Goncalves Farias Silva Comparison Diesel Natural Gas

8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas

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COMPARISON OF DIESEL AND NATURAL GAS

URBAN BUSES ENERGY EFFICIENCY AND

ENVIRONMENTAL PERFORMANCE

Gonalves, G. A.*, Farias, T.L. and Silva, C. M.Instituto Superior Tcnico, Av. Rovisco Pais, 1, 1049-001Lisbon, Portugal

Abstract The present work evaluates the energy and environmental impact of urban transporttechnologies by using a methodology where firstly the different technologies arecompared using numeric methods, and this includes technologies covering pre-Euro

vehicles to modern Euro 3 vehicles.On the second phase, Diesel and compressed natural gas (CNG) vehicles are directlycompared by using the results of a series of on-road measurements, in which a Dieselvehicle was monitored by registering the fuel consumption, altitude and speed on a

second by second sampling rate. Passenger load was also estimated by counting thenumber of passengers on board along the selected bus line.These results were used to simulate a CNG and a Diesel vehicle (using EcoGest, aproprietary simulation tool) on the same route, thereby enabling a direct comparison of

the two technologies in terms of fuel consumption, GHG emission and pollutantemission. The results where further complemented by a WTW (well-to-wheel) analysison the production, distribution and filling of each fuel.Overall, the newer Diesel Euro 3 vehicles have significant advantages in terms of

pollutant emissions, be it NOx, CO or HC, with expected reductions in the order of70.90% (Euro3 vs Pre-Euro). Particle emission is also greatly reduced (73%). CO2emissions have no significant change due to the similar levels of fuel consumption for

different generation vehicles.The CNG (compressed natural gas) vehicle has significant advantages in all criteriapollutants, with reductions in NOx (-99%) and CO (-88%). HC emissions are similar.Concerning energy efficiency and fuel consumption, CNG vehicles used significant

more energy (20-30% more). Considering the total life cycle of the fuel, the Dieselvehicle still uses less energy per km (-2.8%), however, due to the lower carbon content,the CNG vehicle has a lower GHG emission (-14%).

Keywords Diesel bus, CNG bus, Gaseous Emissions, Well-to-wheel analysis

INTRODUCTION

Under the Kyoto protocol, Portugal was allowed a 27% increase ofemissions of GHG (relative to the values of 1990) by 2010. By 2000 emissions

were already 30.4% above 1990 levels. The transport sector is the largest


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contributor to the global GHG emission, with about 30% of the total national

emissions. Within the transport sector, 70% of the emissions result from road

transports.

With the objective of improving the image of public transports andreducing the environmental impact of their fleets, public transport companies

have been introducing alternative cleaner technologies in operation. One such

example are compressed natural gas (CNG) buses, of which STCP (Sociedade

de Transportes Colectivos do Porto), the major transit bus operator in the city of

Porto, Portugal, operates one of the largest fleets in Europe. However, the

advantages of using such technologies are not perfectly clear, especially in what

regards to fuel consumption and GHG emissions.

Another option of reducing pollutant emissions (but not for fuelconsumption) is fleet renewal. More recent Diesel technologies (Euro 3) have

significantly reduced pollutant emissions, which must be compared both to older

technologies and CNG vehicles.

OBJECTIVE

The objective of the present work is to analyze and quantify the energy and

environmental impact of urban transport bus technologies, comparing directly

CNG and Diesel buses, considering the following aspects:

Estimate, using numerical methods, fuel consumption, emissions (CO2,

CO, NOx, HCs and particles) of the buses on the STCP fleet considering thefollowing classes:

CNG vehicles; Pr-Euro vehicles; Diesel Euro I vehicles; Diesel Euro II vehicles; Diesel Euro III vehicles;

Characterize, for a typical line, vehicle dynamics, passengers on board,

and route topography to input in the equations of the numerical methodology; Estimate the emissions of pollutants and fuel consumption for the

vehicles under study considering the effect of:

Type of fuel; Hour;

Calculate the total energy use and CO2 equivalent emissions for the

production, distribution and filling of Diesel and compressed natural gas;


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Analyze the environmental impact of Diesel and CNG on a well-to-

wheel perspective;

METHODOLOGY

For the duration of the study, two vehicles (one Diesel, one CNG)

circulated in the selected line. Daily mileages and fuel consumption where

registered for around one month of regular operation. This assured the vehicles

where subjected to the same traffic and passenger loads. These results where

complemented with calculations of fuel consumption and CO2 equivalent

emissions for the production of the fuels using a life-cycle analysis software

(Gabi, IKP, 1992-2002), the flow of information and tools used are presented onfigure 1.

