3. Hybrid land yacht concepts
3.2 Concept selection
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3.1.13 Concept B12 – Mid Drive Motor to Rear Wheels with SLA Battery
In this concept mid drive motor #1 is used to propel the rear wheels of the vehicle. The transmission is done using a chain connecting the output of the motor and the axle where a sprocket is installed. The battery is put in the same location as in concept A2. It is used a lead acid pack (battery #1).
3.1.14 Concept B13 – Mid Drive Motor to Rear Wheels with Li Ion Battery
The same as B12 except battery #3 (48V lithium ion pack) is used.
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Concepts A3 and B3 would require a much higher investment to have all the components necessary for the electric system than other concepts, due to the fact they are retrieved from e-bikes. Obviously, in actual production the parts would be specifically bought for the vehicles and the cost would be smaller but still they would always be much more expensive than the constituents used (for example) on A1/B1.
Once again, the use of components retrieved from e-bikes instead of getting them separately is done because this is a preliminary study of the integration of an electric propulsion system on a land yacht, and there could be compatibility problems when buying the components separately that could endanger the main goal of the project (this is much less likely to happen when just adding to a kit a battery sold separately). In subsequent works it would be interesting to study the economic advantages of building the hybrid vehicle with components sold separately and to evaluate and solve potential incompatibilities that may exist.
The electric components also take twice the volume (space to accommodate them is very limited) and make the vehicle heavier, mainly because two battery packs are used (for the reasons already mentioned). Additionally, since the batteries are put close to each other they will with greater probability change in a significant way the center of gravity of the kart, and therefore its dynamic behavior in comparison with what is expected from the traditional wind powered land yachts: this is an important drawback since the objective of this project is to include the assistant electric energy power source with minimized interference in the rider’s experience in terms of maneuverability, stability and safety. For these reasons A3 and B3 are also excluded.
Concepts B12 and B13 use a transmission system instead of directly propelling the wheels: for this reason they are intrinsically more complex than the other models and have an extra associated cost because of that (even if it is small and surpassed by other costs in competing models). Two major problems are common to all of the mentioned concepts: (1) they would require a protective covering to keep sand grains from entering the mechanism, which could still not guarantee to certainty this would not happen and cause damage; and (2), the use of a chain would (in a best case scenario) dramatically increase the assemble and disassemble time of the vehicle and could even compromise the goal of having a vehicle where it is possible to remove the electric system and use the kart only having the wind to power it. For these reasons the mentioned concepts are rejected.
Lead acid batteries are substantially larger and heavier than the lithium based ones. In some concepts they are put in the tube connecting the front wheel to the rest of the chassis which may alter the difficulty of the rider’s steering. In addition, the placement of the battery and its large volume complicate the positioning of the rider’s legs, making the experience more awkward and uncomfortable. This brings concepts A5 and B5 to be discarded.
As mentioned before, there is a problem in accommodating lead acid batteries due to their volume and weight. In concepts A8 and B8 two of these are used and put together and for that reason this concepts are going to be excluded.
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3.2.2 Selection criteria
At this point there are 12 concepts that remain (A1, A2, A4, A6, A7, A9, B1, B2, B4, B6, B7 and B9).
This is an acceptable number to use the screening methods. Before that, however, it is necessary to define what are the criteria being considered and why they are important to the selection of the winning concept. For it, consider the following:
-Portability. As it was mentioned before, it would be important not only to be possible to remove the electric components and use the vehicle as a regular land yacht (something that happens with all the models still under scrutiny after the first stage of the selection process) but also to have the ability to completely disassemble the chassis in order for it to be easier to transport.
-Cost. Two things are going to influence the total cost: the part related to the structure chosen and the added budget due to the implementation of the electric system (both the components and the structural changes and additions). Although no exact values are known at this point it is possible to have reasonable estimations, and besides the most relevant thing at this point is being able to make comparisons between the costs of each model. Structure A is considered to be cheaper due to the lower number of components and lower complexity. In what concerns the cost of the electric components necessary, this is given by the price of the bicycle or kit they come from.
-Weight. The total mass of the vehicle is always going to be bigger than the value of the traditional land yacht due to the electric components, being the battery the most relevant. A lower weight means in general terms better maneuverability and easier transport. Structure A is lighter.
-Position of the center of gravity (C.G.). This is a crucial factor in having high maneuverability, safety and stability in the vehicle. It should be as low and as centered as possible in order to achieve these goals.
-Range. The motors and batteries used vary among the different concepts, which means each vehicle will be able to obtain different maximum durations of use of the electric system. It is necessary to take in consideration the values of the nominal power of the motor (W) and the energy storage capacity of the batteries (Wh). Dividing the latter by the former we get an approximation of the expected time of use of the vehicle at full power.
