Design of an Innovative Off Road Hybrid Vehicle by Energy Efficiency Criteria

IEA [1] clearly recognizes that transport sector is the one with the highest final energy consumption and, without any significant policy changes. Fig.1 clearly shows that transport is not the main global contributor to GHG emissions but it has a very important contribution.

In 2008, the IEA [2] published 25 energy efficiency recommendations for different sectors. Four of them regard directly the transport sector and include improving tyre energy efficiency, fuel economy standards for both light-duty vehicles (LDVs) and heavy-duty vehicles (HDVs), and eco-driving. The IEA report “Implementing Energy Efficiency Policies: Are IEA member countries on track?” [3] describes the level of implementation of the energy efficiency recommendations by IEA countries. In particular, IEA recommendations about transport are the following:

Recommendation 5.1. Tyres - US has adopted tyre pressure monitoring systems (TPMS) on all passenger cars, multipurpose vehicles, trucks and buses since 2007. EU integrated tires as part of the EU’s integrated approach to reduce CO2 emissions from light duty vehicles (EU, 2009), including mandatory fitting of TPMS, and requirements for rolling resistance and other essential tyre performances. Japan has introduced a voluntary tyre-labelling scheme based on fuel efficiency and wet grip performance by January 2010.

Recommendation 5.2. Fuel economy standards: light-duty vehicles – US introduced the first fuel economy in cars with the Corporate Average Fuel Economy (CAFE) in 1975. The CAFE update (2009) introduced a function of vehicle size or “footprint” by 2011, with the objective of raising the average fuel economy of the fleet to 35.5mpg1 in 2016, with 30% reduction in fuel consumption compared with 2005.

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Figure 1. Global GHG Emissions per sector and per county (source EPA based on IPCC data 2010)

Labelling of vehicle fuel economy and associated costs has also been a requirement in the US for more than 30 years. Canada has recently switched from a voluntary to mandatory fuel economy system and aligned with the United States’ revised CAFE standards. Vehicle CO₂ emissions and fuel economy labelling has existed in the EU since 2001 with differences in labelling among member states. In 2009, the EU adopted CO2 emissions regulations for passenger cars (implementation over 2012-15), with an average objective of car fleet emissions around 130 g CO₂/km by 2015. They were 161 g CO₂/km in 2005. Further reduction is expected through tyre efficiency, gearshift indicators, air conditioners and low carbon fuels.  In Japan, the Top Runner programme sets standards in energy efficiency across a wide range of equipment, including passenger cars. These standards are based on the “best in class” technology and are a function of vehicle weight. Positive labelling is used in Japan where vehicles are rated by how much they exceed their target.

Recommendation 5.3. Fuel economy: Heavy-duty vehicles - Japan introduced fuel economy standards for heavy-duty vehicles in 2006. They are similar to that for passenger cars in Japan and are based on vehicle weight, in this case gross vehicle weight, and the best in class principle of the “Top Runner” programme. Fuel economy levels are mandated to improve 12% from 2002 to 2015. The problem of emissions’ testing has been solved by measuring engine emissions in the laboratory and simulating full vehicle emissions electronically. The EU is has adopted in 2014 methods and standards for CO2 emissions from HDVs based on simulations. In April 2009 in the US, the EPA initiated rule-making procedures on greenhouse gas (GHG) emissions from HDV. The implementation of fuel economy standards for HDVs is difficult because of the wide range of chassis/engine variations, but the  Combined fuel economy standard (34.1mpg) and new EPA standards for air conditioners = 35.5mpg (equivalent to 156 grams CO2/km).

