planespotting
Posts: 3026
Joined: Sat Apr 17, 2004 4:54 am

Mpg, Other Ratios To Determine Efficiency

Hey dudes,

I was just watching the History Channel, and the programme on was "Modern Marvels: Jet Engines."

Near the end of it, they were talking about the 747, and they mentioned that the 747-400 gets around 746 feet per gallon, which works out to about 1/7 miles per gallon.

Now that sounds pretty low, obviously. But, it got me thinking about how you can't really compare mpg between different modes of transportation.

For instance, my 1997 Oldsmobile Cutlass Supreme gets about 25 mpg on the highway in good conditions. If I use my car as efficiently as I can, that means I can transport 4 human beings (myself and 3 others) fairly comfortably across a certain distance at 25 mpg. So, I think of that as the passenger mileage, which works out to 80 passenger miles per gallon of gasoline (PMPG).

A 747-400 can carry approximately 415 passengers in a 3 class configuration. At 1/7 mile per gallon, that works out to about 59 PMPG. Still though, my old Cutlass is more efficient than the venerable 4-holer.

However, once you throw time into the system, the 744 becomes quite a bit more efficient than my car.

Consider a 500 statute mile trip. My Cutlass would be able to make this trip in a driving time of about 7 hours and 9 minutes (68 mph average speed). The 744 would be able to do this trip (considering taxi, approach and landing) in around 1.5 hours (45 mins cruise @ 480 knots [552mph], 45 mins taxi, departure, approach and landing).

So, if you divide the PMPG by the trip time of a given distance, you get the "true" efficiency of the mode. For the Cutlass, for this distance of a trip, that works out to 11.19 PMPG/Hour. The 744, in this instance, comes in over 3x better than my car, at 39.33 PMPG/H.

Am i correct in thinking that this would be a truly accurate way of comparing the "true" efficiency of different modes of transportation? I just thought of it a few mins ago and only had some non-aviation friends to relay my thoughts to (online), and none of them seemed to really care, haha. I know all about CASM and RASM, but this takes the length of the trip into account, so it would seem like a fairer way to measure the cost of a trip in a more general way, as opposed to airline/system specific.

Does anyone have any suggestions or comments?

Bobster2
Posts: 1523
Joined: Mon Jan 17, 2005 3:04 am

RE: Mpg, Other Ratios To Determine Efficiency

The scientists nowadays measure efficiency by the amount of carbon generated. Time doesn't matter to them. Cost doesn't matter. Nothing matters but carbon. Airplanes have low carbon efficiency compared to cars, trains, buses, or most other forms of travel over land. I'm not sure about ships, maybe planes are better for getting across the ocean.

[Edited 2006-08-10 03:02:18]
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison

planespotting
Posts: 3026
Joined: Sat Apr 17, 2004 4:54 am

RE: Mpg, Other Ratios To Determine Efficiency

 Quoting Bobster2 (Reply 1):The scientists nowadays measure efficiency by the amount of carbon generated.

haha, I didn't really mean most chemically efficient mode of transportation.

I am talking about the kind of efficiency that may matter more to business or economics. In my example above, I am speaking about efficiency measured by total miles per person traveled per gallon of fuel over a variable amount of time.

The unit I came up with was "Passenger Miles Per Gallon Per Hour."

liedetectors
Posts: 323
Joined: Thu Jul 21, 2005 10:44 am

RE: Mpg, Other Ratios To Determine Efficiency

 Quoting Bobster2 (Reply 1):The scientists nowadays measure efficiency by the amount of carbon generated.

Or if you want to sell an engine, you advertise its SFC.
If it was said by us, then it must be true.

OldAeroGuy
Posts: 3298
Joined: Sun Dec 05, 2004 6:50 am

RE: Mpg, Other Ratios To Determine Efficiency

 Quoting Planespotting (Reply 2):The unit I came up with was "Passenger Miles Per Gallon Per Hour."

