Impressive clean diesel technology from those clever engineers at Mazda



Impressive clean diesel technology from those clever engineers at Mazda
Mazda is claiming its ‘Skyactiv’ diesel is the world’s cleanest.  And it doesn’t need an expensive and troublesome exhaust particulate filter, either.

It’s got trick stuff such as variable lift exhaust valves, a two-stage turbo, an exhaust manifold that’s IN the head and very sophisticated piezo-electric injectors.

But the key to the engine’s cleanness is a low compression ratio of 14:1, about the lowest of any diesel and this is this clever engine’s Achilles heel, because that MUST mean it’s less fuel-efficient, thermodynamically at least.

Diesels typically have compression of ratios of between 14:1 and 16:1 for direct injection engines, and between 18:1 and 23:1 for indirect injection.  It’s the compression that explodes the mixture and it’s the high compression that gives the diesel better thermal efficiency than petrol-engined vehicles.

Oxides of nitrogen and soot are the bugbear of diesels and the higher the compression ratio – of diesels or petrol-engined vehicles – the more of these pollutants there are.  They are also more prevalent in exhaust fumes when the engines are warming up, so the Mazda’s quick warm-up system with the exhaust manifold in the head is key here.

Despite the physics of low compression engines, Mazda are claiming increased miles per gallon.  How can this be?

Here’s one reason – direct injection diesels inject fuel just before top dead centre, to prevent pre-ignition, or ‘knock’. That’s before an adequate air-fuel mixture can be formed for ideal combustion.  Not enough oxygen, in other words. So, paradoxically, diesels can’t normally be as efficient as they should be.

Also, other Mazda technologies will have a significant effect - the quick warm-up manifold, for example.

AND, A low(er) compression engine means lower combustion pressures and, therefore, less stress on mechanical components, so Mazda have been able to use light-weight materials.  The cylinder block is aluminium – normally iron in a diesel – saving 25kg!! The aluminium head is 3kg lighter and the pistons 25 per cent lighter.

Low(er) compression could mean misfiring during warm-up, as the compression temperature and pressure are lower than ideal, and this is where Mazda’s variable lift exhaust valves come in. They . are opened slightly during the intake stroke to regurgitate hot exhaust gas back into the cylinder, which increases the air temperature.
Mazda’s ‘multi-hole piezo injectors’ allow for a wide variety of injection patterns and timings - pre-injection, main injection, and post-injection.  There are even, different injection patterns will be set according to driving conditions.

I’m a fan.

More info from Mazda here http://www.mazda.com/mazdaspirit/skyactiv/engine/skyactiv-d.html

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