bigger engines at low power settings

[prkaye]

A number of times people have suggested that to save fuel, instead of getting a smaller engine a better choice would be to get a bigger engine and just typically run it at lower power settings. I'm curious at to what extent this actually works.

Consider two engines, say a 120hp and a 160 hp engine. Now set both engines to generate the same amount of thrust (maybe have the 120hp engine running at about 75% and the 160hp running at about 50%?). My guess would be that the bigger engine would be burning more fuel than the smaller engine to generate the same amount of thrust. A smaller engine should be more efficient at generating thrust within its normal power range than a big engine throttled back. True?

On the other hand, perhaps the difference in fuel consumption between these two cases is so small that it wouldn't make a difference.

A related question - it has been suggested that you can run MOGAS in a 160hp O-320. Is this also true for a 118hp O-235?

[Mike S]

Just a wild guess here, but if you are using a constant speed prop, then you can probably get better efficiency from the bigger engine. You have to think in terms of the torque output.

Bigger engine will make more torque at lower RPM, then load the prop for speed.

I have never flown a c/s so this is from what I have read, and a bit of deductive logic.

Mike

[rv8bldr]

The other thing to remember is, on average, a bigger engine = less payload (i.e. the engine weighs more). As long as you are willing to live with that, you're good. In the U.S. you can set your max gross at just about anything you want. Up here in Canada, the inspector will want paperwork from Van's giving the OK to up the gross weight to compensate for the lost payload with the bigger engine. I doubt very much that you'd get that OK, especially in writing.

When I submitted my paperwork to the RAA to say I'm starting the -8, I put in a max gross of 1950. I got a phone call from the RAA saying that unless I was willing to go through all of the hoops to change it from the published numbers, then I should just list the max gross as what Van's states.

I don't the CG status with the -9, but more weight up front will also mean that you are more noseheavy. Certainly an issue in the -8.

Cheers

[RV6_flyer]

I have a 160 HP Constant Speed prop on my RV-6 (1,933.3 hours flying). I fly a lot of formation. When I fly lead, I set up a power setting of 2,100 RPM and 22 inches. That give 122 KIAS (140 MPH IAS). An 160 HP RV flying off my wing will burn more fuel than I do. A 180 will burn about the same. Dan Checkoway with his 200 HP RV-7 burns LESS gas than I do. We are doing LAZY EIGHTS with up to 60 degrees of bank and 30 degrees of pitch. I run a constant power setting. Dan has repeatedly burned less gas than I do.

On trips, as lead, I will set up 23 squared power setting. That give about 7 GPH. The guys with 180s will burn the same or slightly less fuel than I. The 200 HP will burn about 0.5 Gallons per hour less. As I increase RPM, I burn more fuel than any of the 180s or 200 HP airplanes.

Originally Posted by prkaye

A number of times people have suggested that to save fuel, instead of getting a smaller engine a better choice would be to get a bigger engine and just typically run it at lower power settings. I'm curious at to what extent this actually works.

Consider two engines, say a 120hp and a 160 hp engine. Now set both engines to generate the same amount of thrust (maybe have the 120hp engine running at about 75% and the 160hp running at about 50%?). My guess would be that the bigger engine would be burning more fuel than the smaller engine to generate the same amount of thrust. A smaller engine should be more efficient at generating thrust within its normal power range than a big engine throttled back. True?

On the other hand, perhaps the difference in fuel consumption between these two cases is so small that it wouldn't make a difference.

A related question - it has been suggested that you can run MOGAS in a 160hp O-320. Is this also true for a 118hp O-235?

The above experience is after 8 years and 11 months of flying experience in my RV-6.

[cobra]

IMHO you will only push the power up when you are in a hurry or really need it, othewise, and most of the time, you pick whatever throttle setting works best in your plane and gives the best compromise of efficiency and speed.

In defference to Van, I do not think anyone ever has too much power when it is badly needed or when conditions are not the best, provided it does not come with too much of a weight penalty. But like anything that is good, it can be misused way up high.

The other extreme, needing power that is not there, can kill you and others that you love.

