The Effects of Water/Methanol Injection on Diesel Combustion,
Cooling, and Lubricant Quality
By Matt Snow
There is considerable data showing the effects of water and
methanol on diesel combustion. The data focuses on heat release
and combustion pressure (power), emissions (NOx-nitric oxide,
and PM - particulate especially), fuel efficiency (brake
specific fuel consumption - BSFC), engine cooling, and the
impact on lubricant quality. This is not an exhaustive study but
a cursory look at some of the more recent data.
Power
The data on power production reveals some interesting points:
1. Injected water slows combustion thus reducing effective peak
cylinder pressure unless injection timing is advanced to
compensate.
2. Injected methanol slows combustion if cylinder temps are too
low, but increases the combustion rate when cylinder temps are
high.
These facts would indicate that to inject water and methanol
before normal operating temperatures or at too low a boost level
will result in a net power reduction unless fuel injection
timing is advanced to compensate for the ignition delay. After
normal operating temperature is reached at high boost (over
15psig) and high load conditions (86%), Methanol speeds-up the
timing event while water conditions combustion by slowing it.
3. Water when combusted produces steam during the power stroke.
This increases combustion pressure, "A calculation indicates
that one can expect a 3-psi increase in mean effective pressure
(assuming about 75psi IMEP)."1. One issue is that water will not
convert to steam during peak combustion pressures. This would
indicate that the pressure/power effect of vaporization occurs
only during the beginning of the compression stroke and the last
half of the power stroke. "When water is injected between 85-30°
BTC and therefore just prior to fuel injection results in a
substantial increase in ignition delay, stronger effective
cooling, and a substantial power increase." 1. Whereas peak
combustion pressure is less, total work done in the power stroke
is increased.
4. Methanol when combusted along with Diesel contributes to
power as evidenced by increased heat release and peak combustion
pressures. The combination of diesel seems to act as a catalyst
to light off methanol as sufficient heat is needed for methanol
combustion.
5. Methanol is an oxidizer as well as a fuel in that oxygen is
donated to the combustion process increasing the fueling
potential of a combination. This is of interest in combinations
that are over fueled in an attempt to extract all the power out
of the available oxygen.
6. Power and economy varies with when water is introduced in the
combustion process. Increases ranges from 5.2% for injection
during the exhaust stroke to 20% for injecting at the beginning
of the power stroke. For 100% duty cycle-boost/load dependent
water injection systems, it would be reasonable to assume that
the percentage gains would be in this range although more
research is needed.
Emissions
The data on emissions is mainly focused on the effect on NOx and
particulate. To increase power and reduce fuel consumption,
higher mean cylinder pressures are attained by increasing the
timing of the injection event. This generally results in
increase NOx formation and soot formation rates.
1. "The vaporization of water as well as a local increase in
specific heat of the gas around the flame resulted in lower
Nitrogen Oxide emissions (NOx) and soot formation rates."
2. Both NOx and PM are decreased in low (20%) and med. (44%)
load cases with water injection. Interestingly, at high load
(86%) NOx is decreased as expected, but soot remains unchanged
possible due to the late injection of fuel in the full power
fueling strategy. Advancing the injection timing to compensate
for the additional ignition delay with water injection improves
the soot output markedly.
3. Methanol offers potential for reduced emission of both NOx
and particulates (PM) in compression ignition engines, but only
when used as a replacement fuel (power output is kept constant
through the reduction in diesel injection)
4. Water injection increases both hydrocarbon (HC) and (CO)
carbon monoxide emission slightly due to incomplete combustion
caused by delayed ignition.
Fuel Consumption
Specific fuel consumption is lowest in 44% load cases with water
injection. Although the peak cylinder pressure is less, the
pressure is higher often about 30 degrees ATDC resulting in
increased overall work output of the combustion event.
Interestingly, at high load (86%) this effect is neutralized.
