This will attempt to discuss the differences between a Pulse Width Modulated – Valve (PWM-V) and a Progressive Pump Speed (PPS) water/alcohol injection (WAI) system and why I believe PWM-V systems are far superior to PPS ones.

It has to be noted that this IS NOT my write-up. It is a very in-depth explanation that was made quite some time ago that still holds relevance and can be found at various places around the internet if doing a search for “Indepth study of WAI injection systems“. I simply coalesced the information into a single link and made some grammatical and structural modifications. It is a long read, with some of it dry, but is helpful when trying to make the decision of what route to go when sourcing a WAI kit.

• Overview
• Price Comparison
• Single Stage System
• Dual Stage System
• PPS Controlled System
• PWM-V Controlled System
• Summary
• Addendum 1
• Addendum 2

Overview

Who manufactures what?

• PPS System: Devil’s Own, Snow, Coolingmist, ProMeth, AEM, Alky Controls, etc.
• PWM-V System: Aquamist

Delivery method:

• PPS system: controls flow by changing pump speed.
• PWM-V system: controls flow by pulsing an inline valve (similar principle as an OEM fuel injection system)

Atomization:

• PPS system: low flow equals low pressure equals poor atomization (see video). Poor atomization creates large droplets resulting in uneven fluid entering each cylinder. Modern manifolds are not designed for transporting fluid loaded air.

• PWM-V system: constant pressure at any flow equals consistent atomization (see video).

Response time:

• PPS system: slow response of the system due to rotational inertia of the pump. It can typically take between 0.1 to 0.5 seconds, which may result in inconsistency of the air/fuel ratio under transient load due to the pump’s inability to change speed quickly enough. The rotating mass of the pump during ramp-down still has ample inertia, which results in “after-spray” (see videos comparing the below). The LED strip in the middle identifies when the system is being commanded to inject, a visual indicator of response time. A small detail to pay attention to…notice the pumps for each system. Despite both pumps being identical, the PWM-V pump maintains a constant speed and does not see the shock-loads of the constant ramp-up and ramp-down of the PPS pump. The PPS pump on the right can be seen to “jump” when ramping up or down. This can greatly influence life-span of the pump.

Also notice in the video below, when the valves are closing or during ramp down, the amount of “dribble” the PPS systems tend to have. This heavily loaded, poorly atomized methanol will not properly perform its functions. Again, notice the injection command being identified by the LED strip.

In the video below, you see the LED strip indicating when the WMI system is being commanded to inject and the poor response time of the injectors ramp-down time. A PWM-V injector is on the left and PPS injector on the right. Notice that the injectors used are also identical. You can again see the poor atomization when at those ramp-down, low flow periods.

• PWM-V system: fast response time using PWM fast-acting valve, typically within 0.003 second (see video).

Dynamic range:

• PPS system: only produce 2x between 50-200 psi, which is a typical PPS system operating pressure range. Without the dynamic range, PPS system can only operate in that narrow power range of 2x which isn’t ideal for a daily driver.
• PWM-V system: comfortably manages 10x minimum flow range between 0-100% duty cycle.

Linearity:

• PPS system: a linear “pressure” increase does not produce a linear “flow” through an atomization nozzle. The error can be as much as 120%. An inconsistent AFR under different engine load and speed is common, which can be a nightmare to tune.
• PWM-V system: flow linearity is in the region of 5%, and can be improved by a custom designed valve upon request.

Failsafe:

• The Aquamist kit employs what I would consider one of the best and most-secure failsafe protections for a methanol kit.
• If the vehicle is tuned with the methanol as an integral part of it (i.e., using methanol as an octane booster), then a shortage of methanol, whether from running out or a failure of some part of the system, could be catastrophic. For this, many systems use a failsafe of some sort to protect the engine in the event of a no-methanol scenario.
• As the Aquamist system is integrated into the vehicles PCM, it has a very direct connection to what’s going on. When the system sees a problem, whether a lack of flow or an open line, it immediately cuts boost using the factory boost solenoid via the PCM down to wastegate pressure, giving you the ability to save your engine from catastrophic failure and “limp” using no more than wastegate boost pressure.

