Physics
In a diesel engine the high temperature needed to ignite the fuel is achieved by

Using heat from exhaust
An electrical spark
Compressing air in the cylinders
The battery

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Physics
Conservation of energy refers to the fact that

The entropy of the universe is always increasing
Energy cannot be created or destroyed but can be converted from one form to another
If you conserve energy you will not be as tired
No chemical reaction is 100 percent efficient

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