Three Strokes and Out

In this post, I will discuss the working of a four stroke internal combustion engine such as the one used in most automobiles.  The engine of most modern cars runs on gasoline/petrol.  It does this by burning petrol in air and using the energy of the hot gaseous by-products to produce mechanical movement and motion of the car.  We shall explore how fuel and air are combined in the engine, how the controlled explosion is initiated and how all the heat is converted into rotational energy for the wheels.

Properties of Gases

When a gas is enclosed in a container, it applies a force on the walls of the container in an effort to escape it.  The force that it applies per unit area of the container is known as the pressure under which the gas is held.  If we try to hold more gas in the same container, the pressure increases.  This is because there are more gas molecules rattling around in the container and they hit the walls of the container more often.  If we increase the size of the container (and hence its volume), the gas expands to fill the container and hence exerts a lower pressure.  Again, we can understand this by imagining the gas spread out through a larger volume and hence, for each unit area of the container, fewer gas molecules are striking it.  Throughout this discussion, we assumed that the temperature of the gas remains the same.  If we heated up a gas in a closed container, its pressure increases.  This is because the gas molecules gain more energy.  The temperature of a substance is a direct measure of the amount of energy in the atoms and molecules of that substance.  The higher the temperature, the faster the gas molecules are traveling and hence, the harder they strike the walls of the container.  Another effect that can be observed is that if a container with a gas in it is squeezed rapidly (reducing its volume), either the pressure or the temperature must go up.  Usually, they both go up by smaller amounts.  The equation that relates all these quantities is known as the ideal gas law .  It works as a reasonable approximation for gases that always remain gases – unlike real gases that go funny things like break down at high temperatures or condense into liquids at lower temperatures.  If a certain quantity Q of gas in a closed container of volume V has a pressure P and a temperature T, then

PV = QT

Anatomy of a Car Engine

Labeled parts of a four stroke internal combustion engine

Parts of an engine

On the left, you can see a labeled picture of the cross section of a 4-stroke internal combustion engine (ICE).  An internal combustion car engine, in its simplest form, consists of an airtight cylinder that is closed at the top and with a piston P at the bottom.  The piston is free to move vertically and is lubricated to reduce friction.  It is further connected through a rod R to a crankshaft C which is connected to the rest of the transmission and eventually the wheels.  As the piston moves vertically in an oscillating manner, the crankshaft is rotated.  The connections are made using low friction bearings to minimize energy loss.  The piston is sealed such that gases cannot escape around its edges.  When the piston is higher in the cylinder, the engine encloses a smaller volume.  When it is lower in the cylinder, the engine encloses a larger volume.  At the top of the cylinder (also known as the cylinder head), there are two valves – an exhaust valve E and in intake valve I.  Also mounted at the top of the engine is a spark plug S.  The intake valve is connected to the fuel injection system that is responsible for providing a fuel-air mixture to the engine.  The fuel air mixture consists of vaporized gasoline mixed in with air at just the right ratio so as to cause complete combustion.  Give too little fuel and the engine doesn’t get enough energy.  Give too much fuel and the engine chokes due to lack of air and simply throws away partially burnt fuel, lowering fuel efficiency and polluting the environment.  The exhaust valve allows spent gases to be vented outside the vehicle.  The spark plug is a device the generates an electric spark at regular intervals of time.  It is used to ignite the fuel-air mixture to cause a controlled explosion in the engine.

All the Parts in Motion

Animated scheme of a four stroke internal comb...

Image via Wikipedia

During normal operation, a 4-stroke engine goes through four distinct phases known as strokes (explaining the uncreative name for the engine).  The first stroke is known as the induction stroke and occurs when the piston moves down, expanding the volume in the cylinder.  During this phase, the intake valve is held open.  The increase in the volume causes a low pressure in the cylinder.  This in turn forces a fuel-air mixture to rush into the engine.  The next stroke is known as the compression stroke.  The piston, with the rotational momentum it already has, moves upwards.  All the valves are held tightly shut.  This causes the fuel-air mixture in the cylinder to compress and heat up.  As the piston reaches its highest point in the cylinder, the spark plug fires, causing the already hot fuel-air mixture to ignite and explode.  This explosion pushes the piston downwards as part of the third stroke known as the power stroke.  This is the only stroke during which energy is imparted to the piston and the crankshaft.  The force with which the piston is pushed away causes the piston to continue rotating through the other strokes.  Finally, as the piston returns back to the top of the cylinder after completing its rotation, the exhaust value is opened to let the combusted gases and by-products out of the cylinder.  This is known as the exhaust stroke.  Since the piston is compressing the gases in the cylinder, it causes the gases to have a higher pressure than the air outside and the gases rush out of the cylinder through the exhaust valve.  This completes all four strokes and the engine is ready to begin its cycle again at the induction stroke.  The entire process is repeated in a continuous smooth manner in a car engine to the tune of thousands of times per minute (measured as revolutions per minute or  rpms) to deliver power to the car’s wheels.

The curious reader might want to know how the entire process is started.  After all, the engine begins with nothing moving at all.  An engine like this is initially started by using an external power source such as a starter motor powered by a car’s battery or through mechanical effort.  Old cars at the turn of the last century used to have a crank at the front of the car which the operator could turn rapidly to start a car’s engine.  Once the engine is spinning rapidly enough, the energy gained through the explosions in the cylinder is enough to keep the process going until either the engine breaks down or the engine runs out of fuel.

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2 responses to “Three Strokes and Out”

  1. Kia says :

    Loved this article! it was really interesting, and the diagrams helped put it together. I could understand this pretty well, a few sentences were a little confusing, But i fully understood the whole thing!!!! :)

  2. Simple Scientist says :

    @Kia: Yay! Tell me what was confusing. I can go in and fix the sentences, make a follow up post if the subject warrants a long discussion, or post a comment here with a follow up. Also, feel free to suggest topics!

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