What’s under the hood?

You don’t have to be a mechanic to understand what’s going on under the hood of your car.  If you learn the basic concepts you’ll be equipped to carry on an informed conversation with your mechanic. You’ll also understand why you need to replace some car parts on a regular basis.

The engine

The engine is the main component that propels your car forward. To simplify matters, the engine works something like an air pump.

  1. The air comes in
  2. It gets compressed and mixed with gasoline
  3. The fuel and air mixture explodes, moving the pistons that make the engine turn.
  4. The exhaust is expelled through the exhaust pipe

Of course, many parts are necessary to run the “air pump.”  Here are some of the basics.


The air filter

The filter prevents dust from entering the engine. It has to be changed regularly to ensure good air circulation.

Filtre à air


The fuel injection system

This is what allows fuel to be sent to the cylinders. There the fuel is atomized by the injectors and gets mixed with air.

Système d’injection


The cylinders

Each cylinder contains a piston attached to a rod that turns the engine’s crankshaft when the air/fuel mixture explodes. But a spark is needed for the mixture to ignite. That’s where the spark plug comes in.  Today’s engines typically have four, six or eight cylinders.

Les cylindres


The valves

The valves can be thought of as small doors in the shape of trumpets that open and close at just the right moment, some to let the air/gas mixture into the cylinders and others to release the exhaust gases. If the valve is damaged, it won’t create an airtight seal and will prevent the explosion from moving the piston.



The transmission

The transmission, which can be can be automatic or manual, allows you to shift gears (like on your bike), in addition to backing up and staying in neutral.  To put it very simply, it transfers energy from the engine to the wheels.



The exhaust system

The exhaust system uses a number of pipes to expel gases created when the fuel and air mixture is burned. It has two main components: the catalytic converter and the muffler. The catalytic converter burns and eliminates as much gas as possible to reduce pollution, while the muffler reduces noise. It’s essential that there be no leaks that would allow exhaust fumes to enter the passenger compartment.

Systeme d'échappement


Why does your mechanic need to change certain parts even when your car seems to be running fine?

Each part plays a specific role in ensuring that your engine runs well and doesn’t consume too much gas. Here are a few examples of the risks of worn or damaged parts:

  • Worn spark plugs don’t always produce the spark needed to ignite the air/fuel mixture.
    • Your engine will have less power
    • You will generate more pollution because you will be releasing the fumes of unburned fuel into the atmosphere.
  • A dirty air filter can prevent sufficient air from entering the engine. The result is the same as when your vacuum cleaner is clogged with dust and dirt, the engine has trouble getting the air it needs to perform.
  • Worn or damaged injectors will also interfere with the delivery of atomized fuel to the cylinders. This saps the engine’s strength, increases fuel consumption and creates more pollution.


Why is there electricity under the hood?

The car’s electronic components, such as the radio, the ventilation system, the computer, the sensors and the spark plugs, all depend on electricity.  The battery and the alternator are two key components of the electrical system.

The battery supplies the current to the starter. It’s a small electric motor that turns the engine when the car is started. But it also supplies electricity to the computers and the spark plugs to ignite the explosion in the cylinders.


Once the engine is running, the alternator, which is connected to the engine by a pulley, supplies the electricity to keep everything running and to charge the battery so that it will be ready the next time the car is started. So the alternator is like a little generator.


What kind of problems can arise when one of these parts is defective?

  • A weak battery or one that doesn’t hold its charge will not have enough power to turn the starter. The trouble often starts in the winter. The cold saps the strength of even a good battery, so imagine what it can do to an old and weak one.
  • A defective alternator won’t charge the battery or power the electrical system.


We hope that this overview will help you understand what’s happening under your car’s hood and make it easier for you to talk to your mechanic.


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