-Time of assembly. Related with the previous item is the estimation of how long it would take to assemble completely the vehicle and have it ready to use and to dissemble and store it. However, these points cover different aspects since “portability” refers to how easy it would be to do the transport once the disassembly is completed.
-Power. When the electric power available increases there is usually also an increase of negative factors such as the weight and the cost, and for this reason (among others) it was set a limit of 500W to the totality of the motors. However, within this limit and considering all other parameters equal, it is
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somewhat preferable to have the value of the available power closer to the upper limit since it will help to overcome steeper terrain and will allow to effectively use the vehicle in weaker wind conditions.
-Maintenance. The “cost” variable will determine the price which represents the initial investment for the costumer. However, it cannot be neglected the fact that lead acid batteries have considerably less available charge cycles in their life time compared to the lithium ion based batteries (e.g. 200 versus 700 cycles in the considered 36V batteries). Having in mind that the life cycle of the structure and electric components other than the battery is a lot larger than the 200 days of use of the lead acid battery (assuming a discharge per day of use), it will be necessary for the user to periodically buy a new battery to keep using the vehicle. There is, therefore, an underlying cost in vehicles using this technology that must be taken into account since it constitutes a significant disadvantage for the concepts that use them.
3.2.3 Reference product for the selection
During the process of generation and selection of concepts for this work it was found for sale in a website [44] a product (figure 26) from the company Blokart that consists in an addition to their land sailing vehicles: a wheel with an incorporated hub electric motor and corresponding battery pack, controller and throttle. It is therefore clearly a product that aims to solve the same identified shortcomings of land yachts and that seems in a first analysis to follow the same product generation steps (namely having as a base the technology found on e-bikes and adapting it to the land yacht).
Figure 26 – Blokart electric motor [44]
This product will be useful to make a comparison with the developed concepts, namely in the selection process, where it will be the reference used. The motor in the Blokart product is sold separately, which means the vehicle can be used in its traditional way or having the electric power assistance: this characteristic was also sought on the prototypes imagined in the present thesis (to be more clear, the reference is the traditional Blokart with this addition installed). It has a power of 500W which is the upper limit set in this work (250 to 500W). The battery used is lithium-based and has 36 volts, is inside a protective alloy case and is positioned inside a bag behind the Blokart seat with the system’s controller placed in the side of the structure of the kart. No data on the capacity of the battery was found, however given its chemistry and the fact the equipment is so expensive, and since the battery is always the most expensive component of this type of electric systems, it is going to be assumed a value of 750Wh, which
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is on the higher end of the spectrum of what is seen on e-bikes (the comparison point) but still fairly common and adequate to this type of application.
Its price (approximately 2540€) is considered very high, which could indicate this type of product has in its production higher costs than it was initially thought, due to variables that were inadvertently overlooked in the present process. However, this value could probably be justified having in consideration this product was recently introduced, being common in these situations for companies to try to recoup the development investment more quickly by charging a higher price to the first consumers.
The lack of similar competing products on the market and the fact the brand in question is dominant in the land yacht commercialization area may also play significant roles. Unfortunately, it was not possible to find the patent presumed to have been issued for this product. This may be due to the recent nature of the item.
3.2.4. Screening: second stage
In table 1 the concepts under examination are compared to the reference product and rated according to their performance in each of the selection criteria in relation to it. Plus (+) means the concept has a better performance in that category, minus (-) inferior and zero (0) means there isn’t a relevant difference between them. Then it is done the counting of the number of occurrences each concept has in each grade and based on that information it’s made the ranking of the concepts.
Concepts Selection
criteria
A1 A2 A4 A6 A7 A9 B1 B2 B4 B6 B7 B9 REF
Portability - - - 0 0 0 0 0 0 0
Cost + + + + + + + + + + + + 0
Weight + + 0 + + 0 0 0 - 0 0 - 0
C.G. position 0 - 0 0 - 0 0 - 0 0 - 0 0
Range - - + 0 0 0 - - + 0 0 0 0
Time of assembly
+ + + + + - 0 0 0 0 0 - 0
Power 0 0 - - - 0 0 0 - - - 0 0
Maintenance 0 0 - 0 0 0 0 0 - 0 0 0 0
Pluses 3 3 3 3 3 1 1 1 2 1 1 1
Same 3 2 2 3 2 5 6 5 3 6 5 5
Minuses 2 3 3 2 3 2 1 2 3 1 2 2
Net 1 0 0 1 0 -1 0 -1 -1 0 -1 -1
Rank 1º 3º 3º 1º 3º 8º 3º 8º 8º 3º 8º 8º
Continue? Yes No No Yes No No No No No No No No Table 1 – Concept Comparison
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Some other parameters, like the safety or comfort of the passenger, could also be considered. However, it is not expected substantial variation of these characteristics among the concepts. It would also be important to assess the maneuverability of each of the karts but this would be something that could only be done satisfactorily by testing each of the models (as said before, it is suggested to test the resulting vehicle of this project in subsequent works). Nonetheless, albeit is not easy to establish differences between the concepts, it is generally expected that the introduction of the electric motor will improve this feature of the land yacht, both by providing to the vehicle a much more stable and controllable power source and by allowing the rider to address a higher focus to the steering of the vehicle.
