Shell Eco Marathon Project - Need Your Opinion

[sohaibafzal]

Hi

I am part of a team hoping to make an electric prototype for the 2011 Shell Eco Marathon Asia. I am posting in 'E-Bike Technical' because you guys have more experience with the kind of motors we will be working with.

The vehicle is very light and aerodynamic, and our analysis shows that we would need around 500W continuous power from our motor. A single motor will power the solitary rear wheel, while the front two wheels will be un-powered and used for steering.

Since the Eco Marathon is all about efficiency, we are interested in finding light motors with their peak efficiency in the power range 200W - 500W. We are restricted to using a 48V battery. Currently we are looking at the Cute 128 SX hub motor. Is there a better (more efficient) option?

My second question is about LiPo batteries, since I have never used them before. I have read in this forum that we can discharge them to 80%. Does this mean that I can use 8Ah from a 10Ah battery?

Thanks!

Our project page: http://emenents.com/?page_id=173

 

[Jeremy Harris]

Hi

I am part of a team hoping to make an electric prototype for the 2011 Shell Eco Marathon Asia. I am posting in 'E-Bike Technical' because you guys have more experience with the kind of motors we will be working with.

The vehicle is very light and aerodynamic, and our analysis shows that we would need around 500W continuous power from our motor. A single motor will power the solitary rear wheel, while the front two wheels will be un-powered and used for steering.

Since the Eco Marathon is all about efficiency, we are interested in finding light motors with their peak efficiency in the power range 200W - 500W. We are restricted to using a 48V battery. Currently we are looking at the Cute 128 SX hub motor. Is there a better (more efficient) option?

My second question is about LiPo batteries, since I have never used them before. I have read in this forum that we can discharge them to 80%. Does this mean that I can use 8Ah from a 10Ah battery?

Thanks!

Our project page: http://emenents.com/?page_id=173

Welcome to the ES Forum.

First off, as this is a competition, you can run the battery pack right down to cut-off (100% of capacity) as you're not going to be that worried by losing some cycle life from the pack (my guess is that you will only use the vehicle a few times).

An off-the-shelf hub motor won't be very efficient if used in a wheel as intended. They are probably around 80 to 85% efficient at best, less when run at speeds below their peak efficiency point and it will be hard to get them to work at their most efficient rpm when you can't change the gearing.

If you have the budget, then the Australian CSIRO motor is probably the very best available for what you want to do, but it is very, very expensive (many thousands of dollars for the kit). If you are on a budget, then you may be better looking at using a direct drive hub motor driving the rear wheel through a chain or toothed belt reduction drive, rather than as a hub motor. If you were to use an over-rated motor, and then under-run it, you will probably get the motor efficiency up to around 88 to 90%. I've done this with smaller motors on an electric boat drive system and it's worked well, giving much higher efficiency than the motor specifications would suggest.

Others here probably know more about direct drive hub motors than I do, but I would try and find one with the highest Kv (rpm/volt) that you can and the lowest possible winding resistance. This will minimise losses, because the core losses at the low rpm and power level such a motor would be working at would be fairly low and your overall electrical losses will be dominated by resistive power loss in the motor and electrical system.

Looks like an interesting project, I wish you well with it.

Jeremy

 

[sohaibafzal]

Welcome to the ES Forum.

First off, as this is a competition, you can run the battery pack right down to cut-off (100% of capacity) as you're not going to be that worried by losing some cycle life from the pack (my guess is that you will only use the vehicle a few times).

An off-the-shelf hub motor won't be very efficient if used in a wheel as intended. They are probably around 80 to 85% efficient at best, less when run at speeds below their peak efficiency point and it will be hard to get them to work at their most efficient rpm when you can't change the gearing.

If you have the budget, then the Australian CSIRO motor is probably the very best available for what you want to do, but it is very, very expensive (many thousands of dollars for the kit). If you are on a budget, then you may be better looking at using a direct drive hub motor driving the rear wheel through a chain or toothed belt reduction drive, rather than as a hub motor. If you were to use an over-rated motor, and then under-run it, you will probably get the motor efficiency up to around 88 to 90%. I've done this with smaller motors on an electric boat drive system and it's worked well, giving much higher efficiency than the motor specifications would suggest.

Others here probably know more about direct drive hub motors than I do, but I would try and find one with the highest Kv (rpm/volt) that you can and the lowest possible winding resistance. This will minimise losses, because the core losses at the low rpm and power level such a motor would be working at would be fairly low and your overall electrical losses will be dominated by resistive power loss in the motor and electrical system.

Looks like an interesting project, I wish you well with it.

Jeremy

Thanks for the reply.

The CSIRO motor is too expensive, and its power rating and size are also not appropriate... Our vehicle can work easily with a 500W rated motor.

