Time for a spot of science. This Mazda3 i-stop is the company’s first crack at stop/start technology. But unlike other
similar systems, it's not as simple as you might think. Here's how it works.

When the engine is killed, i-stop knows which piston is in the best position to
restart quickest - 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 the piston down and
- along with a small pulse from the starter motor - firing the whole engine
into life. It's like placing bicycle pedals just past 12 o'clock, making it
easier to push down and move off.
 
In other words, it starts like any engine, only more precisely and therefore
quicker.
 
In fact, when you press the clutch it takes just 0.35 seconds to get going
again. By comparison, BMW's stop-starters take 0.7 seconds, as the starter
motor has to churn the engine conventionally until it catches.
 
Phew. That's the theory, but does it actually work? The short answer is yes. It
cuts out subtly as you come to a stop and grab neutral, and restarts quickly
with less judder than usual.
 
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-degree heat, selecting a 20-degree cabin temp, and it could only manage
about four seconds before calling on the engine again. That's just not good
enough in less-than-strenuous conditions - the sort of conditions a BMW system
would breeze through.
 
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 (as 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, which ain't bad, but 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.
And generally speaking, the Mazda3 is impressive - lovely ride, fun handling
and decent interior.
 
But i-stop just doesn't 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 aircon,
will i-stop really start to shine.

Dan Read