| Author | Message | ||
     Ingemar |
Is there a difference in fuel used at the same RPM’s between 1st gear and 5th gear under full throttle? Oke. I got this question and gave the wrong answer. My theory has always been that rpms in combination with throttle position determine fuel usage. Appearantly I'm wrong. The story is that LOAD requires more fuel, but HOW does more fuel get in if the rpms stay the same and throttle isn't opened any further? In other words, when I in my car, keep the throttle constant, and put my left foot gently on the brake, it will use more fuel? Basically I see an engine as an air pump. It just wants to pump air and the throttle valve controls the amount of air flowing. This in turn sucks more or less fuel. I don't see how load changes airflow or vacuum in a carb to change fuel usage. Acually, I tested my theory in car with board computer that showed me realtime fuel usage. I would keep the car a steady speed on a straight road. Fuel usage would be x. I then shifted (while keeping the throttle perfectly steady) in a higher gear. The car would speed up a little bit and use LESS fuel. Maybe not be the same thing though ... computer controlled fuel injection etc. Who oh who will explain this great mistery to me? | ||
| Hootowl |
If you hit the brake the car will slow down, so you'll have to press on the gas more to maintain speed. (just like going up a hill) The larger opening in the throttle body allows more air to come in. At a given RPM, the amount of air entering the engine can and does change based on the throttle opening because air is compressible. | ||
| Ingemar |
Thanks Hootowl. But the question was at full throttle. You then cannot add more throttle. In my example in the car I said to keep the throttle constant. And that's my point. If I keep the throttle constant, how can fuel usage go up under load?  | ||
| 99buellx1 |
The difference is load, the more load the more work that the engine has to perform to keep the RPMs steady. In order to do the extra work it will need more fuel. In the car scenario you describe not adding more throttle, but that is not the variable, its the RPM. Keeping the throttle constant does not necessarily mean that the RPM didn't change, thats the flaw with that scenario. But the part that I think is getting you, and seems to not be hard to swallow is: How if I'm at full throttle does more fuell get delivered while still at full throttle in another gear. Which I cannot answer. Craig (hopefully I'm on the right track, and this made sense.) | ||
| Bomber |
vacuum guages in the intake tract can help you see this happen so can vacuum operted windshield wipers | ||
| Jlnance |
Ingemar - I think you are right | ||
| Swampy |
But.......With a carb that has an enrichening system such as on most automotive carbs, with the throttle being at any set opening the fuel delivery is dependent on engine vacuum, so when the engine is brought under load(Going uphill) the manifold vacuum drops and the enrichener(Powervalve) opens allowing more fuel past the main jets. Quadrajet carbs used needles in the main jet seats, Holley carbs used a screw in powervalve, most other automotive carbs use some sort of vacuum operated valve arrangement. So the answer is yes, fuel consumption increases with load even at full throttle. Now alot of the industrial and farm engines use speed governors, set the throttle to desired RPM and the governor keeps varying the throttle to keep engine RPM constant, typically no need for a powervalve arrangement. Does anybody need any pictures? | ||
| Evaddave |
I have a similar question at the opposite end of the throttle. Which uses less gas, coasting down a hill in neutral, or coasting down a hill in gear (pick a gear)? And does it matter whether the vehicle is carb'd or FI'd? | ||
| Ingemar |
Evaddave, With a carb, I would *guess* in neutral. With FI, it is definitely in a gear. Any gear that keeps the rpms above, lets say 2500 rpm, because that's when the injectors stay closed. Fuel usage is then NONE. | ||
| Ingemar |
If fuel usage would be dependent on rpms at full throttle, then under load it would slow down compared to no or less load, which brings the rpms down and consequently use less fuel. I can't see any intelligence in a mechanical object that would automagically spit more fuel in the cylinders because you're going uphill at full throttle. The rpms go down, and so does fuel usage, in my theory. But I could very well be wrong, that's why I asked. | ||
| 99x1 |
Most motorcycle carbs are CV (constant velocity), the engine vacuum compared to throttle opening determines the movement of the main needle via the slide diaphragm. (The air under the slide is at a constant velocity). Cutaway at http://www.nightrider.com/biketech/hd_cv_mods.htm | ||
| Benm2 |
The only reasonable explanation that I can see is that its practically impossible to maintain a motorcycle at a fixed rpm at full throttle in first gear. The motorcycle should be accellerating quite rapidly, eh? So the fuel situation for this scenario would be a little richer to account for the fact that each successive revolution is at a higher rpm. The fuel usage at full throttle in fifth at WOT should be more "steady state". The question is how does the typical fuel controller know what gear the engine's in? Aren't typical WOT inputs throttle position, rpm, and air volume (by whatever method; temp/pressure, mass air, etc)? | ||
| Ingemar |
Yeah, but I think we ought to keep it simple. Lets asume a simple bike with ordinary CV carbs. No sensors and computers. WOT 1st gear, WOT 5th gear. Asume in both gears the bike would hit the revlimit. THEY say, 5th gear puts a higher load on the bike (and compare that to you on a bicycle) and uses more fuel. | ||
| Daves |
Vacuum operated windshield wipers. Had those on a 66 Bronco I had about 8 years ago. Going up hills, in the rain, you had to let off the gas every so often so the wipers would wipe! | ||