This would suggest that in order to improve fuel efficiency with
water injection, a system needs to inject a smaller quantity at
lower boost and load continuously thus requiring a large
injection reservoir.
Since SFC is directly related to power production, it stands to
reason that since water/methanol increases power at high
boost/load, that SFC would also be improved. The effect maybe
masked by an aggressive fuel strategy when large injection duty
cycles necessitate injecting diesel late in the combustion
process.
Engine Cooling
Cooling a diesel through water injection has been demonstrated
through various injection methods - direct injection, manifold
injection and manifold induction. Direct injection requires a
dual injector where diesel and water are injected using
different orifices of the same injector. Manifold induction,
diffusing water into the intake manifold in a non-atomized
state, proves adequate for slight cooling. Negative combustion
effects using anything but small quantities makes significant
cooling with this method impractical. Manifold injection - the
injection of water in an atomized state is effective in that
when sufficiently atomized, engine cooling is attained through
reduced air charge temps as well as combustion effects. Some
facts:
1. Since the heat of combustion is reduced as evidenced by lower
EGT's and this is the largest source of heat in a compression
ignition engines, it is reasonable to assume that overall engine
heat will be significantly reduced with water/methanol injection
in the high load state. This is especially significant for
towing heavy loads for sustained periods where the engines
coolant system is over stressed.
Effects on Lubricant Quality
Oil contamination occurs with water injection under the
following conditions:
1. The engine oil temp is less than 212°f (prior to the engine
reaching peak operating temp).
2. A direct injection (DI) system is utilized where water is
continuously injected especially if injection hits the cylinder
water under high pressure.
Conversely there is no evidence of oil contamination if:
1. The engine oil is above 212°f. (Water injection is utilized
only after the engine is warm). Since oil temperature exceeds
212°f, E any water bypassing the rings will not accumulate but
will be evaporated and expelled."1.
2. Manifold injection is used when injection occur only
intermittently under high load/boost (heat) conditions. With
these conditions, water is vaporized immediately when coming in
contact with the heated intake charge and through the heat of
combustion
3. Manifold injection where water isn't injected directly at the
cylinder walls. "It is hypothesized that when direct injected
under high pressure, water cools the cylinder liner where it
hits and the rings pump this into the crank case during the next
stroke."1.
4. Manifold injection with proper atomization. The finer the
atomization, the greater the total surface area of the injection
medium and the greater the evaporative effect.
Bibliography:
1. Lestz, Milton, Jr., and Rambi - Feasibility of Cooling Diesel
Engines by Introducing Water Into the Combustion Chamber. SAE
Document No. 750129
2. Bedford, Rutland, Dittrick, Rabb, and Wirbeleit - Effects of
Direct Water Injection on DI Diesel Engine Combustion. SAE
Document No. 2000-01-2938
3. Ryan, III, Maymar, Ott, LaViolette, and MacDowell -
Combustion and Emissions Characteristics of Minimally Processed
Methanol in a Diesel Engine Without Ignition Assist. SAE
Document No. 940326
4. Kahn, Gollanhalll - Performance and Emission Characteristics
of a Diesel Engine Burning Unstabalized Emulsions of Diesel Fuel
with Water, Methanol, and Ethanol. SAE Document No. 811210
5. Sriram, Udayakumar, and Sundaram - Reduction of NoDx
Emissions by Water Injection in to the Inlet Manifold of a Dl
Diesel Engine. SAE Document No. 2003-01-0264
6. Christensen, and Johansson - Homogeneous Charge Compression
Ignition with Water Injection. SAE Document No. 1999-01-0182
7. Kegl and Pehan - Reduction of Diesel Engine Emissions by
Water Injection. SAE Document No. 2001-01-3259
8. Iwashiro, Tsurushima, Nishijima, Asaumi, and Aoyagi - Fuel
Consumption Improvement and Operation Range Expansion in Hccl by
Direct Water Injection. SAE Document No. 2002-01-0105
