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dougknight

Well Known Member
Probably a dumb question but why the difference in fuel consumption per hour between different engine types.

RV-9A, 1100 lbs, O-320 Lycoming, carbureted, 2300 RPM (fix pitch wood sensinitch prop) = 8 Gallons/hr (give or take).

VS.

1971 ? Ton Dodge Power Wagon Pick up, 4000 lbs, 318 cu. In. V-8, two barrel carburetor, 2300 RPM (14 miles/gallon and 60 miles/hr) = 4.3 Gallons/hr.

Similar engines running the same duration and RPM, yet the Auto engine burns half the fuel while pulling 4 times the load. Just curious and figured someone here could clue me in.
Thanks
doug
 
I don't want to state the obvious but the truck only goes 60 miles in one hour where the -9 will probably go 160 miles in an hour.
Truck 14 mpg where as the RV gets 20 mpg.
 
dougknight said:
Probably a dumb question but why the difference in fuel consumption per hour between different engine types.

RV-9A, 1100 lbs, O-320 Lycoming, carbureted, 2300 RPM (fix pitch wood sensinitch prop) = 8 Gallons/hr (give or take).

VS.

1971 ? Ton Dodge Power Wagon Pick up, 4000 lbs, 318 cu. In. V-8, two barrel carburetor, 2300 RPM (14 miles/gallon and 60 miles/hr) = 4.3 Gallons/hr.

Similar engines running the same duration and RPM, yet the Auto engine burns half the fuel while pulling 4 times the load. Just curious and figured someone here could clue me in.
Thanks
doug
Click to expand...

It's horse power, Doug.

The V-8 is not producing near the power the 0320 is.
 
Dude, help the economy buy a new truck!

Dave has it right, I'm gonna guess the truck is running at 35% power?
 
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I would also like to add that you would be burning a lot more than 8 gallons an hour in the pickup truck if it was going(if it could) 160 mph. The biggest energy cost is Drag and drag is related to the velocity squared.
 
TSwezey

Not talking about time vs distance, talking time vs fuel consumed while running @ same RPM.

Geico266.Dude, help the economy buy a new truck
Spent all my $ on the plane. ha ha!

I guess I still don't understand how the truck can pull 4 times the load on half the fuel @ 35% power

doug
 
OK, The drag thing helps as does the amount of work, I guess.

If both engines were taken out of their respective vehicles and placed on engine stands side by side and run at the same RPM with no load. Would they burn the same amout of fuel??

doug
 
Compare power not RPM

First, it's a fact that a good airplane engine like a IO-320 can produce 1 hp per hour on 0.4 pounds of fuel. The truck, even with FI, doesn't accomplish that or at least no better.

Second, you are not looking at the power factors, just RPM. You need to look at MAP, too if you want to make a comparison. Your MAP at 60 mph will be about 12 inches, more or less, maybe even less. You need a vacuum gauge which is kinda like an MAP meter.
 
dougknight said:
OK, The drag thing helps as does the amount of work, I guess.

If both engines were taken out of their respective vehicles and placed on engine stands side by side and run at the same RPM with no load. Would they burn the same amout of fuel??

doug
Click to expand...

Think about it this way, Doug.

At the same rpm, the engines are not moving the same amount of air through the cylinders because the stroke and bore of the Lycoming is larger than the V-8.

For the V-8 to suck the same amount of air, it would have to turn at a higher rpm. That's why the fuel burn for the V-8 is less than the 0320 at the same rpm. The fuel/air ration is about the same, for the V-8 it means less air, less fuel burned, and less horse power. One engine is an apple the other an orange.

Your original premise "Similar engines...." is not quite the matter of the situation. They are not similar when it comes to rpm vrs power. You could prove it by installing the 0320 in the truck and properly gearing it. I predict it would not require 2300 rpm to move that truck down the highway but will do it around 1800 rpm and the burn would be about 4.3 gph. Conversely, you could install the V-8 in your airplane at the same time and you will find when it is properly geared it will turn up about 4000 rpm and burn about 8 gph. :)

Don't waste your time installing that V-8 in your airplane expecting to take off and fly around at 2300 rpm with a burn of 4.3 gph with or without a gear reduction. It won't work.
 