| Benm2 |
There may be parallels, but I don't know for sure. The slide in a CV carb is controlled by intake vacuum, so its relative position (fully open) would be dictated by how much air is rushing by. This is a stretch, but again if the engine is accelerating in first gear it would be gaining RPM (and airflow volume) quite rapidly. The slide height MIGHT be slightly behind due to inertia. SO, the "steady state" WOT engine at redline might have drawn the slide all the way up, wheras in first gear the positions are more transient. Mostly though, I think I agree with you that the answer is BS. | ||
| Hootowl |
You guys are not taking into account the compressibility of air and the resulting change in the amount of air being drawn into the engine at different load levels even though the throttle angle stays the same. | ||
| Benm2 |
Huh? If the engine is seeing a steady state load, then throttle position must be fixed. If the load changes on the engine, the throttle position MUST change, OR the engine RPM will change. I'm really not sure at all what you're saying. | ||
| Ingemar |
Swampy, I need to chew on your words a few more times. Not sure if I get it. Nevertheless, the load in your example is going uphill. The question was fuel usage in 1st and 5th gear with a fully opened throttle. The load on the engine would then be from a higher gear, all other things being equal (except SPEED!  ). I would think most asume that the load is going to decrease rpms, which would result in a lower manifold vacuum (right?). The question however stated that the rpms are the same, just a different gear. Would you still say fuel usage is going to be higher then? | ||
| Wyckedflesh |
The amount of effort the motor has to put in to move the bike in first gear at WOT is less then the amount of effort the motor has to put in to keep the bike moving in 5th gear at WOT. Therefore the engine will use more fuel in keeping the bike moving in 5th then in 1st. In the real world, you use a carb/FI system that is bigger then what a motor needs for base operation. Therefore a carb/FI system has room at the top to move even more fuel when needed for an extra load situation. IF you used a carb/FI system that was perfectly equal to the base needs of the engine then when the higherload situation arose, the engine would sputter by leaning out to much and therefore be less efficient. | ||
| Swampy |
I think if you are asking about a CV carb the fuel delivery will remain constant at WOT in 1st or 5th. The diaphragm slide will vary by engine speed, and the slide is what controls the fuel delivery. | ||
| Evaddave |
Ingemar, Thanks for answering my question. I don't really have an answer for yours, but over the last day have pondered it a bunch and have some thoughts (anyone with actual *knowledge* on the subject, please correct me): - In a no-load situation, fuel consumption would be identical. Since the transmission is a load, it would be out of consideration, so WOT == WOT. No gears would come into play. - It looks like there's a consensus that a load decreases the RPM. So your initial question is difficult to translate into a real-world scenario. Since in 5th gear there's always more load on the engine than when in 1st gear, there's no place where the throttle position - RPM curves would meet, except for 0 throttle & 0 RPM. - I suggest that the carb's job is to try to add enough fuel for the engine to reach its no-load RPM for a given throttle position. You can see this at less than WOT. Twist the throttle to a particular position, and you'll eventually get to a point where your speed equalizes. The engine will be very near its no-load RPM at that throttle position. How does it do this? By adding more gas until said RPM is reached. I'd guess this is accomplished by the diaphragm slide Swampy mentioned above. - So I think at WOT in 5th, the carb would be adding more fuel than it would at the same RPM with no load (less throttle). However, would that be more fuel than if the bike were running at WOT in 5th at its no-load RPM? That's tough to tell. It's probably dependent on the size of your main jet. Like Wycked said, if you've got room to spare, it might end up using more gas than in first gear. Wow, that's a lotta typing. I'll be quiet now.  | ||
| Ingemar |
I knew there was a reason I shouldn't have bailed outta school .... Thanks guys! | ||
| Hobanbrothers |
The mystery to me is where is there a load sensor and how does it sense a load, can't find one anywhere in the parts catalog? | ||
| Ingemar |
Yeah, that's exactly my problem. The engine just runs and I would think it really doesn't care about load or 1st or 5th gear, or going uphill or downhill. So, although I'm far from an authority on this, I still believe rpm and throttle position determine fuel delivery to the engine. | ||
| Swampy |
Ingemar: I would believe you are correct with a CV carb. | ||
| Buells Rule! (Dyna in disguise) |
Huh? If the engine is seeing a steady state load, then throttle position must be fixed. If the load changes on the engine, the throttle position MUST change, OR the engine RPM will change. I'm really not sure at all what you're saying. I believe Ben is correct. Lets say we use 3000 rpms as our figure. It wont matter if its 1st gear or fifth gear...its the same constant rpm..the motor doesnt know what gear you are in nor does it care. A motor is simply an air pump & the only way fuel consumption will increase is by upping the rpms....or by changing out the jetting. | ||
| Vegasbueller |
HB: I would think it would use a combo of rpm and o2 sensing to detrmine the load, that along with TPS would it not? OR... were you really looking for the answer, and now I look like an azz?  | ||
| Wyckedflesh |
