For Questions That Don't Merit Their Own Thread

[magnemoe]

9 hours ago, Aperture Science said:

Internal combustion engine-related question.

From what I've heard, two valves per cylinder allow for greater power in the lower RPMs, whereas four valves per cylinder push the power to the high end of the RPMs.

In order to get the best of both worlds, what complications would arise from making a "hybrid" camshaft, as in half of the cylinders having 4 valves, and the other half having two?

I imagine the overhead camshaft would be more complicated, but not by much. Could there be any problems with balancing forces, or something else that would make this undoable?

Don't think two valves has any major benefits over 4 outside of simplicity, modern diesel engines is 4 valve, yes its an trade off on timing on the then to open it valves so some engines can change this by manipulating the camshafts. Freevalve is kind of the endgame here.

[Spaceception]

How long does it take to get from Moon to Moon in a gas giant system? à la Cassini?

[StrandedonEarth]

10 hours ago, Aperture Science said:

Internal combustion engine-related question.

From what I've heard, two valves per cylinder allow for greater power in the lower RPMs, whereas four valves per cylinder push the power to the high end of the RPMs.

In order to get the best of both worlds, what complications would arise from making a "hybrid" camshaft, as in half of the cylinders having 4 valves, and the other half having two?

I imagine the overhead camshaft would be more complicated, but not by much. Could there be any problems with balancing forces, or something else that would make this undoable?

First, a disclaimer: I'm not a mechanic of any sort, but I did read a lot of Car Craft and Hot Rod Magazine during my car-nut teenage years. Unfortunately, I never had the budget for all the fun things I read about. My first car was a CheVette. So I'm heavy on the theory but short on the practical experience.That said...

The quick answer to our question is that I don't expect any balance issues with a hybrid camshaft, as long as it's not an oddball engine with 2 (Alfa Romeo), 3 (GM), or 5 (Audi) cylinders, but it might be easier to have a rocker arm that activates two valves from one cam lobe. As for the long answer, I'm going to delve into some theory:

Contrary to what most people think, an ICE runs on air, not fuel. The more air you can get into a cylinder, the more power it will produce, if you add the proper ratio of fuel. The fuel is just there to heat the air and make it expand. A running engine produces a vacuum that sucks air into the cylinders (okay, so it's actually being pushed in by outside air pressure; semantics), so you have a moving column of air running into the cylinder. The bigger the intake area (intake runner cross-section and valve area), the more air you can get in with a given airstream velocity. The poppet valves interrupt this flow, so the airflow backs up behind the valve, but the air farther back in the intake is still moving. In a properly tuned intake, at the RPM the intake is tuned to, this momentum "charges" the air (like a turbocharger or supercharger) so when the valve opens again there's more pressure pushing more air into the cylinder. Again, this ram effect is at its strongest at a certain RPM, and that  RPM requires a certain speed of the airflow, which depends on the area available for the air to flow through.

So, back to more air = more power. To get more air in at higher RPM, you need a bigger intake runner and more valve area. Having two valves for intake (or exhaust) provides more valve area in less space than a larger diameter valve, so you can fit more valve area into a cylinder by having more valves. But at lower RPMs, the engine doesn't need nearly as much air, so the airflow is slower for a given intake area, so the "ram" effect disappears when the intake area is too large for the RPM. So much so that the airflow through the intake can (nearly) stop when the valve closes, and it's harder to get a larger mass of air moving again. This results in a poor signal to the fuel metering system (carburetor; or airflow sensor in an EFI system) and poor throttle response, not to mention less power because the less air gets in the cylinder due to the slower speed. If you've ever slammed the throttle open at idle on a four barrel carb with mechanical secondaries, you've experienced this bogging out. Vacuum secondaries delay the opening of the secondary barrels until the RPMs are high enough to generate enough vacuum to get the air moving quickly.

