Waterinjection and stuff.
"Why on earth would I want to inject water into the engine of my car ?" you say.
The point of waterinjection is to eliminate detonation (as in premature ingition of the fuel/air mixture in the cyclinders). Detonation or engine knock appears when the inlet air becomes too hot. Modern engines have engine knock sensors that retards the ignition timing when knock is sensed and advances timing when there is no knock. The problem is that when ignition timing is retarded the power output of the engine decreases. And when an engine uses more boost pressure than it was originally designed for the chances that detonation will occur increases. That is the problem that waterinjection can solve.
This is how it works:
Water exists mainly as a liquid. When water is heated it absorbs lots of heat due to its high specific heat capacity; six times that of gasoline. As water evaporates it consumes even more heat. That is the whole point of waterinjection: as it is injected into the relatively hot airstream (maybe around 75°C) after the intercooler it will use the heat from the charge air to evaporate, thereby lowering the intake temperature. Then, inside the cylinder the heat increases rapidly, and the water that hasn't evaporated to cool the chargeair before the cylinders will now evaporate to help control in-cylinder-temperature. When the intake temperature and the temperature in the cylinders are lowered more boost pressure can be used without detonation occurring. On standard cars the high in-cylinder-temperature is kept below the critical level using excess gasoline as a coolant. But this only works to a certain degree. When the mixture becomes too rich the power will be reduced instead of increased. When water is used instead the mixture will stay correct while the temperature is still acceptable.
To allow the waterinjection to be used in the winter too (below 0°C) about 20% methanol is added do it won't freeze. An additional effect of this is an increase in the octan rating of the fuel. The reason for this is that methanol has an octan rating of 113, which is 15 points above the best normal fuel available.
Components:
The waterinjection system consists of these main items: An injector that is placed in the intake of the engine, after the intercooler. This injector is pressurized by a high-pressure waterpump in the engine compartment that sucks the water/alcohol mixture from a tank placed in a cool environment (preferably the trunk). The amount of water injected is controlled with a system similar to that of the fuelinjection. It senses the pressure in the intake manifold and revs of the engine and injects water as required. There are also lots of small bits and pieces but those mentioned here are the most important.
This is the result:
The vertical axis on the chart represents charge air temperature, and the horizontal axis is the time running.
As one can see the charge air temperature is lower both on idle and on WOT - wide open throttle (and the points between). The drop in charge air temperature at WOT is 30°C. How many extra HP this lead to no one knows but the Maserati Quattroporte Evoluzione on which the test was done had a 3.2 Litre V8, twin turbos and 32 valves. Power was 335BHP and torque was 450Nm, before the waterinjection was installed.
External waterspray:
This is another way of cooling the chargeair. Less complicated than the waterinjection it just sprays water on the outside of the intercooler. There the water will evaporate because of the heat from the chargeair that passes through the intercooler. The evaporation requires a lot of heat which it will get from the metal of the intercooler. This will result in a lowering of the temperature of the intercooler which in turn will cool the chargeair. External waterspray is used on WRC cars, but might not be too pratical for everyday cars.
For an even more in-depht look at how waterinjection works go to Aquamist which produces waterinjection systems suitable for most turbocharged cars.
Go to the tuning page or to the turbocharging page.
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