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Advantages And Disadvantages Of Hybrid Cars Engineering Essay

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Even by recently introducing hybrid vehicles to the worldwide transportation system, the need to reduce transport - generated CO2 emissions is still a matter of high significance. One promising and at the same time environmentally friendly solution in terms of limiting the greenhouse gas (GHG) emissions is considered to be the introduction of hybrid electric vehicles (HEVs). In this technical report HEVs will be compared to conventional internal combustion engine vehicles (ICEs) and battery electric vehicles (BEVs), by surveying their technical characteristics and performance, their total cost of ownership (TOC) and their GHG and air pollution (AP) emissions. HEVs can be classified either as parallel or series due to differences at their powertrain configuration. They both use an electric motor and an engine but only parallel HEVs can use simultaneously either of them as a main power source. At series HEVs the engine charges an on-board battery unit that transmits power to the electric motor. Reduced engine capacity, regenerative braking ability and engine shut-off capability are the main discernible characteristics of HEVs in confrontation to their equivalent conventional models.1Some of the most generally acceptable advantages of the HEVs are their low local emissions combined with a high fuel economy, the long driving range and their commercial availability but they still depended on fossil fuels and they are more expensive than conventional ICEs.2

Technical characteristics and performance

Vehicle efficiency and primary energy efficiency, or otherwise well-to-wheel efficiency are the measures used in this study to compare those different drivetrain vehicles. We define the Vehicle efficiency: and, the Primary efficiency where = the useful energy at the wheels, = the energy supplied to the vehicle and = the primary energy.3

Hybrid Electric Vehicle (HEV): For both parallel and series HEVs the vehicle efficiency is 29%.

Internal Combustion Engine Vehicle (ICEV): The max efficiency ay ICEs is achieved near the max load point. The mean efficiency is relatively low since no max power can be achieved in normal driving conditions. At mean required power of 10kW the efficiency is low around 18% whereas around 60-90kW reaches up to 35-40%.4

Battery Electric Vehicles (BEV): An electric motor, connected with a generator and a system of transmission forms the main function of BEVs. Due to the development of advanced electronic control systems, the mean energy efficiency over a normal drive schedule has increased both for generators and electric motors.5 The potential vehicle efficiency is 61%.

The difference in efficiency between hybrid and conventional vehicles can be partly justified by the use of Atkinson-cycle in the hybrid vehicle engines instead of the Otto cycle in the ICEs.6 In cases where the Atkinson cycle is applied to a well modified Otto cycle engine it results to high fuel economy that can be explained by the lower per displacement power than the traditional ICE four stroke engine.

When more power is needed, an electric motor can supplement the engine power which is the basis of an Atkinson cycle working hybrid-electric drivetrain. Bigger work output and higher thermal efficiency than the Otto cycle while operating under similar conditions leads to higher primary efficiency in HEVs.7

In terms of acceleration, BEVs are better than both HEVs and ICEs but in high speed performances ICEs are faster than HEVs with BEVs to be the slowest.8

Total Cost of Ownership

The total cost of ownership is by estimation the sum of the purchase price (Components, retail margin, battery, initial on-road costs), the operating costs (fuel, electricity, servicing) and the resale value. The purchase price is fixed for each vehicle (excluding the uncertainties in the battery prices) but in order to define the operational cost we first have to settle a representative drive cycle. In this study we will work with the AUDC (Australian Urban Drive Cycle) which is a bit more intense in the driving behavior than the common ones but still close to the NEDC (new European drive cycle) and the ARTEMIS cycle (150000 km travelled per vehicle lifetime) .9,10

Due to the large uncertainty in the vehicle battery prices we took a baseline value of $800(kWh)-1 or $16.800 [brooker,4] Furthermore, we estimated a base fuel price at $1.4 L-1 as well as a base electricity price at $0.175 kWh-1.11

In order to determine the operational cost of each vehicle we need to define the fuel and electricity consumption of our modeling vehicles. For a Class E parallel HEV the fuel consumption in L/km was calculated 5.7 whereas for the same category the CV had a consumption of 9.4 L/km. The electricity consumption of a Class E BEV is 0.11 kWh/km. It is clear that despite the entailed increase in vehicle electrification in the purchase price it is compensated with a decrease in the operational costs.

