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Energy Transfer

Energy - Petrol and Diesel Engines for Vehicles.

What do Petrol and Diesel Engines do?

An engine burns a hydrocarbon (petrol or diesel)
which has come from a fossil fuel. The engine
uses the released energy to rotate a steel shaft.
The rotational energy is used to move the vehicle
and to generate electricity for the vehicle's needs.

What is the Efficiency of a Petrol or Diesel Engine?

An engine will transfer some of the available
chemical energy
into rotational energy and some
into heat. We can calculate the efficiency of the
engine using the equation

efficiency (%) = (useful energy out ÷ total energy in) x 100.

If the available chemical energy is 270,000J
and the rotational energy produced is 70,000J, then

efficiency = (70,000 ÷ 270,000) x 100
= 25·9 %.

A petrol engine has a typical efficiency of 20 to 30 %.
A diesel engine has a typical efficiency of 30 to 40 %.

The fossil fuel (petrol or diesel) is called "chemical energy"
because energy is released during
chemical reactions with oxygen. The fuel is burned.

Other examples
of chemical energy are electrical cells and food.

A cell (or battery)
transfers chemical energy into electrical energy.

Living organisms eat food and food is also called
"chemical energy" because energy is released during
respiration. Respiration is the reaction of oxygen
with glucose and is the reverse of photosynthesis.

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