Fiat Powertrain’s new two-cylinder engine family made its debut at the Geneva Motor Show in March. A 500 with the engine was unveiled in Turin in early July and will go on sale shortly, reaching the U.K. at the end of the year.
The twins are called Twin-Air, not least because they use Fiat’s Multi-Air valve lift control for intake throttling. Fiat describes the new engines as ‘a brand-new concept on the worldwide auto scene’, though perhaps they’re more of a rediscovery of an old concept. After all, Fiat built vertical twins for a quarter of a century.
The decision to reinvent the twin-cylinder car engine comes from one of the inescapable truths of engine design. The greater the ratio between the total surface area of the combustion chambers and their total volume, the less efficient the engine will be (all other things being equal). The more surface area, the more energy is lost as heat. So reducing the number of cylinders will give you a head-start with thermodynamic efficiency. Friction is reduced, too.
The only slight problem is refinement, but auto makers are clever with that sort of thing these days. At least, we’re hoping so. Fiat has used a balancer shaft to control vibration, and claims that the unit’s NVH performance is equivalent to that of a four-pot but with a characteristic sound.
The Twin-Air family will deliver outputs from 65PS to 105PS. Fiat claims that the Twin-Air units emit 30 per cent. less CO2 than an (unspecified petrol) engine of equal performance. They use a single overhead camshaft, driven by a chain, and indirect cylinder fuelling.
The first application of these new engines, this autumn’s Twin-Air 500, is a turbocharged 85PS unit of 875cc. Fiat anticipates that the 500 twin will have the lowest CO2 yield of any petrol engine — 95g/km.
Fiat is confident that, compared with the current petrol-engined versions, the newcomer provides good performance with a major economy benefit. The new turbo two-cylinder 85PS engine uses up to 15 per cent. less fuel and performs better than the 1.2 8v; against the 1.4 16v, fuel consumption is 30 per cent. better with comparable performance.
Next to a four-cylinder power unit of equal performance, the new engine is obviously significantly shorter: 23 per cent. shorter, says Fiat. Weight also drops, by roughly 10 per cent. These characteristics open the door to interesting applications. For example, a petrol-electric hybrid powertrain in a small car is a lot easier to package with a vertical twin than with an in-line four.
A version of the Twin-Air designed to run on methane will soon be available. This was achieved by adding an extra pair of injectors for the gas on the intake manifold rails.
The 500 Twin-Air will be available with a new economy device. It’s a button marked ‘Eco’ which remaps the engine with the intention of improving running efficiency in urban driving.
There are two driving modes on a 500 Twin-Air. In Normal mode, the engine will deliver its substantial 145Nm if the driver feels the need. The servo assistance provided to the steering is set to provide relatively firm weighting. But switch the driving mode to Eco and the maximum torque is capped at 100Nm (at 2000rpm) while steering assistance is increased.
We’re not absolutely convinced that limiting torque is the best way of making urban driving more efficient. After all, the most efficient way to accelerate is at full load while keeping the engine speed as close as possible to the torque peak. Heat engines really aren’t very efficient at part load: perhaps the 500’s Eco mode would really be of more use out of town.