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MAX Update No. 62: Throttle Management

11/6/2010 10:57:13 AM

Tags: MAX, 100 mpg, Kubota, diesel, throttle, Jack McCornack, Jack McCornack

I got an e-mail from a guy building his own MAX, asking about how I hooked up the throttle — which turned out not to be quite as trivial as I thought it would be back in Twenty Ought Seven, when this project began.

The problem is, diesels don't usually have throttles, and this Kubota is no exception. Oh sure, we call it a throttle, but it's not. We tend to call anything that controls power a “throttle” — my gosh, airline pilots call their jet engine power controls “throttles” and boy is that a misnomer on a jet. Throttles are usually found on spark ignition engines, such as the gasoline engines that motivate our automobiles, but throttles are not a terribly efficient way to regulate engine power output, and one of the reasons that diesel engines are more efficient than gas engines is diesels don't have throttles.

Maybe Webster's can give us a clue here. Ah yes...

Throttle \Throt"tle\, v. i.

--1. To have the throat obstructed so as to be in danger of suffocation; to choke; to suffocate.
 

--2. To breathe hard, as when nearly suffocated. 

We've forgotten what the word means, over the last hundred years. We talk about giving a car more throttle to make it go faster, when we mash our foot to the floor we call that ”full throttle,” but the truth is, throttling is a way to slow the engine down. If you had a spirited draft horse that pulled so hard that you had trouble jogging along behind the plow, you could make that horse ease up by partially strangling it. Well, that's how your car engine works. When you let off the gas pedal completely, the engine is so throttled that all it can do is idle, until you un-throttle it by stepping on the gas.

MU62aTypically, the throttle is a disk inside the air intake with a round shaft bisecting the disk (it's called a “butterfly” because it looks like one, presuming you have a rich imagination — the shaft is the body and the two halves of the disk are the wings). The shaft sticks through the wall of the air intake tube, so the butterfly can be turned from the outside — turn it one way and the disk blocks the airflow, turn it 90 degrees and the disk is edge-on to the air and doesn't block airflow much at all. In between the two extremes, you have varying levels of strangulation, and no, it isn't very efficient, and at low throttle settings, the engine spends more energy gasping for air than it gives out through the crankshaft. That's the main reason we get worse mileage in city driving than we do on the highway...but back to the subject, how do we hook an engine throttle to a gas pedal?

The standard practice for cars nowadays uses an inner cable in an outer sleeve. Pull the cable out an inch from one end of the sleeve, the cable goes in an inch at the other end of the sleeve, no matter how circuitous the path of the sleeve. One end of the cable connects to the throttle pedal and the other end wraps partway around a “cable cam”, which is a half round metal stamping with a groove on the edge to guide the cable.. If you don't know what I'm talking about, look under the hood of your daily driver; I'm pretty sure you'll find a cable cam in there. You push the throttle pedal, it pulls the inner cable, the other end of the cable is attached to the outside edge of cable cam, and the center of the cable cam is attached to the butterfly shaft. The pedal assembly and the cam are designed so when the driver pushes the pedal flat against the floorboard, the butterfly turns ninety degrees, and air can flow into the engine unobstructed.

Like most diesel engines, the Kubota D1105T that powers MAX doesn't have a throttle. Instead, it controls power by limiting the stroke of the fuel pumps (there's a tiny little fuel pump for each cylinder); less fuel = less power.

Since this “throttle” controls a small amount of fuel instead of a large amount of air, it doesn't have to move very much.

MU62bIn fact at full travel, that fuel pump controller only turns about 30 degrees. Compare this with the 90 degrees of a butterfly throttle and you see the problem: if we use the throttle pedal, cable, and cable cam that came out of the donor car (an ancient Toyota Corolla, in MAX's case) the pedal will only move 1/3 as far and be three times more sensitive than it ought to be.

