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Old 05-04-2007, 08:28 PM   #1
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Required Horsepowr at 60 mph

When things are going well my Saturn uses 1.2 gph at 60 mph for 50 mpg. That is roughly 7.2 lbs of fuel per hour. It will use .6 gph at the same RPM in neutral. I would like to know how much hp my car is requiring at 60 mph.

Maybe I can find a "calibrated" hill where it will coast at the same speed and work from the weight of the car and the slope of the hill.

What is your best guess of the specific fuel consumption of this Saturn engine at this light load? It is turning right at 2150 rpm.

If I assume the bsfc of this lightly loaded engine is about .55 lbs/hphr that works out to about 13 hp. Primarily I am looking for a best guess for bsfc or a different way to answer the horsepower question.

Thanks, usedgeo.
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Old 05-04-2007, 10:38 PM   #2
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Your car's weight is ~2500lbs=11000N, ~CdA=~.6m^2, ~Crr=.012 and speed is ~24.5m/s we plug and chug into (weight)Crr+.5(ro)CdA(V^2) and get that the instantaneous force on your vehicle at 60mph/sea level/flatground/no bad weather is 11000N(.012)+.5(1.225kg/m^3)(.6m^2)((24.5m/s)^2)=~353N. To get power, we just multiply that by speed, and get that your car requires 353N(24.5m/s)=~8648W or ~8.65kW, which is roughly 11.6 horsepower. Assuming 90% transmission efficiency, this means your car needs 8/65kW/.9)=~9.52kW at the flywheel. Your car uses 7.2 lbs of gas per hour, and a gallon of gas weighs ~6.25lbs, depending. So you're getting (60miles/hour)/((7.2lbs/hour)/(6.25lbs/gallon)=~52mpg. So far everything seems to check out. Going on to BSFC, if your car only needs ~8.65kWh to go 60 miles at 60mph, and your car uses ~7.2lbs=3266grams to do this, BSFC is roughly 3266/8.56=377g/kwh at 60mph. Provided both of our figures are correct, this means your engine is operating at roughly 22% efficiency. For most cars built within the past decade, ~250-300g/kwh is a decent operating point that would bump up fuel efficiency while still allowing for decent top speed on the highway. So, assuming you geared your car so the engine was in the ~275g/kwh range, you would probably see ~70mpg@60mph. Of course, I'm not including any energy lost due to slippage and the suspension, but I'm guessing a safe bet would be ~65mpg@60mph. To put that into perspective, a Prius engine uses ~220g/kwh when it's operating most efficiently but only gets ~50mpg because it's "geared" to provide "acceptable" acceleration by sacrificing ~10-15mpg at ~60mph.

P.s. Imo, an electric fifth wheel definitely makes economic sense as long as you use lead acid batteries and only use it in crowded city traffic.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 05-05-2007, 11:56 AM   #3
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Originally Posted by omgwtfbyobbq View Post
P.s. Imo, an electric fifth wheel definitely makes economic sense as long as you use lead acid batteries and only use it in crowded city traffic.
Thanks, it seems like we are in the same ball park. Hope we are not both out in left field. I was not sure what efficiency to assign at light load.

You made the right assumption about what I was thinking. A 72 volt system on a PERM 132 motor would do it. However as I get my car cleaner aerodynamically there is less and less payback for a major change like batteries and electric motor. I am leaning more and more toward putting that money into an electric trike. The first one was a cheapy for an older brother who can no longer drive. My wife was emphatically opposed to the BugE when I brought up buying one of those though.

http://blueskydsn.com/BugE_Concept.html

On the other hand I was wondering what it would do to the overall efficiency of the vehicle to have a second small engine generating 8 hp with the 8 hp engine optimized for 45 mph or so. This really is just a thought experiment but it seems like it would be easier to optimize a small Honda engine to that load rather than the 1.9 liter car engine. This is a spin off of Displacement On Demand.

The rational thing would be to buy a Jetta TDI but I was just day dreaming about ways to increase low power efficiency.

usedgeo
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Old 05-05-2007, 01:50 PM   #4
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Well, the problem with a small engine is BSFC usually isn't much better than what you're getting anyway. For instance, the minimum BSFC for a small Honda engine is ~310g/kWh, and that's only at maybe ~1500rpms. So, you may be able to put a little direct drive GX 390 on there and cruise at ~35-45mph depending on wind/elevation/etc, while at peak engine efficiency, and maybe use ~1400grams of fuel to go 40 miles and get ~80mpg. But, why bother when you can gear your engine right and get ~60mpg@60mph/70mpg@50mph/etc... I don't really know what it'd be since we would need a BSFC map to determine that, but, I do know that small engines tend to have worse minimum BSFC compared to larger ones, and if you're mostly just cruising along at ~40-60mph, they won't really help out w/ mileage much because they'll be outside of their peak BSFC range, and even if they were in it, they'd still get worse mileage.