By over-rated, do you mean that I should look for, say a 60V rated motor and run it at 48V?

If a motor is geared down internally, will that be ok? Or should I always prefer getting a direct drive motor and then gear it down using a chain as you said?

Thanks again!

 

[Jeremy Harris]

Thanks for the reply.

The CSIRO motor is too expensive, and its power rating and size are also not appropriate... Our vehicle can work easily with a 500W rated motor.

By over-rated, do you mean that I should look for, say a 60V rated motor and run it at 48V?

If a motor is geared down internally, will that be ok? Or should I always prefer getting a direct drive motor and then gear it down using a chain as you said?

Thanks again!

Sorry, I wasn't very clear about the motor rating. By over-rated I meant a motor that was rated to deliver maybe three or four times more power than you need. This will mean that it will have a lower winding resistance, so if you then run it at it's best efficiency speed, but at a lower torque, which probably means using toothed belt or better still chain drive to the rear wheel, you will have lower motor losses and better overall efficiency.

You still need to match the motor to the battery pack voltage, just run it at a lower current than it was designed for. More often than not this will improve efficiency, particularly as motors tend to be over-rated my the manufacturers anyway. Try and keep the motor spinning fast by gearing it carefully, as most motors are more efficient when run at the upper end of their speed range, around 80 to 90% of maximum speed is often best for efficiency.

If you can do some basic calculations on motor parameters you should be able to get a good idea of the sort of motor and gearing combination that will give you the best overall efficiency, at the speed you need to go in order to be competitive. The basic motor calculations are hard, just tedious, because you will need to do several iterations to get the combination that gives the best compromise for your needs.

An internally geared motor might work, but I think you will find the gearbox losses too great. A chain or toothed belt would almost certainly be more efficient, and would give you the option to change the ratio to tune it to the vehicle.

Jeremy

 

[MitchJi]

Hi,

One of the following should work well but will require a lot of reduction:
http://www.astroflight.com/index.php?main_page=product_info&cPath=20&products_id=122
http://www.astroflight.com/pdfs/3210WEB.pdf

http://www.xeramotors.com/index.php...&category_id=1&option=com_virtuemart&Itemid=1

The 4035/5y-240 series is a low output power low loss motor for low rpm direct drive applications or small EV's.

 

[sohaibafzal]

Thanks Mitch, thanks Jeremy! You guys are great.

 

[Lock]

Hi sohaibafzal...

Is this the vehicle you will be entering?:



Not a lot of time to nail down these details?

Phase II: Opens 16 November 2010 - Closes 15 December 2010
Login to the registration area on the Shell Eco-marathon Asia website using your team login details that you received after completing Phase I. Teams are asked to complete Phase II by 15 December 2010.
Complete the final team member details, indicate lodging requirements, and upload each team member’s student identification and passport.
Download and complete the Technical Submission spreadsheet http://www-static.shell.com/static/.../2011/asia/S11-Part2_Technical_submission.xls(XLS, 628 KB) - opens in new window and upload the completed spreadsheet no later than 15 December 2010.

Other bits from the Rules:

Article 71: On-Board Electrical Energy For safety reasons, the maximum voltage on board of any vehicle must not exceed 48 Volts (this includes on-board batteries, external batteries, super capacitors, fuel cell stack, etc). Only one battery per vehicle is allowed.

A ‘battery’ is defined as a source of electrical energy, which has exactly two connectors and comes as a single unit. This single unit may contain more than one sub-unit.

This on-board battery must operate all safety devices (e.g. horn, hydrogen sensor) for the duration of the competition may also operate the starter motor, the ignition, the instrumentation and electronic management systems. All other additional sources of electricity are forbidden.

Competitors are required to provide the main characteristics of the battery in their technical documentation: maximum voltage that can be supplied, capacity in ampere-hours (i.e. the quantity of electricity that the battery can theoretically provide when new), dimensions and weight.

The on-board battery is not allowed to power compressors, blowers, engine cooling systems, motors, etc. It may however be used to power a ventilation/cooling fan for the driver.

Competitors must provide the Organisers with a precise description and technical drawing of the vehicle’s electrical circuitry. The Organisers reserve the right to request additional information from Teams using high-capacity batteries.

The Organisers reserve the right to request Teams to install one joulemeter, intended to measure the quantity of energy provided by the battery. If this amount of energy exceeds the power typically required to operate the starter motor, horn and safety devices the competitor will be disqualified.

Batteries must be installed outside of the driver’s compartment behind a bulk head.

The following devices may be powered by additional batteries provided they use built-in batteries: radio communication system, GPS system, data loggers excluding engine management units, driver ventilators. If a Lithium Polymer battery is used, a Battery Monitoring System (BMS) must be installed to control and protect the battery against risk of fire.

