Virtual Real Driving Emissions (vRDE) Part 1: New Technologies Inspire New Emissions Tests

The goal of automotive regulation has always been to walk the line between offering OEMs an implementable procedure for certification and representing real-world driving conditions that drivers will experience with a vehicle. The inherent variance of operating conditions and drivers makes it difficult to predict the actual performance, fuel economy, and emissions levels of vehicles when they are sold to customers. On the other hand, OEMs must be able to ensure that multi-billion-dollar vehicle programs will be able to meet the regulatory burden before reaching production and therefore they need clear testing procedures and guidelines to use during development.

The Need for a Better Way

Historically, vehicle certification for fuel economy and emissions has been done on chassis dynamometers with fixed-profile driving cycles. European regulation was based around the New European Driving Cycle (NEDC) while the US used the Federal Test Procedure 75 (FTP-75). The cycles were designed to mimic the vehicle speed and load range that a typical customer might expect to impose on the vehicle during normal operation.

The NEDC driving cycle is characterized by regions of constant acceleration, deceleration, and cruising at constant speed. These qualities render it a poor representation of real world vehicle operation, which is subject to variable acceleration and speed ranges. The FTP-75, on the other hand, includes more diverse transients, with distinct regions aimed to mimic both city and highway driving, but features a relatively low average speed and moderate acceleration requirements.

In addition to the inherent deficiencies of the driving cycles themselves, the testing procedures offer opportunities for OEMs to optimize vehicle and engine operation around the driving cycle requirements, even if the performance isn’t representative in real world settings. This can range from well-trained drivers who minimize vehicle acceleration throughout the driving cycle within the allowable error window all the way to electronic defeat devices that use different calibration maps when the vehicle detects it is being tested on a specific cycle.

On-Road Certification of On-Road Performance

One of the main motivating factors for chassis dynamometer testing has historically been the bulky nature of emissions measurement equipment. However, in recent years, improvements in the accuracy and downsizing of Portable Emissions Measurement Systems (PEMS) has enabled affordable emissions testing and measurement outside of the laboratory test cell and directly on the road by carrying the measurement equipment in the vehicle.

These advances have prompted rethinking of current vehicle testing procedures with the objective of more accurately capturing actual operation, performance, and pollution. PEMS have also been used in academia to independently verify claims about emissions levels produced by vehicles. With on-road measurement now becoming affordable, Euro 6c regulations include an updated chassis dynamometer driving cycle coupled to an on-road verification test called “Real Driving Emissions.”

The first phase of the overhaul involves replacing the synthetic NEDC driving cycle with an all new Worldwide Harmonized Light Vehicles Test Procedure (WLTP), which includes 3 variants of an all new driving cycle. The appropriate cycle is chosen based on the tested vehicle’s power-to-weight ratio and scales the maximum speed and acceleration demand accordingly. These new cycles aim to cover a broader region of the engine’s operating domain, both in terms of speed and load.

For the purposes of new emissions regulations, these driving cycles serve as a baseline, which must adhere to increasingly stringent limits on various constituents. However, the vehicle cannot be certified until a “Real Driving Emissions” test also demonstrates that the vehicle is in compliance.

“Real Driving” – Procedural Unpredictability

Rather than simply certifying a vehicle based on its WLTP results, recently-enacted Euro 6c emissions regulations now also require that the vehicle be instrumented with a PEMS and conduct a long, “realistic” drive on the road. The main criteria that must be met during this “real driving test” include:

• Duration of 90-120 minutes
• Three distinct phases (urban, rural, motorway)
• Average and maximum speed conditions
• Minimum stop duration in the urban portion
• Ambient conditions approximately nominal
• Minimal change in elevation between start and end point

Because no specific vehicle speed profile is mandated as long as all regulated conditions are met, no two “RDE” driving cycles will look the same, but their general characteristics will be similar.

Because violating the mandated RDE criteria might result in a void test, it is important to choose a route that is readily available and bears the right characteristics. Yet all the planning might be rendered useless due to events such as road construction, accidents, or traffic that prevent the driver from meeting the cycle requirements.

The challenges in meeting RDE requirements are most pressing in early development stages when designs are still in flux, prototype equipment may not be available yet, or testing vehicles are in high demand. Not only does each iteration of the test take a substantial amount of time, the work is conducted away from testing facilities in the event a vehicle suffers a mechanical problem. This has created demand for alternative means of ensuring product compliance early in the development process without the need for costly on-vehicle testing, and computer simulation is emerging as the primary solution.

Join us for Part 2 – Virtual Real Driving Emissions Solutions

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Written By: Damian Sokolowski