Saturday, March 05, 2005

Engine Recipes

Old Lady 1: I've been shopping!
Old Lady 2: What d'you buy?
Old Lady 1: A piston engine!
Old Lady 2: How d'you cook it?
Old Lady 1: You don't cook it.
Old Lady 2: You can't eat that raw!

    --- Monty Python's Flying Circus, Episode 43

Actually, you can cook a piston engine. One stellar way is to lean out the mixture at high power and run it for several hours. You still can't eat it, but the company has to eat the substantial cost of replacing the cylinders. What a company pays a pilot in a year is a fraction of what the pilot can cost the company through poor engine handling.

In a piston machine, the pilot controls the ratio of vapourized fuel to air that is burned in the cylinders, using the mixture knob. It's usually red, an attention-getting colour that should underscore its importance. By sliding, twisting or pulling (aircraft manufacturers didn't get together on these things) the mixture control, the pilot decreases the flow of fuel to the engine. Pull it all the way out and there is no longer enough fuel flow to sustain combustion. That's the normal way to shut down a piston engine. The fuel actually has two purposes. One is to participate in the combustion reaction and the other is to provide cooling.

I could, and will later, write a whole post on combustion but essentially it's a recipe. Like a screwdriver: two ounces of vodka, six ounces of orange juice. You can slide that ratio a fair way in either direction. Excess orange juice serves to quench your thirst without getting you drunk, while excess vodka gets you drunk without filling your belly. An exceptional excess of vodka leaves you passed out on someone else's floor with one of your eyebrows shaved off.

The pilot's job is to adjust the mixture such that fuel is not wasted, but the engine does not overheat. At high power, that usually means full rich. The engine will burn a lot of fuel, producing a lot of heat, and the extra provides cooling through evaporation. That cooling is especially needed in the climb, when the forward speed and thus cooling (most piston engines are air cooled) is reduced. In cruise, the mixture can be reduced ("leaned out") so that the fuel flow more closely matches the available air, according to the altitude. This saves fuel and produces a smoother running engine. Before increasing the power, the pilot should increase the mixture. Lean the mixture too much or leave it lean while applying full power and you have a recipe for a cooked piston engine.

Turbine engines are even easier to cook, if the internal turbine temperature limits are not adhered to, but Monty Python doesn't have a sketch about them.

6 comments:

david said...

The best way to cook a piston engine is to cruise with the mixture just a wee bit rich of peak exhaust gas temperature, which will produce the hottest possible cylinder head temperatures and (ironically) is exactly where a lot of POHs say to operate it.

To keep the cylinders cool without an engine monitor, you can cruise as lean as possible, if your engine will run smoothly (my preference); cruise very rich, if you're not worried about all the extra CO seeping into the cabin; or fly at 65% power or lower, where there's really nothing you can do to hurt the engine (often the only realistic choice).

Lord Hutton said...

Ah. I think you mean Mrs Smoker and Mrs Non Smoker. And it cost 3 quid!

Aviatrix said...

Yeah, I admit to a touch of editing, to make it fit the context better. Once you've cooked one it's not worth much more than three quid. But raw, they're considerably more costly.

Lost Av8r said...

After spending a year behind an IO-520, the PT6 is way easier to care for.

Aviatrix said...

In flight it's a workhorse. No shock cooling, no leaning, just keeps going around. It's just at start up. Watching the temperature gauge is like watching the spin pointer on Wheel of Fortune when it's approaching the "bankrupt" square. Oh please don't let it tick over any more. Except that at stsrt up it would be my fault, not random fate, if it exceeds the unmarked value for more then two seconds. Do I even have a concept of how hot 1090 degrees is?

John said...

Shhhh! It's a secret that turbin engines are easier to fly than piston engines. You giving it all away ... ;-)

Seriously, chatting with some other pilots at my company about the PT6, the concensus was that the only time these pilots ever saw a hot start or a hung start was in the simulator. Total PT6 time of these pilots combined to over 25,000 hours of operation.

My 2 quid worth ...