a small coil.
A small coil can produce less power, or must operate at a higher frequency.
Some form of C-D ignition would allow decreasing the size of the coil as low as the voltage requirements allow.
Gap controls voltage required (along with compression). Voltage and current are teh power required. Current is what provides the heat to "light" the charge. The power is normally far above the "ignition power", both due to inefficiency, and the need to be "certain" it lights.
A small gap and similar current WOULD reduce power somewhat, but not THAT much.
So some form of electronic driver to produce a high frequency current is beeded to get a small coil.
I don't remember where I read it, but probably in Strictly IC mag, there is a limit to how small components like distributors can be scaled down. If you get too small, the spark will have a tendency to jump wherever it takes a notion, rather than the terminal the rotor is pointing at...I'm no electrician, but the idea seems to be that voltage, insulation requirements, etc., do not scale. This is why distributors or mags on multi-cylinder model engines seem to be oversize for the scale.
Ron in CO...
Ron in CO...
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tornitore45
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I am well familiar with the electrical design aspect, my problem is to understant how much energy is necessary to light the charge in a 5cc cylinder compared to a 500cc cylinder. Linear scaling 100^0.333=4.6
Energy = Points Arc Voltage x Current x Time
Voltage depends on Gap, can we divide the gap by 4.6? Probably not.
Current determines the number of atoms stripped by electrons detrmining the spark temperature, can we say we need less because the charge is smaller?
Time does not scale, the ignition must take the same angle and RPM are rougly the same for models and full size. If anything will have to scale down.
These are the real questions to answer before a design, otherwise we are left with experimentation. Nothing wrong with trial and error methods but are less efficient.
My preliminary calculations convinced me that a Capacity Discharge can be built smaller because the cap can be charged to a fixed energy quickly and held charged until the spark is needed.
The conventional coil keep storing more and more energy as the RPM goes down, thus must be dimensioned to handle excessive idle current just to provide enough at high RPM. Delaying the current start time is not practical because it requires predicting the ignition time cycle by cycle.
The CD charging converter can be made reasonably small because the frequency is not related to RPM and can be high, the voltage is in the 200V range which does not create all those insulation and creepage issues you find above 1000V
Energy = Points Arc Voltage x Current x Time
Voltage depends on Gap, can we divide the gap by 4.6? Probably not.
Current determines the number of atoms stripped by electrons detrmining the spark temperature, can we say we need less because the charge is smaller?
Time does not scale, the ignition must take the same angle and RPM are rougly the same for models and full size. If anything will have to scale down.
These are the real questions to answer before a design, otherwise we are left with experimentation. Nothing wrong with trial and error methods but are less efficient.
My preliminary calculations convinced me that a Capacity Discharge can be built smaller because the cap can be charged to a fixed energy quickly and held charged until the spark is needed.
The conventional coil keep storing more and more energy as the RPM goes down, thus must be dimensioned to handle excessive idle current just to provide enough at high RPM. Delaying the current start time is not practical because it requires predicting the ignition time cycle by cycle.
The CD charging converter can be made reasonably small because the frequency is not related to RPM and can be high, the voltage is in the 200V range which does not create all those insulation and creepage issues you find above 1000V
Mauro Gaetano
in Austin TX
in Austin TX
The large cylinder must have a good spark because the fire must be lit over the entire volume quickly. Either multiple spark plugs, or else a very hot spark to light a good volume quickly.
Thinking about it, a smaller motor has less flame propagation distance, and a lower volume, so that tends to counteract the higher speeds somewhat.
But, I think the best practice is to have as hot a spark as you can produce, regardless. because conditions are not always perfect....
Thinking about it, a smaller motor has less flame propagation distance, and a lower volume, so that tends to counteract the higher speeds somewhat.
But, I think the best practice is to have as hot a spark as you can produce, regardless. because conditions are not always perfect....
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tornitore45
- Posts: 2078
- Joined: Tue Apr 18, 2006 12:24 am
- Location: USA Texas, Austin
The consideration about volume and flame propagation was my last hope that some scale factor could apply.
Agree that a hotter the spark can't hurt and will provide margin for all the variables working agains ignition with a minimum energy spark.
My concerns are due to the ability to design and built smaller ignition components to mantain a scale.
Perhaps a perfect scale is not achievable.
Some fudging may be acceptable and some large component may be hidden but what I want to avoid is to be tied to automotive or motorcycle components.
A possibility I see is to start with an automotive coil and then reduce the spark current and/or the duration until performance starts to drop.
Once we know for sure what is the optimum energy W=V x I x T then we can desigh the smallest ignition electrically possible.
The arc voltage is not a variable, is what the gap needs at that compression and current level.
Agree that a hotter the spark can't hurt and will provide margin for all the variables working agains ignition with a minimum energy spark.
My concerns are due to the ability to design and built smaller ignition components to mantain a scale.
Perhaps a perfect scale is not achievable.
Some fudging may be acceptable and some large component may be hidden but what I want to avoid is to be tied to automotive or motorcycle components.
A possibility I see is to start with an automotive coil and then reduce the spark current and/or the duration until performance starts to drop.
Once we know for sure what is the optimum energy W=V x I x T then we can desigh the smallest ignition electrically possible.
The arc voltage is not a variable, is what the gap needs at that compression and current level.
Mauro Gaetano
in Austin TX
in Austin TX