By Richard L
It is my attempt of explaining how different type of systems work and its advantage and dis-advantage, based on current systems offered. It is a my view and findings, please do chime in and discuss.
1) Single-stage
2) Two-stage
3) Progressive Pump Speed system (PPS)
4) PWM Valve controlled system
5) System that will integrate well with third party controllers.
6) Direct port
The single stage:
The single stage WAI (water alcohol injection), as the heading implies, is not as basic as most people expects. It some cases, it will out perform a two-dimension progressive system. Please do not underestimate it. I will try to explain briefly why after the next few paragraphs.
Having a single trigger point and a fixed flow rate, one will get to know its effect on your engine very quickly. Due to its consistent repeatability, it is very easy to tune. This type of system is normally set to start spray in the peak torque region, where the engine is most likely to knock.
As the RPM climbs, the ratio of water to mass air tends to decrease. This may not be a bad thing because the tendency to knock is also lessen as the wastegate starts to open and prevent the boost pressure from increasing further. The volumetric efficiency of the engine also decreases as RPM climbs, breathing in less air. This also has the effect of reducing the engine's tendency to knock, demand of WAI flow is less. Unfortunately some engines do require continuous WAI flow at higher RPM due to heat build up through friction and turbo efficiency.
A 2-D pump speed system based on manifold pressure is a little bit tricky to tune compared to the single stage system. The user has to set the start and finish pressure points, those points are sometimes set at a considerable distance apart. Matching those operation points in a 3-D environment such as RPM/Boost ramp (nonlinear) is quite difficult. We will be discussing it in more details later.
FOR:
1) Low cost, simple and dependable.
2) Easy to tune
3) Very effective on a stock factory set up with a few pounds of boost extra.
AGAINST:
1) Dynamic operating range is narrow, may not be as effective on a high RPM knock suppression.
2) For high power/ high % alcohol applications, considerable fuel has to be taken out (boost clamp) to make the afr tolerable. Some sort of failsafe mechanism is necessary to prevent engine destruction when the WAI fails to delivery the correct flow.
The 2-stage system
At present, adding a second manifold pressure switch to active an additional solenoid valve at a higher manifold pressure is the definition of a 2-stage system.
This arrangement gives the system greater flexibility as well as extending the flow range. It addresses the problem associated with the single stage system, too much flow at the start and not enough when RPM climbs beyond the wastegate setting.
As the system is based on boost trigger, it still won?t address the RPM related flow. For a turbo charge engine, the most significant active regions are the boost ramping stage and engine?s maximum torque range. A two-stage system fits these two regions nicely, allowing the some form of cooling demand during the ramp-up stage. The second stage provides the in-cylinder cooling and knock suppression as the engine is under the most stress or highest BMEP (Brake Mean Effective Pressure).
FOR:
1) Relatively low cost to give mark improvement to the single-stage system.
2) Provides well defined triggering points during the boost cycle.
3) Minimising the under/over flow problem.
AGAINST:
1) Trigger points requires some time to set up.
2) Triggering points may differ on each gear if you have a fast spool up turbo
3) Require a bit more care during tuning
Propressive Pump Speed system (PPS)
Does the pump speed controller perform better than a two-stage system, you are about to find out.
Changing pump speed merely put more pressure behind a nozzle, hence more flow. This type of system is commonly known as a progressive system (pump-speed).
Let us examine how much a M5 nozzle will flow between 40psi to 160psi. According the chart below (Published by Hago, a well know US oil heater nozzle manufacturer), the flow starts from 200cc/min and ends at 400cc/min., when pressure is increased from 40psi to 160 psi.
Almost all PWM pump controller on the market uses Shurflo pump, designed to operate between 0-150psi. The heart of the system is an electronic motor speed controller, vary the speed according to a sensor. It could be a MAP sensor, a MAF or any sensors that read engine load. It is normally a 2-dimensional system. A manifold-pressure type system does not take into account of any RPM change.
A swirling type atomizing nozzle requires a head pressure of at least 30psi to produce a decent mist. Droplet size is very important to the inlet cooling ability and even cylinder distribution. Let say the system pressure starts at 40psi (as shown on the chart) and ends at 160psi. One can assume you will get a 4x flow range? In practice, not so, according to the chart, you will only get a flow change from 200cc/min to 400cc.min (see M5) instead of 200cc/min to 800cc/min. Flow/pressure obeys the square-root law.