[Brian Denk]

Originally Posted by RV6_flyer

I have a 160 HP Constant Speed prop on my RV-6 (1,933.3 hours flying). I fly a lot of formation. When I fly lead, I set up a power setting of 2,100 RPM and 22 inches. That give 122 KIAS (140 MPH IAS). An 160 HP RV flying off my wing will burn more fuel than I do. A 180 will burn about the same. Dan Checkoway with his 200 HP RV-7 burns LESS gas than I do. We are doing LAZY EIGHTS with up to 60 degrees of bank and 30 degrees of pitch. I run a constant power setting. Dan has repeatedly burned less gas than I do.

On trips, as lead, I will set up 23 squared power setting. That give about 7 GPH. The guys with 180s will burn the same or slightly less fuel than I. The 200 HP will burn about 0.5 Gallons per hour less. As I increase RPM, I burn more fuel than any of the 180s or 200 HP airplanes.






The above experience is after 8 years and 11 months of flying experience in my RV-6.

I concur. Flew formation with a -4/O-320/FP in my -8/O-360/FP and burned about .5gal less for the flight, which was about an hour in duration.

[bumblebee]

Originally Posted by prkaye

My guess would be that the bigger engine would be burning more fuel than the smaller engine to generate the same amount of thrust. A smaller engine should be more efficient at generating thrust within its normal power range than a big engine throttled back. True?

As a general rule ... and I mean "general"....for the same power, larger/slower is more fuel efficient than smaller/faster. Mainly due to higher friction losses and pumping losses in spinning smaller engine faster to make same power.

Prop efficiency probably plays a role here too when a f/p flies alongside a c/s.

[dan]

Originally Posted by RV6_flyer

Dan Checkoway with his 200 HP RV-7 burns LESS gas than I do.

In general, having more HP comes at a higher up-front and periodic cost. I'm pretty sure I spent close to 1.5x or 2x as much on my powerplant as Gary did. So if I'm burning 0.5 gph less than Gary on every cross country trip, how long will it take to recoup that difference in up-front cost? A looooooooong time...and I fly a LOT!

Don't get me wrong, I'm a huge proponent of "going big" on the powerplant (as big as the airframe designer allows). And I'm a proponent of spending up-front money on systems that lower operating costs (FI + EI). But just be realistic about how long it will take to "recoup" any higher up-front costs.

Ok, all that said...ask Gary what engine he would like to have in his RV-6. It ain't the 320. If he upgrades, should he go with a 390? There's some law of diminishing returns, and there's a balance to be struck...and it will be different for each builder/pilot/airplane. Depends on your mission, your budget, how much you plan to fly in the next N years, etc. YMMV.

[gmcjetpilot]

Originally Posted by prkaye

A related question - it has been suggested that you can run MOGAS in a 160hp O-320. Is this also true for a 118hp O-235?

MOGAS in high compression engines? You can't use the MOgas at the airport. You need buy premium at a gas station (w/o alcohol, ethanol), hauled it to your aircraft in jugs, pour it into your tank. Prem national avg is $3.31, AVgas is $4.91, so looking at max min of each, you could save may be 50 cent min to a $1.50 max per gallon, avg about $1. So flying at 8g/h you save $8 an hour for all the hassle and potential dangers (read below). Of course premium is not available on the road since the Auto gas at the airfield is only 82 octane. Also the availability is not great. In fact some FBO's will not sell it to you unless you have your STC on file that you can use it.

Remember many 235's, 320 (160HP), 360 (180, 200) HP engines are certified for 91/96 octane. The pumped autogas at an airport (MOgas) is about 82 octane. Premium auto gas may say 94 octane but is really 89 octane on the AVgas scale. Will the engine run? Yes, but what are the draw backs. (read on).



The MOgas for use with early STC's for factory planes was squarely for low compression engines (in the 7's to 1 ratio). Here is a list of approx CR's for Lyc's.