Article 81: ‘Battery Only’ Powered Vehicles (‘Plug-In’s’)
The ‘Battery Only’ energy category is a new addition for the 2011 season.

Competitors will be able to participate with both Prototype and UrbanConcept vehicles in the new category.

The drive train in the ‘Battery Only’ category is restricted to a maximum of one electric storage device, electric motor(s), one control unit and the required connections.

Only Super Capacitors and Lithium Polymer batteries are permitted as electric storage devices.

If a Lithium Polymer battery is used, the vehicle must be equipped with a Battery Monitoring System (BMS) to control and protect the battery against risk of fire.

Any type of electric storage device and on-board system is subject to the maximum voltage of 48 Volts.

The competitors must provide a description and a precise technical drawing of their electrical drive train with the application.

The entire drive train must be easily accessible for inspection and measurements.

Batteries or Super Capacitor must be placed outside the drivers compartment behind the bulkhead.

All electrical / electronic cases must be made of transparent material or at least have a transparent top.

All vehicles must be equipped with one joulemeter to measure the electric motor energy consumption.

The Organisers will provide this joulemeter for the duration of the event. A security deposit may be required for the joulemeter. The joulemeter must be positioned so that its display can be easily read from outside the vehicle. The joulemeter must be inaccessible to the Driver in his or her normal driving position. In line with the technical specification of the joulemeter, the electric current must not exceed 50 amperes permanent and 150 amperes peak.

The entire installation must be adequately fused.

The vehicles will go to the starting line with their batteries charged.

On the starting line, Fuel Marshals will reset the joulemeter to zero, and then the vehicles will have access to the track to start their attempt under the same distance and time conditions as specified for their respective vehicle class. At the finish line, Fuel Marshals will read the joulemeter display.

All ‘Battery Only’ powered vehicles which complete a successful run will be classified in ascending order of energy consumed, expressed in km/kWh.

An additional on-board battery as defined in Article 71 is permitted. It must not be connected to the electric circuit(s) involving any power train components and must only be used to power safety related components and those mentioned specifically in Article 71.


Article 118: PROTOTYPE GROUP
Participants in the Prototype category must complete four (4) laps of the circuit in the normal racing direction.
Minimum speed: For their attempt to be validated, teams must complete the six laps in a maximum time of 28 minutes with an average speed of approximately 30 km/h. The total distance to cover is 11.2 km (4 laps of 2.8 km minus the distance between the start and finish lines).
The time limit or average speed may be changed prior to the event at the discretion of the organiser.
Each team will be limited to five (5) official attempts: The best result will be retained for the final classification.

I'm just adding these details to this thread for context/others interested in your project.

sohaibafzal, did your team construct the chassis and shell just assuming a hub motor would be used?

Good Luck!

 

[MitchJi]

Hi sohaibafzal,

You might get some good ideas in this thread. Its more about power than efficiency but for the power and how Matt drives it its very efficient.
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=19290

3220 has twice the power of the 3210 which I posted the link to earlier.

Anyway, I am using a 7 turn wye 3220 (Mitch's original motor I swapped him for :wink: ). I will be running 12S 10ah 25C Flightmax lipos through a HV160 controlled by Fechter's current limiter. I am innitially building this with a belt drive reduction unit. So, I will limit it to 4kw. If the belt does not like the torque, I will go with a chain (it is in the case anyway). But, for efficiency sake, I want to try the belt with the 3220. I will probably move to a chain. But we shall see. This build is more about efficiency and light weight than power. But, hey, a little power isn't a bad thing!...

It is being built for high efficiency and smooth running with a touch of muscle...

It looks like this trike is good for 11wh per mile at 20mph....

How does this trike compare to your original recumbent in terms of efficiency, power, and ride?

That is a very good question. They were parked next to each other today and I was wondering the same thing.

Here are a few comparisons;

Actionbent Midracer--- 13wh per mile at 20mph.
CT700--- 11wh per mile at 20mph.
Midracer--- 48 pounds.
CT700--- 60 pounds? (still have to weigh it)
Midracer--- 3 stages from motor to wheel.
CT700--- Two stages to the wheel.
Midracer--- Extremely complicated drive unit.
CT700--- Very simple drive unit.

Midracer--- Top speed 40mph.
CT700--- Top speed 26mph (in current configuration. I may increase this to 30+ soon).
Midracer--- Relatively comfortable.
CT700--- VERY comfortable.
Midracer--- A bit fragile, but not too bad (too many parts in the drive unit).
CT700--- Very reliable. I would trust this thing across the state or country.
Midracer--- Cost to build--- $3,800.
CT700--- Cost to build--- $4,600 (roughly). Of course, I get stuff at cost or I make it myself. That helps. This thing is worth about $6,500 to $7,000 as it sits if you consider retail price of everything and the time spent.