Now, as we all get closer to what MetroMPG has done to his vehicle, assuming we keep gearing the same, we end up making the engine run more and more inefficiently, so efficiency mods will have limited effect. Short of making the engine run more efficiently, we can use P&G/etc to break up the engine's operating time, so it makes the same amount of power overall, but by making more in a shorter amount of time, it does so more efficiently. Or, just gear it so it's always in the efficient power band when cruising at out desired speed.

An electric motor/pack would be great for those of us stuck in traffic, and maybe even suitable for someone who can cruise at ~35mph. Electrics are nice, since unlike gassers, and to a lesser extend diesels, as you make your car efficient, range increases by more than that amount for lead acid batteries, because of Peukert's effect. For instance, lets say that you start with a stock Saturn, and have enough batteries to go 15 miles at 30mph, or however far on gas at the same speed. If you cut the Crr and CdA in half, you'll more than double the range on electricity, but only increase gasoline range but maybe 30-50%, because you're making the gasoline engine more inefficient, and having the opposite effect on the electric system. This is why The Toecutter and I will occasionally rant about automotive drag. If we have cars with a CdA of ~.3m^2 and LRR tires, compared to something with a CdA of ~.6M^2 and normal tires, they would go more than twice as far with the same amount of batteries. Or, to put it another way, to go ~100 miles at 55mph as an EV, your saturn would need ~16 60lb T-105 batteries. While the engine weighs a few hundred pounds, the batteries would still only let the car carry one person according to the GVRW. Otoh, with half the CdA/Crr, you would only need 8 batteries to go the same distance, the system would only add a couple hundred pounds, and the cost of the pack per miles would be cut in half. To put it another way, an efficient EV I just described using T-105s would need ~$800 for battery costs every ~50k miles, plus electricity, which is ~ 2 cents per mile. In order for a gasoline vehicle to compete with this, it would need to get ~150mpg at 55mph. It can, but then gradeability/acceleration suffers because the gearing needs to be so tall. Once we get down into the CdA=~.3-.4 and Crr=.005-.007 range, it becomes very hard for a gasoline vehicle to compete with an electric in terms of cost per mile or acceleration. Otoh, with the inefficient designs we have today, it's harder for an electric to compete due to low energy density.

Anyway, you probably already know this stuff, but I might as well be thorough. The best bet for you depends on what you can do and what your routes are like.
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Old 05-06-2007, 07:59 AM   #5
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I was wondering about the whole electric assist thing as well, but in the context of increasing glide only. Under certain conditions, a "small" increase in power might mean significantly longer glide times... I have a 48 V system in my boat... 8 T105's but 4 would work fine with the motor... say attached to a thin bicycle wheel (or something like that, you get the idea)... there might be a high return on energy point in their somewhere. Thoughts?
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Old 05-06-2007, 08:11 AM   #6
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Like the opposite of this? I dunno... 4 T105s plus motor/etc would weigh in at ~300-350lbs, so maybe a 500lb trailer. As long as it was built right, it may even improve aerodynamics? Turn it into a boat tail by having fabric that could bridge the gap between car/trailer? With ~3kWh available, it'd probably work out great for city traffic and extending glides. Plus, the boat tail end should allow for some more storage space... I think it'd work out very well. Just make sure to pick the best combination of capacity/dod/lifetime so battery cost is minimized.
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Old 05-06-2007, 08:23 AM   #7
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So let's take this idea one step further in a different direction. I noticed an older Formula 1 car that had a fan installed in the rear to suck air out from underneath to increase down force. Take this idea.... the fan provides some thrust... but maybe also it can be installed to balance the overall air pressure around the car (like air supplied from the grill) or even from the perimeter of the body where the flow changes from streaming to turbulent, but before it becomes separated, thus somehow extending the streamlined/turbulant flow longer getting more "soon to be bad" air back into the low pressure zone behind the car. When I say "bad" I mean air flow that is about to separate and cause all sorts of drag. Again, only at cruising speed and maybe only during glide. On paper you could get two beneifts... electrical energy converted into a few pounds of thrust, plus better aero.
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Old 05-06-2007, 08:50 AM   #8
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Shoot. That's a tuff one. It may work, but it'd take a lot of testing to figure out when to use it and where to put it. Are you good with CFD software?
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Old 05-06-2007, 10:04 AM   #9
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The SW I've seen is not sophisticated enough, or I suspect powerful enough on home computers, to get to this level of detail. But, I have not made a good exhaustive search so I can't really say... if you know of some interesting SW let me know... I would love to look at it.

I am just thinking that from "feel", if you expend energy to solve two problems, you should be able to do it more efficiently than just one problem... nothing more than that really... so maybe the "add thrust/alter aero" idea could be made to get a ROI of over 50% or higher if the batteries were re-charged with unused potential energy, like the re-generative braking thing.
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Old 05-06-2007, 10:28 AM   #10
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I've messed around w/ gmsh, and it can create pretty fine meshes, but I haven't gotten around to checking out openflower, although with the time I have to me, I probably should. If you ever get around to using it, post up your results. I asked about battery regeneration in one of my earlier posts, and it's doable, but may not be worth the extra complexity. Of course, if you're doing everything on a trailer it'd probably work, but only be worthwhile if you're doing a lot of stop and go.
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