So at low RPMs, a smaller intake area  can help maintain the ram effect, and gives better cylinder filling (more power) and better throttle response. Manufacturers have come up with several ways to try and get the best of both worlds (good low RPM response and more high RPM power). The simplest may have been Chevy's LT-5 engine which debuted in the 1990 Corvette. It was an all-aluminum, 5.7 liter 32 valve V8. It had 16 intake runners, and half of them had a butterfly valve which only opened at higher RPMs (if not locked closed by the "valet mode"). Another aftermarket solution I saw in those old magazines had matched intake manifolds and exhaust headers with different sized runners, which effectively split an engine into two smaller engines tuned for different RPMs. This split the torque peak and resulted in a flatter, wider power curve, with overall more power under the curve but less peak power. Your hybrid solution (some 4V cylinders and some 2V cylinders) would have the same effect as this approach, and would be more effective if the intake and exhaust are hybrid-tuned accordingly.

Many modern engines use Variable Valve Timing technology (like in the 3.6L 24V Pentastar V6 in the Caravan I used to own) to extend the power curve. Advancing the cam timing moves the power curve to the higher RPMs (IIRC) and retarding it does the opposite. VVT gives the best of those worlds.

The biggest potential downside I can see with a spilt torque peak approach is vibration issues if the power output between different cylinders is too great, but beefier (and likely heavier) engine mounts and flywheels might soak that up.

 

 

[tater]

Cool article:

https://www.theatlantic.com/science/archive/2019/01/unprecedentedly-thorough-evolution-experiment/581521/

 

[ARS]

Could we use Nitrous Oxide on propeller plane's engine for a speed boost (aka improvised afterburner for propellers)? Even if the engine itself is already on WEP (War Emergency Power)?

[Xd the great]

8 hours ago, Spaceception said:

How long does it take to get from Moon to Moon in a gas giant system? à la Cassini?

From days to months.

[kerbiloid]

4 hours ago, tater said:

https://www.theatlantic.com/science/archive/2019/01/unprecedentedly-thorough-evolution-experiment/581521/

Quote

They photographed each rodent, took a DNA sample, implanted a tiny radio chip between its shoulders, and released it into one of the enclosures.
As time passed, many of the mice fell prey to owls

They should also study the evolution of the owls forced to (evacuate) the undigested chips.

Spoiler

A new species has appeared in the nature: rear-square owls.

 

Edited February 1, 2019 by kerbiloid

[DDE]

5 hours ago, ARS said:

Could we use Nitrous Oxide on propeller plane's engine for a speed boost (aka improvised afterburner for propellers)? Even if the engine itself is already on WEP (War Emergency Power)?

WEP is merely going over what’s considered safe performance. NoX is added oxygen in the mixture, which should boost power further.

Göring Mischung 1 (GM-1) was a Luftwaffe NoX booster program.

[Terwin]

5 hours ago, kerbiloid said:

They should also study the evolution of the owls forced to (evacuate) the undigested chips.

  Hide contents

A new species has appeared in the nature: rear-square owls.

 

Owls expel indigestible parts(bones, teeth, feathers, fur, etc) orally as pellets, I can't imagine that microchips would be treated much differently than teeth or feathers with this system.

[wumpus]

8 hours ago, DDE said:

WEP is merely going over what’s considered safe performance. NoX is added oxygen in the mixture, which should boost power further.

Göring Mischung 1 (GM-1) was a Luftwaffe NoX booster program.

I don't think I need to remind you on a rocket board that oxidizers are heavy.  Afterburners consume something like ten times the fuel for twice the thrust, and I think cars have an optimal air/fuel ratio of 16:1*.  Nitrous oxide should have a similar value for air (roughly 2 atoms of nitrogen for every atom of air) and I'd assume the weight issue would be worse than an afterburner.  This would be something only used while dogfighting or doing a final bombing run (possibly taking off a short/juryrigged runway), and when it runs out you will be in trouble.

* I think cars use a value closer to 12:1 because an ideal (for the first reaction) is bad for secondary NOX emission production.

