Motorsport Plumbing Systems: Sizing
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Sizing
13.21
| 00:00 | The size of the fuel line supplying the fuel to the injectors has a big impact on the amount of fuel flow we'll see. |
| 00:07 | If the lines are too small, we won't be able to flow enough fuel to support our engine's power potential, but if the lines are too big, the system will take a lot longer to pressurise, as well as being unnecessarily heavy and expensive. |
| 00:20 | The system including the pump needs to keep up with the flow requirements or the pressure in the lines will drop below our requirements, potentially causing the injectors to underdeliver and the engine to run lean, risking damage. |
| 00:35 | So how do we determine our flow requirements? First, there's some fundamental knowledge about fuel injectors we need to understand. |
| 00:43 | Firstly, the injectors will have a flow rating at a specific pressure, this is commonly 3 bar or 43.5 psi, but often aftermarket injectors will have flow ratings at several pressures so we can choose a suitable fuel system pressure to achieve the required injector flow. |
| 01:02 | Injector flow will change with the pressure it's supplied with, so this does give us a tuning tool we can use to get more or less flow from the injectors however there are limitations. |
| 01:13 | Increasing the fuel pressure too far will mean the pumps need to work harder in order to generate fuel pressure, so their flow capability will be reduced, meaning we may just max out our pumps. |
| 01:25 | High fuel pressures also make it harder for the injector to open as it's trying to open against the fuel pressure, which in turn is trying to force the injector shut. |
| 01:35 | This is most notable at low battery voltage when we have less electrical energy available to open the injector. |
| 01:42 | In extreme cases the injector may not be able to open at all when cranking the engine in order to start it. |
| 01:49 | On the other hand, reducing the fuel pressure will not only reduce the flow through the injector but also can negatively impact the spray pattern and fuel atomisation. |
| 02:00 | Lastly, while outside of the scope of this course, the fuel pressure we choose will affect the characteristic of the injector, which we refer to as dead time or offset. |
| 02:11 | In basic terms, the dead time or offset defines the difference between the time that the on signal is sent to the injector by the ECU and the amount of time the injector is actually open and flowing fuel into the engine. |
| 02:25 | This is a key metric that we need in order to correctly tune the engine. |
| 02:30 | So with all of that being said, we typically want our fuel pressure to be in the range of about 3-4 bar or 43.5-58 psi in the majority of situations. |
| 02:42 | The injector sizing and maximum flow requirements, and therefore the flow requirements of our system can be calculated based on this equation. |
| 02:52 | Injector size in pounds per hour equals the expected horsepower multiplied by the brake specific fuel consumption. divided by the number of injectors. multiplied by the maximum injector duty cycle or more specifically, the overall fuel flow equals the horsepower times BSFC. |
| 03:12 | The brake specific fuel consumption is a measure of an engine's efficiency in burning fuel and producing power, measured in pounds per horsepower per hour, or grams per kilowatt per hour. |
| 03:25 | You'll find some typical BSFC values below which can be used to get a good estimate of the fuel flow requirements. |
| 03:33 | This information is covered in more depth along with the engine's operating principles we've been discussing in the HPA EFI Tuning Fundamentals course so I'd recommend checking that out if you're interested in picking up some more skills and understanding in this area. |
| 03:48 | Let's really quickly look at an example. |
| 03:51 | If we have a 250 horsepower Honda K20A engine, which is one of the more efficient production based engines with a BSFC of around 0.3 pounds per horsepower per hour. |
| 04:05 | This means our fuel flow will be around 75 pounds per hour. |
| 04:10 | Let's say this is running on some form of pump gasoline, which has a specific gravity of 0.74 which is the density relative to water where the specific gravity equals the density of our fluid over the density of water. |
| 04:26 | More simply, since the density of water is one gram per centimetre cubed, the density of gasoline is about 0.74 grams per centimetre cubed. |
| 04:36 | 75 pounds per hour is the equivalent of 34,019.4 grams per hour, or 566.99 grams per minute and dividing this by 0.74 grams per centimetre cubed gives us about 766 centimetres cubed per minute or cc per minute. |
| 04:58 | This is the overall fuel flow, so dividing this by the number of injectors show we need around 200 cc per minute injectors for our max fuel flow. |
| 05:08 | Without getting too far outside the scope of this course, this would require a 100% injector duty cycle, and we'd normally recommend capping this at around 90%. |
| 05:19 | So 200 divided by 0.9 equals 222 cc per minute, which would most likely not be available so we'd always choose the next size up being 250 cc per minute meaning we'd also choose a 1000 cc per minute pump for some extra headroom. |
| 05:40 | Our fuel pump will of course need to be capable of providing at least this flow and at a pressure higher than what we require. |
| 05:47 | For clarity, 1000 cc per minute is the equivalent of 60 litres per hour, which is the standard unit for fuel pumps. |
| 05:55 | Next we need to determine the size of fuel line required to achieve this and let's say our fuel pressure will be the standard 43.5 psi for a return style port injection system that's common on the majority of modified engines and of course the Honda K28. |
| 06:13 | Calculating the size of plumbing required to achieve this flow and pressure is the tricky bit, and the maths is going to get a bit more complicated here but don't stress too much because at the end we'll give you a list of the general sizes you can use depending on your requirements. |
