Calculate a carbon footprint

A carbon footprint is defined as the total amount of greenhouse gases produced by an activity, usually expressed in equivalent metric tons (1,000 kilograms) of carbon dioxide (CO2). Any time you operate an engine which burns a carbon-based fuel, you are producing CO2. A vehicle’s carbon footprint is the sum of all of the equivalent CO2 emissions produced over a given time frame (usually a carbon footprint is calculated for the time period of a year).

In addition to carbon dioxide, vocational trucks emit nitrous oxide (N2O) and methane (CH4) as the result of incomplete combustion. Some also emit hydrofluorocarbons (HFCs) from air conditioning or refrigeration systems. While nitrous oxide, methane and hydrofluorocarbons are more powerful greenhouse gases than carbon dioxide, they are emitted in relatively small quantities compared to carbon dioxide. In fact, CO2 emissions compose more than 95percent of the greenhouse gas emissions from most work trucks.

Greenhouse gas emissions are typically expressed in a common metric so that their impacts can be directly compared, as some gases are more potent (possessing a higher global warming potential or GWP) than others. The international standard practice is to express greenhouse gases in carbon dioxide equivalents. Emissions of gases other than CO2 are translated into CO2 equivalents using global warming potentials. The Intergovernmental Panel on Climate Change recommends using 100-year potentials (see chart below).

  GWP
Carbon dioxide (CO2) 1
Methane (CH4) 21
Nitrous oxide (N2O) 310
Hydrofluorocarbon (HFC)-134a 1,300

 

 

 

 

Vehicles account for 28 percent of U.S. greenhouse gas emissions. Since fleets account for nearly 9 million U.S. vehicles and emit millions of metric tons of greenhouse gases annually, many fleets are taking steps to reduce their emissions, beginning with understanding their current level of emissions or footprint. 

Carbon Dioxide
Total emissions of carbon dioxide are calculated by multiplying the volume of fuel consumed by the appropriate fuel-specific carbon dioxide coefficient.

Carbon Dioxide Coefficients (per U.S. Environmental Protection Agency)
Gasoline 8.81 kg / gallon
Ethanol (E-85) 6.05 / gallon
Diesel 10.15kg / gallon
Biodiesel (B-100) 9.46 kg / gallon
Biodiesel (B-20) 10.01kg / gallon
Compressed natural gas (CNG) 0.054 kg / standard cubic foot (scf)
Liquefied natural gas (LNG) 4.46 kg / gallon
Propane (LPG) 5.79 kg / gallon
Electricity (average) 7.18 x 10-4 metric tons CO2 / kWh











For example, when a vehicle burns 1,000 gallons of gasoline, the amount of carbon dioxide released is calculated as follows: 1,000 gallons x 8.81 kg/gallon = 8,810 kg (8.81 metric tons) of CO2.

 
Methane (CH4) and Nitrous Oxide (N2O)
Calculating emissions of CH4 and N2O is more complicated than calculating CO2 emissions. Emissions of CH4 and N2O depend on vehicle drive cycles, miles traveled and pollution control technology. To more accurately calculate these emissions, the U.S. Environmental Protection Agency (EPA) provides coefficients for CH4 and N2O emissions. Fleets need unit-specific mileage data along with pollution control technology (the industry-preferred method) or model year to utilize these coefficients.
 
Hydrofluorocarbons (HFCs)
HFCs are chemicals used as alternatives to ozone-depleting substances. HFC-134a (CF3CH2F) is utilized in most vehicle air conditioning systems. Each unit of HFC-134a emitted has the same global warming impact as 1,300 units of CO2. To fully account for emissions of HFC-134a, fleets need to track data on the capacity of vehicle air conditioning systems, rate of leakage, system recharges and charges at time of disposal.
 
Calculating a Carbon Footprint
Numerous computer-based programs can help you calculate carbon footprint. The Environmental Defense Fund developed a Fleet Greenhouse Gas Emissions Calculator for determining vehicle emissions. 

Reducing a Carbon Footprint 

Reducing your carbon footprint, as well as the total amount of fuel consumed by a vehicle, in most cases, can be accomplished in a number of ways. 

Using Low-Carbon Alternative Fuels
Using low-carbon alternative fuels such as propane, natural gas and electricity will have the greatest impact on your carbon footprint. When calculating reductions, take into account each fuel’s relative energy content. A gallon of liquefied natural gas (LNG), for instance, has approximately half the carbon of a gallon of gasoline, but due to its lower energy density, LNG does not replace gasoline (or diesel) on a one-for-one basis. The gallon equivalent factor for fuels such as propane and natural gas (LNG or CNG) will vary depending on the engine, fuel delivery system and exact composition of the alternative fuel (bio-methane vs. natural gas, etc.).
 
