What engine is this???

Sean,

When the exhaust valve opens, the piston chamber is exposed to ambient air pressure. So even though it may only be sea level ambient that the intake system is achieving, this is very high pressure compared to ambient at high altitude. It takes a lot of “blown in air” to return the cylinder to that pressure at each intake stroke. That movement of air can blow the spark out.  

"When the exhaust valve opens, the piston chamber is exposed to ambient air pressure"

What happened to exhaust back pressure, present in all exhaust systems, particularly those driving a turbo charger. 

I can see that the reduced air pressure, would also reduce exhaust back pressure (could this be adjusted in flight?) but how can it be so reduced that the incoming air "blow the spark out".

I accept there is an fuel ignition problem, at very high altitude but am struggling to understand the mechanism for this problem.

OK, think about what air is....at sea level it has a weight of .08 lbs per cubic ft.  Each cubic foot (volume) has 21% oxygen.  To burn this, we need a ratio of air to fuel, at 14.7 (air) to 1 (fuel) Now part of the issue is the engine has to breath by volume but to burn air we need the ratio to be by weight, not volume.  Liquid, our fuel, will weight the same at every flyable altitude.

You can convert this to KG and metric values, but the result is the same.  As an exercise let's just play a bit.  

To burn one gallon (US, not Imperial gallon) the fuel weight is about 6 lbs, just to keep it simple.  To maintain our ratio, we then need a weight of air that is 14.7 X 6 = 88.2 lbs of air to burn our 6 lbs of fuel.  We know that 1 cu. ft. of air has a weight of approx. .08 lbs so 88.2 / .08 will give us 1102.5 cubic feet of air at sea level.  This gives us the correct ratio we need for a nice burn.  

As we start to look at this it is obvious that the air as we go up will lose weight for the volume, the higher we are the less dense the air is and the less available oxygen in each cubic ft.  In general, we allow about a 3% loss of density for each 1000 feet we go up.  To make this easy we can look at say 10,000 feet.  This will give us about a 30% loss of air density.  Our carbureted engine with no boost loses power proportionately and we must lean the fuel to balance off the weight of the air.  (At least the Bing will try but after about 6 to 8000 feet will have a hard time to keep a good mixture) The prop has less air so if you run a fixed pitch the power loss and the thrust loss are about equal.  The rate of climb drops as a result.  

For running at altitude, the supercharger or in our case turbocharging of an engine can make up for the power loss by stuffing the volume into the engine and gaining the power back by higher pressure.  As Jeff noted the ambient air however is very low.  When the engine is running under very high pressures the incoming packing of air literally blows out the spark if it is not good enough.  Many race engines even at sea level can even see this problem, think turbocharged F1, supercharged drag race engine and the like.  For electrical power increases there are many companies who make very high-powered ignition parts to overcome this problem of spark inadequacies. Indeed, the special secondary coils on the new Rotax 916 have a number of improvements to allow for more power at the spark plug.  

As for the back pressure in the turbocharger it is still reliant on the ambient outside air as well.  Indeed, at very high altitudes as we have seen the engines turbochargers are much larger and most will be more than single stage.  2 stage or as in the picture shown even a stacked system.  The picture shown was a very early exercise and the newer versions went to special large 2 stage units to reduce weight and complexity.  (Altus) 

I hope that explains it a bit for you. 

Cheers

Hey Sean

We may need a more highly educated person to explain this and set me straight, maybe some with a major in physics and a minor in mechanical engineering.  

I know the same phenomena happens in highly boosted engines at sea level. It’s been my understanding that it’s due to increased airflow through the cylinder.  I don’t think it matters what the actual cylinder pressure is, what’s important is the rise in pressure the turbocharger or supercharger provides in relation to ambient. In other words, the change in pressure.  The more the pressure is boosted, the more air that must move through the cylinder. To get sea level cylinder pressure at 60,000’ you need almost 14 lbs PSI of boost.  By contrast, a typical turbocharged automotive engine operates between 6-8 PSI of boost.  

I am sure you realise I am trying to make intelligent comment, from almost no background/knowledge - here goes;

It seems to me that no matter, naturally aspirated or boosted, the gas in/out pressure will remain relative. 

Boosting increase volumetric efficiency (VE) the amount of fuel burnt per cycle, therefore power delivery.

High pressure in/high pressure out & visa versa.

The reduction in air density (particularly the O2 component) with altitude may be overcome (to a point) by pressurising (making denser) the air. 

As an example;

In a turbo normalised system (TNS), the air in, is sea level pressure, the out exhaust gas, does not go straight to ambient (low) pressure, as it is contained within a restricting exhaust system and further restricted by a turbocharger turbine (back pressure). IF this statement is correct, there must be some other factor at work, which suppresses the ignition spark itself or the efficiency of the burn.

Boosting only increases the air flow (VE) if the intention is to increase power ie its a comparative statement. Using an effective TNS the air flow, through the combustion system, should be the same at sea-level as at 80,000ft.

"........what’s important is the rise in pressure the turbocharger or supercharger provides in relation to ambient."

Hmmm!

I always understood that its the rise in combustion chamber pressure. Sure high (boosted) pressure in will contribute, however its what happens within the combustion chamber that is the significant factor in power delivery.

😈