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How Does An Engine Work (4 Stroke/Otto Cycle Engine)


A technical drawing cross section of a DOCH (dual overhead camshaft) V engine showing the pistons, crankshaft, camshafts, valvetrain and more.

Engines are incredibly fascinating devices. The sheer fact that us humans, as mindless apes, have been able to create a device that can control and contain thousands of tiny explosions per minute over a lifespan that can last over 100 years and millions of kilometres travel is truly mind boggling.


But how does a normal car engine work? And why do some people call them 4 stroke or Otto cycle engines?


Well, an engine works on a very simple base principle overall. It's main job is to turn heat energy, produced by burning a fuel, into rotational or kinetic energy to turn the wheels.

In fact, the whole idea of using heat and pressure to move things is not a new concept at all. Hot air balloon have used heat to generate lift (another type of movement) since 1783, and the first steam engine was invented in the 1st century AD (though it wasn't what we think of until around 1712).


In fact, there are a lot similarities between these steam engines and a modern petrol engine, but more on that later.


Now, we've covered the basic principle of the petrol engine (turning heat into kinetic energy), how does an explosion create power?

Here's the thing. Almost all substances on this planet have a calorific content or a calorific value. This doesn't mean that you could eat these things as a part of your 'recommended daily intake', in fact it's quite the opposite.


Calorific content simply refers to the innate amount of energy any substance can give off. It's measured by burning a specified quantity of a substance in a container surrounded by water and measuring how much the temperature changes.


For example, in 1kg of celery there is just 190 calories, compared to 3,870 calories in 1kg of sugar and almost 11,000 in 1kg of petrol. Calories are a measurement of energy and seeing as petrol has so much energy, it's a perfect candidate.


The next part of this equation relates to the explosion part of the process, which we're quite fond of.

When you burn something incredibly quickly (or "blow it up" scientifically), it expands at an extremely rapid pace. This creates massive amounts of pressure resonating around the blast epicentre and exerts force onto anything in its way. The same as the shock waves leaving the epicentre of a bomb.


At some point in history, we decided that just blowing things up wasn't aggressive enough and started exploring the "what if's" around the explosive process and we discovered that if the pressure from the explosion process was contained inside of something, the amount of energy rises exponentially until it finds a way to escape. The first time this container failed was coincidentally the first grenade in recorded history.


We also figured out that if the fuel for the explosion was under pressure to begin with, the output explosion and pressure would be much greater than the volume of space it originally occupied.


What does all this mean to a simpleton like you and I though?

For those of us not blessed with the necessary social skills prerequisite to becoming an engineer, if you put a bomb in a container it makes a big boom and if it's under pressure it makes an bigger boom.


Does a bomb and a grenade have anything to do with engine? Well yes, the same way that steam engines do, they both use the expansion of a hot gas to move a thing.


Now that you understand how something exploding creates energy, you really have the basis of the modern car engine. In essence, with every firing of the engine, a small petrol powered bomb is set off inside of a container called a cylinder (or combustion chamber) creating a massive expanding force.


The real question then is how this is harnessed.


This is all going to get a little complex, but we'll add in some pictures below to make it easier to read for us common folk. At the bottom of this container (or cylinder), is a metal plunger (or piston). Seeing as the expanding gas and force from the explosion has no where else to go but in the direction of the plunger, this plunger is forced down in a rather violent manner.

A diagram showing the cross section of a 1 cylinder diesel motor with the different components individually labeled.

This 'downwards' motion is then transferred through a connecting rod into the crankshaft, which is a device that changes a linear input (a motion in a straight line) into a rotational output. The added beauty of this is that as the crankshaft continues to rotate through a full turn, it eventually returns the piston to its original starting point.


The most incredible part about this is that this explosion actually starts under quite a large amount of pressure which means that the output of force is much higher than expected. This is managed by making the lowest point of the stroke (linear motion of the piston) occur at basically atmospheric pressure. As the piston rises back to the top of the stroke, the pressure increases significantly to increase the potential of the explosion.


We get this is all quite complicated, so here's a funny YouTube video to give your mind a rest.


Back to the show.


So far we know that fuel burns explosively, that explosions in a container gain force at an incredible rate, that the force can be used to turn a rotating device called a crankshaft that allows the system to reset to it's starting position after each explosion and that the explosion happens under increased pressure making it a much more significant event.

But how do you repeat this whole shebang over and over at multiple thousands of times per minute?


This is where our man Nicolaus Otto comes in. Nicki Otto was blowing up stuff all over Germany in the mid-late 1800's (before it was problematic for a German to blow things up) and was struggling to find a truly good use for his arsonist-ic tendencies.


He and business partner Eugen Langen had some mild success with a thing called an atmospheric engine throughout the 1860's however, little Nicki cracked the code to when he came up with the Otto cycle.

Now, what are the problems with the engine we've describe above?


There are actually a few, but most of all, it's reloading the cylinder with fuel and setting off the explosion at an appropriate time in the cycle.


Little Nicki Otto had a couple of tricks up his sleeves to deal with this and the biggest one was the use of valves. By having a mechanical device that could open and close at regulated times, Nicki was able to add the fuel mixture whenever necessary.


He also came up with the 4 main components of the Otto cycle, suck, squeeze, bang and blow. We swear this isn't a joke.

Nicki discovered that by using a flywheel (a big lump of metal that acts as a force to keep the engine spinning even when the explosion wasn't happening) and by separating the main components into different strokes of the piston, he was able to add the fuel mix to the engine under low pressure, increase the pressure drastically, burn the fuel and expel the waste all in a controlled manner.


How can all of this happen?


Picture that piston going up and down, over and over in the cylinder. Now, the first time it goes down, a valve opens and a bit of vacuum is formed in the cylinder which sucks in a fuel mixture.


After this, all of the valves close and the piston rushes back up the cylinder and puts pressure on the fuel mixture.

Diagram of the 4 different strokes of the Otto cycle engine. From left to right it reads, intake, compression, combustion and exhaust.

Just as the piston reaches the top of the stroke, a small spark ignites the fuel mixture and pushes the piston back down the cylinder with a massive amount of force.


Finally, one of the valves open again and allow the used up fuel to be exhausted from the engine.


This repeats over and over, thousands of times per minute and in multiple cylinders at the same time and that's basically how a 4 stroke engine works. Of course, there are a lot more complicated principles involved, such as spark timing, but that's enough of a brain fart for now.

 

If you have any questions about this blog post, would like a second opinion from a mechanic or would like to find an honest mechanic in Brisbane, check out Kashy here.


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