C172 fuel efficiency - how to maximize 0300 MPG

Leo O'Farrell

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Leo O'Farrell
Lately, have been shifting my Jeep into neutral on downhills to save fuel, and am wondering how to save fuel while flying.

Is is safe and OK for the engine to operate at the lower end of the green range, leaned to the max? Say 2,250 RPM.

Other tips?
 
Have you actually noticed a difference in mileage with your Jeep? I'd guess not. Also if you have an overdrive you're probably better off letting it stay in gear and coast - fuel injection will turn off the injectors.

Look at the 172 POH and see what it has as far as speeds and fuel burns at best economy. You're not likely to get any better than that.
 
Do you have power steering or power brakes on that Jeep?

Think about the engine failure scenario. It's much more likely in a Jeep than in a Cessna. And the results will be much, much more immediate, especially in mountainous terrain.

If the engine quits, you will still get some power assist in both systems even with an unlocked torque converter. With a manual transmission, you'll get full capability in those systems right up until you come to a stop. If the check valve is working, you'll even get a couple more stops after that (but the power steering pump will be idle).
 
as far as the Jeep, if it is the least bit modern i.e. fuel injection, you're probably not saving any gas by going into neutral. The computer senses fuel demand and cuts back fuel flow. Look at a car that has instant MPG display. It skyrockets when you let off the gas, even when it's still in gear.
 
Is is safe and OK for the engine to operate at the lower end of the green range, leaned to the max? Say 2,250 RPM.

Other tips?

You must remember what killed the GO-300, do not run your engine at a low RPM.

2450 and lean aggressively at all altitudes and all throttle settings.

Pulling the throttle back simply makes you go slower, and take longer to get there, fuel usage will be the same.
 
as far as the Jeep, if it is the least bit modern i.e. fuel injection, you're probably not saving any gas by going into neutral. The computer senses fuel demand and cuts back fuel flow. Look at a car that has instant MPG display. It skyrockets when you let off the gas, even when it's still in gear.

Except you may experience some "engine braking" and not maximize the fuel savings.
 
Except you may experience some "engine braking" and not maximize the fuel savings.

If the downhill is so wimpy as for that to be a factor, he's trading "fuel savings" for wear on the clutch or torque converter, either of which will make his savings negative.

More likely, the tradeoff is whether the excess heat goes out the tailpipe/oil pan/radiator or the brakes. As a very frequent mountain driver (and also a frequent witness to flatlander inexperience), the brakes are a bad choice.
 
I was in a Camaro near Asheville, NC and put in neutral at the crest of a mountain. Was doing about 115 near the bottom, tapped the brakes a little to corner, but just let the rest of the speed bleed off as we started to go back up the next hill.
 
As to whether shifting into neutral saves gas on not, it depends on the car. I device called a scangauge which hooks into the computer while driving and displays all kinds of useful information. From it, I have determined that my 2000 Honda Accord, if you coast in neutral, gets great gas mileage, like over 100 mpg depending on speed. But if you leave it in gear, it will do one of two things - 1) get similar gas mileage, or 2) shuts off the fuel flow entirely, giving me basically infinite gas mileage. It seems to kick into (2) when it is gradually decelerating.

My 2001 Ford F-150, however, has no (2), meaning the fuel never completely shuts off.

So it depends on your car, its computer, and the algorithm it uses for fuel flow.
 
On a related note, it's a good exercise to do the math at different power settings to see what you're really saving on a trip. It sounds wise to throttle back and burn 7-8gph instead of 10gph on a O-360, but you also need to consider the dry cost of the aircraft. I did some quick calculating in Excel one day and figured out that at 75% power with no wind it cost me almost the exact same price per mile as it did at 55% power because of the extra dry cost incurred. It required a significant tailwind in order for 55% power to really make a difference worth caring about.
 
DFCO is present on most all cars from about 2000 and later. Stick and auto both do it. There is some benefit on a long descent if you can maintain speed by coasting in neutral and removing the engine braking from the equation(it's complicated). Autos have a different braking effect that manual of course.

As for the plane, I can tell you that I'm getting very good fuel economy in my bigger Conti engine by reducing RPM and leaning aggresively. I run around 2000-2100 revs, and lean till it hurts. Sure, I slow down some, but I plan to have my engine go over 2000 hours provided nothing catastrophic happens.
 
I would think someone willing to go downhill in neutral would also be willing to fly his plane at L/Dmax speed at all times, leaned to the hilt.
 
You must remember what killed the GO-300, do not run your engine at a low RPM.

2450 and lean aggressively at all altitudes and all throttle settings.

Pulling the throttle back simply makes you go slower, and take longer to get there, fuel usage will be the same.

Whut? The GO-300 isn't an O-300. For one thing it has gear box, was designed to be run at over 3,000 RPM and has 1200hr TBO.

