Dwell time

[GBRAVO]

Hi,
How I calculate the dwell time for a boxer 2 cylinders engine.
Thanks,
German

[dynosor]

For a boxer engine the cylinders reach top dead center together (TDC), but while the first cylinder has just fired its plug, the second cylinder is just starting its intake stroke. The cylinders take turns firing around TDC, but on alternating revolutions or 360 crank shaft degrees apart.

My understanding of dwell time is it represents the time while the ignition points are closed. It is usually expressed as an angle of rotation, but translates to time at a given engine speed.

The higher the engine speed, the less time there is for the magnetic field in the coil to saturate, before the points open and the collapsing field generates the high voltage pulse that fires the spark plug.

If dwell time is too short, engine speed too high, or plug gap too large, there is no spark and the engine miss-fires.

All that said, I don't think dwell time is very important to just get an engine running. More fundamental is spark timing. Too early and the engine won't start, will knock and possibly damage itself. Too late and the engine will also run poorly, have low power and overheat.

As you know, spark timing for maximum torque and efficiency throughout the rev range depends on the delay in the start of combustion and the increase in combustion speed with higher volumetric efficiency. Simply put, you need to fire the spark plug earlier as engine speed increases, up to a point. You also need to fire it earlier under light load (lower gas pressure and torque) than with the throttle wider open.

My suggestion is that you study the timing diagram for full-size engines and build your model with the ability to adjust the timing so that you can custom fit its ignition advance to produce the best torque and smoothest running under all conditions you care about.

Apart from nominal compression ratio and fuel used, optimal timing is also affected by valve timing, valve size, lift, runner length, combustion chamber shape & volume and distance from the spark plug to the exhaust valve (hot spot). Mixture strength has a huge effect on the ignition advance that can be run without knocking - rich mixtures cool the inlet charge through evaporation, while lean mixtures can be difficult to ignite and then knock easily once lit.

I have created a sample chart in Excel that plots the timing advance for engine speeds over an expected useful range of 2000 to 10,000 RPM with a typical timing advance curve shape for engine speed intended for full throttle operation. The total timing will typically also add more advance for part throttle operation; perhaps another 20 degrees of ignition advance for very light throttle operation.

Note the negative timing at very low engine speed - this is to make starting easier and to prevent stalling.

The bottom line is that dwell timing is not very important, except when you don't have enough.

[GBRAVO]

Dynosor,
Many thanks for your interesting response.
I will try with the rule of thumb for 4 cycles engines, dwell angle= (Camshaft RPM)*0.0075 and will check the effects together with the other variables.
Thanks,
German

[Mago]

Dynosor,

A well thought out and articulated response.

Nice to see a positive reply to a question.

Hope others take note.

Mago

[svenakela]

Dwell time is not important to get the engine running, yes. But it's important to get maximum out of the coil without overheating.
You can read more at: http://www.megamanual.com/ms2/configure.htm pretty down by the bottom, search for "Maximum Dwell Duration".

[dynosor]

Originally Posted by svenakela Dwell time is not important to get the engine running, yes. But it's important to get maximum out of the coil without overheating. You can read more at: http://www.megamanual.com/ms2/configure.htm pretty down by the bottom, search for "Maximum Dwell Duration".

Good point, but the risk of overheating goes down as the number of cylinders goes up - you can't keep the coil energized for too long without interruption if it has to fire multiple plugs per revolution. This assumes one coil and a distributor, if not actual contact points.

Then again my car has a coil directly on top of each sparkplug. I am sure that with electronic ignition dwell time stays constant at the optimal value even with rising engine speed.