"Turbineguy: Nom de Zoom" (will-alib)
09/03/2018 at 08:50 • Filed to: None
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"Turbineguy: Nom de Zoom" (will-alib)
09/03/2018 at 08:50 • Filed to: None | 0
| 17 |
A happy Labor Day to all my gearhead bros/broettes. I’ve spend the last hour or so on YouTube watching motor videos and can’t wrap my noggin around this one question. If the fuel/air charge explodes during the power stroke why would the exhaust stroke be so loud? All that’s happening is the piston pushing out spent gases right? What generates the exhaust noise? And why do you see flames popping out of short-stacked motors? Is it just unburnt gases igniting in the exhaust?
McLaren M8C for your viewing pleasure.
pip bip - choose Corrour
> Turbineguy: Nom de Zoom
09/03/2018 at 09:11 |
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how it travels through the exhaust is my guess.
TorqueToYield
> Turbineguy: Nom de Zoom
09/03/2018 at 09:18 |
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Exhaust gas is under pressure. Pressure released suddenly causes loud noise. Kind of like how popping a balloon is loud but it’s only air being released.
Furthermore exhaust tubes are long metal pipes. Long pipes resonate at certain frequencies easily.
Nibby
> Turbineguy: Nom de Zoom
09/03/2018 at 09:18 |
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honestly it all depends on how much blinker fluid you have
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Turbineguy: Nom de Zoom
> TorqueToYield
09/03/2018 at 09:22 |
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I hadn’t considered that. So there’s a decent amount of lag after BDC before the exh valve opens?
kanadanmajava1
> Turbineguy: Nom de Zoom
09/03/2018 at 09:26 |
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BiTurbo228 - Dr Frankenstein of Spitfires
> Turbineguy: Nom de Zoom
09/03/2018 at 09:27 |
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You can test that hypothesis by looking at cam timings. Exhaust valve timings usually open a little before bottom dead centre so they would still be open while the mixture is expanding. Here’s a diagram from wikipedia (we’re interested in the EC-exhaust closes and bdc-bottom dead centre):
However, there’s still a pressure differential between the cylinder and the exhaust just before the exhaust valve opens. If you tune your exhausts right, you should have a wave of negative pressure resonating back up the manifold from the collectors hitting the port when the valve is open at a given rpm range . If you’ve done it really right, you should time this between the inlet opening and the exhaust closing so you can draw through even more mixture and gain a mild supercharging effect from resonance.
sdwarf36
> Turbineguy: Nom de Zoom
09/03/2018 at 09:31 |
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Because what is happen ing is the when the spark plug ignites the mixture, it starts expanding. That expanding is pushing the piston down. I doesn’t stop just because its at BDC. And with everything happening so quick, you have to start opening the valves early. So some of the heat and expanding mixture goes out the pipe. https://www.youtube.com/watch?v=xflY5uS-nnw
sony1492
> Turbineguy: Nom de Zoom
09/03/2018 at 10:48 |
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In regards to exhaust flame; The most power is made when all the oxygen has been combusted which leaves unburnt fuel in the mixture , when it’s lean all the fuel has been used and there’s excess oxygen still available. So the flames are fuel igniting on the exhaust, not just because it's hot, but also because there is oxygen availble for it to burn.
wafflesnfalafel
> Nibby
09/03/2018 at 11:00 |
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and the viscosity of said fluid
AuthiCooper1300
> Turbineguy: Nom de Zoom
09/03/2018 at 11:16 |
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If the fuel/air charge
explodes
during the power stroke why would the exhaust stroke be so loud?
Common misconception. In an internal combustion engine working properly there is no explosion, but a rapidly-advancing flame front.
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gogmorgo - rowing gears in a Grand Cherokee
> Turbineguy: Nom de Zoom
09/03/2018 at 11:43 |
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Pretty much what you’re hearing is the exhaust valves opening. Like popping the cork on a champagne bottle, the hot gasses under pressure “pop” when the valve opens. On a production car t he muffler uses baffles to slow down the exhaust gasses and smooth out the pulses so they’re not so loud exiting the tailpipe , but a more free-flowing design leads to more noise because faster-flowing exhaust tends to self-evacuate to a certain degree — one of those Bernouilli things, faster moving air is at a lower pressure. You can also wave-tune w header to line up the pulses from one exhaust valve with the next one, helping to draw the exhaust out, but I digress.
You can also get a substantial amount of noise on the intake side beyond the sucking of drawing a ton of air with a highly tuned intake breathing through a restrictor, like on a race car. The pulses from each intake valve popping open travel up to the air inlet and make noise. Production cars aren’t generally going to do this, as not having to account for sucking as much air as quickly as possible through a restrictor means slower, lazier air through the intake, and modern vehicles will have chambers off the main intake flow to further address pulsing.
Generally though, on most “loud” street vehicles, you only have exhaust noise.
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Turbineguy: Nom de Zoom
> AuthiCooper1300
09/03/2018 at 21:44 |
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I meant to say 'burn'
XJDano
> Turbineguy: Nom de Zoom
09/04/2018 at 00:46 |
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Good question. I may have learn something from the comments. One of these “I read this on the internet” things that in a year from now will not be accurate how I tell it.
Distraxi's idea of perfection is a Jagroen
> Turbineguy: Nom de Zoom
09/04/2018 at 01:16 |
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No, it’s not that there’s lag, it’s that the pressure at BDC on the power stroke is still pretty high. Hot air expands, and if it can’t expand it pressurises: it’s that pressure differential that actually pushes the piston around . If the cylinder is full of cold (ish) air at 1 (ish) bar at BDC on the inlet stroke, by the time you get back to BDC on the power stroke you’ll have a cylind er full of very hot exhaust at quite a few bar .
The other as pect is that each time you have an exhaust stroke a wave of exhaust is pushed into the pipe by the piston . At 6000rp m this is happeni ng 5 times per second (for a 4 stroke) , multiplied by however cylinders you’ve got, And because the pulses aren’t a ll nice and sinusoidal and the flow path has cor ne rs and whatnot, y ou also get waves bouncing around at harmonic multiples of that frequency. Waves in air at “from tens to thousands per seco nd” frequencies are technically known as “ sound”.
Demon-Xanth knows how to operate a street.
> Turbineguy: Nom de Zoom
09/04/2018 at 08:00 |
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It has to do with the compression and pressure inside the cylinder. It’s really noticeable when you listen to engines before the 30s when octane was in the 20-30 range and the pressure was far lower to prevent detonation. Those engines exhale with a whistling whoosh or a light pop whereas after that the cylinder pressures are far higher as octane quickly rose to the 70s when they figured out cracking to get 100s then mixed it to get a medium. After that time you end up with the familiar bang as the exhaust valve opens and the cylinder pressure pushes out with a sharp bang.
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Turbineguy: Nom de Zoom
> BiTurbo228 - Dr Frankenstein of Spitfires
09/04/2018 at 09:21 |
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I think it’d be cool to rig up a motor with electronically actuated valves and play with the exh valve timing (advance/retard) too see how it affects the engine in real time
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BiTurbo228 - Dr Frankenstein of Spitfires
> Turbineguy: Nom de Zoom
09/04/2018 at 10:57 |
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Oh yeah that would be awesome :) I remember seeing a video from K oeni gsegg doing some testing of electronically actuated valves on an old Saab 9-5 (which gave it a significant bump in horsepower).
You could probably do a similar thing for cheaper if you got a car with proper graduated variable valve timing and hook those up to a program where you can alter them on the fly. Not sure if VANOS does that (I know MultiaAir does). VTEC I think is more on-off. Certainly older iterations are.