Recommendation 5.4. Eco-driving - Eco-driving can reduce fuel consumption and CO2 emissions by up to 20%.  Most countries now have programmes of eco-driving, either at national or sub-national level. Programmes can be supported with technical aids such as in-car feedback instruments. EU adopted Gear Shift Indicators (GSI) in all new cars from 2012. Eco-driving is implemented at member state level and countries have different programmes. Japan promotes ecodriving informative campaigns and manufacturers generally offer feedback instruments even though they are not required to do so. In 2009 more than 70% of new cars contained such instruments. In the US, several programmes at state level are supported by the auto industries. Eco-driving is a low cost method of reducing vehicle fuel consumption without the need of vehicle technology improvements. An advantage is that it can be implemented with drivers of both new and old passenger cars, as well as those of all sizes of commercial vehicles.

Trancossi [8] has analysed a general energetic model of a vehicle and discussing how the can be used as a design instrument from the preliminary phases of the design. Trancossi [9] has also compared different methods for evaluating the overall Life Cycle Assessment of the impacts of different transport modes. Dewulf and Van Langenhove [10] have defined an innovative exergetic analysis based on an effective analysis of the productivity of the resources. They extend the basic concept of MIPS (Material Input Per Unit of Service) in terms of the second law of thermodynamics. This approach leads to define the concept of EMIPS (Exergetic Material Input per Unit of Service). With respect to transport it takes into account the total mass of the vehicle, the payload to be transported, the total distance, and the speed. This methodology allows on one side an effective evaluation of different transport modes, but also to support an effective design methodology which is based on an effective analysis of vehicle performances.

$E M I P S=\frac{\mathrm{E} \mathrm{x}_{\text { Resources }}}{\mathrm{E} \mathrm{X}_{\text { Service }}}$(1)

The amount of resources extracted from the ecosystem to provide the transport service has quantified defining an inventory of all exergetic resources which are used over a single transport mission but also at whole life cycle level.

This flexibility allows using EMIPS methodology as an integrated design method for an effective energetic and environmental analysis and design, which allows an effective optimization of the system with comparable results. The EMIPS method allows evaluating the cumulative exergy consumption for a specified service with an effective distinction between non-renewable and renewable resource inputs according to Gong and Wall [11]. A model, which can be suitable for design optimization, will be considered and it is much more detailed than the original model by Dewulf. This model will consider two terms one related to the vehicle and one related to the payload. It has been then been possible to understand the energy which is necessary to move the vehicle and the one to move the payload.

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Figure 2. Losses in a ground vehicle

With respect to Dewulf the friction with the ground has also been considered. The general expression of the dissipations during service is then

$E x_{\text { Service }}=E_{R o l}+E_{K i n}+E_{D r a g}+E x_{S t a n d l y}$(2)

where:

1.    Kinetic term:

$E_{K i n}=\frac{1}{2} \cdot M_{t o t} \cdot v_{\max }^{2}$(3)

because the kinetic energy to be acquired depends on the maximum speed v_maxduring the trajectory.

2.    Rolling term:

$E_{R o l}=\int_{0}^{t_{d}} \mu \cdot M_{t o t} \cdot g \cdot v \cdot d t$(4)

3.    Aerodynamic term:

$E_{D r a g}=\int_{0}^{t_{a}}\left(\frac{1}{2} \cdot C_{D} \cdot \rho_{a i r} \cdot A \cdot v^{3}\right) \cdot d t$(5)

4.    Standby term:

$E x_{\text {Standby}}=\dot{m}_{f, \min } \cdot L H V_{f} \cdot T_{s}$(6)

Rolling and aerodynamic terms depend on the velocity profile. Different speed profiles can be investigated. The most efficient profiles should be without reacceleration in order to avoid loss of kinetic energy. In almost all cases of transport technology, kinetic exergy is completely lost, although in electrically driven cars and trains a fraction can be recovered.

Kinetic and rolling terms can be expressed as the composition of two terms because the mass M_tot is the sum of a mass of the payload M_p and the mass of the vehicle M_v:

$E_{K i n}=\frac{1}{2} \cdot\left(M_{v}+M_{p}\right) \cdot v_{\max }^{2}$(3')

$E_{R o l}=\int_{0}^{t_{d}} \mu \cdot\left(M_{v}+M_{p}\right) \cdot g \cdot v \cdot d t$(4')

These considerations allow defining a specific energy balance for the vehicle and for the payload to be transported.

