The choices are typically a demand switch pump or a by-pass pump. Either pump should be capable of delivering water/methanol.
Demand switch pump can easily identified by the switch assembly in front of the pump. The by-pass pump does not have it, just a flat plate.
The most important reason for the by-pass valve incorporation is the delivery is in absolute smoothness. A demand switch pump tends to give a pulsating delivery at low flow (relative to the full flow capability), pressure fluctuation can be as much as 20-25psi swing - when the demand switch goes on/off/on/off/on/off.... etc. It is not nice to the pump motor, even less than ideal for WI application.
Here is an image of the size difference between the pumps
The picture shows the range of Shurflo pumps used on 90% of the water injection system. This has changed and this post will be updated with images of the new pumps being used.
At low flow, the pump with demand switch tends to produce high pressure pulses as the water pressure reaches the switch setting. They can vary as much as 20-30psi !!!
The following video link shows this phenomenon:
Image hosting, free photo sharing & video sharing at Photobucket
The video showed a progressive system at work - the owner had no-knowledge of this "on/off/on/off" pulsing" until he installed the DDS3.
Pump cam angle:
Cam angle dictates and governs the final specifications of pump's flow rate, pressure. Shurflo offers: 2, 2.5, 3.0, 3.5 or possibly more profiles. Depending on the application, WAI makers can select the most suitable cam for their system.
At first glance, using the highest lift cam will produce the most flow and pressure. But if this cam is matched with a small motor, it will cause undue stress on the motor winding. This is very 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 prefers using the lower angle cam because of the following (as long as it delivers the specified flow) :
1. Less stress to the diaphragm - long term mechanical reliability
2. Less stress to the motor - lower running temperature.
3. Within a flow range up to 1000cc/min or so, there is ample pressure generated to hold the system at 150psi.
4. A much smoother control range from a PPS controller. High cam lobe tends to ramp up too much pressure with the same duty cycle applied -making the ramp-up abrupt - similar to using 2nd gear to get the car moving.
5. The pressure spike is also much smaller, this allows the peak pressure closer to the "150psi demand switch". Overshooting 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. In these days, hypes rules the market.
By-Pass pump (no demand switch):
Shurflo Offers a huge range of internal by-pass valve to overcome the necessity of using a "demand switch" . Excess pressure is being "by-passed" internally by a set of spring load 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 numbers of PWM valve WAI manufacturer uses 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 application where the usage is intermittent and not cyclic use.
Against the recommendation, there are WAI systems on the market employ 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+ exists.
If this method is being used to regulate water pressure on a PWN valve system, it is a completely different story. The "intermittent" use now becomes " cyclic" usage. This will create long term problem on the switch as well as the pumping mechanism. Water pressure will also suffer from pressure spikes, sometimes as much as 20+psi spike. Using a low hysteresis switch may reduce the ripple but the other half of the problem still remains.
TO BE CONTINUED
W/A Injection Pumps
Linear Mode