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Not True!

David-aviator said:
Think about it this way, Doug.

At the same rpm, the engines are not moving the same amount of air through the cylinders because the stroke and bore of the Lycoming is larger than the V-8.

For the V-8 to suck the same amount of air, it would have to turn at a higher rpm. That's why the fuel burn for the V-8 is less than the 0320 at the same rpm. The fuel/air ration is about the same, for the V-8 it means less air, less fuel burned, and less horse power. One engine is an apple the other an orange.
Click to expand...

They are both around 320 cubic inches which means for every 2 rotations of the crankshaft in a perfect world with 100% volumetric efficiency they would both pump 320 cubic inches of air through them. The difference in this example is the truck is probably running 1/4 throttle while the lyco is pretty much wide open.

The others have it right, it is all about the work being produced which we know as horse power.
 
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Horsepower

The real question is how much horsepower (and therefore fuel burn) does it take to fly a 4000 pound pickup truck. With better aerodynamics :rolleyes: my guess is around 500 HP, which will result in a burn of around 30 to 35 an hour at RV9 speeds. :D

John Clark ATP CFI
FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
Sounds Familiar!

John Clark said:
The real question is how much horsepower (and therefore fuel burn) does it take to fly a 4000 pound pickup truck. With better aerodynamics :rolleyes: my guess is around 500 HP, which will result in a burn of around 30 to 35 an hour at RV9 speeds. :D

That sounds just like my venerable T-6G !! :p
Click to expand...
 
You also have to realize that air makes a pretty lousy transmission as far as efficiency is concerned. The truck can transfer more of it's power to the ground better than even a constant speed prop can transfer power to the air.
 
Aerodynamic drag versus weight

Doug, the difference in weight doesn't matter much when you are comparing fuel consumption at cruise. When your truck or airplane are being accelerated to crusing speed, the rate of fuel burn is affected by the weight, but after you reach cruise speed, the rate of fuel burn is primairly a function of aerodynamic drag. Simply put, the faster you go, the more fuel you burn because you are pushing more wind.

While your truck may weigh 4,000 lbs, once you get that weight up to speed it doesn't take any extra power to keep it at that speed (unless you have to pull a hill, and then your fuel mileage will drop significantly).

So, the short answer is that the airplane burns fuel faster because it is going faster.
 
Don Jones said:
They are both around 320 cubic inches which means for every 2 rotations of the crankshaft in a perfect world with 100% volumetric efficiency they would both pump 320 cubic inches of air through them. The difference in this example is the truck is probably running 1/4 throttle while the lyco is pretty much wide open.
Click to expand...

I don't believe the amount of pumping air is the same at 2300 rpm in the example offered.

If the V-8 throttle is 1/4 open and the 0320 is near WOT, they are not at the same manifold pressure therefore not moving the same amount of air.
 
Yes, but...

David-aviator said:
I don't believe the amount of pumping air is the same at 2300 rpm in the example offered.

If the V-8 throttle is 1/4 open and the 0320 is near WOT, they are not at the same manifold pressure therefore not moving the same amount of air.
Click to expand...

David-aviator said:
At the same rpm, the engines are not moving the same amount of air through the cylinders because the stroke and bore of the Lycoming is larger than the V-8.
Click to expand...


This is true, however, IF both were running wide open they would be flowing approximately the same amout of air, with all else being equal, such as port design, camshaft profile, etc. The V-8 is no where near 100% volumetric efficiency with partial throttle in your example, but the bore and stroke have nothing to do with it's diminished airflow. The bore and stroke being the cause was the only thing I took exception to;).
 
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Don Jones said:
This is true, however, IF both were running wide open they would be flowing approximately the same amout of air, with all else being equal, such as port design, camshaft profile, etc. The V-8 is no where near 100% volumetric efficiency with partial throttle in your example, but the bore and stroke have nothing to do with it's diminished airflow. The bore and stroke being the cause was the only thing I took exception to;).
Click to expand...

