Time for some science for piston heads. When this
Mazda 3’s engine is stopped, the car’s new i-stop system – the company’s first
crack at stop/start – knows which piston is in the best position to restart
quickest. It’s the one in the combustion stroke phase, where air and fuel are
in the cylinder and ready to be ignited. Sensors pinpoint which cylinder that
is, before the mixture is sparked and ignited, forcing that piston down first
and firing the engine into life faster. It’s like placing bike pedals just past
noon, making it easier to push down and move off.

You might think that’s the most pointless factoid
ever, but Mazda is justifiably quite proud of that kind of detail. And it makes
a difference. When you press the clutch, the engine takes just 0.35 seconds to
get going. By comparison, BMW’s stop-starters take 0.7secs, because the starter
motor has to churn the engine in a conventional fashion. If you’re first at the
lights and you’re determined to use your stop/start, that speck of time is
significant.

The problem is keeping it stopped. If you want to
run the aircon or wipers or de-misters, it’ll only switch off for a few seconds
or sometimes not at all. Despite having a second battery to run the starter
motor, i-stop is reluctant to do its thing unless you switch everything off. We
tested the car in 25° heat, selecting a 20° cabin temp, and it could only
manage about four seconds before calling on the engine again. That’s not good enough in less-than-strenuous
conditions, but it’s a problem we’ve experienced with other makes of car.

And then there’s the engine
itself: a naturally aspirated 2.0-litre with direct-injection. Not the obvious
choice for a stop/start system, which would usually be fitted to a diesel or
smaller petrol. Trouble is, i-stop works most efficiently with a
direct-injection engine (because less fuel is wasted in the restart process),
and Mazda only has two of those on its books – this one, and the 2.3-litre from
the CX7. Diesel is incompatible with i-stop in its current state, due to a lack
of spark ignition and an extra phase to its combustion cycle. The 2.0-litre
manages 41.5mpg and 159g/km – not bad, but it doesn’t feel substantial
enough (consider that a BMW 120i with a conventional stop/start system hits
44.1mpg).

Yes, it’s an interesting
system, and the speed of the restart is impressive, but i-stop doesn’t quite
make sense with this engine. Mazda is working on a solution to that diesel
problem and is designing some smaller direct-injection engines too. Only when
that happens, and when it works better with the air-conditioning, will i-stop
really start to shine.

Dan Read