Furthermore, it is important to notice that, as explained before, this work is intended to be a preliminary effort in developing this kind of vehicle and for that reason it is acceptable to introduce some approximations in the comparison of concepts done in the previous table, as well as overlook some less fundamental characteristics that could otherwise be included in this sort of analysis.
Some of the considered selection criteria used in the previous selection table are quantifiable while others are more subjective. For the first type it is presented below table 2 with the respective values, while for the second it is put forward the justification for the attributed comparative rating.
Concepts Selection
criteria
A1 A2 A4 A6 A7 A9 B1 B2 B4 B6 B7 B9 REF
Cost (electric component s only) (€)
630
[37]
630
[37]
507 [40], [41]
814 [40], [42]
814 [40], [42]
1628 [40], [42]
630
[37]
630
[37]
507 [40], [41]
814 [40], [42]
814 [40], [42]
1628 [40], [42]
2517
[44]
Range (hours at full power)
0.96 0.96 1.73 1.5 1.5 1.5 0.96 0.96 1.73 1.5 1.5 1.5 1.5
Power (W) 500 500 250 250 250 500 500 500 250 250 250 500 500 Table 2 – Quantifiable Criteria Values
In what concerns “portability”, it is considered as a first approximation that all the concepts that use structure A are less portable than the reference model, since the reference is a traditional Blokart where it is possible to add the wheel with the motor, and it was considered in this work that structure A cannot be as easily put inside the carrier bag and put in the trunk of the car as it happens with the Blokart and its electric motor addition. This is mainly because the individual parts of this structure are bigger (A has fewer parts) and take more space. As an example, the central part of structure A (as seen in figure 27), where the mast is connected to the vehicle, has a shape that makes it somewhat harder to transport.
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A standard Blokart model weights around 29kg. Unfortunately, it was not possible to find a source for the weight of the electric motor system used in the reference model. However, in order to make the comparison with the reference it is enough to note that the Blokart electric addition uses a lithium battery and just one motor (and these have approximately the same mass as the corresponding elements used in the generated concepts, in which this value is known), and that structure A is lighter than B (which is also the reference). Note: a “+” sign means a positive characteristic, so in this case it refers to a lighter model.
The cost of the reference product, since they are sold separately, must include the price of the Blokart classic model (2465€) as well as the price of the electric system (approximately 2517€) for a total of almost 5000€. The cost of the concepts comprise the electric components (for which the prices are known from the market research) and the cost of manufacturing the structure, which is not known at this point and would require a detailed analysis for both types of chassis (for structure B only the retail price of the model it is based on is known at this point). However, for this study it is enough to understand that based on the known values, and even including others like those related with the changes necessary to accommodate the electric propulsion system, it is reasonable to assume both the total cost and the consequent price to the costumer would be lower than that of the reference.
The time of assembly of each vehicle is difficult to determine exactly, so there are going to be considered estimates based on the type of structure and on the number of components of the electric system.
Concepts that use structure A have lower assembly times, but the opposite is assumed for those with a higher number of electric components (A9 and B9).
Since both types of structure provide a similar position to the rider, the most significant characteristic in influencing the alteration of the position of the C.G. from one model to the other is the type of battery (consequently, its weight) and its location.
3.2.5 Final selection
In order to have a greater degree of certainty that the winning concept is the best possible solution among the existing ones considering the set criteria, the number 1 concept in the first table is not immediately picked as the final. Instead, the 2 best ranked are going to be selected to go through to a next phase.
At this point is necessary to attribute a value (a scale of 1 being the worst and 5 the best) to each selection criteria of each concept that reflects its place in the ranking among the totality of the concepts.
Simultaneously, to each selection category is going to be assigned a value that translates the relative importance of said criteria in the selection of the vehicle. Although the two final concepts are similar in terms of general structure and disposition of the elements of the electric system, there are differences
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between them in terms of components (namely motor, battery and front wheel). The result is presented in table 3.
A1 A6
Criteria Weight
Rating Weighted Score
Rating Weighted
Score
Portability 15% 3 0.45 3 0.45
Cost 15% 4 0.6 3 0.45
Weight 5% 4 0.2 4 0.2
C.G. position 15% 4 0.6 4 0.6
Range 10% 3 0.3 4 0.4
Time of assembly
15% 5 0.75 5 0.75
Power 10% 4 0.4 3 0.3
Maintenance 15% 4 0.6 4 0.6
Total Score 3.9 3.75
Rank 1º 2º
Develop?
YesNo
Table 3 – Final Selection