Question for the Motor Heads

[MadBlaster]

Don't disagree with anything said. Backpressure plays a role in the deal too (e.g., number exhaust valves, header/manifold/cat). Each engine is engineered out for a specific task. I guess I was thinking about the example more literal. The plane is on the ground with the chocks on no aero forces on prop, the throttle valve on carb is fixed at some position and the only thing being done by the pilot is toggling the prop pitch from fine to course to increase the thrust load and lower the rpms. If the rpms on the supercharger pump are directly proportional to turns on the driveshaft (i.e., the belt drive gearing is a fixed ratio), what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged? I think what blackdog is saying makes sense. Backpressure on the exhaust side holds the air in the intake manifold longer for that particular engine at that range of rpm/load change and fuel rate. Another thing to consider, the supercharger being belted to the driveshaft. I guess is a variable load on the engine. It takes energy to pump air. So while the prop is increasing load to lower engine rpms, the lower rpms on the supercharger pump are decreasing load (still assuming fixed fuel meter). That's intuitively what I was thinking about above where cranking that supercharger pump at certian rpm range with the increase in load that it brings to do that work doesn't pay off verses normal aspirated. Jeez, what a brain drainer this one is.

[ATAG_Bliss]

Originally Posted by MadBlaster

what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged?

Manifold pressure is going to increase when you compensate for the loss in RPM's through engine load and throttle increase. But that same metered fuel that required 50% throttle to maintain the RPMs you wanted to achieve, is now doing it at less RPM's. This creates positive pressure - boost (but only until the engine is normalized) then the boost guage (depending on the engine / charger combo) will remain fairly steady, but again, this is under a load. Superchargers do take HP to turn and cause a load on themselves, but they are generally designed for the application it's being used on.

A boat is the easiest way to see a supercharger in action with regards to boost, because the prop is always submerged in water at any RPM.

As far as back pressure is concerned, I think you meant to say exhaust manifold instead of intake, but your points are exactly right. That all factors into the equation of a boosted combustion engine.

[Babi]

Maybe i don't undestand because english is not my language, but i don't really grasp the fact that pressure output from the supercharger is dependant on "load"; probably because i dont understand the term.

The pressure should just depend on the engine RPM (wich determine the supercharger's RPM) and the throttle position (and of course external pressure). In the example of the car, the supercharger itself couldn't care less if the car is going uphill or downhill. Then if more "load" means that the engine RPM decrease, this will affect pressure output from the supercharger, but it's not DIRECTLY tied to load.

Then again maybe i undestand because of my bad english.

I think the discussion was about the fact that with constant throttle position and constant altitude manifold pressure drops with RPM

[ATAG_Dutch]

I understand the 'load' part.

Back in the late 80's, I owned a Kawasaki GPZ750 turbo (what an animal!) and the boost gauge rarely got up to full boost unless I gave it 'loads' of throttle in a higher gear.

Keeping it at or near the rpm red line resulted in lowish readings on the boost gauge, not that I could look at it too much at those speeds, as it was mounted on the tank.

But I think I've got it now. Put simply, at lower rpm, the engine isn't sucking as hard so the pressure in the inlet manifold goes up, and with no alteration to fuel/air metering would result in a dangerously weak mixture.

Conversely, increasing rpm makes the engine suck harder, lowering the pressure in the manifold, and without alteration of fuel/air metering would result in an overly rich mixture.

Have I got it?

Thanks for all the posts if so!

[swiss]

Originally Posted by Dutch_851

I understand the 'load' part.

Not really, turbos need exhaust pressure to spin up the turbine.
So flooring it is the same action but for a different purpose.

How engineload and sc-pressure on a mechanical driven SC are related is beyond my understanding - that is, unless the Sc engines have the pressure sensor behind the TB, or SCs use a clutch(?).


http://www.superchargersonline.com/content.asp?id=21

Boost builds exponentially with engine rpm, meaning that boost comes on very quickly in the upper half of the powerband.

confused,
swiss

[ATAG_Dutch]

I think I'm getting it, if you have a look at my edited post.

Increased rpm results in lower manifold pressure, but a mechanically driven supercharger would compensate for this pressure loss to an extent by raising the level of forced induction exponentially.

I think!

Also, yes I know turbos need exhaust velocity, hence the dreaded 'lag' which superchargers don't suffer from too much, but it's still driving a turbine (impellor?) to force-feed air into the manifold.

[ATAG_Bliss]

Originally Posted by Babi

Maybe i don't undestand because english is not my language, but i don't really grasp the fact that pressure output from the supercharger is dependant on "load"; probably because i dont understand the term.

The pressure should just depend on the engine RPM (wich determine the supercharger's RPM) and the throttle position (and of course external pressure). In the example of the car, the supercharger itself couldn't care less if the car is going uphill or downhill. Then if more "load" means that the engine RPM decrease, this will affect pressure output from the supercharger, but it's not DIRECTLY tied to load.

Then again maybe i undestand because of my bad english.

I think the discussion was about the fact that with constant throttle position and constant altitude manifold pressure drops with RPM

Well what you just said "throttle position" is in effect partial engine load. To give you an idea, you could generally redline (max RPM/rev limiter) an engine with the transmission in neutral far before you ever reached 100% throttle. It's quite easy to spin that engine up with no load on it. This means that it doesn't take as much fuel or the atmospheric "air" to go to max RPM. In this case the engine is running at max RPM (say it's 10,000 RPMs) yet will be fed the same amount of fuel and air that an engine lugging or loaded @ 2000 RPMs will.