You're on the right track. If you substitute Lbs of Fuel for Gallons, this indeed a measure of efficiency used by the airline industry.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Mpg, Other Ratios To Determine Efficiency

A standard used for reference for many years was 'specific range' which was usually expressed in nautical air miles per thousand pounds of fuel. (NAM:K)

A nautical air mile is determined without reference to wind. So it uses TAS and not GS to determine. Somewehre I have a nifty four-part equation for solving it on either hand-held calculator or whizwheel.

The fuel burn of each engine on each airplane in the fleet is monitored by the company. Mostly the trend data is downlinked automatically.

It might go something like this: Thirty minutes after leveloff above FL300 on the first flight of each GMT day, the system downlinks for each engine the EPR (if applicable) N1 N2 N3 if applicable, EGT, fuel flow, oil temperature, and for the airplane, the FL, TAS, M# as well as SAT/TAT. All of these things are plotted in trend-monitoring software and sudden excursions in any of these performance factors are investigated as early signs of impending problems. I'd certainly welcome comment from the maintenance side of the house on this.

Probably every US air carrier has an office that does nothing but study fuel usage and how to improve those stats.
Happiness is not seeing another trite Ste. Maarten photo all week long.

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Mpg, Other Ratios To Determine Efficiency

The basic measure of passenger airliner economics is the cost per seat-mile. The lower this is the better. You can do that calculation for your Cutlass too, come to that.

Your 500 mile drive uses 20 US gallons of gas. At around \$3.00 per gallon that's \$60 for the journey (2000 seat-miles) and so \$0.03 per seat-mile (as the driver counts as a passenger too, unless you drive a taxi   ).

Jet A fuel is \$2.21/gal, so the seat-mile cost is \$0.04 for the 744. However, a 744 is not suitable for 500nm sectors (except of necessity in Japan where pax volume is crucial).

The 746 feet/gallon is a very crude average of fuel capacity divided by range at maximum fuel. On a 500 mile flight you wouldn't have anywhere near full tanks, and the last 120 miles would be at idle in descent. So over 500 miles the 744 might beat the Cutlass on seat-mile costs alone and get there much quicker.

Coast to coast, the Cutlass would start to look more attractive cost-wise, but only if you have the time and energy to make the drive.

Time doesn't really enter the economic equation, as slower cruising Airbus and 767 would not seem so attractive. Also Concorde would have appeared highly efficient, and sold in 100s.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

planespotting
Posts: 3026
Joined: Sat Apr 17, 2004 4:54 am

RE: Mpg, Other Ratios To Determine Efficiency

 Quoting Planespotting (Thread starter):So, I think of that as the passenger mileage, which works out to 80 passenger miles per gallon of gasoline (PMPG).

and I made a mistake...that would be 100 passenger miles per gallon (25 x 4 = 100, not 80...don't know what i was thinkin)...so that works out to 13.99 PMPG/H.

 Quoting OldAeroGuy (Reply 4):You're on the right track. If you substitute Lbs of Fuel for Gallons, this indeed a measure of efficiency used by the airline industry.

hmmm, okay. have to rework the calculations a bit now, but that makes sense.

I believe a gallon of Jet-A weighs around 7 pounds/gallon...sooo, that means the 747-400 gets 106 feet per pound of fuel burned (746/7). I'm gonna make an assumption that typical automotive gasoline weighs about 6 lbs per gallon, so that means my car gets about 3.57 miles per pound (m/p) of gas burned (25/7). Divide the 106 feet by 5280 and you get about 1/50 m/p --> (.02007).

