Called the Smart Idling Stop System, it injects fuel directly into a cylinder while the engine is stationary and ignites the fuel to create downward piston force that serves to start the engine. Mazda claims that the Smart Idling Stop System is more energy-efficient than an electric motor restart, and also restarts the engine more quickly and quietly than a conventional idle-stop system.

Direct-injection technology makes it possible for fuel to be supplied directly to the engine’s cylinders while the engine is stationary and then ignited to turn the engine over.

The three stages of the Smart Idling Stop System operation. The stages correspond to the numbered descriptions to the right. Click to enlarge.

However, since the cylinders soon return to atmospheric pressure when the engine stops, a special technique is needed to achieve enough expansion energy to restart the engine. The Smart Idling Stop System operates as follows:

1. The system injects a small amount of fuel into the cylinder whose piston is part-way through its compression stroke and ignites it, causing the engine to run slightly in reverse.

2. The system then injects fuel into the cylinder that has begun to undergo compression owing to the engine’s reverse operation.

3. Finally, the system ignites the latterly injected fuel, causing powerful expansion that drives the engine in the forward direction, thereby starting the engine.

For this to work, the compression-stroke piston and expansion-stroke piston must be stopped in positions that create the right balance of air volumes. Consequently, the Smart Idling Stop System effects precise control over the piston positions during engine shutdown.