Friday, June 10, 2011

Mazda: the new engine technologies Skyactiv Sky-D

After having talked about the new technologies that SkyActiv Mazda apply to gasoline Sky-G, we focus now on the Sky-D turbo-diesel engines and their technology. With 14,0:1, the new engine has the same compression ratio of gasoline: this makes the diesel with the lowest compression in the world. It is known that diesel engines do not require spark plugs: the fuel mixture injected turns itself at high pressure and high temperature due to compression which is near the "top dead center," or when the top of the piston is closer to the cylinder head.

To allow a reliable cold start and a stable combustion during heating, td Conventional high compression ratios ranging from 16:1 to 18:1. The engine Sky-D instead, because of the low compression ratio, valve timing allows for the optimization of combustion. When the compression ratio is reduced, decreasing the pressure and temperature in the compression top dead center.

Consequently, the ignition takes longer, even when fuel is injected near TDC, allowing better mixing of air and fuel. This reduces the formation of NOx and soot, since the combustion becomes more uniform, without high temperature or localized areas of oxygen deficiency. Moreover, the injection and combustion closer to top dead center make it a highly efficient diesel engine: the expansion ratio (or amount of actual work carried out) is greater than that of a diesel engine at high compression.

The timing for the optimization of combustion in practice due to better use the energy contained in fuel, this is the main reason for the 20% reduction in consumption on the new Sky-D. Because of its low compression, Skyactiv-D also burns more cleanly, emitting fewer nitrogen oxides in the exhaust.

It may therefore dispense with the post-treatment of NOx and yet meet the stringent Euro 6 emissions regulations. However, it is known that a low compression on a diesel could generate problems: for example, the temperature of compression-ignition in the case of cold start and during the cold operation is usually too low for a diesel engine which has a relationship compression of only 14:1.

Work smoothly, especially in winter conditions, losing shots during the heating phase. And at extremely low temperatures, the engine may not start the first time. Therefore, to improve cold start and low temperature operation, Mazda has equipped its Sky-D with ceramic glow plugs, as well as with variable exhaust valve lift.

The role of the latter is to allow the internal recirculation of hot exhaust gases into the combustion chamber. It works like this: a candle bulbs is used to produce the first combustion cycle, which is able to carry the exhaust gases at a temperature sufficient. After starting the engine, the exhaust valve does not close as usually happens during the intake stroke.

On the contrary, it remains slightly open to allow some exhaust gas to return. This increases the air temperature in the combustion chamber, which in turn facilitates the subsequent ignition of the air / fuel mixture and prevents the engine misfires. The lower compression ratio of the engine means a reduction of the maximum pressure and therefore less stress to engine components compared to conventional diesel.

Consequently, there are more opportunities to make structural changes to reduce weight: the cylinder heads with thinner walls and the exhaust manifold is integrated 3 kg lighter than before, while the cylinder block, now aluminum, saves others 25 kg. With the addition of a 25% reduction in the weight of pistons and crankshafts, Mazda has managed to reduce the friction inside the engine a total of 20% over the current MZR-CD diesel.

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