1997 Oldsmobile Cutlass Supreme
3.57m/p x 4 passengers = 14.28 passenger miles per pound (pm/p)
14.28 pmpp/7.15 hours (trip time) = 1.997 pm/p/hour

Boeing 747-400
.02m/p x 415 passengers = 8.3 pm/p
8.3 pm/p/1.5 hours = 5.533 pm/p/h

planespotting
Posts: 3026
Joined: Sat Apr 17, 2004 4:54 am

RE: Mpg, Other Ratios To Determine Efficiency

 Quoting Jetlagged (Reply 6):Time doesn't really enter the economic equation, as slower cruising Airbus and 767 would not seem so attractive. Also Concorde would have appeared highly efficient, and sold in 100s.

Ahh but yes it does, from a macro-economics point of view.

Mainly, I was trying to make a point and see if it was valid or not, from a plane v. car point of view. I only am using the 747-400 for this because it was the only specific miles per gallon thing that I had. I realize it is an extremely general number, but it was mainly used to represent the air transportation mode in general.

But, time does factor into the equation in terms of the economic benefits added to the economy as a whole. For instance, if a business man in Omaha had something to attend to in Indianapolis that would only take one day, it would benefit him, his company, the other company, his family, etc...if he were to be able to go do so without having to spend an extra 2 days of driving. His efficiency goes up overall if he can be there and back within a time frame of 13 hours or so. Figure about 6 hours spent at the airport, in the airplane (both ways), and driving to/from...that leaves him 7 hours to finish whatever business he had in Indianapolis. If he can leave his house at 6am and be back by 9pm, he will be able to live his life without any kind of major disruption (hotel, renting a car, etc...).

That is the efficient benefit of flying an airplane. I've come to realize that this isn't necessarily something an airline would really look at that much, but to an economist or someone looking at the big picture, this could really play a major factor.

But, anyway, just my 2 cents...

OldAeroGuy
Posts: 3298
Joined: Sun Dec 05, 2004 6:50 am

RE: Mpg, Other Ratios To Determine Efficiency

 Quoting Jetlagged (Reply 6):Time doesn't really enter the economic equation, as slower cruising Airbus and 767 would not seem so attractive. Also Concorde would have appeared highly efficient, and sold in 100s.

Time, in the form of speed, does matter. It appears in the Breguet Range Equation and is an important factor in economic terms such as crew costs.

Consider that airplane efficiency can be expressed in terms of M*(L/D).

For the 777: M*(L/D) = .84*20 = 16.8

For Concorde: M*(L/D) = 2.2*4 = 8.8

From the airframe stand point alone, a 777 is almost twice as efficient as the Concorde. When you throw engine TSFC, you probably double 777 efficiency again. (I'm too lazy to research Olympus TSFC for this thread)
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Mpg, Other Ratios To Determine Efficiency

 Quoting OldAeroGuy (Reply 9):It appears in the Breguet Range Equation and is an important factor in economic terms such as crew costs.

Of course speed is part of any range calculation, so is specific fuel consumption. The two units of time cancel each other out.

As for crew costs, and other time based operating costs, point taken.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

redcordes
Posts: 244
Joined: Wed Jan 18, 2006 6:28 am

RE: Mpg, Other Ratios To Determine Efficiency

Here are some comparative numbers for types of engines:

Engine type SFC
Ramjet 1.0 lb/(hp·h) (0.61 kg/(kW·h))
Turbo-prop 0.8 lb/(hp·h) (0.49 kg/(kW·h))
Otto cycle 0.5 lb/(hp·h) (0.3 kg/(kW·h))
Diesel cycle 0.4 lb/(hp·h) (0.24 kg/(kW·h))
Otto-Compound engine 0.38 lb/(hp·h) (0.23 kg/(kW·h))
Turbocharged Diesel 0.38 lb/(hp·h) (0.23 kg/(kW·h))
Turbocharged & Intercooled Diesel 0.36 lb/(hp·h) (0.22 kg/(kW·h))

Also, here is a link to a webpage of the most powerful and most fuel-efficient engine in the world (109,000 hp, 0.26 lb/hp/hr). Check it out.

people.bath.ac.uk/ccsshb/12cyl/

[Edited 2006-08-11 00:12:00]
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