Ah, we are on the same page. :)

I was thinking of the H6 just deleted in favor of the O360. The Subby had a displacement of about 180 inches, it had to run at twice the speed of the 0360 to move the same air.

If the displacement is the same, that is not true.
 
David-aviator said:
I don't believe the amount of pumping air is the same at 2300 rpm in the example offered.

If the V-8 throttle is 1/4 open and the 0320 is near WOT, they are not at the same manifold pressure therefore not moving the same amount of air.
Click to expand...

David,
What you were missing in your analogy is that your truck has 8 cylinders instead of the Lycoming's 4 cylinders. Although the bore and stroke differ, the total amount of air/fuel mixture moved through the two engines (let's say it's a 360 ci Dodge versus a 360 ci Lycoming RV) will be similar.
The V8's throttle does not need to be open as far. It is taking 8 smaller gulps of air, rather than 4 large ones. 360 cubic inches is 360 cubic inches, regardless of # of cylinders, bore or stroke.
Charlie Kuss
 
The engines are producing different HPs. Just install torque gauges on the truck and the aircraft, you will see the HPs output is much different between the two engines.

Use this.

HP = (RPM*torque) / 5252
 
Sounds!

PCHunt said:
John Clark said:
The real question is how much horsepower (and therefore fuel burn) does it take to fly a 4000 pound pickup truck. With better aerodynamics :rolleyes: my guess is around 500 HP, which will result in a burn of around 30 to 35 an hour at RV9 speeds. :D

That sounds just like my venerable T-6G !! :p
Click to expand...

Pete,
Nothing sounds as good as a T-6! ;) I hadn't thought of a T-6 when I was coming up with the comparison, but it sure fits.

John Clark
RV8 N18U "Sunshine"
KSBA
Click to expand...
 
Apples and oranges

dougknight said:
TSwezey
I guess I still don't understand how the truck can pull 4 times the load on half the fuel @ 35% power
doug
Click to expand...
The way the energy is being expended is totally different.
The weight of the load on the truck has almost no effect on it's 'cruising' power because it's rolling, not lifting. Starting and stopping is definitely affected by weight (hence the difference between city and highway mpg). Cruising MPG is driven by drag, and going 60 vs. 160 is a huge difference as noted earlier.
Have the truck drive up Pikes Peak with that load and see what kind of gas mileage it gets :eek:
 
Forget about Pikes Peak or any other peak. Even a small incline (less than a few degree) will make the mileage drop to single digits.
 
Define Work

If you think about the definition of work, it equals force over distance.

Assuming similiar efficiencies on the power produced by a gallon of gas, I give the truck engine more efficiency.

For the truck to travel 500 miles @ 60 mph, it will take 8.3 hours and use 35.7 gallons of fuel. If the RV9 travels at 150 mph, it will use 26.7 gallons of fuel over the course of 3.3 hours.

The truck weighs 3 times what the plane does and is the biggest difference that can be readily measured.

If we trade other differences tit for tat, then parasitic drag is probably close since the truck is considerably slower than the plane but has a much larger frontal area so let's call that even. The truck must overcome friction of the tires but the plane doesn't touch the ground but does have induced drag so lets call that even.

If we could strip just 1/2 the weight of the truck's weight away, then in theory, we should burn about 18 gallons of gas (27 mph) which is just about what a 2,000 pound economy car does. Based on this analogy, then the truck engine is a little over 30% more efficient.

Amazing what you dream up when you're a little bored at work. Now go shoot holes in this theory.
 
chaskuss said:
David,
What you were missing in your analogy is that your truck has 8 cylinders instead of the Lycoming's 4 cylinders. Although the bore and stroke differ, the total amount of air/fuel mixture moved through the two engines (let's say it's a 360 ci Dodge versus a 360 ci Lycoming RV) will be similar.
The V8's throttle does not need to be open as far. It is taking 8 smaller gulps of air, rather than 4 large ones. 360 cubic inches is 360 cubic inches, regardless of # of cylinders, bore or stroke.
Charlie Kuss
Click to expand...

