Hi,

I have been reading up on turbo/super charging and noticed that on Page 4.7 the book there is a graph which says engine power reduces with increased altitude.

However, I also note that at the higher altitudes volumetric efficiency is increased due to more gas being able to be exhausted due to the ambient low pressure at altitude, not to mention also the cold OAT at such heights which increases density for the given pressure. I envisaged that power would increase up to full throttle height.

G'day,

Volumetric efficiency is a measure of the quantity of fuel/air charge induced into the cylinder as compared to how much the cylinder could contain just sitting there under static conditions.

You're right in a way, there is a tendency to improve the volumetric efficiency as you climb since the decreasing outside air pressure reduces the back-pressure on the exhaust which makes it easier for the exhaust stroke to purge the cylinder of burnt waste gases, making more space available for the new charge on the next induction stroke.

However the reduced air density has a much more detrimental affect on the volumetric efficiency and so you will tend to get a net loss in power output as you climb.
It's true that if you lower the temperature at a given pressure altitude the density of the air will increase. However in this case we are not staying at one altitude but increasing altitude as we move to the right along the X-axis of the graph in Fig. 4.8.

As you climb, the cold will make the air molecules move closer together (increase density) but there will be far fewer of them because of the increased altitude. There will actually be a net decrease in density as you climb despite the temperature effect.

Lower air density means less volumetric efficiency which means lower power output.

In a normally aspirated engine, this effect will be noticed as soon as you start your climb. However, if we can find some way to artificially increase the pressure at the inlet manifold, we can delay the onset of this effect significantly. That's the point of turbocharging.

Let's take a closer look at Fig 4.8 to make sure you understand the effects.

Normally Aspirated Engine:
The normally aspirated engine we are considering can produce 200 hp at sea level using full throttle. As it climbs (at full throttle) the air density decreases with altitude and so too does the volumetric efficiency. There's simply fewer air molecules getting drawn into the cylinder. That will mean the engine will steadily produce less power.

If you kept climbing, at some point the engine is unable to produce enough power to maintain any climb and the aircraft will end up hanging in level flight just above the stall.

Ground Boosting:
Let's ground boost the same engine. In this example engine, the inlet manifold of this engine is actually able to cope with higher pressure than that produced at sea level with full throttle, so let's fit it with a compressor to boost the inlet manifold pressure.

Now, at full throttle at sea level the engine is now able to produce 300 hp. However, as the aircraft climbs, the air density still decreases and so too does the volumetric efficiency of the cylinders. This will theoretically start to have an effect on engine power output as soon as we leave the ground.

We still lose engine power as we climb but because we started with a higher max power at sea level, the altitude at which the engine can no longer support a climb is greater than that reached with the normally aspirated engine.

Altitude Boosting:
Let's now consider the same engine but this time with altitude boosting. A beefier compressor is fitted enabling an engine to theoretically produce much more power at sea level - 400hp in this case. Our example engine however can only handle 300hp so we need to protect it by not using full throttle initially and keeping the power output at a maximum of 300 hp.

You will therefore need to climb with reduced throttle setting to prevent overboosting but as you climb and the air density decreases you can steadily increase the throttle setting and maintain the manifold pressure required to produce the 300 hp.

In this engine, the throttle setting for 300 hp maxes out at about 11000 ft and after that, just as with the other two engines, the decreasing air density will start to directly affect the volumetric efficiency and therefore the power output of the engine.

Altitude boosting simply delays the onset of the loss of volumetric efficiency until a much higher altitude. Both the ground boosted and the normally aspirated engines started suffering power reduction as soon as they left the ground. However all of them will suffer at some stage with the thinning air.

No but wouldn't it be great if that was true - as you climbed higher, the power would increase and you could reduce the throttle setting until eventually you were climbing with no power at all. Unfortunately, physics doesn't give us any breaks that way

What you need to remember is that the air density decreases with altitude and lower density = lower power output.

The best we can do is delay the inevitable power loss by using turbines and compressors to artificially increase or maintain the pressure at the inlet manifold despite the decreasing air density outside as we climb.

Cheers,

Rich

Hi,

I also have a question which relates to this topic. On page 4.2 Bob talks about full throttle height with a gear-driven supercharger and that at this height the engine is enjoying its best volumetric efficiency.

What I don't understand is how volumetric efficiency is at its best when its at full throttle height? In this example, isn't the supercharger just maintaining rated boost ie. highest recommended manifold pressure, all the way from sea level to a particular altitude and not actually increasing that which was already available at sea level? Or is he just saying that this is the best volumetric efficiency that can be achieved at this height, and to do that you need to open the throttle? I would have thought best VE would have been obtained with full throttle at sea level with supercharger in full swing. Obviously this would expose the engine to risk of over boosting but theoretically that is when the engine is bringing in the largest volume of charge for the piston displacement.

Any clarification would be great! I'm sure i'm just missing something.

Thanks,

Angus

Hi All,

This is one of my favorite subjects. I used to use this topic as part of assessing a Grade 3's theoretical knowledge for considering suitability for a Grade 2 upgrade.

Volumetric Efficiency (VE) is a measure of how easy that engine breathes. Simply put, the engine breathes it's easiest with the throttle fully open compared to partially open (try running on a treadmill while breathing through a straw). The reason why VE is best with the throttle fully open is because it will have the least amount drag, resistance or blockage caused by the butterfly. It's inline with the airflow allowing (for example, an IO-360) 360 cubic inches of air pass through with each rotation of the crank. It can be increased by either forcing the air in via a pump or ramming it in with forward speed.

This is the important bit!
Best or Maximum VE is achieved by flying at an altitude where full throttle will give you the desired % power setting i.e. F.T.H. for 65% power from the P.O.H. power setting table. Any higher and your % power will start to decrease away from 65%. Any lower and % power will increase away from 65%.

Now let's talk about Superchargers. As we know, a Supercharger (and Turbochargers) is just a pump to force feed the engine with air. The engine still has a throttle butterfly and, importantly, it will still have an altitude where full throttle will achieve max VE for a given % power.
The main difference is you will be much higher than a normally aspirated engine with all the benefits of higher TAS and smoother weather.

Things to remember!
Forced induction aircraft engines were designed for high altitude work.
If you set 65% power at sealevel, you will most likely find that the throttle will not be fully opened thus not achieving a good VE.
Volumetric Efficiency is about achieving % power with the throttle fully open. Be it 45%, 55%, 65% or whatever % power you need.
Also. Yes, opening the throttle fully on the ground with one of these engines will overstress the poor thing as well as lead to detonation.

I hope this helps.

Cheers,
Mister W

Thanks for your excellent reply. That's a concept that many students have trouble with.

Bob

Thanks! That really does help