Efficiency, getting real data

[Oran]

I get a lot of ideas from the internet and living on an island makes me good at finding different ways of doing things. Without having things within easy reach at a local store it’s a case of being inventive with what you’ve got.

[Oran]

It’s been a while since I last posted anything so I thought I would give an update on the efficiency testing.


This is a photo of the test rig I will be using. It consists of a stand for the bicycle and a permanent magnet generator with a roller on one side and a chain drive on the other connected to a drill. In the photo it is powering two 12V, 50 watt bulbs at a voltage of 10V. The next thing to do is make a variable resistor.

To get the mechanical power input to the alternator I will have to take in to account the power loss within the alternator. To do this I will first record the open circuit voltage against rpm. Then when there is a load I can use rpm to calculate the open circuit voltage and multiply this by the current to give the total power dissipated, not just that dissipated by the variable resister.

I will use this setup to record the efficiency of the drill at various speeds and amounts of power output. Once I know this I can add the bicycle and connect the drill to the largest chainring. The rear wheel will drive the alternator via the roller. Then I will hopefully be able to measure the drop in efficiency which should be due to the bicycle transmission.

[Normand_Nadon]

Great rig!

I can't wait to see your results

[Oran]

Update Nov 2013
The results I posted below are now questionable since I discovered how worn-out the hub was (see topic, What’s inside a 2012 model NuVinci N360?). I have no proof that a worn-out hub is less efficient but my experience suggests it is.

Well I have results but of a different kind, here is some more real world data. I have to stress that the following result was obtained under unusual circumstances and is likely to be close to the worst efficiency experienced on a human powered NuVinci bicycle. It does not represent the average efficiency in normal use and throughout the full ratio range. The point was to find the low end of the efficiency scale so that it might be possible to calculate it under more normal circumstances.

This time I have tried to eliminate the unknown factor, power and its relationship to heart rate, by keeping it the same throughout the test. I used the same bicycle and kept swapping the wheel between each test. I tried to keep the same ratio of 24 gear inches for both derailleur and NuVinci which was full under drive for the NuVinci hub. The only difference was the tyres, which on the derailleur wheel was a 2.1 inch with tread and the NuVinci wheel had a 1.6 inch slick tyre.

By maintaining the same effort I managed to keep my heart rates similar enough to make the results of this test viable. My average heart rate with the NuVinci is a bit higher but could be partly due to the NuVinci times being on average 25.5 seconds longer. Therefore the period of time where my heart rate is increasing is a smaller percentage of the total time.

The result is that on average it took 1 min 35 sec to ride the NuVinci hub up the hill instead of under 1 min 10 sec for the derailleur. The resulting power loss within the hub out of the calculated 510 watts input is a shocking 137 watts! The efficiency is working out to be around 73%.

[Oran]

I’m currently sourcing the necessary measuring equipment so it may be a few weeks before I can start any testing.

Here is some more real world data. There is a hill that I ride up on the way to work. It is a climb of 50m in a distance of 550m giving an average gradient of 9%. Before I installed the NuVinci hub I used to climb this hill in the same gear and consistently averaged 7.2mph (11.5km/h). Now with the NuVinci I’m consistently a bit slower averaging around 5.5 mph (8.8 km/h). I use full underdrive which is 24.5 gear inches.
Using the same method of calculation as in the previous post I get a power output of 257 watts for the derailleur and 195 watts for the NV hub. This gives the approx efficiency for the NuVinci hub of 76%.

It will be interesting to see if I can verify these figures with the test rig.

[Normand_Nadon]

Sounds really bad for the N360 in high torque conditions

[FransS]

Oran's results look quite dramatic, but...
I've never done any quantitative testing but used the NuVinci extensively for cycling holidays, incl. climbing with lots of gear.
I've never cycled so comfortable due to the easy shifting and especially with hills the advantage of easy shifting and hence being able to use the right gear-ratio seems to be more advantageous than whatever loss of efficiency. Also I have the impression that when climbing, gravity is the major problem to overcome. This implies that the extra 2kg of the NuVinci doesn't help, but I am still surprised about the low efficiency Oran measured.

Frans

[Oran]

The efficiency of a NuVinci hub is a strange thing. At times I find it’s no issue and I love the way it shifts. In fact I’m often faster not just because I’m using the right ratio but also I know the chain won’t slip. Then sometimes I find myself thinking why is it taking so much more effort to get up a hill I know I could go up faster and with less effort with derailleur gears. Also I would never consider towing 100kg which I can manage with a derailleur bike, the efficiency is too low, 50kg is hard enough.

One thing that has to be remembered is that the efficiency of a CVT is not fixed. It changes with torque and also ratio to far greater extent than a geared transmission.

In some ways the efficiency is close to what I expected and in both cases I was in full underdrive. Fallbrooks efficiency data provided for the wind turbine report stated 87% for a slip ratio of 0.5. However it does seem very low considering that the torque of a pedal stroke approximates a sine wave and what I calculated will be the average. Does this mean the efficiency at the peak power point of the pedal stroke is somewhere around 60%? Can it go that low? There’s still a lot more to learn.

I feel the only way to answer this question fully is to put two power meters on a bicycle and ride it for 1000s of miles while logging the data. A power meter on the crank or built into the chainset to measure power input to the transmission. A second such as the laser spoke to measure power output to the road. Although I have just realised that fitting it to the large diameter of a NuVinci hub will require some modifications.

The cost is an issue and the laser spoke is only a prototype at the moment. If the laser spoke goes into production I might buy one. Then I would seriously consider doing this and rent another power meter.

[Oran]

I shall be able to start testing as soon as I have some time. This is the resister I made the other day using some wire out of an old heater. I cut the strands of wire into lengths with a resistance of 6 Ω. There are to two groups of 6 wires that can be connected in series or parallel depending on which wires are connected to the alternator. By disconnecting individual strands of wire it is possible to get a range of resistances between 12 – 0.5Ω. I know it would be nice to use a variable resistor but finding one with a high enough power rating is difficult and this method gives a surprising number of different resistances. So far it has been able to dissipate 100 watts without any difficulty. If it can manage 200 watts that will be a total of 300 – 400 watts through the transmission which is what I’m aiming for as the maximum I think my rig can handle.

[Normand_Nadon]

wow great work Oran!

I will keep following this thread... don't let go!