Nah, air has inertia, and the plenum/intake manifold on the engines mean that the carb won't see the intake pressure pulses. Velocity through a carb is VERY close to steady state; the air moving through the carb doesn't start and stop as each cylinder begins to move into its intake stroke, so the airflow as a function of time is pretty much independent of the the number of cylinders.

Fuel flow is best measured against time (lbs/min, gal/hr, L/sec, whatever). For two engines of similar design and displacement, they'll use about the same amount of fuel at the same power setting. The power setting is defined by two things; RPM and throttle position. Your truck at 60 mph and 2400 rpm might be at 20% throttle, the plane at 2400 rpm and 150 mph at 80% throttle.......The difference is the amount of air (per second) that is passing through the engine.
 
Lift and drag

Webb said:
Amazing what you dream up when you're a little bored at work. Now go shoot holes in this theory.
Click to expand...

The truck's engine does not have to overcome the drag created by the lift of the wing. Also, at 150 vs 60, remember the drag goes up as the cube of the speed.

John Clark
RV8 N18U "Sunshine"
KSBA
 
Assuming similiar efficiencies on the power produced by a gallon of gas, I give the truck engine more efficiency.
Click to expand...
Umm, assuming "similar efficiencies", they would have similar efficiencies. :D

The truck weighs 3 times what the plane does and is the biggest difference that can be readily measured.
Click to expand...

It is also the difference that matters least. What matters is the amount of work each engine is being asked to produce.

The truck is asked to produce sufficient power to overcome rolling and aerodynamic resistance at 60mph.

The RV9 is asked to produce sufficient power to overcome aerodynamic resistance at 150mph. This is the greater load. It requires a lot more HP to overcome, thus more fuel burn.

I'll bet the Lycoming is making slightly more efficient use of each pound of fuel, assuming 65% power, LOP, etc. etc...

If we are trying to compare apples to apples, the efficiency we are trying to compare is BSFC. Period. The relative disparity in weights, speeds, etc. are just distractions and complications. ;)
 
Ted Johns said:
...If we are trying to compare apples to apples, the efficiency we are trying to compare is BSFC. Period. The relative disparity in weights, speeds, etc. are just distractions and complications. ;)
Click to expand...
Yep, and it is real hard to get the data but I would wager that there is very little difference in BSFC of the two engines when they are operated near their design point. Both would be near what is attainable with a gasoline powered piston engine.

The truck engine would probably be quite a bit worse at 75% power and the airplane engine would be worse at 30% power.

Yeh sure, I know it's Wikipedia, but they have a pretty good overview http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

What I can't get my mind around is that one of the most efficient piston engines ever is that giant two stroke one, up to 0.260 lbs/hp/hour. Two stroke and efficiency just don't belong in the same sentence, to me.

rta96c_crank.jpg

http://people.bath.ac.uk/ccsshb/12cyl/​
 
Yes, size does matter, but more specificially, combustion chamber surface/volume ratio is the key (heat transfer = losses!). Increasing the swept volume improves this, dramatically so when you're talking BIG engines like the Wartsila.

Take a look at the bore-stroke ratio of the Wartsila too. It's very "undersquare" at 0.384, compared to the Lycoming's 1.3+ish. This is the best way of controlling your thermal efficiency for a fixed displacement, so the Lyc's don't fare very well in this regard.

As far as two-strokes go, they can be very efficient, especially as diesels You just can't judge them by lawnmower engine standards where simple and cheap lord it over fuel efficiency. It all depends on how you design it. BTW, the Napier Nomad was also a two-stroke engine (at least for one of its working cycles!)

A
 
The truck engine would probably be quite a bit worse at 75% power and the airplane engine would be worse at 30% power.
Click to expand...

