Once the define, measure, benchmarking, and top-flop analysis phases are completed, the process defined by the QFD can be concluded, with the analyze phase, creating the “WHAT/HOW relationship matrix”.
4.1. WHAT/HOW Relationship Matrix
Through this table the “thing”, or the needs of the product according to the customer’s point of view, are compared with the “how”, that is the technical requirements necessary for the realization of the product, the same employed in the top-flop analysis.
For this stage the parameters used are those that have emerged, most significantly from the previous tables, which are indicated by cells colored in yellow.
Also, in this case numerical evaluations are used on the type of relationship: Nothing (empty box, equivalent to 0), weak (1), medium (3), strong (9). Finally, the sums are executed for rows and columns for these numbers.
From the reading of this “WHAT/HOW matrix” (Table 5
), it is therefore possible to understand which are the most significant values, to be improved, for the innovative design of an electric motorcycle.
The sum of the line values indicates which requirement is most affected by the technical specifications.
The sum of the column values indicates the priority of the actions that must be implemented, to achieve the maximum effect on the row requirements.
On the basis of the values obtained, we note that the priority in the development of an innovative electric motorcycle is mainly linked to the specifications relating to: Vehicle weight and battery capacity, range of autonomy, sales price, and finally, to the installed power.
It means that if the designer wants to please, the customer should emphasize the above-mentioned characteristics (i.e., weight, battery capacity, autonomy, price and power) during the design process.
Before starting with the design phase, it is necessary to analyze how the most important technical specifications are influenced. In this way, it is possible to establish how to intervene on the single characteristic to determine the best innovative solution of the product.
The first characteristic analyzed, in order of importance, is the weight. The analysis of this specification focuses mainly on two aspects: The number of components used and the type of material used to make the parts.
Regarding the number of components used, it is obvious that a reduction of the parts can be a good solution, however, the functionality of the motorcycle must still be guaranteed and it is not always possible to remove components without penalizing other aspects. Furthermore, there are parts that have a very high impact compared to the rest of the parts, such as the battery and the electric motor, even if their effect cannot be reduced, except with further technological advances.
On the other hand, if we consider components such as, for example, the rims, the forks, the frame or the tail, it is possible to intervene on the weight by choosing the type of material used.
In general, the choice of a material for a given component depends on various factors: The price, the mechanical characteristics, the specific weight, the aesthetic aspect, the workability, and the disposal possibilities.
If, however, we want to restrict the analysis only with respect to weight, the mechanical characteristics and specific weight are the determining values. In particular, we introduce the specific stiffness, or specific modulus, or the ratio between the elastic modulus and the density of the material. The higher this value is, the greater the weight reduction for structural rigidity.
If you want to choose which material to use to make a motorcycle frame, the main choices are analyzed in the table below.
The values shown are those relating to the most relevant mechanical characteristics (Table 6
Steel is one of the most economical and common materials used. It is available in numerous shapes and sizes and is often used in the form of hollow, welded section tubes to create a trellis frame.
Titanium is very similar to steel, however, it has a very high cost and its application is limited to a few special components, in particular, it is used to create special screws on racing bikes.
Aluminum makes it possible to create structures that are more rigid and lighter than steel, due to its higher specific modulus, although it is slightly more expensive and requires much more complex welding methods. Furthermore, this material is particularly suitable for extrusion and casting processes, making it a valid solution for perimeter or monocoque frames.
Magnesium, on the other hand, despite having the same specific resistance as aluminum, has a low resistance to oxidation, if specific surface treatments are not carried out, and its properties easily degrade with high temperature values.
Carbon fiber reinforced polymers are extremely light and resistant materials, characterized by particularly high specific stiffness values. However, the application on motorcycle frames still needs further development, while there are already several examples of swingarms for racing bikes.
The other specification, of equal importance with the weight, is determined by the size of the battery. The choice for this specification is identified by how much energy can be stored per kilogram, which implies that attention is placed on the intrinsic limit of how light a battery can be.
This limit is characterized by two main aspects: The weight of the material used and how much energy is transmitted for each electron exchanged. Therefore, the optimal solution is that of a light material, which generates as much energy as possible.
At the moment, the standard for a light, rechargeable, and safe battery, available on the market, is to use lithium ions (li-ion batteries). To generate electricity, a transfer between atoms that tend to give up their electrons is necessary, such as alkaline metals, placed to the left of the periodic table, and atoms that tend to receive them, like non-metals. In fact, lithium, one of the lightest materials on the periodic table, is used as a material for the batteries of electric vehicles [4
Specifically, on the one hand there is lithium and graphite (LiC6), with the possibility of different options for the other, usually using lithium oxide and cobalt (LiCoO2). Lithium atoms are what dissolve or deposit in order to transfer electrons, hence the name “lithium ions”. The other materials that do not actively participate during the transfer, despite having an important role from the chemical point of view, significantly increase the mass for each electron transferred.
The complete chemical reaction is:
Considering the year 1990, the amount of energy stored per kilogram was about half of its current value, so it is legitimate to ask how much this type of batteries can become read.
