Caculating boost pressure loss after critical altitude?

[Ryanjames17]

Is there a way of Caculating manifold pressure after critical altitude? Say a engine was boosted for 35 inmg with a critical altitude of 1500feet what would the max boost be at say 5000 feet?

 

[Shortround6]

Is there a way of Caculating manifold pressure after critical altitude? Say a engine was boosted for 35 inmg with a critical altitude of 1500feet what would the max boost be at say 5000 feet?

well, you can try this.

Air pressure at 1500ft is 28.34in. so your supercharger has a 1.235 pressure ratio (manifold pressure divided by ambient air pressure, does not include RAM)
air pressure at 5,000ft is 24.89in so times 1.235 (supercharger pressure ratio) gives you 30.74in manifold pressure (not including ram).

 

[wuzak]

Just to clarify....

inmg should be inHg, as in inches of mercury.

SR used inches, but he meant inches of mercury.

 

[fliger747]

For various reasons the falloff in MP for supercharged engines doesn't precisely follow the change in pressure lapse rate. Close, but not exactly. A lot goes on in induction systems, including the presence or non presence of ram effects. Some of the aircraft manuals will have charts showing performance vrs Altitude, MP and RPM. Other graphs of HP vrs altitude are available as well in such references as "Amerca's Hundred Thousand". If you have a specific example in mind, attempt to find the flight manual. In the above calculation air density is perhaps more useful than air pressure, as temperature falls generally with altitude, which is a function of the decreasing temperature.

 

[K5083]

A little better guess can be achieved by realising that the total indicated horsepower is the number that should (all things being equal) vary with density altitude. Then you have to take off a figure for mechanical losses, pumping losses and (maybe) supercharger power requirement. That's gonna be something like a fixed number. Say you have a 100hp non-supercharged engine. It has an IHP of, say, 120HP. 100hp at the prop and 20 taken up by losses. At altitude corresponding to a loss of 20% of density you lose 20% of 120hp that leaves 96hp indicated. But you have to take off that fixed 20hp in losses. So you end up with 76hp. You lost 24% of your power at the prop. Different engines will have different figures, of course. I've seen an estimate of 13.2% of IHP in losses for a typical non-supercharged engine. Seems about right, but only typical not specific. With a supercharger, mechanical or turbo, things get more complicated.

 

[fliger747]

A mild further complication is exhaust scavenging is increased with reduced atmospheric pressure at altitude, a small compensation in power loss.

 

[Shortround6]

In a booklet put out by General Motors (owner of Allison) they figured a hypothetical 1000hp engine would pick up 80hp at 20,000ft if it could be supplied with sea level air pressure at no cost to the engine (mechanical or turbocharger).

An early Allison was using up a bit over 100 hp in friction while a late model Allison was close to 200hp in friction. Different piston rings needed to stand up to higher cylinder pressures and strong valve springs creating more friction/drag on the cams among other things.