iculus;
Here is what I understand, hopefully an expert can fill in the gaps of what I miss.
Put an internal combustion piston engine through one revolution and it attempts to pump a set volume of air. This amount of air is based on the combined volumes of each of its combustion chambers. For the engine to pump a volume of air, it must also intake the same volume of air.
For most automobiles this is also how the engine is rated. IE: 2.5 liters.
The engine makes horsepower (Hp) in proportion to the amount of air that flows to it. Restrict the air flow, and you control the Hp output. A valve is placed on the engine intake line, and is able to restrict the amount of inflowing air. This valve is generally called the "Throttle" or "Throttle valve" and serves to control engine Hp.
The restriction of air flow, by the "Throttle Valve", often causes the engine intake pressure to be at a pressure below atomspheric. The engine is actually trying to suck air faster than it can get past the restriction of the "Throttle Valve". However, with the "Throttle Valve" wide open, there is little restriction and intake pressure approaches atomospheric.
An operator or linkage is connected to the "Throttle Lever" in the cockpit, and moves the "Thottle Valve". Allowing the pilot to control the engines Hp output.
A gauge is placed downstream of the "Throttle Valve" and upstream of the engine air intake. This guage measures intake pressure in absolute pressure. <edited>.
<edited>
The intake pressure indication (MAP) is a very good way to determine total engine output or engine loading(Hp).
Picture this:
Throttle valve nearly closed --> airflow is severly restricted --> Engine load is low (idling) -or- Throttle valve is wide open --> airflow is not restricted --> Engine load is maximum (Mil Power).
I believe the engines / propellers modeled in AH are "Constant Speed". This means that the engine and propeller do not change RPM. What actually changes is the pitch of the propeller.
The propeller has a mechanism that detects RPM. Any attempt to increase RPM and the propeller increases pitch. Increased propeller pitch increases the load on the engine and consequently RPM returns to the previous value. Same, but the oposite, for attempting to lower RPM's.
So lets say you open the throttle. MAP goes from 30" to 40". The engine now recieves more air, and thusly makes more Hp. More Hp means the RPM's raise. Higher RPM is detected by propeller, and the propeller pitch is increased. RPM drops, due to the increased work load placed on engine from the propeller. Kind of a balancing act.
Normally sea level atomosheric pressure is 30 inHg. <edited> As altitude increases this pressure drops. To maintain the same engine Hp at 15,000 feet, the air pressure to the engine intake must be raised or boosted.
So say at sea level the atomospheric pressure is 30" and at 15,000 feet it is 24" (these numbers from my kazoo
). You have lost 6" (20%) of air pressure and effectively 20% available engine Hp. <edited>
This is where "Super or Turbo Charging" is applied. An air pump, called a "Charger", makes up for those lost inches of air pressure. So with 6" of boost applied, instead of having 24" applied to the intake, you have 30". The engine will not suffer dramatic loss of Hp, because it has the same air pressure applied as it had at sea level. <edited>
I am not sure, but I suspect that boost is applied upstream of the "Throttle Valve". Essentially fooling the engine into believing it is always at sea level and the Hp it produces remains fairly constant over various altitudes.
You can also boost the intake air pressure above sea level air pressure. However, now you must drastically increase engine component part strength to do so. This was often done however; for high performance engines used in WW2 aiplanes.
For automobiles, this added boost can give a V-6 engine the power of a larger V-8.
Merry Christmas Everyone!
Mino
[This message has been edited by Minotaur (edited 12-14-1999).]