Electricity may help you achieve the greatest reduction in your carbon footprint, but the electricity source used to charge the vehicle’s batteries must be considered in order to obtain a more accurate footprint value. The carbon value listed here for electricity (7.18 x 10-4 metric tons CO2 / kWh) is an average which includes coal, oil, gas, hydroelectric and nuclear generation. Actual values may vary depending on the primary source of electricity. 
 
Downsizing Your Vehicles
The more a vehicle weighs, the more energy is required to move it. Downsizing a vehicle reduces the weight and, in doing so, reduces the amount of energy used (fuel consumption). This strategy for reducing fuel consumption is only effective if the vehicle can accomplish the same amount of work as the heavier vehicle it replaced. If downsizing creates the need for more trips or for more vehicles to complete the job, there will likely be no net reduction in fuel consumption. In some cases, downsizing may actually result in increased fuel consumption.
 
Vehicle safety and regulatory compliance should also be considered when downsizing. It is important to ensure that the resulting vehicle is not overloaded or unsafe to operate. Below is a chart detailing the fuel economy effect of truck downsizing by vehicle class: 

 
Truck Class GVWR Average Fuel Economy (MPG) Typical Fuel Savings
3 10,001 – 14,000 10.5 24percent (from class 4)
4 14,001 – 16,000 8.5 8percent (from class 5)
5 16,001 – 19,500 7.9 3percent (from class 6)
6 19,501 – 26,000 7.0 9percent (from class 7)
7 26,001 – 33,000 6.4  

 

 

 

Source: Environmental Defense Fund

Improving Vehicle Designs, Specifications and Technologies
Vehicle design and specification can significantly impact fuel economy, especially in the following areas:

  • Vehicle weight reduction
  • Powertrain optimization
  • Reduction of accessory and other parasitic loads

Weight reduction
Even if you cannot downsize a truck to reduce weight, there are many ways to reduce the tare weight of a truck (i.e., using lightweight components and eliminating unnecessary vehicle content). The chart below provides the fuel economy effect of truck tare weight reduction.

Vehicle Class Weight Range Average Tare Weight (Lbs) Fuel Economy Improvement per 1000 # of Tare Weigh Reduction
4 14,001 – 16,000 10,343 5.6percent
5 16,001 – 19,500 10,413 4.7percent
6 19,501 – 26,000 13,942 3.9percent
7 26,001 – 33,000 18,094 2.8percent
8A 33,001 – 60,000 23,525 1.9percent
8B 60,000+ 28,979 1.1percent

 

 

 

 

 

 


 

Source: Environmental Defense Fund

If the vehicle tare weight is reduced and payload remains constant, fuel economy will increase, as noted in the graph above. An alternative application of tare weight reduction is to utilize the weight savings to increase the vehicle’s usable payload. If the increase in payload can be used to reduce the number of trips needed to accomplish a task or, for example, replacing four trucks with three (without requiring additional trips), there may be a net reduction in fuel consumption, and thereby, your carbon footprint. 

Powertrain Optimization
The design of a truck’s powertrain (engine, transmission and differential) is often based on past experiences or “educated guesses” as to what is required. As a result, the engine may have a higher (or occasionally lower) horsepower and torque rating than what is needed to efficiently accomplish the job. Likewise, the transmission may not be properly matched to the engine, which is necessary to ensure maximum efficiency.
 
Another common powertrain design issue occurs when vehicle performance criteria are not properly defined. If performance factors (such as starting gradeability, reserve gradeability, desired road speed, etc.) are not accurately defined from the start, the resulting powertrain will likely be inefficient, no matter how carefully components are selected.
 
Most medium- and heavy-duty truck dealers now have computer programs to help perform their calculations and to offer guidance in selecting appropriate performance criteria. Aftermarket programs are also available to help designers optimize powertrains. 
           
Reduction of Parasitic Losses 
A significant portion of the energy produced by a vehicle’s engine is consumed by
parasitic losses (the portion of engine power that is not available to actually propel the vehicle). These non-engine losses can account for as much as 45percent of the total power generated by the engine and can include all of the following items:
  • Vehicle accessory component and system loads
  • Aerodynamic drag
  • Friction losses (non-engine)

Many of these parasitic energy losses can be reduced through the use of high-efficiency components and careful vehicle design, as detailed below.

Accessory Loads
Accessory loads include items such as radiator cooling fans, water pumps, alternators, air compressors, heating and air conditioning systems, lighting equipment, and other electrical loads. There are numerous high-efficiency accessory components available to help reduce these loads. 
 
Improved Aerodynamics
Wind resistance and drag represent a large portion of the energy required to move a vehicle. With commercial vehicles, which typically operate at highway speeds for extended periods of time, aerodynamics can play a key factor in the fuel economy equation. It is well documented that components such as air deflectors, side fairings, nose cones and skirts can significantly impact fuel economy in this scenario. Conversely, it is generally accepted that vehicle aerodynamics have no measurable impact on fuel economy at operating speeds below 25 MPH.
 