My solution was to cut a quarter circle's worth of cable-attachment-and-guide off the Toyota's cable cam, weld it to a strip of steel, and bolt the assembly to the “throttle” lever on the engine. Of course I had to figure out how long to make that strip and where to put the bolt holes, but fortunately, I remember everything I learned in 7th Grade Geometry (wow, am I dating myself, or what?) and it was child's play (hey, I was 11 back then) to calculate that I needed four inches from pivot point to cable to get the Toyota's throttle pedal in synch with the Kubota's fuel pump lever.

I could have found the same answer through trial and error, but it would have taken me a lot longer...unless you count the time I spent in geometry class.


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Post a comment below.

 

123gt
12/7/2010 11:52:38 PM
Late to the discussion but the reason for using a kubota engine is very simple and the same issue I had doing a similar project. Try and find an efficient computer controlled diesel engine that small in the USA. To have a fair comparison you have to take into account that its more then just the computer that makes newer computer controlled diesels more efficient

jeff dean
12/1/2010 8:21:16 AM
Used tires would work to see how a "taller" gear would "feel". You could find out if dropping 400 rpm at cruise speed caused lugging of the kubota. I found on the Cummins engine website that worn tires can be more fuel efficient than new tires,up to 7% better for otr trucks. I don't think it would be that much for passenger vehicle tires because of the tread depth. Just one more variable to consider.

Jack McCornack
11/30/2010 8:16:39 PM
Re changing wheel diameter and... > You wouldn't need new tires to try it out I think I would. I think the most we could hope for with a 4% tire diameter change is a 2% fuel economy change, and I want to keep the variables as singular as possible. Different tire designs can make an easy 5% mileage difference, even in the same size tire. I'm very fortunate that Goodyear is taking an interest in MAX, and makes high mileage tires (the aptly named "Fuel Max" line) that are identical in the 14" and 15" sizes, with the singular exception of diameter. > Did you add some framework when you added some body to max? Indeed I did, back in July. There's a pic and an explanation in... MAX Update No. 53: Taking Sides In the Impact Wars ...and you'll find a link to all the MAX Updates just below the blog post, right near where you click for Comments.

jeff dean
11/30/2010 1:13:14 PM
I believe a 1991-96 Escort gt came with 15 inch wheels with the 4 x 100mm bolt circle to fit max. You wouldn't need new tires to try it out, surly the local tire store has some different size used tires to try out to see what works best. It does not look like larger tires would hit the frame in the original pictures. Did you add some framework when you added some body to max?

Jack McCornack
11/29/2010 10:32:16 PM
> If max has rear body damage, now might be the time to try > larger rear tires (different gear ratio). Tires are much > easier to change than diff. gears or complete differential. An excellent point, Jeff. It'll be a tiny bit complicated, but you're right, now is the time for it. The complication is, the axle will have to be moved back a bit so the front of the tires don't hit the frame, which means a new drive shaft will have to be built, but that's the price of research. Now I have to find a set of 15" wheels that will fit the Corolla axle. If I do, I can try the Goodyear Fuel Max 185/65R15 to compare with the 185/65R14 set I got from them recently--it would be a smidge over 4% gearing increase and would drop my cruise revs from 2300 to 2200.

jeff dean
11/27/2010 10:32:06 AM
I believe the kubota diesel in max is similar to a diesel in a farm tractor. The operating range of 1200 rpm is about the same as a farm tractor or diesel pickup truck. Most diesel semi tractors operating range is 300 rpm, but they have more gears to select from. Farm tractor manufactures recommend running the tractor at the lowest rpm within the operating range without lugging the engine. " shift up, throttle back to conserve fuel " Farm tractor diesels don't seem to have any oil problems using them at various rpm. They do save a lot of fuel running them at low rpm for light loads.( Some larger farm tractors use semi tractor engines and they will have oil problems if they are run outside of their operating range. They are generally used for maximum pulling at maximum rpm. ) For maximum loads maximum rpm is best for power and fuel. Cruising on the highway at a constant speed is a fairly light load so the kubota should be at the lowest rpm without lugging. When accelerating, more power needed, more rpm, downshift. If max has rear body damage, now might be the time to try larger rear tires (different gear ratio). Tires are much easier to change than diff. gears or complete differential.

