Answer is:
ignition of the charge before the passage of flame front
Explanation:
Auto ignition is defined as the self-ignition of the vapors emitted by a liquid heated above its ignition temperature.
Answer is:
allow for lengthening of the valve owing to the heat of combustion
Explanation:
It is necessary to maintain the valve clearance as they allow for lengthening of the valve owing to the heat of combustion.