# Rx1/Apex fuel consumption

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#### aircraftbuilder

##### Well-Known Member
I been doing some research on these engines and have not been able to find out how much they consume. Does anyone have an idea how many gallons per hour they are doing?

#### rv7charlie

##### Well-Known Member
Short answer: The same as any other 4 stroke in the same airframe at the same speed, ignoring differences in cooling drag if going from air cooled to water cooled.

The practical, real world answer is that they are 4 stroke, internal combustion piston engines. For all practical purposes, 4 stroke IC engines will burn somewhere between 0.4 & 0.5 pounds of gasoline, per horsepower, per hour. Perfectly tuned water cooled engines under low to moderate load will be at the low end, and closer to 0.5 at full power. Air cooled engines under moderate to moderately high load *might* get close to the low end, if they're running optimized mixture and electronic ignition which can advance/retard timing automatically. Air cooled engines running at near full power typically consume at 0.55-0.6, because they use excess fuel to cool the exhaust valve.

Above is driven by the basic physics & poor thermal efficiency of IC engines.

To do the math:
For easy round numbers & typical midpoint efficiency, let's assume 100 HP at cruise.
100*0.45=45 pounds of gas per hour
Gas weighs ~6 lbs per gallon, so,
45/6=7.5 gallons per hour.
If you're burning less fuel than that, you're almost certainly making less power than 100 HP.
If you're burning more fuel than that, you *might* be making more power, but only if mixture and ignition timing are optimized.

Claims that exceed those parameters are almost without exception 'fish stories'. Example: Cessna 172 drivers are fond of claiming that their 160 HP Skyhawk 'burns 6.5 gallons per hour at 75% power.' It doesn't. If they looked at the operational manual, or Lycoming's operator's manual, they'd see that it can't. What's happening is that they're running at well under 75% power, because pushing the throttle open any further at cruise doesn't net any speed increase in a C172 (airframe drag). 75% power in that engine will be right at 9 gallons per hour; slightly lower if fuel injected with electronic ignition.

Or I could be wrong.
Charlie

#### lear999wa

##### Active Member
Short answer: The same as any other 4 stroke in the same airframe at the same speed, ignoring differences in cooling drag if going from air cooled to water cooled.

The practical, real world answer is that they are 4 stroke, internal combustion piston engines. For all practical purposes, 4 stroke IC engines will burn somewhere between 0.4 & 0.5 pounds of gasoline, per horsepower, per hour. Perfectly tuned water cooled engines under low to moderate load will be at the low end, and closer to 0.5 at full power. Air cooled engines under moderate to moderately high load *might* get close to the low end, if they're running optimized mixture and electronic ignition which can advance/retard timing automatically. Air cooled engines running at near full power typically consume at 0.55-0.6, because they use excess fuel to cool the exhaust valve.

Above is driven by the basic physics & poor thermal efficiency of IC engines.

To do the math:
For easy round numbers & typical midpoint efficiency, let's assume 100 HP at cruise.
100*0.45=45 pounds of gas per hour
Gas weighs ~6 lbs per gallon, so,
45/6=7.5 gallons per hour.
If you're burning less fuel than that, you're almost certainly making less power than 100 HP.
If you're burning more fuel than that, you *might* be making more power, but only if mixture and ignition timing are optimized.

Claims that exceed those parameters are almost without exception 'fish stories'. Example: Cessna 172 drivers are fond of claiming that their 160 HP Skyhawk 'burns 6.5 gallons per hour at 75% power.' It doesn't. If they looked at the operational manual, or Lycoming's operator's manual, they'd see that it can't. What's happening is that they're running at well under 75% power, because pushing the throttle open any further at cruise doesn't net any speed increase in a C172 (airframe drag). 75% power in that engine will be right at 9 gallons per hour; slightly lower if fuel injected with electronic ignition.

Or I could be wrong.
Charlie
Couldn't agree more.

#### TFF

##### Well-Known Member
The description of 75% power is such a misnomer. 75% is still full throttle, but at the altitude where it only makes 75%, correctly leaned. Where I live, being at 230 MSL, most 172s are going to be putting around at 1500 MSL with the throttle pulled back to something like 75% of the knob, full rich. A savvy flyer will lean and putt around at 6 gal an hour, but they are not trying to go anywhere fast, just spend time in the air cheap.

#### pictsidhe

##### Well-Known Member
BSFC has a strong dependency on piston speed. I believe that Yamahas run well north of 2000ft/min, where BSFC is somewhat above and rapidly increasing from it's lowest value.

#### rv7charlie

##### Well-Known Member
If the interwebs tell the truth, the Apex has a 2.29" stroke. The source for that info failed to disclose rpm, but IIRC, the output shaft is 7500 rpm with ~1.2-1 reduction within the case, meaning about 9,000 rpm @ full power. So, doing the math, that's 3435'/min. A Lyc 360 has a 4.375" stroke, so doing the math at 2700 rpm, 1968'/min. But...
Most Lycs 'live' at around 2400 rpm (conservative operators), so, 1750'/min
Where will the Yamaha live? I don't know, but I'll bet they live below 6000 rpm; once the typical STOL plane gets off the ground good, it's aero drag means that power has little affect on speed. At 6000 rpm, 2290'/min. Not that far from your number. (A clean, fast airframe might well result in different numbers.)

Question: I don't know enough about the BSFC/speed relationship. Is it driven purely by piston speed, or does stroke length figure in the equation? Is it a friction thing, or a combustion time vs stroke thing, or...?