Abbey Bend
11/26/2010 12:37:52 PM
Jack,Thanks for your answers, helps me understand a bit more about how you are approaching the problem you have set out. I was looking at the issue as getting the highest amount of mileage for each B.T.U. available. I see you are taking a different approach, more of a using the fewest B.T.U.s you can with a large margin left over for things like acceleration and I assume pulling. Not a bad way to go but the inherent problems with this approach will most likely keep you from the 100 MPG mark I think. I think two problems will keep you short of your goal, engines work most effectively at the point of maximum torque on the RPM curve and as time goes on an industrial engine being operated at less than recommended RPM tend to fail from improper lubrication over time. Not an absolute but a tendency. Have you done an oil test on the engine? With a 10k budget I do understand about many compromises being needed! Best of luck with this, and thanks again for answering my questions.

Jack McCornack
11/18/2010 2:43:35 PM
Hi again Jeff, > The kubota engine may be most efficient at 2300 rpm for > making 32 hp, but it sounds like you don't need that much > power at 55. The brochure's efficiency curve doesn't tell the whole story, because it doesn't show what horsepower is the most efficient at a given rpm. Engines run least efficiently at high power and low rpm, and low power at high rpm. To keep engine efficiency (horsepower produced divided by fuel consumed) high, as power needs go up, rpm should go up too. So the most efficient rpm for 32 horsepower is 3000 rpm, the most efficient rpm for 2 to 4 horsepower is in the 1200-1500 rpm range, and 2300 rpm is optimal at around 12 to 15 horsepower. With the slippery body, MAX doesn't need 15 horsepower to cruise at 55, so engine efficiency has gone down a smidge with the streamlining, but overall vehicle efficiency (miles traveled divided by fuel consumed) has gone up, since no matter what the revs, less power consumes less fuel. And though the engine isn't terribly efficient below 1500 rpm, if I'm only looking for a couple to a few horsepower, it's more efficient that running at 10% power at high revs, and if I'm loafing through town at 30mph, MAX doesn't need any more horsepower than that. But MAX won't cruise at 55 on four horsepower, so raising the revs raises the overall vehicle efficiency.

jeff dean
11/17/2010 10:00:51 PM
Jack From my experience most vehicles get better mpg with lower engine revs at cruising speed. If max still has power down to 1200 rpm, cruising rpm should be a bit higher than that. The kubota engine may be most efficient at 2300 rpm for making 32 hp, but it sounds like you don't need that much power at 55. If you could increase rear tire size by 35% (from 23" to 31") that would bring your rpm at 55 mph from 2300 to 1500. That sounds about right since you said you run between 1200 and 1500 rpm around town. This would be an inexpensive experiment to see if it would help. You could try different sizes to fine tune it, then you could get the appropriate diff. ratio and tire size. i.e...2.73 gears/29"tires, 2.47 gears/26.4"tires. It doesn't sound like this would be a problem taking off since you said you often start in 2nd gear. 35% less revs at 55mph would have to help mpg in my opinion.

Jack McCornack
11/17/2010 3:28:00 PM
Jeff, I have some answers, but for more details, go to kineticvehicles.com and click on the first picture of MAX, it'll tell you more about ratios and such than you ever wanted to know. Of necessity (1500 character limit) I'll be brief here. ...2700, but I usually cruise at 55 (2300). ...3.30 ...23" diameter ...1800 to 3000 at full pedal, down as low as 1200 rpm if I'm loafing, but the very best is around 2300, 2400 or so. ...with the sreamlined body it has enough power and acceleration to get into trouble if you're not paying attention. ...Yes. See my web site. ...I haven't made changes in tire diameter; bigger tires would need bigger fenders. ...Tolerable, not slow enough to be a hazard but it's sure not MAX's best feature. ...Yes. No. 100mpg is the target, and what makes it an interesting target is the budget; we know we can do it with unlimited money but can we do it for ten grand? I still have some work to do. Still, MAX is currently besting 80mpg for less than eight grand, and if everybody had an inexpensive 80mpg car for easy trips on nice days, it would greatly reduce our nation's fuel consumption. P.S. I'd like to keep this blog a flame-free zone. We have a variety of opinions here and there's room for all of them, let us not jab at each other. Besides, with a 1500 character comment limit, there's not enough word count for a good flame wa...

