Petrol engines (Skyactiv-G)
Thermodynamic improvement has been the focus for the petrol engines, the compression ratio has been increased to 14:1, until now only reached in competition engines.
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| Petrol Skyactiv engine |
The advantage of increasing the compression ratio is that it increases the thermal efficiency, which brings with it a reduction in fuel consumption.
The disadvantage in a conventional engine is self-ignition, so various measures have been taken to prevent it:
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| Skyactiv exhaust manifold in a petrol engine |
• Long exhaust manifold.
• Multi-hole injectors.
• Improved pistons.
The development of the longer exhaust manifold prevents the exhaust gases returning to the engine, thus reducing the hot exhaust gas pressure. A lower temperature in the combustion chamber inhibits self-ignition.
The duration of the combustion has been reduced to prevent the mixture remaining at high temperatures for too long. To achieve this, multi-hole injectors are used and a special cavity in the piston which enables good atomisation of the fuel and good flame propagation.
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These innovations result in an increase in engine torque while reducing fuel consumption.
Another of the improvements involves the reduction of pumping losses. This phenomenon occurs when the intake throttle is nearly closed at low loads. Under these conditions, a vacuum is produced in the manifold that the piston must overcome which reduces the engine's efficiency.
A variable valve timing system regulates the opening and closing of the valves, which controls the entry of air into the cylinder. During the compression phase at low load, the intake valves are kept open to return the air and produce more efficient combustion. In this way, the pumping loss is reduced and optimal performance is achieved at low engine speeds.
Lastly, with a weight reduction and less friction between the main internal engine components, such as piston and rods, a fast engine response is achieved with lower energy consumption during operation, thus reducing fuel consumption.
Petrol engines (Skyactiv-D)
Skyactiv-D was one of the first diesel engines to meet the Euro 6 standards, without resorting to additives such as AdBlue or Eolys. To achieve this, Mazda has reduced the compression ratio to 14:1, which is lower than any other current diesel engine.
This compression ratio reduces the temperature and pressure at top dead centre, with the advantage of achieving a more uniform explosion. It also allows the moment of injection to be advanced, thus increasing the expansion stroke compared to engines with higher compression, this reduces fuel consumption and generates lower quantities of nitrous oxides and soot.
To prevent the low compression ratio affecting the engine operation when cold, there is a variable valve lift system on the exhaust side.
This system keeps the exhaust valve slightly open during the intake phase, allowing hot gases to enter the cylinder and thus increase the combustion chamber temperature. In this way, while the engine is reaching its optimal temperature, the ignition of the mixture is assisted and ignition failures prevented.
The new engine incorporates a two stage turbocharger, this involves the application of two different sized turbochargers that act together in accordance with running requirements. At low engine load, the small turbo offers a faster response to avoid lag in the delivery of engine torque. At higher engine load, the larger turbocharger provides more power. This system provides a more linear torque delivery throughout the entire rpm range.
Lastly, weight has been reduced as the engine is subjected to smaller stresses. Reducing the weight of the components such as the cylinder head, pistons and block, among others, results in a better engine response with reduced consumption.
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