[FresnoR]

I have been wracking my brain trying to figure this out. Maybe the great minds around here can help me. Anyway, in any given car or truck, the size of the engine will directly influence its gas mileage. For instance, the exact same model car (size, aerodynamic qualities), but with a V6 instead of an I4, will always get a few less highway mpg. This is theoretically, on the same road, at the same speed.


So why is it that an RV with an O-360 gets better gas fuel economy at a given speed than an O-320? What is it that makes the automobile have a negative correlation with engine size, and an aircraft a positive?

 

[danielhv]

I have been wracking my brain trying to figure this out. Maybe the great minds around here can help me. Anyway, in any given car or truck, the size of the engine will directly influence its gas mileage. For instance, the exact same model car (size, aerodynamic qualities), but with a V6 instead of an I4, will always get a few less highway mpg. This is theoretically, on the same road, at the same speed.


So why is it that an RV with an O-360 gets better gas fuel economy at a given speed than an O-320? What is it that makes the automobile have a negative correlation with engine size, and an aircraft a positive?

I can tell you this... in relation to cars, my old truck was an 03 F150 with the 4.2L V6 in it. I was LUCKY if I got 16 mpg. The 4.6L and 5.4L variants got much better than that. Now Im in a GMC Sierra with a 6.2L in it, and I get 16.3 all day long... 1.9 more Liters, and the gas mileage is a tad better!

My only explanation is that the bigger engine has to work less to achive the same result, thus burning less fuel. sounds good to me!

 

[Geico266]

In the 03 Ford the transmission of power to the ground was the killer. It all depends on how hard the engine has to work to get to that speed. The new Fords have a 6 speed trans and claim 17-21 MPG, 2WD.

Getting back to RV's, the same MP setting on the 0-320 should be more efficent than the same MP setting on a 0-360. Chasing a speed will not be an apples to apples contest. For the 0-320 to keep up with the 0-360 it will need to burn more fuel.

 

[L.Adamson]

Getting back to RV's, the same MP setting on the 0-320 should be more efficent than the same MP setting on a 0-360. Chasing a speed will not be an apples to apples contest. For the 0-320 to keep up with the 0-360 it will need to burn more fuel.

I don't have all the answers; but a friend of mine likes to tweak his RV9A (0320) for fuel economy rather than speed. On a cross country, he flew my 6A (0360) at fuel economy speeds while someone else flew his 9A. He ended up using less fuel in my 6A on this particular trip.

While we often fly cross countries together. I usually prefer to fly faster, and take a little more fuel. But at other times, our fuel usuage has been quite close. At full throttle, my 6A is around 20 mph faster. We both have C/S props.

L.Adamson

 

[rwhittier]

Its about power production

Anyway, in any given car or truck, the size of the engine will directly influence its gas mileage. For instance, the exact same model car (size, aerodynamic qualities), but with a V6 instead of an I4, will always get a few less highway mpg. This is theoretically, on the same road, at the same speed.

Piston engines have an optimum air fuel ratio of around 15 to 1. They can be run richer or leaner and there are good and bad trade offs doing either (different subject). But the point is ALL piston engines need that, its physics whether its your lawnmower, car or airplane.

Long ago engineers figured out using BSFC (Brake Specific Fuel Consumption) as a measurement of HP produced per unit of fuel consumed. Large and small engines tend to end up with a BSFC (ie HP output per unit of fuel) that are very close (BTW, Lyc and TCM aircraft engines have a very good BSFC rating). So, why are they different in real life (leaving a lot out for simplicity sake)?

One big factor is the vehicle gets heavier. Larger engines weigh more, they need stronger transmissions/drivelines, bigger brakes, etc... The big engined vehicle needs to produce more HP to drag itself around. It kind of like you always wearing a backpack full of bricks, you will burn more calories to go the same distance. If a 3000 lb car gains 200 lbs in this process it has a permanent 6.6% weight disadvantage. It all adds up.

Oh yeah, and the bigger the engine the more rush you get from pressing on the accelerator ;-) so you self induce a HP demand that uses more fuel.

 

[Ron Lee]

I don't know what your concern is but if it is fuel economy that "may" drive you to a smaller engine...rethink it.

I recommend the largest approved engine for your RV model. Maybe not 200hp for those that take it but at least 180 hp. You can always throttle back, run lean of peak (maybe), use electronic ignition, etc to save fuel. You can't make up for POWER when you need it with a smaller engine.

 

[FresnoR]

I am definately not looking at strict fuel economy, if I was, a 7 would not be my first aircraft choice. I am actually looking to by a new car, and comparing the different engine choices for any particular model and of course I have to relate everything I am doing in my life to an RV!

 

[rtry9a]

Three issues at play: gearing/rpm, engine efficiency, and tuning (for leaner mixtures). Most of the btu fuel energy in an internal combustion engine goes to heat, not necessarily to producing work.

 

[JHines]

I have been wracking my brain trying to figure this out. Maybe the great minds around here can help me. Anyway, in any given car or truck, the size of the engine will directly influence its gas mileage. For instance, the exact same model car (size, aerodynamic qualities), but with a V6 instead of an I4, will always get a few less highway mpg. This is theoretically, on the same road, at the same speed.