jeff dean
11/16/2010 8:56:26 PM
Jack I have some questions about max: ...rpm at 65 mph ...differential ratio ...tire size ...recommended "sweet spot" rpm ...feel of power at 65 mph...does it have any acceleration? ...have you tried different ratios? ...larger/smaller tires? (I don't remember reading about any changes in this area) ...how is acceleration from stand still? ...have you considered different transmission? maybe a second trans? Thanks. I hope we can get this thing up passed 100mpg. P.S. abbey may want to go back and read about keeping things simple before commenting again.

Jack McCornack
11/13/2010 3:08:59 PM
The first line of my post below was (blush supposed to be... "Hi Abbey, those are interesting comments." (And Roo and Jan, that doesn't mean your comments aren't interesting, but I only have 1500 characters per comment.) Regarding... > This type of engine is designed to run at a constant RPM > not the variable RPM of an automotive engine. ...and... > this engine has a very narrow operating RPM range, only > 1,200 making it not as flexible to use for automotive use. > http://kubotaengine.com/products/pdf_en/12_d1105t_30.pdf I don't see why this engine would care about variable RPM any more than an automobile engine does. The power curve on the brochure (thanks for the link, Abbey) shows an engine that runs strongly and efficiently from 1800 RPM to 3000 RPM, why would it suffer more than an automotive engine does by being run up and down that RPM range? I suspect that graph doesn't show power output below 1800 RPM because few customers care about lower revs than that, but even if the chart defined that 1200 RPM as the full operating RPM range, it's 40% of maximum RPM and it's easy to stay in the "sweet spot" by shifting gears. In town it seems I do most of my driving in the 1200 to 1500 RPM range (MAX doesn't need much power to go 35 mph) but I downshift if I need quick acceleration

Jack McCornack
11/13/2010 1:47:59 PM
Hi Abbey, those interesting comments. > You do realize all modern transportation engines are > completely computer controlled and the throttle butterfly > has not controlled the air-fuel ration being fed to the > engine since the mid 90’s. My realization has plenty of weak points, so it's never a good bet to assume I realize anything. All I've ever known throttle butterflies to do was control airflow. I know there are a few fly-by-wire automotive systems out there that have servo actuated (electronic) butterflies, but my own mid-90s cars have cable actuated (mechanical) throttle butterflies controlling the air, and electronic fuel injection (EFI) controlling the fuel. In essence, the driver controls the butterfly directly, and the computer reviews a number of inputs (including a sensor on the throttle butterfly, to see where the driver put it) to determine when and how much fuel to add to the mix. Fuel injected diesels have a simpler job than fuel injected gas engines, since they don't need to determine throttle position 'cause same as with mechanical fuel injection, EFI diesels no throttle. > why is he using such an inefficient engine for the job at hand Many engines that look better on paper have low efficiency at low throttle openings, and since MAX cruises on 6 to 8 horsepower, it lives most of its life at low throttle. If you know of a better engine choice, and it fits in MAX's $10,000 budget, you might talk me into making a swap.

