Combinations of RPM & Manifold Pressure

[USMCmech]

High throttle/low RPM increases the pressure in the cylinders. Basically, the lower RPM means the valves will be open for a longer period of time, and more air will enter the cylinder on the intake stroke.

Come again?

Unless the laws of physics have changed, I'm pretty sure that 320 cubic inches of air/fuel gets sucked into an O-320 every four strokes regardless of how many times that happens in a minute.

The only thing that changes the amount of air flowing into the cylinders is the throttle valve. Check out the definition of "throttle", as in to choke off the airflow. If your throttle is not wide open, you are choking the engine.

 

[Orange Anchor]

From that last link:

This discussion about lean-of-peak operations has been going on an awfully long time, said Braly, who began evangelizing on its benefits 10 years ago at the Advanced Pilot Seminars he taught at the companys Ada, Oklahoma, headquarters. Weve got some NACA research from 1943 that was classified as top secret at the time because they wanted to know if they could run lean of peak to extend the range of bombers and other U.S. military aircraft.

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And as the old joke went, if you wanted your own B-29 all you had to do was buy land in Kansas. This joke was later upgraded to German F-104s due to crashes. Engine FIRES and failures were considerable. And as noted, even if the intent was reduced fuel consumption, going from 30hrs to more than 2500 is rather noteworthy. And treating a big recip right was an art. 56 spark plugs on each engine and if one fouled all 4 engines... 224 plugs to change. And THEY were not cheap either.

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I'm puzzled why you'd think that anecdotal evidence published by people who have a financial stake in your being convinced are a reliable source of information.

I know George in passing and Deakin fairly well. If nothing else and maybe oddly old-fashioned, both are men of integrity in my contact with them. Yes, they have financial incentives but that alone does not dismiss their claims. And George has spent a considerable sum on engine test stands to study what is happening in the engine.

You are welcomed to your skepticism. Today that is probably healthy. and you can vote with your wallet and advise others to do the same.

 

[DaveC]

Come again?

Unless the laws of physics have changed, I'm pretty sure that 320 cubic inches of air/fuel gets sucked into an O-320 every four strokes regardless of how many times that happens in a minute.

The only thing that changes the amount of air flowing into the cylinders is the throttle valve. Check out the definition of "throttle", as in to choke off the airflow. If your throttle is not wide open, you are choking the engine.

Same volume: yes. Same mass: no.

At a given throttle position, there will be a greater quantity of air/fuel in the cylinder at a lower RPM, resulting in higher overall cylinder pressure during combustion.

 

[tgrayson]

I know George in passing and Deakin fairly well. ...

I spent years on another forum with both of them; Deakin is a bit of a blow-hard and knows little about engines that Braly didn't teach him. Braly is an engineer, so he has much more credibility than Deakin does, but I accept the claims of both to the degree they have the evidence to support them.

Braly surely knows what good evidence looks like; the fact he doesn't provide it, but instead relies on snake-oil type marketing literature with testimonials is somewhat discrediting. I'm not saying their engine life claims are false, I'm saying they are unsupported and shouldn't be represented as fact by anyone who values accuracy.

 

[Orange Anchor]

I spent years on another forum with both of them; Deakin is a bit of a blow-hard and knows little about engines that Braly didn't teach him.

I guess it is not surprising that we see Deakin differently. We see many things differently. But I like John so maybe I grant him more leeway. I figure a guy with his experience knows a thing or two. How did he get crosspoints with you? Or you with him?

Braly is an engineer, so he has much more credibility than Deakin does, but I accept the claims of both to the degree they have the evidence to support them.

So you also know George? So which is it? George is telling the truth with embellishment or George is a liar. Which are you pointing towards?

 

[USMCmech]

It's up those asserting some position to supply the evidence. All they have is anecdotes, which is what psychic healers and UFO trackers have. It may or may not be true, but you can't honestly assert something is true just based on stories. Personal testimony is very, very, very unreliable.

In my experiance the advocates for "traditional" engine operations rely soley on anecodatal information that has been barely understood, much less verified. "I heard from a mechanic once that if you run your engine oversquare, it will bend the driveshaft." that's a direct quote.

15 years ago when GAMI were just starting out selling their injectors and crazy techniques, I would buy your argument but thousands of planes have flown millions of hours since then creating a fairly large data pool. I don't doubt that GAMI is selling their product with their data, but that's better than "I heard it from my 300hr CFI"

 

[tgrayson]

George is telling the truth with embellishment or George is a liar. Which are you pointing towards?

Let's just say he's extrapolating beyond the data.

 

[tgrayson]

In my experiance the advocates for "traditional" engine operations rely soley on anecodatal information that has been barely understood, much less verified. "I heard from a mechanic once that if you run your engine oversquare, it will bend the driveshaft." that's a direct quote.

I totally agree. I'm not really sure that anyone knows for sure the statistically best way to operate an engine. And the "statistically" is the important part. If there is an optimal engine management strategy, there will still be SOME engines that won't make TBO for reasons that had nothing to do with how the engine was operated. And there will be some engines that will go way past TBO even though they were terribly abused. That's why anecdotal evidence is useless; you need fleet data so that you can identify trends that would otherwise be masked by the data points that appear random when you only have a handful.

By far what I consider the greatest service that Deakin et al perform is at least making the reader aware that you're aren't going to destroy the engine by not slavishly following what your CFI taught you.

BTW, I did point out to Deakin that there was one bit of conventional wisdom that he seemed to accept without criticism and that was the idea that when you advance power, prop goes first, then throttle, and the reverse when reducing power. There's really no reason this needs to be so during normal operations.