So why is it that an RV with an O-360 gets better gas fuel economy at a given speed than an O-320? What is it that makes the automobile have a negative correlation with engine size, and an aircraft a positive?

For Lycomings and similar it appears the BSFC is very similar across displacments. Apparently this is related to aspects of the configuration such as bore size, volumetric displacement per cylinder, etc. I don't think this is true for auto engines. I know for sure I have owed vehicles with large displacment engines and the economy didn't vary all that much with output/speed. My current vehicle with a small 4-cylinder does vary a lot depending on load.

 

[Kyle Boatright]

So why is it that an RV with an O-360 gets better gas fuel economy at a given speed than an O-320? What is it that makes the automobile have a negative correlation with engine size, and an aircraft a positive?

I would question the premise. Several 0-360 flyers have claimed that their airplanes get better fuel efficiency than similar aircraft with 0-320's. However, without identical aircraft, pilot technique, etc. is is hard to accept this data as conclusive. One guy could be flying around full rich all the time while the other guy is running extremely lean, at high MP and low RPM...

From an engineering standpoint, there is no reason to believe there would be a meaningful difference in fuel economy until the 0-320 hits the fuel enrichment threshold at ~75%, while the 0-360 remains below that threshold.

Now if we're talking angle valve engines (the 200 HP IO-360), things are different. The combustion chamber shape on those engines with angle valve heads is superior to the parallel valve heads on "normal" 0-320's and 0-360's and does result in improved BSFC.

 

[Finley Atherton]

So why is it that an RV with an O-360 gets better gas fuel economy at a given speed than an O-320?

I don't buy this statement that the 0-360 is more fuel efficient than an 0-320.
Taking data from Lycomings Part Throttle Fuel Consumption curves shows that at our typical cruise power settings of around 100 to 110 hp and leaned to "best economy", the 160 hp I0 320 is about 1.8% to 3% (depending on rpm) more fuel efficient than the 180 hp I0-360 and about 1% to 1.3% more efficient than the 200 hp I0-360. I accept that up to a point, the bigger engines will be more fuel efficient at higher hp as they can operate at greater hp (not % hp) before needing to go ROP.

Fin
9A

 

[twsurveyor]

Fuel Economy / Engine Size?

So why is it that an RV with an O-360 gets better gas fuel economy at a given speed than an O-320? What is it that makes the automobile have a negative correlation with engine size, and an aircraft a positive?

Guys, It's all about horsepower! There is a rule of thumb formula in the "Lycoming Library" that goes like this:

Fuel Burn (in gal/hr) = Horsepower (HP) x .0725, so................. a 180 HP engine @ 65% = (180 x .65) = 117 HP.......... 117 HP x .0725 = 8.48 gal/hr etc., etc.

I have checked this "rule of thumb" many times and found it to be very accurate, usually within 3-5%.

As one poster said "Go with the largest engine recommended". I would hate to think that I gave up 20 (or better) HP for a 2-3% fuel saveings!
Extra horsepower is like money in the bank, it's always there if you need it!

 

[Greg Arehart]

Another thing that happens in cars (for me at least) is when you have an underpowered vehicle, your foot (my foot anyway) is always in the carburetor, typically resulting in some wasted fuel, whereas if the engine is correctly matched to the vehicle, then a larger engine will appear to use less fuel because of the way the engine is operated.

greg

 

[elippse]

But also this

I agree with Roger W., but I also add this. Sliding friction, or coulomb loss, in an engine is pretty much constant regardless of rpm. If you operate the engines of two identical planes with CS props, flying side-by-side at the same power output, and one has full MAP and rpm dialed down, whereas the other uses more rpm and a lower MAP, the one using higher MAP, lower rpm will have better efficiency than the one using lower MAP, higher rpm. The second will have both higher friction power loss due to the higher rpm and greater pumping loss. Friction power is friction force X rpm / 5252; pumping loss is due to the crancase-intake manifold pressure differential acting on the piston on the intake stroke. Lindbergh demostrated this to the pilots in the Pacific in WWII to get them more range. On some engines they list the minimum rpm vs MAP, so you don't go below this number due to bearing load. However, this can work against a comparison of two identical planes flying side-by-side, equipped with identical FP props, and one has an O-320 at WOT and the other has an O-360 at reduced power. There, the O-360 will have to pull back power to stay back with the 320, and so at its lower MAP it will have greater pumping loss.

 

[Andy_RR]

Sone of the real reasons...

...that the O-320 isn't as good as it "should" be is that

a) the bore/stroke ratio, or more importantly the combustion chamber surface/volume ratio is much worse (more than 10% worse) on an O-320 compared to an O-360. This is indicative how much heat you lose during combustion - that is, heat that isn't converted into work for you.

b) when operating at altitude, making equivalent power is possible at lower engine speeds on an O-360. This delivers better BSFC, at least in the speed range that we can use. An O-320 at 75% power will be spinning a couple of hundred RPM faster than an O-360 developing the same power level (not 75%, but at the same hp) If you look at the Lycoming data, you'll see the BSFC is improving as you decrease engine speed. This obviously assumes you have a CSU.

