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 Motorcycle Safety
 Physics and the theoretical
 Gas mileage
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nomad dan
Advanced Member
1276 Posts


Denver, Colorado
USA

Kawasaki

06 Vulcan Nomad 1600

Posted - 05/16/2005 :  5:07 PM                       Like
James R, or anyone else who might know, I just got back from a trip through the most desolate parts of Utah. You really have to watch the mileage in between gas stops because you can easily run out if you choose your route wrong and don’t have a large tank. On one of my tanks, it looked as if I might run out before I got to another town; the distance from one town to the next was 140 miles. And my range is 145 max.

I was running fairly fast which hampers good gas mileage and towards the end I slowed from 80mph down to 55mph to get better mileage. My tank holds 4.2 gal. and I filled up at the first opportunity there was since the last fill up and I put 4.1 gal in, I was lucky.

That got me wondering about trying to get better mileage.

One question that I had was about going up hill and back down hill. If you go up a steep grade and gain say 1000 feet of elevation, then at the top you go back down 1000 feet; is it a wash? Does the amount of extra gas used to go up, get made back up going back down, or is there a reduction in payback going down vs the consumption going up? Let’s assume 65mph going up and down.

And if you are tying to get the best mileage within reason, do you go slower going up, then faster going down, thereby maintaining the same average speed? Say the speed limit is 65mph. I went 75mph up and down before I started getting worried. Once I got worried, I wasn’t sure what the best strategy was, other than just going slower altogether. This was 140 miles of up and down, up and down.

Saving .2 gal could save a 6 mile walk, and I had plenty of time to contemplate such things.


kiddal
Male Advanced Member
1561 Posts
[Mentor]


SE, Indiana
USA

Kawasaki

KLR650

Posted - 05/26/2005 :  11:20 PM
I can't prove it, but my opinion:

I think the most fuel efficient strategy would be to use the lowest possible RPM without bogging down the engine and to judge corners as to minimize brake usage. Every time you touch your brakes, you are wasting some previous acceleration or some future momentum.


Assuming a Fuel Mileage Olympics, there are some other tricks you could use, but they would be dangerous and stupid. Like, you could turn off the ignition on downhills. Carry as much speed as possible through the corners to reduce acceleration required. Lay down on the seat to make it more aerodynamic. Get one of those Tour de France goofy helmets.

Just for emphasis. The previous paragraph contains really dumb and dangerous ideas.
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FRE
Standard Member
219 Posts


Albuquerque, NM
USA

Kawasaki

Ninja 500

Posted - 01/02/2006 :  12:42 PM
There is a trick that has been used to get the maximum fuel mileage on cars, and it would work with motorcycles too, but cannot be recommended.

Accelerate in top gear to about 60 mph at almost full throttle, then coast in neutral (or with the clutch disengaged) until you get to the lowest speed at which the engine will run properly in top gear. At that point, start the engine and accelerate again. Using that technique, it is possible to get dramatically higher fuel mileage; it might even double.

An engine is most efficient when heavily loaded. If the fuel mixture remained constant regardless of throttle position, the engine would be most efficient at full throttle and at approximately the speed at which it delivers its maximum torque. However, there are complicating factors. With some engines, the mixture may exceed 1.0 stoichometric at full throttle thereby reducing fuel efficiency. Also, while engine efficieny increases with load, the force required to overcome aerodynamic drag increases with the square of the velocity thereby causing the power required to increase with the cube of the velocity. That's why fuel mileage decreases as the velocity is increased beyond a certain point.

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Ross
Starting Member
8 Posts


Comox valley, BC
Canada

Yamaha

XJ900XT

Posted - 01/21/2006 :  11:40 PM
I found to get best gas milage is to set your throttle, while accelerating, to the position where you will obtain the maximum speed desired. In other words accelerate very slowly. Maintain that throttle setting for up hill, letting speed bleed off, and gaining extra speed on the downhills. Rolling the throttle off on the down hills is the same as putting the brakes on...it wastes energy. Go over the top of the hill very slowly and let the bike gain as much speed as safe on the down hill. Use the gears to keep the engine running smooth. The main thing is to maintain one throttle setting and don't go beyond it. Now having said that , I don't ride that way at all unless I am really short on gas. I find it is a lot safer, and fun, to often change my speed to match potential hazards.
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Stan
Standard Member
146 Posts


Tempe, AZ
USA

Kawasaki

Nomad 1500

Posted - 01/22/2006 :  8:04 PM
N-Dan,
My carb version of the 1500 has given me a max ave of around 40 mpg, but that's on smooth trips running moderate slopes... like between PHX and Tucson let's say.

On trips into the mountains, the downhill runs do not seem to make up for the extra burn going on the ups... so I can only count on 38 mpg with two-up plus luggage at best.

