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MAX Update No. 70: MAX's Theoretical Max

4/4/2011 11:44:34 AM

Tags: MAX, 100 mpg, fuel economy, engine efficiency, Jack McCornack

What a miserable month, as is typical of the end of a Western Oregon winter. Rain rain rain snow rain, definitely not road test weather. But thanks to the HBFCI I've been able to learn a thing or two in the relative comfort (it's not raining indoors and that's good enough for me) of my well ventilated shop.

Two updates ago (MAX Update No. 68: Idle Speculation) I reintroduced the Honey Bear Fuel Consumption Indicator, and did some engine efficiency comparisons between MAX and my store-bought minivan. I was curious what we'd save if we shut our engines down at idle, plus I was curious what MAX's minimum fuel consumption might be. Well, I learned that it takes a fair bit of fuel just to make an engine run, and to nobody's surprise, as engine size and horsepower increase, so does the fuel required, even if the engine is just sitting there idling. 

But we're interested in fuel economy here — the fuel it takes to get someplace — so fuel burn at idle isn't the main issue. A more important question is, how much fuel does it take to run the engine at cruise? 

So with the van idling in park, I gradually increased throttle until my ScanGauge showed I was burning 1 gallon an hour, and noted the rpm. I did it a few times and 2150 looked about right; I double checked by holding 2150 rpm and sure enough, the gallons per hour nibbled around between 0.98 and 1.02 so I'm calling it 1 gph.

The sobering thing about this is, this van goes 54 miles an hour at 2150 RPM, so even if the van had no aerodynamic drag and no rolling resistance and no transmission losses at all, the best mileage it could ever hope to get would be 54 miles per gallon. There is no car available — not even in theory — that is so light and streamlined and friction free that it could get 55 mpg with that engine and gearing.

Interestingly, it takes less fuel for each turn of that engine at 2150 than at idle. Idling at 760 rpm on 0.43 gallons per hour, it takes about 1.21 ounces of fuel to turn the engine over 1000 times (you can trust me or do the math: gph divided by 60 gives gallons per minute, divide that by rpm for gallons per revolution, times 128 converts gallons to fluid ounces, times 1000 gives ounces per thousand revs, so 0.43 / 60 / 760 x 128 x 1000 = 1.2070... and since my measurements aren't likely to be perfect, I've rounded to the nearest 1/100 of an ounce). At 2150 RPM and 1 gallon per hour, it's 0.99 ounce of fuel per thousand revolutions (1 / 60 / 2150 x 128 x 1000 = 0.9922...) so for a very simplistic estimate, that engine is about 20% more efficient at cruise than at idle. That's what we want, of course, we want best efficiency while driving, not while idling, and presumably the folks at Kubota also aim for best efficiency in the engine's normal operating range.

So back at the Honey Bear, I found it took MAX an average of 8 minutes and 12 seconds to burn 4 ounces of fuel at 2150 rpm, which is less than ¼ ounce of fuel (0.23) per 1000 revolutions. At idle (3.425 minutes per ounce at 940 RPM) each thousand revolutions sucks up 0.275 ounces, which is still pretty good (an ounce ever 3.425 minutes? In college, I knew guys who consumed Jose Cuervo at that rate, though we hid their car keys first), but (what a surprise) like my minivan motor, MAX's burn per rev was also about 20% better at 2150 RPM than at idle.

Well I know you're curious: if the engine were MAX's only fuel economy limitation, if MAX-the-car had no drag at all and all the engine had to do was turn itself over, what mileage would it get? Since it's geared a little lower than that minivan of mine (MAX goes 52 mph at 2150 rpm) MAX would go 228 miles on a gallon if the engine had no load but its own.

From these tests, a couple of interesting hypotheses spring. For one, presuming these are typical internal combustion powerplants, it doesn't look like driving an ordinary car real real slow is going to improve mileage—and in this case, I'll have to list MAX as an ordinary car—because going very slowly decreases engine efficiency, and as long as the engine is running, it is consuming fuel. So for in-town driving, hybrids have a clear advantage over straight ICE...presuming the hybrid system is light enough and inexpensive enough.

A couple of blog posts ago, I'd concluded that hybridization would do MAX more harm than good, since it looked like the electrical bits (motor, controller, starter/generator, batteries) would increase MAX's weight by about 20%, but a commenter pointed me toward a system he thinks would only bump my weight up by 100 pounds. Unfortunately it's not for sale, and if it was it would cost about five grand, but it's an interesting concept and worth brooding over.

But first, let's see if we can hit 100 mpg the old-fashioned way; it's time to get back to road testing.

Let me conclude with this brain teaser: the first time I tried to time 4 ounces of fuel at 2150 RPM, I barely broke 7 minutes (7:09). I had to take the shirt off my back to get the fuel to last more than 8 minutes, and if you think you know why, click Comments below and send in your best guess.

