Fuel

The energy needed to propel a combustion vehicle is stored as chemical potential energy in a liquid fuel that is burned by the engine. The fuels available at US competitions are gasoline of octane ratings 93 and 100, and E85. Fuels available in FS competitions are 98RON gasoline and E85 . Fuels and the fuel system are covered in IC.5 of the FSAE Rules, and CV 2 in the FS rules. No fuel additives can be used.

Chemistry

Gasoline

Although the gasoline readily available in the US, and in US competition, is 5-10% ethanol (check about in other countries), gasoline is typically approached chemically as pure octane.

2C₈H₁₈(l) + 25O₂(g) → 16CO₂(g) + 18H₂(g)

If we use atomic weights for carbon, hydrogen, and oxygen, we find that 2 mol of octane is 224 grams, 25 mol of oxygen is 800 grams which comes from 3809 grams of 21 percent air. This yields an air fuel ratio of 17. Since gasoline is not pure octane and air is not exactly 21% oxygen. Experimentation yields the stoichiometric ratio of 14.7 grams of air per gram of fuel. Gasoline sold in America has varying levels of ethanol content varying a few percent around a nominal 10% with no way of knowing what exactly is being offered besides testing. Usually the 100 Octane gasoline is more consistent in this respect.

Ethanol

The ethanol used in competition is E85, nominally 85% ethanol and 15% gasoline,.

System Design

Fuel Storage

I.C.1.2 Packaging Restrictions

The fuel tank design rules can be found in IC.5.2 for FSAE and CV 2.3 for FS.

There are two main compromises that the fuel storage system must navigate. The first is Quantity of Fuel. A car will always be faster with less weight, but cutting too close to the minimum fuel level for your car can lead to disastrous consequences such as running out of fuel or temporary fuel starvation, as well as minor inconveniences such as cg changing with fuel level. The second compromise is Fuel Sloshing. As the car accelerates, the fuel itself can move about in the tank, possibly uncovering the fuel pickup and making the car more difficult to drive. Combatting this usually involves a system of internal baffles as well as tank geometry, but the cost is weight, CG height, and the possibility that you design a tank that prevents the fuel from making it back to the pickup fast enough to supply the engine when needed.

Volume Determination

There are two ways to size a fuel tank. The first is to carefully restrict the volume of fuel as to ensure a lighter car. The second method is to ensure the fuel tank has enough fuel to be used during extended drive cycles such as those found on test days. Because refueling a car takes so little time and is generally regarded as a safe practice, the first method is often chosen for combustion cars, while the second is more commonly found in EV batteries.

The minimum quantity of fuel should be enough to barely finish the endurance race at competition. However, this limits the functionality of the car by limiting run time, and increases the likelihood that you will run out of fuel early. The quantity of fuel used in an endurance competition should be determined by experimental data, but can be estimated based on past usage, or usage of similar teams, or if masochistic, be predicted based on average speed of the vehicle, the track length, and the consumption of your engine.

Below are tables showing average fuel consumption by race finishers in the 2019, 2021, and 2022 Michigan Competitions.

Average FSAE Michigan May Competition Fuel Use [L] by Type

Year	93 Octane	100 Octane	E85
2022	3.7	3.8	5.4
2021	4.0	4.3	5.6
2019	4.0	3.9	5.5
2019 (4 cyl engines)	4.4	4.6	5.9

Average FSAE Michigan May Efficiency Score by Fuel Type

Year	93 Octane	100 Octane	E85
2022	56.5	49.4	57.0
2021	77.2	66.8	77.3
2019	59.1	62.0	66.0

Fuel Sloshing

Similar to oil sloshing, fuel will move around in the tank as the car goes around corners or accelerates/brakes. The design of the fuel tank should reduce this phenomenon. Each team will face different levels of sloshing and some may never encounter a problem. Generalized advice cannot really be furnished here except to consider it.

Internal baffles are a frequently employed solution to this. The baffles can be solid, sometimes made of the same material as the tank itself, or the baffle can be a gasoline-resistant open cell foam that resists the movement of fuel inside the tank.

An alternative to, or in addition to an internally baffled fuel tank is placing an absorbent mat at the bottom of the fuel tank connected to the fuel pickup. Anecdotal evidence indicates that a fuel mat like Holley's HydraMat seem to solve fuel pickup issues for teams of all levels.

Rigid Container

Most fuel tanks in FSAE are solid. The important considerations for rigid fuel tanks are mounting and material.