Midracer links:
http://www.wisil.recumbents.com/wisil/shumaker/default.htm
http://endless-sphere.com/forums/viewtopic.php?f=6&t=3904#p57685

 

[jbond]

An off-the-shelf hub motor won't be very efficient if used in a wheel as intended. They are probably around 80 to 85% efficient at best, less when run at speeds below their peak efficiency point and it will be hard to get them to work at their most efficient rpm when you can't change the gearing.

You really have to judge efficiency over the whole drive train. I do wonder how these compare.
- DD Hub motor
- Geared hub motor
- Geared hub motor driving by chain to the rear wheel
- DD Hub motor
- RC motor, reduction gearbox, chain to rear wheel
- RC motor, reduction gearbox, friction drive

I don't have the detail, but descriptions of people running DD hub motors seem to need a lot of electrical power to go the same speed as geared hub motors. The advantage seems to be that they can tolerate much more power than geared hub motors. But at the same speed they appear to be less efficient. Is that correct? Although RC motors may be more efficient on their own, by the time you've added a reduction gearbox and the chain drive I wonder if they are really any better than a geared hub.

For both the geared hub and RC builds, the actual motor is running much too fast to drive a bicycle wheel so there's inevitably a gearbox in there. So how does a planetary gearbox compare to a belt gear reduction for efficiency? And what are the real losses of driving a chain as compared with a hub motor built into the wheel?

For the competition, I'd look at first starting with the fastest, most efficient velomobile. The basic vehicle should be light and aerodynamic with very low rolling resistance. I'd then look at ways of using a mid mounted motor driving a 7 speed derailleur. That should give a roughly 2:1 range in overall gearing between motor and rear wheel. At that point, there's a lot to be said for a geared hub mounted centrally, with a pair of sprockets. One fixed wheel to the rear and a freewheel to the front chainset. In an economy competition, acceleration isn't that important so we could choose the motor sprocket sizes for speed/efficiency. The 2:1 range will then let them work up hills and accelerating from rest.

The real trick here for economy, is to reduce weight and wind resistance. And secondly to avoid using any more power from the assist than is absolutely necessary. That implies work on the controller and human controller interface. For instance, tricks to deliberately limit the power available via current limits unless the rider deliberately over-rides it. There's a lesson here from the Panasonic system. That gets a large range from the same battery by deliberately linking assist to the amount of human input via a torque sensor on the crank. If you don't put any effort in, you don't get any assist.

 

[Jeremy Harris]

An off-the-shelf hub motor won't be very efficient if used in a wheel as intended. They are probably around 80 to 85% efficient at best, less when run at speeds below their peak efficiency point and it will be hard to get them to work at their most efficient rpm when you can't change the gearing.

You really have to judge efficiency over the whole drive train. I do wonder how these compare.
- DD Hub motor
- Geared hub motor
- Geared hub motor driving by chain to the rear wheel
- DD Hub motor
- RC motor, reduction gearbox, chain to rear wheel
- RC motor, reduction gearbox, friction drive

There are some very general principles that help sort this lot out.

1. Direct drive hub motors have fairly poor efficiency when run at low speed/high torque. A quick glance at Justins simulator on the ebikes.ca site shows this very clearly.

2. Geared hub motors just shift the position of the best efficiency point and add in extra losses from gears.

3. Planetary gears are almost always less efficient than either roller chain or even toothed belts, but do have the advantage of being more compact for a given torque handling capability.

4. Some RC motors are extremely efficient (up around 90% or more) when run under the right conditions and not run beyond a sensible power level.

5. Lots of motors intended for hobby use have wildly optimistic ratings, especially ebike and some RC motors. If run at their sellers maximum ratings their efficiency will be poor.

6. Friction drive is likely to be comparable to vee belt drive in terms of efficiency, so slightly lower than gears, toothed belts or roller chain.

To put these figures into perspective, here's my take on some approximate drive train efficiency numbers for the sort of speeds and torque levels an ebike might use:
- A roller chain that is clean, well oiled and running at the correct tension and alignment can be around 98 to 99% efficient.
- Good toothed belts (HTD or GT2 for example) and a decent spur gear set can be around 97 to 98% efficient.
- Helical gears will be maybe 96 to 97% efficient.
- A vee belt/friction drive will be anything from around 94 to 97% efficient, depending on the coefficient of friction (which will be affected by wet/dry conditions) and the applied roller pressure/tyre pressure/vee belt tension/alignment.

Bikes have stuck with roller chain for years because not only does it come out on top for efficiency, but it also tends to remain pretty efficient even when it's a bit dirty and worn.

I don't have the detail, but descriptions of people running DD hub motors seem to need a lot of electrical power to go the same speed as geared hub motors. The advantage seems to be that they can tolerate much more power than geared hub motors. But at the same speed they appear to be less efficient. Is that correct?

