Turbo Nozzle Shrouds

[Bdflies]

I’ve recently learned why one individual put the turbo shrouded injectors on his N/A engine. He didn’t like the fuel stains caused by fuel boiling and dribbling at his injectors. The shrouded turbo injectors eliminated the issue. His engine stays spotless!


Bill

[DanH]

Originally Posted by robert@jonesrv10.net

The aircraft you have seen them on most likely had a Sam James or Showplanes cowl. The problem is that the velocity of the air coming in the small opening is quite high, which drops the pressure, especially for the number 1 and 2 cylinders.

My understanding also, and very likely. Recall a small inlet area generally means a inlet high velocity ratio and internal diffusion. If the inlet duct doesn't expand sufficiently prior to the front cylinders, velocity will still be high at the nozzle bleed. Some of the earlier James plenum inlets are like that, with long narrow necks.

I've been flying turbo nozzles several years now. This cowl is a low Vi/Vo system, so velocity near nozzles 1 and 2 should be low, but I was still curious to see if I could get a little further LOP if I balanced bleed air pressures front to rear. The kicker was that Don was curious about bleed air pressure vs manifold pressure. So, each side got a rail with a pitot entry, tying a pair of nozzles together. Tapping the aft end of one bleed air rail provided a relatively stable measurement source. The other side of the Honeywell pressure sensor was connected to the primer port of #1. Analog sensor output went to a Dataq https://www.dataq.com/products/di-1100/ and my laptop. Flight engineer was a teenager in the back seat. None of my older airport buddies seemed to be able to run the software

Ran another channel to pick up the #1 spark, providing a reference marker for event timing.

Collected a lot of data at different power settings and altitudes. As I'm sure you've heard, real time manifold pressure isn't the steady value you see on your cockpit MP gauge. It oscillates considerably due to wave activity and valve action, which is why we wanted to see what deltaP looked like across 720 degrees of crank rotation. I've attached a sample below.
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[DanH]

Some readers may be a bit hazy regarding constant flow nozzles, so here's the short version...

The restrictor is a precision orifice. This is the part we swap for slightly smaller or larger diameters to achieve a small GAMI spread, i.e. to make all the EGTs peak at the same time when leaning.

The nozzle body has a passage somewhat larger than the restrictor ID. There is an air bleed hole in the side of the body. The restrictor shoots a stream of solid fuel into the passage, where it mixes with air from the bleed. The result breaks up more readily in the intake port, in particular at part throttle.

A standard nozzle body has a simple guard shroud and screen to keep the bugs and trash out of the bleed hole. The air source is upper cowl pressure, which is usually higher than manifold pressure, even at full throttle.

The variables here are (a) pressure loss through the intake system and (b) upper cowl pressure recovery, i.e. how much of the available dynamic pressure was converted to increased static pressure in the upper cowl. An installation with great intake tract performance (ram, no filter, big throttle throat, etc) and lousy cooling pressure will have poor bleed air delta. A system with uneven upper cowl pressures will have unbalanced bleed air supply; the nozzles may flow different quantities of bleed air, and the proportions may change at different airspeeds.

A turbocharged installation has a more significant problem; manifold pressure is routinely higher than upper cowl pressure. Obviously that would make the bleed air holes flow backwards. So, a sealed shroud is installed over the nozzle body, and air is piped to the nozzles from a source near the turbocharger outlet. That keeps the bleed delta positive.

Assume (or suspect) a normally aspirated engine shows symptoms of poor or uneven bleed pressure. We can install the sealed shroud turbo nozzles, and supply them with a pitot tube (usually in the cowl inlet) to capture total pressure, the sum of static and dynamic. The result should be higher bleed pressure, equal at connected nozzles, thus better atomization in the nozzle passages. At least that's the theory.

The plot in the previous post was a sample from a live measurement of bleed air delta with turbo nozzles. In the photo below, you can see the pitot opening feeding bleed air to cylinders 2 and 4, and a black hose connecting the pitot tube to the cyl #2 nozzle shroud.

Do you need a setup like this? Maybe, maybe not. Note available dynamic pressure, a function of velocity and density, drops with altitude gain. There is less bleed air pressure to entrain air in the nozzles, so at low flow rates (think LOP and WOT cruise at 10.5K) the nozzle delivery into the port is more blobby, for lack of a better term. Higher, or more uniform bleed air pressure may improve LOP smoothness due to better atomization. Again, so sayeth the theory.

Education and recreation.
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[rv8ch]

Originally Posted by Bdflies

I’ve recently learned why one individual put the turbo shrouded injectors on his N/A engine. He didn’t like the fuel stains caused by fuel boiling and dribbling at his injectors. The shrouded turbo injectors eliminated the issue. His engine stays spotless!


Bill

Thanks Bill - I don't like the stains either, even if mine are not blue. Not to mention the smell of fuel after shutdown.

Anyone know if these shrouds can be retrofit?

@DanH - thanks for the explanation and graphs - very interesting if these shrouds have even more than the cosmetic benefit. Any concerns about bugs flying into the pitot?

[DanH]

Originally Posted by rv8ch

@DanH - thanks for the explanation and graphs - very interesting if these shrouds have even more than the cosmetic benefit. Any concerns about bugs flying into the pitot?

I have considered installing screens on them, but it has gone quite a while without trouble.

[DanH]

Bit of follow up...

I like a reality check when collecting data. For example, look at underlying principles (the laws of physics don't vary for anyone), or compare with measurements from other sources.

Below I've aligned two intake port pressure plots. The upper plot was taken from an old CAFE report (EPG Part IV, IO-360 angle valve on a Mooney), the lower from my own records (IO-390 angle valve on an RV-8). The intake tracts are similar, the differences being recording method, RPM, and density. The CAFE plot was taken at 2700+ near sea level, while mine was at 2400, at altitude. The difference shifts the wave shapes and timing a bit, but still, the correlation is obvious.

BTW, in considering the desired length and diameter of the intake tract, those 1960's Lycoming guys did OK. Ignore the left side, when the intake valve is closed. Our interest is high port pressure at valve opening, to get the flow moving into the cylinder, and again as the valve closes, to shove in the last bit even as the piston is beginning to rise from BDC.
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[SHORTRV7]

Dan thanks for the information! I am right in the decision state with my IO-540, 9:1, Barrett CAS. I just can't get LOP without a rough engine. Don at AFP suggested I go to Turbo Injectors and based on your analogies looks like that be best for me.
My question to you is getting away from bugs why not pick up Pressure in the Air Plenum?

[DanH]

By "air plenum", do you mean a pickup location somewhere above the engine but further aft, not in the inlets?

The tube in the inlet is a total pressure pitot, the sum of both static pressure and whatever dynamic pressure remains in the inlet velocity. A unidirectional source somewhere in the aft cooling plenum would be exposed to flow without much velocity, i.e. less dynamic pressure.

[SHORTRV7]

[quote=DanH;1715453]By "air plenum", do you mean a pickup location somewhere above the engine but further aft, not in the inlets?

No I meant in the air inlets to the Servo....

[DanH]

Originally Posted by SHORTRV7

No I meant in the air inlets to the Servo....

An airbox pressure tap will supply less bleed pressure than the pitot tube in the cooling inlet.