Cost considerations:

• PPS system: it is basically a motor speed controller, with design cost being low. Regardless of how many gizmos are included.
• PWM-V system: each PWM valve is individually calibrated by technicians to ensure flow consistency. PWM-V system production cost can be high with PPS systems being roughly 30% lower in cost to produce. However, overall performance of a PPS system can be as high as 5x as poor.

Price Comparison

And for those curious about how the prices of this type of system compare to some others, take this cost comparison chart into consideration. It’s somewhat dated as well, but it will give you an idea of what to look for in the auxiliary costs of each system and the benefits you are paying for.

Single Stage System

The single stage WAI is not as basic as most people would expect. In some cases, it will outperform a two-dimensional progressive system so please do not underestimate it.

Having a single trigger point and a fixed flow rate, a tuner can get to know it’s effect on the engine very quickly. Due to it’s 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 lessened as the waste gate 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 and demand of WAI flow is less. Unfortunately some engines can require continuous WAI flow at higher RPM due to heat build up through friction and turbo efficiency.

A two-dimensional pump speed system based on manifold pressure can be tricky to tune compared to the single stage system. The tuner must set the start and finish pressure points, whose points can sometimes be set a considerable distance apart. Matching those operational points in a three-dimensional environment such as an RPM/boost ramp (nonlinear) is quite difficult and will be discussed in more detail later.

Pro:

1. Low cost, simple and dependable.
2. Relatively simple to tune.
3. Very effective on a stock factory set up with a few pounds of boost extra.

Con:

1. Dynamic operating range is narrow, may not be as effective as a high RPM knock suppression tool.
2. For high-power/high-percentage alcohol applications, considerable fuel has to be taken out (boost clamp) in order to make the AFR tolerable. Some sort of fail-safe mechanism is necessary to prevent engine destruction when the WAI fails to deliver the correct flow.

Dual Stage System

Adding a second manifold pressure switch to activate an additional solenoid valve at a higher manifold pressure is the current definition of a dual 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, which can be too much flow at the start and not enough when RPM climbs beyond the waste gate setting.

As the system is based on a boost reference, it still can’t address RPM related flow. For a turbocharged engine, the most significant active regions are the boost ramping stage and the engine’s maximum torque range. A dual stage system fits these two regions nicely, allowing 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).

Pro:

1. Relatively low cost to give a marked improvement over the single stage system.
2. Provides well defined triggering points during the boost cycle.
3. Minimizes the under/over flow issue.

Con:

1. Trigger points require considerable set up time.
2. Triggering points may differ on each gear if you have a fast spooling turbo.
3. Requires a bit more care during tuning.

PPS Controlled System

Changing pump speed merely puts more pressure behind a nozzle, hence more flow. This type of system is commonly known as a progressive system, using pump-speed to modify system flow.

Let us examine how much an M5 nozzle will flow between 40psi to 160psi. According the chart below (Published by Hago, a well-known US oil heater nozzle manufacturer), the flow begins at 200cc/min and ends at 400cc/min, when pressure is increased from 40psi to 160 psi.

Virtually all PPS pump controllers on the market use either an AquaTec or Shurflo pump, designed to operate between 0-150psi. The heart of the system is an electronic motor speed controller, varying the speed according to a sensor. This sensor could be a number of different types, depending on whose system it is. It could be a manifold absolute pressure (MAP), mass air flow (MAF) or any other sensor designed to read engine load. PPS systems are typically two-dimensional systems. A manifold-pressure type system does not take into account 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 capability as well as even cylinder distribution. Let’s say the system pressure begins at 40psi (as shown on the chart) and ends at 160psi. In theory, one could assume a 4x flow range, but in practice, this isn’t the case. According to the chart, only a flow change from 200cc/min to 400cc/min (see M5) can be achieved 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. But how progressive? Almost no one ever questions this. Most people simply assume 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 to serve a wider operating range between 5-25psi by merely changing the dial, the range is governed by the laws of fluid dynamics and not by a technically advanced motor speed controller.