So, I once thought an IO-340 was the thing to have, but I'm convinced now that an IO-375 would be the best compromise of power and fuel consumption of the Lyclone range in my RV-8.

 

[frankh]

Do not throttle back!!!

Throttling is a big MPG killer..Its called "pumping losses"...that is..when your pushing air through a tube it takes power to do this..if you add a restriction (throttle) it takes more power to do this.

In fact the more you throttle the proportionally more power it takes and your true BSFC number will drop.

The BSFC curve shown in the Lycoming manual is almost certainly with the throttle wide open.

So if you wan't max MPG (I know I do) then leave the throttle wide open..How do you do that?

well with a CS prop its much easier...you just fly up around where you get 24" mp at full throttle, dial the prop back to 2400RPM and then lean to 25 to 50Deg lean of peak.

With a FP prop..well I dunno cus I've never flown an RV with FP prop..but I would guess that the same rule applies...just fly high to get to 24" MP or less (this is to get well away from the detonation zone BTW) leave the throttle wide open and haul back on the mixture..I don't know where your prop speed will end up.

Now this all assumes your engine will run LOP of course.

This is partly why small cars have really high 5th gears for highway cruising...the engine speed drops and the throttle ends up being wide open...as long as its rigged such the accelerator pump is not running then it gets better highway MPG.

Note diesel engines don't have throttles..add to that a more energy rich fuel and they get very decent MPGs usually.

Frank

 

[frankh]

did I answer the question?

I guess the actual question of "why is a bigger engine more efficient" is a much bigger question than just engine size.

a Lyc running 25 deg LOP will make the same percentage of power for the same amount of fuel proportionatly...I.e at 24^2 a lyc makes about 55% power (from memory)..in the case of an O360 that actual HP is higher than an O320.

But...what does the airplane do with the extra power..it certainly flies faster..but does the extra speed (and higher fuel flow) translate into better MPG????

I honestly don't know..It will be dependant on drag...but airframe drag goes up as a square of the speed..so in theory the O360 should be slightly less efficient.

I think though these airplanes are very efficient anyway and personally I'd rather have the extra power, altitude capability, speed and bragging rights the O360 has...

Frank

 

[C-FAH Q]

fuel economy

Last November, Myself(rv7 200hp) 2 friends in Rockets(IO 540) went to Texas and back, 1100 nm each way in formation. The rockets with the IO 540's used less fuel than me at every leg.

 

[frankh]

welll

two things..what wing were the Rockets running?..the Rockets were presumably tandem seating..and thus less drag compared to your 7.

Were they running LOP and you ROP? (trying to keep up)..In other words there were a number of differences. it wasn't an apples to apples comparison.

A bit sickening though eh?..

Frank

 

[Finley Atherton]

Further to my post #12 where I disputed that the 180 hp IO- 360 is more fuel efficient than the 160 hp IO-320 here are some figures from the Lycoming "Part Throttle Fuel Consumption" graphs from the respective owners manuals. The graphs are small and hard to read so I enlarged them and hopefully I have got reasonably accurate figures.
Figures show US gal/hr for 110 hp leaned to "Best Economy":

RPM , IO-320 , IO-360
2,200 , 7.75 , 7.86
2,400 , 8.00 , 8.01
2,600 , 8.20 , 8.44
2,700 , 8.35 , 8.59

At all rpm the IO-320 was slightly more fuel efficient. The differences are small and I think in the real world it would be fair to say that the two engines essentially have about the same fuel efficiency. I used 110 hp as the measure as this would be about the hp that many of us would use in cruise. A similar trend occurs at 100 hp with the IO-320 being slightly more efficient.

Fin
9A

 

[frankh]

Hmm

Further to my post #12 where I disputed that the 180 hp IO- 360 is more fuel efficient than the 160 hp IO-320 here are some figures from the Lycoming "Part Throttle Fuel Consumption" graphs from the respective owners manuals. The graphs are small and hard to read so I enlarged them and hopefully I have got reasonably accurate figures.
Figures show US gal/hr for 110 hp leaned to "Best Economy":

RPM , IO-320 , IO-360
2,200 , 7.75 , 7.86
2,400 , 8.00 , 8.01
2,600 , 8.20 , 8.44
2,700 , 8.35 , 8.59

At all rpm the IO-320 was slightly more fuel efficient. The differences are small and I think in the real world it would be fair to say that the two engines essentially have about the same fuel efficiency. I used 110 hp as the measure as this would be about the hp that many of us would use in cruise. A similar trend occurs at 100 hp with the IO-320 being slightly more efficient.

Fin
9A

Well this is interesting too, because two different sized engines that are throttled to produce the same HP..everything else being equal..
Well..the 360 has more displacement than the 320...so..the throttle would actually be more closed than the same throttle on the 320..i.e if you opened the throttle all the way the 360 would flow more air.

So to get the same hp, the 360 must be throttled more than the 320.

We know from my previous post that throttling is bad for efficiency..so more throttling is more bad than less throttling.

so by reason the 360 would have to burn more fuel at part throttle.

So to make the best mpg..don't throttle..fly high enough to keep it wide open and run LOP if possible.

