Pumping losses?

[RVbySDI]

Ok, this question has been nagging at me for quite some time now. I have read all the threads I can concerning engines as I have not yet made up my mind concerning what I wish to do in that part of building this airplane. I have heard a great deal of discussion on smaller engines v larger engines and I would like some clarification from the mechanical engineers, the chemical engineers, the train engineers and the general "I think I am" engineers out there.

There is a great deal of discussion about whether to run a high horsepower engine at a lower RPM in order to conserver fuel and decrease wear and tear on the engine. Some out there are in favor of this over running a lower HP engine wide open. Those in this camp believe it is better to run an engine at higher RPM's because it is built to run "at a certain speed". This "certain speed" is supposedly around 75% of max HP and will gain its best efficiency at this rate.

In the ongoing debate over large vs. small displacement engines there is much talk about pumping losses. Those in the camp of running a smaller displacement engine at higher RPM's uses this argument of pumping losses to support their stance.

So after all of this here is my question. I understand this principle of pumping losses so I would like to ask that any answers to my question would be specific in nature and not generalizations. I would like to know if anyone knows the specific delta difference in pumping losses between the 320 engine vs the 360 engine. Are these pumping losses so great that a difference in displacement of 40 ci will be enough to have a drastic enough pumping loss to offset running the 360 at slower RPM's for efficency over the 320?

So as an example, if one were to run an O-360 engine at 55% of max RPM and compare it to an O-320 engine running at 75% of max RPM what would be the difference in pumping losses for the two engines. Would the pumping loss in the O-360 be enough to offset any gains in running the larger engine at a slower RPM?

I would very much like to hear from the experts on this issue as it seems to me that the size difference in these two engines would not be great enough to see a drastic difference in efficiencies due to these pumping losses. my thoughts are that if I am comparing an O-320 with an IO-740 then perhaps this pumping loss issue would have merit. If I am wrong then I ask to be enlightened. If my supposition is correct I would like to have some specific information to support this supposition.

So please give me some numbers to chew on!

[osxuser]

The Angle Valve 360 series is more efficient than the Parallel valve on the top end, on the lower end it's about the same as the Parallel valve.

I don't know much about effciency other than that. I do know however that Lycoming suggests that for best life, the engine should be run at lower power settings most of the time. Therefore, running an O-320 hard will shorten it's life. At 75% power the O-320 will put out 120HP, the O-360 will be at 66.67% HP and a IO-360 (Angle Valve) would be at 60% power.

This would lead to longer engine life according to Lycoming. Fuel effciency would be higher with the IO-360 assuming you don't have a problem running LOP, which you wouldn't be able to do at 75% with the O-320 (Or even with an IO-320).

[David-aviator]

Steve,

I've never heard the term pumping loss, but I can tell you about running at less than 75%.

A first run 0360A4M I once owned had its oil changed every 25 hours, was flown at 2200 rpm most of its life and otherwise pampered like a baby. When it was overhauled it at TBO, the crank was in new limits, the valves were in very good shape and the cam had some slight wear. I did install new cylinders and pistons, but the overhaul guy said the original ones could have been cleaned up and run some more. In fact I sold the old cylinders to an overhaul shop and they probably are running today.

I wasted some money on that overhaul.

.....standing by here to learn about pumping loss.

[videobobk]

Pumping loss stems from the fact that almost all gasoline engines have a throttle that restricts incoming air from going directly into the cylinder, at least when not wide open. When wide open, the engine only has to overcome the friction losses of the induction system; air and fuel passing through a filter and some tubes. When the throttle is partially closed, the manifold pressure drops, and the drop in pressure in effect is working against the piston on the intake stroke. The engine is working to produce a certain amount of vacuum.

With a higher hp engine, in general you would run with the throttle closed off more to lower the hp. That, coupled with either larger diameter pistons or longer stroke, equates to more power absorbed in pumping the air/fuel mixture. Diesel engines have no air throttle and do not suffer these losses. I understand some BMW engines also eliminate this with valve timing that allows air to pass out of an open valve on compression at anything less than full throttle. I understand they have 15-20% better economy at low hp. Neat idea, but hardly new. Old "hit and miss" engines of the late 19th/early 20th century used this to regulate engine speed.

No, I'm not an engineer, but somewhat of a historian. I also was senior tech writer for Cummins Engine Co. Hope the explanation helps.

Bob Kelly

[hevansrv7a]

The pumping loss is the direct result of partially closed throttle. With a CS prop, can't you slow it down and open the throttle and thus eliminate the pumping loss? Wouldn't this also save wear with fewer piston miles? Of course, those fewer miles will be at higher pressures which means more ring-cylinder friction. I know this raises the spectre of "over-square" operation, but hasn't that been discredited as long as you are under 75%? Experts, please feel free to critique this.

