Question

which describes what happens when fuel is ignited in a heat engine?
chemical energy is converted into thermal energy.
thermal energy is converted into solar energy.
solar energy is converted into chemical energy.
thermal energy is converted into chemical energy.

Explanation:

When fuel is ignited in a heat engine, a chemical reaction (combustion) occurs. In combustion, the chemical bonds in the fuel are broken and new bonds are formed. This process releases energy in the form of heat (thermal energy). So, chemical energy (stored in the fuel) is converted into thermal energy.
Solar energy is not involved in the basic operation of a heat - engine when fuel is ignited. Also, thermal energy being converted into chemical energy would be a non - spontaneous process (opposite of combustion) and not what happens when fuel is ignited.

Answer:

Chemical energy is converted into thermal energy.