Frank

 

[Andy_RR]

Figures show US gal/hr for 110 hp leaned to "Best Economy":

RPM , IO-320 , IO-360
2,200 , 7.75 , 7.86
2,400 , 8.00 , 8.01
2,600 , 8.20 , 8.44
2,700 , 8.35 , 8.59

At all rpm the IO-320 was slightly more fuel efficient.

Nice data Finley, and in the theoretical world, it's true that a smaller engine will yield better FC for a given power level, but...

... when you add altitude (air density) into the equation, you'll find that a larger engine allows you to produce the same power at a lower engine speed. Note well how the BSFC in your example is improving for decreasing engine speed!

If you're looking for best MPG's, altitude is where it's at, due to the TAS advantage.

This, IMO is where the IO-360/375 will gain over the smaller engines!

Andrew

 

[frankh]

ahh

Now we're getting it.....more power=more altitude=less drag...and were not even talking tailwinds..

 

[elippse]

Altitude

Here's something else to consider about altitude. Even though the drag goes down as altitude goes up due to the reduced air density, the speed doesn't remain the same. Now you would think that they would go hand in hand, but here's the rub. An engine will run more efficiently if the inlet temperature goes UP. Yes! That's right. Increasing the inlet temperature increases the efficiency by the square-root of the absolute temperature ratio, or 1% for 10.4F. BUT, on the other hand, decreasing the air temperature increases the inlet density by 1%/ 5.2F and increases the power. As a result, the power only increases with the square-root of the absolute temperature ratio as the temperature goes down, so the power decrease is greater than the drag decrease. And there, my forum communicados, is why, along with increased induced drag, your plane's speed will drop off with altitude. Power drops faster than drag. A simplified method to determine WOT altitude power is to multiply sea-level power by the density ratio to the 1.135 power at the same rpm. And so, as so many have written, if you want good mpg, have a high efficiency prop that allows the engine to run WOT at altitude.

 

[Finley Atherton]

Nice data Finley, and in the theoretical world, it's true that a smaller engine will yield better FC for a given power level, but...

... when you add altitude (air density) into the equation, you'll find that a larger engine allows you to produce the same power at a lower engine speed. Note well how the BSFC in your example is improving for decreasing engine speed!

If you're looking for best MPG's, altitude is where it's at, due to the TAS advantage.

This, IMO is where the IO-360/375 will gain over the smaller engines!

Andrew

Granted, if you want to go high (and fast) then the big engine will have the fuel consumption advantage as the smaller engine will need to increase rpm to maintain HP once at an altitude where full throttle is reached.

For me at least, (and I suspect the average RV pilot) this is not really an issue as I don't carry oxygen and I am able to operate my O-320 at low cruise revs (2,250 rpm) at my usual cruise altitude between 7,500 and 9,500 ft and still get my desired cruise speed of about 160 kts TAS leaned to "best economy" and at about 7.1 US gal/hr (verified, accurate figures). This puts me at about 62% power which is about as much as I want to go without going ROP (Lycoming says 75% is OK).

Conceded that with the bigger engine I could go faster (maybe 165 kts??) before needing to go ROP but I would not be willing to accept the significant decrease in mpg even if I did have the big engine. 160 kts and 7.2 g/h seems a pretty respectable cruise to me!

Now let's get to the real matter of the argument. You guys with the big engine want to have your cake and eat it to! Big engine, Big power, Big climb, Big status, Big machismo and you also want to claim Big fuel economy
Fin
9A
O-320 Hartzell C/S

 

[L.Adamson]

Now let's get to the real matter of the argument. You guys with the big engine want to have your cake and eat it to! Big engine, Big power, Big climb, Big status, Big machismo and you also want to claim Big fuel economy

9A

Yes to all of the above....

If Bill R. would just switch to a C/S prop, now that he's got a real engine.......he'd prove it!

L.Adamson --- RV6A

 

[Andy_RR]

Conceded that with the bigger engine I could go faster (maybe 165 kts??) before needing to go ROP but I would not be willing to accept the significant decrease in mpg even if I did have the big engine. 160 kts and 7.2 g/h seems a pretty respectable cruise to me!

But Finley, you're withdrawing the comparison now. 160kts and 7.2g/h seems pretty good, but a bigger engine will allow you to go the same speed with even less fuel!

 

[Andy_RR]

Here's something else to consider about altitude. Even though the drag goes down as altitude goes up due to the reduced air density, the speed doesn't remain the same. Now you would think that they would go hand in hand, but here's the rub. An engine will run more efficiently if the inlet temperature goes UP. Yes! That's right. Increasing the inlet temperature increases the efficiency by the square-root of the absolute temperature ratio, or 1% for 10.4F. BUT, on the other hand, decreasing the air temperature increases the inlet density by 1%/ 5.2F and increases the power. As a result, the power only increases with the square-root of the absolute temperature ratio as the temperature goes down, so the power decrease is greater than the drag decrease. And there, my forum communicados, is why, along with increased induced drag, your plane's speed will drop off with altitude. Power drops faster than drag. A simplified method to determine WOT altitude power is to multiply sea-level power by the density ratio to the 1.135 power at the same rpm. And so, as so many have written, if you want good mpg, have a high efficiency prop that allows the engine to run WOT at altitude.

