View
217
Download
0
Category
Preview:
Citation preview
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
1/14
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
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
2/14
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;
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
3/14
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
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
4/14
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.
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
5/14
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
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
6/14
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).
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
7/14
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
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
8/14
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.
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
9/14
Figure 5 Influence of hour in fuel consumption
Figure 6 Influence of average speed in fuel consumption
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
10/14
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
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
11/14
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
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
12/14
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
13/14
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
8/3/2019 Goncalves Farias Silva Comparison Diesel Natural Gas
14/14
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
Recommended