O235 (115HP) 6.75:1 80 octane
O235 (115HP) 6.75:1 80 octane
O235 (116HP) 8.10:1 80 octane
O235 (125HP) 9.50:1 91/96 octane (now 100/100LL)
O320 (150HP) 7.00:1 80 octane
O320 (160HP) 8.50:1 91/96 octane (now 100/100LL)
O360 (180HP) 8.50:1 91/96 octane (now 100/100LL)
IO360 (200HP) 8.70:1 91/96 octane (now 100/100LL)

Mogas at airports (82 octane) is great for low compression engines but not so much for high compression engines. Also RV's have tight hot cowls with tight exhaust. All of these things are not good for using autogas. Just saying be careful.

Not all aircraft that are tested for autofuel STC's pass. The Mooney M-20-C and Piper Comanche with the 180HP, O-360-A1A (my engine), could not pass. They got pass the vapor lock problem, but the fuel kept boiling in the carb. Quote Petersen: "Pneumatic lock takes place when the fuel boils as it enters the carb. The engine then dies due to an over-rich mixture. This is just the opposite of a vapor lock where the engine quits or runs poorly due to a lean mixture. The better an airplane performs, the more difficult it is to get it through the flight test program." (Note, the last line. This is from a group that does autogas STC's.)

So your custom RV is an unknown. If there is a STC out there for your engine, it's valid and tested only for the installation. Of course we don't need a STC, but it sure is an indication of caution when a plane can't get the STC due to a tight cowl and exhaust.

Vapor lock is caused by not only high temps of the fuel but altitude. If you intend on flying autogas high and hot, consider the following:

-Insulate/firesleeve all your fuel lines especially fwd of the firewall
-Heat shield / air blast tube on the gascolator and mech fuel pump
-Vapor return line from just before the carb back to the gas tank

Some people laugh and say, "Ahwww you don't need all that". OK. There are about 250 NTSB reports where autogas was on-board and vapor lock was named or suspected, about 80 reports in experimentals. Vapor lock like carb ice is hard to prove, since the evidence either melts (carb ice) or condenses as in the case of vapor lock. The description of erratic and low fuel pressure and loss of power are in many more reports without the words vapor lock mentioned. BTW, carb ice is thought to be more likely with autogas. That is my experience.

Vapor lock affects factory planes, experimental's and even planes using AVgas alike. So vapor lock can happen to anyone. Autogas no matter how you paint it has much lower resistance to vapor lock than AVgas. Bottom line auto gas has much higher vapor pressure than AVgas, especially when car gas goes to the "winter blend". The winter blend has even high vapor pressure to assure better starting of cars in cold weather. Of course you need to test (worry) about alcohol and less than 10% ethanol which is becoming an issue.

Also lower octane means lower detonation margins. That is a fact without dispute. Are you running electronic ignition with timing advance? If you want to risk detonation or retard your timing (at loss of power) you can do that. It is experimental after all.

My advice or comment is engines that are designed for 81/86 octane are better suited for autogas. If you want to mess around with buying premium fuel at the corner gas station and hauling it to your 91/96 octane engine, go a head, but read about all the hazards of fuel contamination and fire danger. NTSB reports that involved AUTO GAS, many accidents involved fuel contamination due to miss-handling of the fuel.

Octane at the Q-mart gas pump is 5 points lower than the AVgas equivalent, so "Super Duper Premium" that's labeled 94 octane is really 89 octane in AVgas land. When you fly cross country you will not able to get premium fuel from a car gas station, since it's unlikely you'll be able to haul it. The MOgas available at airports is only about 82 octane. That's great for engines certified for 80 octane, by not high compression engines.

You get what you pay for. Just be careful. It is serious business. The easy way and safer in my opinion is use AVgas and pay the extra $4 to $8 an hour. If you go for lower cost fuel, educate yourself and talk to folks that have do it, especially in a RV. I had experience with MOgas long ago in a C-182 and I was not impressed. As gas prices rise further, the incentive is great enough to use Mogas. Each to their own. Safety first, cheapness second is my motto.

No where did I say autogas is bad, but there are real issues and you are messing with your "margins of safety".

[prkaye]

Thanks! That's a great bunch of information. I think I will probably end up going for a low compression engine, like that 150hp O-320 (does Vans OEM pricing apply to all these models?). With the low compression engine it sounds like I would ultimately have more options for the fuel.

What is an STC?