lml@lancaironline.net Mailing List Archive

Many thanks to Mark for clarifying the event and the configuration of the engine. The engine was set up "properly" with a common intake manifold for all cylinders. And the fuel servo(throttle body) was "pressurized", meaning downstream of the turbos. It would then measure the net air flow into the engine and not the air leaking out of the turbo discharge. Therefore, it is not surprising that the three cylinders left running were not in any particular firing order and that, I suspect, was the reason for the extreme roughness. If Mark had been lucky and 2,4, and 6 were running it would have remained quite smooth and he could have added more power. The boost pump operation is of no consequence, as it has no effect on air fuel ratio. The Lycoming fuel system, without the altitude compensation option, will have an air/fuel ratio variation, or error, proportional to square root of the density of the air. Being downstream of the turbo, the air density stays nearly constant as altitude changes, requiring little or no mixture adjustment. But when the manifold pressure changes from 35 inches to 14 the mixture will likely richen to beyond the combustible range. The Continental system doesn't measure air flow at all, but meters in proportion to throttle position. That means that the mixture changes in direct proportion to manifold pressure - twice the "error" of the Lycoming system. For that reason many, or most, of the high-horsepower Continental systems are sold with the optional altitude compensation(actually manifold pressure compensation). If that were the case, the Continental system would have corrected for the MAP loss better. Except for the operation of the boost pump, which DOES have a significant effect on fuel flow in a Continental system.

Hope that explains it a little more.

Gary Casey

From: Mark Patey <markpatey@me.com>

2. Why did it fail when Lynn's plane didn't fail or foul plugs? This is a multi part answer. First, there is big difference between going from boost to 26 inches MP like Lynn did, and going from boost to 14 inch MP like I did.

...

3. What might be different between the Continental and Lycoming in this situation? Well one big thing might be the electric fuel pumps. On my Cirrus (Turbo Contintinal 550) and on my Continental powered Legacy (Also turbo), I have a "High Boost/Prime" and "Low Boost" settings on the pump. High to prime and low for the climb. On the Lycoming you have one pump setting and it is "High/Prime" and you leave it on for the climb. So when my failure happened, I was Power Lever Full, Mixture Full, Boost/Prime pump ON and very little air in the cylinders. Would a Continental quit at altitude? I don't know; I would only guess not, unless you had the fuel pump on high/prime but that is not the normal engine management procedures so I doubt anyone would find themselves in that situation. My other legacy has automatic waist gates so I can't take that one to altitude and pull the turbos out to test that theory. I also don't know if there is any difference between how the servos meter fuel. It may be that the Continental will just push the extra fuel back to the wing, the Lycoming can't do that, there is no fuel return to the wing.

...

5. Why as you got lower didn't you advance the power beyond 14inch MP? Well as the power increased in only three cylinders the vibration was frightening. I made one attempt to push the power up and it started "kicking" I don't really know why the "kick", my only guess is that one or more of the dead cylinders where somehow trying to fire as the piston was on the upstroke. I knew I had 700FPM required to hit my target and I was descending at around 600-650FPM. So if I could keep the engine from blowing itself apart I would make the field.

6. How is the turbo system set up? Standard twin turbo setup other then manual waist-gates (they are coming off, don't like 'em) The boost lines "Y" together before the servo, the exhaust pipes are separate, 3 cylinders to each turbo, two waist-gates.