SKYACTIV-D (Diesel engine)

SKYACTIV-D (Diesel engine)

Today we will focus on the all-new SKYACTIV-D Diesel engine. Here are some detail information from Tokio Ishino, Staff Manager Powertrain Development Division.

At 14.0:1 the all-new common rail SKYACTIV-D has the same compression ratio as the SKYACTIV-G petrol unit. This makes it the lowest-compression diesel engine in the world. SKYACTIV-D is also one of the first diesels to comply with strict Euro 6 emission regulations (which don’t take effect until 2014) without needing expensive SCR (selective catalytic reduction) aftertreatment or an LNT (lean NOx trap catalytic converter).

Higher expansion ratio due to lower compression ratio

Diesel engines do not require spark plugs. The injected fuel mixture ignites on its own at high pressure and the resulting high compression temperature near the “top dead centre” (TDC), or when the top of the piston is closest to the cylinder head. To ensure reliable cold starting and stable combustion during the warm-up phase, conventional diesel engines have high compression ratios of 16:1 to 18:1. But not Mazda’s unique SKYACTIV-D.

SKYACTIV-D Fuel economy improvement

SKYACTIV-D Performance improvement

Its low 14:1 compression ratio enables combustion timing to be optimised. When the compression ratio is lowered, compression temperature and pressure at TDC decrease. Consequently, ignition takes longer even when fuel is injected near TDC, enabling a better mixture of air and fuel. This alleviates the formation of NOx and soot, since combustion becomes more uniform without localized high-temperature areas and oxygen insufficiencies. Furthermore, injection and combustion close to TDC make a diesel engine highly efficient: The expansion ratio (or amount of actual work done) is greater than in a high-compression diesel engine. Simply put, optimised combustion timing means the SKYACTIV-D makes better use of the energy contained in the fuel. And that is the main reason for the 20% reduction in fuel consumption.

Euro 6 without NOx aftertreatment
Thanks to its low compression, the SKYACTIV-D also burns cleaner, discharging far fewer nitrous oxides while producing virtually no soot. It can thus do without NOx aftertreatment and still meet tough emissions standards the world over (like Euro 6) with their strict NOx limits.

The fact that Mazda’s SKYACTIV-D is still considered a pilot development today — no other manufacturer has attempted to emulate it thus far — can be attributed to the system-related drawbacks of low compression.
For example, the compression-ignition temperature for cold starts and during cold operation is normally too low in a diesel engine with a compression ratio of only 14:1. It would run rough, particularly in winter conditions, misfiring during the warm-up phase. And at extremely low temperatures, the engine might not even start in the first place.

Exhaust VVL (variable valve lift)

Variable Valve Lift

So, to improve cold starting and cold running, Mazda furnished its SKYACTIV-D with ceramic glow plugs as well as exhaust variable valve lifts. The role of the latter is to allow the internal recirculation of hot exhaust gas into the combustion chamber. This is how it works: a glow plug is used to carry out first combustion cycle, which is enough to raise the exhaust gas to a sufficient temperature. After the engine starts, the exhaust valve does not close as usual during the intake stroke. Instead, it remains slightly open to allow some exhaust gas to reenter. This increases the air temperature in the combustion chamber, which in turn facilitates the subsequent ignition of the air-fuel mixture and prevents misfiring.

Reducing weight and internal engine friction
The SKYACTIV-D’s lower compression ratio means lower maximum pressure and thus less strain on engine components than in conventional diesels. As a result, there is more scope for structural modification to reduce weight: Cylinder heads with thinner walls and an integrated exhaust manifold are 3kg lighter than before, while the cylinder block, now made of aluminium, saves another 25kg.

Reduced weight

Reduced friction

Add in a 25% decrease in the weight of the pistons and crankshafts, and Mazda has managed to reduce internal engine friction by 20% overall in the SKYACTIV-D relative to the current MZR-CD diesel. For the driver, this translates into superior responsiveness, more pulling power and better fuel economy.

Friction reduction due to lower compression ratio.

Two-stage turbocharger
Turbochargers not only help diesel engines deliver high torque, but also improve fuel economy while reducing harmful emissions. The SKYACTIV-D uses two-stage turbocharging.

Two-stage turbocharger

It features one small and one large turbocharger, which are selectively operated according to driving conditions. The small, quick-responding turbo feeds air to the combustion chambers at low engine speeds to provide low-speed torque and eliminate “turbo lag”. Turbo lag is characterised by abnormally low torque and poor throttle response.

It is caused by a lack of exhaust pressure to rotate the turbocharger’s turbine up to a speed necessary to supply boost pressure. Together, the two turbos ensure high torque and responsiveness at low engine speeds and high power even at unusually high rpms, enabling the SKYACTIV-D to easily reach its 5,200 rpm redline. There is no compromise to power, driving dynamics or driving enjoyment, despite the engine’s extraordinary efficiency. And the synergetic effect of the two-stage turbocharging and low compression ratio enables optimal timing for combustion. Since there is a sufficient supply of air (oxygen), NOx and soot emissions are kept to a minimum.

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