I believe so, at least at low speed/high torque. The trade off is that direct drive has to run a bigger diameter (for greater torque) motor at a higher current so incurs penalties in terms of weight, rotational inertia affecting acceleration (a significant effect, talk to any race car driver about the impact of lightening a flywheel) and I²R losses in the motor, controller and wiring. The upside is that they have a big motor that's capable of running at higher power levels at high speeds.

Although RC motors may be more efficient on their own, by the time you've added a reduction gearbox and the chain drive I wonder if they are really any better than a geared hub. For both the geared hub and RC builds, the actual motor is running much too fast to drive a bicycle wheel so there's inevitably a gearbox in there. So how does a planetary gearbox compare to a belt gear reduction for efficiency? And what are the real losses of driving a chain as compared with a hub motor built into the wheel?

This depends a great deal on the reduction drive efficiency. The typical planetary spur gears used in hub motors will be pretty inefficient when compared to roller chain or even toothed belts. My guess is that they will struggle to be better than around 96% efficient, and, due to poor running tolerances and grease lubrication could be worse than that. An RC (or any other motor) driving through a chain or toothed belt is likely to have around half the losses that a typical Chinese geared hub might have. Add in that you can better match the reduction ratio to the vehicle requirements, by a simple change of sprocket, and the chain or belt drive wins hands down for this requirement (unless the hub motor is specifically designed for the vehicle, which isn't going to be the case here).

For the competition, I'd look at first starting with the fastest, most efficient velomobile. The basic vehicle should be light and aerodynamic with very low rolling resistance. I'd then look at ways of using a mid mounted motor driving a 7 speed derailleur. That should give a roughly 2:1 range in overall gearing between motor and rear wheel. At that point, there's a lot to be said for a geared hub mounted centrally, with a pair of sprockets. One fixed wheel to the rear and a freewheel to the front chainset. In an economy competition, acceleration isn't that important so we could choose the motor sprocket sizes for speed/efficiency. The 2:1 range will then let them work up hills and accelerating from rest.

The real trick here for economy, is to reduce weight and wind resistance. And secondly to avoid using any more power from the assist than is absolutely necessary. That implies work on the controller and human controller interface. For instance, tricks to deliberately limit the power available via current limits unless the rider deliberately over-rides it. There's a lesson here from the Panasonic system. That gets a large range from the same battery by deliberately linking assist to the amount of human input via a torque sensor on the crank. If you don't put any effort in, you don't get any assist.

This pretty much sums up the motor arrangement I was suggesting in the first reply. The key thing for the drive train is the ability to adjust the gearing to ensure that the motor is running at it's peak efficiency rpm for as much of the course as possible.

Jeremy

 

[jbond]

Reading all the experiments on ES, it seems to me that one of the big limitations of RC at the moment is the state of controller technology for RC motors when used on a full sized vehicle. And I don't fully understand yet how RC ESC manage to be so much smaller and lighter than controllers designed for hub motors. There's a gap in the market for the equivalent of the Lyen modified controllers but designed to work with the much higher rpm of an RC motor.

The second problem is the lack of off the shelf gear reduction for them. Part of this is that mid mounted motors just don't look very elegant when bolted onto conventional bicycles. It's hard to build in all the various freewheels required to cope with pedal only, motor only and pedal plus motor. Perhaps they work well, but I'm uncomfortable with spinning freewheeled chain wheels for instance. This stuff gets easier on a recumbent bike or trike just because there's more room available.

 

[Jeremy Harris]

Reading all the experiments on ES, it seems to me that one of the big limitations of RC at the moment is the state of controller technology for RC motors when used on a full sized vehicle. And I don't fully understand yet how RC ESC manage to be so much smaller and lighter than controllers designed for hub motors. There's a gap in the market for the equivalent of the Lyen modified controllers but designed to work with the much higher rpm of an RC motor.

The controllers that Lyen builds and sells are based on the same boards etc as Keywin Ge sells, the standard XieChang range. These aren't particularly speed restricted, I've run a 7 pole pair, 12 slot RC motor at over 8000 rpm with no sign of the controller having a problem keeping up.

The only problem I'm aware of is with the sensorless adapter fitted to some versions of this controller. Because this needs to filter out the 15kHz PWM frequency, it can't work at high commutation frequencies. If the motor is fitted with sensors then the XieChang controllers will drive them very well indeed.

The second problem is the lack of off the shelf gear reduction for them. Part of this is that mid mounted motors just don't look very elegant when bolted onto conventional bicycles. It's hard to build in all the various freewheels required to cope with pedal only, motor only and pedal plus motor. Perhaps they work well, but I'm uncomfortable with spinning freewheeled chain wheels for instance. This stuff gets easier on a recumbent bike or trike just because there's more room available.