On my commutes, I can't seem to get better than 36 - 37 riding solo no luggage. For some reason, my mileage right now is real crummy-- not sure if it's the winter gas here, the colder temps, or what.
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John Henry
Male Advanced Member
535 Posts
[Mentor]


Houston, TX
USA

Harley-Davidson

2005 FLHRCI

Posted - 01/22/2006 :  11:53 PM
OK, I don't profess to know any of the right answers on the uphill/downhill scenario, and I would like to know what the actual truth is based on real data. My thought would be that to maximize mileage uphill(let's take them separately, since downhill, the best way is to let off the throttle and just go if you can handle the resulting speed) would be to minimize the fuel usage over that fixed uphill distance, right? OK, no rocket science there, but how is that best done?

Clearly, the most efficient engine performance range would have to be used. This would be in the highest gear I assume, since that gives the highest power output/distance traveled ratio. OK, that's also obvious. There is a fixed amount of "work" (in the physics sense) to get a bike to the top of the hill. That can't be changed. So what is the proper behavior while on the hill? Start fast? Accelerate? Maintain constant speed?

Variables are the speed coming into the hill, and the accelerate/decelerate actions while on the hill. If we assume that the hill is long enough that no matter how fast you start at the bottom, your initial speed will be insignificant to the overall fuel economy.

OK, I don't know the math on this, but my gut feeling is that a constant RPM at the most efficient engine RPM would be the most efficient. This is true on a flat surface, so why not on a hill? This would result in zero acceleration while on the hill, i.e. constant speed. Again, this assumes a very long hill.

What do you experts think?
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James R. Davis
Male Administrator
17372 Posts
[Mentor]


Houston, TX
USA

Honda

GoldWing 1500

Posted - 01/23/2006 :  12:32 AM Follow poster on Twitter  Join poster on Facebook as Friend  
quote:
OK, I don't know the math on this, but my gut feeling is that a constant RPM at the most efficient engine RPM would be the most efficient. This is true on a flat surface, so why not on a hill? This would result in zero acceleration while on the hill, i.e. constant speed. Again, this assumes a very long hill.



I honestly don't know the answer to the question but I can find no fault in your logic. The math turns out to be quite simple as the difference between running on a flat surface and climbing a hill is only the added amount of force needed to overcome the grade - and that is a fixed amount based on the grade.



(I drew a wheel to represent the entire scoot just to save time. Obviously the CG does not go through the center of either the front or rear wheel.)

The blue arrow is Normal gravity force (weight), say 500 pounds. If the angle of the grade (a) is, say, 20 degrees, then the force that must be added to that which is required on a level road is merely mg * sin(a). Thus, in this example, it would be 171 pounds (500 * sin(20) = 500 * .3420) The power required to climb the slope is calculated using P = V * mg * sin(a) where V is the desired velocity.
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Niebor
Ex-Member

Posted - 01/23/2006 :  8:17 AM
I cant prove it with facts available to me here. But I have had to stretch a gallon of fuel on occasion. I believe the most efficient way is to accelerate easily through the gears reaching the desired speed. The engine should be most efficient between the torque and power curves. Therefore, if your desired speed is produced in a gear less than top, so be it. Let it slow to the bottom of the torque curve as you climb a hill. If it goes below, you must shift to stay within the curve. As you pass the apex of the hill, and begin to gain speed, grab the next gear. If no power is required going down, let it roll in top gear. As you near the bottom of the hill, if you are near the bottom of the torque curve, grab a gear as you start in to the next hill. On and on, until you reach the plains, or the next gas station Again, cant prove it with real numbers, just my .02
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FRE
Standard Member
219 Posts


Albuquerque, NM
USA

Kawasaki

Ninja 500

Posted - 01/23/2006 :  10:15 PM
Way back in the early 1960s, BMW did a test to determine the most economical way to drive their cars; that was before fuel injection.

They found the most economical technique to be to accelerate at 2/3 to 3/4 of full throttle and to upshift at about 2000 rpm. With fuel injection, the most efficient way might be somewhat different since FI probably wouldn't deliver an over-rich mixture at full throttle. With many motorcycles, upshifting at such a low speed might not be a good thing to do.

The worst thing was to run in a lower gear than necessary to provide the power being used.

In general (there may be exceptions), an engine is most efficient when heavily loaded at approximately the speed at which it produces maximum torque.

So far as hilly terrain is concerned, fuel efficiency would be improved by coasting downhill with the clutch disengaged. However, in some places, that would be illegal and, if it results in excessive speed or overheated brakes, it would be dangerous.

The throttle valve is a serious source of inefficiency with an Otto cycle (4-stroke, spark ignition) engine. It wastes considerable power to draw air through a partially open throttle valve which is the principal reason that a Diesel engine is more efficient; in general, Diesel engines have no throttle valve.

