Supercharging & stuff - losses, improvements, good vs. bad. vs. excellent etc

[Daggerr]

In high gear mixture is more heated by the supercharger rotating faster ( delta temp is proportionnal to the square of the impeller tip speed).

So higher temperature of mixture at same manifold pressure (=boost) gives lower quantities in the cylinders, and lower power.

Basic !

Exactly.

And on top of that the S/C power consumption in FS is higher than in MS, so in FS the decreased SHP (due to higher manifold temperature) has to deliver more power to the S/C (due to higher speed), so the remaining BHP is lowered for two reasons.

 

[Bretoal2]

Exactly.

And on top of that the S/C power consumption in FS is higher than in MS, so in FS the decreased SHP (due to higher manifold temperature) has to deliver more power to the S/C (due to higher speed), so the remaining BHP is lowered for two reasons.

Yes. And we may add that the relative losses (% of loss on charge density and % of extra power to actuate th S/C impeller) depends on the S/C performances and settings. So, no absolute rule !

 

[Simon Thomas]

In the first post, there is a figure that states this:

Dropping it into Google translate gives this:
Fig. 11. — Official results obtained at the Chalais-Meudon Test Station in June-July 1938, on a Hispano-Suiza 12 Y GRS engine at a speed of 2'|10 rpm, and an inlet pressure of 885 mm of mercury

What is "2'|10 t/min"? Is it a typo that meant to say 2110 rpm, or does 2' and 10 mean something else?

 

[Simon Thomas]

Ignore me - it's a 4, i.e. 2410 rpm.

From AEHS Torque Meter v5n1

 

[Bretoal2]

Ignore me - it's a 4, i.e. 2410 rpm.
View attachment 792061
From AEHS Torque Meter v5n1

It is curious and regrettable that the great AEHS specialists were not able to read or understand correctly all the curves presented here.

The Chalais-Meudon installations were ground-fixed benchs....

The "A" curves are the curves recorded on the bench without any correction : very simply, the engine is breathing through a large air tank whose internal pressure is controlled by a valve. So the pressure upstream of the engine is reduced to simulate the loss of atmospheric pressure corresponding to altitude.

Without any surprise, while the upstream pressure decreases, the supercharger regulation system acts to maintain a constant boost : so the Hispano-Suiza engine delivers a constant power of 760 hp from "0 to 3,100 m." (in reality, with a suction pressure upstream of the engine simulating these altitudes). Above 3,100 m., the S/C has reached its maximum performance and the actual boost pressure begins to decrease. The power too!

For the S/P (Turboméca) supercharger, it is a little more complex because the particular nature of its regulation leads to a slight decrease in the intake pressure, inducing a slight loss of power which goes from 860 hp at the ground to 780 hp at "3,200 m". Above is a gradual and normal decrease in power.

And the B curves? We have just seen that the "A" ones are the direct readings with an engine whose suction is simply reduced. Two essential factors are missing to really simulate altitude : the drop in intake temperature and the drop in exhaust backpressure (see all our exchanges since the beginning of this thread).

As these two data cannot (could not) be recreated on the Chalais-Meudon test bench, the initial readings are interpoled by calculation, following a standardized formula. Which obviously gives a higher power than the actual measurements, since the two missing parameters of temperature and back pressure both go in the direction of improving the performances observed on the bench.

Don't say that this procedure is barbaric, all the countries of the world did the same in the 1920s and 30s!