Only by comparing each vehicles purchase price, the CV is the most cost effective solution of both HEVs and BEVs with the lasts to be the most costly ones mainly because of the high battery costs. On the other hand even though the BEVs have the lower running costs it is shown that the parallel HEVs are the ones with the lower Net Present value. Finally in a recent study it was suggested that even hybrid cars are a quite more expensive than the conventional ICE vehicles thay may reduce fuel consumption by 34-47% compared to them which decreases their NPV even more.12

Environmental evaluation

In order to determine the environmental impact of each vehicle we will examine their air pollution and greenhouse gas emissions. To estimate the total CO2 emissions we use the product of carbon intensity (CO2e/MJ) by fuel producers, energy intensity (MJ/km) by car producers and demand (km) by car drivers. In Hybrid (gasoline) vehicles the CO2 emissions are 20 gCO2/MJ and 220 gCO2/MJ delivered to vehicle wheels during production and vehicle life cycle respectively. In ICEs the emissions during production and life cycle are 50 gCO2/MJ and 300 gCO2/MJ whereas in BEVs (electricity production from coal) are 320 gCO2/MJ and approximately 0 gCO2/MJ respectively. It is interesting to notify that in case were electricity production comes from renewable sources (wind) the emission at the production stage of BEV are almost defeasance.13,14

Table1 Environmental impact associated with vehicle production stages

Type of car

GHG emissions (kg)

AP emissions (kg)

Conventional

3595.8

8.74

Hybrid

4156.7

10.10

Electric

9832.4

15.09

In both HEVs and BEVs we must also consider the environmental impact of batteries. We assume that both vehicles use NiMeH batteries of 53kg (1,8kWh capacity) and 430kg( 27kWh capacity), respectively. The production of those batteries require 1.96MJ of electricity and 8.35MJ of liquid petroleum gas.15 With those data and considering that the number of batteries per life of vehicle is 2 for hybrids and 3 for electrics, the total GHG emission per life of vehicle are more than 12 times higher in BEV's.

Finally in order to compare the total GHG and AP emissions for ICE, BEV and HEV's we will consider the scenario that electricity is produced only from renewable energy sources. In that case ICE vehicles are the most polluting ones with almost double GHG and AP emissions than hybrid vehicles and 10 times more than BEV vehicles (450/235/40 g CO2,equivalent /mile respectively).16

Table2 Total environmental impact for different vehicles

Car Type

GHG emissions(kg) /100 km of travelling

AP emissions(kg) /100 km of travelling

Conventional ICE

21.4

0.0600

Hybrid HEV

13.3

0.0370

Electric BEV

2.31

0.00756

The average travelling distance during a 10 year vehicle life time is 241,350km.17

We must say here that in any scenario for electricity production the BEV are still the most environmentally friendly vehicles. Furthermore, hybrid cars may reduce Well-to-wheel GHG emissions to 89-103 gCO2 comparing to conventional ICE gasoline vehicles.18

Georgios Fontaras, Panayotis Pistikopoulos, Zissis Samaras, 2008, Experimental evaluation of hybrid vehicle fuel economy and pollutant emissions over real-world simulation driving cycles, Atmospheric Environment 42, 2008, 4023-4035.

C.C.Chan, Fellow, IEEE, Alain Bouscayrol, Member, IEEE, and Keyu Chen, Member, IEEE, 2010, Electric, Hybrid, and fuel-Cell Vehicles: Architectures and Modeling, IEEE transactions on vehicular technology, Vol.59, No.2, February 2010.

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Kopf et al, 1997, development of a multifunctional high power system: meeting the demands of both a generator and traction drive system, Electric Vehicle Sympozium 14, Orlando (FL), 1997.

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criteria of an irreversible Atkinson heat-engine, Applied Energy 83,2006, 789-800.

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Michel André, 2004, The ARTEMIS European driving cycles for measuring car pollutant emissions, The Science of the total environment, 334-335, 2004, 73-74.

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Oscar P.R van Vliet, Thomas Kruithof, Wim C. Turkenberg, Andre P.C. Faaij, 2010, Techno-economic comparison of series hybrid, plug in hybrid, fuel cell and regular cars, Journal of Power Sources, Vol.195, Issue 19, 2010, 6570-6585.

Felix Creutzig, Emily McGlynn, Jan Minx, Ottmar Edenhofer, 2011, Climate policies for road transport revisited (1): Evaluation of the current framework, Energy Policy, 39, 2011, 2396-2406.

Mikhail Granovskii, Ibrahim Dincer, Marc A.Rosen, 2006, Economic and environmental comparison of conventional, hybrid, electric and hydrogen fuel cell vehicles, Journal of Power Sources, 159, 2006, 1186-1193.

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