True. Matt seems to be the only one making reduction drives for RC motors, although there are a few one-offs out there that could be productionised, like Thud's two speed drive.

Jeremy

 

[jmygann]

Hi

I am part of a team hoping to make an electric prototype for the 2011 Shell Eco Marathon Asia. I am posting in 'E-Bike Technical' because you guys have more experience with the kind of motors we will be working with.

The vehicle is very light and aerodynamic, and our analysis shows that we would need around 500W continuous power from our motor. A single motor will power the solitary rear wheel, while the front two wheels will be un-powered and used for steering.

Since the Eco Marathon is all about efficiency, we are interested in finding light motors with their peak efficiency in the power range 200W - 500W. We are restricted to using a 48V battery. Currently we are looking at the Cute 128 SX hub motor. Is there a better (more efficient) option?

My second question is about LiPo batteries, since I have never used them before. I have read in this forum that we can discharge them to 80%. Does this mean that I can use 8Ah from a 10Ah battery?

Thanks!

Our project page: http://emenents.com/?page_id=173

what motors are the competition using ?

which motor won in 2010 ?

 

[Jeremy Harris]

what motors are the competition using ?

which motor won in 2010 ?

The solar power class (as far as I can tell the only electric class at the inaugural Asia event last year) was won by the Venture II car from Nanyang Technology University, Singapore with a range of 316km on the equivalent energy of 1 litre of fuel. Details of their cars are here: http://www.nanyangsolarteam.com/. It looks like they used a 98% efficient Mitsuba hub motor and 52Ah of Panasonic cells, arranged as 28S, 18P for 110V. These were charged by 6m² of solar cells. Top speed (tested) seems to be 96km/h.

Mitsuba is a Japanese motor manufacturer best known for making accessory motors for cars (wiper motors, electric power steering pumps etc). I can't find anything specific on their hub motors, it was possibly a one-off for this project. It may be related to this motor mentioned by Docbass earlier in the year: http://endless-sphere.com/forums/viewtopic.php?f=30&t=16345

Jeremy

 

[Hillhater]

6. Friction drive is likely to be comparable to vee belt drive in terms of efficiency, so slightly lower than gears, toothed belts or roller chain.

do we have any data to verify this ?/
somehow i just feel a well set up friction drive would have much less loss than a belt or chain drive if for no other reason than the higher speed, low torque , nature of the peripheral drive and zero extra moving components.
Unless i am mistaken, this competition is all about maximising efficiency, which basically means a fixed speed situation..( minimum allowable to minimise aero loss).. so a specifically designed friction roller drive may be a good choice.

 

[Jeremy Harris]

6. Friction drive is likely to be comparable to vee belt drive in terms of efficiency, so slightly lower than gears, toothed belts or roller chain.

do we have any data to verify this ?/
somehow i just feel a well set up friction drive would have much less loss than a belt or chain drive if for no other reason than the higher speed, low torque , nature of the peripheral drive and zero extra moving components.
Unless i am mistaken, this competition is all about maximising efficiency, which basically means a fixed speed situation..( minimum allowable to minimise aero loss).. so a specifically designed friction roller drive may be a good choice.

There will always be a degree of slip in any drive that depends purely on friction, under some load conditions, it's inherent in the principle, and this will absorb energy (it's what wears out the tyres on your bike from just rolling along the road). When you add in the tyre hysteresis loss (deflecting the tyre in contact with the roller absorbs energy that isn't wholly returned as useful work as the deflected segment passes) I strongly suspect that a friction roller drive will be worse than a vee belt under some load conditions. The test would be to try and spin the wheel by hand, off the ground and see how much resistance to motion there is compared with another drive system.

In the eco marathon there is a fair bit of time spent accelerating. They change drivers every four hours and it takes a fair while to get up to speed. The best vehicles also tend to use a 'pulse and glide' technique, similar to that used by Prius hyper-milers, as this tends to give better efficiency than running at constant speed. They are also going around a race circuit, so have to deal with bends which means accelerating very gently out of each bend and regeneratively braking into the next one.

Jeremy

 

[sohaibafzal]

Hi sohaibafzal...

Is this the vehicle you will be entering?:
:
:
I'm just adding these details to this thread for context/others interested in your project.

sohaibafzal, did your team construct the chassis and shell just assuming a hub motor would be used?

Good Luck!

We participated last year with this car. It runs with a simple motorcycle gasoline engine. Shell has allowed battery powered electric vehicles for the first time in the 2011 Marathon.

For this year, we need to submit the technical details which you mentioned by the 15th of December. Shell will short-list the teams based on the quality of their designs by the 15th of January, after which they would start building their vehicles.

The prototype vehicle we are planning has three wheels, the front two for steering and the rear one powered. The body has been designed and major work on the chassis is complete, just the details near the rear wheel (which depend on the motor we decide to use) remain and hopefully will be completed soon.