To recap, good dynamic range (pressure/flow span) is the main factor one should expect from a “progressive” WAI system. Let see what a 150psi system can really offer. We will take into account the effects of manifold pressure, an inline check-valve, as well as minimum pressure for a well-atomized spray.

For example:

1. Manifold pressure start: 10psi
2. Manifold pressure ends: 20psi
3. Inline check-valve crack pressure: 20psi (updated)
4. Minimum pressure of the atomizing nozzle: 40psi (Hago chart).

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 what one would imagine a 150psi pump system should be performing at.

There are other factors that could also affect the performance of a two-dimensional progressive pump system. The chart below is a predicted performance of a progressive system compared to a single and two-stage system. At first glance, it doesn’t appear there is a distinct advantage to adding a progressive controller. Adding a larger nozzle doesn’t alter the dynamic range, it just shifts the entire curve higher.

Pro:

1. Easy to set the start and end point.
2. Some correlation between manifold pressure and flow.
3. Cost effective.

Con:

1. Limited dynamic range, system becomes less effective after waste gate pressure (see addendum).
2. Extra cost can easily be spent on a higher performance two-stage system with greater dynamic range .
3. If the two-dimensional system is used to replace high percentage fuel with alcohol, re-mapping the three-dimensional 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 systems bypass this switch, at the cost of risking system pressure beyond design limits).
5. Response time due to inertia of a rotating mass. Very “laggy” at start and possible overrun on stop.

PWM-V Controlled 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 closing time to meter flow. Before getting too deep into the subject, note that there are two types of inline solenoid valves on the market.

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 engine management system (EMS) with a spare PWM channel. Delivery rate can either be mapped or mirror the fuel injector duty cycle (IDC). 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 opening and closing characteristic of a fuel injector. This is important for the fuel flow mirroring algorithm since a modern EMS has a correction stage to compensate for the opening and closing 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 higher the valve lifts from the seat. 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 (see paragraph below) and lift variations (hysteresis of the magnetic circuit), approximate +/- 10-15% flow deviations.

Generally, it will deliver liquid comparable to a PPS system as there are some similarities between the two. The nozzle tip pressure is directly proportional to the flow. This is because the proportional valve acts as a variable restriction upstream of the nozzle tip. This results in restricted flow which equals low pressure. And low pressure equals poor atomization.

Summary

It is important to know some basic facts between the two PWM valve systems before choosing which type.

This is an illustration of the difference in construction between the two valves, both made by Clippard. Notice the “PWM” (full on/off”) has a softer spring rate than the “proportional lift” valve, enabling the PWM valve to perform “full on” and “full off” positively.

Because of the way a PWM system meters its flow based on a simple pulse width, it is very accurate. Further precision can be increased by introducing 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 to create your own MAP via a third part system – only the “PWM-Valve” can be driven directly by an EMS, matching the principle of a modern “fuel injection system” in every respect.

WARNING: Before rushing off making your own system, Clippard valves are 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. Lastly, the valve mentioned has a typical 4mS on and 4mS off time.

Addendum 1

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):

• Retain some pressure in the line to compensate the next injection event. A 20psi loaded check valve will keep 20 psi of pressure in the line after injection.
• Stop water being siphoned into the engine if the water jet is installed in the vacuum side of the manifold.
• 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.
• Stop 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):

• The presence of a check valve has a very significant impact on 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.
• A normal nozzle requires ~30psi in order to produce a satisfactory atomized spray. An inline 20psi check valve means the pump has to produce 50psi to produce a decent spray.
• Let say the PPS system’s starting point is at 12psi boost, the system will now require 62psi to produce a decent spray. Some vendors will tell you a check valve will not impede flow once it is opened, which is true. However, it will tax the pressure heavily where the PPS system relies heavily upon.
• When the PPS system arrives at 24psi boost (end point) manifold pressure, the dynamic pressure is now further taxed. Before 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.
• From a reputable PPS system manufacturer, a 150-psi 60W Shurflo flow pump has the follow characteristics (important thing to remember is that nozzle size determines output pressure):

• M1 – Untested
• M2 230psi
• M3 225psi
• M3 215psi
• M5 190psi
• M7 180psi
• M10 170psi
• M12 – Untested
• M14 155PSI

From the figures above, the pump is only capable of sustaining 135psi system pressure on an M5 nozzle. Despite claiming the pump is capable of flowing 3-4 liters 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.

