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Chapter 43
Fuel Consumption and Emission
Studies
Abstract
This report is an attempt to provide a basic knowledge about the fuel consumption and vehicular emissions. The concepts of air pollution and automobile pollution are also given due importance. Various types of numerical models related to fuel consumption and air pollution are discussed briefly. The report aims to identify the necessity of understanding the impact of vehicular pollution on the environment. In order to bring the fuel consumption and emission levels to a minimum, various mitigation measures are to be implemented, which are also pointed out in the report.
43.1 Introduction
Urbanization has paved the way for higher levels of comfort and standard of living. Rapid urbanization has thus caused an increase in the number of vehicles and this, on the other hand, is causing another set of problems including lack of space, reduction in natural resources, environmental pollution, etc. We need to consider the existence of a future generation and plan the utilization of our environment and resources wisely. The following sections discuss how the transportation engineer is related in bringing about welcome changes in the development of a sustainable environment. For this, we need to have a basic knowledge about fuel consumption and air pollution, which are discussed briefly below.
43.1.1 Fuel Efficiency
Fuel efficiency or Fuel Economy is the energy efficiency of a vehicle, expressed as the ratio of distance traveled per unit of fuel consumed in km/liter. Fuel efficiency depends on many
parameters of a vehicle, including its engine parameters, aerodynamic drag, weight, and rolling resistance. Higher the value of fuel efficiency, the more economical a vehicle is (i.e., the more distance it can travel with a certain volume of fuel). Fuel efficiency also affects the emissions from the vehicles.
43.1.2 Fuel Consumption
Fuel consumption is the reciprocal of Fuel Efficiency. Hence, it may be defined as the amount of fuel used per unit distance, expressed in liters/100km. Lower is the value of fuel consumption, more economical is the vehicle. That is less amount of fuel will be used to travel a certain distance.
43.1.3 Air Pollution
Air Pollution maybe defined as “The disruption caused to the natural atmospheric environ- ment by the introduction of certain chemical substances, gases or particulate matter, which cause discomfort and harm to structures and living organisms including plants, animals and humans”. Air pollution has become a major concern in most of the countries of the world. It is responsible for causing respiratory diseases, cancers and serious other ailments. Besides the health effects, air pollution also contributes to high economic losses. Poor ambient air quality is a major concern, mostly in urban areas. Air pollution is also responsible for other serious conditions such as acid rain and global warming. The substances causing air pollution are collectively known as air pollutants. They may be solid, liquid or gaseous in nature. The pollutants may be natural or manmade (anthropogenic). Pollutants are classified as primary and secondary air pollutants. Primary pollutants are those which are emitted directly to atmosphere, whereas, secondary pollutants are formed through chemical reactions and various combinations of the primary pollutants. Some of the major primary and secondary air pollutants are given in Table. 43:1 and Table. 43:. The sources of air pollution may be natural or anthropogenic. The anthropogenic sources of air pollution are those which are caused by human activity. The major anthropogenic sources include Stationary sources (such as smoke stacks of power plants, incinerators, and fur- naces), Mobile sources (e.g. motor vehicles, aircraft), Agriculture and industry (e.g. chemicals, dust), Fumes from paint, hair spray, aerosol sprays, Waste deposits in landfills (which contain methane) and Military (e.g. Nuclear weapons, toxic gases). The natural sources of air pollu- tion may be Dust from areas of low vegetation, Radon gas from radioactive decay of Earths crust, Smoke and CO from wildfires, and volcanic activity which produces sulfur, chlorine and particulates.
Evaporative Emissions Refueling Losses
ExhaustEmissions
Figure 43:1: Losses of fuel in vehicles, Source: [4]
43.2.1 Types of Vehicular Emissions
The fuel loss of vehicles may be due to emissions or refuelling. The emissions maybe evaporative or exhaust emissions. The fuel losses in a vehicle are shown in Fig. 43:1.
- Exhaust emissions: Exhaust emissions are those which are emitted through the exhaust pipe when the vehicle is running or is started. Hence, the exhaust emissions maybe of 2 types - start up emissions and running emissions.
