The International Cessna^® 170 Association

Dear Members:

For several years my practice has been to cruise at an RPM setting at the bottom the the green arc. My purpose in doing so was to 1) go easy on the engine 2) conserve fuel 3) extend range and 4) reduce cockpit noise.

However, when my wife found out that she was sitting in the plane longer than necessary, she said that if I was lucky enough for her to be riding along, the least I could do was make the trip as short as possible! (It's the airplane she isn't thrilled about, not me...at least that is what I tell myself.)

Anyway, my oil consumption at the lower RPM's was a pretty consistent 1 quart about every 3-4 hours, but, at these higher RPM settings, I've notice lower oil consumption. For example, last week we took a 3 hour round trip with RPM in the higher 1/4 or the green arc, and I only used 1/4 qt. of oil.

What would account for lower oil consumption at higher RPM?

Regards,

Curt - N4288V

That's very interesting about the lower oil consumption!

Without variable timing like the modern auto engines have, the timing on our engines are optimized at the 2450 cruising setting. Maybe the better fuel burn in the cylinders helps with the oil consumption at that RPM?

At lower rpm you have a higher manifold pressure (vacuum) above the rings pulling oil up into the combustion chamber and you burn it at a high rate at higher rpm you have lower manifold pressure and higher cylinder pressure not restricting the flow of oil to the combustion chamber. The rings like higher cylinder pressure to seal better also, (lower manifold pressure
that my opinion
Dave

daveramp is thinking right....he's just saying it differently.
The manifold pressure is LOWER at lower cruise rpms with a fixed pitch prop. The higher rpm (caused by higher MP) results in higher combustion chamber pressures. The pressure above a piston applys outward pressure of the rings against the cylinder walls, resulting in lower oil consumption. (The pressure gets beneath the ring in it's lands and presses it outward.)

You might also consider this: A low rpm results in a low groundspeed. This results in a longer flight time to cover the same distance. This results in higher operating hours. Resulting in fewer miles covered per engine-hour per overhaul period. The gas you save, is actually costing you more per mile in engine overhaul costs.

Guys:

Thanks...your explanations make sense.

Regards,

Curt

Curt,
What Dave and George said is right on! I especially liked George's cost compairison! We used a C-172F for flight training (O-300D) We told the students to run the A/C at around 2300 RPM on X-countries. (YOU WANT TO BULID TIME, NOT GET THERE FAST) We also told them to fill it with oil and check the fuel at each stop. When I'm going some where I run at the top of the green and lean. (But I have to, To keep up with any 170 I run with, running at 2300!!) I also put more fuel in than they do!
brad

I don't know about aircraft engines, but on some automotive engines, operating the motor in a higher vacume range lower RPM) can result in increased oil consumption via the valve guides. This can occur on engines with worn intake valve guides or seals.

I would presume that an aircraft engine may experience the same thing.

Scott