20 hours ago, magnemoe said:

Don't think two valves has any major benefits over 4 outside of simplicity, modern diesel engines is 4 valve, yes its an trade off on timing on the then to open it valves so some engines can change this by manipulating the camshafts. Freevalve is kind of the endgame here.

The biggest advantage I can think of for two values be cylinder is to use a OTV (pushrod, as opposed to overhead cam) engine.  This allows a relatively smaller engine, especially relative to the displacement, and especially for "V-n" engines.  Having two valves won't give any advantages at low rpm, but having more displacement certainly will.  It generally will have issues spinning at high rpm (although GM used things like sodium filled and titanium valves to get a 7.0l OTV V-8 up to a 7000rpm redline [LS-7]) so typically won't be tuned for such things.  It is also more difficult to use variable valve timing on such things (although the current corvette and some Dodge cars seem to manage variable valve timing), so that makes tuning for high rpms harder.  But if you encounter such an engine in a car "in the wild", don't be surprised if it makes a "chug, chug, chug" sound at a redlight and takes off when the light turns green: obviously tuned for that high rpm (because the owner changed out the cam for a hot one (yes, one cam for an entire V8)).

OTV engines have a distinct advantage in places that don't regulate engine displacement.  On the other hand, even places that have such regulations tend to use the smaller mass produced low displacement SOHC/DOHC engines when you don't need the power of a big V8 (and Ford V8s have 32 valves regardless of the market).  Kerbanauts from outside of America and/or under sixty may have a hard time understanding why these engines, at least when made between 5 to ~7 liters, are called "small blocks".  Perhaps comparing the external size to a 5.0l (32 valve) Ford Coyote engine (typically found in a Ford Mustang) would explain it, but I think they just needed to contrast the engines to the 7-8 liter monsters also in production (mostly obsoleted by modern "small blocks", but often still loved by old school drag racers).  Just remember that a "small block" engine is one of the biggest you'll find not powering a truck or industrial usage when you see the term.

https://www.youtube.com/channel/UClqhvGmHcvWL9w3R48t9QXQ

Link goes to Engineering Explained.  I would go so far as to say that Jason Fenske is the Scott Manley of automotive engineering.

Edited February 1, 2019 by wumpus
added the car bit.

[DDE]

1 hour ago, wumpus said:

and I'd assume the weight issue would be worse than an afterburner

We’re talking about a piston engine, so an afterburner isn’t really an option - and a superperformance rocket engine (like RATO but streamlined) turned out to be even worse.

[tater]

 

[DDE]

11 hours ago, tater said:

*snip*

Wrong thread? We have one for that bolide.

https://forum.kerbalspaceprogram.com/index.php?/topic/181908-bright-bolide-over-the-florida-explosion-over-cuba/

Edited February 2, 2019 by DDE

[tater]

5 hours ago, DDE said:

Wrong thread? We have one for that bolide.

https://forum.kerbalspaceprogram.com/index.php?/topic/181908-bright-bolide-over-the-florida-explosion-over-cuba/

Except I posted that video 16 hours ago, and the bolide thread appeared 14 hours ago. So you're right, assuming I have a time machine

 

[kerbiloid]

15 hours ago, tater said:

Except I posted that video 16 hours ago, and the bolide thread appeared 14 hours ago. So you're right, assuming I have a time machine

Now we know, who has launched that bolide.

[tater]

3 minutes ago, kerbiloid said:

Now we know, who has launched that bolide.

Shhhh!

[kerbiloid]

1 minute ago, tater said:

Shhhh!

... and why.

https://en.wikipedia.org/wiki/Timescape_(1992_film)

[mikegarrison]

BTW: "NOX" is not nitrous oxide. NOX (or more correctly, NO_x, refers to "oxides of nitrogen", which are mainly NO and NO₂.) Nitrous oxide (which is used as an oxidizer and also a gas used for recreational, medical, and food service purposes) is N₂O.

