Diesel motors are becoming better, cleaner and more sophisticated over time. However, it takes more than just advancements to build the best example that reigns supreme over others. And that one very maker to do so is Mazda, and this is why the Skyactiv-D remains as the world’s most refined turbodiesel engine yet to hit the growing consumer market.

emember the good old days where you’re quickly apprise with the arrival of your school bus in the early morning as it gets close to your house? Yes, that clattery, noisy and occasionally smoky yellow bus that hauls you to school, which is driven by that one elderly uncle bragging about saving so much in fuel bills despite charging you quite a fortune in monthly transportation charges?

If you do, then here’s why diesel technology matters so much in the automotive world. You might conceive diesels as a waste of exercise, but it plays a pivotal role that has served millions in saving them a lot of money in the long run, and it’s here to stay with us through its ever-growing technology; rivalling alongside its leaner and more refined cousin, the petrol engine.

When started, diesels were the moving tour-de-force during the infancy of the motoring world. It all began back in 1885 where Herbert Akroyd Stuart thought paraffin oil would be the next best alternative for conventional petrol engines. Unlike early petrol-powered engines where fuel volatility is low and it enables greater combustion; the diesel engine was born with a high-thermal internals to smoothen all progress to exude lean burning, and dissipating heat through excess air.

Contrary to petrol engines which uses the Otto cycle to keep all things going — the internals of a diesel engine opts for a method that involves highly-compressed hot air to ignite the fuel. There are no spark plugs involved in this matter, and it channels air into the combustion chamber with a compression rating of no less than 15:1 or more. With this, the high compression causes the temperature in the air to rise.

Contrary to petrol engines which uses the Otto cycle to keep all things going — the internals of a diesel engine opts for a method that involves highly-compressed hot air to ignite the fuel. There are no spark plugs involved in this matter, and it channels air into the combustion chamber with a compression rating of no less than 15:1 or more. With this, the high compression causes the temperature in the air to rise.

The use of better fuel injectors are vital in this case, and due to this, it allows all combustion to take place without an ignition system. It greatly increases the engine’s efficiency in the end — but on the deficient side — producing lots of harmful greenhouse gases after passing through the exhaust system. Solutions were quickly rectified with the introduction of diesel particulate filters, best known as DPFs, and NOx adsorbing catalysers.

It was proven good, but albeit a little costly in the long run as most diesel-powered cars would need to put up with the local source of diesel fuel, depending on the region you’re in. With low quality diesel, parts such as these will deteriorate over time. As wear and tear increases no thanks to the use of low-quality diesel fuels, an inevitable replacement is due in a varying time frame, resulting in irregular maintenance follow-ups.

Technologies such as EGR, known as exhaust gas recirculation, and selective catalytic reduction are also added into the diesel motor palette, but all is proven to be less effective eventually. Somehow, these are the modern-day diesel engines you can get from various manufacturers, but with our mindset associating diesels as a suitable choice for commercial purposes rather than your own private commute, it remains a very left-field segment in the Malaysian market.

‘When started, diesels were the moving tour-de-force during the infancy of the motoring world.’

The mission to change every single perception

Solutions are definitely a stone throw away, but to defy convention, Mazda has taken the unorthodox road by offering a world-leading diesel engine. Surprising for a manufacturer famed more for its Rotary motors, this is where the Skyactiv-D steps in by being the world’s most advanced diesel engine — proving others that you don’t need excess compression to combust diesel fuel, which saves you more on wear and tear in long run without expelling harmful emissions.

Famed for being the world’s first diesel engine with the lowest compression ratio, the Skyactiv-D complies with any modern-day exhaust gas regulations. It even lacks a NOx after treatment system, all thanks to its 20-percent better efficiency that is combined with a remarkably low compression ratio of 14.0:1.

Unlike high-compression diesel engines that suffers from extreme pressure at the piston’s top dead centre (TDC), Mazda has decided to reduce compression in order to reduce TDC stress — and consequently — enabling better air and fuel mixture. During progress, it assuages all formation of soot and NOx build up through its uniformed combustion technique.

Momentum-wise, the Skyactiv-D sprays fuel during the piston’s upward movement that is close to the TDC. Due to this, it results in a more efficient burning process that not only boasts leaner combustions, but reduces excess exhaust gases upon exiting the chamber. It beats the traditional combustion method of a conventional diesel engine, which burns fuel in between the gap, forming NOx due to insufficient oxygen.

But what if the performance is conferred? Then this is where the two-stage turbochargers take place by adding more low to high-end torque with a high 5200rpm limit. The turbos are designed to reduce emissions and fuel consumption, and the two-stage architecture is charted in a way to selectively respond according to continuously changing driving conditions.

Being a diesel, it is no surprise that cold starts are one of the challenges due to its use of low volatility fuel. To ensure cold-start capability, Mazda has fitted in a set of multi-hole Piezo injectors that automatically adjusts itself through a wide variety of injection patterns. Precision plays an important role here as accuracy matters when it comes to timing and mixture of concentration control that assures a smooth cold start.

The Piezo injectors are able to produce nine injections in one combustion cycle, and it comes with three types of basic injections; starting from pre-injection, main and post injection. It changes according to driving patterns. It doesn’t only function as an injector, but also sprays the right amount of fuel into the chamber during a cold start, which is partly-helped by the use of ceramic glow plugs.

Mazda Motorsports has been supplying its Skyactiv-D clean diesel race-tuned engine to teams competing in the LMP2 class of international motorsport.  The engine has made its Le Mans 24hours debut with Dempsey Racing back in 2013.  Mazda remains the only Asian car maker to have won the world’s toughest endurance race, in 1991.

DID YOU KNOW?

'It is currently the best diesel engine to hit production, bar none.'

Compression ratio and diesel combustion

Friction reduction due to lower compression ratio

Lower compression has also given Mazda the upper hand of incorporating more lightweight materials during the Skyactiv-D’s manufacturing process. The structure of the engine has been optimised to aluminium as opposed to metal, and with that alone, the Skyactiv-D motor is 25kgs lighter than any conventional diesel engine on the market.

The cylinder head is now lighter, as it comes with thinner walls that is topped-off with an integrated exhaust manifold. To complete its focus on shedding unnecessary weight, Mazda has even reduced the weight of the pistons and crankshaft by 25 percent. This leads to a lower mechanical friction on the internals — which matches close enough to a conventional petrol-powered engine.

Higher expansion ratio due to lower compression ratio

Two-stage turbocharger

Increase in air temperature assisted through its VVL Exhaust during warm up also plays a role in reducing any misfirings. A single combustion cycle is what it needs to warm up the exhaust gas temperature to rise — all thanks to its valves that open slightly during the intake stroke to exude hot exhaust air back into the cylinder. All in all, this helps in the event of compression temperature, and it stabilises ignition.

Here’s why we love the Skyactiv-D. Being different from the rest, it also plays a vital role in the growing diesel market with an utmost purpose in mind: to reduce emissions and boost efficiency without the need of any expensive after treatment systems. It is currently the best diesel engine to hit production, in my opinion. And it looks like it will be for a very long time.

Variable valve lift (VVL)