Figure 1 Flow of information

Experimental characterization

To characterize in detail the fuel consumption and dynamics of the vehicles

in operation a Diesel vehicle was selected from the STCP fleet and equippedwith several measuring devices (figure 2).

Experimental

characterization

Daily fuel

consumption

EcoGest

model

Corinair

Methodology

Emission of

pollutants Diesel

and GNC

Emission of

pollutants Diesel

Pre-Euro-Euro IV

Information on

operation and

infrastructure

Lyfe-cycle

software

Well-to Wheel

impact


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Figure 2 Layout of the monitoring system

The system was developed from a VDO Kienzle set that included a

flowmeter and a command/display unit. For this application, the system is

connected to a data acquisition board and a laptop computer in order to record

(with a 1 Hz frequency) fuel consumption and distance. Additionally, a GPS unit

with a barometric altimeter is connected to register the route topography. The

information is recorded in a Laptop running proprietary software.

The monitoring system allows the bus to run in regular operation, with

minimal interference. The whole system and operators occupy only two seats

(one of the operators does the passenger count).

Adding to the detailed measurements, the daily fuel consumption for the

vehicles being compared was recorded for an extended period in order to verify

the measured values.


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Simulation with the EcoGest model

EcoGest is a simulation model for fuel consumption and pollutant

emissions for road vehicles (Silva et al, 2004). It has the possibility to analyze

real driving cycles (as the ones measured), considering the vehicle

characteristics like weight, transmission and exhaust after-treatment. The model

also takes into consideration the topography and load. The model uses the

measured parameters from the on-board monitoring system: speed, grade, fuel

consumption and number of passengers. The model has a database with several

engine maps for specific fuel consumption and pollutant emission (HC, CO and

NOx), for stationary, warm operating conditions. These maps are either supplied

by engine manufacturers or generated by a sub-model for conventional andalternative fuels. The internal calculations of the EcoGest model are based on

the determination of engine load and rpm at each moment according to vehicle

speed and gearbox management. These values are used as inputs for specific fuel

consumption and emission maps (in this case supplied by MAN). Another sub-

model deals with exhaust after-treatment for vehicles equipped with catalytic

converters and computes the emissions to the atmosphere. For the present work

the maps available allowed the simulation of Euro 3 Diesel and CNG vehicles in

the routes monitored, allowing a direct comparison between the two

technologies under the same operating conditions (speed, load and grade).

Corinair Methodology

The Corinair methodology was developed by a consortium of European

research centers (Ntziachristos et al, 2000) and provides an estimation of fuel

consumption and pollutant emissions for a fleet based on average speed, average

grades and vehicle type. The methodology includes vehicles from Pre-Euro to

Euro V but does not CNG vehicles.

The results from these two methodologies (EcoGest and Corinair) are not

directly comparable. The Corinair methodology is based on a typical fleet,

where the vehicles are subjected to a wide array of operation conditions. When

comparing fleets with different ages (and emission regulations) this provides a

good overall result. When the objective is comparing two vehicles in a specific

situation, the methodology is limited due to its global approach. This is where

the EcoGest model has the advantage of comparing two vehicles under the sameoperating conditions. In this specific situation, the routes selected are verydemanding, and therefore not the average route that the Corinair methodology


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considers, and the emissions calculated by EcoGest will very likely be higher

that the results from Corinair.

Life Cycle analysisWhenever one estimates the emissions for a given combination of fuel and

propulsion system, it is necessary to take into account not only the emissions

associated with vehicle operation but also those resulting from the production,

transport and filling of the fuel. Taking as example electric propulsion vehicles,

although not having any local gaseous emissions, they use the available electric

grid, and the power mix not only changes from country to country but also along

the year.

For the present work life cycle software was used - Gabi (IKP, 1992-2002)that, for the production of each fuel, calculates the energy input and the emission

of greenhouse gases. The tool takes into account the origin and nature (crude oil,

natural gas) of the primary energy source, its transport and processing and final

filling to the vehicle tank.

For the production of Diesel fuel, the inputs of the tool require the

geographic origin of the imported crude (with the different transport costs, both

in energy and emissions), processing at the refinery in Leixes (located near

Porto) and final filling. There is no distinction between imported and locally

produced Diesel fuel.

For CNG, the fuel comes from Algeria and the tool takes into consideration

extraction, processing and transport through pipeline (liquefied natural gas from

Nigeria is not taken into consideration for this work), transport within borders tothe STCP depot and electricity consumption in the compression to fill the

vehicle (the onboard takes store the natural gas at a pressure of 200bar, the

compressors supply the gas at approx. 240bar.

