## 3 Results and Discussion  
The DEA method model has been implemented by using the Diviz software. Figure 2 shows the Diviz workflow to implement the DEA model with restrictions over the weights [77]. Table 4 shows the result of the analysis of which types of buses are efficient from the point of view of pollutant emissions, by applying the previous DEA model with \"Set 1 of restrictions\", to assess which engines are able to cover the same kilometers while emitting fewer pollutant gases. In order to obtain more accurate results, it has been decided to include more restrictive conditions on the weights based on the relationships that exist between the mean values assigned by the experts to the weights:  
\[w_{1}\geq 2.5w_{3}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Criteria** & **Mean Weights** & **SD** & **CV1** & **CV2** & **CV1–CV2** \\ \hline NO\({}_{\text{c}}\) emissions & 0.5 & 0.228 & 0.23 & 0.23 & 0.00 \\ Particular matter emissions & 0.3 & 0.279 & 0.37 & 0.28 & 0.09 \\ CO\({}_{\text{2}}\) emissions & 0.2 & 0.354 & 0.34 & 0.35 & 0.01 \\ \hline \hline \end{tabular} SD = standard deviation; CV1 = 1 st round coefficient of variation.  
\end{table} 
Table 3: Delphi model: consensus on elements, weights, and statistical data. Figure 2: Diviz workflow to implement the DEA model with restrictions over the weights. \[w_{2}\geq 1.5w_{3}\]  
The previous more restrictive conditions will be hereafter named as \"Set 2 of restrictions\". By applying these conditions, the solution provided by DEA model is shown in the Table 5.  
According to DEA model theory [68] a DMU will be efficient if the efficiency value is equal to 1 with non-zero weights. As can be seen in Tables 4 and 5, in both cases the efficient solution is the plug-in electric motor. Hence, the only efficient engine, from the point of view of pollutant emissions, is the plug-in electric. This means that it is not worth investing in induction electric vehicles from a pollution point of view because they are not efficient. In addition, the infrastructure required to operate induction electric vehicles is complex in built-up areas. A graphical representation of the DEA model solutions has been created, aiming to provide a clearer and visual solution. Two axes have been considered: one represents the output (which is intended to be maximized) and the other one represents the input (which is intended to be minimized). To construct a single function that represents the three initial inputs, a principal component analysis, from now on PCA, has been performed. The values for this PCA of the inputs are shown in the Table 6.  
The Table 7 shows that the first component, which has been denoted F1, accumulates 95.975% of the total information. Therefore, without loss of generality, it is possible to work with it to graphically represent the inputs on a plane. \begin{table}
\begin{tabular}{c c} \hline
**Alternative (DMU)** & **First Component** \\ \hline Diesel & 0.87377 \\ GNC & 0.90508 \\ Diesel hybrid & 0.35636 \\ Plug-in electric & \(-1.14098\) \\ Induction electric & 0.99424 \\ \hline \end{tabular}
\end{table} 
Table 6: First principal component analysis values for inputs. \begin{table}
\begin{tabular}{c c} \hline
**Alternative (DMU)** & **Efficiency Value** \\ \hline Diesel & 0.600127256137285 \\ GNC & 0.520363376365661 \\ Diesel hybrid & 0.754288155408271 \\ Plug-in electric & 0.754288155408271 \\ Plug-in electric & 1 \\ Induction electric & 0.886544716896212 \\ \hline \end{tabular}
\end{table} 
Table 4: Maximum efficiency of motors with “Set 1 of restrictions”. \begin{table}
\begin{tabular}{c c} \hline
**Alternative (DMU)** & **Efficiency Value** \\ \hline Diesel & 0.600127256137285 \\ GNC & 0.520363376365661 \\ Plug-in electric & 1 \\ Induction electric & 0.886544716896213 \\ \hline \end{tabular}
\end{table} 
Table 5: Maximum efficiency of motors with “Set 2 of restrictions”.