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
