What is a start-stop system? PH Explains


Billions of gallons of fuel are wasted each year by idling engines. Be it in traffic, at junctions, on or peoples' driveways, these engines that aren't otherwise doing anything useful continue to consume fuel and produce emissions. Idling engines cause several key issues. Besides costing the driver money by burning fuel in a wasteful fashion, they also generate unnecessary pollution and noise. This can be a significant headache for many, particularly in urban areas that suffer from congestion.

Consequently, many manufacturers now use so-called 'start-stop' systems to alleviate these problems. These systems can automatically shut the engine off when the vehicle has come to a stop, preventing wasteful idling. When the driver is ready to set off again, the engine will be automatically restarted and the car can continue onwards. This means that, while the car isn't going anywhere, the engine won't burn any fuel, make any noise or emit unnecessary emissions - greatly reducing carbon dioxide and particulate emissions in congested areas and saving the driver money.

Start-stop systems are not a new development, that said, with early versions arriving the 1980s - but they are now far more commonplace and, in many cases, increasingly unobtrusive in their operation.


How do start-stop systems work?

A start-stop system will monitor various parameters, such as wheel speed and the driver's inputs, in order to decide whether the engine should be shut down or restarted. In a car with a manual transmission, shutting down the engine automatically in a safe fashion typically requires three conditions to be met - the car needs to be stopped, the transmission in neutral and the clutch to be released.

In that situation, the start-stop set-up can shut off the engine when it would otherwise be unnecessarily idling. When the driver intends to set off again, and begins to press the clutch pedal, the engine will be promptly restarted to allow the car to continue on.

Start-stop functions in a similar fashion in automatic cars, although the transmission can often be in drive or neutral - with the car being held in position, enabling the shutdown of the engine, by the driver keeping their foot on the brake. Alternatively, an auto-hold feature may secure the car.


Modern start-stop systems will often account for several other variables in order to ensure the best functionality, too. For example, most 'auto-stop' setups will only shut down the engine when it is up to temperature, once the climate control has met its interior temperature targets and if the battery has reached a sufficient state of charge.

In order to ensure durability and proper operation, manufacturers may also fit uprated components that can endure the repeated on/off cycling and higher electrical loads. Bosch, for example, supplies starter motors with upgraded bearings and pinion assemblies, batteries that are designed to withstand the heavier electrical loads and more efficient alternators.

Many may make further changes to ensure smooth operation. Mercedes-Benz, among others, equips its automatic cars with an electrically operated transmission oil pump for start-stop applications. This allows pressure to be built in the transmission prior to the engine restarting, so that first gear can be engaged quickly; this allows for a much swifter response and less hesitation off the line.


Just how beneficial can a start-stop system be?

During NEDC fuel economy trials, according to manufacturer Bosch, a start-stop system reduced fuel consumption and emissions by eight per cent. It also claims that, in real-world traffic, a start-stop system can offer improvements of up to 15 per cent on both the economy and emissions fronts.

Other reports, such as those published by the technology firm Schaeffler, state a saving of around five per cent in NEDC tests - and more in urban traffic. Reportedly, the amount of fuel necessary for a warm start is equivalent to that consumed when the engine is idle for just 0.7 sec; as a result, the start-stop system does not have to shut the engine down for long for there to be a benefit.

In any case, the potential economy and emissions advantages are notable. As a result, the system is of great use to manufacturers attempting to meet increasingly stringent emissions regulations - including ever-lower CO2 emissions targets. That aside, the reduction of noise from traffic is of considerable benefit to those living alongside the roads.

PH Explains homepage

P.H. O'meter

Join the PH rating wars with your marks out of 10 for the article (Your ratings will be shown in your profile if you have one!)

  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
Rate this article

Comments (43) Join the discussion on the forum

  • scarble 23 Oct 2018

    But, during a stop phase is the cool down of aftertreatment sufficient that emissions of other pollutants is worsened? Is this another case of the CO2 crusades negatively impacting local emissions?
    Would stop-start be a better name?
    Also modern systems are increasingly able to stop the engine juuust before the car stops and if the driver then tips in before fully stopped can restart an engine while it's still running down, or with 48v systems maybe even just drag the car along under electrical power while the engine runs up, in theory.
    And full on coasting, stopping the engine at speed (even at 70), might be along soon.

  • Gary C 23 Oct 2018

    Christ, I'm suprised PH thought that this needed explanation.

    Next, fuel, makes your car go. pH explains smile

    Anyway, I notice that on my commute, having ss turned on saves me a few mpg, so it must do something.

  • buggalugs 23 Oct 2018

    In before some codger goes on about starter motors and batteries as if there the first person ever to think of that...

  • techguyone 23 Oct 2018

    buggalugs said:
    In before some codger goes on about starter motors and batteries as if there the first person ever to think of that...
    haha yes, every time SS is mentioned that's the first thing 'I'm not doing that, it'll wreck my... battery/alternator/starter motor....'

  • Lancashire Lad 23 Oct 2018

    I simply can't be doing with it. I've an SL63 which has it fitted. It means theres no such thing as a "Smart" getaway. The car has to think about starting then it decides to go. Its so frustrating.

View all comments in the forums Make a comment