Dumb Questions. Engine RPM.

GregP · Oct 10, 2011

Gotta' disagree with you Siegfried.

Deflagrtion is entirely subsonic. Detonation is supersonic. That is the basic difference.

Deflagration cord could make just as big an explosion as det cord, but it was marginally slower, that's all.

As qualification, I was test manager at Unidynamics Phoenix and we made explosives that both detonated and deflagrated.

The difference, when applied correctly was amazing. When similar amounts were used in similar circumstances, the difference was depenent on the shape of the detonation charge. Shaped charges are very interesting, if you need to blow up something, but leave another thing undamaged.

Mostly deflagration cord was used to set of detonations subsonically. It LOOKS simulataneous, but is not when filmed at 20,000+ frames per second.

PWR4360-59B · Oct 13, 2011

Siegfried said:
The combustion process in a SI or spark ignition engine is known as 'deflageration' it is not an explosion but a controlled burn along a flame front at at sonic speed emanating from the sparking plug. Simultaneous combustion at multiple points throughout the cylinder is known as detonation, is damaging to the engine and occurs at supersonic speed at multiple points in the mixture propagated by infrared radiation.

Simultaneous combustion at multiple points? Hmmm how about 2, 3 spark plugs per cylinder, detonation (no)? And yes sometimes there is a timing shift for aircraft 2 plug arrangements.

You also mention diesels would have been in the airliners if.... I say they should be now, to save fuel.

GregP · Oct 14, 2011

Vanir,

What is an "easy good throttle altitude?" from a few posts back ...

Just curious.

Siegfried · Oct 16, 2011

GregP: Yes, your right, deflageration is subsonic, though I didn't say supersonic, I wrote sonic, which technically is at the speed of sound, so a little sloppy.

From my understand of 'full throttle height' it refers to the altitude at which the supercharger produces 1 atmosphere pressure at the inlet manifold at full throttle setting. It gives and indication of the relative merits of the supercharger.

engguy: I believe multiple spark plugs are a way of helping to achieve a more stable burn, so is creating a stratified charge around the spark plug region by piezoelectric or other injectors or by bouncing a jet of fuel of the top of the piston into the spark plug area. Other techniques include hemispherical combustion chambers. Lots of work on combustion chamber swirl.

diesels are showing up in light aviation, I believe they tend to run of jet fuel rather than diesel. If they could run of light marine diesel they would be perfect for flying boats which are expected to make somewhat of a comeback due to the corrosion resistance of composites. I suppose it might be possible to fly something about the size of a DASH-8 using diesels. The diesels might be cheaper to purchase and run even if they are heavier or would be useful for aircraft with light load factors.

Shortround6 · Oct 16, 2011

Siegfried said:
From my understand of 'full throttle height' it refers to the altitude at which the supercharger produces 1 atmosphere pressure at the inlet manifold at full throttle setting. It gives and indication of the relative merits of the supercharger.

"full throttle height" is the altitude at which the throttle can be fully opened. Opening the throttle fully below this height can lead to engine damage, either through detonation of the fact that a particular engine cannot stand up to the BMEP generated.
As an example the Merlin III had "full throttle height" of 16,250ft on 87 octane fuel at which point it's supercharger could still produce 6lbs (or 6 1/4) of boost in the intake manifold. At take off the engine was limited to 880hp instead of the 1030hp at FTH at the SAME rpm and manifold pressure because the throttle was part closed (and a few other minor factors). With the coming of 100 octane fuel the throttle could be opened more without detonation and the engine/supercharger could make 1310hp at 9000ft with 12lbs of boost in the intake manifold with the throttle fully opened. As the plane climbed higher with the throttle fully opened the intake pressure fell along with the power until, at 16,250 ft the engine/supercharger was making 1030hp with 6lbs of boost. Later, for short life engines the Merlin III was cleared for 1440hp at 5500ft with 16lbs of boost which became the new "full throttle height" although it did not change the power at higher altitudes and the supercharger remained unchanged.

mikewint · Oct 16, 2011

There was a thread a while back on props. There is a practical limit to prop RPM as the tips approach the speed of sound. In my opinion the key here is not how much HP you can produce but the power-to-weight ratio. My Sky has a 4-cylinder 2.0L engine but it turns 270HP

PWR4360-59B · Oct 16, 2011

Siegfried said:
GregP: Yes, your right, deflageration is subsonic, though I didn't say supersonic, I wrote sonic, which technically is at the speed of sound, so a little sloppy.

From my understand of 'full throttle height' it refers to the altitude at which the supercharger produces 1 atmosphere pressure at the inlet manifold at full throttle setting. It gives and indication of the relative merits of the supercharger.

engguy: I believe multiple spark plugs are a way of helping to achieve a more stable burn, so is creating a stratified charge around the spark plug region by piezoelectric or other injectors or by bouncing a jet of fuel of the top of the piston into the spark plug area. Other techniques include hemispherical combustion chambers. Lots of work on combustion chamber swirl.

diesels are showing up in light aviation, I believe they tend to run of jet fuel rather than diesel. If they could run of light marine diesel they would be perfect for flying boats which are expected to make somewhat of a comeback due to the corrosion resistance of composites. I suppose it might be possible to fly something about the size of a DASH-8 using diesels. The diesels might be cheaper to purchase and run even if they are heavier or would be useful for aircraft with light load factors.

Reason I mentioned multi spark plugs, is.
It was said and is common knowledge that ignition occuring at some other point in the combustion space, and then colliding with the normal burn from the spark plug kernel is a cause of detonation. Well then I'm saying purposely do that. And normal 2 plug arrangements do it. So I'd say it kinda puts some holes in the theory. I'd say its more of a high pressure and thus high temperature related rapid burn, or detonation, and nothing to do with a colliding flame front. And the way it was limited by TEL is the combustion process was cooled. I know TEL will cool cylinder head temperatures in an aircooled engine, I've done it. I forget the numbers but it was quite a bit.

vanir · Oct 17, 2011

I know there are tricky things you can do with spark mapping fine tuned for a fuel and running condition, using the dual magnetos and multiple sparks you can really fine tune it for some highly customised results, raise overboost tolerances at the cost of a little continuous rated torque and things like that. You could make a dozen performance variations with the same basic engine doing this, add to it the various blower options and you wind up with what the engineers were actually doing at the time I believe.

Dumb Questions. Engine RPM.

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