Official emissions and fuel-consumption figures explained

How do auto manufacturers arrive at those fuel consumption and emissions figures?

When auto makers give their product titles like 'Australia’s most fuel-efficient car', from what are they sourcing their claims? Where do they get those fuel consumption and emissions figures from? How can you be sure they're arrived at in some neutral, objective way that makes them worth taking seriously?

In your travels through Carsales or CarPoint, you've most likely encountered the terms 'city cycle' and 'highway cycle'. You may even have wondered what these cycles are. In a nutshell, they're the yardstick testing procedures by which light-duty vehicle manufacturers and motoring organisations measure fuel consumption and emissions.

There's more to city and highway figures than a drive around town, a drive in the country and calculating odometer readings against fuel receipts. In fact, the car doesn't leave the workshop. These testing cycles take place not on the road but on a roller test bench, on which the drive wheels run a chassis dynamometer calculating power and torque output by rotational speed. The dyno is set up to simulate aerodynamic drag and inertia -- resistance to movement caused by the vehicle's mass. A fan mounted ahead of the vehicle emulates the flow of air into front-end intakes at varying speeds.

To date in Australia, the official testing method comes care of ADR37 (Australian Design Rules) cycle, derived from the US FTP-75 (Federal Test Procedure) test (see Figure 1 below, speeds in MPH). It's an 1874-second cycle divided into three phases: a 505-second cold-start phase reaching US highway speeds of up to 55 mph (88 km/h), then a transient phase of 864 seconds at urban speeds of up to 35 mph (60 km/h), followed by a hot-start repeat of the first phase. It was modified in 2000, with two Supplemental Federal Test Procedures (SFTP) added to cover more aggressive, high speed driving patterns and substantial rises in the use of air conditioning systems.

All up, ADR 37 covers 17.77 km at an average speed of 34.1 km/h (see Figure 1). Because some European models are made to run on PULP, Australian testing uses 95RON fuel for all petrol cars, to level the playing field.

More recently, however, a more comprehensive testing procedure, the New European Driving Cycle (NEDC), has provided a more up-to-date procedure.

The NEDC entails putting the vehicle through four urban ECE-15 cycles and an extra-urban EUDC cycle (see Figure 2 below). The ECE-15 phase was conceived to emulate urban driving conditions in large, crowded European cities. Each urban cycle lasts 200 seconds, covering 1.013 km (a total 4.052 km) at speeds of up to 50 km/h, averaging 18.7 km/h.

The single-phase EUDC extra-urban cycle, designed to emulate open highway driving, is twice as long -- 400 seconds, covering 6.955 km at speeds of up to 120 km/h. It averages out at 62.6 km/h. There is an alternative EUDC for low-powered vehicles -- limited to 90 km/h.

Testing through the urban cycle takes place in several ways. The car starts cold, in an ambient temperature of 20 degrees. As happens in city driving, there's little load placed on the engine, which in turn means exhaust gas temperatures remain low and emissions relatively high. CO testing takes place at the exhaust outlet with the car idling while warm, while other emission levels are measured at the engine end at idle and 50 km/h.

Rather than test every variation of a specific platform, testers use a look-up table to account for differences in weight (etc) between sedan, hatch, wagon, coupe and convertible variations of the same model.

And what do they glean from all this? For fuel economy, the first 800 seconds -- the four ECE-15 phases combined -- produce urban fuel consumption figures. The highway cycle comes from the last 400 seconds. The combined-cycle figure comes from the complete 1200 second cycle, as do CO2 emissions figures.

European legislation requires detailed analysis of exhaust emissions not only for CO2 but for carbon monoxide, particulate matter, unburnt hydrocarbons, and oxides of nitrogen

The Federal government is currently in negotiations with the local industry over its plans to implement Euro 5 emissions standards by 2013, working towards Euro 6 by 2016. The FCAI says it's likely local manufacturers will need more time to achieve those. Even with Euro standards in place, it's likely the current ADR37 testing won't be supplanted by NEDC.

Government proposals for mandatory fuel consumption levels and, more importantly, CO2 emissions levels for 2011 are still being debated with the FCAI and other industry bodies. These will push towards fleet averages of 190g/km by 2015 and 155g/km by 2020. To date, the local industry has worked comfortably ahead of its voluntary targets, reaching 218g/km by the end of 2009 against a benchmark of 222g/km.

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