Analyzing these theoretical calculations, with a certain degree of approximation, it appears that the minimum possible weight of lithium ion batteries is about half their current value.
However, considering other materials that transfer more energy per electron for a given weight, there are theoretically better alternatives using lighter elements from the periodic table.
The Li-S battery, lithium sulfur, has a quantity of energy per electron similar to that of Li-ion batteries, but lithium and sulfur together are lighter than lithium, cobalt, oxygen, and carbon, equivalent to about 30%.
Approaching the theoretical limit of batteries based on chemical reaction, there is the Li-O2, or Li-air, lithium-air battery, which is equivalent to ≈25% of a Li-S, but still remains a technology still very advanced and technical difficulties are discouraging, bearing in mind that the two materials are in different physical states.
Finally, the limit is given by Li-F batteries, lithium-fluorine, but combining these two elements together is almost impossible. However, theoretically its weight would be only 90% of a lithium-air battery.
So, at best, the limit would be about 6% of current lithium ion batteries, but even a Li-air represents a technology that is still too advanced.
Furthermore, to guarantee the correct working condition of the batteries, the introduction of a cooling circuit may be necessary. In fact, during operation heat is generated due to I2R losses while the current flows through the internal resistances of the battery, both during charging and discharging. This is known as the Joule effect.
In case of discharge, the total energy of the system is limited and the temperature increase is limited by the availability of energy. However, this can cause high localized temperature increases, even in low power batteries. Already with internal resistance in the order of 1 m Ohm heating can be significant.
For this reason, a cooling system is required to minimize hot spots and distribute temperatures, ensuring a longer battery life.
This can be achieved by inserting refrigeration pipes between the various battery cells, then the coolant is cooled through the passage for a radiator, positioned in the front of the vehicle (Figure 2
The analysis on autonomy is mainly focused on two aspects: The weight of the vehicle and the capacity of the battery, both have already been discussed previously.
In any case, it should be noted that as the capacity of the battery increases, the overall weight of the motorcycle increases, so it is not always said that as capacity increases there is a directly proportional increase in the range of autonomy.
Therefore, the propulsion group is analyzed. Usually, to observe how power is articulated, three fundamental factors are examined, namely motor, power supply, and transmission. However, since it is an electric vehicle, the power supply is supplied by the battery pack, while the heat engine is replaced by an electric one.
In particular, there is no “standard” transmission, but a single-gear reduction is used instead. The reason for this choice is due to the fact that this is not necessary and also for simplicity.
Internal combustion engines are characterized by a limited useful torque curve, in a given range of engine revolutions, so it is necessary to change the transmission ratio, to allow the engine to operate efficiently. Instead, electric motors are capable of delivering high torque values, almost constantly up to the maximum engine revs.
Since useful values are obtained throughout the range of use there is no need for a transmission. And, furthermore, since the torque is available from 0 RPM, it is not even necessary to use a clutch.
If a multi-speed transmission is inserted, weight, complexity, friction, and inefficiency would be increased, compared to an otherwise simple system. In this way, the power produced and the overall efficiency would be reduced.
Considering the electric motor, the two main types present in electric vehicles are analyzed: Brushless DC motors, or alternatively, AC permanent magnet synchronous motors, and AC induction motors. The reason for this choice is due to their high range of use (the useful torque is available from zero speed up to over 10,000 RPM) and potentially, since they do not require additional components, they do not have parts subject to particular wear, allowing around millions of uses.
The brushless DC motors and the AC permanent magnet motors are grouped together because they have a virtually identical mechanical construction, however, they have slight differences in the waveform. They have a relatively high cost, as they use rare earths within their magnets, but they guarantee a small advantage in terms of efficiency and the size/weight ratio. This becomes all the more important the smaller the dimensions become, as in the case of an electric motorcycle, therefore, they are particularly suitable for almost all electric wheeled vehicles [4
On the other hand, alternating current induction motors, resulting in less expensive, can fill the small size/weight disadvantage for large vehicles, such as buses or trucks.
However, an electronic device must be introduced to control the motor. By inserting an inverter or a buck converter, you can adjust the power that is generated, otherwise you would always have the condition of operation at maximum speed.
The battery pack produces the current directly, so a conversion is necessary before being transferred to an AC motor. For this purpose, an inverter is inserted, which converts direct current to alternating current. Moreover, this device allows to vary the amplitude and frequency parameters, controlling the power output from the motor. It can also be used to regenerate current during slow-downs or braking.
In the case of DC motors, a conversion is not necessary, however, a buck converter is still required. This device allows to lower the voltage, while increasing the current, and vice versa, from the input to the output. So, its use is essential to control the torque produced by the DC motor.
Finally, the total price of the motorcycle is determined both by the quality and quantity of the components used, which affect variable costs, and by project and management costs, or fixed costs, compared to the number of volumes, all multiplied by a coefficient k
(typically values less than 3 are not used).
In particular, under the heading of fixed costs all expenses related to the phases of research and development must be included, therefore costs of prototyping, analysis of the materials and work of the engineers, then the expenditure on production plants and the cost of services, electrical, logistics, and marketing.