(a) Startup emissions: Emissions when the vehicle is started initially. Based on how long the vehicle had been turned off after use, they may be cold start and hot start. Cold start refers to when the vehicle is started suddenly after a long gap of use, whereas, hot start refers to when the vehicle is started without the vehicle getting enough time to cool off after its previous use. (b) Running emissions: Emissions during normal running of the vehicle, i.e., when the vehicle is in a hot stabilized mode.
- Evaporative emissions: These include running losses and hot soak emissions produced from fuel evaporation when an engine is still hot at the end of a trip, and diurnal emissions (daily temperature variations). [4]
43.2.2 Exhaust Pollutants
The pollutants which are emitted from the exhaust pipe of the automobiles are known as exhaust pollutants. They are formed as a result of combustion of the fuel in the engine. These pollutants are harmful to the atmosphere and living things in particular. The major types of exhaust pollutants are discussed in the following sections.
Sulphur Oxides (SOx)
Combustion of petroleum generates Sulfur Dioxide. It is a colorless, pungent and non flammable gas. It causes respiratory illness, but occurs only in very low concentrations in exhaust gases. Further oxidation of SOx forms H 2 SO 4 and thus acid rains.
Nitrogen Oxides (NOx)
Combustion under high temperature and pressure emits Nitrogen dioxide. It is reddish brown gas. Nitrogen oxides contribute to the formation of ground level Ozone and acid rain.
Hydrocarbons and Volatile Organic Compounds (HC and V OC)
Hydrocarbons result from the incomplete combustion of fuels. Their subsequent reaction with the sunlight causes smog and ground level Ozone. V OCs are a special group of Hydrocarbons. They are divided into 2 types methane and non methane. Prolonged exposure to some of these compounds (like Benzene, Toluene and Xylene) may also result in Leukemia. V OCs are a special group of Hydrocarbons. They are divided into 2 types methane and non methane. Prolonged exposure to some of these compounds (like Benzene, Toluene and Xylene) may also result in Leukemia.
Carbon Dioxide (CO 2 )
It is an indicator of complete combustion of the fuel. Although it does not directly affect our health, it is a greenhouse gas which causes global warming.
Carbon Monoxide (CO)
It is a product of the incomplete burning of fuel and is formed when Carbon is partially oxidized. CO is an odorless, colorless gas, but is toxic in nature. It reaches the blood stream to form Carboxyhemoglobin, which reduces the flow of Oxygen in blood.
Lead (P b)
It is a malleable heavy metal. Lead present in the fuel helps in preventing engine knock. Lead causes harm to the nervous and reproductive systems. It is a neurotoxin which accumulates in the soft tissues and bones.
more horsepower and more weight, these factors also contribute to the emission rates. Another important factor is the age of the vehicle. Older vehicles have higher emission rates.
Other Factors
- Ambient Temperature Evaporative emissions are higher at high temperatures.
- Type of engine Two stroke petrol engines emit more amounts of pollutants than the four stroke diesel engines.
- Urbanization Congestion is higher in urban areas, and hence emissions are also higher. [1]
43.2.4 Bharat Stage Emission Standards
Bharat Stage emissions standards are emissions standards instituted by the Government of the Republic of India that regulate the output of certain major air pollutants (such as nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), particulate matter (P M), sulfur oxides (SOx)) by vehicles and other equipment using internal combustion engines. They are comparable to the European emissions standards. India started adopting European emission and fuel regulations for four-wheeled light-duty and for heavy-dc from the year 2000. For two and three wheeled vehicles, the Indian emission regulations are applied. As per the current requirement, all transport vehicles must carry a fitness certificate which is to be renewed each year after the first two years of new vehicle registration. The National Fuel Policy announced on October 6, 2003, a phased program for implementing the EU emission standards in India by 2010. The implementation schedule of EU emission standards in India is summarized in Table. 43:3. [3] Some of the important emission standards for different vehicle types are given in the following tables (Table. 43:4 - 43:7).
