From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Lehanover@aol.com
Sent: Friday, July 09, 2010 9:55 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Dyno

Don't know the answer - however, my view is at 5500 rpm even with 100% Ve
the engine is ingesting only so much air mass - Which means you can only
throw in so much fuel before you are burning all the ox in the air.  I don't
think that any WOT operation at/near full power is going to vary the BSFC
much from the 0.55 - but, ignore my response lets see what Lynn says.

Ed

As so many quickly point out to us, the rotary uses more fuel than a piston engine. So you need a snappy comeback to counter that attack. What could that be?

Oh yeah, well I can run way lean of peak EGT and you can't. .........And so you can.

Piston engines collect the fuel / air charge and mashes it into a tiny space that contains the heat of compression and the hot exhaust valve face and the hot spark plug tip. This provides a charge temperature not far from detonation. This allows the piston engine to operate close to the edge of doom for its whole life. The only way to get closer to doom would be to add a turbo-charger, and they do that too. Detonation is charge temperature dependant.

But having that mixture in a small hot package allows for some very complete burns with only small amounts of spark advance. The closer you get to using all of the fuel in the burn the more efficient the engine will be. The piston engine has a slight advantage in this area.

In the rotary the charge is squeezed gently into a large cold combustion chamber that has no squish areas to drive mixture to the plugs. Near the apex seals the rotor face and housing are close enough to stop the flame front. So mixture in this area does not burn.

Much of the heat of compression is lost to the chamber before ignition. The fuel is trying to condense into droplets for lack of heat. For the most part, not a good show.

The problem seems less so when we see that the poor chamber heating makes any fuel appear to have a higher octane rating than it does. The mixture contains less energy, and the flame front speed is low. Pressure build up is slow and constant. The better dwell holds the chamber closed for a long time, and this improves the burn. The exhaust gas temperature

is high compared to a piston engine, until you remember the piston engine is heating an exhaust valve to orange on each cycle, while the rotary is dumping gas at the speed of sound right onto the EGT probe.

If you mix enough fuel to reduce the available oxygen in the chamber during the burn, the unburned fuel cannot combust on top of the EGT probe. So as you lean the rotary, less fuel burning in the chamber makes additional oxygen available to burn escaping fuel on top of the probe, and we see the high EGT. This is happening at just lean of peak power.

See Sky Ranch page 143.

So leaning to peak EGT is already past best power. Our concern here is the apex seals passing over the peripheral exhaust port where it is exposed to the exhaust gasses leaving the chamber. Renesis owners my leave the room now. Over heating the apex seal is very bad Mojo. So we stay well rich of peak EGT until a few minutes into cruise, and lean very quickly past Peak, and into lean of peak EGT. (Or, switch to the "B" controller that you have set up for lean cruise. The engine will smooth further. The burn rate slows, because the clumps of fuel and air are further apart. (Acts like higher octane fuel) The EGT going down saves the muffler. Reduces noise. Extends range. Reduces cooling load.

How so? Less fuel=less energy=Less heat. But also less power. (So the plane slows down)

The BSFC in Pounds (Of fuel) per Horse Power Hour goes down. Could get very close to piston engine numbers.

The difference is that BSFC is mostly a function of surface area exposed to the flame. The piston engine has very little, and the rotary has a bunch. So the piston engine looses less HP as you reduce BSFC. The rotary looses more HP as you reduce BSFC.

So you compare BSFC at the RPM you plan to use. So if you have one built you want a copy of the dyno sheet, or look at a sheet from a similar engine. This will give you BSFC for best power at whatever RPM.

A number of gags are used to reduce the poor burn. Like a very high energy  multi strike ignition system.  Using more ignition advance. In the 16X dream engine you see fuel injected late, and directly into the chamber. Unless a turbo is involved, high octane fuel is less effective than low octane fuel. Plus low octane fuel generally has more BTUs per pound.

Leaning past best power (Rich of peak EGT) takes you to peak EGT, and if you stay right there for a while, this is where the Lycoming swallows an orange exhaust valve head, and blows a rod out through the case, and the rotary begins to overheat the apex seals in non Renesis engines.

Power can be (and often is)  controlled by leaning the mixture, with the throttle left wide open.

In the olden days, all of the big radial engines were run well lean of peak EGT. The only way to get to England from the Azores, was lean of peak. They carried drums of engine oil in case of a problem. But never had extra fuel.

Lynn E. Hanover