I'm still trying to get my head around your argument Paul, but I think you're being too simplistic with the factors involved. Engine efficiency will improve with lower inlet temperatures, provided you don't have to throttle to control power. Lower inlet temp = lower peak cylinder temp = lower thermal losses = higher thermal efficiency.

Also, you have to remember that exhaust pumping work decreases with increasing altitude! It's a complicated set of interacting influences.

 

[Finley Atherton]

But Finley, you're withdrawing the comparison now. 160kts and 7.2g/h seems pretty good, but a bigger engine will allow you to go the same speed with even less fuel!

Andy, you have lost me here so you may have to elaborate if I am missing something? I am already running at "best economy" mixture and low rpm so the only way the bigger engine could give the same speed (same HP) and better fuel consumption is if you reduced the rpm below my 2,250 and increased MAP to compensate (relative to what it was at 2,250 rpm). I don't know if this is realistic as I think most pilots would not want to go much under about 2,250 rpm in a 160 kt cruise. I know that my engine gets noticeably less smooth under about 2,200 rpm in cruise.

Fin
9A

 

[Andy_RR]

No, you're not missing anything Finley. You're just adding your own additional constraints, which are "not too high", "not too fast" and "not too rough".

All I'm saying is that for a given scenario of getting from A to B and without any "artificial", personal preference or installation-specific constraints, the larger engine will give a better overall MPG, all other things being equal.

For me, that's the reason why I'll take an IO-375 over an IO-340, which I was originally thinking to do on my RV-8.

A

 

[Finley Atherton]

No, you're not missing anything Finley. You're just adding your own additional constraints, which are "not too high", "not too fast" and "not too rough".

All I'm saying is that for a given scenario of getting from A to B and without any "artificial", personal preference or installation-specific constraints, the larger engine will give a better overall MPG, all other things being equal.

For me, that's the reason why I'll take an IO-375 over an IO-340, which I was originally thinking to do on my RV-8.

A

We will have to agree to disagree. All other things being equal they will give similar fuel economy as per my figures in post #21.

Fin
9A

 

[AK4x4]

Post 21 ?

In post 21 you list part throttle fuel burn at specific RPM. You do not list manifold pressure and engine HP at those RPM's. It seems probable that with the same RPM and similar fuel burn the 360 is producing more HP and with a constant speed prop would result in a faster airspeed. Russ

 

[Mike S]

Gee, this is fun

Matching load to powerplant is a big factor. What work is actually being done, in other words.

My pickup gets 16 mpg or so. My wifes Prius gets close to 50.

If I put the Prius in the back of the truck, and drive around, I will still get 14 or 15 from the truck.

If I put the truck in the back---well, maybe strap it on the roof--- of the Prius, and drive it around, I doubt if the Prius will get even the 14 or 15 of the truck, yet both are doing the same work----that being moving the total of the truck and Prius. If you look at the mileage of the combined units as a %, then the Prius will fair even worse.

Mechanical systems like to be matched correctly. And yes, bigger is often better, at least for efficiency.

 

[N941WR]

Yes to all of the above....

If Bill R. would just switch to a C/S prop, now that he's got a real engine.......he'd prove it!

L.Adamson --- RV6A

Never! Unless someone gave me a REALLY good deal on a Whrilwind two bladed prop. Even then I would have to figure out where to put that blue knob in my panel.

BTW, with all the changes I'm making to the plane, there will be no meaningful comparison between the 135 HP O-290-D2 with a climb prop and Van's cowl and the O-360 w/ cruise prop, and Sam James cowl and plenum. Not to mention the additional eight pounds I'm adding with the Clasic Aero leather side panels.

As for this thread, some time back (decades) someone actually gave me an engineering degree and from what I vaguely remember that education would support Finley's assertion. However, classroom theory doesn’t always support real world application.

Still, no one has tested these engines as much as Lycoming, so I would have to go with their numbers (which I assume came from a fully documented and instrumented test, in a test cell, and adjusted for barometric pressure, temperature, humidity, etc.).

The problem is the engines are not the same. The carbs are different, the air path, while similar, is different, etc. Add to that each of our installations. Two seemingly identical aircraft could have very different power outputs due to miss-matched exhaust ports, dirty injectors, oil (Using a thinner oil can increase hp. Spec Miata racers understand this very well.), cooling (ring expansion and thus compression efficiency is supposed to be optimal at around 360*F), and more.

Keep arguing people, this thread has been educational.

 

[Ron Lee]

Previous posts shows similar fuel burn at comparable HP. Close enough. Discuss if you wish but face reality. Put in a small engine with the intent to save a little fuel and most people will regret that decision every time they fly with another RV...or try to sell it.

IMO the best option is a large engine (at least 180 HP for RV6, 7, 8 RVs), CS prop and probably an electronic ignition.

A Sam James cowl may be better than the Vans cowl. I would like documented proof before I ever change. Build it straight, good fairings, as light as possible, etc and it will be good.