43.3 Fuel Consumption Models
Fuel consumption models are mathematical functions relating the various factors contributing to the fuel consumption. The influencing factors may be no. of vehicle trips, distance travelled by the vehicle, no. of stops, vehicles average speed, etc. The major fuel consumption models are discussed in the following sections.
Table 43:3: Indian Emission Standards (4-Wheel Vehicles), Source: Emission Norms, SIAM India [3] Standard Reference Date Region India 2000 Euro 1 2000 Nationwide 2001 NCR, Mumbai, Kolkata, Chennai Bharat Stage II Euro 2 2003.04 NCR, 13 Cities** 2005.04 Nationwide Bharat Stage III Euro 3 2005.04 NCR, 13 Cities* 2010.04 Nationwide Bharat Stage IV Euro 4 2010.04 NCR, 13 Cities*
- National Capital Region (Delhi) ** Mumbai, Kolkata, Chennai, Bengaluru, Hyderabad, Ahmedabad, Pune, Surat, Kanpur, Lucknow, Sholapur, Jamshedpur and Agra
Table 43:4: Emission Standards for Diesel Truck and Bus Engines, g/kWh, Source: Emission Norms, SIAM India [3] Year Reference Test CO HC NOx P M 1992 - ECE 17.3- 2.7-3.7 - - R49 32. 1996 - ECE 11.2 2.4 14.4 - R 2000 Euro I ECE 4.5 1.1 8 0.36* R 2005** Euro II ECE 4 1.1 7 0. R 2010** Euro III ESC 2.1 0.66 5 0. ETC 5.45 0.78 5 0. 2010# Euro IV ESC 1.5 0.46 3.5 0. ETC 4 0.55 3.5 0.
- 0.612 for engines below 85 kW ** earlier introduction in selected regions, see Table. 43:1 # only in selected regions, see Table. 43:
Table 43:7: Emission Standards for 4 - Wheel Gasoline Vehicles (GVW 3,500 kg), g/km, Source: Emission Norms, SIAM India [3] Year Reference CO HC HC + NOx NOx 1991 - 14.3- 2.0- - 27.1 2. 1996 - 8.68- - 12.4 3.00-4. 1998* - 4.34- - 6.20 1.50-2. 2000 Euro 1 2.72- - 6.90 0.97-1. 2005** Euro 2 2.2-5.0 - 0.5-0. 2.3 0.2 0. 2010** Euro 3 4.17 0.25 - 0. 5.22 0.29 0. 1 0.1 0. 2010# Euro 4 1.81 0.13 - 0. 2.27 0.16 0.
- for catalytic converter fitted vehicles ** earlier introduction in selected regions, see Table. 43:1 # only in selected regions, see Table. 43:
120 110 100 90 80 70 60 50 40 30 20 10 (^0 0 20 40 60 80) 100 120 140 160
PSVs
Overall OGV
Speed kph
Litres/100m
Figure 43:2: Fuel consumption as a function of speed, Source: [4]
43.3.1 Average Speed Model
Average speed models are macroscopic in nature. They are concerned with the traffic network as a whole, on a large scale. Individual vehicles are not considered. This model relates the fuel consumption directly with the travel time (or indirectly with vehicle speeds). This model is not valid for speeds higher than 56 km/hr. as the effects of air resistance become increasingly stronger. The fuel consumed is related to the average speed (or travel time) using the relation below:
F = k 1 + k 2 T (43.1) F = k 1 + k v^2 (43.2)
where, F = Fuel consumed per vehicle per unit distance (liters/km) T = Travel time per unit distance, including stops and speed changes (minutes/km) v = Avg. speed measured over a distance including stops and speed changes (10 ≤ v ≤ 56kmph) k 1 = parameter associated with fuel consumed to overcome rolling resistance, approximately proportional to vehicle weight (liters/veh- km) k 2 = Parameter approximately proportional to fuel consumption while idling (liters/hr) [2] Fig. 43:2 gives the relation between fuel and consumption and speed of the vehicle. It can be inferred from the figure that fuel consumption is high for lower speeds and is the minimum for intermediate speeds. Fig. 43:3 shows the relation between bus fuel consumption and number of stops. It is clear from the graph that fuel consumption increases as the number of stops of the vehicle increases.
commuters use a 50 seater bus service.