When you have a huge load on the engine, it may require full throttle, meaning much more fuel and air, than the same engine turning the same RPMs or it may be such a load that the engine simply can't rev up that high. This creates more boost simply because the inlet is not restricted and more air can be pumped into the intake manifold.

Once you see that a loaded engine at 2500 RPMs can use more air and fuel than an unloaded engine at 10,000 RPMs - meaning RPM's are just part of the equation, then you can start to understand the load part of it.

Another way to think about it - drag racing. There's obviously many boosted cars both supercharged/turbocharged cars in high HP racing. Well when you get to a certain limit, most people prefer an automatic transmission, but hear me out. If those guys racing, simply raised their engine RPMs to max while the car was in neutral waiting for the go light to come on, they would have no boost built up. Because it doesn't require hardly any throttle to make an engine go to max RPM's without a load on it. That's the reason there are things such as tranny brakes. In a sense, it locks 2 gears together (generally 1st and reverse) and puts a massive load on the engine, which in turn cranks the boost up, so when they finally release the tranny brake, their car shoots off like a rocket. This allows you to go WFO (wide f'n open) or floored on the throttle. Without any boost that car would launch horribly. So basically you can sit there and rev the engine all day long and won't gain hardly any boost until it's under a load. This helps much more with a turbocharger as it's even more dependent on base engine and engine load. The more load, the hotter the exhaust, the more the impeller will spin to spool up the turbo.

There's guys that will use a line lock method (a solenoid that will hold the brake pressure - say you press on the brakes and you have 1200lbs of brake pressure, if you press the solenoid it now holds the brakes for you - don't need to press the brake pedal anymore) to create a load for boosted apps. All you do is lock the brakes and press in on the gas (load it up / viola - boost!) The good thing about superchargers is the power is available almost instantaneously. So, many guys will run a blower car with a 2 step module. This will electronically kill the spark at a certain RPM so you can keep the throttle pedal held down without blowing the engine up, and therefore create boost. But those that do this usually are the one's that have some pretty nasty blow off / bypass valves, because the pressure will be enormous (WOT at almost max RPM help constantly) and fuel delivery is not stopping. Supercharger explosions aren't fun.

I'm sure if you just googled superchargers or engine load with superchargers it will say the same thing.

Edit: I just did. http://www.superchargersonline.com/faq.asp

@swiss - you need the part about understanding how a SC works though

Is the supercharger always working?

Answer: While the supercharger is always spinning and moving air, it is not always producing boost in the engine. Boost is a function of engine load and RPM. The majority of the time your supercharger will not be producing boost. The supercharger produces boost under high load conditions which may include heavy acceleration, going uphill, passing another vehicle or under towing conditions. Superchargers offer the power you need on demand, the reminder of the time the engine is working just like a normally aspirated engine.

What exactly does a supercharger do?

Answer: A supercharger forces additional air and fuel into the engine. This occurs when the engine is under full throttle or under load, not at normal cruise or most normal driving. A large displacement engine makes more power than a small displacement engine because it can convert larger amounts of fuel and air into energy. A supercharger allows a smaller engine to do the same thing but only when extra power is actually needed.

In an airplane (supercharged engine), your engine load is determined by how much air the prop is pulling. That is why less RPM because of increased load will = more boost. If you want the physics of the engine, I think Blackdog_kt's links will give you a thorough understanding of that.

I haven't played flown IL2COD all that much because I'm deployed, but I'm assuming with full CEM switched on, you're probably not able to run at full throttle very long with an incorrect prop pitch correct? If that's the case, load is modeled. But I'm not saying it's modeled correctly. It's just if you can hammer the throttle wide open and set a bad prop pitch (ie - less load / higher RPMs) I bet the engine will be damaged in a short while, which is what would really happen. Does it work like that in game?

[ATAG_Dutch]

Does it work llike that in the game?

Yes it does as far as I can tell.

And thanks hugely for the input Synbliss, and Blackdog for the links, and Thor et al.

Some of this stuff was in my brain somewhere already, it just needs a good prod now and again to bring it to the fore!

[Babi]

Originally Posted by SYN_Bliss

Once you see that a loaded engine at 2500 RPMs can use more air and fuel than an unloaded engine at 10,000 RPMs - meaning RPM's are just part of the equation, then you can start to understand the load part of it.

I think we are expressing the same thing with different approaches. To me it's more intuitive to think that the intake pressure is dependant on "throttle" (throttle butterfly valve position) which causes a pressure drop, and engine RPM because it mechanically drives the supercharger (the relation between supercharger's RPM and its compression ratio may depend on the type of sc, not sure though).

A loaded engine at 2000 rpm achieves more boost (and consumes more fuel) than an unloaded one at 2000 rpm just because in the unloaded one the butterfly valve (throttle) is "more closed" (otherwise RPMs would go up) which causes a bigger pressure drop.

the point is that, ignoring the external pressure dependance, the intake pressure should depend on 2 variables, not three.

Do we agree on this?

Regarding the game, at constant altitude and throttle, if the prop pitch is coarsed (load increases, RPM decrease) the intake pressure goes up, which is not what one would think since the supercharger spins slower.

[swiss]

this is exactly how i understand it.

The output of the SC is only dependent on rpm but the pressure(boost) measured behind the butterfly on load.

Regarding the game, at constant altitude and throttle, if the prop pitch is coarsed (load increases, RPM decrease) the intake pressure goes up, which is not what one would think since the supercharger spins slower.

I just tried that on the FW190 1.65.
If you decrease pp(lower rpm) boost decreases too.

Not sure on the turbo planes, I'll try later.