Understanding stop/start automobile – engine design, Part 1: The idea

Today’s motor vehicles are vastly different compared to those of just a decade ago. This article will look at a relatively new “feature” (some say an unneeded and unwanted one) called “stop/start” which is increasingly standard on many internal combustion and hybrid electric vehicles (HEVs).

Q: What is stop/start?

A: It’s a simple idea, at least in concept: turn off the gasoline engine when the car is stopped, such as at a traffic light, and very quickly restart it when the driver moves his/her foot from the brake to the accelerator, so fast that there is no noticeable lag in resuming motion. The formal names for this feature are Engine Stop Start (ESS) or Auto Stop.

Q: Is it just the internal combustion engine which is turned off, or are other accessories and subsystems also de-activated?

A: Although the engine is off, all the remaining accessories (lights, A/C, infotainment) remain powered on as normal. In stop/start mode, the engine is automatically restarted after about 60 seconds (a long traffic light or super-slow traffic) to prevent the battery from being drained by these other functions.

Q: How much gas does it actually save?

A: That answer depends on many factors, with the primary one obviously being the nature of the driving situation. For those who drive almost entirely on highways with few stops at lights or traffic, the savings are modest at best, about one mile per gallon (mpg). The reported benefits are greater at about 10% or 3 to 5 mpg for those who drive in urban or heavy stop-and-go traffic.

Q: It seems like a simple enough idea – but is it?

A: Like many ideas, it’s simple only in principle. It has implications on starter-motor design, engine oil, battery type, and more. It stresses the car’s electrical system and power train, although designers have managed to design compensation for that. Plus, the basic idea of stopping the engine and then restarting it under the intended circumstances has many exceptions and special cases that must be taken into account by the control algorithms.

There are also non-technical “human factors” which must be acknowledged. Although the idea is simple to express, actually making to work smoothly in practice involves sophisticated algorithms and tradeoffs to process the many related sensor inputs and accommodate the many real-world situations.

Q: Do hybrid electric vehicles (HEVs) have the stop/start feature?

A: Yes, but used in a different set of cases.  In hybrid cars, the same system may also shut down the combustion engine when the car is cruising at low throttle loads, when descending gradients and when decelerating from higher speeds. The engine starts again when the clutch is engaged, or the brake is released, or when the driver is ready to move or accelerate again

Q: Which cars have it?

The answer depends on the car make (vendor), the specific models, and, of course, the model year. Right now, about half the new models have it, and the number keeps increasing.

This article will look at various technical and human factors related to stop/start operation, including its impact on starter-motor design and use, battery technology and battery management system, internal combustion engine characteristics, and driving attitudes and preferences. Unlike the conventional starter subsystem, which mostly involves the path from the ignition key (or keyless activation), to the battery and the starter motor, stop/start involves many more functions and subsystems of the vehicle (Figure 1).

In Part 2, we’ll start at the obvious place: the starter motor itself.

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External References

There’s a lot of information available about stop/start, as there is for almost anything related to cars: it’s a big industry which affects just about everyone in one way or another. Perhaps because stop/start is relatively new, contrary to tradition, and has many technical and driver-related issues, these sites’ content ranges from personal opinion, to technical discussion, to in-depth technical analysis. These are the useful sites and sources I reviewed for this article:

• Green Car Reports, “Don’t start-stop systems wear out your car’s starter?”
• Autocar, “Stop-start systems: is there a long-term impact on my car’s engine?”
• CarPro USA, “Understanding Stop/start Systems”
• Driving, “How It Works: Starters and automatic start-stop technology”
• Varta (Clarios), “Reasons why the start-stop system does not work”
• Varta (Clarios), “What is automatic start-stop and how does it work?”
• Varta (Clarios), “Why do I need a special battery for the automatic start-stop system?”
• Varta (Clarios), “Start-Stop”
• The Jag Wrangler, “SmartStartStop”
• The Truth About Cars, “The Problem With Start-Stop Systems”
• Best Ride, “Does Auto-Stop/Start Technology Wear Out Engine Components?”
• Practical Motoring, “Myth-busting: Does stop-start damage your engine?”
• NAPA, “Stop-Start Technology — Pros and Cons”
• Idle Smart, “Pros and Cons of Automatic Engine Stop/start Solutions”
• How Stuff Works, “Love It or Hate It: Stop-start Technology Is Here to Stay”
• Wikipedia, “Start-stop system”
• STMicroelectronics, “48V Start-Stop System”
• Daimler, “Under the microscope: ECO stop/start: Sophisticated technology gives the highest levels of efficiency”
• Argonne National Laboratory, “Start and Restart Effects on Modern Vehicle Starting Components”
• Bosch, “Full power for stop/start”
• How a Car Works, “How the starting system works”
• Your Mechanic, “How Does a Starter Motor Work?”
• General Motors, “Cadillac’s Electric Self Starter Turns 100”
• Elreg Distributors, “The Evolution of the Starter Motor”
• PR Newswire, “ZMJ and CRCI to Acquire Bosch’s Starters and Generators Business”
• TechLink, “Enhanced Starter Motor Operation in Engine Stop/Start Systems”
• JeepFan, “How Electronic Start Stop ESS Works on the Jeep Wrangler JL”
• Autoblog/Verizon Media, “Mazda introduces clever start-stop system with direct injection”