Auto Fuel Economy

Measuring Fuel Economy
Fuel Economy vs. Emissions Standards
Parameters Affecting Auto Fuel Economy

What is Auto Fuel Economy?

In the field of transportation, the fuel efficiency/fuel economy/fuel consumption of a certain vehicle or stock of vehicles is calculated as a ratio of distance travelled per unit of fuel consumed (e.g. miles per gallon or km/L) or, inversely but equally, fuel consumption per travel distance (e.g. L/100 km). Thus, the greater the distance travelled per unit of fuel, the greater the fuel economy/efficiency. In other words, the less energy consumed to travel a certain distance, the more efficient that vehicle is.

Simply put – the more mileage a car gets per liter of petrol, the more economic/efficient, and the lower the CO₂ emissions, from that vehicle.

Measuring Fuel Economy

The unit of measurement used to express fuel economy can be in miles per gallon (mpg), kilometres per litre (km/L), litres per hundred kilometers (L/100km, also referred to as fuel consumption), and rate of carbon dioxide (CO₂) or CO₂ equivalent emitted per unit of distance (in grams per mile or km or gCO₂/km).  The choice of unit varies by region and country.  However, it is important to note that the metric used carries with it certain perceptual and policy implications.  For example, the Global Fuel Economy Initiative aims to cut global average automotive fuel consumption from the current global average of 8 L/100 km to 4 L/100 km; this translates into a doubling of mpg from the current 29.4 miles per gallon to 58.8 miles travelled per gallon by 2050 globally, or going from 12.5 km/L to 25 km/L. 

When expressed in g CO₂/km reaching the global GFEI target of 4 L/100 km means going down to a global average for all light duty vehicles of 90 g/km of CO₂by 2050 – basically, halving the unitary emissions of CO₂currently emitted by cars.  This unit will often be used when the aim of a fuel economy standard is to highlight greenhouse gas emissions from vehicles.  Fuel economy and fuel consumption units (L/km, mpg) are used to emphasize energy savings and oil use reductions.

In the European Union, a mandatory auto fuel economy standard is in place to reduce emissions to 130 g CO₂/km by 2015 and 95 g CO₂/km by 2020. In the United States, the federal Environmental Protection Agency has proposed a standard of 250 grams per kilometer for light-duty cars and trucks and medium-duty passenger vehicles, beginning with new vehicle models for 2012, going down to 102 gCO₂/km in 2025.

 

Fuel Economy vs. Emissions Standards

It is important not to confuse fuel economy/greenhouse gas standards with conventional pollutant standards.

Fuel economy and/or greenhouse gas emission standards (including CO₂) are usually governed by separate (but related) regulations at the national level.  For example, in the U.S. the Tier 2 Vehicle and Gasoline Sulfur Program governs vehicle pollutant emissions (sometimes called standard or conventional emissions) including NOx, SOx, PM, CO in addition to parameters of vehicle fuels (both diesel and petrol) that affect emissions, e.g. sulphur levels.  The U.S. rule on automotive fuel economy was enacted through the separate CAFÉ standards enacted in 1975 during the oil crisis; more recent Truck CAFE standards of 2008 incorporate heavy duty vehicles.

Similarly, in Europe conventional vehicle pollutants are governed through the Euro standards (the EU is currently at Euro 5, transitioning to Euro 6 from 2014 for cars), whereas fuel economy is governed separately by EC Regulation No 443/2009 on the reduction of CO₂ from light-duty vehicles.  Similar legislation for commercial vehicles was passed in 2011, via Regulation No 510/2011.

In summary, the term ‘vehicle emission standards’ usually refers to well-established programs that address conventional tailpipe pollutants.  Auto fuel economy/CO₂ standards that govern greenhouse gas emissions and energy use from car and/or trucks are usually governed by separate (usually more recent) legislation.  This is an important clarification to make.

Conventional pollutants and CO₂ and fuel consumption measures are performed on the same test cycle that is common to all vehicles tested for a given country and/or region. The fuel consumption is measured directly from the fuel tank, while the tailpipe emissions (including CO₂) are measured by analyzing the exhaust gases.

There are different test cycles in each country and/or region that have emission regulations; the one cycle that seems to have been adopted the most widely is the New European Drive Cycle (NEDC) adopted first in the EU. However, this cycle is to undergo renewal after 2015 in the EU as it is not widely considered to be representative of real-world driving conditions.

Test conditions, such as ambient temperature, humidity and fuel specifications for the homologation of vehicles (or, verification of their emissions) are well defined and are equal for all vehicles homologated (or verified).

Parameters Affecting Auto Fuel Economy

There are a number of aspects that affect the amount of fuel used and the greenhouse gas emission of any given vehicle.  These include

• The efficiency of its engine, which varies with engine type (e.g. diesel vs. petrol, or spark-ignition vs. compression), the mass of the car and its load, and the engine speed (usually noted in RPM);
  

• The quality and type of the fuel used – for example high sulphur diesel lowers vehicle efficiency as it taxes emission controls;
  

• *Driver behavior, which can be addressed through eco-driving;
  

• *Driving conditions (including idling and stop-and-go traffic) and road infrastructure;

• Aerodynamic drag, which is related to the design of the vehicle;

• Rolling friction or resistance, which is related to the tires used;

• Braking frequency and technology, which can be addressed through regenerative braking technology now being used in hybrid vehicles;
  

• Transmission type – manual versus automatic;


• *Air conditioning;


• *Power steering;


• Engine cooling;


• Electrical systems, including headlights, battery charging, active suspension, circulating fans, defrosters, media systems, speakers, and other electronics.

Most of those aspects are covered during the homolgation test of the vehicle, but some of them are not (indicated by an asterisk *), so acting on those parameters will not impact the displayed fuel economy label value at dealerships or for values reported by manufacturers.