Water Injection

Optimizing Power by Controlling Temperature

In the realm of high-performance engines, "knock" (or premature detonation) is a major issue that limits a motor's ability to develop its full power. Knock occurs when the air/fuel mixture ignites uncontrollably before the spark plug initiates the combustion, leading to increased wear, or even damage, to internal components. To address this issue without resorting to an excessive mixture enrichment (which would reduce performance), some manufacturers, such as BMW with its M4 GTS F-series, have opted for water injection.

Principle of Water Injection

Water injection involves spraying a fine mist of water directly into the intake manifold or combustion chamber. When the water vaporizes, it absorbs a large amount of heat, which reduces the temperature of the air/fuel mixture. This cooling effect not only pushes back the threshold for detonation (knock) but also allows for advancing the ignition timing and increasing the boost pressure. In the BMW M4 GTS, for example, this system raises the boost from 1.19 bar to 1.49 bar—a roughly 26% increase—resulting in a power increase from 431 to 500 hp.

Technical Implementation on the BMW M4 GTS

Technically, the water injection system of the M4 GTS stands out due to several innovative aspects. Three injectors are strategically positioned on the intake manifold to ensure even distribution of the fine mist, thereby optimizing the cooling of the compressed air. A reservoir located at the rear of the vehicle supplies the necessary water. However, to prevent improper use, the engine's mapping prevents full boost pressure from being developed when the outside temperature is below 5°C. This limitation protects the system from risks related to excessive cooling. Additionally, the injection pump is reversible, allowing the system to purge the injection lines after use, eliminating stagnant water that could cause corrosion or blockages.

A Brief History of Water Injection

These technical solutions are rooted in historical research: as early as the 1930s, Harry Ricardo studied how to control knock in supercharged combustion chambers. In Hugh MacInnes' book "Turbochargers" (H. P. Books, 1978), we see how injecting a fine mist of water helped absorb heat and achieve higher combustion pressures. This technique was also used during World War II, notably in certain variants of the BMW 801 radial engine used in aviation.

Water and Methanol

While water injection is at the core of this technology, adding methanol to the mixture increases its octane rating. This practice, common in high-performance tuning, allows for further optimization of settings without the risk of detonation. It combines the cooling effect of water with the anti-knock properties of methanol, pushing the limits of engine efficiency and power.

Conclusion

In summary, water injection is a particularly interesting technical-scientific solution for pushing the boundaries of engine performance. The system on the BMW M4 GTS, with its three injection points, optimized management based on ambient temperature, and reversible pump, perfectly illustrates how a concept developed nearly a century ago remains a relevant technology for improving boost and power. At WOT Engineering, we are passionate about engineering, especially solutions that combine performance and reliability to harness an engine's full potential without compromising its durability.