1. An engine, comprising:a plurality of cylinders;
each cylinder comprising an ignition coil;
each ignition coil comprising at least one electronic circuit that:
detects a spark event of a spark plug,
measures the ion current from said spark plug, and
calculates a dimension free relative measure of the ion current from each of said plurality of cylinders based on a time or crank angle from the detected spark event; and
an engine control unit comprising a central processing unit that receives said dimension free relative measures from each cylinder and uses said relative measures from said plurality of cylinders to improve a performance of said engine, wherein said calculation includes dividing the ion current signal into a plurality of crank angle subintervals, calculating the integral of ion current in each crank angle subinterval; and performing a calculation on the integrals of ion currents from different subintervals such that the result of the calculation is dimension free.