The True Purpose and Efficiency of Stop-Start Cars

Stop-start technology has been marketed as a means to save fuel and reduce CO2 emissions. However, its true purpose is much more nuanced. This article delves into the history, technology, and real-world impact of the stop-start system. We will explore how this technology was primarily designed to reduce CO2 output in standardized tests, and how its effectiveness varies depending on driving conditions.

Understanding the Stop-Start System

The stop-start system, often marketed as a fuel-saving solution, is a key feature in modern vehicles. This system can shut down the engine during periods of idling, such as at traffic lights, and restart it when the driver is ready to move. While its marketing suggests substantial fuel savings, the reality may be quite different.

The Primary Purpose: Reducing CO2 Emissions

Stop-start systems were introduced to help vehicles pass new emission tests. These tests are standardized to replicate common driving conditions, including periods of idling. The primary goal is to reduce CO2 output, which is relevant for tax brackets and government regulations.

For example, in the UK, certain vehicles fall into tax brackets based on their CO2 emissions. A Ford with a CO2 output of 121 grams per mile would be taxed higher than a Vauxhall at 119 grams per mile. Such a small difference can translate into millions of pounds for car manufacturers. The stop-start system helps reduce the overall CO2 figures, thus keeping the vehicle in a lower tax bracket.

How Stop-Start Works

The stop-start system on a vehicle like the Ram 1500 with the E-Torque stop-start system operates through a 40-volt electrical system and a belt-driven motor/generator. When the vehicle comes to a stop, the engine shuts off. When the driver releases the brake, the motor/generator starts the engine. This system provides a significant torque (150 lb. ft.) to start the vehicle, and uses regenerative braking to recover electricity and reduce energy usage.

Real-World Impact and Practicality

While the stop-start system may sound like an effective fuel-saving feature, its real-world impact varies greatly depending on the driving conditions. For instance, if you drive predominantly on highways, the stop-start system may not save much fuel. This is because highways often lack traffic lights and other stop-and-go situations where the system would be most effective.

I own a Ram 1500 equipped with the E-Torque stop-start system. In testing, the 5.7L Hemi engine is expected to save approximately 2 ounces of fuel per 90-second stoplight. However, my driving habits in Phoenix, AZ, have limited the number of stoplights I encounter. This has meant that the system has not significantly reduced my fuel consumption to date.

Moreover, the stop-start system is not always activated when the cabin temperature is not at a comfortable level. In Phoenix, where the air conditioning is often running, this feature can limit the system's effectiveness in saving fuel.

Conclusion

The stop-start system is designed to reduce CO2 emissions in standardized testing conditions, rather than to provide significant fuel savings in everyday driving. Its effectiveness depends on the driving environment and the specific conditions of each vehicle. Drivers should be aware that the system may not consistently save fuel, especially in areas with minimal stop-and-go traffic.