BMW Improves Powertrain with Water Injection

The 2016 BMW M4 GTS features a 493-hp, 3.0-liter engine with a water injection system. Water is injected as a fine mist into the intake manifold plenum where it evaporates and reduces the ambient temperature of the air, thereby providing a means by which combustion can be improved for greater performance. This is said to be the first application in a production vehicle of such a system.

In addition to its clever powertrain setup, the M4 GTS is a study in lightweighting through the extensive use of carbon fiber reinforced plastic (CFRP). For example, the hood, roof (said to be 25% lighter than the aluminum roof on the standard M4), and decklid are CFRP. The CFRP spoiler is mounted on CNC-machined aluminum struts. Even the driveshaft is CFRP.

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Although the carburetor that runs on water is the stuff of urban myth, BMW has introduced the 2016 M4 GTS with a 493-hp, 3.0-liter M TwinPower Turbo in-line six that uses . . . water injection.

Yes, water.

No, this is not in place of the fuel.

Rather, the water is injected as a fine spray into the intake manifold plenum chamber. It evaporates, thereby reducing the temperature of the intake air by as much as 80°F (in addition to the temperature reduction of the intercooler, which is generally on the order of 160°F).

Cooler air is more dense, so the amount of oxygen in the combustion mixture is increased, so there is a higher mean combustion pressure, which translates into optimized power and torque development.              

In addition to which, the spark timing can be advanced closer to the optimal value. The risk of knock is minimized.

What’s more, the cooler operation means there are reduced thermal stresses on components ranging from the pistons to the exhaust valves to the turbocharger to the catalytic converter.

There are three water injectors used. Each injector supplies water to two cylinders.

There is a 1.3-gallon water tank in the trunk. The water pump used supplies water to the injectors at approximately 145 psi, with the injection quantity based on load, engine speed, and temperature. There are sensors and actuators as part of this injection system and the control is provided by the engine management system.

BMW engineers anticipate that the water tank wouldn’t need refilling much more often than at every fifth refueling for everyday driving conditions—although they do point out that if the M4 GTS is running on a track (it ran a lap time of 7 minutes 28 seconds around the Nürburgring-Nordschleife), it might need to get refilled with water every time it is refilled with gasoline.

If the tank runs dry? Boost pressure and spark timing are adjusted as necessary to protect the engine.

Incidentally: BMW acknowledges this as a special-edition car. Only 700 are going to be manufactured, of which 300 are slated for the U.S. market. (Chances are by the time you read this, all 700 will be gone.)