HM

I believe the term "service ceiling" technically is where the climb rate of an aircraft drops to 100ft/min. As such this is a combination of the engine AND airframe.

For instance my Sting S4 912IS has a service ceiling of 15,000 feet. Personally I have only been to 13,500 and it was still climbing at about 500 ft/min so I suspect this is correct

The IS Operators manual does not state a service ceiling. However it does have a performance graph and equation of power vs Altitude in section 5.

To know the service ceiling you will need to know the power required to produce 100 ft/min climb at each height and then calculate whether the IS can to this.

I think there might be other engine considerations you would need to check if you wished to fly really high on a regular basis but i’m no expert. One consideration is restart ability, then there may be ECU, fuse box, cooling and fuel matters.

The aircraft has a service ceiling defined as where the rate of climb falls below 100fpm.

The aircraft can fly higher than its service ceiling up to its Absolute Ceiling where the rate of climb falls to Zero.

The Engine does not have a service ceiling.

The 912is engine will continue to run up to maybe 50,000 ft.

It would be making a low 1500 rpm idle and might deliver 1hp to the prop, but it would still be running.

It is unlikely you could design anything other than a balloon to carry the engine to that altitude.

The charts in section 5 of the 912is Operators manual show how the power reduces for every foot you go above sea level.

It is below 40hp at 25,000 ft.

- - -

You might be thinking of the Critical Altitudes of a Turbocharged Engine.

This would be the altitude where the engine can no longer produce 100% of its rated power.

For Example...

The Rotax 914 turbo engine can maintain..

Maximum Rated Power at 5800 rpm and 39"hg MAP up to 8000ft. and...

Continuous Rated Power at 5500 rpm and 34.9"hg MAP up to 16000ft.

I noticed the critical altitude of the 915 iS is 23,000 FT. Does that mean it can still produce 45” of manifold and 141 HP at that altitude? 

The 915is has a Max CONTINUOUS Power (133hp) Critical Altitude of 15,000 ft.

So, Yes???, Full Continuous power of 133hp up to 15,000 ft.

It has a Max Operating Altitude of 23,000 ft. (Whatever THAT Means??? ... 75% Power ??? )