 

[sohaibafzal]

what motors are the competition using ?

which motor won in 2010 ?

The first Shell Eco Marathon Asia was last year, and battery electric vehicles have been introduced for the first time this year. However, in the European and American versions, teams have been using hub motors e.g. Golden motors magic pie and the CSIRO motor that Jeremy mentioned.

 

[sohaibafzal]

There will always be a degree of slip in any drive that depends purely on friction, under some load conditions, it's inherent in the principle, and this will absorb energy (it's what wears out the tyres on your bike from just rolling along the road). When you add in the tyre hysteresis loss (deflecting the tyre in contact with the roller absorbs energy that isn't wholly returned as useful work as the deflected segment passes) I strongly suspect that a friction roller drive will be worse than a vee belt under some load conditions. The test would be to try and spin the wheel by hand, off the ground and see how much resistance to motion there is compared with another drive system.

In the eco marathon there is a fair bit of time spent accelerating. They change drivers every four hours and it takes a fair while to get up to speed. The best vehicles also tend to use a 'pulse and glide' technique, similar to that used by Prius hyper-milers, as this tends to give better efficiency than running at constant speed. They are also going around a race circuit, so have to deal with bends which means accelerating very gently out of each bend and regeneratively braking into the next one.

Jeremy

I have another question about LiPo batteries. I have read that they do not support trickle charging. Does that mean that regenerative braking will reduce their life span? We are hoping that our 2012 team might be able to re-use the 2011 battery if possible.

 

[Hillhater]

In the eco marathon there is a fair bit of time spent accelerating. They change drivers every four hours and it takes a fair while to get up to speed. They are also going around a race circuit, so have to deal with bends which means accelerating very gently out of each bend and regeneratively braking into the next one.

Jeremy

Jeremy, i am confused over exactly what this competition is, but from Lock's posting of the rules...

Article 118: PROTOTYPE GROUP
Participants in the Prototype category must complete four (4) laps of the circuit in the normal racing direction.
Minimum speed: For their attempt to be validated, teams must complete the six laps in a maximum time of 28 minutes with an average speed of approximately 30 km/h. The total distance to cover is 11.2 km (4 laps of 2.8 km minus the distance between the start and finish lines).
The time limit or average speed may be changed prior to the event at the discretion of the organiser.
Each team will be limited to five (5) official attempts: The best result will be retained for the final classification.

At an average speed of 30km/hr round a track for 28 mins max, its unlikely there will be the need for much accelerating or braking and no driver changing.
With a low power friction drive onto a very hard tyre, friction losses would be minimal and certainly no sprocket /chain inertia and friction losses that may be seen if using a high efficiency (high rpm ?) BLDC motor.

 

[dogman dan]

If you go hubmotor, I'd be tempted to use a direct drive with a low speed winding. I find my slow wound 9 continent motor just as efficeint as gearmotors I've used. But a gearmotor does have an advantage, you can pulse and glide better with a gearmotor. A direct drive gets efficient when you can hit a set speed, and then pedal a few watts to lower the amp draw. This won't apply to you, I think. No pedals right?

So go with a gearmotor like the cute if you go hubmotor. Then you can pulse and glide like a hypermiler does in his car.

 

[Jeremy Harris]

Jeremy, i am confused over exactly what this competition is, but from Lock's posting of the rules...

Article 118: PROTOTYPE GROUP
Participants in the Prototype category must complete four (4) laps of the circuit in the normal racing direction.
Minimum speed: For their attempt to be validated, teams must complete the six laps in a maximum time of 28 minutes with an average speed of approximately 30 km/h. The total distance to cover is 11.2 km (4 laps of 2.8 km minus the distance between the start and finish lines).
The time limit or average speed may be changed prior to the event at the discretion of the organiser.
Each team will be limited to five (5) official attempts: The best result will be retained for the final classification.

At an average speed of 30km/hr round a track for 28 mins max, its unlikely there will be the need for much accelerating or braking and no driver changing.
With a low power friction drive onto a very hard tyre, friction losses would be minimal and certainly no sprocket /chain inertia and friction losses that may be seen if using a high efficiency (high rpm ?) BLDC motor.

I was looking at last years competition, where the rules seem to have been different. The race lasted hours, with driver changes and the electric vehicles ran at speeds up to 95km/h.

The answer to the friction drive efficiency question is to try the experiment I suggested. I pretty much guarantee that it'll be harder to turn a friction drive wheel unpowered that it would one driven by the same motor and reduction ratio but with a chain drive. There's a very good reason that we don't have friction drives all over the place, and that's because they are lossy. That's not to say their not a good solution for some requirements, but this one really needs to wring out the last fraction of a percent from all the components to be in with a good chance.

