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The TwinAir engine, a £300million investment by Fiat, signposts the way forward for internal-combustion engine technology over the next few years. Smaller, lighter, less friction, lower emissions - yet it also has more power than a conventional engine and drivability that's every bit as good.

The TwinAir engine combines a number of technologies which enhance its efficiency, performance and drivability. The most significant are the two-cylinder architecture, the MultiAir valve control system and the turbocharger installation.

Two-cylinder architecture: balancing efficency, power and refinement

Fiat went for a two-cylinder engine to achieve optimum thermal efficiency, low friction losses and low weight. TwinAir is 23 per cent shorter and 10 per cent lighter than the existing 1.2-litre, four-cylinder eight-valve engine.

Fiat paid special attention to noise, vibration and harshness (NVH) to deliver four-cylinder levels of refinement with a two-cylinder engine. To achieve this the TwinAir unit has a balance shaft which runs at the same speed as the crankshaft (rather than twice engine speed, as with balance shafts used on four-cylinder engines). The shaft runs in needle roller bearings to reduce friction.

MultiAir valve control: reducing losses and improving drivability

TwinAir was the first engine designed specifically to incorporate Fiat's MultiAir system of inlet valve control, which is the key to engine's efficency. The system was introduced on the MultiAir four-cylinder engine which was introduced in the Alfa Romeo Mito in 2009.

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In a conventional petrol engine, a throttle is used to regulate the amount of air going into the cylinders - which in turn controls the engine's power output. MultiAir does away with the throttle, and instead regulates the air flow into the engine by fine control of the intake valves. If less than full power is required, the valves do not open fully and close early.

MultiAir controls the valves using an electro-hydraulic system. The valves are operated by a camshaft, as in a conventional engine, but in between camshaft and valve is an oil chamber. The motion of the camshaft pressurises the oil, and this increase in pressure causes the valve to open. The clever bit is that the volume of oil in the chamber can be varied using an electrically-controlled valve (red in the picture above, which shows the Alfa MultiAir engine) and as a result the valve lift can be infinitely varied.

The system provides several advantages:

  • 'pumping losses' caused by a part-open throttle are eliminated, improving thermal efficiency
  • the obstruction in the intake caused by even a wide-open throttle plate is eliminated, giving an improvement in power
  • the air in the induction system is at a constant high pressure all the way to the ports, which improves accelerator response and low-speed torque

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Small capacity, turbocharged for power and torque

A new generation turbocharger from Mitsubishi increases maximum torque as well as making it available at very low revs. The maximum 145Nm of torque is on tap at just 1900rpm, as opposed to the 102Nm at 3000rpm of the Fiat 1.2-litre eight-valve engine. CO2 emissions are reduced by 15 per cent while power is up by 23 per cent. It gives the 85bhp 500 TwinAir with Duologic robotised transmission the lowest CO2 emissions of any production petrol vehicle at 92g/km.

Turbocharging ensures the TwinAir engine is flexible and responsive, despite its small capacity, yet it remains reliable and simple to manufacture.

The Future for TwinAir

TwinAir is sure to become available in a wider range of Fiat group vehicles in the future. In addition to the current 85hp specification, a 65hp normally-aspirated version is due soon. There will also be an 80hp bi-fuel version and a 105hp performance option with 120bhp/litre.

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