If you connect an engine to a dynamometer that is capable of motoring the engine (driving the engine at any desired speed with no fuel provided), you will find that the power required to motor the engine varies with the throttle position. The power required is least with the throttle totally closed or fully open, and greatest with the throttle partly open. Contrary to what many people would expect, the power required to motor an engine with the spark plugs removed is quite high.

Incidentally, the wide-spread idea that an engine provides compression braking is simply wrong, with the exception of some truck Diesels which have special equipment to provide compression braking. Engine braking results from pumping losses and mechanical friction within the engine.
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pmdean
Junior Member
71 Posts


Burnsville, MN
USA

BMW

K1200LT

Posted - 01/27/2006 :  1:13 PM
Great information FRE, and I am just anal enough to test it for myself. It goes against intuitive thought that accelerating and coasting is the way to be more efficient. I've always heard I should just lower my speed. When it gets warm enough I think I'll wander out to South Dakota and see how far I can get on one tank of gas. My normal range is about 220. If it works as you say I may see Montana.
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Dean
New Member
11 Posts


Mesa, AZ
USA

Yamaha

81 Seca 750

Posted - 02/02/2006 :  9:57 PM
quote:
Originally posted by FREIncidentally, the wide-spread idea that an engine provides compression braking is simply wrong, with the exception of some truck Diesels which have special equipment to provide compression braking. Engine braking results from pumping losses and mechanical friction within the engine.

I am not sure I agree. When decelerating, the engine is still drawing in air and fuel (at least on a carbureted bike) and then compressing it (intake and exhaust valves are closed during the compression stroke). That's why some high performance motorcycles reduce engine braking by holding valves open during deceleration in order to prevent the rear tire from losing traction.

Back on the fuel mileage topic, a guaranteed way to get much better mileage is to draft another vehicle. I don't recommend you ride too close to a vehicle, but you can stay a ways back from a tractor-trailer rig and dramatically reduce the amount of power required to maintain your speed.
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FRE
Standard Member
219 Posts


Albuquerque, NM
USA

Kawasaki

Ninja 500

Posted - 02/03/2006 :  2:35 PM
quote:
I am not sure I agree. When decelerating, the engine is still drawing in air and fuel (at least on a carbureted bike) and then compressing it (intake and exhaust valves are closed during the compression stroke). That's why some high performance motorcycles reduce engine braking by holding valves open during deceleration in order to prevent the rear tire from losing traction.


Actually, I'm not sure that there is a disagreement. The matter is just a bit more complicated than one would expect.

If the fuel or ignition were cut off, it would be found that there is a certain position of the throttle valve which provides maximum engine braking. Probably, at least with a carbureted bike, the throttle is never closed completely; I don't know about fuel injection - it might depend on the particular bike. I would expect that if the throttle were closed completely, there would be less engine braking than when it is in idle position.

On the intake stroke, it takes power to pull the piston down against the vacuum. With the throttle partly open, the vacuum cannot pull the piston back up all the way on the up-stroke (compression stroke) because some air has leaked into the cylinder through the slightly open throttle valve. However, if the throttle valve were totally closed, then there would be no leakage of air into the cylinder and the vacuum would be able to pull the piston up again thereby reducing pumping losses.

With the spark plugs removed, there is still considerable vacuum above the piston on the intake stroke, at least at high speeds, because the spark plug hole is not very large. On the upstroke, because air has entered the cylinder, the vacuum cannot pull the piston up all the way, so the power lost pulling the piston down against the vacuum cannot be completely recovered. In fact, at some point, there would be pressure above the piston while the piston forces air back out through the spark plug hole, and that also takes power. There would be a certain hole size that would maximize pumping losses and engine braking.

If the cylinder heads were removed, then the pumping losses would be almost zero and any engine braking would be the result of mechanical friction.

The valves are able to permit more air to flow than are the spark plug holes and, if there is very little resistance to air flow in both directions through the open valves, then the pumping losses would be minimal with the valves open and the piston would not have to act against either vacuum or pressure.

Some late-model car engines deactivate the valves on some cylinders when the power from them is not required. With the valves totally closed, pumping losses are virtually zero. Any power required to pull the piston down against vacuum on a downstroke would be recovered on the upstroke. Likewise, any power required to force the piston up against pressure would be recovered on the downstroke. The cylinders which remain active would have reduced pumping losses because the throttle would be open farther thereby reducing vacuum. Thus, cylinder deactivation improves fuel efficiency.

I hadn't known that some bikes provide for keeping the valves open to reduce engine braking, but it certainly makes sense. Perhaps they find that simpler than trying to close or open the throttles completely since that could affect engine response when the rider calls for power and has to control it precisely - there could be a slight lag. It might also unduly complicate the throttle linkage. It's an interesting way to reduce engine braking.