 

[USMCmech]

I totally agree. I'm not really sure that anyone knows for sure the statistically best way to operate an engine. And the "statistically" is the important part. If there is an optimal engine management strategy, there will still be SOME engines that won't make TBO for reasons that had nothing to do with how the engine was operated. And there will be some engines that will go way past TBO even though they were terribly abused. That's why anecdotal evidence is useless; you need fleet data so that you can identify trends that would otherwise be masked by the data points that appear random when you only have a handful.

In my experiance, the ABSOLUTE best "engine management strategy" is to fly your airplane often. Everything else is a distant secondary consideration.

Flight school trainers are misused and abused, yet their engines useualy last to and past TBO.

The engines on the Beech Duke sit in the hangar for 6 weeks then are run way too hot for a couple of days. The owner/pilot is VERY careful not to "shock cool" his engine, yet it constantly requires work.

 

[tgrayson]

Same volume: yes. Same mass: no.

I think the concept you're referring to is volumetric efficiency. In normally aspirated engines, it will almost always be less than 100%, meaning that the cylinders don't fill with all the air they could hold under static conditions. Higher RPM tends to decrease the volumetric efficiency for the reasons you say.

 

[Corporate Pilot]

In my experiance, the ABSOLUTE best "engine management strategy" is to fly your airplane often. Everything else is a distant secondary consideration.

I agree, we flew our Cessna 340 more hours this year and the cylinder pressures came up this annual compared to the last 2 annuals.

 

[mshunter]

In normally aspirated engines, it will almost always be less than 100%, meaning that the cylinders don't fill with all the air they could hold under static conditions. Higher RPM tends to decrease the volumetric efficiency for the reasons you say.

It will always be less than 100%. And depending on cam profile, higher RPM may lead to higher efficiency. The only way to achieve 100% efficiency is to force feed an engine.

Here is an example of better than 100%. Yes, thats a MPH record, 212 in 1320 feet with a 10.5" tire in less than seven seconds. Get a ruler, tape measure and see just how wide those tires are. And then go out and drive your car and see how fast it get's to 60. We leave the line with 6lbs boost and go through the traps with over 30. On a small block chevy(well, it looks like on atleast). Forced induction FTW!
[YT]jyQTi6ofUMo[/YT]

 

[tgrayson]

It will always be less than 100%. And depending on cam profile, higher RPM may lead to higher efficiency. The only way to achieve 100% efficiency is to force feed an engine.

According to Wikipedia, not so:

With proper tuning, volumetric efficiencies above 100% can also be reached by naturally-aspirated engines. The limit for naturally-aspirated engines is about 137%[1]; these engines are typically of a DOHC layout with four valves per cylinder.
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[mshunter]

According to Wikipedia, not so:

With proper tuning, volumetric efficiencies above 100% can also be reached by naturally-aspirated engines. The limit for naturally-aspirated engines is about 137%[1]; these engines are typically of a DOHC layout with four valves per cylinder.
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Really, your going to quote a Wiki article that states there is no reference material.

This article does not cite any references or sources

I have personally dealt with some of the biggest names in engine design, including Crane, Edelbrok, Hawaii Racing, Brodix, etc. I also personally worked for, and was friends with Rocky Childs untill his recent passing, which afforded me meeting some of the biggest naames in drag racing (Don Garlits, Shirley Muldowney, and Don Purdhomme) . It is physically impossible for a four stroke internal combustion engine to draw in more air and fuel than it's displacement because of restrictions of valve angle, valve size, valve design, port size, port shape, intake manifold shape, etc. It's all in how efficient you can make the intake system in an NA engine which gets you a higher VE. And the peak VE is going to be through an extremely narrow RPM range, hence a torque/hp curve seen on an engine dyno, or the "power band" you feel in the seat of your pants. Historically, DOHC has been they type of engine that has to rev to the moon to make any kind of power to get the velocity in the port high enough to make any kind of power due to it's more efficent flow, but it still doesn't work out to better than 100%. Many more factors go into than mentioned here, but I don't feel like writing a book.

I don't feel like going round and round with you about this. I'll give you aerodynamics. But on this one, from my experince, I think I got ya licked. Racing/engines have been in my blood as long as I've been on this earth. I grew up around it. Lived it, tested it, played with it, and been bitten by it.

It's late, I had to get up at 3am for work, and I'm tired. Goodnight.

 

[Orange Anchor]

Let's just say he's extrapolating beyond the data.

...but what put you at odds with John, the 'blowhard'?

 

[tgrayson]

Really, your going to quote a Wiki article that states there is no reference material.

Wikipedia is very accurate source for non-political subjects. But you'll find sources all over the internet making the same assertion, so I expect there's something to it. Engineers, in general, should never say something is impossible, only "I don't know how it's done." Some exceptions to that rule, of course, are observed physical limits of the universe, such as the laws of thermodynamics. I don't think this one ranks up there with that one. If you ask around with your contacts, I bet you'll find someone who knows how it works.

 

[SteveC]

[modhat]
Moved the checklist and human factors posts to a new thread here:

http://forums.jetcareers.com/technical-talk/110833-checklists-human-factors-breakaway-thread.html[/modhat]

 

[ryan1234]

I think it's extremely important for CFIs to differentiate between Normally Aspirated and Turbo-Charged engines - and what works as far as RPM vs. MAPs....

You can't fly a 421 like a Seneca in regards to throttle and prop settings. There are different tubro charged engines that have different characteristics. Turbo charged engines present new effects from fuel/mass flow, OAT, Oil Pressure and Waste gate operation, bootstraping, etc

 

[shdw]

when you advance power, prop goes first, then throttle, and the reverse when reducing power. There's really no reason this needs to be so during normal operations.

Sure there is, but it has nothing to do with preserving engine life; it's human physiology. Force a pilot to always operate it this way and the chances the pilot will ram the throttle forward, emergency or not, without touching the prop will decrease significantly.