Other noteworthy factors when considering aerodynamic features for a vehicle include vehicle type (straight truck vs. combination), body style and drive cycle. In general, aerodynamic components are more effective on combination vehicles (tractor-trailers) than on straight trucks. 
 
Similarly, aerodynamic components are more effective on fully enclosed box vans and trailers than on flatbeds, service bodies and other open-top bodies. Even if a vehicle does operate at highway speeds during a portion of its drive cycle, it may be better to concentrate on other fuel reduction technologies if they offer greater potential for overall fuel consumption reduction. This may be the case if a vehicle’s drive cycle includes frequent idling periods and/or extended stationary power take-off (PTO) operations. 
 
Non-Engine Friction Losses
Friction is the force resisting the relative lateral (tangential) motion of solid surfaces, fluid layers or material elements in contact. With a vehicle, the energy required to overcome these forces is not available to propel the vehicle and is reflected as heat within the various components where friction is present. While it can never be eliminated, friction can be reduced through effective vehicle design, such as:
  • Properly sizing components for the application
  • Using gear drive accessories in lieu of belt drives where available
  • Replacing mechanical drive components with electric drive components as available
  • Using proper synthetic lubricants in engines, transmissions, axles and wheel bearings
  • Maintaining proper axle alignments (front and rear axle)
  • Using low rolling resistance tires
  • Applying low-drag disc brakes where available
  • Minimizing drive line angles
Utilizing New Vehicle Technologies
Hundreds of advanced vehicle technologies are available for increasing fuel efficiency and, in turn, reducing your carbon footprint. Those with the greatest potential for reducing greenhouse gas production fall into the following categories:
 
Auxiliary Power Units (APUs)
The use of APUs is closely related to the use of both idle reduction and E-PTOs. APUs utilize a small, efficient auxiliary engine to provide the limited amounts of power needed to operate vehicle-mounted equipment and systems (such as cab heating and cooling). They typically consume significantly less fuel than what would be used by the vehicle’s primary engine to accomplish the same task.

Electric Power Take-offs (E-PTOs)
The use of traditional mechanical PTOs requires the vehicle’s engine to provide power to operate accessory components. Typically, accessory power requirements are only a fraction of the engine’s potential output, so full engine operation is inherently inefficient. In addition, there are usually periods of time during PTO operation when no power output is required, so the engine is, in effect, idling. Electric PTOs can significantly reduce, or even eliminate, the operation of the vehicle’s engine while still addressing auxiliary power requirements.
 
Hybrids  
Many see this technology as being at the forefront of greenhouse gas reduction. Hybrid vehicles typically utilize two or more power sources that work together to provide for the most efficient vehicle operation. In addition, most hybrids incorporate some means of on-board energy storage and on-board energy recovery. When properly utilized, this often allows for the use of a smaller, more energy-efficient engine (prime mover) and also provides for a means of powering auxiliary equipment and other loads without having to continuously run the engine. 
 
Idle Reduction Technologies  
When idling, a vehicle is unproductive. In fact, the reduction of engine idling time is one reason that hybrid vehicles are more efficient than conventional vehicles. Numerous technologies have been (or are being) developed which have the potential to significantly reduce engine idling time for conventionally-powered vehicles. 
           
Limiting Vehicle Speeds
As noted above, aerodynamic drag consumes a significant portion of the energy used to move a vehicle. It is important to note that the energy needed to overcome aerodynamic drag increases by a cubed function in relation to vehicle speed. This means that doubling the speed of a vehicle (e.g., 35 MPH to 70 MPH) takes eight times the energy (2 x 2 x 2) to overcome the difference in aerodynamic drag. The maximum road speed of a modern electronically-controlled vehicle can be set without the use of high-maintenance mechanical governors or by gear-binding. In this way, fleet managers can control vehicle speed without incurring higher maintenance costs or limiting performance.
           
Improving Vehicle Utilization
Improving vehicle utilization is a way to reduce your carbon footprint without replacing existing vehicles or incurring major retrofit costs. Developing a more efficient method of loading and unloading a truck can shorten the time needed to accomplish the overall job and may allow you to eliminate one or more trucks from your operation. Likewise, the routes followed in conjunction with a given task (especially with delivery vehicles) may not be the most efficient.   
 
Computer-based linear processing programs, such as “What’s Best” and GPS-based vehicle routing and dispatching systems offer some of the best opportunities to improve vehicle utilization. Even if you cannot reduce fleet size, you may be able to reduce the total number of miles driven and vehicle idle time associated with traffic congestion through the use of utilization improvement programs.
 
Educating Drivers
The driver is ultimately responsible for vehicle operation. Training programs that stress the benefits of proper vehicle operation have been shown to provide a measurable reduction in fuel consumption, especially when attendance is tied to a driver bonus compensation program.