| 06:28 | There are two things to consider here, the first being flow velocity. |
| 06:32 | We don't want to get too deep into the fluid dynamics, but keeping the flow under about 1.2 metres per second significantly reduces the risk of turbulence in the fuel flow. |
| 06:43 | We want to get the flow laminate where it's smooth and streamlined as opposed to turbulent as this can increase the friction or pressure loss we're about to discuss. |
| 06:54 | As well as the heat of conduction and thermal mixing. |
| 06:58 | Essentially, we want to keep the fuel cool and minimise the pressure loss so turbulent flow is not ideal. |
| 07:05 | The standard sizes of fuel line plumbing are as follows: 5/16ths of an inch which is dash five, 3/8ths of an inch or dash six, 1/2 inch or dash eight, 5/8ths of an inch or dash 10 and for really extreme cases we might even use 3/4 of an inch or dash 12, all of which are the dimensions of the internal diameter, at least for flexible hose anyway. |
| 07:30 | The velocity is related to the flow rate and the internal plumbing diameter by the following equation: Velocity equals four times the flow rate divided by pi times the diameter squared. |
| 07:43 | So, for a 1000 cc per minute flow rate and a 5/16th inch or dash five line, which has a diameter of 0.79 centimetres, the velocity will be about 2040 centimetres per minute, or 0.34 metres per second. |
| 08:02 | Much slower than the 1.2 metres per second, so no risk here. |
| 08:07 | Increasing the line size would reduce the velocity further, but clearly there's no need. |
| 08:12 | The other thing to consider is the pressure drop due to resistive forces on the fluid flow in the plumbing. |
| 08:18 | This results in the fuel at the engine end of the line having a lower pressure than the fuel at the pump end of the line. |
| 08:25 | What's important is whether or not this will make our pressure at the fuel rail lower than our required 43.5 psi. |
| 08:32 | For example let's say our pump is a Walbro unit, rated at 190 litres per hour at 50 psi. |
| 08:41 | Clearly more than enough flow for our requirement. |
| 08:44 | Let's say we have four metres of plumbing between the tank and the engine, which is not a bad approximation for a car with a 2.6 metre wheelbase, accounting for all the bends and routing. |
| 08:57 | The Darcy-Weisbach equation allows us to calculate the pressure loss in a length of pipe due to friction. |
| 09:03 | Where the pressure loss equals the density multiplied by the Darcy friction factor, multiplied by the length of pipe, multiplied by the velocity squared, all divided by two times the diameter. |
| 09:17 | The Darcy friction factor is a dimensionless coefficient and a good average value for fuel flow through the type of piping we'd typically use for automotive applications is 0.08. |
| 09:30 | Density in this equation needs to be in kilograms per metres cubed, which for our fuel is 740. |
| 09:37 | The length and diameter of the plumbing is in metres, so four metres long and 0.0079 metres in diameter. |
| 09:46 | And the speed is in metres per second, so 0.34 in our case. |
| 09:51 | So, the pressure loss at 0.34 metres per second velocity, which is the worst case scenario, can be calculated at 1.73 kPa which is only 0.25 psi. |
| 10:05 | Obviously this isn't going to be an issue for our fuel pump, providing more than enough flow at 50 psi. |
| 10:12 | But if we instead had a 650 horsepower turbocharged engine with a BSFC of 0.6, we would require a flow rate of around 390 pounds per hour, which is the equivalent to about 4000 cc per minute. |
| 10:30 | Using the same dash five line, this would result in a flow velocity of 1.36 metres per second, which could start to cause turbulence. |
| 10:40 | The pressure loss due to friction in this case would be 27.75 kPa or about 4 psi which is starting to become more significant. |
| 10:51 | We'd likely want to increase the size of the line to reduce the velocity of the flow alone, but also to help lower the pressure drop. |
| 11:00 | The other thing to consider is the pressure loss we gain as our plumbing gains height, basically due to the change in gravitational potential energy. |
| 11:09 | The change in pressure equals the change in height multiplied by the density in kilogram per metres cubed again, multiplied by the acceleration due to gravity being 9.81 metres per second squared. |
| 11:24 | So, if our fuel rail was 0.5 metres above the pump, for gasoline we'd have a pressure loss of 3.6 kPa or 0.5 psi on top of our pressure loss from the friction in the plumbing. |
| 11:38 | With all this in mind, there are some common industry standards for the size of the plumbing depending on the engine's power and output. |
| 11:46 | Although these aren't set in stone, they should provide a good guide for what's required. |
| 11:52 | For around 300 horsepower or less, plumbing with a 5/16th inch internal diameter can be used, which is the equivalent to dash five. |
| 12:01 | For up to around 450 horsepower, 3/8th inch or dash six will be suitable, whereas half an inch or dash eight line will be best for up to 650 horsepower or so. |
| 12:14 | Getting to the upper ends of what's common, 5/8th of an inch or dash 10 will be good up to about 1000 horsepower and above this might even use 3/4 of an inch or dash 12. |
| 12:25 | Obviously, the BSFC will have an impact here as well where less efficient engines will need more fuel flow, perhaps pushing them up to the next size range. |
| 12:35 | For ethanol blends, we'll usually need to flow around 35% more fuel than gasoline and methanol requires as much as 2.5 times the flow of gasoline. |
| 12:47 | So, lines are generally a size or two larger, while the return line is generally a size smaller than the feed line. |
| 12:54 | To sum it all up, the pump must of course be rated to provide the fuel flow and pressure we require to achieve our target horsepower, but we also need to consider the size of the fuel lines. |
| 13:05 | We want to avoid turbulent flow from too high velocity in small lines, as well as pressure drop from friction and moving the fluid against gravity as this could put us below our injectors required pressure. |