[David-aviator]

Originally Posted by videobobk

Pumping loss stems from the fact that almost all gasoline engines have a throttle that restricts incoming air from going directly into the cylinder, at least when not wide open. When wide open, the engine only has to overcome the friction losses of the induction system; air and fuel passing through a filter and some tubes. When the throttle is partially closed, the manifold pressure drops, and the drop in pressure in effect is working against the piston on the intake stroke. The engine is working to produce a certain amount of vacuum.

With a higher hp engine, in general you would run with the throttle closed off more to lower the hp. That, coupled with either larger diameter pistons or longer stroke, equates to more power absorbed in pumping the air/fuel mixture. Diesel engines have no air throttle and do not suffer these losses. I understand some BMW engines also eliminate this with valve timing that allows air to pass out of an open valve on compression at anything less than full throttle. I understand they have 15-20% better economy at low hp. Neat idea, but hardly new. Old "hit and miss" engines of the late 19th/early 20th century used this to regulate engine speed.

No, I'm not an engineer, but somewhat of a historian. I also was senior tech writer for Cummins Engine Co. Hope the explanation helps.

Bob Kelly

Ah-ha! Thank you very much, Bob.

So the technique of flying around at WOT, but regulating HP with rpm is very valid. Jan Eggenfellner developed the idea with his engines and most of us do it.

Set WOT at take off and forget about it. Want to fly at max economy, bring the rpm back to 1700 and watch the fuel flow drop to 6-7 gph, need to climb, reset the rpm to 2300-2500 and fuel flow goes up to 12-14 gph along with HP.

So, the pumping loss would be zero with this technique.

Obviously, it only works with a constant speed prop. Pumping loss is most apparent with a 0360 and fixed pitch flying around at 2200 rpm. However, with CS prop, rpm could be set at 2200 at WOT, just like the Subaru technique.

Years ago, there was a rule to never run less rpm than manifold pressure, i.e., 23 inches, 2300 rpm minimum. Wonder if that still is a rule with Lycoming? It may be WOT and low rpm is possible only with a knock sensor.

[Ironflight]

Originally Posted by David-aviator

Years ago, there was a rule to never run less rpm than manifold pressure, i.e., 23 inches, 2300 rpm minimum. Wonder if that still is a rule with Lycoming? It may be WOT and low rpm is possible only with a knock sensor.

From all I have read, the "over square rule" was pretty much a myth (at least until you reached extremes). After all, the numbers (25/2500, 24/2400, 23/2300....) only look "square" if you are measuring with "inches" - it's just an accident of the units. What if you were using bars, millibars, or radians per second?

Thanks Bob K for the best explanation of pumping losses that I have ever read - I think I even understand it!

Paul

[Bryan Wood]

Originally Posted by Ironflight

From all I have read, the "over square rule" was pretty much a myth (at least until you reached extremes).
Paul

In Deacon's articles "Pelicans Perch" he has a section on props driving the engine which addresses this. It's been a long time since reading it, but he explains why it is a no no if your flying a radial. If you haven't read it Deacon explains thoroughly.

http://www.warmkessel.com/jr/flying/td/jd/78.jsp

Regards,

[BOBM]

Originally Posted by David-aviator

Set WOT at take off and forget about it. Want to fly at max economy, bring the rpm back to 1700 and watch the fuel flow drop to 6-7 gph, need to climb, reset the rpm to 2300-2500 and fuel flow goes up to 12-14 gph along with HP.

If you look at the Cafe Foundation flight test of the RV9A with 160 carb Lyc, pages 6-7 http://cafefoundation.org/v2/pdf/rv-9a.pdf
you will see that 2600rpm produces 30mpg while 2300rpm only gives 27mpg. Both tests at 8500 wot & lop. Lower rpm gives 3 mph more tas but .6gph higher ff. Note that they are burning 6.1 gph at 180 mph with a carb O320. These Cafe tests are the most meticulous & highly instrumented flight tests that I know of.

[Russ McCutcheon]

Now WOT at sea level or 1000? with the prop pulled back to 2300 rpm would be extreme right? I guess its no different then taking off with fixed prop running 2300 rpm and WOT but is this acceptable for extend cruse?

My angle valve 200 HP CS RV-4 never sees WOT except take off and above 9000? where my cruse manifold pressure reaches WOT and then begins to fall off from there on up. Also 9000? and up is where this package shines in both speed and economy over its 160 HP fixed brothers, I?m typically about 20mph faster then my Dads -4 at 10,000? and on a two hour leg burn a gallon less fuel, two hours for me but 2 hours 15 for Dad so he is burning fuel for longer and also spends more time climbing at a slow ground speed then I do. He has a new fuel flow gauge that is not yet calibrated so when he gets this going it will be interesting to see how they compare at 1000? and the same speed.