Powertrain Features for Fuel Consumption and CO 2 Requirements

The reduction of CO2 produced by the vehicle can be achieved with two main actions, see Figure 4-6:

 the substitution of fuel used for the ICE propulsion, with its carbonization (e.g. CNG) or de-fossilization, for instance using bio-fuels or other energy curriers (e.g. electricity);

 decreasing the vehicle energy consumption.

Figure 4-6 Pathway for a de-Fossilized Vehicle

The first item will be discussed in the Chapter #9, the second one is analyzed in this chapter.

The energy/fuel consumption and CO2 targets in HoQ1 is the more challenging for the vehicle due to even more stringent norms, as presented previously. In the current legislation they are defined as integral Tank-to-Wheel targets on standard driving cycles (e.g. NEDC in EU). It is not immediate to transform these requirements in quantitative powertrain specifications,

To fulfil the fuel consumption and CO2 requirements, a global system approach has to be employed for the vehicle design, with the following main actions, see Figure 4-6 and Figure 4-7:

 reducing the energy for the vehicle movement, that depends on mass, aerodynamic characteristics of the vehicle and wheel rolling resistance, according to equation (4.2);

 reducing the energy used on board by the accessories (e.g. AC, etc.) and other electric devices (e.g. heaters, lamps, etc.) or producing it in more efficient way;

 improving the energy conversion efficiency from fuel to mechanical energy for propulsion;

 recovering the vehicle kinetic energy, otherwise dissipated during braking.

67 Figure 4-7 Energy requirements for vehicle with gasoline engine in US combined Highway/Driving cycle (source:

www.fueleconomy.org).

Reduction of Vehicle Energy Needs for propulsion

The energy needs for the propulsion is mainly dependent on vehicle segment and chassis characteristics.

The powertrain impact on energy needed to move the vehicle is due to the weight and volume. The second feature sets constraint for vehicle shape and aerodynamic resistance. Due to these reasons higher specific power and power density for the powertrain are preferred.

Furthermore the mass and volume of energy storage systems are particularly critics, especially in P-HEVs and BEVs, as already explained in the previous section.

The electric hybrid powertrains are disadvantage compared with pure ICE powertrains, since the mass increase causes fuel penalty, which reduces the electrification benefits.

Reduction of Energy Needs for Auxiliary Functions

The reduction of energy not used for propulsion is another challenging item to improve the fuel economy and the CO2 emission. In [21] the main technology options to face this point at vehicle level are presented.

However the request of new vehicle functions, for instance autonomous driving and connectivity, implies the addition of many sensors and electric devices [24,25], that contrasts the improvements due to efficiency increase of the conventional electric systems on board the vehicle.

The vehicle energy needs not used for propulsion has an undirected impact on powertrain systems, and the following Figure 4-8 can clarify it.

For instance, the cabin heating is an important requirement for the end-user, that depends on engine features. In fact the high efficiency engines (e.g. Diesel ones), due to lower heat losses, cannot be able to perform the fast heat-up of cabin in worst cold conditions and electric heating systems or engine exhaust heat recovery systems have to be adopted, see Chapter #6.9.

68 Figure 4-8 Energy flow and functions of an internal combustion engine in a vehicle

In the real use of vehicle, the AC system is another great energy consumer device. It can affect significantly the fuel consumption and vehicle autonomy. Focusing only on the aspect related to powertrain, that can improve the AC function, the heat to cooling energy conversion should have high advantage. In this field, the more promising technology is based on adsorption principle and it will be described with more details following.

Vehicle Kinetic Energy Recovery

The recovery of braking energy is an important function to improve vehicle efficiency and it has great impact on powertrain architecture, because it forces to introduce devices that are able to harvest the braking energy of vehicle. Among the technology options (e.g. hydraulic based, etc.) the recovery based on an electric system is the most widely adopted solution, favored by the technological progress of the ETD and batteries in the last decades.

Table 4-6 shows the typical values of energy requested to move two different vehicles and the energy dissipated during braking, considering different driving cycles. It can be noted that 20÷35% of energy used during the trip is dissipated by means of the braking and as consequence its harvesting can lead to an important advantage in terms of fuel consumption and emission reduction.

Table 4-6 Energy for vehicle moving and dissipated during braking, considering two different vehicles

Improvement of Propulsion Efficiency

The last contribution affecting the fuel consumption and CO2 emission is the conversion efficiency of fuel to mechanical energy for propulsion, known as Tank-to-Wheel efficiency of the engine/motor. With focus on the SI engines, favorable for lower cost of exhaust gas after-treatment, the study [216] analyzes the critical areas of engine to be improved, as shown in the engine operating map in Figure 4-9:

 pumping losses at partial loads in zone n.1;

69

 limited thermodynamic efficiency due to reduced compression ratio, to avoid knocking damages at high loads, mainly in zones 2 and 3;

 combustion chamber and exhaust system thermo-mechanical stresses at high loads in zone n.4, where the enrichment of the mixture is necessary to protect the engine components, such as the turbine.

In [5,216] and in Chapter #6 the engine technologies options are presented, showing that maximum Brake Thermal Efficiency (BTE) over 40÷45% can be achieved. High efficiency is needed in an area of the engine map as large as possible, especially to ensure low fuel consumption and CO2 emission in real use conditions, where a wide area of operating conditions are involved.

Figure 4-9 Strategic zones of improvements for SI Engine

The use of automated transmissions (e.g. DCT or AT) with high number of gears and proper shifting strategies can bring the engine to work in the most efficient area. The market penetration of automated transmission is expected to increase in the near future, to fulfil new comfort requirements and to allow the implementation of autonomous driving.

In addition to energy recovery, the electric propulsion helps to overcome the limit of engine at part load, below the blue line in Figure 4-9, reducing the engine operation points with low efficiency in zone 1 and shiting the engine operation at high load with higher efficiency. Considering the blue line as an iso-power curve, its position depnds on the electric traction drive and the engine efficiency. The higher the efficiency gap among the electric system and the internal combustion engine, the higher is the power level where electric propulsion will be convenient.

However to fulfill the even more stringent requirements on fuel consumption and CO2,considering driving cycle with higher operating load (e.g. WLTC and in the near future maybe EU RDE cycles), the electrification of the powertrain doesn’t allow to relax the specifications target for the engine. In Figure 4-10 this concept is clarified. The electric propolsion zone, considering its estansion due to use of kinetic energy recovery, covers a lower part if the driving cycle load is extended, with lower benefits. To improve global powertrain efficiency, ICE has to increase its efficiency in area not covered by the electric propulsion.

As consequence of these consideration, the engien technologies that overcome the ICE limits in the zone 2, 3 and 4 have higher priority in the design and they will be presented in a follwing chapter. With simulation models, presented in the Chapter #10, it is possible to obtain a quantitative target for the efficiency of ICE, and to evaluate how much such area has to be extended.

Electric Propulsion zone

ICE Propulsion

70 Figure 4-10 Operating points of WLTC and RDE cycles in SI Engine maps