Edited February 4, 2019 by mikegarrison

[mikegarrison]

On 1/31/2019 at 5:02 PM, StrandedonEarth said:

Contrary to what most people think, an ICE runs on air, not fuel.

Obviously, this is not really true. All such engines are powered by the difference in heat between their hottest point in the cycle and where they reject the heat to. This is basic thermodynamics. You could have all the air in the world and if you didn't have a heat difference, that engine would be inert.

But given a heat source (and a heat difference), more massflow through the engine equals more power. And that's why the more air you can ram through the engine, and the denser that air is, the more power the engine can deliver for a given size.

[Racescort666]

 

On 1/31/2019 at 8:48 AM, Aperture Science said:

Internal combustion engine-related question.

From what I've heard, two valves per cylinder allow for greater power in the lower RPMs, whereas four valves per cylinder push the power to the high end of the RPMs.

In order to get the best of both worlds, what complications would arise from making a "hybrid" camshaft, as in half of the cylinders having 4 valves, and the other half having two?

I imagine the overhead camshaft would be more complicated, but not by much. Could there be any problems with balancing forces, or something else that would make this undoable?

What you describe is sort of how VTEC works, other manufacturers have different technology that does similar things but Honda had the patent on their system. VTEC is probably one of the less expensive methods to implement variable lift as well so that probably explains why it's stuck. StrandedonEarth does a pretty good rundown of engines below so there's a more detailed discussion down there.

On 1/31/2019 at 8:02 PM, StrandedonEarth said:

<snip, but worth the read>

 

The main takeaway is volumetric efficiency. Most of the tricks to get air in and out of the cylinders is improvements to volumetric efficiency. With variable valve timing, it affects things like cylinder scavenging, the exhaust gas leaving the cylinder can pull the intake air into the cylinder as it leaves. The efficiency of this depends on engine RPM and the amount of valve overlap so they change valve overlap to adjust how much scavenging they get in a particular engine across the RPM range. There is a lot more going on than just scavenging though, valve timing can also affect how the air interacts with the plenum volume and intake runner length. Butterfly valves can be used to adjust both by blocking or adding another passageway/volume depending on engine RPM and load. 

Not all engine management is for performance though, sometimes they add VVT to help with low RPM emissions. It's hard to know without being involved in the development though. 

On 2/1/2019 at 2:46 PM, wumpus said:

The biggest advantage I can think of for two values be cylinder is to use a OTV (pushrod, as opposed to overhead cam) engine.  This allows a relatively smaller engine, especially relative to the displacement, and especially for "V-n" engines.  Having two valves won't give any advantages at low rpm, but having more displacement certainly will.  It generally will have issues spinning at high rpm (although GM used things like sodium filled and titanium valves to get a 7.0l OTV V-8 up to a 7000rpm redline [LS-7]) so typically won't be tuned for such things.  It is also more difficult to use variable valve timing on such things (although the current corvette and some Dodge cars seem to manage variable valve timing), so that makes tuning for high rpms harder.  But if you encounter such an engine in a car "in the wild", don't be surprised if it makes a "chug, chug, chug" sound at a redlight and takes off when the light turns green: obviously tuned for that high rpm (because the owner changed out the cam for a hot one (yes, one cam for an entire V8)).

OTV engines have a distinct advantage in places that don't regulate engine displacement.  On the other hand, even places that have such regulations tend to use the smaller mass produced low displacement SOHC/DOHC engines when you don't need the power of a big V8 (and Ford V8s have 32 valves regardless of the market).  Kerbanauts from outside of America and/or under sixty may have a hard time understanding why these engines, at least when made between 5 to ~7 liters, are called "small blocks".  Perhaps comparing the external size to a 5.0l (32 valve) Ford Coyote engine (typically found in a Ford Mustang) would explain it, but I think they just needed to contrast the engines to the 7-8 liter monsters also in production (mostly obsoleted by modern "small blocks", but often still loved by old school drag racers).  Just remember that a "small block" engine is one of the biggest you'll find not powering a truck or industrial usage when you see the term.

https://www.youtube.com/channel/UClqhvGmHcvWL9w3R48t9QXQ

Link goes to Engineering Explained.  I would go so far as to say that Jason Fenske is the Scott Manley of automotive engineering.