LEts put this whole question into a quick perspective...Ran a little test this morning on the way to work inadvertently but double checked to be certain. Half throttle in 1st gear will hit redline...half throttle in 5th will hit oh about 85...so WOT in first gear is on my bike atleast well beyond redline but since I have a limiter, its basically overkill. Just thought I would throw that out there. | ||
| Evaddave |
I think the "load sensor" is the vacuum. | ||
| Sflabuell |
The amount of fuel to get a bike or any engine, to full throttle is a set number, based on the load. What might produce full red-line RPM under NO LOAD, may be half throttle. In the example given; Full throttle, first gear, you are only producing so much work. We also realize our triumphant rev limiter kicking in rather quickly, because sustained first gear work being performed is less than a full throttle work load at 5th (or 6th) gear. It takes significantly more gas to constantly produce the force to move the bike through the higher speeds. We can realize this by counting on our fingers how often the rev limiter plays into our 5th gear "full throttle" runs. Someone want to run 100 miles at 3000 RPM in second gear, starting with a topped off tank, then measuring the refill. Now do another 100 miles, 5th gear, same 3000 RPM. It'll take a lot less time, but the fuel consumption should be measurably higher in the second run. Same amount of work (FxD)= 100 miles, but the high speed run requires more energy to achieve and sustain the speed. If you check the throttle openings for both 3000 RPM runs, they won't match. | ||
| Opto |
Is there a difference in fuel used at the same RPM’s between 1st gear and 5th gear under full throttle? The key words are same rpm's and full throttle. The load and the gear selected are immaterial, on a given day at full throttle at 5k rpm for example, the amount of fuel injected/used will only change with inlet air temp on an XB, or air density with a carb. | ||
| Hootowl |
"I think the "load sensor" is the vacuum." BINGO! | ||
| Hobanbrothers |
OK, where is the vacuum sensor? We need to go back to the original question, is there a difference in fuel used in a BUELL at the same RPM between 1st gear and 5th gear. Yes, load will determine fuel usage, but what I believe Ingemar is asking is "does the current Buell system, adjust fuel delivery for a given load?" At this point, I will have to say no as there is no way the current FI system has of determining load or gear selection, well I guess gear selection would be easy enough, but it does not bring that into its process. The system monitors O2 at the pipe, intake temp and head temp. It knows RPM and throttle position, that is it to my knowledge. If the box would sense the period of time it is at a certain rpm and throttle position and does a calculation of the estimated load and then changes fuel delivery, maybe, but, to my knowledge, that is not avaliable currently from Buell. | ||
| Ingemar |
Is there a difference in fuel used at the same RPM’s between 1st gear and 5th gear under full throttle? That was the question asked in some questionaire (test your tuning knowledge). No specifications were given, so I asumed a mechanical system with no electronics and 'intelligence'. That is a 4-stroke engine with CV carbs. I answered 'NO', which was wrong. The answer was 'YES', but they failed to explain 'WHY'. To me, these are keywords: full throttle, same rpm, 1st gear & 5th gear. Since rpm are the same, the same amount of air is being pumped, thus the same vacuum is created and the same amount of fuel used. In my theory, that is. Now if you disagree with that, please explain how the gear box can influence that fuel usage. I understand that 5th gear will give a higher resistance on the crank. Nevertheless. the pistons reach the same speed as in first gear, thus pumping the same amount of air. How does this higher resistance (the load we talk about) on the crank result in a higher fuel usage? | ||
| Hobanbrothers |
That really is a loaded question, now that I think about it, there are not enough points to draw a fair comparison. If it would be over the RPM range that it requires more fuel, I could see that as it will take longer to get from 2,000 RPM to 6,000 RPM because of the load applied in different gears, but length of gears will also determine that, so you would have to throw that out?. (But that is not the question, is it?) What FI system? I look at WO throttle (throttle position) and a specific RPM as you have and for that specific instance in time, fuel delivery-injector pulse width (which I would believe to be the definition of usage)is exactly the same, or should be every time in a Buell FI system. I do not think I would put too much weight in that questionaire as there is not enough info to make an informed decision, but given that exact question, you are correct! IMHO | ||
| Aesquire |
I read in a Cycle world article on a "cheater box" that tells the black box the bike is in first, to let you go over the 5th gear limit on speed in a 'Busa. The map is different in different gears, to account for the extra work the bike is doing to overcome drag. I don't know how Buell does it. I do know because our rev limit is lower, there should be no programing to limit top speed, as there is on The faster Suzuki's, etc. | ||
| Hootowl |
The vacuum sensor in FI bike is the MAP sensor. The "vacuum sensor" in a carburetor is the many passages in the air horn that affect the fuel flow to the jets. | ||
| Ingemar |
We will see. I asked techlusion why my answer was wrong and showed him this thread. Just got his answer: By friday they will have a video online that will explain it all in detail. If that's not cool then I don't know what is! | ||
| Opto |
The Oz Buells are delivered with the speed sensor wire to the ecm disconnected, so there is no way the ecm would know what gear my bike's in. It could measure the engine load by measuring how fast the engine spins up compared to throttle opening IF it was programmed to do so. There are just too many things we don't know about a Buell ecm. |