 

[elippse]

Inlet temp vs eta

I'm still trying to get my head around your argument Paul, but I think you're being too simplistic with the factors involved. Engine efficiency will improve with lower inlet temperatures, provided you don't have to throttle to control power. Lower inlet temp = lower peak cylinder temp = lower thermal losses = higher thermal efficiency.

Sorry, Andy, that's what I used to think. But I was telling a friend, relative to supercharged engines, that the engine power went down 1% with a 5F increase in temperature. He said that he'd heard that it was 1% per 10F. Well I trust this old f**t, and even though he's been known to tell some pretty outlandish stories; he does have an aero degree from Cal Poly. Well, back to the books, and who to consult but CF Taylor! And there it was, staring me in the face, engine efficiency increases with the square-root of an increasing absolute temperature ratio. Horrors! Now I not only had to go back and change my airplane-prop evaluation equations, but I had to tell Tom S. that he was right and I was wrong. BUT, volumetric efficiency, or the density of the charge entering the cylinders, decreases with an increasing absolute temperature ratio. So if you multiply the two together, what you get is TR/sq rt(TR), so that as you climb and the temperature goes down, the power decreases about 1%/per 10F, whereas drag decreases 1%/5F. So if you are looking for more mpg at altitude, pull on carb heat. Your power will go down 3%/30F, but your speed will only go down 1%, and your SFC will be better, you will be able to lean the engine even more and it will be smoother. And at altitude, assuming you have a carb temp gauge, you ought to be able to increase your inlet to 80F-90F, the same as on a hot California day. So how's that, Bill R, for keeping it going! But there is nothing like a super-efficient prop for converting those BTUs into MPH and GPH!

 

[Finley Atherton]

In post 21 you list part throttle fuel burn at specific RPM. You do not list manifold pressure and engine HP at those RPM's. It seems probable that with the same RPM and similar fuel burn the 360 is producing more HP and with a constant speed prop would result in a faster airspeed. Russ

Russell, to quote from my post #21; "Figures show US gal/hr for 110 hp leaned to "Best Economy" so at each rpm both engines are producing 110 HP. Obviously the smaller engine will require a higher MAP to produce the same HP at the same rpm. Which ever way you cut it, the smaller engine will use slightly less fuel to produce 110 HP if both engines are at the same rpm.
However, having said that, in this case of producing 110 HP, the bigger engine does have a trick up it's sleeve. It can reduce rpm to 2,000 rpm, increase MAP and still produce 110 HP. Now it's fuel consumption will be 7.64 g/hr which is about 0.1 g/hr better than the smaller engine as Lycoming's graph shows it is limited to 2,200 rpm to produce 110 hp. Presumably in an attempt to produce 110 hp the smaller engine runs out of throttle at 2,000rpm?? However if you want to produce 100 hp then the smaller engine can match it with the bigger engine and produce 100 hp at the same 2,000 rpm with slightly better economy than the big engine . As I said in a previous post, 2,250 rpm is my lower limit for cruise so operating at 2,000 rpm is academic for me. YMMV

Fin
9A

 

[Andy_RR]

However, having said that, in this case of producing 110 HP, the bigger engine does have a trick up it's sleeve. It can reduce rpm to 2,000 rpm, increase MAP and still produce 110 HP. Now it's fuel consumption will be 7.64 g/hr which is about 0.1 g/hr better than the smaller engine as Lycoming's graph shows it is limited to 2,200 rpm to produce 110 hp.

Yay! Fin's made my point for me!

Of course, you can get better MPG's if you go slower and slower, at least until you run past best L/D, and many people, Finley included, will apply other constraints to help make a decision. Fin chooses to limit engine speed and TAS. I would take the larger engine and climb higher to where I'm back to WOT - least pumping work and higher TAS at the same AoA.

All we really want to do on this thread though is make an apples-to-apples comparison.

Paul L, far be it me to contradict the esteemed Mr Taylor! I know he's right, except I suspect you need to take a close look at his assumptions. I believe (but I don't have him handy) that he's holding everything else constant and moving only inlet temperature. I'd argue that we're changing more than one thing since by climbing higher we can reduce MAP and avoid having to pay for it (no throttling). On top of that, we get the TAS dividend. If we close the cowl flap that we all should have, we can benefit from reduced thermal losses from reduced inlet temperatures too.

 

[videobobk]

Sounds to me like there are things more important than engine size in relation to fuel economy. I think we can agree on that. Operating MAP, rpm, altitude, temperature, etc. These are things we have some control of. I almost always pull carb heat as I can lean more and save about .2 gph at 130 kts (yeh, I'm cheap.) It doesn't seem to work as well at or above 150 kts. I went to electronic ignition and saved 1.2 gph at 75%, far more than the difference between equally-equipped 320 vs. 360. Truth is, there is no bad setup (Lycoming, anyway) on the front of an RV. I'd love to have more hp, but I don't need it. I'd love to have FI, but a carb works well. Learn you own setup, find the sweet spots, and go enjoy.

Bob Kelly

 

[petehowell]

Now this is Cool!

I have known from experimentation that I get better MPG when I pull the carb heat. My carb temp data says I can get it up to about 100 degF. I thought I was just getting better fuel atomization, allowing me to lean to the bottom of the BSFC bucket, but maybe more is going on.

