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Why is it so efficient?

  • Mazda's new compact SUV driven
  • Priced from 21,000 (est)
  • On sale spring 2012
Words BySteve Huntingford

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The compression ratio of an internal combustion engine is the difference between the volume of the combustion chamber when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke.

In Mazdas Skyactiv petrol engine, this ratio is unusually high (14:1), which increases thermal efficiency and lets the engine extract more energy from a given amount of fuel.

The downside of a high compression ratio has traditionally been premature ignition of the fuel-air mix, which leads to a drop in torque. However, the CX-5 has a new exhaust manifold that disperses waste gases more efficiently. The resulting reduction in compression temperature cures the problem of premature ignition.

Diesel engines work differently air alone is compressed, instead of a fuel-air mix, then at the appropriate time the diesel is injected into the extremely hot compressed air where it ignites.

As a result, diesel engines tend to have much higher compression ratios than petrols. However, Mazdas Skyactiv diesel has the same compression ratio as the petrol, making it the lowest-compression diesel engine in the world.

The main advantage here is that ignition takes longer, allowing a better mix of air and fuel. In addition, the lower pressures mean theres less strain on the engine and it can be built from lighter materials.

The downside of a low-compression diesel would usually be an engine that runs rough when cold, but the CX-5 has ceramic glow plugs and clever exhaust valves that raise the air temperature enough to prevent misfire.

Mazda has also used a higher proportion of high-tensile steel in the CX-5's body to keep weight down without sacrificing rigidity.

Meanwhile the new manual gearbox is smaller and lighter than the ones in other Mazdas, and the auto sends drive to the wheels very early, preventing engine output loss. The standard stop-start system completes the efficiency measures.