The service term, which refers to the vehicle, is

$E x_{\text {Service,v}}=E_{\text {Rol}, v}+E_{K i n, v}+E_{\text {Drag}}+E_{\text {Standby}}$(7)

and the service term, which is expressly referred to the payload is

$E x_{\text {Service,p}}=E_{\text {Rol}, p}+E_{K i n, p}$(8)

Some further considerations can be performed about the terms, which are based on speed. In first approximation, the speed v is assumed constant and equal to the average value of speed v_av and they become:

$E_{R o l}=\int_{0}^{t_{d}} \mu \cdot m_{t o t} \cdot g \cdot v \cdot d t=\mu \cdot\left(M_{v}+M_{p}\right) \cdot g \cdot v_{a v} \cdot t_{d}$(4")

$E_{D r a g}=\int_{0}^{t_{d}}\left(\frac{1}{2} C_{D} \rho_{a i r} A v^{3}\right) d t=\frac{1}{2} \cdot C_{D} \cdot \rho_{a i r} \cdot A \cdot v_{a v}^{3} t_{d}$(5")

It is possible to express the two service terms as follows:

$E x_{\text {Service,v}}=\frac{1}{2} \cdot M_{v} \cdot v_{\max }^{2}+\mu \cdot M_{v} \cdot g \cdot v_{a v} \cdot t_{d}+$$+\frac{1}{2} \cdot C_{D} \cdot \rho_{a i r} \cdot A \cdot v_{a v}^{3} t_{d}+\dot{m}_{f, \min } \cdot L H V_{f} \cdot T_{s}$

$E x_{\text {Service,p}}=\frac{1}{2} \cdot M_{p} \cdot v_{\mathrm{max}}^{2}+\mu \cdot M_{p} \cdot g \cdot v_{a v} \cdot t_{d}$

During service, the exergy supplied to the system is equal to the one of the necessary fuel.

By dimensional analysis front area of a defender can be estimated 3.2 m². Autobild Magazine [13] has experimentally evaluated drag coefficient by wind gallery testing. For defender it has been obtained CD=0.59 and CD A = 1.9. Aerodynamic Drag effects are presented in Figure 3.

Preliminary calculations have been performed against Sovran and Bohn [14]. The results have been presented in Table 3. They show the full energetic value of the fuel and results in-line with Sovran and Bohn ones. Calculations have been performed by considering the data of a Defender.

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Figure 3. Power dissipated by Drag as a function of speed

This important issue is fundamental for improving the vehicle performance and energy efficiency.

The reshaping activity is currently in an optimization phase. The results allow keeping the same dimensions of the Defender with major aerodynamic improvements without radical changes into the styling.

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Figure 9. Preliminary land rover defender reshaping

A preliminary draft is presented in Figure 9. CFD activity which is still running to ensure an effective system optimization is producing a CD of 0.425 with major improvements with respect to the actual configuration. An objective of this activity around 0.38 is expected as a final result.

Considering a pessimistic CD of 0.425 aerodynamic performances of the vehicle can be completely recomputed.

Data have been evaluated according to equations 4,5,6. Energy conversion losses have been extimated about 5%.

This paper has demonstrated that a very well defined redesign of a mechanical system, such as a car is, can be performed by considering an effective energy optimization process, which can produce both minimization of impacts but also minimization of emissions.

The obtained layout is an initial configuration for the recursive design process. In particular, it allows thinking a very innovative design of possible future hybrid vehicle specifically designed with the generous dymensions of the Land Rover Discovery. 

The results clearly show that outstanding results in terms of energy efficiency can be obtained. The comparison between the original vehicle and the new design is presented and a general reduction of consumption and emissions around 50% can be extimated.