Speed is a function of the cube root of power.. It takes a lot of fuel to get an extra knot and a lot more to get the 2nd knot. There's a law of diminishing returns. There's a graph somewhere that shows the sweet spot. At some point, you're just wasting money.
 
Whut? The GO-300 isn't an O-300. For one thing it has gear box, was designed to be run at over 3,000 RPM and has 1200hr TBO.

Furthermore, the problem with the GO-300 was that it was driving a constant speed prop, right? So when pilots were flying it at "low" RPM they still had high power output which loaded down the gearbox with a ton of torque which was the real problem. At least, that's what I've always assumed. I could be wrong though. I don't know that there's anything similarly dangerous or "wrong" with running a O-300 with fixed pitch prop at low RPM (and correspondingly low power output).
 
DFCO is present on most all cars from about 2000 and later. Stick and auto both do it. There is some benefit on a long descent if you can maintain speed by coasting in neutral and removing the engine braking from the equation(it's complicated). Autos have a different braking effect that manual of course.

Cars with CVT have a really annoying habit of downshifting when going downhill, presumably to maintain speed, but it kills the reclamation of potential energy paid for when going up the hill. I hate it. :mad3::mad3:
 
Lately, have been shifting my Jeep into neutral on downhills to save fuel, and am wondering how to save fuel while flying.

Is is safe and OK for the engine to operate at the lower end of the green range, leaned to the max? Say 2,250 RPM.

Other tips?

Absolutely safe and will save you money. I have run my IO540 2100 rpm for best economy when just farting around.

Any plane you fly will get better mpg at 65% than at 75%.
 
Where did all this GO300 stuff come from? The op has a 172
 
Fuel efficieny is only one part of the equation for saving money. If you rent wet than go as fast as you can as that will be the cheapest for you. However, if you own then there is a subtle balance between fuel efficiency, and speed. I did a excel spread sheet for my 182 and found that was most fuel efficient for me was not was was cheapest when I figured in fixed costs based on an hourly breakdown. It cost me about $170 an hour to run the plane(including all repairs, annuals, 25 hr oil changes, fuel, insurance, training, etc). When I factored this number in it turns out if I run at about 26 inches MP with 2000 to 2200 RPM, leaned for peak TIT(as recommended by POH) I do the best.
 
may not matter in your situation, but my mooney has an RPM limitation. No continuous operation below 2300 RPM based on my prop and engine.
 
may not matter in your situation, but my mooney has an RPM limitation. No continuous operation below 2300 RPM based on my prop and engine.

Yes, but you have a CS prop, right? Different beast. You can use a higher RPM and lower MP to avoid that. Fuel consumption and top speed for consistent leaning will be proportional to the product of the two. CHT will be happier with the higher RPM (so will detonation, lubrication, etc.).
 
Furthermore, the problem with the GO-300 was that it was driving a constant speed prop, right?

Not all of them, 1 year 1960 was a fixed pitch. it still had problems

The 0-300 and the GO-300 are the same engine, one has a gear reduction unit the other does not.

they both suffer the same problems from running low RPM, and slow airspeeds, the CHT goes up the cylinder fail early.

The GO-300-D was also found in the 1963 P172 other wise known as the powermatic 172, with the C/S prop, which became the 172XP.
 
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Yes, but you have a CS prop, right? Different beast. You can use a higher RPM and lower MP to avoid that. Fuel consumption and top speed for consistent leaning will be proportional to the product of the two. CHT will be happier with the higher RPM (so will detonation, lubrication, etc.).

sorry, brain fart. :rolleyes:
 
may not matter in your situation, but my mooney has an RPM limitation. No continuous operation below 2300 RPM based on my prop and engine.
No such restriction on my plane. Though this would not be the first time I thoought something was okay, and somewhere in the bowels of the documentation there is a restriction I (and the mechanics and others familiar with my plane thatI consulted on on how to run my engines) did not know about. CHT's are always less than 350, TIT usually about 1550, and EGT around 1450. Burns about a quart of oil every 20 hours or so.
 
Fuel efficieny is only one part of the equation for saving money. [...] I did a excel spread sheet for my 182 and found that was most fuel efficient for me was not was was cheapest when I figured in fixed costs based on an hourly breakdown.

Yeah, this. Obviously differs for every plane, and you really want to do the math with actual measured TAS and GPH numbers at a few altitudes and power settings, but even the POH numbers will get you close enough to finding the "sweet spot" in your plane. I don't have my numbers handy, but it was something like $.77/nm at 75% and $.77/nm at 55% with no wind, and with a 20kt tailwind something like 2 cents cheaper per mile to fly at 55%.

Those numbers included only the dry rate we put into aside for the plane, and not the fixed hangar/insurance/database costs of course.
 
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