I know how engines work, what makes them work etc. but this question has been on my mind for a while. How can a bunch of controlled explosions create the sound characteristic for a specific engine type? For example, a V6 engine sounds distinctly different than a V8 or I4, but how do they get their sound & why can’t the sound be different, for example an I4 engine with the sound of a V12?
In: Engineering
1) If you think of a car engine and car exhaust as being akin to an instrument, then you can see that the tone, timbre, and harmonics of a car’s engine noise will change depending on the precise geometry and materials of the exhaust side of the engine.
2) There’s actually an interesting thing going on with respect to human perception which is fundamentally why you hear a continual “buzzing” tone from your car’s exhaust despite the fact that combustion is actually a large number of discretized combustion events.
In music theory, there’s a thing called the “inter-onset interval,” which is basically the time between discrete notes, measured in milliseconds. The fastest reliable inter-onset interval is generally held to be around 100 ms, and something weird happens as you push beyond it to 50 ms; instead of hearing the discretized notes, you begin to perceive the notes as singular constant tone. This is because and inter-onset interval of 50 ms corresponds with a signal at 20 Hz, and *20 Hz is the lower limit of human hearing*. 20 Hz also corresponds with an engine operating at 1200 RPM, which is basically a high-idle engine speed. As a result, despite the fact that you’re hearing individual combustion and exhaust events coming down the exhaust pipes, because these individual sounds are coming at you at such a high rate of speed and high frequency, your brain interprets them the same way it would a continual soundwave. This is also why engine sounds change pitch linearly with RPM; RPM converts linearly to exhaust event frequency, which corresponds directly to a “tone” coming from your cars exhaust.
It’s important to keep in mind that ICE engines do not (unless there is something severely wrong!) create explosions. Think of it as a controlled burn.
As such, the sound you hear is a combination of sound waves interacting with each other as they leave the header and interfere. The number of cylinders, header design, and exhaust design all contribute to how this interference occurs.
The firing order of an engine can also determine its sound.
I.e. the v8 engines in german cars usually use cross plane crank shafts and have an even firing order allowing exhaust waves to flow smoothly one after another. Some american brands have uneven firing order and cause exhaust waves to collide and create either constructive or destructive interference. This results in the loud gurgling sound you hear from a classic v8.
The valve lift, cam profile, etc. can all also have an impact on engine sound.
The frequency harmonic differences between i4s, v8s, and v12s are due to the fact that as the engines increase cylinder count, there are (typically) more combustion cycles per rotation of the engine. So 3000rpm in an i4 engine has a lower harmonic than 3000rpm in a v12 (since there are 3 times the number of combustion cycles occuring per revolution).
Because the explosions are happening in very, very, very, extremely fast succession. The sound blurs together creating the sound. Why do different types of engine sound different? Because they operate slightly different and thus sound slightly different. You asking why a v6 doesn’t sound like an I4 or whatever is like asking why a piano doesn’t sound like a guitar. Because they’re different things.
think of your exhaust the same as a brass musical instrument. When playing a trumpet or a tuba, the sound is generated the same, however, the output is totally different.
an exhaust system is the same way. Its ‘tuned’ for performance and sound based on length of the expansion chamber, the diameter of the pipe, the baffles etc….
There’s a lot of info here but I feel like no one is talking about how crankshaft geometry affects the sound.
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