Question for the Motor Heads

[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?