• blog@thephotonmagazine

How does your car work? — Rithvik Papani

Updated: Sep 16, 2019



Automobiles are among the most necessary technologies required for the functioning of our modern society. From the bus that takes you to school to the truck delivering shipments across the country, automobiles drive the world around us, and they have come a long way since the first automobiles created by Karl Benz in 1886. So how do cars really work? It’s simple. An engine that turns around wheels. But how does that engine work? Let’s explore the working of the internal combustion engine.




So, what is it that can make a few tons of metal on wheels move around? The answer is the engine. More specifically, controlled explosions facilitated by the engine which turn chemical energy into mechanical energy. But this is a superficial explanation. What really goes on behind the hood? Let’s find out.


The Slider-Crank Mechanism:

As stated previously, an engine works on the principle of converting an explosion into motion, namely rotational motion. How is it then that an explosion that propagates in all directions can be controlled to turn the wheels of a car? The answer to this is the Slider-Crank Mechanism. The parts of this system include:


-Piston: A small metallic cylinder

-Connecting rod: A rod that connects to the cylinder

-Crankshaft: A mechanical part that can covert linear motion into rotational motion, or in this case, turns the piston’s up-down motion into the rolling of the car’s wheels

-Cylinder: A case which encloses all the other parts


When the piston is pushed down, the connecting rod that it’s attached to it pushes down on the crank which rotates. The piston, later, goes back up due to the inertia of the crank and is ready to be pushed down again, either due to another explosion or the inertia possessed by the rotating crank. The cylinder, in which this entire process takes place in, prevents the piston from rotating about the crank due to gravity.


The Four Stroke Cycle:

Now that we’ve covered how the linear motion provided by the explosion is converted into rotational motion, let’s explore the process by which gasoline is converted into linear motion- the Four Stroke Cycle. The Four Stroke Cycle consists of four steps or ‘strokes’:

1) The Intake Stroke- The piston moves downward, thereby lowering the pressure of the airspace in the cylinder to create a vacuum. Simultaneously due to a process that will be discussed later in the article, the intake valve opens up and thus a large amount of air fills up the vacuum created by the piston moving down. It is also at this stroke when the fuel injector sprays a small amount of fuel into the airspace.

2) The Compression Stroke- The piston moves upwards due to the inertia of the rotating crank. This causes the piston to compress the airspace recently filled up with air. This causes the pressure and temperature of this gas to rise tremendously.

3) The Power Stroke- When the piston is at the top of the cylinder and the air in the cylinder is maximally compressed, the spark plug ignites the air-fuel mixture and thus causes an explosion to occur. This pushes the piston downwards.

4) The Exhaust Stroke- The piston moves upwards due to the inertia of the crank which was provided angular momentum by the explosion that occurred in the power stroke. The exhaust valve is open during this stroke and so as the piston moves upwards, the combusted gas filling up the airspace is expelled.

Now, you might be wondering how the piston was able to move downwards in the first stroke. Where did the energy for the motion come from? It came from the explosion that occurred in the preceding cycle. Hence, the explosion in the power stroke is the only source of energy for the piston to move and the other strokes occur due to the inertia of the crank. But this begs the question, how is the first cycle initiated? The answer to this is the starting motor which starts the very first intake stroke by turning the crank around with the power of a battery. This is why cars have jumper cables. If the battery of a car is dead, it is unable to power the starter engine and thus start the car. Jumper cables allow for the dead car battery to be recharged and thus capable of powering the starter engine.


Putting it all together

If you’ve been reading so far, you might’ve noticed that the way the intake valve and exhaust valve are able to be in sync with the piston system hasn’t been discussed. How exactly does the intake valve know to open up at the intake stroke to let in fuel into the cylinder or how does the exhaust valve know to open up during the exhaust stroke to let the exhaust out? The answer to that is the camshaft. The camshaft is a cylinder fitted with “cams”, or lobes, placed above the cylinders which push on the valves to open them up. The camshaft is connected to the crankshaft with a chain known as the timing belt in such a way that the camshaft rotates in sync with the crankshaft such that its lobes press down on the valves of the cylinders at the right time. So, the cam shaft is connected to the crankshaft in such a way that that one of its cams goes over the intake valve during the intake stroke and its other cam goes over the other valve during the exhaust stroke. As for the timing of the ignition sparks, older models of engines used to have the ignition sparks timed to spark right after the compression stroke using the inertia and rotation of the crankshaft, much like how the camshaft times the opening and closing of the valves with the rotation of the crankshaft, but nowadays the process is automated digitally.


Power uniformity of an engine

One thing you might have noticed is that the entire four-stroke cycle is not very smooth. The piston is at an extremely high velocity during the power stroke, directly following the ignition of the fuel in the cylinder, and at a relatively lower velocity, moving upwards against gravity, in the intake stroke. What this would mean is that your car wouldn’t have a constant speed and would instead be jittering between different velocities in a very rough manner. However, this is obviously not the case. So, how is it then that your car is able to run smoothly and with a constant velocity? The answer is multiple pistons. If instead of just one piston, there are multiple pistons working in tandem, the power is able to made significantly more stable. This is because in a multiple piston system, when one piston is at a high velocity following the combustion of fuel in the cylinder, another cylinder will be completing its intake stroke, which is much slower. Hence, the power outputs of all the cylinders are able to average out to a significantly more stable power output.


Transmission

The engines are attached to the wheels of the car. The rotational motion of the crankshaft is shared with the wheels by means of the car’s transmission, which consists of gears connecting the crankshaft and wheel in such a way that they turn together.

Putting this all together, you can see how an automobile fundamentally works. Granted, the engines we have now are incredibly complex and not as simple as the one described in this article, they all do follow the basic principles outlined. So, there you have it! This is how a car works.

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