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Note: this was inspired by [n81641]'s idea for a low power car conversion.
A light car with low power admitedly is a very good idea. But it made me wonder how such a low power/ high economy an vehicle could be made more driveable. And I came up with a theory that with the right gearing, very decent
acceleration and top speed could be available from a diesel engine with 50-60HP.
Take the 1.2L Inline-3 turbodiesel from the Volkswagen Lupo for example. This car has the potential to get 78mpg combined driving because the engine is so small and efficient. Yet the low power causes the car to be anemic and have a 0-60 time of around 18 seconds, a time that very slow cars can usually have complete the quarter mile in. I would imagine that this would be extremely frustrating, especially to those wishing to go green having previously owned cars with decent acceleration. However it seems like there is a way to get the best of both worlds: it's all in the gearing.
Volkswagen's DSG transmission can shift in 8 milliseconds, meaning that the shift time is virtually nonexistent. So what if the gears were spaced extremely closely, such that the top speed would be in increments of 10? Well sure there would be 5 shifts required to get to 60, but only adding 40 milliseconds. This is why I believe it is a good idea; I know that for example big rigs have manual transmissions with over 10 gears and very short spacing, but the problem is that a human cannot shift even close to as quickly as a computer, and with that many gears to row to get up to speed the time really adds up.
So the DSG sounds good alright, but the problem is that it would need more of 10 gears, and there is no way that the current DSG could accomodate that many. That is why I propose a 2-speed final drive ratio, also in the DSG format. This way you could have the main transmision with speeds of, say, 10, 30, 40, 50, 70, and 90, then if the second gear on the final drive was half the ratio of the first hence doubling the speed, it would allow for 20, 60, 80, 100, 140, and 180. Obviously it would require some trick computer programming to sequence the shifting just right, but that is definately doable. And while the 140 and up gears would not add to speed, they would most definately be good overdrives. The great thing about this all is also that the DSG allows skipping of gears, meaning that as long as you don't require the maximum acceleration, only a few of the gears actually have to be utilized.
So basically my idea is for a very small-engined diesel car conversion with trick transmission upgrades. It might not sound like all that much of an improvement, but I would have to believe that it would drastically improve acceleration and if anything help fuel economy.
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Your idea is for and an overdrive DST box on a DST. Why not just make the leap to a MVT which is baked, but still just a delay for taking the last step to the CVT, which is also baked. Heck, I patented one. Zero shift times, infinite gear ratios within a range. |
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But also baked in the Subaru Justy which had about 40 HP and a CVT, though it wasn't a diesel. The downside of diesels in really small cars is the weight, which now becomes significant portion os the weight of the whole car. |
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I guess the problem I see is size and weight. To handle an amount of torque a gear has to be a certain size. In a manual trans you need two of those for each shift. In a DSG you need three. You want to add to that by adding more gears in something that already has so many to begin with. And because you are adding it as an overdrive, you also need to add two more clutches to the two you already had before. |
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So you started with a 7 speed DSG with 21 gears and two clutches and propose to add 6 more gears and two more clutches? The CVT seems easier, lighter and cheaper. |
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I really can't see how this would help acceleration much. I mean the torque curve for a diesel is bloody flat - a good diesel will develop peak torque way down in the 1200-1500rpm range, and hold it until near red-line. That's a very wide power band - why do you nead the ultra-close gearing? Scondly, I can't imagine shift time is a significant factor. |
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Your extra weight you want to use on the final drive would be better used on a hybrid system, a hydraulic over nitrogen regenerative brake system, or a nice paintjob. |
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Well I know that CVTs are far more inefficient than geared, so while it may be able to hold the engine at a speed that propels the car harder, the amount of power that translates will be lower. Also, sure the diesel has a very flat torque curve, but the torque multiplication of the low gearing is what would provide such good acceleration. Think of it logically: it doesn't matter if the torque is the same from idle to redline, it is easier for the engine to pull the car from 20 mph to 30 mph than from 10 to 30. |
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even if the trans can hold the engine at the most efficient load/speed at all times the frictional losses to the transmission itself are very important. Since most time is spent cruising in one gear it could be argued that a very simple light weight trans with minimal friction loss would be most efficient (for a given speed). A direct shaft leading to a differential of the appropriate ratio is the most efficient transmission. At cruise the transmission should behave nearly like this. |
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I keep reading this as "Disco Evil Granny
Conversion." |
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.... which is a quite different, and much more frightening, idea ....... |
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60HP is still 60HP no matter how you gear it. F=MA and all those other goodies from physics class will still apply. |
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Right, but there is still a reason your car doesn't do 100mph in first gear, because it is easier for the engine to lug the car up to smaller increments at a time. Don't get me wrong I'm not saying with this that the car would be a barn burner, but it would undoubtebly shave a few meaningful seconds off the acceleration times. |
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//60HP is 60HP no matter how you gear it// It's not that simple. That 60HP is 60 peak HP, at idle you may get 5. HP is torque x RPM, so although diesels have nice flat torque bands, that doesn't equate to even HP across the band. Also that flat torque curve is a compromise. If you didn't need one, you could probably raise that peak considerably by tuning all passages for a single RPM. So everytime you shift you generally drop down before peak HP climb up to the peak and down the back and then shift again. An ideal CVT matched to a specifically tuned engine will make a huge difference. Ask any trucker pulling a heavy load how much those extra gears mean or just tell me why there are sometimes 24 gears on what used to be called a 10 speed bike. |
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