Being "progressive" implies a reasonable dynamic range between start and finish. How progressive? Almost no one ever questions this. Most people just assumes it covers all the flow requirement between 10psi to 20 psi of boost once the range-dials are set on their pump speed controller. In practice, you cannot expect the same M5 nozzle will serve a wider operating range between 5-25psi by merely changing the dial, the range is governed by the law of physics and not a technically advanced motor speed controller.
If one would want to delve deeper into the subject, as the title demands. So the subject will continue?
Just to recap, good dynamic range (pressure/flow span) is the main factor one should expect from a "progressive" WIA system. Let see what a 150psi system can really offer. We shall take into account of the effect of manifold pressure, inline check valve as well as minimum pressure for a good atomized spray.
For example:
1) Manifold pressure start: 10psi
2) Manifold pressure ends: 20psi
3) Inline check valve crack pressure: 30psi
4) Minimum pressure of the atomizing nozzle: 30psi.
When the system starts: it will instantly see an initial back-pressure of 60psi and a final back-pressure of 70psi (extra manifold pressure). The actual dynamic pressure range is now from 60psi to 110psi. The system can now only manage a 35% change in flow, far from one would imagine a 150psi pump system should perform.
There are other factors that could also affect the performance of the 2-D progressive pump system. It may be a subject for a later discussion, depending on the interest of the readers. Chart below is a predicted performance of a progressive system compared to a single and two-stage system. I hope there will be people chiming in to add to this. At first glance, it doesn't appear there is a distinct advantage for adding a progressive controller. Adding a bigger nozzle doesn't alter the dynamic range, it just shifts the whole curve higher.
FOR:
1) Easy to set the start and end point.
2) Some correlation between manifold pressure and flow
3) Cost effective.
AGAINST:
1) Limited dynamic range, system becomes less effective after wastegate pressure. (see addendum)
2) Extra cost can easily be spent on a higher performance two-stage system with greater dynamic range .
3) If the 2-D system is used to replace high % fuel with alcohol, re-mapping the 3-D fuel map will be very difficult due to the wide dynamic flow range demanded by the engine.
4) Pulsing due to demand switch ~20psi ripple. (some system by-pass this switch, but risking system pressure beyond design limits). May require further explanation
5) Response time due to inertia of a rotating - laggy (start) and over-run (stop). May require further explanation
PWM valve water injection system:
These systems require a stable system pressure, normally held between 100-125psi. An inline solenoid valve and a PWM controller that modulates the opening and shutting time to meter flow. Before getting too deeply into the subject, note that there are two types of inline solenoid valves on offer.
Type #1
Pulse width modulation type:
(Optimum operating frequency range: 30-80Hz)
This type of system resembles the modern automotive fuel injection system. The system can also be controlled by a third party EMS with a spare PWM channel. Delivery rate can either be mapped or mirroring the fuel injector duty cycle. The latter makes tuning very simple.
The valve behaves similar to an on/off gated button on a garden hose. The longer the gate is opened, the more the flow (duration). Alternative, rapid opening/shutting the gate per second (frequency) also control the flow. The common EMS uses duration for load change and frequency for RPM change. The dynamic flow range is extremely wide, 100:1 is normal.
A WAI valve should closely match the closing and shutting characteristic of a fuel injector. This is important for fuel flow mirroring algorithm since the modern EMS has a correction stage to compensate the opening delay and shutting delay.
Type #2
Proportional lift type:
(Chopped DC (~400Hz) or DC current)
This type of valve resembles the action of a rotary water tap. As more current is applied to the valve coil, the valve opens more. It is a very nice way to control flow.
There are a few minor problems associated with this type of valve: Atomization at low flow and lift variations (hystersis of the magnetic circuit), approximate +/- 10-15% flow deviations.
All in all, it will deliver liquid well compared to the PPS system. There are some similarities between the two. The nozzle tip pressure is directly proportional to the flow. This is because the proportional valve acts like a variable restrictor upstream of the nozzle tip. Resulting in: restricted flow = low pressure. Low pressure = low atomization.