Abbey Bend
11/12/2010 7:49:29 PM
Jan, I would suggest you might want to do a bit of studying yourself! While I agree with your statement about of the computer requiring some more exotic materials than someone with your obvious knowledge has any idea of how to handle, if you take the time to look at all modern constructed engines, they all need things like vanadium, chromium and several other “exotic” materials to build and maintain them. While you do make some interesting claims, none of them would hold up to any kind of scrutiny. As I mentioned the modern gasoline engine has no problem breathing around the butterfly valve, there is no mixing of fuel and gasoline near the valve because there is no venture, has not been since 1996 when all engines became OBDII compliant. If the only way you or people you know can figure out what is wrong with a modern engine, diesel, or gasoline is by using a $20,000 code analyzer, then I suggest you find a better mechanic. All you need is a cheap book, and a cheap, less than $160.00 code reader to learn what is going on in the engine. I have used them extensively, my mechanic brother does too, and it never takes more than a few minutes for someone that knows what they are doing to figure out what the computer is telling you. Sorry if people you know cannot read. The reason I asked why he is using such an inefficient engine has to do with using engine Kubota D1105T, an engine designed for use in industrial equipment. (continued)

Abbey Bend
11/12/2010 7:47:56 PM
In addition, this engine has a very narrow operating RPM range, only 1,200 making it not as flexible to use for automotive use. http://kubotaengine.com/products/pdf_en/12_d1105t_30.pdf Therefore, my question remains, why is he using such an inefficient engine for the job at hand, when there are more flexible, more effective automotive engines available for the job. I am in no way condemning the project, just do not understand the reasoning for the choice, and am curious about the reasoning.

Abbey Bend
11/12/2010 7:46:43 PM
This type of engine is designed to run at a constant RPM not the variable RPM of an automotive engine. This leads to less efficiency of use and shorter life of the engine. This has to do with the camshaft profile and timing curve of the engine. Check into what a timing curve is and why diesel or gasoline, it is important for both acceleration, pulling ability, and fuel economy. Therefore, I am not talking about the absolute efficiency of any particular engine, and in automotive use, this has less to do with its maximum potential efficiency and much more to do with the calories of energy contained in the fuel. Look up the difference in calories or joules of energy contained in alcohol, gasoline, and diesel. This is where most of the difference of mileage comes from for automotive use all other things being equal, like weight, co-efficient of friction, tires, and weather. The last computer controlled diesel engine I drove, had no butterfly valve at all, it has no need for such an item. The fuel pump is controlled electronically and only meters enough fuel for the load being used at that moment. As I stated earlier all the butterfly valve does in a modern gasoline engine is control the airflow into the engine and in no way affects the efficiency, one of the reasons modern gasoline engines are able to run easily for 300,000 miles and get very good mileage during their life with minimal care. They never run in a condition of excessive amounts of fuel as a carbureted engine does, washing oil from the cylinder walls, and diluting the crankcase oil. Non-computer controlled diesels also do this when there left to idle, one of the many reasons all modern trucks use them, because besides improving mileage, the computer control extends the life of the oil and the engine making them more efficient in the grand scheme of things. In addition, this engine has a very narrow operating RPM range, only 1,200 making it not as flexible to use for automotive use. http://kubotaengine.co

Jan Steinman
11/12/2010 3:20:39 PM
Abbey, a computer belongs on my desk, not under the hood! I fix my diesels with a wrench, not a $20,000 fault-code analyzer. If something serious happens, a local machinist can make a new part -- it doesn't require electronic components and exotic materials, gathered from all across the earth. If the company goes out of business, I still have a fixable engine -- your computer-controlled engine is a piece of junk. You wrote, "Diesel or gasoline all the butterfly controls is the air reaching the engine." I'm pretty sure you're wrong here, even with computer-controlled diesels. A goal of any diesel engine designer is to get it to "breathe" as easily as possible, which (as Jack mentions) is one reason diesels are more efficient. You don't have to get the mixture just right -- just give it as much air as possible, and control only the fuel. This results in more complete combustion than throttled (computer-controlled or not) gas engines. You wrote: "I do not understand why you are using such an inefficient engine in your car..." Again, I think you're wrong here. The least efficient diesel from 50 years ago is probably more efficient than the most efficient computer-controlled gasoline engine. Diesels often achieve a Carnot efficiency of 40% or more -- the best a gas engine can do is about 33%. You need to do some homework before writing such things!