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#### pictsidhe

##### Well-Known Member
At low speeds, thermal losses from the combustion gas dominate, these decrease with speed. At high speeds, friction dominates. Friction increases with speed. In the middle is the sweet spot. Centred around about 1500ft/s for most engines. The peak is fairly broad so you can stray +/- 500 ft/s and not lose too much efficiency. After that, it is downhill. Stroke or rpm don't come into it, other than to calculate piston speed. Kind of like the balance of induced and parasitic drag on planes.
I suspect Yamaha people are a little embarrassed by their consumption numbers. They have great power to brag about instead...

##### Well-Known Member
If the interwebs tell the truth, the Apex has a 2.29" stroke. The source for that info failed to disclose rpm, but IIRC, the output shaft is 7500 rpm with ~1.2-1 reduction within the case, meaning about 9,000 rpm @ full power. So, doing the math, that's 3435'/min. A Lyc 360 has a 4.375" stroke, so doing the math at 2700 rpm, 1968'/min. But...
Most Lycs 'live' at around 2400 rpm (conservative operators), so, 1750'/min
Where will the Yamaha live? I don't know, but I'll bet they live below 6000 rpm; once the typical STOL plane gets off the ground good, it's aero drag means that power has little affect on speed. At 6000 rpm, 2290'/min. Not that far from your number. (A clean, fast airframe might well result in different numbers.)

Question: I don't know enough about the BSFC/speed relationship. Is it driven purely by piston speed, or does stroke length figure in the equation? Is it a friction thing, or a combustion time vs stroke thing, or...?
I'm in the process of reaching out to Teal Jenkins to get some good data of these engines; I know he has mentioned 10k rpm at full power, but saw this on a Fakebook post the other day (I posted this in another thread):

I finally got around to checking ACTUAL fuel burn in the Kitfox-4 / Apex today. I did a small XC that totaled 132 nm that included 3 take-offs & 3 landings. I completely topped tanks before, and measured fuel back in tanks to full when completed via cans. I only climbed roughly 1000-1500 AGL typically. Airports were at 4000' (Susanville, CA) & 5000' (Stead/Reno) elevation and made 1 stop at Dead Cow Lake Bed.
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Conclusion:
I think my calculated fuel burn based on injector duty cycle is pretty accurate. I might be a bit high but always better to land with extra fuel right!
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Actual Fuel Burn, In Flight Average: 3.8 gph
Actual Fuel Burn, Total (startup, taxi, run-up, etc): 4.6 gph
Calculated (AEM EFI) Fuel Burn, Cruise: 4.8-5.1 gph
Safe Range, Time: 5-6 hours
Safe Range, Miles: 400+ miles
Note: Range decreases with increased take-off's, landings, and climbs obviously!
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Here is the data if you want to dissect it some other way:
Airplane: Kitfox 4/1200
Engine: 2006 Yamaha Apex, AEM EFI
Total Engine Run Time: 2.12 hours
Total Flight Time: 1.75 hours
Total Fuel Used: 8.0 gallons
Distance: 132 nm
Altitude: 1000-1500 AGL (4000-6500 MSL)
DA: Approximately 6500'
Wind: Variable direction, less than 5 knots typically

#### rv7charlie

##### Well-Known Member
Same data appeared in another thread. No MAP, leaning, or rpm included, so no way to even wildly estimate power from engine info. What would the same airframe do in the same conditions with a different engine?

#### pictsidhe

##### Well-Known Member
If it had been at full power, it would have been by far the most economical piston engine on earth. It's a lot more likely that it was throttled way back.

#### aircraftbuilder

##### Well-Known Member
So the engine is not a gas guzzler. I know the Japanese motors were very fuel efficient but wasn't sure if it would be the same while flying.

##### Well-Known Member
Same data appeared in another thread. No MAP, leaning, or rpm included, so no way to even wildly estimate power from engine info. What would the same airframe do in the same conditions with a different engine?

When we get our bird up flying with that Apex, our goal is to perform all sorts of real data collection. Everything...temperatures, pressures, fuel flows, vibration spectrums, will be catalogued, and we will release all raw data we get.

As an engineer, I am liking the feedback and information that I have been seeing, but what seems to be missing is genuine context. Yes, people are telling me ad nauseum that "it works, is so much better than, and is way cheaper than a Rotax". But, when I ask these operators for data, pure numbers, weights, conditions, etc, many go quiet. So, even though I have one of these engines in my shop that I scored for dirt cheap & absolutely plan to overhaul, mod and test (\$350 for a YG4i to be exact), I am resolved to answer the biggest question of all- "What's the Catch?"

So I have to request of the forum... we are starting to piece the engine part of our test plan together, so please let me know what kind of data you would like to see that will be genuinely useful to the Yamaha aircraft conversion community as a whole? We are looking at data collection from the following channels so far:

1. All basics (RPM, coolant temp, oil press/temp, EGT, fuel flow, fuel press, etc)
2. Vibration Engine, 2 axis
3. Vibration PSRU pinion, 2 axis
4. Vibration PSRU idler, 2 axis
5. Vibration PSRU spur, 2 axis
6. Vibration prop, 2 axis
7. PSRU oil temp
8. PSRU case press
9. Throttle position
10. OAT
11. Press Alt
12. Airspeed, ktas

We are still working on specific test conditions, but implied at present are startup, taxi, runup, takeoff, climb to altitude, altitude change, slow flight, cruise, performance in turns, max level speed, decent & landing. Also considering in-flight shutdown & restart.

On the record, we will be using a SkyTrax PSRU and a Sensenich ground-adjustable pitch prop.