I seem to get best results with full carb heat below 5K ft and partial carb heat above 5K. I'm not sure if this is due to the effect Paul is siting, but testing (on my plane at least) seems to concur with what he has stated.

Sorry, Andy, that's what I used to think. But I was telling a friend, relative to supercharged engines, that the engine power went down 1% with a 5F increase in temperature. He said that he'd heard that it was 1% per 10F. Well I trust this old f**t, and even though he's been known to tell some pretty outlandish stories; he does have an aero degree from Cal Poly. Well, back to the books, and who to consult but CF Taylor! And there it was, staring me in the face, engine efficiency increases with the square-root of an increasing absolute temperature ratio. Horrors! Now I not only had to go back and change my airplane-prop evaluation equations, but I had to tell Tom S. that he was right and I was wrong. BUT, volumetric efficiency, or the density of the charge entering the cylinders, decreases with an increasing absolute temperature ratio. So if you multiply the two together, what you get is TR/sq rt(TR), so that as you climb and the temperature goes down, the power decreases about 1%/per 10F, whereas drag decreases 1%/5F. So if you are looking for more mpg at altitude, pull on carb heat. Your power will go down 3%/30F, but your speed will only go down 1%, and your SFC will be better, you will be able to lean the engine even more and it will be smoother. And at altitude, assuming you have a carb temp gauge, you ought to be able to increase your inlet to 80F-90F, the same as on a hot California day. So how's that, Bill R, for keeping it going! But there is nothing like a super-efficient prop for converting those BTUs into MPH and GPH!

 

[Finley Atherton]

Yay! Fin's made my point for me!

Happy to oblige Andy. I am willing to concede that a big engine is not all bad.
As you want to fly high and fast then your choice of the 205 hp IO-375 would be the way to go. I do not know the fuel consumption figures for the IO-375 but be aware that if you ever want to fly below Flight Levels or at more conservative and economical cruise speeds then your fuel economy will suffer as you will have to throttle back considerably and accept the relatively big hit (compared to a smaller engine) that the pumping losses will have on your fuel consumption.

Fin
9A

 

[hecilopter]

Not swinging the same prop

In all this comparison about same airframe and the engines putting out the same power, no one has mentioned that all else being equal, they don't have the same prop. If you put 160 HP from an O-320 to a prop that is made for it (ex. Hartzell C2YL-1BF/F7663-4), it will probably be slower than getting 160 HP from an O-360 to a prop that is made for it (ex. Hartzell C2YR-1BF/F74972 or C2YR-1BF/F7497).

I guess the point is, if it only took 155 HP from the O-360 with the appropriate prop to go the same side by side speed as the O-320 putting out 160 HP with its prop, the O-360 could actually burn less fuel on the same trip. This is where I think the real world differences vs. theory is coming in to play.

There are several comparisons by Van's on various prop efficiencies and that alone could make a significant difference. The 360 also has more torque, if that matters.

 

[Finley Atherton]

If you put 160 HP from an O-320 to a prop that is made for it (ex. Hartzell C2YL-1BF/F7663-4), it will probably be slower than getting 160 HP from an O-360 to a prop that is made for it (ex. Hartzell C2YR-1BF/F74972 or C2YR-1BF/F7497).

I can see what you are getting at but I can't logically see why, " it would probably be slower". You could compare the relative efficiencies of one prop verses another and conclude that one prop may give a higher speed than another at a particular rpm and HP, but I think it is a bit of a broad brush to generalize that props on smaller engines are less efficient if this is what you are saying? I suppose you could say that the O-320 prop being smaller would need to be at a coarser pitch relative to the bigger prop to produce the same HP at the same rpm, but I don't know if this necessarily means it is less efficient??
Any prop experts out there?

Fin
9A

 

[elippse]

prop comparisons

I suppose you could say that the O-320 prop being smaller would need to be at a coarser pitch relative to the bigger prop to produce the same HP at the same rpm, but I don't know if this necessarily means it is less efficient??
Any prop experts out there?

Fin
9A

You really can't compare CS props designed for different HP and TAS; the twist won't be the same. Actually, the prop designed for the 320 might be better in this comparison than the 360's since if it was designed for a given rpm and TAS, then if it's properly designed, its twist will be optimum. The 360 prop might be operating at a slower speed than its twist is designed for. But these are just pear-tangerine comparisions; in the end what really counts is SFC!

 

[F1Boss]

MAYBE!!

Granted, if you want to go high (and fast) then the big engine will have the fuel consumption advantage as the smaller engine will need to increase rpm to maintain HP once at an altitude where full throttle is reached.

For me at least, (and I suspect the average RV pilot) this is not really an issue as I don't carry oxygen and I am able to operate my O-320 at low cruise revs (2,250 rpm) at my usual cruise altitude between 7,500 and 9,500 ft and still get my desired cruise speed of about 160 kts TAS leaned to "best economy" and at about 7.1 US gal/hr (verified, accurate figures). This puts me at about 62% power which is about as much as I want to go without going ROP (Lycoming says 75% is OK).

