July 25, 2016 @ 06:29 PM

Feeling the D, the SkyActiv-D

We get down and dirty with Mazda’s diesels to discover they aren’t down on power nor are they dirty.

You would imagine that with more than 130 years of continuous development and technological ameliorations, the internal combustion engine; the heart and soul of a motorcar, would be pretty efficient in dispatching its duties.

Essentially a large air pump, the internal combustion engine is in actuality, staggeringly incompetent at converting the chemical properties of the fuel into mechanical energy that turns the wheels. In fact, petrol engines only transform 25-30 per cent of the energy into actual mechanical force with the rest lost through thermal, friction, parasitic, rolling resistance and drivetrain losses. Diesel engines check in a smidgen better at 30-40 per cent efficiency but the fact that a Toyota engine with a 38 per cent efficiency is touted as greater than any other mass-produced combustion engine speaks volumes.

Fundamentally though, the losses are thermal in nature; meaning the prime suspects are frictional and a lack of cooling. This has been the crux of the Mazda R&D team’s focus in their SkyActiv engines.

There are six principal factors focussed on in their approach:
- compression ratio
- mechanical friction loss
- combustion timing
- combustion duration
- air-to-fuel ratio
- pumping loss

By optimising these factors, Mazda aims to increase the efficiency of these engines and their research pointed towards the compression ratio as the backbone to achieving this although the manner in which they did so was completely conflicting.

Kiyoshi Fujiwara, Senior Managing Executive Officer of Oversight of R&D, mentioned off the record that the engineers are optimistic of approaching a thermal efficiency close to 60 per cent for light duty passenger vehicles.

While most manufacturers went down the downsizing route and supplementing the loss of displacement with forced induction, Mazda switched its focus to optimising the compression ratio. This meant increasing it for petrol engines and decreasing it for diesel engines, the latter leading to the lowest compression ratio for a diesel-engined passenger vehicle.

The 2.2-litre SkyActiv-D engines that have just made their way here in the Mazda 6 and CX-5 is one of the first diesel engines to comply with the Tier II Bin 5 North American emission regulations without the use of expensive and complex selective catalytic reduction (SCR) aftertreatments or a lean NOx trap catalytic converter (LNT).

It represents a monumental leap forward in terms of efficiency with fuel consumption reduced by 20 per cent due to the lower 14.1:1 compression ratio. The models here both sing to the tune of 173bhp and 420Nm of torque so they are not sacrificing performance in the name of efficiency either.

According to the Mazda boffins, the lower compression ratio results in a lower temperature and drop in pressure at top dead centre (TDC). Additionally, the injection of fuel closer to TDC allows for a better air and fuel mixture for a cleaner combustion; something the longer ignition process also contributes significantly to.

With the cleaner combustion and lower compression ratio, the issue of NOx and soot is diminished as the uniform combustion with an even temperature spread alleviates the snag. Mazda adds that the injection and combustion closer to TDC increases the efficiency of a diesel engine significantly.

Nonetheless, it is not without disadvantages. A lower compression ratio would generally lead to issues with cold starts in cold climates with misfiring a common stumbling block during the warm up phase or in a worst case scenario, inability to start completely.

Mazda alleviated this with ceramic glow plugs and variable valve lift for the exhaust side. The former ignites the first combustion cycle while the latter remains open slightly during the intake stroke for exhaust gases to re-enter the chamber and increase the ambient temperature to catalyse the subsequent ignitions.

One of the cardinal by-products of the lower compression ratio is the reduction in maximum pressure and strain on engine components as compared to conventional diesel engines. As a result, the engineers could introduce significant design elements that reduced weight.

Thinner walls for the cylinder head and an integrated exhaust manifold slash around 3.0kg although the biggest pruning in poundage is courtesy of the aluminium block that accounts for 25kg alone. The weight reduction also extended to the pistons and crankshaft, which in turn dropped internal friction by 20 per cent.

Turbocharging is handled by a sequential twin-turbo setup that sees a smaller, more responsive turbo eliminate lag and bump up low-speed torque before the larger unit kicks in for some top end grunt.

Here’s an interesting nugget of information. Mazda has even gone racing with this very 2.2-litre SkyActiv-D mill. It dominated its class in the 2013 Rolex Grand-Am GX class with the Mazda 6 SkyActiv-D racecars taking nine out of 12 wins.

Even the engine was a true production-based engine with 51 per cent of parts found in the production vehicles. Its output was 378bhp and 610Nm of torque with a redline of 5500rpm. So rest assured, these engines will take you far and fast too.

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