The use of energy and efficiency at different driving patterns

In section 9.1 the need for computer simulations were discussed and also their practical use.

To use only a test matrix and tests in a laboratory as a basis for taking decisions as to whether a series hybrid or a parallel hybrid is the best alternative is both time consuming and costly and therefore hardly any car manufacturer will do so. Today the planning of for example a new model is based on an investigation of many factors and not only on the design of the vehicle but also on a study of the market, cost factors and so on. In the case of systems such as hybrid vehicles the situation may be somewhat different. Here some irrelevant factors may give a need to present new ideas and expectations to achieve such result of the development

which are unrealistic in reality. On the other hand many new ideas have been realized by trial and errors and in the case of hybrid vehicles there still is a lack of hardware with as good performance as it is possible to achieve. One example is the batteries, which with today’s knowledge could be much better.

Careful planning of the different factors, not only the hardware but also the use of the vehicle, can result in more sound and realistic expectations. This will go a long way to achieving a certain result. As a basis for the decision to be taken as to whether a series hybrid or parallel hybrid is the best system for the fulfillment of the expected goal, computer simulations can be carried out. These should use factors such as the driving pattern for the area, where the vehicle will be operated and the basic parameters for different hybrid systems. One such computer simulation in connection with laboratory measurements is presented and discussed in the following paragraph.

9.2.1 Studies of hybrid systems for BMW

In order to get an idea of how an evaluation of hybrid systems can be carried out the model used by Dresden Technical University during their investigations and studies carried out for BMW presented is Section 8.1.5 and Section 9.1 respectively. The studies were especially concentrated to the evaluation of the energy efficiency of series hybrids and parallel hybrids and were, for the computer simulations, based on calculations, in one phase, of the difference between a series hybrid vehicle and a conventional vehicle. A similar calculation and comparison was carried out for the evaluation of a parallel hybrid system. Then a series hybrid vehicle was compared with a parallel hybrid vehicle. The authors of the report (Friedman et al, 1998) point out that there is a loss of efficiency in the electric drive train.

They say that, in the case of a parallel hybrid, the electric drive (which transmits force parallel to the mechanical drive train) works like the drive train in a series hybrid.

The result of the evaluation is presented in Table 20. From the table it can be seen that the regenerative braking has a considerable influence on the fuel economy of both the series hybrid and the parallel hybrid. However, the authors claim that regenerative braking (which is engaged during decelerations) shortens the “exhaust emission free driving” achieved by free rolling of the vehicle during decelerations. According to the authors of the report (Friedman et al, 1998) regenerative braking does therefore not favor the reduction of exhaust emissions (here it should be added that this problem could be handled by the hybrid control unit). From the table it can also be seen that for the series hybrid, according to the calculations there is a decrease in fuel consumption (or energy used) for only one combination, namely “0” for electric operation range, 0.92 for Degree of coupling and “Yes” for BERG (braking energy generation). For all other combinations there are increases in fuel consumption from +8% up to 50% when compared with a conventional vehicle.

In the case of the parallel hybrid the picture is somewhat different concerning the savings and losses in fuel consumption where savings are from 0% up to 15% and the losses from 0% up to 10% according to the calculations.

It should be noted that these results are based on computer simulations using data or experiences from their own experiments or data from the literature. The example is interesting in that some of the important parameters of the hybrid vehicles are defined and used in a mathematical model. In this case it has been shown that a parallel hybrid vehicle is more efficient than a series hybrid vehicle for a driving pattern such as the driving cycle used in this example. This is in line with what that others have pointed out, as for example Iwai (Iwai, 1998) who underlines that it is more difficult achieve high efficiency for a series hybrid vehicle than for a parallel hybrid vehicle. However, it should be noted that the figures

presented by the authors of the report (Friedman et al, 1998) will certainly be different to those from other driving cycles and other efficiency estimations.

Table 20. Comparison between a series and a parallel hybrid vehicle with a conventional vehicle.

*Fc: Fuel consumption (Use of energy). Minus sign: Reduced Fc. Plus sign: Increased Fc.

9.2.2 Energy use and efficiency of Mitsubishi hybrid trucks

In Section 8.3 two hybrid trucks – one delivery truck and one aerial working truck -developed by Mitsubishi were presented. In the report (Horii et al., 1998) two figures showing the fuel consumption and energy efficiency were also presented. The values presented by Mitsubishi have been slightly rearranged and are shown here in Figure 36 and Figure 37.

Figure 36. Mitsubishi service truck. Comparison of energy used between a hybrid truck and a diesel truck. Source: Horii et al., 1998.

Three different driving conditions have been compared – idling, 40 km/h and 80 km/h and in the figures even the efficiency, η, has been filled in for the two trucks and the different conditions. The values for the energy used was calculated in the way quoted from the report (Horii, 1998). The energy consumption of the HEV was calculated by the use of simulated values. “The actually measured efficiency values of the various parts of the delivery truck

HEV were partially modified to match the work truck HEV, and the efficiency values of all the components were multiplied together to work out vehicle efficiency. In addition, theoretical running energy was divided by vehicle efficiency to calculate energy consumption”. The energy used was presented in kWh/km and “Theor” and “Act” in the figures below means “Theoretical” and “Actual” respectively.

Figure 37. Mitsubishi working truck. Comparison of energy used between a hybrid truck and a diesel truck. Source: Horii et al., 1998.

Figure 38. Energy efficiency of same important component of a HEV. Source: Horii et al., 1998.

Figure 38 shows the power/energy flow and the efficiency of the individual components used by Mitsubishi as a basis for the calculation of vehicle efficiency. Since the details of regenerated power and generated power that are used (that portion of the energy which is used without passing though the batteries and that which is used for recharging the batteries) vary according to the operation mode, the regeneration efficiency and battery efficiency were changed according to the mode of operation.

10 FUEL AND DISTRIBUTION

It is only in the case of electricity that distribution differs from that for conventional vehicles, as long as conventional and common alternative engines are used. New alternative fuels can come into existence if other types of internal combustion engine and sources of power (e.g.

fuel cells) come to be used. This will most likely not occur for a long time (10-20 years or more). One question which needs to be thought about is how will electric energy for hybrid vehicles be distributed, so that this will be as effective as possible and is going to meet the demands which must be placed on it.