Summary:

Some PPS system makers offer an inline solenoid upgrade so that the dynamic range is improved by a good margin. Many PPS systems run their pumps up to 200-300psi – and as Per Shurflo, this exceeds their design limits.

Here are Abner’s check valve tests:

Addendum 2

Knowing the heart of a WAI system: The pump.

As Shurflo is the main pump supplier of the majority of water/alcohol injection systems manufacturers, it is only natural to take a good look at them first. There are many pump variations they offer over the standard “off the shelf” configuration.

Pump motor:

There are three frame sizes: short, medium and long stack, covering three power ratings: 60W, 100W and 150W.

Pump cam angle:

Cam angle dictates and governs the final specifications of pump’s flow rate, pressure. Shurflo offers: 2.0, 2.5, 3.0, 3.5 and possibly more profiles. Depending on the application, WAI manufacturers can select the most suitable cam for their system.

At first glance, using the highest lift cam would appear to produce the most flow and pressure. However, if this cam is matched with a small motor, it can cause undue stress on the motor winding. This is similar to going up a steep incline on high gear where the car’s engine and gearbox is being stressed.

On the other hand, a low cam angle will produce less pressure and flow. Some PPS manufacturers prefer using the lower angle cam because of the following (as long as it delivers the specified flow) :

• Less stress to the diaphragm, which contributes to long term mechanical reliability.
• Less stress to the motor, leading to lower running temperatures.
• Within a flow range up to 1000cc/min or so, there is ample pressure generated to hold the system at 150psi.
• A much smoother control range from a PPS controller. High cam lobes tend to ramp up too much pressure with the same duty cycle applied, making the ramp-up abrupt and similar to using 2nd gear to get the car moving.
• The pressure spike is also much smaller. This allows the peak pressure closer to the “150psi demand switch”. Overshooting this will cause the infamous “pulsing” often associated with a PPS system.

Note: a system advertised as 150psi@3 litre/min may NOT out-perform a 150psi@1 litre/min system. Often, the latter is a much better system. As far as the raw material cost difference is concerned, there is none.

By-Pass pump (no demand switch):

Shurflo offers a huge range of internal by-pass valves to overcome the necessity of using a “demand switch”. Excess pressure is being “by-passed” internally by a set of spring loaded poppet valves (x3).

This type of configuration is more suited for PWM valve system rather the PPS system. The pump is switched on just before injection, attains a “steady” line pressure continuously through out the entire delivery cycle.

Only a very small number of PWM valve WAI manufacturers use this set up.

Regulating pump pressure steady (with on-demand switch):

Other than employing the bypass valves, the water pressure can be limited by using an “on-demand” switch. This method is simple. Every time the pump hits the “set pressure” of the on-demand switch, the 12V feed to the pump is interrupted. Shurflo recommends this method should only be applied to an application where the usage is intermittent and not cyclic in use.

Against the recommendations of pump manufacturers, there are some WAI systems on the market employing this technique to control water pressure. Most PPS systems only hit this pressure at peak injection pressure intermittently, so long term damage to the pump is not severe. Pressure spikes of 20psi+ can exist.

If this method is being used to regulate water pressure on a PWM-valve system, it is a completely different story. “Intermiittent” usage now becomes ” cyclic” usage. This will create long-term problems on the switch as well as the pumping mechanism. Water pressure will also suffer from pressure spikes, sometimes as high as 20+psi spikes. Using a low hysteresis switch may reduce the ripple, but the other half of the problem still remains.