Ininternal combustion engines,water injection, also known asanti-detonant injection (ADI), can spraywater into the incoming air orfuel-air mixture, or directly into thecombustion chamber to cool certain parts of the induction system where "hot points" could produce premature ignition. Injet engines—particularly earlyturbojets or engines in which it is not practical or desirable to have anafterburner—water injection may be used to increase enginethrust, particularly at low-altitudes and at takeoff.
Water injection was used historically to increase the power output ofmilitary aviation engines for short durations, such as duringaerial combat ortakeoff. However it has also been used inmotor sports and notably indrag racing. InOtto cycle engines, the cooling effect of water injection also enables greatercompression ratios by reducingengine knocking (detonation). Alternatively, this reduction in engine knocking in Otto cycle engines means that some applications gain significant performance when water injection is used in conjunction with asupercharger,turbocharger, or modifications such as aggressiveignition timing.
Depending on the engine, improvements in power andfuel efficiency can also be obtained solely by injecting water.[1] Water injection may also be used to reduceNOx orcarbon monoxide emissions.[1]
Many water injection systems use a mixture of water andalcohol (often close to 50/50), with trace amounts of water-soluble oil. The water provides the primary cooling effect due to its greatdensity and high heat absorption properties. The alcohol is combustible, and also serves as anantifreeze for the water. The main purpose of the oil is to preventcorrosion of water injection and fuel system components.[2]
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Water injection has been used in both reciprocating andturbineaircraft engines.
In areciprocating engine, the use of water injection, also called anti-detonation injection or ADI, is used to preventengine knocking also known as "detonation".[3] Commonly found on largeradial engines withpressure carburetors, it is a mixture of water and alcohol injected into the carburetor at high power settings. When using arich mixture, the engine runs cooler, but cannot reach maximum power, and a leaner mixture means detonation is likely. With the use of ADI, the injected water and alcohol (which is mixed with the water to prevent it from becoming ice) absorbs the excess heat to prevent detonation while still allowing for a leaner and more powerful mixture.[3] Notable engine with water fuel injection:BMW 801,Daimler-Benz DB 605,Junkers Jumo 213,Pratt & Whitney Double Wasp.
When used in a turbine engine, the effects are similar, except that preventing detonation is normally not the primary goal. Water is normally injected either at the compressor inlet or in the diffuser just before the combustion chambers. Adding water increases the mass being accelerated out of the engine, increasing thrust; and it also serves to cool the turbines. Since temperature is normally the limiting factor in turbine engine performance at low altitudes, the cooling effect lets the engine run at higher RPM with more fuel injected and more thrust created without overheating.[4]
Prior to the widespread adoption ofafterburning engines, somefirst-generation jet fighters used water injection to provide a moderate boost in performance. For example, the late-model variant of theLockheed F-80 Shooting Star, the F-80C, used water injection on itsAllison J33-A-35 engine. Water injection increased thrust from 20.5 to 24.0 kN (4,600 to 5,400 lbf), a 17% thrust increase (at sea level).[5]
Early versions of theBoeing 707 fitted withPratt & Whitney JT3C turbojets used water injection for extra takeoff power, as didBoeing 747-100 and 200 aircraft fitted withPratt & Whitney JT9D-3AW and -7AW turbofans;[6] this system was not included in later versions fitted with more powerful engines. TheBAC One-Eleven airliner also used water injection for itsRolls-Royce Speyturbofan engines. Filling the tanks with jet fuel instead of water led to thePaninternational Flight 112 crash.[7]
In 1978,Olympic Airways Flight 411 had to abort and return to its take-off airport due to a failure of the water injection system or its processes.[8]
A limited number of road vehicles with forced induction engines from manufacturers such asChrysler have included water injection. The1962 Oldsmobile Jetfire was delivered with theTurbo Jetfire engine.[9]
Saab used water injection in the 1978Saab 99 Turbo S for an extra 15–20 hp (11–15 kW) on its 2-litre engine and in the 1980Saab 900 Enduro.[citation needed]
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In 2015,BMW introduced a version of their high performance coupe, theBMW M4 GTS with the straight-6 2,979 cc turbochargeBMW S55 engine that combined water injection with intercooling. With the aid of the system, BMW stated that the GTS got a 37 kW (50 hp) boost, increasing power to 368 kW (493 hp) and torque to 600 N⋅m (443 lb⋅ft). Three water injectors were used to allow boost pressure to be increased from 17.2 to 21.6 psi (1.19 to 1.49 bar) over theN55 engine that it was based on.
BMW had previously used water injection on itsBMW 801 aircraft engine using theMW-50 methanol-water system for low altitude boost with the addition ofGM-1 nitrous oxide system for high altitude performance.
The BMW M4 GTS was featured in the 2015MotoGP season as the official safety car for the series and was released for the commercial market in 2016.[10]
By the mid 2020s, engine development was shifted focus also on improvedfuel consumption, due to the pressure onCO2 emissions reduction and related regulations.[11][12]
Co-developed withBMW, Bosch offers water injection systems named WaterBoost for all manufacturers.
Bosch has a heritage of water injection with BMW as early as 1942. First appeared in the BMW 801D in 1942, it never went into production because the cylinder heads developed micro-cracks whenMW50 was used. Instead, theDaimler-Benz DB 605 engined later versions of theMesserschmitt Bf 109 were fitted with an MW 50 injection system, beginning in early 1944. Today Bosch claims up to 5% increase in engine performance, up to 4% decrease in CO2 emissions and up to 13% improvement in fuel economy.[13]Similar results[14]
Water Injection and cooledexhaust gas recirculation (EGR) could be seen as competitive technologies. It has been demonstrated that at medium load, a 40-50 % Water-to-Fuel Ratio (WFR) with Port Water Injection (PWI) has the same effect as an EGR-rate of 10%, which is seen as relatively limited, even for petrol engines.[15]
Surveys asking customers about their willingness to regularly fill up an additional operating fluid have demonstrated that the acceptance level is limited.[12] Therefore, the need for refilling is considered as one of the main barrier for the mass adoption of Water Injection. A key enabler is the development of on-board water generation system to run in close loop system, especially in order to guarantee consistent low level of emissions (engine CO2 emissions will be raised if the water supply is exhausted). Three major sources can be investigated:
The first two variants are highly dependent on weather ambient conditions with sufficiently high humidity levels or driver habits (no A/C operation wanted). Consequently, an adequate supply of water cannot be ensured. In contrast, condensing of water vapour formed during the combustion of gasoline is a reliable source of water: there is approximately a volume of 1L of water vapour in exhaust per each liter of gasoline fuel consumed. In October 2019, Hanon Systems together with FEV presented anAudi TT Sport demonstrator equipped with water injection operating as a closed system thanks to a Hanon Systems "Water Harvesting System".[16]
A 2016 study combined water injection withexhaust gas recirculation. Water was injected into the exhaust manifold of adiesel engine and, by opening the exhaust valve during the induction stroke, the injected water and some of the exhaust gas was drawn back into the cylinder. The effect was up to 85% reduction inNOx emissions and also significant reduction in soot emissions.[17]