h a l f b a k e r yVeni, vedi, fish velocipede
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Many car companies now have engines (mostly V-8's) that have control systems to shut down half of the cylinders when at cruising speed and the power of a V-8 is not neccesary. The problems associated with this type of system include burning out the unused cylinders (not so much a problem now, but was
in the past), and frictional losses which limit the power that the now "4-cyl" engine can produce.
While it could potentially cost more than its worth, I propose a clutch that would disengage the half of the engine not running, and would re-engage when the fuel and spark systems are turned back on and the computer senses that the 2 halves are operating at the same RPM (to avoid any unneccesary jerkiness).This would allow for a more efficient V-4 that would make the power of a 4-cyl with the friction of a 4-cyl, and not the power of a 4-cyl with the friction of a V-8 to overcome as well.
A potential problem could be operating the front accessories (A/C, alternator, power steering...) when the front half of the engine is disengaged. Maybe a stronger camshaft that could transfer the power to the front drive accessories...?
Or maybe have the front half run at idle at all times and have the rear half power the car under low power/high efficientcy mode, untill all 8 cylinders are needed and the clutch engages.
To be even more efficient, the part time half (which would run on basically a seperate fuel and ignition system anyway) could run on ethanol or some other "green" fuel source.
I know, I know, we already have systems like this called "electric hybrids" that use electric motors instead of inefficient internal combustion, but most Americans want the "power of a V-8 with the efficientcy of a 4 cyl", not the "power of a 4 cyl and a (insert number, because I really have no clue) watt motor with the efficiently of a 4 cyl."
1958 Citroen 2cv Sahara
http://www.supercar.../CitroenSM/4v2.html
The power of twincylinder boxer engines. - 24 hp [jmvw, Jul 21 2006]
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I think you may be over estimating the impact of frictional losses of the extra cylinders. I would be willing to bet that the added weight of the 2 engine blocks and clutch system would be much more costly to move around than the losses from an unpowered Cylinder(s). Really this is just a far less efficint and heavier version of a Northstar engine. |
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Why not add a gigantic 2nd electric motor to a hybrid car, along with a large capacitor bank. The small engine might take awhile to charge everything, but it'd have great acceleration. |
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I think you may be underestimating the frictional losses in an engine. Its widely know by many old hot rodders that an easy way to gain a few horses is to get a better oil (which helps reduce friction). Hot Rod Magazine gained something like 12 hp on a motor by changing to a synthetic oil, granted it was a lower viscosity, there was still an improvement due to reducing friction.
I know this is a little oversimplified, but if an engine is consuming 4 cylinders worth of air, it should get 4 cylinder feul consumption. If a car is only using 4 of its cylinders, the car should get (approximately) 4 cylinder mileage. Why then does GM's Impala (I'm just picking a car that I am somewhat familar with) get better mileage with its 6 cylinder on the highway than its 8 cylinder, which is only running on 4 on the highway? It's not due to the added weight. That would be like saying that you lost 4 miles a gallon due to carrying two other people in the car with you. The answer is in the frictional losses. |
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I really would be interested in seeing a dyno sheet of anyone who has a D.O.D. engine and could make it run in either mode to compare power. Theoretically, with the same bore x stroke and all that, and half the displacement, you should get half the power. Anything less would show just how much has been lost to friction in the non-running portion of the engine. |
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And I wasnt proposing 2 seperate engine blocks, just a clutch mounted in the middle of the crankshaft inside the crankcase of an enlongated block (to make room for the clutch). |
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- was just wondering if the inlet and outlet values valves are left open in the unused cylinders |
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- if not this would explain why the greater loss is not frictonal --- it is the engergy required to compress the inlet gasses in the unuesd cylinders.... |
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You need to remember that the pressure in a non firing cyclinder does not get very high at all. maximum pressure might be in the 100 to 140 psi range for a decenct engine. This is very little presure and requires very little energy to obtain. |
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In the OE systems, the intake valve doesn't open, so there isn't fuel and air going through the empty cylinder. On this proposed system, this would be irrelevant anyways because the unused cylinders wouldn't be turning anyways. |
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I just realized that this would throw the firing order off and unless you could get the crank clutch to mate to the same point on either side, you would throw everything off (camshaft/crankshaft relation, ignition timing, firing order...). So apparently it is a bad idea. |
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Wow, it sucks having to bone your own idea... |
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Im still not convinced of the increase in efficiency that is claimed. Twice the weight just doesnt add up to the frictional losses that are being cited. The HP loss in the Motor oil example has as much to do with pumping the oil around the engine than it does with reducing friction of the operating engine. You have also neglected to account for the Frictional Losses in the Clutch system. |
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Effeciency is different than HP output. In general Bigger engines get lower milage because they produce more power (which is used to accelerate the car) If those two engines were tuned to the same level, and were given the same load you might find that they are nearly equal in efficeincy. At Idle the 4cyl will win, Under utilized the 4 Cylinder would win, but under power you would find the 8 probably has the edge. |
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I wish I could find a source for the actual number, but something like 25% of an engines power is consumed by internal friction. Have you ever tried to turn an engine by hand? There is quite a bit of resistance. And an oil pump only consumes 5 or so HP at most, probly more like 2 or 3. (If it were any more, you would see electric oil pumps more in aftermarket performance parts catalogs.) |
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And there would not be that much more added weight. The clutch would be the only new piece of equipment, and it would not be very big. There wouldnt be another engine block (in my design... others have proposed something more like this) or anything, Just a two-piece crankshaft with a clutch in between the two halves and possibly a slightly longer block, but it wouldnt weigh that much more. The difference could most likely be made up in useing a lightweight starter that would start the primary half of the engine first until the other half was needed. |
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Earlier, I was referring to fuel efficiency in a vehicle mainly, but efficiency in general also changes. If you took three engines, all tuned the same with the same head design, etc., one of them a four cylinder, one of them a V-8, and one of them a 4 cyl throwing around twice as many moving parts as it needs to with all of the extreemly close tolerances used in modern engines, the V-8 and the true 4 cyl will make equal power and efficiency (relative to displacement), but the 4 cyl with the extra 4 rods, 4 pistons, 2 main bearings, 12 piston rings, 8 (or 12 or 16) lifters, pushrods, and rocker arms will make noticeably less power with less efficiency. |
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And [Lt], whatever happened to "Don't ask, don't tell?" |
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The reason an engine is so hard to turn by hand is due to compression in the cylinders. Take out the spark plugs and the engine will spin quite easily. I dont know where you came up with that 25% number but that seems outragously high I dont think you lose that much horsepower throught the entire drivetrain out to the wheels. |
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But wouldnt the compression be offset by the decompression in the opposing cylinder? |
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And I read that figure somewhere, for the life of me I don't know where. It seemed incredibly high to me too, but I was convinced. The power is lost before the crankshaft output, so it would be difficult to measure compared to the crankshaft vs. axle HP comparison we are used to. The number may have included all internal forces as well (change in momentum of the pistons and such), I really don't know. And I'm too tired to think too much about it. |
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But if the 25% figure is correct, then the gains would be quite noticeable. a 300 HP v-8 would actually produce 400 HP. If you "cut the engine in half," you would expect a 150 HP v-4, but you would in reality only have 100 HP because the motor would produce 200 HP and lose 100 HP before the crankshaft. |
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Slightly off the subject, but I have heard of drivetrain losses of up to 40% on older automatic transmission equipped cars, much higher than the now accepted 10-20% on modern cars. |
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It would work better if the two engines had different displacements: that way you could get 3 different optimal power/fuel-consumption peaks. |
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