Motor exhaust noises

[CobyRupert]

This is one of those dumb questions. My car is getting loud because it needs a new muffler. While driving it, I started to wonder “When (and why) is my car loud without one? “

Should be a simple question, right?

When I press on the gas, I assume it gets louder because more gas is being poured in, bigger explosions in the engine, more energy, a louder rumble. Yes?

When I’m up to speed and the engine doesn’t have to work (apply any torque), momentarily coasting, engine is a quiet as having a muffler, even if throttle is being applied for the engine rpms to match the coast speed (i.e. foot is on the pedal, gas is still pouring in).
Why is it quieter?

When letting foot completely off the throttle, and near zero gas is going in, why does the engine breaking sound almost as loud as accelerating ? And louder than coasting at 1/2 throttle?

What are we really hearing when we hear an engine? And engine braking? Why is noise more a function of the engine torque (accelerating and engine braking), than throttle position or rpm? Are we hearing more detonation as timing differences change (relative to valve positions) as torque (or rpm, vacuum in the old days, etc.,) changes?

I can’t believe I’ve reached this age and understand nothing about what I’m hearing.

 

[BravoXray]

Depending on the power level the engine is operating at, the pressure in the cylinder can be low or pretty high when the exhaust valve opens. High cylinder pressure makes a high pressure wave when the valve opens in the exhaust pipe and thus louder exhaust note.
That's also why trucks with Jake brakes, exhaust brake system that opens the exhaust valves on the compression stroke, letting out the compressed air, make the loud burbling sound.

 

[CobyRupert]

Depending on the power level the engine is operating at, the pressure in the cylinder can be low or pretty high when the exhaust valve opens. High cylinder pressure makes a high pressure wave when the valve opens in the exhaust pipe and thus louder exhaust note.….

Ok, but isn’t that high pressure a result of exploding/expanding gas when the throttle is pressed?

When engine braking, and no/little fuel being applied, isn’t the motor just an air pump, getting a restricted amount of air blocked by the throttle plate? Where would the additional pressure come from to make the additional noise. More noise than when the motor was “coasting” at no torque (but same or higher rpms and throttle partially pressed)?

 

[BravoXray]

Exhaust brakes on diesels will not come on unless the throttle is at idle, and if the engine speed is above idle, the fuel pump will be in cutoff and no fuel will be delivered to the injectors.
Diesels have no throttles plates like gassers do, thus the cylinders are getting filled on the intake stroke even if the throttle is at idle position. So on the compression stroke when the valves a popped open, there is still a lot of pressure in the cylinder, considering that most diesels have a compression ratio of 14:1 to 20:1.

On a gas engine, at idle very little air is being drawn into the cylinders, and and thus at the beginning of the exhaust stroke there will be a lower than atmospheric pressure in the cylinder and when the exhaust valve opens some air in the exhaust system may be drawn back into the cylinder, that my cause unburden air/fuel in the cylinder to ignite.

As far as a gas engine goes, that's how an engineer explained it to me a long time ago.

 

[CobyRupert]

Hmm… so maybe engine braking noise (which is louder than an engine receiving fuel, but at near zero torque) is because the ignition is happening when the exhaust valves are opening (or in the exhaust manifold)?

Perhaps, I mean, in some cases you actually hear vehicles “backfire” when that ignition occurs farther down the exhaust system.

 

[ericm979]

Perhaps, I mean, in some cases you actually hear vehicles “backfire” when that ignition occurs farther down the exhaust system.

On over run the FI shuts off fuel which makes the mixture too lean to burn. The remaining hydrocarbons go out the exhaust pipe. Sometimes they ignite, making a backfire.

These days its usually on poorly tuned engines though some sports engines are tuned to make the exhaust 'burble' (small backfires not explosions) because racecar. Not that I'm entirely immune to that.

 

[CobyRupert]

On over run the FI shuts off fuel which makes the mixture too lean to burn. The remaining hydrocarbons go out the exhaust pipe. Sometimes they ignite, making a backfire.

These days its usually on poorly tuned engines though some sports engines are tuned to make the exhaust 'burble' (small backfires not explosions) because racecar. Not that I'm entirely immune to that.

Yeah, for sure. …backfiring is pretty recognizable.

I still don’t know why engines are louder when the throttle is closed, and engine is “braking” than when at (say) 1/4 throttle but “coasting”(zero torque) (and rpms are nearly identical).