Abbey Bend
11/12/2010 11:12:53 AM
You do realize all modern transportation engines are completely computer controlled and the throttle butterfly has not controlled the air-fuel ration being fed to the engine since the mid 90’s. Diesel or gasoline all the butterfly controls is the air reaching the engine. Amount of gasoline or diesel injected is completely controlled by the computer, all engines used for transportation have been fuel injected in the United States since the late 80’s. The lack of city mileage has nothing to do with the style or position of the air butterfly valve, (throttle plate) since fuel injection took over as the only method of metering fuel into a modern engine. The computer controls air-fuel ratios in the engine, diesel, or gasoline. The reason for lower city fuel mileage is simply because of the starting and stopping and the engine idling time. The elimination of the engine idling is the main method enabling hybrids to get better city mileage, and the same reason they do not get as high a mileage rating on the highway as the same non-hybrid vehicle. The stop and go mileage, (city) is related to mass being accelerated from a stop, nothing to do with a butterfly in the airstream, partially restricting the airflow. It takes more horsepower to accelerate a vehicle than it does to maintain a vehicles speed, the main reason for high mileage numbers on the highway compared to the city! Granted I only follow your project periodically, but I do not understand why you are using such an inefficient engine in your car, all of the more modern computer controlled engines would be much cleaner to use from a pollution standpoint and many would give you higher mileage, gasoline, or diesel. Also for Roo Trimble, look at motorcycle cables or older style PTO cables for something that will work better for you. The motorcycle cables would work well with a double cable, push-pull system, the PTO cables for single cable system.

Roo Trimble_2
11/8/2010 6:46:55 PM
Hi Jack, good throttle advice, I had similar problems to figure out with Roopod (see www.roopod.com), even though I made my own pedal, I ended up having more pedal travel than I wanted (even with geometry!) now I am redoing with less... BTW, I am using bicycle brake cables for the throttle and shift cables (very nice lined stainless and avail in long lengths) such a simple detail, but has to be right, as Toyota has begun to notice lately...now the shift cable a whole n'other can of worms! can't wait to see MAX back on the Road and hopefully in the next Rally Green along with Roopod... -roo trimble

Roo Trimble_2
11/8/2010 6:46:52 PM
Hi Jack, good throttle advice, I had similar problems to figure out with Roopod (see www.roopod.com), even though I made my own pedal, I ended up having more pedal travel than I wanted (even with geometry!) now I am redoing with less... BTW, I am using bicycle brake cables for the throttle and shift cables (very nice lined stainless and avail in long lengths) such a simple detail, but has to be right, as Toyota has begun to notice lately...now the shift cable a whole n'other can of worms! can't wait to see MAX back on the Road and hopefully in the next Rally Green along with Roopod... -roo trimble

Roo Trimble_2
11/8/2010 6:42:17 PM
Hi Jack, good throttle advice, I had similar problems to figure out with Roopod (see www.roopod.com), even though I made my own pedal, I ended up having more pedal travel than I wanted (even with geometry!) now I am redoing with less... BTW, I am using bicycle brake cables for the throttle and shift cables (very nice lined stainless and avail in long lengths) such a simple detail, but has to be right, as Toyota has begun to notice lately...now the shift cable a whole n'other can of worms! can't wait to see MAX back on the Road and hopefully in the next Rally Green along with Roopod... -roo trimble

Roo Trimble_2
11/8/2010 6:41:59 PM
Hi Jack, good throttle advice, I had similar problems to figure out with Roopod (see www.roopod.com), even though I made my own pedal, I ended up having more pedal travel than I wanted (even with geometry!) now I am redoing with less... BTW, I am using bicycle brake cables for the throttle and shift cables (very nice lined stainless and avail in long lengths) such a simple detail, but has to be right, as Toyota has begun to notice lately...now the shift cable a whole n'other can of worms! can't wait to see MAX back on the Road and hopefully in the next Rally Green along with Roopod... -roo trimble







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