Conceded that with the bigger engine I could go faster (maybe 165 kts??) before needing to go ROP but I would not be willing to accept the significant decrease in mpg even if I did have the big engine. 160 kts and 7.2 g/h seems a pretty respectable cruise to me!

Now let's get to the real matter of the argument. You guys with the big engine want to have your cake and eat it to! Big engine, Big power, Big climb, Big status, Big machismo and you also want to claim Big fuel economy
Fin
9A
O-320 Hartzell C/S

Hey Fin:

Don't rely on what I say -- ask others.

On the far 'bigger is better' side of the equation, I am running an IO550-N in my plane. I will suggest that RPM is a large factor in this equation. When flying with Falcon Flight, I generally see 1850-1900 RPM and about 19"MP. Yes, the engine is throttled -- to what I would call min HP, or about 35% on the Dynon (~110HP?). This setting varies a bit, but generally I see 7.4GPH @140-145KIAS, and of course this is LOP operation (and 7hrs endurance!). Temps are on the low end of the scale, so I wouldn't want to operate the engine much slower.

Not knowing what temp you get your 160KTAS at, it looks like my plane is very close to yours in MPG at the above power settings, tho it is carrying a few hundred pounds more weight -- I'd guess it would be starting out at about 1875lbs.

If we all top off prior to departure, and make an enroute fuel stop, I generally take on less than any of the RVs I would be in formation with.

Thus my opinion: you can have your cake, and eat it too. At times, it is a big, thirsty cake -- no argument there. If the pilot can be restrained, the machine will be very efficient in turning 100LL into miles behind you.

In the final analysis, the problem is restraining the pilot!

Carry on!
Mark

 

[Finley Atherton]

Hey Fin:

Don't rely on what I say -- ask others.

On the far 'bigger is better' side of the equation, I am running an IO550-N in my plane. I will suggest that RPM is a large factor in this equation. When flying with Falcon Flight, I generally see 1850-1900 RPM and about 19"MP. Yes, the engine is throttled -- to what I would call min HP, or about 35% on the Dynon (~110HP?). This setting varies a bit, but generally I see 7.4GPH @140-145KIAS, and of course this is LOP operation (and 7hrs endurance!). Temps are on the low end of the scale, so I wouldn't want to operate the engine much slower.

Not knowing what temp you get your 160KTAS at, it looks like my plane is very close to yours in MPG at the above power settings, tho it is carrying a few hundred pounds more weight -- I'd guess it would be starting out at about 1875lbs.

If we all top off prior to departure, and make an enroute fuel stop, I generally take on less than any of the RVs I would be in formation with.

Thus my opinion: you can have your cake, and eat it too. At times, it is a big, thirsty cake -- no argument there. If the pilot can be restrained, the machine will be very efficient in turning 100LL into miles behind you.

In the final analysis, the problem is restraining the pilot!

Carry on!
Mark

Hi Mark,
Fully agree with what you are saying. A big engine will give the flexibility to go high and fast and can also be run at low power for good economy. All I am trying to argue against is the idea that if the O-360 is operated at low power settings then it must somehow be more fuel efficient than the O-320 producing the same power. As the figures in my post #21 show, this just ain't so!
Now we all know there has to be limits to how big the engine can be. I would love to have a bigger engine, however in my case the heavier O-360 with the heavier Hartzell C/S prop (personal preference) would unacceptably cut into my allowable load and move the c of g too far fwd.

Fin
9A O-320, Hartzell C/S

 

[F1Boss]

No argument there!

Hi Mark,
Fully agree with what you are saying. A big engine will give the flexibility to go high and fast and can also be run at low power for good economy. All I am trying to argue against is the idea that if the O-360 is operated at low power settings then it must somehow be more fuel efficient than the O-320 producing the same power. As the figures in my post #21 show, this just ain't so!
Now we all know there has to be limits to how big the engine can be. I would love to have a bigger engine, however in my case the heavier O-360 with the heavier Hartzell C/S prop (personal preference) would unacceptably cut into my allowable load and move the c of g too far fwd.

Fin
9A O-320, Hartzell C/S

Hey Fin:

Sounds like you seek the Holy Grail: a larger engine that is more efficient. Large engines are capable of larger HP numbers, but (generally) not greater efficiency. Greater efficiency comes from a better design (lots of parameters there!) or better operating conditions.

It may be possible for you to have an O-360 with a FP prop that would operate at a lesser FF than your 320, given like airspeed, loading conditions, etc. But, that gonna require a better prop (likely larger diameter, or better design) turning at a slower RPM, and higher MP setting.

This is an old formula! A fella named Lindbergh proved it to the US Air Army Corps with the P38 -- you've probably heard of that. I use that exact formula when I fly with my pals, and it seems to work the same.

But, having your cake, and being able to eat it to, is most likely gonna require a CS prop. The Whirlwind 200 is very light -- and it seems to be a good product (I have not flown one). One of those on an O-360 would seem to get you closer to the search for the Grail.

The weight gain won't be THAT much -- your wallet will be much lighter.

I will also suggest that a fwd CG is far easier to work with compared to an aft CG.

Carry on!
Mark