Everything I have read online seems to indicate that start / stop technology reduces air pollution, is (marginally) better for the engine, reduces emissions and is more economical (link1, link2, link3). My calculations, if correct, show that the difference between idling and not idling could save 30% of pollution at major junctions.

So, why is it not law for start/stop technology to be fitted as standard on all new cars (UK / EU)? (the 2019 Hyundai i10 has no option for start/stop at all, neither does the Citroen C3 2020 French model)

  • The real question is: Why are new cars built at all?
    – Erik
    Commented Oct 15, 2019 at 9:31

3 Answers 3


The simple answer is because any feature costs money, so car makers won't include it unless they're forced to. On some models it's available as an option for extra cost - which relies on the purchaser wanting it. On others, where there's no option, it hasn't been designed for that model, so there's design cost as well as per-unit cost to consider.

As this is a fairly simple technology, and has been around for over 10 years (I first came across it in a hired BMW 1-series diesel, in a job I left in 2009), the question becomes why hasn't it been mandatory in new cars for the last few years? That's more of a political question, but small partial solutions are often overlooked in favour of a perceived panacea later (electric cars in this case). Hybrid cars of course include auto-stop, and for a while it was thought that we'd all be driving those. The actual benefit is limited by the number of years cars typically last anyway - it takes a long time for a mandatory feature to become a significant fraction of cars on the road


There is another possible reason, just a theory mind. Small cars may pass emissions testing without the need for the technology whereas large cars must have it to pass. In other words, it is not necessary for small cars to have the technology to meet emission requirements.

(although of course, the vehicles could, with start-stop, still benefit from a 12% reduction in emissions)


The best start-stop automotive system is a small hybrid system that does nothing more than roll the vehicle the first five to ten feet from a stop. Of course the internal-combustion-engine starts up during that roll.

But current hybrid systems are larger, heavier, and more expensive than just a system for beginning a roll from a stop.

Now the best hybrid is the BMW i8 because it uses the hybrid system to make an overall four-wheel-drive. However, even that car can be used for a short range in an all-electric mode such that after the sort range then the hybrid system is no longer available for assisting the internal-combustion-engine.

These larger hybrid systems work best as assisting the internal-combustion-engine such that there is then less load on the internal-combustion-engine. However, these hybrid systems are very large so as to have a short all-electric range which of course quickly depletes the hybrid system. Also, the weight and cost is too much with the larger hybrid systems.

The car I recommend for the average driver is a rear-wheel-drive sedan or coupe with a 2.0 turbocharged engine and no hybrid system. The turbocharged engine makes good torque which is a feeling that most drivers like. Also, the peak torque is at low RPM which helps city driving and improves city MPG. Just watch out for vehicle weight even here.

Well, the cities could limit non-commercial vehicles to 2.0 engine size during business hours and could limit commercial vehicles to 3.0 engine size during business hours. The quickest and easiest way to reduce vehicle pollution would be for the cities to pass laws.

  • 2
    Thank you for answering, but do you know why the UK or EU is not making start-stop technology mandatory? Because that's the OP's question.
    – THelper
    Commented Oct 13, 2019 at 7:06
  • There's a conflict between small vehicle hybrid systems, which can be start-stop technology, and large hybrid systems which are much more than a start-stop technology. The popularity of the large hybrid systems is why start-stop technology is not featured. But I warn about the weight, expense, and chosen method of use of large hybrid systems.
    – S Spring
    Commented Oct 13, 2019 at 15:42

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