Hence the number of buses will be 4000/50, which is equal to 80 buses. Remaining (20000 - 7000 - 4000 = 9000) are single car drivers. The total consumption by car will include the consumption of cars of single occupancy and the cars in the carpool. Hence, the fuel consumption by cars is [0.085 * 15] + [(1.5/35) * 15], that is 1.917 liters/vehicles. So, for all the cars, the total fuel consumption will be 1.917* (9000 + 2333), which is 21725. liters. Similarly, the bus fuel consumption for a bus with 7 stops will be 0.3 *2.35 * 80 * 15 which is 846 liters.
Fuel consumption corresponding to 7 stops is obtained from Fig. 43:3. 2.35 is a conversion factor to bring the fuel consumption in terms of liters/km instead of gallons/mile. Total fuel consumption will be the sum of fuel consumptions of bus and car. That is 21725+ = 22571 liters.
The total amount of fuel saved will be the difference of fuel consumptions in both the cases. Hence the amount of fuel saved is 43500 - 2257, which is equal to 20929 liters.
43.3.2 Drive Mode Elemental Model
Unlike the average speed model, the drive model elemental model is a microscopic fuel con- sumption model. It considers the movement of a single vehicle. This model is used to obtain the fuel consumption rates during various vehicle operating conditions or drive mode. The different drive modes include cruising, idling, accelerating and decelerating, which together form a driving cycle. The important assumptions used in this model are that the driving mode elements are independent of each other and the sum of the component consumption equals the total amount of fuel consumed. The advantages of this model are that the model is simple and general and there is a direct relationship to existing traffic modelling techniques. The disadvantage of this model is that the variation in the behavior of different drivers and behavior of the same driver under different situations is ignored. [2]
The component elements considered here are various drive modes such as cruising, idling and accelerating. The total fuel consumed for the drive mode elemental model is given by the relation: G = f 1 L + f 2 D + f 3 S (43.3)
where, G = fuel consumed per vehicle over a measured distance (total section distance) L = total section distance traveled D = stopped delay per vehicle (time spent in idling) S = number of stops f 1 = fuel consumption rate per unit distance while cruising f 2 = fuel consumption rate per unit time while idling f 3 = excess fuel used in decelerating to stop and accelerating back to cruise speed
Numerical Example 2
The total fuel consumption by a vehicle travelling on a stretch of road is 0.0735 liters/veh-km. The average stopped delay for the vehicle is 6s. The vehicle stops thrice during its journey. Assume f 1 = 0.0045, f 2 = 0.0035 and f 3 = 0.002. Calculate the length of road considered. If the vehicle is cruising throughout the stretch of the road, what is the decrease in fuel consumption?
Solution: From the equation. 43.3, the fuel consumed per vehicle over a measured distance is given by
G = f 1 L + f 2 D + f 3 S
Step 1: It is given that fuel consumed per vehicle is 0.0735 liters/veh-km, average delay is 6s and the number of stops are 3. The values of f 1 , f 2 and f 3 are given as 0.0045,0.0035 and 0.002 respec- tively. It is required to find the length of the road. The length L can be computed from the above equation as given: 0 .0735 = (0. 0045 ∗ L) + (0. 0035 ∗ 6) + (0. 002 ∗ 3) Therefore, Length, L is equal to 10 km. Step 2: When the vehicle is cruising throughout the length, there will not be any delays or stops. Therefore, total fuel consumption: G = f 1 L = 0. 0045 ∗ 10 = 0. 045 liters/veh − km Step 3: The decrease in fuel consumption is will be the difference in fuel consumptions as obtained in steps 1 and 2, which is 0.0735-0.045 = 0.0285 liters/veh-km.
