PDM Installation & Configuration: Electric Water Pump Control
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Electric Water Pump Control
05.37
| 00:00 | - A common modification in a performance vehicle is to do away with an engine driven water pump for the cooling system and instead move to an electric water pump. |
| 00:09 | Electric water pumps offer a couple of advantages, the first is that mechanical water pumps take power to spin and when that power is taken from the crank, it's power that isn't being sent to the wheels which is where we really want it. |
| 00:21 | By moving to an electric water pump, we eliminate this power draw from the crank. |
| 00:25 | Now I say eliminate but in reality, there is still some power loss as now the alternator will need to work harder to generate the electrical power that new water pump needs. |
| 00:35 | But this will be a smaller mechanical load than a crank driven water pump. |
| 00:40 | A second benefit is an increase in controlability. |
| 00:42 | The speed a mechanical water pump spins at is directly linked to the crankshaft speed. |
| 00:47 | And this can mean the size of the pump is a bit of a compromise where it needs to be large enough to circulate water effectively at engine idle speeds but can be too large and quite inefficient at high engine speeds. |
| 00:59 | There's another element in a conventional cooling system that controls the flow rate and that is the thermostat. |
| 01:05 | When we replace a mechanical water pump with an electric one, the thermostat is also removed, meaning that we now have just a single element determining the flow rate of the coolant in the engine. |
| 01:16 | And if we have a PMU capable of speed control, we can optimise that flow rate for whatever the conditions are that the engine is currently operating under. |
| 01:24 | With the reasons we might want to install an electric water pump established, let's get into how we would use our PMU to control it. |
| 01:31 | We're going to make an assumption here that the hotter the engine is, the more coolant flow is required to keep the temperature under control. |
| 01:38 | In reality this is actually a pretty complicated relationship but without getting into some pretty intense modelling of the system, we'll never be able to hit on an optimised solution first time. |
| 01:49 | Instead, we're going to make this assumption, set the system up, test it, adjust it and iterate from there. |
| 01:56 | The other feature we're going to include in this function is a run on period where the coolant pump will continue to run, circulating coolant, after the engine has been switched off. |
| 02:05 | This will help alleviate heat soak in the engine bay and ensure the engine cools more evenly. |
| 02:10 | This is particularly important with turbocharged engines as turbos are very prone to heat soak after the engine is shut off. |
| 02:17 | Traditionally this has been taken care of with careful plumbing of the turbo coolant lines to encourage convective flow but using an electric water pump is going to give us a better result. |
| 02:27 | Thinking about these requirements, the inputs we're going to need for this function are a signal to tell us if the engine is running or not and the engine coolant temperature. |
| 02:35 | The output is going to be the channel connected to the water pump which will need to be PWM capable at the pump's maximum rated current. |
| 02:44 | For the engine running function, we'll set up an "if" function and have it give us an on output if the engine speed is above 600 RPM and an off signal if it is not. |
| 02:55 | These signals will go to a couple of places, one of which is another "if" function that's specifically looking for the event of the engine going from running to not running. |
| 03:04 | This sort of function is commonly called an edge detector as it's looking for the edge when it's input goes from on to off. |
| 03:12 | The output of the edge detector is then used to trigger a timer which we've set with a period of 10 minutes. |
| 03:18 | The output of the original engine speed based "if" function and this timer are then fed into an "or" function. |
| 03:25 | This provides a signal that tells us that the engine is either running or it was switched off within the last 10 minutes. |
| 03:33 | This is where we finally bring in the engine coolant temperature, the variable that we're actually trying to control. |
| 03:38 | We can use the output of the above logic to enable a speed control table that'll look at the current engine coolant temperature and run the water pump at a defined PWM duty cycle and thus speed based on this. |
| 03:51 | Earlier, we made an assumption that the hotter the engine coolant temperature is, the faster we want to run the pump and while this is almost certainly true, the optimal relationship between these two variables will not be a linear one. |
| 04:04 | When the engine is cold, we want to warm it up as fast as possible but we still need the pump to run slowly to prevent any hot spots from forming. |
| 04:12 | So the pump duty cycle might be fixed at a very low value until the water termperature reaches say 70°C. |
| 04:19 | Now we can start ramping up the pump speed to make sure the temps stay under control. |
| 04:24 | We don't want to be too slow to respond and have the temperature run away on us. |
| 04:28 | We've got a factor on our side here that the physical flow of water around the system will respond very quickly to changes in water pump speed which we can control almost instantly. |
| 04:39 | The speedy response means there isn't any lag in winding up the flow rate and we're not likely to see any temperature overshoot issues due to the system responding slowly. |
| 04:48 | That being said, there are a lot of thermal effects in this system and as we've mentioned, we're not equipped to understand all of them without some pretty intense mathematical modelling. |
| 04:57 | Instead, we'll tune the water pump speed in response to engine coolant temperature iteratively to get the fastest warm up time we can but making sure that the temperature never runs away out of control. |
| 05:08 | This will give us a robust electric water pump control system that combined with the rest of the cooling system, should keep engine temps well under control. |
| 05:16 | In this module, we've looked at how we can use our basic functions, PWM speed control and a tunable table to implement an electric water pump control system that ensures we will have a short warm up period with no hot spots occurring as well as an even cool down period to help deal with any heat soak in the engine bay. |