Depends how the airplane engine is configured when putting out 30% power. If it's at 18K MSL, WOT, LOP, pumping exhaust into a much lower than sea level pressure atmosphere... well, I'll bet you it's got the better BSFC. Besides, there's not too many places you can drive your truck at 18k MSL. ;)

If we're talking part throttle operation at sea level, yeah, automotive engine control systems are optimized for that condition.

Seriously cool engine photo, by the way!
 
gtmule said:
Nah, air has inertia, and the plenum/intake manifold on the engines mean that the carb won't see the intake pressure pulses. Velocity through a carb is VERY close to steady state; the air moving through the carb doesn't start and stop as each cylinder begins to move into its intake stroke, so the airflow as a function of time is pretty much independent of the the number of cylinders.

Fuel flow is best measured against time (lbs/min, gal/hr, L/sec, whatever). For two engines of similar design and displacement, they'll use about the same amount of fuel at the same power setting. The power setting is defined by two things; RPM and throttle position. Your truck at 60 mph and 2400 rpm might be at 20% throttle, the plane at 2400 rpm and 150 mph at 80% throttle.......The difference is the amount of air (per second) that is passing through the engine.
Click to expand...

You are probably correct regarding air flow through a carburetor. I haven't seen a carburetor on a light truck or car in almost 20 years!
Power is determined by more than two things. Load also enters the equation. At higher loads, the spark timing must be retarded. No matter how you cut it, this is an apples to oranges comparison. Each engine was optimized for the job it was designed to do.
Charlie Kuss
 
chaskuss said:
You are probably correct regarding air flow through a carburetor. I haven't seen a carburetor on a light truck or car in almost 20 years!
Power is determined by more than two things. Load also enters the equation. At higher loads, the spark timing must be retarded. No matter how you cut it, this is an apples to oranges comparison. Each engine was optimized for the job it was designed to do.
Charlie Kuss
Click to expand...

Well, in a steady state condtion (const speed) the load on two engines making the same power is.....the same.

You retard timing at a given rpm with more load, I agree, but to pull that load along you need.....more throttle. Throttle position, MAP, Boost, whatever, are all measures of load. If there's not enough load there to hold the engine at 2400 rpm and 50% throttle, then you'll either end up with more rpm, or less throttle to maintain speed. The engine doesn't care what the load looks like.
 
Older carbed engine such as the 318 Dodge in question could run very lean at cruise. This produced a lot of NOx which is tightly controlled on modern engines. Modern engine controls make fuel consumption nearly linear with power output not displacement.

An engine like the 0-320 is highly optimized for running at 2400 to 2700 RPM and at high power output. Automotive engines run over a much broader power range than aircraft engines that more resemble generator service.

My 340 ci truck engine (5.6 l) at peak torque puts out 250 HP at 3400 RPM. Its maximum power is 320 at 5200 RPM. When I am towing I am often at 3400 and full power but almost never at 5200. The camshaft and manifolds are designed to produce power over this range. When I am towing and pulling a 5000 lb trailer up an 8 to 10% grade I wish I was burning 4.3 gal/hour my quick calc using 0.52 BSFC (a guess but should be close to full power BSFC) I come out at around 22 gal/hour. Total weight of truck and trailer is about 11,000 lbs. I pull this particular hill I am thinking of at 45 MPH this results in 2 MPG. **** why did I figure this out, that hurts!!

Those enormous marine 2 cycles are quite different from small gas 2 cycles. They do not use the crankcase to pump the air into the cylinders, it is blown in by an air compressor and also turbo charged as well. The bore is approx 1 meter (39 inches) and the stroke is about 2.5 meter (100 inches) per cylinder. Maximum engine speed is about 100 RPM. As mentioned the heat losses is very low resulting in very high efficiencies.

Interestingly this efficiency exceeds the best projected (but never achieved) efficiency of the Hydrogen Fuel Cells that I worked on for years.