I spent over 6 months getting rid of frictional losses on my boat drives, some of it documented here. Some of the power absorbed by various components of the drive was surprisingly high. I ended up getting the total no load, max rpm, lost power in the motor and drive train down from around 22% to about 5%, after all those months of chasing down friction. These guys need to do better than that.

Jeremy

 

[sohaibafzal]

If you go hubmotor, I'd be tempted to use a direct drive with a low speed winding. I find my slow wound 9 continent motor just as efficeint as gearmotors I've used. But a gearmotor does have an advantage, you can pulse and glide better with a gearmotor. A direct drive gets efficient when you can hit a set speed, and then pedal a few watts to lower the amp draw. This won't apply to you, I think. No pedals right?

So go with a gearmotor like the cute if you go hubmotor. Then you can pulse and glide like a hypermiler does in his car.

Yup, no pedals.

We are hoping to use the special Michelin tires for the Marathon. This means that our wheel dia is 406mm. Rough calculations show that we would need a 15:1 gearing to get the desired vehicle speed using the Astro RC motor mentioned earlier. Unfortunately, here in Pakistan, we would not be able to get good quality sprockets made for the drive. If we go for a two stage chain transmission, perhaps we would be able to manage it, but that would mean more losses.

The RC motor idea is very good, and we will keep looking into it. Perhaps import a good quality chain-sprocket set. But for now, we are registering with a hub motor based design. Fortunately we can change the drive-train later when we start fabrication, so we will keep researching.

Thank you all for the guidance!

 

[MitchJi]

Hi,

We are hoping to use the special Michelin tires for the Marathon. This means that our wheel dia is 406mm. Rough calculations show that we would need a 15:1 gearing to get the desired vehicle speed using the Astro RC motor mentioned earlier. Unfortunately, here in Pakistan, we would not be able to get good quality sprockets made for the drive. If we go for a two stage chain transmission, perhaps we would be able to manage it, but that would mean more losses.

The RC motor idea is very good, and we will keep looking into it. Perhaps import a good quality chain-sprocket set. But for now, we are registering with a hub motor based design. Fortunately we can change the drive-train later when we start fabrication, so we will keep researching.

One potential problem with the Astro in a vehicle without pedals is it doesn't have sensors. You can probably manage to get sensors installed if you really want to use it.

With a 15:1 reduction you will get roughly a 15:1 multiplication of torque so the Astro will really be loafing.

I'd avoid a single stage 15:1 reduction. If nothing else I think you will lose efficiency with a drive sprocket of under 14t which means a 210t driven sprocket! I'd buy one of Matt's single stage reduction units for a 4:1 reduction using #25 chain.
http://endless-sphere.com/forums/viewtopic.php?f=28&t=16741

Then use #25 chain (should be fine for 500 watts) for a second stage reduction of 3.75:1 and check what your efficiency is as compared to your other options. I think an Astro with 2 stage #25 chain reduction will be pretty good.

If you are not happy with the Astro and Matt's drive unit I think you could sell them on the forum and recoup a big percent of the cost.

 

[curious]

The requirements for a competition vehicle are very different from a daily rider, even if you have the same budget for the prototype. For example you can use stage 1 spur gear reduction which can be very efficient (if done properly) yet unusable for daily riding because of unpleasant high-pitch whine. Also a daily rider is usually designed for very high peak to average power ratio (dealing with traffic, hills, convenience of transportation, or pure fun) yet any electric motor has fairly narrow power/rpm range where it's efficiency is high. A marathon competitor OTOH has to run in the motor/controller efficiency sweet spot most of the time. It may accelerate like crap by this forum standards and stall up a medium grade hill but it will win the efficiency competition.

Speaking mathematically you have to convolve the estimation of probability distribution of your vehicle power demand and rpm range P(rpm,power) with dyno-measured motor/controller efficiency curve (ideally also a two variable function E(rpm,power)) and optimize it for the maximum, varying both motor options and gear ratio.

I strongly advise against oversizing the motor for competition application. Motor efficiency is dominated by two types of losses - copper loss and stator eddy currents. The later can kill efficiency when using an oversized motor. I understand the temptation of using multi-KW Astro or similar high-power motor but for specific competition you may be better of with a much smaller motor that achieves peak efficiency at desired power/rpm interval. Take a look at Kontronik Tango (coreless, very low magnetic losses, but needs very high speed PWM which partially negates the design benefits, unless extra effort spend on controller) or smaller offerings in the Astro or Neu product lines.

To some degree motor efficiency sweet spot can be shifted down if you reduce rpm, hence I suggested using 2D map in optimization strategy but you need to have a good motor testing dyno for measurements (it is a good thing to have for a competition entry). You can shortcut it using usual motor efficiency formula for estimating the 2D map but I do not believe it will give you very reliable results, but maybe you should try it anyway.