Both the three-cylinder and four-cylinder petrol engines get BMW’s ‘Twin Power’ turbo set-up, which consists of direct fuel injection, Valvetronic variable lift for the inlet valves and Double Vanos variable timing for the exhaust and intake valves.
The exhaust manifold and turbocharger are now housed in the cylinder head and a new direct fuel injection system operates a maximum pressure of 350 bar – rather higher than in the current generation of petrol engines. BMW says this allows for more accurate metering of the fuel and, as a result, lower emissions of pollutants.
The new cooling system should also reduce pollutants by optimising the combustion process. This features a coolant pump with two separate outlets – one for the head and one for the block. The two parts of the engine ideally need to be cooled at different rates.
Both engines still have balancer shafts, but the unit for the three-cylinder engine has been redesigned. A single-piece timing chain is used to help reduce noise.
The new diesel engines also promise a 5% drop in CO2 emissions and an emphasis on reducing exhaust pollutants. All of these new four-cylinder diesel engines now get twin-turbochargers - something previously reserved for the most powerful BMW diesel engines.
The low-pressure turbocharger (used at lower engine speeds) has electrically adjustable vanes and the high-pressure turbocharger is integrated into the exhaust manifold. There’s also a switchable cooling system for the low-pressure turbocharger.
New exhaust gas recirculation (EGR) systems have been introduced for both the three-pot and four-cylinder diesel engines. These are intended to further cut exhaust pipe emissions of nitrogen oxide (NOx), a polluting gas that has proven to be difficult to reduce in real-world driving conditions.
As a result, all of the engines get a selective catalytic reduction system and AdBlue urea injection into the exhaust stream to further reduce NOx emissions.
The common-rail fuel injection system of the new diesel family has been redesigned and gets new injectors with a "range of upgraded" sensors to ensure even greater accuracy of the amount of fuel injected into the cylinder.
Fuel pressures have been further increased to 2200 bar for the three-cylinder engines and an extraordinary 2700 bar for the four-cylinder engines.
Perhaps the most unusual aspect of the new diesel engines is that the cylinder bores are not the same diameter along their whole length. Instead, the bores on the new engines are fared sightly at the lower end.
BMW engineers say "thermal and dynamic forces" when the engine is assembled and especially during operation mean that straight-sided bores are not ideal.
Either the piston crown becomes ‘loose’ at the top of the bore or 'tight' at the bottom, increasing friction on every piston stroke. The new bore design should eliminate this problem.