RESULTS

Line characterization

Detailed measurements where made for a complete day of operation (19

August 2004) in line 20 (figure 3) of STCP network. This is a circular line

without terminal stop and constant circulation. The length is ~7.8km of entirely

urban circuit, with low speeds and high gradients (figure 4).


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Figure 3 Line 20 of STCP network

Figure 4 Route profile

For each trip the monitoring system records distance and fuel consumption.

In he day of the trials 19 trips where made, of which 14 are considered valid (the


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remaining where not complete). The vehicle was driven by two different drivers,

one in the morning and one in the afternoon.

Table 1 shows relevant data for the valid trips. Load estimations are based

on passenger counts with an average weight of 70kg.

Table 1 Measured data for valid trips

Starthour

Duration(h:m)

Distance(m)

Fuel(l)

Average speed(km/h)

Consumption(l/100km)

Average load(kg)

6:41 29:38 7847.9 3.56 15.9 45.3 544

7:12 32:33 7844.7 3.62 14.5 46.2 526

7:46 37:04 7841.7 3.96 12.7 50.4 786

8:25 36:30 7840.4 4.46 12.9 56.8 1368

9:47 45:54 7828.5 5.42 10.2 69.3 1685

10:35 38:18 7851.4 5.45 12.3 69.4 260611:14 42:05 7844.4 5.83 11.2 74.3 2576

11:58 38:33 7838.2 5.27 12.2 67.2 1822

14:56 41:03 7836.7 5.83 11.5 74.4 1840

15:39 45:53 7835.7 6.09 10.2 77.7 1572

16:27 46:41 7827.8 5.31 10.1 67.8 1510

17:16 52:25 7843.4 5.85 9.0 74.6 2651

18:11 42:09 7833.5 5.53 11.2 70.6 1591

Influence of operating conditions

From the data collected and presented in table 1 it is possible to correlate

fuel consumption with other variables such as time of day (figure 5) and average

speed (figure 6). The results also show the different driving stiles of bothdrivers.


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Figure 5 Influence of hour in fuel consumption

Figure 6 Influence of average speed in fuel consumption


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Comparison of emissions and fuel consumption for different

generations of Diesel vehicles

The Corinair methodology was used to compare different generations of

Diesel vehicles. The fuel consumption is considered equal for all the

technologies, which was confirmed by the operators from field experience. For

more recent vehicles, any improvements in fuel economy are wasted due to

increasing vehicle weight and auxiliary power consumption (air conditioning).

Table 2 shows the results for the selected route.

Table 2 Emissions for different Diesel vehicles according to the Corinair methodology

Pre-Euro(g/km) Euro I(g/km) Euro II(g/km) Euro III(g/km) Euro IV(g/km)

CO 3.93 1.97 1.57 1.10 0.80

NOx 31.37 21.96 15.68 10.98 7.69

HC 3.31 2.48 2.32 1.62 1.13

PM 1.22 0.79 0.49 0.34 0.06

For this case, Euro III vehicles show a reduction of NOx of 62% (the

reduction for Euro IV vehicles would be 75%) in relation to Pre-Euro vehicles.

For CO emissions, the reductions are of 75% (85% for Euro IV vehicles). For

particulate matter emissions, the reductions are of 73% (95% for Euro IV

vehicles).

Comparison Diesel CNG

The Corinair methodology does not include CNG vehicles so a different

methodology was used: the EcoGest model. One of the main input of EcoGest

are engine maps for specific fuel consumption and pollutant emissions, and for

the present work two such maps where used, one CI (compressions ignition)

Diesel and one SI (spark ignition) CNG, both Euro III, allowing direct

comparison between the two technologies using the EcoGest model. Results are

summarized on table 3.

Table 3 Comparison between Diesel and CNG using the EcoGest model.

Euro III Diesel EcoGest (g/km) Euro III GNC EcoGest (g/km)

CO 2.93 0.35

NOx 18.72 0.124HC 0.31 0.24


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The results obtained are generally in agreement with other studies (Nylund

et all, 2003, Pelkmans et all, 2002), with the exception of the NO x emissions for

the CNG vehicle, which are exceptionally low. Although CNG vehicles tend to

have lower NOx emissions than Diesel vehicles, the lowest values found in theliterature are still in the order of 1g/km, and the engine simulated here is also

common to several of the CNG buses tested on the literature. The values

calculated are based on emissions maps supplied by the engine manufacturer,

and those values are obtained in test bench for stationary conditions. When

vehicles run in normal operation most of the time the engine runs in transient

mode, and that can explain some differences between calculated and measured

values: if the pollutant generation mechanism is very dependant on transient

operation, calculated values will be lower than measured. This however does notseem to influence emissions of CO and HC, which follow very closely measured

values.