NOTE: This type of valve has a typical open time of 4ms and closing time of 4ms at its designed voltage. It is too slow to be used as a true PWM valve. Open/close speed can be increased by over-voltage pulses. The design of spider valve is also vastly different from the PWM valve.
Summary:
It is important to know some basic facts between the Proportional valve and PWM valve systems before choosing this type of system.
Here is an illustration of the difference in construction of the two valves, made by Clippard. Notice the proportional lift has a stiffer spring rate than the PWM valve, enabling the PWM valve to perform full on//full off a great deal easier.
Because the way the PWM system meters its flow based on a simple pulse width, it is very accurate. Further precision can be increased by introduce a suitable RRFPR to maintain Manifold pressure against water pressure. It is also possible to factor in a small duty cycle increase to the valve relative to boost increase.
Final consideration: If you are planning in future to create your own MAP via a third part system - only the "PWM-valve" can be driven directly by the ECU, matching the principle of a modern "fuel injection system" in every respect. Warning, before rushing off making your own system, Clippard valve is only rated up to 100psi, even with the smallest orifice version. The larger orifice type can only sustain 25psi. Multivalve is needed for flows over 500cc/min.
END
To be continued...
EDIT:
Addendum: Action of a check valve under dynamic conditions:
Almost all progressive systems use a check valve between the pump output and nozzle for the reasons listed below:
Positive effects (well documented):
1. Retain some pressure in the line to compensate the next injection event. A 20psi loaded checkvalve will keep 20 psi of pressure in the line after injection.
2. Stop water being siphoned into the engine if the water jet is installed in the vacuum side of the manifold.
3. Prevent emptying the entire tank into the inlet tract if the tank location is higher than the jet (gravity fed) or the car is parked on an incline.
4. Stop some dribble after an injection event. Even when the power of the pump is switch off, the inertia of the rotating mass keeps the pump running for a second or so.
Negative effects (less well documented):
5. The presence of a check valve has a very significant impact of the dynamic range of a 150psi progressive pump speed system. A 20 psi check valve inline will instantly drop the 150 PPS system down to a 130psi span.
6. A normal nozzle requires ~30psi for produce a decent atomized spray. An inline 20psi check valve means the pump has to produce 50psi to produce a decent spray.
7. Let say the PPS system’s starting point is at 12psi boost, the system will now require 62psi to produce a decent spray. Some vendor will tell you a check valve will not impede flow once it is opened, true. But it will tax the pressure heavily where the PPS system relies heavily upon.
8. When the PPS system arrives at 24psi boost (end point) manifold pressure, the dynamic pressure is now further taxed. Before not too long later the dynamic range of a PPS system is now from 62psi to 126psi – translate it to flow: 176cc/min to 326cc/min, a mere 84% percent increase.
9. I have taken some data from a reputable PPS system manufacturer, a 150-psi 60W Shurflo flow pump has the follow characteristics:
Most important thing to remember, nozzle size determines output pressure (updated on the 21st January 2008) .
M1 (not tested)
M2 230psi
M3 225psi
M3 215psi
M5 190psi
M7 180psi
M10 170psi
M12 (not tested) did not have one handy.
M14 155PSI
From the figures above, the pump is only capable of sustaining 135psi system pressure on a M5 nozzle. Despite claiming the pump is capable of flowing one 3-4 litre per minute, conveniently missing the pressure parameter. The above PPS system maker is the only one that published these figures in public – thumbs up for them.
NOTE: The above data is from one maker only (as stated) Please read post #3 to understand why different WAI makes uses differ pumps, some make more flow and pressure than others.
Summary:
Some PPS system makers are now offering an inline solenoid upgrade so that the dynamic range is improved by a good margin. Not sure why they didn’t include it in the kit at the beginning. Many PPS systems run their pumps up to 200-300psi - something I have questioned Shurflo, they said NO, NO and NO - no "ifs" or "buts". They are looking into a higher pressure pump but not yet done and will not be released until they are comfortable with it. I have a meeting with the Shurflo's director of engineering at our works two months ago during his UK visit - he confirmed the imaginary 200psi+ pump (8000 series).
Indepth study of WAI injection systems
Linear Mode