I'll have to think about why 2-stroke engines provide little engine braking. I can see why there would be almost no pumping losses above the piston, but one would think that there would be pumping losses below the piston since the crankcase and the bottom of the piston act as a pump. These things always follow well-established physical principals, so I'm sure that there is a scientific explanation.

It is known that Diesel engines, unless they have special equipment, provide little engine braking. That's because, having no throttle valve, they have minimal pumping losses. That's also the main reason (though not the only reason), that they're more efficient than gasoline engines.

When I studied physics, I found that quite often I had to read explanations like this several times before it would sink in. I always envied people who could get it the first time.
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FRE
Standard Member
219 Posts


Albuquerque, NM
USA

Kawasaki

Ninja 500

Posted - 02/03/2006 :  8:31 PM
Here is a link to a web page on engine braking:

http://www.humvee.net/hid/engine/engbrake.html

Although it is only indirectly related to fuel mileage, it provides some insight into pumping losses.
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Dogugotw
Male Standard Member
117 Posts


Bristol, NH
USA

Kawasaki

2011 Kawisaki EX650R

Posted - 06/17/2006 :  4:21 PM
Some experimental data. When I purchased my bike last fall, the break-in instructions were to keep the rpms below 4000. I was getting 60 - 65 mpg over low traffic back roads (not a lot of lights, I could maintain 4000 rpm most of the 65 mile commute).

I'm now running highway speeds on a freeway (so almost no lights at all) and typically pulling 5500 - 6000 rpm. Mileage is averaging 55.

My conclusion is for a given bit of road, a consistent, low rpm will provide better mileage.
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Indiana Randy
Moderator
2118 Posts
[Mentor]


Fort Wayne, Indiana
USA

Honda

2000 Magna V4 750

Posted - 06/18/2006 :  6:39 AM
Very good info on the efficient operation of an engine, BUT, it seems to me the single biggest factor in gas mileage is drag from wind resistance.

IMHO, the most effective way to increase gas mileage is to reduce speed and wind resistance. Without regard to the way we operate our bike or where our particular engine operates most efficiently, slowing down will yield the biggest increase in mileage, IMO.
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John Henry
Male Advanced Member
535 Posts
[Mentor]


Houston, TX
USA

Harley-Davidson

2005 FLHRCI

Posted - 06/18/2006 :  7:01 AM
quote:
Originally posted by Indiana Randy

Very good info on the efficient operation of an engine, BUT, it seems to me the single biggest factor in gas mileage is drag from wind resistance.

IMHO, the most effective way to increase gas mileage is to reduce speed and wind resistance. Without regard to the way we operate our bike or where our particular engine operates most efficiently, slowing down will yield the biggest increase in mileage, IMO.



"or where our particular engine operates most efficiently"

Randy, I think your post could be inherently conflicting. While I have no data to back it up at this point, I know there is some out there, and it is also intuitive...if you operate your motorcycle in the band where the engine is most efficient, then it has to follow that that speed is where you get the best gas mileage, isn't it? Since all of us operate bikes at a speed higher than the most efficient one (as in highway speeds), your "slow down" comment will be true. But it doesn't follow that going 30 mpg will be more efficient than going 45 mph.

The most efficient speed depends on the bike, the engine and the transmission. I would venture to say that the top gear is the most economical, probably at the lower end of it. If one had 6 or 7 gears, then the most efficient speed would be higher, though at some point, increasing wind resistance would lead to diminishing and even zero returns in the mileage area.

Your points on wind resistance are very valid, though I'm not sure that resistance is the most important factor. It is clearly a factor.
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Dogugotw
Male Standard Member
117 Posts


Bristol, NH
USA

Kawasaki

2011 Kawisaki EX650R

Posted - 06/18/2006 :  9:11 AM
John,

Wind resistance is a huge factor. In cars, the change in mpg going from 55 to 70 is almost entirely due to increased resistance. Granted a bike has a smaller front section pointing into the wind, but dropping resistance makes a major difference. I'm guessing if I started riding layed out on my tank, my mileage would increase by 5 - 10%. I'm just not in the mood to drive an hour with my head tilted up that far!

Doug
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Indiana Randy
Moderator
2118 Posts
[Mentor]


Fort Wayne, Indiana
USA

Honda

2000 Magna V4 750

Posted - 06/19/2006 :  12:34 PM
John Henry, we agree more than we disagree.

Shame on me for not mentioning to be in the highest gear.

I found this on the CarTalk website PBS' Click & Clack;

RAY: Wind resistance increases as a square of the speed at which you're traveling. So, for example, at 70 mph, wind resistance is double what it is at 50 mph (70 squared is almost twice as much as 50 squared). That's murder on your mileage.
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