BoyntonStu
06-04-2009, 11:59 AM
7.13 What does water injection achieve?.
Water injection, as a separate liquid or emulsion with gasoline, or as a
vapour, has been thoroughly researched. If engines can calibrated to operate
with small amounts of water, knock can be suppressed, hydrocarbon emissions
will slightly increase, NOx emissions will decrease, CO does not change
significantly, and fuel and energy consumption are increased [113].
Water injection was used in WWII aviation engine to provide a large increase
in available power for very short periods. The injection of water does
decrease the dew point of the exhaust gases. This has potential corrosion
problems. The very high specific heat and heat of vaporisation of water
means that the combustion temperature will decrease. It has been shown that
a 10% water addition to methanol reduces the power and efficiency by about
3%, and doubles the unburnt fuel emissions, but does reduce NOx by 25% [114].
A decrease in combustion temperature will reduce the theoretical maximum
possible efficiency of an otto cycle engine that is operating correctly,
but may improve efficiency in engines that are experiencing abnormal
combustion on existing fuels.
Some aviation SI engines still use boost fluids. The water-methanol mixtures
are used to provide increased power for short periods, up to 40% more -
assuming adequate mechanical strength of the engine. The 40/60 or 45/55
water-methanol mixtures are used as boost fluids for aviation engines because
water would freeze. Methanol is just "preburnt" methane, consequently it only
has about half the energy content of gasoline, but it does have a higher heat
of vaporisation, which has a significant cooling effect on the charge.
Water-methanol blends are more cost-effective than gasoline for combustion
cooling. The high Sensitivity of alcohol fuels has to be considered in the
engine design and settings.
Boost fluids are used because they are far more economical than using the
fuel. When a supercharged engine has to be operated at high boost, the
mixture has to be enriched to keep the engine operating without knock. The
extra fuel cools the cylinder walls and the charge, thus delaying the onset
of knock which would otherwise occur at the associated higher temperatures.
The overall effect of boost fluid injection is to permit a considerable
increase in knock-free engine power for the same combustion chamber
temperature. The power increase is obtained from the higher allowable boost.
In practice, the fuel mixture is usually weakened when using boost fluid
injection, and the ratio of the two fuel fluids is approximately 100 parts
of avgas to 25 parts of boost fluid. With that ratio, the resulting
performance corresponds to an effective uprating of the fuel of about 25%,
irrespective of its original value. Trying to increase power boosting above
40% is difficult, as the engine can drown because of excessive liquid [110].
Note that for water injection to provide useful power gains, the engine
management and fuel systems must be able to monitor the knock and adjust
both stoichiometry and ignition to obtain significant benefits. Aviation
engines are designed to accommodate water injection, most automobile engines
are not. Returns on investment are usually harder to achieve on engines that
do not normal extend their performance envelope into those regions. Water
injection has been used by some engine manufacturers - usually as an
expedient way to maintain acceptable power after regulatory emissions
baggage was added to the engine, but usually the manufacturer quickly
produces a modified engine that does not require water injection.
BoyntonStu
Water injection, as a separate liquid or emulsion with gasoline, or as a
vapour, has been thoroughly researched. If engines can calibrated to operate
with small amounts of water, knock can be suppressed, hydrocarbon emissions
will slightly increase, NOx emissions will decrease, CO does not change
significantly, and fuel and energy consumption are increased [113].
Water injection was used in WWII aviation engine to provide a large increase
in available power for very short periods. The injection of water does
decrease the dew point of the exhaust gases. This has potential corrosion
problems. The very high specific heat and heat of vaporisation of water
means that the combustion temperature will decrease. It has been shown that
a 10% water addition to methanol reduces the power and efficiency by about
3%, and doubles the unburnt fuel emissions, but does reduce NOx by 25% [114].
A decrease in combustion temperature will reduce the theoretical maximum
possible efficiency of an otto cycle engine that is operating correctly,
but may improve efficiency in engines that are experiencing abnormal
combustion on existing fuels.
Some aviation SI engines still use boost fluids. The water-methanol mixtures
are used to provide increased power for short periods, up to 40% more -
assuming adequate mechanical strength of the engine. The 40/60 or 45/55
water-methanol mixtures are used as boost fluids for aviation engines because
water would freeze. Methanol is just "preburnt" methane, consequently it only
has about half the energy content of gasoline, but it does have a higher heat
of vaporisation, which has a significant cooling effect on the charge.
Water-methanol blends are more cost-effective than gasoline for combustion
cooling. The high Sensitivity of alcohol fuels has to be considered in the
engine design and settings.
Boost fluids are used because they are far more economical than using the
fuel. When a supercharged engine has to be operated at high boost, the
mixture has to be enriched to keep the engine operating without knock. The
extra fuel cools the cylinder walls and the charge, thus delaying the onset
of knock which would otherwise occur at the associated higher temperatures.
The overall effect of boost fluid injection is to permit a considerable
increase in knock-free engine power for the same combustion chamber
temperature. The power increase is obtained from the higher allowable boost.
In practice, the fuel mixture is usually weakened when using boost fluid
injection, and the ratio of the two fuel fluids is approximately 100 parts
of avgas to 25 parts of boost fluid. With that ratio, the resulting
performance corresponds to an effective uprating of the fuel of about 25%,
irrespective of its original value. Trying to increase power boosting above
40% is difficult, as the engine can drown because of excessive liquid [110].
Note that for water injection to provide useful power gains, the engine
management and fuel systems must be able to monitor the knock and adjust
both stoichiometry and ignition to obtain significant benefits. Aviation
engines are designed to accommodate water injection, most automobile engines
are not. Returns on investment are usually harder to achieve on engines that
do not normal extend their performance envelope into those regions. Water
injection has been used by some engine manufacturers - usually as an
expedient way to maintain acceptable power after regulatory emissions
baggage was added to the engine, but usually the manufacturer quickly
produces a modified engine that does not require water injection.
BoyntonStu