Well not just on cars but all internal combustion engines. The normal ICE is 4 stroke with the ignition stroke being the the one that makes the bang, yet at that point of the cycle the valves are closed and the cylinder effectively sealed so how does the sound get out?
From the point of ignition the piston travels down the cylinder and is on it’s way up again before the exhaust valve opens and lets the exhaust gas and sound escape so has the sound of the ignition bang been waiting to be let out?
If it’s possible to contain sound this way the possibilities are endless, so why don’t we have tins of canned sound effects available?
The cylinders are sealed in terms of gas not getting out, but that that doesn’t mean that sound can’t get out. You do realise that sound can travel through solids, including the metal your engine is made out of?
I ran my car ('75 MGB) with a straight 2 foot pipe one time. It’s a tiny little 100HP four cylinder engine.
The level of sound coming out of the engine was the kind of noise you feel – it was unbelievably loud. Pulses of exhaust gas come out of your engine 1500 or more times per minute, and the gas is pressurized when it is ejected into the manifold. A good portion of the sound is a bunch of hot gas shooting out of the engine several times a second, not just the sound of the fuel igniting.
I don’t think that sound escaping through the solid of the engine is the issue, since if it were putting a muffler on the exhaust wouldn’t muffle. The valves may be closed when each cylinder fires, but when they open compressed gas escapes violently. A series of violent escapes of gas is called “a loud noise”.
I’m under the impression that the combustion itself is relatively quiet - detonation (which you can hear from the driver’s seat) is a sign that something is amiss.
Rather, like Princhester and others have said: the loud sound out of the exhaust pipe comes from the exhaust valve opening. There’s still a considerable pressure difference between the just-combusted gases in the cylinder and those in the exhaust manifold, and each time the exhaust valve begins to open there’s a loud “pop”.
Figuring on the fly…
(Engine rotational frequency) * (number of cylinders) / 2 = (frequency of exhaust note)
…the factor of two is because (on a four-stroke engine) there’s only a combustion event every other revolution of the crankshaft. Just watch units - engine speed is usually measured in revolutions per minute, while sounds are typically quoted in cycles per second; account for this appropriately when performing the calculation and it ought to work.
Think about the biggest, loudest most annoyingly ear splitting motorcycle you’ve ever heard.
The engine in that bike was smaller than the engine in a Volkswagon Beetle. Not the new sleek and plastic Beetles. The old ones that hippies drove around in. The engine that you and your friend Steve could take out of the car and carry into your parents’ basement to tinker with.
That’s right, when the ignition in the cylinder takes place both intake and exhaust valves are closed so there’s no sound coming out except what little travels through the solid which isn’t much since there is a lot of metal to absorb it and the elastic engine mounts will kill the rest.
Around the time the cylinder reaches bottom dead center (BDC) the exhaust valves start to open. At that time the exhaust gasses still possess quite a lot of energy (pressure) and to make things worse they are forced out by the movement of the cylinder. For a comparison, piston-type air compressors make a helluva noise and there’s no combustion taking place. It is just the rapid movement of air that causes it.
Also for the same reason there’s quite some noise from the intake side of the engine. Drive a car around with the filter box removed and it becomes rather loud.
I guess I could actually add info to this from my own experience…
When cranking over an engine slowly you can hear the different sounds of intake, compression, ignition, and exhaust…
Intake, you can hear the, well, sucking, intake of air into the engine…depending on the engine, you can make out this sucking noise as it runs.
Well, you can’t really hear the compression…
but ignition is audible…on a diesel it can be a very distinct “knock” or “rattle” inside…though it is loud, that is not the sound you hear out of an exhaust. When cranking I can feel the release of pressure as the exhaust valve opens, and that release of pressurized air makes a VERY audible distinct “POP” out the exhaust…when the engine is actually running on it’s own, the "pop"s all blend together into a smooth roar.
Exhaust systems have other purposes besides reducing noise. Here’s a video of a racing chainsaw which obtains extra power due to a large “tuned” exhaust system.
Two stroke engines (like that chainsaw) are very dependent on the backpressure provided by the exhaust because they do not have valves to keep the fuel from escaping to the atmosphere. Without an exhaust the engine won’t work at all. There’s a nice animation showing the operation here: Two-stroke engine - Wikipedia
4 stroke engines are not that sensitive but still removing the exhaust completely will alter the power band characteristics. Usually it will hurt performance at low to mid RPM (due to fuel escaping but for different reasons compared to 2-stroke engines) but it will improve at high RPM.
