Fuel level management

I'm building a 120L tank for a budget endurance car and wanted to understand the best way to perform fuel level management and the sender types that are most accurate. Knowing how much fuel left for the stint is crucial for timing pit stops and driver changes. Using AIM dash (as part of the PDM) and MoTec m130 ECU. Our class has a mandatory 5 minute stop for refuelling so maximising range is key.

- the tank will have fuel foam so I assume I can't use any type of float level sender in case it fouls on the foam and that a capacitive type is best?

- giving the fuel is sloshing around how do you get an accurate reading from the sender? Sample every second and put it through a running average filter over - say - 30 seconds or more?

- or dont bother with a sender and just measure the fuel going in and rely on the 'fuel used' functions of the ECU.

Keen to know how this is done in the big budget teams.

Fuel Used calculations are the most accurate. The key is knowning how much fuel went in. That is usually accomplished by measuring the fuel containers, only putting in a known amount (i.e. multiple of 5-gallons), or always filling the fuel cell.

If you don't have an ECU that reports the fuel usage (i.e. stock OEM ECU), you can measure the duty cycle (or pulse width) of a fuel injector, multiply by the RPM and scale by a correction factor to get almost the same answer. I have helped someone do that with a MoTeC dash.

Thanks David

We will either completely fill the tank, or- say near end of session - part fill with known volume. Would you bother with a level sender in this case?

Hi Adrian,

Providing you have accurate injector calibrations you may be able to make use of the fuel usage values transmitted on CAN, If these values are reporting incorrectly there's a correction factor you can adjust to align the reported usage with the actual fuel usage.

Normally this is calibrated by filling the tank with a known quantity and taking measurements before and after recording a fuel usage value to determine the error. you can measure either the weight of the vehicle or the quantity of fuel remaining to determine the actual usage vs the calculated usage.

In a MoTeC dash there are fuel prediction calculations which can be used for what you want to achieve. Does the AiM dash have similar functionality?

Which firmware are you using in your M130?

I think a fuel level sensor is only really useful for a street car. Where you would look at the level when stopped, or driving in a smooth / steady fashion. It can still be useful for a race car, but with the tools we have, it's not really necessary.

However, you may be limited by your AiM PDM. It doesn't have a "preserved" channel, or an easy way to reset the M130's Fuel Used channel. As mentioned above, these are all thought-through on a MoTeC dash in the Fuel Prediction configuration.

Thanks David.

Using M130 GPR. Will use the latest firmware version.

The AIM PDM doesn't have any fuel calc functions (other than displaying a channel value and simple maths functions) nor from what I can see any ability to preserve values when the PDM is turned off. M130 GPR does have a fuel-used variable that transmitted via CAN and is reset when the ECU is restarted.

I do have a Podium Connect telemetry module and that has the ability to write custom scripts in a proper programming language (LUA). Also I left space on the CAN keypad for a 'fuel reset' button. I'm sure between the fuel-used variable in the M130, the reset button and a LUA script I'll get a solution together. Might even be as simple as keeping the ECU on during driver changes (without refueling) and turning it off when we refuel. If the Podium can preserve values then I'm set.

I'll omit the sender from the tank design.

Many thanks for the help.

First thing, if you're still to fabricate the fuel tank, and there is a 120l limit you're making it to, I'd suggest making it closer to 135-140l as the foam and other hardware within the enclosure will take up some of the initial capacity and, if required, you can purchase fillers to reduce the fuel fill as required.

2nd thing is to make sure you can use all the fuel that you can fit in the tank - at a minimum use a trap doored sump or with a bigger budget a suitably sized Holley Hydramat - https://www.holley.com/products/fuel_systems/hydramat/

That said, there are fuel sender options that use a float, or potentiometer, in a tube so there is no concern of interference, from many suppliers that can be connected to the common displays. For example - https://fuelsafe.com/motorsports/fuel-delivery-systems/fuel-tank-sending-units-gauges/ the closer you can place it to the middle of the tank, in plan view, the more accurate and consistent the readings will be.

If you have a decent budget, there are the actual fuel flow measurement options which the professionals use, which will tell you exactly how much fuel you're using/used, but they can be expensive. The advantage is they are accurate and can indicate other issues developing. For example - https://www.sentronics.com/motorsport/

Unfortunately, because of the different fuel system constraints, you can't use Smokey Yunick's trick of a hugely oversized fuel line to add an extra lap of two's capacity.

Thanks Gord.

I've left 4L for the fuel foam (I've read 3-4% space taken up by the foam). The tank is a awkward shape to fit as a 120L+ tank starts getting pretty big. There is a lift pump on either side of the saddle and two integral surge tanks, each with their own pressure pump. The idea is you run on one surge tank (backed by the capacity of the main tank) until you get fuel surge. When the pressure drops you switch to the second pump/surge tank with a known 4L volume. That way you know you have exactly 4L left before empty. This takes the risk out of getting the fuel-remaining calculations accurate.

We've been doing something similar with the stock tank and an external surge tank at it works great. You use every drop in the tank and then get a 8-10 minute warning before you're completely out of fuel.

I looked at a Hydromat and a ball valve'd trap but the lift pumps are so cheap. Same price as a ball valve.