What does fuel-saving technology really do?: The truth about start-stop systems

"Everyone hates it, so let's fix it": Lee Zeldin, head of the U.S. Environmental Protection Agency (EPA) since January, recently questioned start-stop systems in cars. What's the truth behind his criticism? Ten questions and ten answers.

Quite simply: to save fuel in cars with gasoline and diesel engines and reduce emissions from the environment and climate. Depending on the model, the start-stop system deactivates the engine when the car is coasting or has come to a standstill, whether in the city or in a highway traffic jam. As long as the driver presses the brake pedal, the drive remains switched off, as far as possible and sensible (more on this below). And standstills are becoming more and more common: In 2023, a commuter in Germany spent an average of 40 hours in traffic jams; by 2024, this figure had risen to 43 hours.

Its roots lie in the 1980s with Audi, Fiat, Opel, and VW, but the technology didn't become widespread until 20 years later. The early systems used traditional technology: During restarting, the starter gear meshed with the flange on the crankshaft's flywheel. This was rather slow and unwieldy, especially in diesel cars – especially if the interaction with the dual-clutch transmission (if available) wasn't perfectly coordinated.

Many modern combustion engine models feature a 48-volt mild hybrid system (MHEV). This typically integrates a starter generator that turns the crankshaft either directly or via a belt. This process is smooth and fast. Furthermore, the vehicle electrical system uses power from the 48-volt battery to power all electrical functions—from power steering and lights to heated seats, windshield wipers, and infotainment. Cars without MHEV continue to use the traditional system; it operates much faster and smoother today than it did 20 years ago. Nevertheless, many diesel taxi drivers still turn off the system—partly to avoid disturbing their passengers.

Start-stop systems are closely integrated into the car's neural network. Based on sensor signals, they allow the system to remain shut down, especially when the pressure in the brake booster is too low, when the combustion engine is not yet warm, when the ambient air is very cold, when the air conditioning is running at full blast, and when the battery is running low on power. Or even if the doors or seat belt buckles are open—in which case the car assumes a parking maneuver.

This was a widespread and not entirely unfounded fear in the early years of technology. Today's technology, however, mitigates many problems. An electric water pump, for example, cools the engine even when it's not running, and the turbocharger continues to be supplied with oil after it's shut down. Many manufacturers are also upgrading the automatic or dual-clutch transmission, the flywheel or belt pulley, and the crankshaft bearing shells. A smart crankshaft sensor makes it possible to shut down the engine in the ideal position for restarting. The 48-volt batteries of MHEV vehicles are deep-cycle batteries, as are 12-volt batteries with so-called AGM technology. In these, the electrolyte is bound in a fiberglass mat, which increases robustness.

That's the crucial question, one that can't be answered in general terms – the influence of driving profile and drivetrain is too great for that. The ADAC (German Automobile Club) cites savings of up to 15 percent in pure city traffic, but this seems quite high. Georgiadis Iordanis of the Baden-Württemberg Taxi Association, with whom we spoke, says: "If you drive 60,000 kilometers a year, you'll save a low three-digit amount in fuel costs with start-stop." That would be a magnitude of only two to four percent.

There are no figures available for this because the system on the roll was not and is not deactivated. However, it must have had a significant impact on the NEDC cycle, which was in effect from 1992 to 2017. The 11-kilometer cycle included 12 stop phases, accounting for 25 percent of the 1,180 seconds of driving time. This suggests that this cycle promoted the spread of start-stop systems because they significantly reduced paper consumption.

The WLTC Class 3, as the passenger car driving cycle is called, consists of four individual cycles, which add up to six stationary phases. In total, the car covers 13.25 kilometers in 30 minutes, of which it is stationary for a total of 242 seconds. That's only 13.4 percent.

No. And the driver isn't obligated to leave it permanently activated. In many cars, they can deactivate it, but they have to do this every time they start the car. Permanently disabling the system with a technical trick is conceivable, but it's illegal. This could cause serious problems during the vehicle inspection, as the vehicle's operating license could be revoked.

Not as long as a gasoline or diesel engine serves as the power source. Even full hybrids and plug-in hybrids have such systems because they repeatedly use the combustion engine.