On 7 May 2021, at 7:46 am, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:

OK, so I hooked up a scope to the fuel pressure sensor.
Sensor has 50mV/psi sensitivity.
Below 20"Hg (secondaries not opening) just very slight pulses.
But above 20"Hg -- about 4100 RPM -- I see 1.24V or 25psi peak-peak  pulses.
So that explains the wildly fluctuating fuel pressure readings. Depending on when the pressure sensor voltage is sampled by the engine monitor there can be 25 psi or more difference in engine monitor readings and data logging.

If you look at the picture I attached on May 4th, you'll see the pressure sensor is located at the end of the secondary fuel rail (solid tube) . Apparently the rubber lines from the T-fitting to primary and secondary rails are enough to somewhat dampen the primary injector fuel pressure pulses. I probably should add an R-C filter between sensor and monitor to get more reliable fuel pressure readings.

With today's 15 deg F lower OAT the engine transitions smoothly at the staging point. I'm again seeing cracks in the JB-weld around primary runners, so possible that they may be collapsing somewhat under high ambient temps -- maybe not. Remember I'm using the stock exhaust manifold. Even with the stock heat shield and an additional 0.016 alum shield, the heat from the exhaust manifold is intense. That is a huge chunk of metal that'll radiate heat with low or no airflow over it after shutdown. A definite drawback from using the stock exhaust manifold. Maybe I don't have a leak in my welds on the exhaust manifold and it was just higher OATs?

Another suspect for odd behavior is the engine controller's temperature sensor. I located it in the left radiator air scoop. Probably a bad idea when running on the ground. 

Finn

On 5/4/2021 2:08 PM, Charlie England ceengland7@gmail.com wrote:

Does the pressure increase have a solid 'step' at the exact staging point, or is it just rising in that area? How much increase on the gauge? Where is the reference for fuel pressure measurement? 

The pressure *regulator* should be referenced to MAP (manifold pressure), and will raise absolute fuel pressure as manifold pressure rises. If the gauge is referenced to atmosphere (or absolute), it would indicate a rise in pressure as the throttle is opened.

Charlie

On 5/4/2021 12:44 PM, William Schertz wschertz343@gmail.com wrote:

Finn

No rx-8 injector experience, but if at the staging point, if the EC-2-3 shuts off the primary injectors, and the secondaries haven't opened, fuel out of the manifold decreases, so the pumps would raise the pressure. Are you confident that the controller is reducing the primaries and starting the secondaries at the same time?

Bill

On Tue, May 4, 2021 at 11:07 AM Bobby Hughes BHughes@qnsinc.net <flyrotary@lancaironline.net> wrote:

Is the fuel pressure really increasing or is it the EM2/3 reading?  Steve B. has posted good advice for tuning around the staging point with different size injectors. 

 

Bobby

 

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] 
Sent: Tuesday, May 04, 2021 11:46 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Renesis secondary injectors

 

I noticed that my indicated fuel consumption appeared to be about 50% higher than actual.

So I decided to start calibrating engine monitor fuel flow.

The first step is to check/adjust fuel flow around the staging point (20"Hg) where the secondary injectors open (secondary injectors are larger than primary injectors on the 4-port Renesis).

I thought I had previously tuned the engine for a smooth transition around the staging point, at least to the point when I didn't notice anything when dvancing throttle through the staging point.

However, now doing stationary run ups around the staging point (especially when engine is getting hot) I get the weird phenomenon of it going very lean when secondary injectors (supposedly) open. Another odd thing is that fuel pressure increases markedly.

On the HomebuiltAirplanes.com forum a guy that raced RV-8s for a couple of seasons mentioned troubles with the stock Renesis injectors.

Now I'm wondering if I have (intermittently) bad secondary injectors -- not opening when they should.

However, I'm confused by the rise in fuel pressure. Is there a injector failure mode where pressure in intake runners could push back through the injector into the fuel line?

Note that these are factory-new, maybe 10 to 20 hours of total engine run (or attempted run) time.

Another possibility is heated secondary fuel rail. But if there were boiling fuel there the fuel pressure should still drop when secondary injectors open...

I tried to look but didn't find a chart of 87 octane 10% ethanol boiling point vs temperature.

Anybody here had any trouble with stock Renesis injectors? Symptoms? Other ideas?

Finn

RX-8 4-port:
Primary: Denso 195500-4430    33.3 lb/hr @ 43.5 psi (350cc/min)    13.8 ohm 
Secondary: Denso 195500-4460    51.4 lb/hr @ 43.5 psi (540cc/min)    13.8 ohm

 

 

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