Hold the fuel for the endurance event without leaking, and keep fuel over the pickup under every acceleration case.
A CFRP sump with an aluminium tophat and filler neck. The sump geometry is carried between years so cells stay cross-compatible; the aluminium tophat is retained specifically because it simplifies the geometry for sealer application.
Sump designed to the acceleration cases: 1 g downward gravity plus 1 g maximum longitudinal gives 1.42 g at 45 degrees from vertical against a rear wall 30 degrees from vertical; 1 g downward plus -2.3 g maximum braking gives 2.51 g at 66.5 degrees from vertical against a vertical front wall. Values derived from prior drive data, assuming constant acceleration and neglecting inertial effects.
There can be absolutely no leaks, ever. Volume target set from the CR21 endurance fuel starvation event. Packaging under the seat, where HydraMat sizes are the limiting factor. From the design reports and one-pagers: From the CR22 Fuel System Design Report: the space available is beneath the firewall between the electrical and powertrain systems; parts availability limits the fuel pump and injector choice; and there must be enough fuel in the system to last an entire endurance. The fuel rules are unusually prescriptive and drive most of the geometry. IC.5.2.1: the tank must fill to capacity without manipulating the tank or vehicle, refuelling on a level surface must not form air cavities that let the sight tube level drop afterwards, and spillage must not contact the driver position, exhaust, hot engine parts or ignition. IC.5.3.1: rigid tanks must be securely attached without letting chassis flex load the tank, and must not carry structural loads from roll hoops, suspension, engine or gearbox mounts. IC.5.3.4: capacity must not be variable. IC.5.3.5: there must be a provision for emptying the tank.
Fuel cell capacity analysis sizes the tank so the vehicle can drive without fuel being the limit while still minimising cell volume. Validation is by measurement, not calculation: fill the car, run a mock endurance, pump the remaining fuel out, measure the mass pumped out and compare against the MoTeC fuel use sheet, repeated over five endurance runs and averaged.
1 layer of Kevlar over 5 layers of stitched carbon twill, laid on HIPS dissolvable molds. Components bonded with 3M DP420. Coated with Caswell epoxy gas tank sealer to prevent leaks and tank wear. From the design reports and one-pagers: CR22 aluminium cell build sequence with logged times: waterjet the shell, bend to shape, weld the seams. Top port: machine the sealing surface (2 hr), weld two pipe halves and two squares to the top port (1 hr), weld the part to the cell (30 min), tap the top port bolt holes (1 hr). Top hat waterjet cut (30 min), weld fittings to the top hat (1 hr, and the report is emphatic about jigging this to minimise warping), waterjet the gasket (30 min). Filler neck: weld the lower neck to the cell (2 hr), cut to length while fitting to the car (30 min), weld the cap threads (30 min), weld the sight tube and vent bungs (30 min), drill and tap the fill cap for the rollover valve (1 hr), drill out the filler neck bungs (5 min), cut clear hose to mate neck and cell (20 min). Weld tabs to the outside of the cell where possible (30 min), drill matching holes in the carbon floor and firewall (30 min), cut and weld the stud plate. Cut sight tube and vent tube to length (1 hr). Then leak check. The CR25 composite cell replaced this with: 3D print the mould with Prototyping Solutions, 5-layer stitched fabric layup, dissolve the mould, post-process the carbon, apply phenol novolac epoxy sealer. Estimated 3 weeks total.
CFRP with a Kevlar outer layer, Al 6061 tophat and filler neck, 3M DP420 adhesive, Caswell epoxy gas tank sealer.
Three Fault Vault entries, all CR26I, all about sealing or venting. 2026-05-17, Manufacturing, Compromised Failure. Major fuel cuts under lateral loading at low fuel level. The cause was narrowed to one of three things: pump wiring under the top hat, the HydraMat sitting too high in the tank, or a leak between the HydraMat and the fuel pump. Fixed by lowering the HydraMat and rewiring. 2026-06-07, Operational Errors, Compromised Failure. Threads in the top hat failed after the component had been removed and installed many times. The initial torque specs were too high and had already compromised some threads; at competition multiple threads slipped completely and the tank leaked after endurance. Corrective action: do not overtorque, use antiseize on the threads, and size up the bolt. 2026-07-12, Operational Errors, Complete Failure. The fuel vent check valve was too small to let enough pressure escape on hot days, so heavy pressure built up in the tank. When the top was unscrewed to relieve it, all the fuel in the tank shot out at once, ending up in multiple people's eyes, face and clothing and over the car and personal gear. Fixed by fitting the old fuel vent cap, whose opening is about four times larger. The historic sealing problems are on the same theme: threads stripping before reaching clamping pressure, fuel leaking through threads unless nylon crush washers were used, and fuel travelling through the wires of the previously selected bulkhead.
Bolts moved from 8-32 to 10-32 with torque raised from 20 to 30 in-lb, average clamping pressure raised from 689.4 to 951.5 psi, and bolts moved from the interior to the exterior of the cell, which eliminated thread sealant. Fittings replaced with -4 AN weld bungs and stud bulkhead terminals so no fuel path runs through wire. After the CR26I thread failures the recorded next steps are antiseize on the threads and sizing the bolt up.
1.4 gal cell capacity, 1.56 gal effective system capacity. 3.73 lb against the previous 4.19 lb including pump and fuel cell hardware, a 0.45 lb (11%) reduction. From the design reports and one-pagers: CR22 aluminium cell: 6061-T6, 1.4 gal capacity, top port height sized to accommodate the fuel pump, length sized for a 3 x 15 in HydraMat on the cell floor replacing the previous slosh foam, total estimated weight 4 lb, 8-32 bolts into tapped holes in the top port. CR25 composite cell one-pager: current tank 2.54 lb, anticipated carbon tank 1.49 lb, weight loss 1.05 lb, at a materials cost of $43.99 for a 0.5-pint Caswell gas tank sealer with 3D printing and carbon fibre coming through sponsorships. Full CR22 fuel system BOM totalled about $1,115 excluding the BMRS order. Quantity per car, as costed: 1