Bob Parry
 
fuel burn

heat makes hp
the trucks internal parts are water cooled
the 0-320 uses 33 % of its fuel as cooling
 
wow!!!!!

wow!!!!! I didn't know I needed to know these things to be able to build and fly my own home built aircraft. LOL
 
turbo super rv-10 said:
heat makes hp
the trucks internal parts are water cooled
the 0-320 uses 33 % of its fuel as cooling
Click to expand...

Both engines are usually 100% air cooled. The truck has an intermediate medium.

During climb the O-320 may use a rich mixture to aid cooling, but probably not at any other time.

cyberpilot10 said:
wow!!!!! I didn't know I needed to know these things to be able to build and fly my own home built aircraft. LOL
Click to expand...

Well, as you suspect. You don't.
 
Ted Johns said:
Umm, assuming "similar efficiencies", they would have similar efficiencies. :D

Man was I asleep at the time - I said when bored at work and in this case, I was also napping at the same time. What I meant to say was assumning that a gallon of auto gas and a gallon of aviation fuel have the potential to produce the same amount of energy, then I think the truck is more efficient at producing power from that gallon of fuel.

John Clark said:
The truck's engine does not have to overcome the drag created by the lift of the wing. Also, at 150 vs 60, remember the drag goes up as the cube of the speed.

John Clark
RV8 N18U "Sunshine"
KSBA
Click to expand...

The real question is which engine is more efficient at producing fun..........

John, don't forget that I called parasitic drag even since the plane's speed is higher but has less frontal area than the truck which is much slower and called rolling friction even with induced drag (lift) just for fun and the sake of a theory for everyone to shoot me down......
Click to expand...
 
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The question...

Webb said:
The real question is which engine is more efficient at producing fun..........
Click to expand...

Now you are talking! I can write a long diatribe on the romance of driving my Dodge pickup in traffic vs going flying in my '8'. Nah, somehow I think most of this crowd will figure out the conclusion.

John Clark
RV8 N18U "Sunshine"
KSBA
 
fuel burn

n5lp said:
Both engines are usually 100% air cooled. The truck has an intermediate medium.

During climb the O-320 may use a rich mixture to aid cooling, but probably not at any other time...
Click to expand...



i dont think so
if that were the case dick rutan would have chose the 0-320 and not the water cooled engine they went to a lot of trouble to use the water cooled engine because it is about 33% less fuel burn air cooled engines are not as efficent as water cooled because of thermal dynamics
 
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turbo super rv-10 said:
i dont think so
if that were the case dick rutan would have chose the 0-320 and not the water cooled engine they went to a lot of trouble to use the water cooled engine because it is about 33% less fuel burn air cooled engines are not as efficent as water cooled because of thermal dynamics
Click to expand...

True. But after the big war recip engines in airplanes went air cooled until the jets came along, probably because liquid cooling fails now and then, air cooling does not. The name of game is failure modes and how many are too many?

I've come full circle and am just pleased to be without a psru, liquid cooling, incomprehensible electronics, and a don't miss the cs prop and yellow fuel except a little.

KISS is not a bad thing in airplanes - or anywhere else. :)
 
turbo super rv-10 said:
i dont think so
if that were the case dick rutan would have chose the 0-320 and not the water cooled engine they went to a lot of trouble to use the water cooled engine because it is about 33% less fuel burn air cooled engines are not as efficent as water cooled because of thermal dynamics
Click to expand...
There is no doubt that the Voyager 200 is a very efficient engine. Much more so than the O-200 from which it was derived. It has an 11.4:1 compression ratio as compared to 7.0:1. It also has many other differences including liquid cooling. I think it would take someone intimately involved with the project to hazard a guess as to how much of the increased efficiency comes from what differences.

Perhaps Dick Rutan had some input, but Burt Rutan designed the Voyager.

http://www.sae.org/technical/papers/871042
 
Fuel Burn

Your all way off everyone knows auto engines can't make the HP so therefore they don't use as much fuel..... Just bring your title.
 
fuel burn

my title
im just a country boy from Coonbottom fl
im not very smart but i have a lot of common sense
my 12 hp riding mower burns more than my 27 hp water cooled
case closed
 
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