Instantaneous Emission Model
The model is similar to the instantaneous fuel consumption model. It describes the vehicle emission behavior during any instant of time. The advantages of the model are that the emission factors can be calculated and generated for any vehicle operating profile, and the model considers dynamics in driving patterns. The model also has some disadvantages such as detailed and precise information on vehicle operation and location is required and the process of data collection is expensive.
Emission Factor Model
This model is useful in macro level where detailed information is not required. A single emission factor is used to represent a particular type of vehicle and general type of driving. Emission is estimated using the equation: E = A ∗ EF (43.4)
where, E = emissions, in units of pollutant per unit of time A = activity rate, in units of weight, volume, distance or duration per unit of time EF = emission factor, in units of pollutant per unit of weight, volume, distance or duration
Numerical Example 3 Using the emission factor model, the amount of CO emitted by a vehicle was estimated as 50 grams per hour. If the vehicle travelled at a velocity of 40kmph, estimate the emission factor for CO for the vehicle.
Solution It is given that the total emission E is 50g/hr. The activity A here is the amount of CO emitted by the vehicle, which is 40km/hr. from the eqn. 43.4, we have, the total emissions is E = A ∗ EF Therefore, the emission factor will be E/A = 50/40 = 1.25. That is, the emission factor of CO is 1.25 grams/km. The variation of exhaust emission factors with speed for the major exhaust pollutants are given in the following figures (Fig. 43:5 to Fig. 43:10).
Cars petrol HGVs rigid
LGVs diesel HGVs articulated Buses Motorcycles
Cars diesel LGVs petrol
For Particulate Matter (P M10) and Volatile Organic Compounds, the emissions steadily
0 20 40 60 80 100 120
0
0.
0.
0.
0.
1
1.
1.
1.
1.
Speed (km/hr)
PM 10
Emission factor, g/km
Figure 43:5: Variation of emission factor with Speed for Particulate Matter, Source: [5]
(^00 20 40 60 80 100 )
2
4
6
8
10
12
14
16
18 NOx
Emission factor, g/km
Speed (km/hr)
Figure 43:6: Variation of emission factor with Speed for Nitrogen Oxides, Source: [5]
0 20 40 60 80 100 120
0
0.
0.
0.
0.
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Speed (km/hr)
Emission factor, g/km
SO 2
Figure 43:9: Variation of emission factor with Speed for Sulphur Dioxide, Source: [5]
0 20 40 60 80 100 120
0
100
200
300
400
500
600
Emission factor, g/km
Speed (km/hr)
CO 2
Figure 43:10: Variation of emission factor with Speed for Carbon Dioxide, Source: [5]
decrease with the speed. In case of Nitrogen Oxides, Sulphur Oxides and Carbon Dioxide, the emission is highest for low speeds, decreases for intermediate speeds and then again increases with the speed. For Carbon Monoxide, the highest emission levels occur for higher speeds and minimum emission occurs for intermediate speeds.
Average Speed Emission Model
Average Speed Models are used in the measurement of emission rates of a pollutant for a given vehicle for various speeds during a trip. Average Speed Emission models, along with the Emission factor models are widely applied in national and regional inventories. The emission factor in this model (EF ) is measured over a range of driving cycle (which includes driving, stops, starts, acceleration and deceleration). It is given in g/veh-km. Though these models are good in measuring congestion, they have certain disadvantages, which are explained below:
- A single emission factor is used for a value of average speed irrespective of the vehicle operational characteristics
- Average speed is a less reliable indicator of estimation of emissions for the newest gener- ation of vehicles ( as they have after treatment devices)
- The shape of an average speed function is not fundamental, but depends, amongst other factors, on the cycle type used. Even though each cycle used in the development of these functions represents a real life driving condition, the real distribution of these driving conditions is not normally taken into account.
- Average speed models do not allow for detailed spatial resolution in emission predictions. [2]
Modal Emission Model
This model is similar to the drive mode elemental fuel consumption model. Emission rates are explained as a function of the vehicle operation mode. The model provides accurate emission estimates at micro level. For each mode, emission rate is fixed for a particular type of vehicle and pollutant. Instantaneous traffic related data is required to estimate the fuel consumption.
