Mazda SkyActiv-D; Low Compression Equals Low Emissions

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Mazda SkyActiv-D

Any diesel-engine manufacturer that confidently claims it produces an oil-burner with the “world’s lowest compression ratio,” will no doubt draw the immediate attention of inquisitive enthusiasts, asking obvious questions like: “Doesn’t diesel normally need a lot of compression to function properly?” and “What, if any, are the benefits of a diesel with less compression?”
Mazda Motor Corporation is the company making the claim, saying it’s done it with the SkyActiv-D engine, an innovative powerplant that efficiently operates on only a 14:1 ratio (diesel manufacturers normally use a 16:1 or higher ratio to facilitate proper burn). This low ratio is a key factor in the engine having 20 percent better fuel efficiency and being able to meet modern emissions standards (Euro Stage 6 and US EPA Tier 2 Bin 5) without the use of expensive NOx aftertreatment, like a selective catalytic reduction system.
Using compression ignition to run an engine means everything relies on when and how the fuel ignites, so compression is of utmost importance. Not enough compression usually means misfires, rough idle, and hard starts on a traditional diesel engine. Mazda overcomes these issues with the SkyActiv-D by incorporating many different technologies.
Mazda SkyActiv D 1

  |   Mazda’s SkyActiv-D is 55 pounds lighter than the engine it replaces, thanks to an aluminum block and cylinder head (with integrated exhaust manifold).

SkyActiv-D uses multi-hole Piezo injectors to provide a wide variety of injection patterns that are adaptable to the engine’s needs. The air and fuel mixture concentration can be controlled to ensure cold-start capability by increasing timing and using a precise injection of fuel. Ceramic glow plugs are used in conjunction to warm the air and provide a good ignition. The high-spec injectors are capable of nine injections per combustion cycle.
A specifically designed egg-shaped combustion chamber on the piston works with the measured amount of fuel the injectors spray to suppress momentum loss and form a vertical vortex that reduces combustion duration and provides cleaner emissions.
If misfires do occur during engine warm-up, the variable-valve lift system on the exhaust valves will (after a single combustion cycle) open the valves slightly during the intake stroke to regurgitate hot exhaust gas back into the cylinder to increase air temperature, which promotes the rises in compression temperature to stabilize ignition.
Mazda SkyActiv D Skyactivd Side View Xray Pistons

  |   Mazda SkyActiv D Skyactivd Side View Xray Pistons

The lack of ignition pressure in the cylinders causes compression temperature and pressure at top dead center (TDC) to decrease, which then slows ignition even when fuel is injected near TDC. This extended ignition period allows for a better mixture of fuel and air to occur. The improved mixture alleviates the formation of NOx and soot due to the more uniform combustion that occurs without localized high-temperature areas and oxygen insufficiencies. Getting injection and combustion close to TDC results in the engine being more efficient, without having high compression.
While turbocharging a diesel is critical to the engine’s ability to produce ample amounts of torque, turbos are also indispensable in the effort to reduce emissions output and fuel consumption. Optimizing SkyActiv-D is done with a two-stage turbocharger system that selectively utilizes one small and one large turbo during different driving conditions, providing greater torque at low speeds, increased power at high speeds, and responsiveness throughout the rpm range. The synergetic effect of the low compression ratio and turbochargers allows for optimally timed combustion, because a sufficient amount of air is secured in the cylinder. This also works to lower NOx and soot emissions even more.
Having a lower maximum in-cylinder combustion pressure made it possible for Mazda to use aluminum as the material of choice for SkyActiv-D’s block, which saves 55 pounds over the Mazda MZR-CD engine it replaces. The aluminum cylinder head is also lighter, engineered with thinner walls and an integrated exhaust manifold that collectively cuts another 6 pounds. The weight reductions put SkyActiv-D on par with gas engines in the same class.
Internally, the reciprocating parts are lighter as well, with piston and crankshaft (main journals shrunk from 60 mm to 52 mm) mass reduced by 25 percent each. Besides saving weight, this also drops mechanical friction to the level of an average gas engine.
Mazda SkyActiv D 3

  |   This is a race-tuned SkyActiv-D developed by Mazda Motorsports to race at Le Mans in the Prototype 2 class.

Mazda SkyActiv D Skyactivd Display Cutaway

  |   Mazda SkyActiv D Skyactivd Display Cutaway

SPECIFICATIONS

Engine: Mazda SkyActiv-D
Displacement: 2.2L (133.7 ci)
Engine Layout: I-4
Valvetrain: DOHC 16-valve
Bore x Stroke: 3.38 x 3.70 inches (86 x 94.3 mm)
Compression Ratio: 14:1
Head material: Aluminum
Block material: Aluminum
Piston material: Aluminum
Power: 175 hp (129 kw)
Torque: 310 lb-ft (420 Nm)
Emissions: Euro Stage 6, US EPA Tier 2 Bin 5
Induction: Two-stage turbocharging
Exhaust: Cylinder-head-integrated manifold
Intercooler: Air-to-air
Cooling System: Liquid-cooled
Fuel System: Common-rail direct injection
Lubrication System: Wet sump
Lubrication Capacity: 5.38 quarts (5.1 L)

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