real-life fuel economy versus manufacturer specification

  • April 29, 2014
  • Marcel Romijn

A current discussion that is frequently heard of in Automotive, is the difference between the manufacturer claimed fuel economy of a passenger car versus the actual achieved fuel economy by the drivers. We asked Brace Automotive Engineer Marcel Romijn to explain some of the basics.

What is your position at Brace Automotive and how long have you been working in Automotive Engineering?

Marcel: "I am a Project Engineer, specialized in On Board Diagnostics and Exhaust gas emissions control. I've been active in the field of Automotive Engineering for around eight years after a short career as an automotive workshop technician."

Why are exhaust gas emissions and fuel economy measured?

Marcel: "Exhaust gas measurements were required by governments with the introduction of exhaust gas standards such as the Euro emissions standards in the EU. The measurements were needed to prove that a vehicle was clean enough to be allowed on the road. The levels were set based on the available technology versus the effects on public health. Second came the fuel economy determinations for informing the public and promoting the purchase of fuel efficient cars as a way to limit oil dependency. The fuel economy numbers became more important when governments started to subsidize or give out penalties to vehicles and/or drivers based on the vehicles fuel economy numbers. Also, CO2 reduction targets are now in place which link directly to fuel economy numbers."

What is measured and how?

Marcel: "The measurements are performed with a car on a chassis dyno. The car is placed on big rollers that have a braking system. The braking system is controlled to brake the vehicle with the same forces as it experiences when driving on a road. These forces are for example the rolling resistance and the wind resistance. The car is then driven according to a vehicle speed profile called the driving cycle. For the EU that currently is the NEDC cycle. While driving this cycle the exhaust gasses from the vehicle are captured and split-up into different chemical components. The components are stored and the total stored quantity from the test cycle is analyzed. These components are exhaust gasses that are directly bad for public health such as CO, HC, NOx and Soot – and their weight is directly expressed in milligrams. Also the amount of CO2 is stored and analyzed. CO2, together with H2O (water), constitute the results of a perfect combustion. CO2 is also an indicator for the amount of fuel used. Fuel economy is then calculated from the measured CO2 values. Since the NEDC drive cycle represents sections of different driving conditions (city, urban, highway) and the analysis is done per section, fuel economy numbers can be provided per section or type of driving condition."

What is the reason of the differences between manufacturers claimed fuel economy and the real world numbers?

Marcel: "First of all this difference is not new however due to websites where many people track their fuel economy it is more visible than it used to be. Secondly due to the stronger incentives and penalties linked to CO2 numbers manufacturers are spending much more money and time in improving their fuel economy numbers that follow from the test. These improvements however not necessarily also improve their real world fuel economy. Now for the differences; as described the chassis dyno brake is controlled to represent the forces acting on the vehicle while driving on the road. These forces are configured through vehicle brake data, collected on a test track. The vehicle is tested to see how much resistance it has. The testing standard allows some room here for inaccuracies which can also be used to achieve more convenient numbers for the chassis dyno. Also when the vehicle is on the chassis dyno it is tested in warmer conditions than the average EU temperature while items such as the air-conditioning stay off and the car starts the testing with a fully charged battery. The last big contributor is the NEDC driving cycle itself. The cycle consists of accelerations and speeds that no longer are applicable to today’s traffic. Also highway traffic is underrepresented. Today's traffic consists of stronger accelerations and higher speeds.”

What are the consequences of these differences?

Marcel: “Generally it has resulted in unsatisfied customers. It has even come to the point that lawsuits have been started against car manufacturers or their local representatives. Since EU trade legislations only allow car manufacturers to advertise with the fuel economy numbers from official tests these lawsuits however have little chance for success. A second consequence is that vehicles that have low exhaust gas emissions on the test cycle are making higher levels of exhaust gas emissions in real-life. This dilutes the effect of more stringent legislation for cleaner cars."

What options are there to solve these issues?

Marcel: “Governments have been alerted by the increasing differences between real world and manufacturer numbers. In the EU a new testing protocol is under development. Many of the changes in the new protocol are inspired by the US and Japan. In these regions the government has had more success lately in balancing between a repeatable laboratory test protocol and real world numbers. Some examples of changes are: a new drive cycle pattern with a better relation to modern traffic, stricter testing conditions, stricter chassis dyno brake settings, and inclusion of use of air-conditioning systems. Since there are so many incentives and penalties associated with fuel economy numbers, changing the test procedure is not an easy task. All of the incentives and penalties have to be reviewed as well.”

What are the capabilities of BRACE in this field?

Marcel: “BRACE has experience in exhaust gas emissions test, including CO2 and fuel economy numbers. BRACE does not have facilities to perform these tests however we work with our clients or with contract parties. Specific discussions linked to differences between test procedures and real world data in fuel economy and exhaust gasses are a frequent topic for us. For example BRACE recently looked at how a potential GPS involvement into a combustion engine control could increase this difference and how this difference could be handled satisfactory for both drivers and governments.”


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