Motorsport Plumbing Systems: Routing & Heat Management
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Routing & Heat Management
06.50
| 00:00 | The fuel tank is more often than not at the opposite end of the vehicle to the engine it supplies, for the purpose of a more balanced front to rear weight distribution. |
| 00:10 | For front engine vehicles, the fuel tank is usually behind, above or just in front of the rear axle, where rear engine cars like a 911 might have it over the front axle. |
| 00:21 | There's some more variety in mid engine vehicles but the key point here is that our fuelling system spans a relatively large distance compared to the other plumbing systems in a car. |
| 00:33 | The majority of the fuel lines run underneath the vehicle, attached to the floor pan or chassis rails, sometimes in the transmission tunnel. |
| 00:41 | One of the key considerations in this routing is avoiding damage, particularly from abrasive road surfaces and the particles it kicks up. |
| 00:50 | But we also want to minimise unnecessary restrictions in the plumbing by trying to keep the length of the system and the bends to a minimum wherever possible. |
| 01:00 | The other thing to consider is the fuel plumbing's exposure to heat sources, as it often runs near the exhaust system and other hot components in the engine bay, eventually attaching to the hot engine itself. |
| 01:12 | Routing and heat management are important for safety, we want to avoid damaging the plumbing and risking leaks and fire, but we also want to keep the fuel temperature as low as possible. |
| 01:24 | Routing options can often be limited by other constraints, so some protection from heat and other elements might be necessary. |
| 01:32 | Unlike our coolant system, there's no purposeful heat exchange to and from the fuel. |
| 01:37 | Our fuel enters the vehicle in its natural cool state determined by our local environment, but by the time it reaches the engine, it's likely been heated a significant amount and we want to minimise this as much as possible. |
| 01:51 | Firstly, the temperature of our fuel lines causes the materials to degrade faster, especially rubber based hose. |
| 01:58 | Essentially the hotter our plumbing gets, the sooner it'll fail. |
| 02:02 | But the temperature of the fuel also has a significant impact on the performance. |
| 02:07 | Without getting too deep into the science here, increases in fuel temperature reduce the density of the fuel and can also increase the overall charge temperature. |
| 02:17 | The effect of fuel density is closely related to the intake air density and the tuning and calibration of the fuel supply system, be it EFI or carburetted. |
| 02:28 | In either case, fuel is supplied to the engine on a volumetric basis. |
| 02:33 | So, if the density of the fuel decreases, for the same volume, less fuel mass is injected into the intake charge, causing the engine to run leaner. |
| 02:43 | The opposite is also true, lower temperature fuel means more density, which can result in the engine running richer. |
| 02:51 | Something a bit less known is the heat of vaporisation. |
| 02:55 | Essentially as we inject the fuel, it changes from a liquid to a gas and this requires energy, which it draws as heat energy from its surrounding, being the cylinder. |
| 03:06 | This cools the cylinder and the higher the mass of fuel, the greater the cooling effect. |
| 03:12 | On a side note, ethanol has a much greater heat of vaporisation than gasoline, which increases the cooling effect and this is helped even further by the 35% or so extra fuel mass required when using it compared to pump gasoline. |
| 03:27 | So, if the fuel temperature increases, this can cause the engine to run lean and the cylinder temperature to rise, both potentially leading to detonation and damage to the engine and other components. |
| 03:40 | Fuel temperature sensors can be used by the ECU to adjust the tune to compensate for this, but it's clearly not ideal and the best bet is to prevent the heating of the fuel in the first place. |
| 03:52 | In extreme cases it's possible for the fuel to even vaporise while in the fuel delivery system, basically losing all fuel pressure and starving the engine of fuel, obviously stalling it. |
| 04:04 | This was more common in carburetted cars as there's often a mechanical fuel pump near the engine rather than an electric unit submerged in the fuel tank. |
| 04:13 | Once the fuel has vaporised, the pump can no longer flow fuel so starting the car again can be difficult to say the least. |
| 04:22 | So, how do we keep our fuel plumbing and therefore the fuel temperature from getting too hot? The same considerations as always apply around heat transfer via conduction, radiation and convection. |
| 04:35 | Firstly, we want to avoid mounting fuel lines to hot components to decrease conductive heat transfer. |
| 04:41 | It can also help to isolate them, using rubber mounting blocks for example. |
| 04:46 | We should avoid routing fuel lines near hot components to decrease heat transfer via radiation. |
| 04:53 | But if this isn't an option due to packaging constraints, then heat shields somewhere between the heat sources and the plumbing are generally the next step. |
| 05:01 | There are two ways to look at convection. |
| 05:04 | We want to prevent hot air flowing over the fuel lines, but cold fresh air flowing over the fuel lines can be beneficial. |
| 05:11 | As we discussed previously, due to the thermal mass of the hard lines, they all reach approximately the same temperature and transfer similar heat to the fluid inside regardless of the materials. |
| 05:23 | For flexible lines however, it's possible to increase the thermal mass significantly due to the thickness of the hose, and stainless steel braids can help reflect radiant heat. |
| 05:36 | Naturally this all comes at a weight penalty. |
| 05:38 | However, for hard lines or flexible hose, we can also use insulation sleeves, usually in the form of a laminated fibreglass with a silicon or reflective outer sleeve which also comes with a small weight and expense increase, but can make a significant difference. |
| 05:56 | Let's round out our discussion by summarising what we've covered around routing and heat management. |
| 06:01 | How well the plumbing materials can hold up to heat is one thing, but we want to make sure the plumbing lasts and remains safe. |
| 06:09 | Most hard lines are up to the task, but some lower quality flexible lines like factory style rubber hoses tend to deteriorate quickly when exposed to high temperatures. |
| 06:20 | What's more important to performance is keeping the temperature of the fuel down and that generally means the plumbing as well. |
| 06:27 | An increase in fuel temperature means a decrease in fuel density, changing the mass of the fuel injected into the engine and this can have a significant impact on the performance and heat generation, potentially leading to engine damage. |
| 06:41 | As usual we want to avoid heat sources with our routing, and use heat shields and insulation where necessary. |