Do you mean overhead valve (OHV instead of OTV)? I've usually called them push rod motors since overhead cam (OHC or SOHC/DOHC) also have the valves over the head to avoid confusion. This is in contrast to a side-valve/flat head engine (images below). Yes, push rod engines have an advantage due to their compact arrangement. Theoretically, they should be worse at higher RPM because of having more reciprocating mass in the valvetrain but drag racers and NASCAR don't seem to have an issue revving them out to 9k-10k rpm. 

Side valve engine (exploded view):

Regarding small blocks, if we're going to be pedantic... Only Chevrolet ever made Small Blocks. Ford retroactively started calling several of their engines "small blocks" since they compete with the Chevy engines. Since Ford Small Block refers to more than 1 engine family usually you have to be specific of which one. These would be the Windsor and Cleveland engines mostly but sometimes other ones get thrown in the mix as well. Chevrolet also had a line of Big Block engines which are distinct from the Small Block; namely, their bore spacing is 4.84" vs the SBC 4.4". By the nature of aftermarket, there is a bit of overlap in the displacements of big blocks and small blocks so displacement isn't really what determines if something is a big or small, it's really just a specific engine family.

At this point, I should point out that Ford Big Block is a term that gets thrown around a bit too but that also refers to at least 3 different engine families of which, very few parts are interchangeable. The 60s-70s were weird times for the US auto manufacturers. 

The last thing I want to say is that I hate, HATE, HATE Engineering Explained. He's right on a few points but misses the mark by a wide margin and every time I watch his videos, it's like watching peak dunning-kruger effect in action. For example, every video he does on suspension:

It's like listening to the cashier from Autozone who learned some buzzwords and can make a techy sounding presentation. 

[tater]

[Xd the great]

13 hours ago, tater said:

"Why is there a fire next to my feet?"

[mikegarrison]

14 hours ago, Racescort666 said:

The last thing I want to say is that I hate, HATE, HATE Engineering Explained. He's right on a few points but misses the mark by a wide margin and every time I watch his videos, it's like watching peak dunning-kruger effect in action. For example, every video he does on suspension:

Took me a long time to understand how to tune my dampers. I heard a lot of BS and a lot of "rules of thumb" that were contradictory.

Finally I understood that the purpose of the suspension is to keep the wheels attached to the road. And damping rates are used to control how fast the car's weight shifts from one wheel to another.

For instance -- if your car understeers too much when accelerating out of an apex, that means the weight is moving to the back too quickly. So ease off the rebound damping on the front and let the front wheels stay attached to the road. (Of course, the weight will move to the back anyway -- the question is how quickly it does that.)

[KG3]

Explosive Bolts,

  I saw recently that the Saturn rockets used retractable hold downs to keep the rocket on the launch pad while the engines powered up but the Space Shuttle used explosive bolts.  What are explosive bolts?  I've heard of them before but don't know anything about them.  I imagine a technician with a torque wrench very carefully tightening a nut, hoping he doesn't loose any fingers!   Are they anything like the bolts I might see at a hardware store?

[Xd the great]

23 minutes ago, KG3 said:

Explosive Bolts,

  I saw recently that the Saturn rockets used retractable hold downs to keep the rocket on the launch pad while the engines powered up but the Space Shuttle used explosive bolts.  What are explosive bolts?  I've heard of them before but don't know anything about them.  I imagine a technician with a torque wrench very carefully tightening a nut, hoping he doesn't loose any fingers!   Are they anything like the bolts I might see at a hardware store?

No. NO. 

This is not KSP.