The maps used in these simulations do not include particulate matter and

no estimations are provided for those values. CNG engines have however a great

advantage in that area, although one might argue that as Diesel technologies

progress, using for example particle traps, new comparisons should be made,

specially when considering that the particles emitted from a CNG vehicle are of

a different nature, much smaller and with unknown effects to the human health,

but that discussion falls outside the scope of this work.

Energy consumption

For these calculations only average values for fuel consumption are

considered, as they are representative of a broader array of operating conditions.

For the vehicles tested data is presented in table 4.

Table 4 Average fuel consumption for the period tested

CNG

4,623 km

3,720.9 Nm3

80.49 Nm3/100km

Diesel

1,958 km

1,321.1 Liters

67.47 Liters/100km

The calculations include a life cycle analysis on the production of the fuels,

using a life cycle analysis tool (Gabi). The tool takes into account all the

processes involved in the extraction, processing, transport and filling of the two

fuels. In these calculations electricity plays an important role, as it is extensively


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CONCLUSIONS

The objective of the present work was to compare the energy and

environmental performance of the two main technologies for urban buses: Diesel

and CNG.

Presently, most fleets are composed of a mix of Diesel buses, with different

ages and emission standards, from Pre-Euro to Euro III. Based on the

comparison of the different Diesel technologies, one can conclude that:

When subjected to the same operating conditions (and assuming the

vehicles are in good working conditions), more recent vehicles have

considerable advantages in pollutant emissions (CO, NOx, HC and PM), whichreflects the effect that emissions regulations, having successfully reduced

emissions of new vehicles by 50-90% in relation to Pre-Euro vehicles.

By replacing Pre-euro vehicles with Euro III vehicles, reductions in the

emission of pollutants are in the order of 62% for NOx, 75% for CO and 51% for

unburned hydrocarbons. For particulate matter, a reduction of 73% is expected.

CO2 emissions are not going to be reduced by introducing more modern

vehicles.

Using natural gas as fuel is an option several operators are taking, and

several manufacturers already offer in their catalogue CNG buses. In comparing

CNG and Diesel buses a numerical simulation model was used complemented

with detailed measurements of dynamics, topography and passenger load in asuburban line of STCP (line 20).

Based on the results for this line, which is characterized by low speeds and

high gradients, one can conclude that:

Based on calculated values, CNG buses have substantial advantages for

all pollutants, especially on CO emissions (-88%), HC emissions are

comparable.

NOx values calculated must be analyzed more carefully, but values from

literature still show a considerable decrease in emissions for CNG vehicles when

compared to Diesel vehicles.

Average fuel consumption for Diesel vehicles in this line is 67.5

liters/100km (24.4MJ/km), while for CNG vehicles consumption is 80.5

Nm3/100km (27.2MJ/km), depending on operating conditions. When considering the whole life cycle, including fuel production, the

overall energy consumption for both technologies is very similar (higher


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efficiency on Diesel vehicle but bigger energy input for fuel production), while

GHG emissions favors the CNG technology, with savings of 14%.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of DGTT Direco Geral

dos Tranportes Terrestres and STCP.

REFERENCES

IKP (1992-2002) GaBi Software-System and database for Life CycleEngineering; (www.gabisoftware.com).

Ntziachristos, L. and Samaras, Z. COPERT III: Computer program to

calculate emissions from road transport - Methodology and emission factors

(Version 2.1) (2000), European Environment Agency.

Silva, C.M., Farias, T. L. and Mendes-Lopes, J. M. C. Cold Start, Part

Warm Start and Warm Up Simulation of Vehicles in Ecogest. FISITA 2004 -

30th FISITA World Automotive Congress. Barcelona, Spain, 23-27 May 2004

Nils-Olof Nylund, Kimmo Erllila, Maija Lappi and Markku Ikonen, Transit

Bus Emission Study: Comparison of Emissions from Diesel and Natural GasBuses, 2004

Luc Pelkmans, Guido Lenaers, Dirk De Keukeleere, Evaluation of

Emissions and Fuel Consumption of Heavy Duty Natural Gas Vehicles in Real

City Traffic, 2002

Goncalves Farias Silva Comparison Diesel Natural Gas

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