I was going to write a review of the latest Supernatural episode (S11E21: All in the Family) today, but my thoughts on the episode can be summed up in a few words. It was a good episode designed to set up the characters for the finale (and when Dean cries – I cry).
Instead, I’ve decided to write a short tutorial on how combustion engines work (I know it’s a bit of a leap from Supernatural – but hey, I’m unpredictable!). Enjoy!
After two and a half years of studying mechanical engineering, I have learned a lot of maths, physics, thermodynamics (easily my favourite subject), problem solving tools, teamwork and leadership skills, a whole lot of material properties, and how to use all that information in manufacturing and design. But, when it comes to the process of how a combustion engine works – I recall only one session where we glossed over a few parts of a lawn-mower engine.
I’m certainly not complaining, because the knowledge I have retained after two and a half years is pretty incredible. Studying engineering has completely changed my thought process when it comes to problem solving, team-work and conflict resolution. It has certainly helped my study habits and I often find myself looking at an object (most recently a ridiculously strong children’s tricycle) and contemplating how it was designed, manufactured and ways it could be improved.
So, I thought it was about time I took the initiative to learn how a car engine works!
At its most basic, a combustion engine ignites high energy fuel (eg. gasoline) in a small, enclosed space. This releases a huge amount of energy that can be converted into motion.
Most cars use a 4-stroke combustion cycle (also called the Otto Cycle – named after Nikolaus Otto in 1867) but commonly can have 1 (lawn-mowers), 6 or 8 (V8 engines) cylinders depending on the performance requirements.
I’m going to focus on the 4-stroke cycle which goes a little something like this (see Figure 1 for visual aid):
- Intake Stroke: The intake valve opens and, as the piston goes down, the cylinder is filled with a mixture of air and gasoline (only a tiny drop of gasoline is required to produce enough energy for each cycle).
- Compression Stroke: The piston is pushed up by the crankshaft to compress the air/gasoline mixture (high pressure), which makes the to-be explosion more powerful. During this stage the valves are closed (and piston rings create a seal between the piston and the cylinder to prevent any leakage).
- Combustion Stroke: The spark plug ignites the compressed air/gasoline mixture and drives the piston down with significant force (this is what releases the energy and drives the vehicle).
- Exhaust Stroke: The exhaust valve opens and the piston pushes out the remaining exhaust (N2, CO2, H2O, O2 + others) from the cylinder and out the tailpipe.
- This is repeated until there is no more fuel… Or something breaks.
As it turns out I had already studied the Otto Cycle somewhat briefly during my thermodynamics class while analysing the efficiency of various energy cycles (and being told why perpetual motion machines just cannot happen – which was a bit of a downer). Here is what I learned:
Below is a Pressure-Volume diagram of the Ideal Otto Cycle. It does make sense – even if it looks intimidating right now. It describes the pressure and volume changes during the 4-strokes which can be used to determine the engine efficiency. Here is how I like to explain it (best to follow along with each step on Figure 2):
- Intake Stroke –The air/gasoline mixture enters the cylinder. There is no change in pressure, but volume increases as the piston goes down.
- Compression stroke – The gas is compressed, increasing the pressure and reducing the volume.
- Combustion – The spark plug ignites the gas, causing a phase change which increases the pressure but does not affect the volume (gasoline + air –> exhaust).
- Power Stroke – The increase in pressure forces the piston down, increasing the volume and reducing the pressure as it expands.
- Heat Rejection – The exhaust valve opens (before the piston moves) and some of the exhaust escapes, returning the system to its original pressure (almost instantaneous).
- Exhaust – The exhaust is pushed out of the cylinder by the piston and the volume within the cylinder decreases. We are now back to point 1.
NOTE: This is for the IDEAL Otto Cycle – in reality there are significant losses and therefore the thermal efficiency of a standard combustion engine is not 100% – not even close – more like 25% – interesting right! This is why I love thermodynamics!
Don’t worry if this second part makes no sense – it is second year engineering and for half the semester it didn’t make sense to me either. If you do understand, awesome!
I hope this was interesting/made sense/was helpful. If you have any questions about engines, or engineering, or science in general feel free to ask and I’ll do my best to answer!
Have a nice day!
References (and good locations for more information):