# Spark voltage in a car engine.

## Main Question or Discussion Point

HI, my friends

Lets imagine a car engine combustion chamber which is fed with an estequiometric fuel mixture (gasoline+air) with a 1mm plug gap.

Is there an aproximate algorithm to solve out what voltage is needed to create a spark, based on gas pressure and temperature?.

I dont want to include variables such a gas turbulence, or other possible parameter that may alter that required voltage.

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mgb_phys
Homework Helper
At very low pressures (ideal gas) there is a simple formula but I don't think it would apply at IC engine piston pressures.

At very low pressures (ideal gas) there is a simple formula but I don't think it would apply at IC engine piston pressures.
Well, really, it is after sparking, when combustion starts what creates the very high pressure to push the pistons. But before the combustion starts, the pressure will be in the range of CR, more or less. Say 150 psi´s ?

In the case of air we need about 5000 volts for sparking over a 1mm gap at 760 torr.

What is that voltage for a fuel/air stoichiometric mixture keeping those conditions?

I just want an approach.

Any help is greatly appreciated

Regards.

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mgb_phys
Homework Helper
Well, really, it is after sparking, when combustion starts what creates the very high pressure to push the pistons. But before the combustion starts, the pressure will be in the range of CR, more or less. Say 150 psi´s ?
The corona discharge formula is for much less than 1atm

Breakdown voltage of dry air at STP is 32MV/m so 32KV/mm but in reality air will break down at much lower voltages and the actual figure is very sensitive to any water or dust present.

A quick google search produces a few papers for modelling engine ignition but they are all behind pay for jouranls.

dlgoff
Gold Member
Breakdown voltage of dry air at STP is 32MV/m so 32KV/mm but in reality air will break down at much lower voltages and the actual figure is very sensitive to any water or dust present.
Funny about this. Yesterday evening while watching the lightning after a storm we had here, I estimated the cloud to cloud lightning "arcs" being about 3 miles. That would take over [STRIKE]58,000MV[/STRIKE] 150,000MV if it weren't for all the water in those clouds.

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Expect a voltage anywhere between 10 and 30 kV, I think?

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I haven't succeeded yet in my search.

regards

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