Photo by Jack McCornack 

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jeff dean
5/12/2011 10:51:15 PM
Jack, You have got a great opportunity to change to a taller gear in Max (at least for testing) because you started with such a short tire. I know you will have to make a few adjustments, but the knowledge will be invaluable, and now looks like the time to do it. If you keep going to a taller tire until you get the rpm down as low as possible, then improvements in aerodynamics will give it drivability along with fuel economy. With that knowledge you could then work on the proper gear ratio for the tires you have to run. Ideally Max should have a very tall differential gear with five or six speed transmission with a 1:1 high gear because overdrives are less efficient than the 1:1. I know that's a lot of change and experimenting for Max but for Max II or Max III or Max XVII in my shop...well I think it would be great. You do all the work, and we get the benefit. Actually I'd love to physically help if we could figure out how to get me to Oregon or Max to Ohio. Keep up the good work.

Jack McCornack
5/6/2011 11:34:26 AM
jeff_58, you may be exactly right. You're definitely right in concept: the most efficient RPM for a given power output moves up as power demand goes up, down as power demand goes down. As far as what RPM is most efficient for the (as yet unknown) power needed to cruise, I can't get any info from the manufacturer except the peak efficiency curve. But if best RPM for 0 horsepower is 900 or so (idle) and the best for 32 horsepower is 3000, then best RPM for the (estimated) eight horsepower I need for freeway cruising could well be 1600 RPM. I could be 2150 too, and it'll take much more testing to and I are both working from educated guesses. Anyway, if I could find a taller final drive gear set for MAX's differential, I'd sure try it.

jeff dean
5/2/2011 8:56:34 PM
Jack, If 2150 is the most efficient rpm,why not set the idle at 2150? Maybe because it's the most EFFICIENT rpm for producing maximum power,but you don't need maximum power for just idling. You need maximum power for acceleration, not idling and not cruising at 55 mph. With max's light weight and aerodynamics, you should only need about half the maximum available hp from the diesel. That amount of power should be available at a much lower rpm than the maximum efficient 2150. 1600 rpm is not the most "efficient" rpm in terms of fuel per 1000 rpm, but it would be more efficient in terms of miles per gallon. The difference of rolling resistance between 35 mph to 50 mph would not be that much but the air resistance would be significant. If you could simulate that extra drag at the slower speed (lower rpm) you could determine the best rpm for fuel consumption and still have drivability. I thought that's what you did with the first non-aerodynamic version. You said you could cruise around at 35 mph in high gear (with the same drag as new version at 55 mpg) at low rpm, very fuel efficient.

Jack McCornack
4/16/2011 1:20:50 AM
Dirty Ray is the winner! And Dirty Ray (can I call you Dirty for short, or do you prefer Ray?), you win a free set of MAX plans as soon as they're ready for release (though of course so does everybody else because the plans will be downloadable from the web site). Yep, you got it. Even convection cooling--no fan, no motion, MAX just sitting there--overcooled the engine enough that it couldn't run at peak efficiency until I draped my shirt over the radiator. It's a subject worthy of a MAX Update; I don't yet know how hot is too hot but it was well over 212F and it seemed it just kept liking it better and better...I was expecting some difference between normal and hot, but I was surprised by how much.

4/15/2011 10:49:27 PM
If an appropriately sized Hydrogen generator could supply enough Hydrogen/Oxygen to help burn the Diesel fuel a little quicker and more completely, it would increase the efficiency and mileage.

Dirty Ray
4/15/2011 8:29:39 PM
Ok Jack, I'll bite, you used your shirt to cover the radiator, because a diesel with no load isn't a real big heat pump.Getting to operating temp and staying there is why we plug the mercedes in before chugging to the Dairy Queen.And that tired old girl will pull 42mpg combined, not bad for 3800lbs. and 270,000 miles.So am I right? Did I win? Is first prize a kubota to toy 5 speed adapter plate? I'm more than a little curious about the diesel/hydraulic hybrid that Big Brown is evaluating. Seems lighter/safer in my twisted mind.

Jack McCornack
4/15/2011 8:16:09 PM
Hi Ronnie, the short answer is, because I tried that in 5th grade and it didn't work then and I don't have any reason to think it'll work now. Somebody able to achieve 125 miles per ounce of water doesn't need to blow their own horn on the internet, the Nobel Prize committee will do it for them.

Ronnie Blair
4/15/2011 3:06:55 PM
I have been wondering why you have not tried to make a hydrogen generator for MAX. I have tried to do some research on the subject and there are lot's of them around but with my limited knowledge of the process, I haven't gotten mine to going. I haven't taken time to really get into it yet though. Anyway, just wondering. There have been generators that have been able to achieve 125 miles per ounce of water supposedly and as small as MAX is, it would probably fly,ha. I appreciate your efforts and aplaude your achievements. Keep up the good work and keep us posted.

Abbey Bend
4/15/2011 11:37:37 AM
No real surprises with your finding. The quick and easy way to find the most fuel effecient RPM for a motor it to simply find out where it develops maximum torque. When an engine hits max torque, it is working at max effeciency and etc... no real testing required. Of course the wrench in the works is all of the other things, like the tranmission and differential, plus tire friction. Several horsepower are required to overcome the losses of these items alone, air drag loss is not that big by comparison. I do not remember what transmission you are using, but the largest horsepower loss in an automobile is in the transmission, however the newest ones are better by several horsepower. Next biggest loss is in the differential, harder to improve on them. My question to you would be how much power are you loosing between these two items? It should be available from the manufacture. Best of luck going forward! :)

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