Mounting
The most important mounting consideration is to ensure the vibration and torsional forces going through the frame cannot pass through the fuel tank. A stressed fuel tank may fracture and leak fuel. The easiest method to ensure no torsional forces can be transferred into the fuel tank is to only use three points for mounting. If the fuel tank is attached by bolted connections, it is necessary to isolate the tank vibrationally. This can be achieved by inserting a rubber washer into the bolt/washer stackup between the frame and fuel tank. If rubber washers are used, ensure that the bolt is not tightened to the point that the connection becomes rigid again.

Material
They can be made of sheet metal, or a gasoline resistant plastic or composite.

Metal fuel cells cannot be re-welded after fuel has been used in the fuel tank without cleaning it of any trace of fuel or by welding it in an oxygen free environment.

Bladder

A fuel bladder is an alternative to a rigid fuel tank. The bladder will expand and contract with the quantity of fuel inside the tank, with the intent of solving sloshing or fuel pickup issues.

The bladder must be enclosed by a non-flexible container that is rigidly connected to the chassis. The rigid casing is not subject to the same constraints as a rigid fuel tank and may be load bearing. This rigid container means that the fuel bladder is unlikely to be lighter than a rigid container.

Fuel Lines

Material	Internal Diameter [in]	Pressure [psi]	Weight [lb/ft]	Min. Bend Radius [in]	Supplier	Notes
Hard Lines
Aluminum	3/8-1/2 (OD)	250*			Russell	link *pressure rating is wall thickness dependent
Aluminum (0.035 wall thickness)	1/4-5/8 (OD)	200*	for 3/8": 0.043 (.651g/cm)		Summit	link *pressure rating not specified formally, only mentioned in Q&A with conflicting answers, trust with caution
Nickel/Copper Alloy (0.028 wall thickness)	.132-0.319 (__ -3/8 OD)	unspecified	for 3/8": 0.012 (.175g/cm)		Summit	link sold as pressure rated comparable to mild steel brake line
304 Steel (0.028 wall thickness)	.257-.319 (5/16-3/8 OD)	3500	for 3/8": 0.010 (.156g/cm)		JEGS	link
Soft Lines
Nitrile Rubber*	3/8	50			JEGS	link
Nitrile* (Neoprene* cover)	1/8	50			Dayco	product details cannot buy direct
Buna-N*	3/16+	50		1.25	McMaster	link yarn reinforced
Buna-N*	3/4+	150		3	McMaster	link steel wire reinforced
Braided Lines
Nitrile Rubber* and Steel	1/4+	50			Spectre	link
Nitrile Rubber* and Stainless Steel	0.22+ (4AN+)	1000		2	Pegasus	link
PTFE and Stainless Steel	0.27+ (4AN+)	1320		0.75	Pegasus	link
PTFE and Aramid	0.27+ (4AN+)	1320		0.92	Pegasus	link
PTFE and Polyester	0.38+	305+		2	Pegasus	link

* Not compatible with e85

Comparison of Line Types

Quality	Hard Lines	Braided Lines	Soft Lines
Weight			-
Cost		-	+
Cost Tables (FSAE)		-	+
Manufacturability	-		+
Pressure Capacity	+*		-
Reusability**	-		+
e85 Compatibility	+		-
*Depends on wall thickness, see table above **New design or small adjustments that need to be made

Filling

Fuel filling is critical to consider in the system design in three ways: safety, function, and rules compliance. To ensure a safe filling procedure, common sense should be used to prevent fuel spillage and ease of access. An easy process is a safe one. A rules mandated splash guard can further protect the vehicle, driver, and fuel filling team-mate.

Filler Neck

The filler neck has a rules specified vertical height, maximum angle, and internal diameter. Meeting these requirements helps ensure the tank can be filled safely, easily, and will pass technical inspection. Because this is often overlooked in design, and creating a new fuel tank or modifying an existing one is quite difficult on the day of competition, it is recommended to strictly follow rules as written, and to ask rule questions if clarifications are needed.

It is unusual but not unprecedented for a technical inspector to measure the internal diameter of the fuel filler neck, or the vertical height if it is visually unclear. Often in these cases, a gas can of the type used by the fuel filling team at competition is used to ensure that the vehicle can be safely fueled even if the rules are not entirely met. If the rules breach is egregious, the car will likely not be allowed to pass tech even if it may be safely fueled.

Sight Tube

There are vertical height and routing requirements to the sight tube. A frequently discussed solution is a clear, fuel resistant plastic used as the filler neck material itself to comply with both filler neck and sight tube rules. The fuel filling team at US competitions ask the teams to not mark the fuel fill line themselves and will mark it themselves at the fuel fill station to avoid incorrectly marked fill levels causing an accident.

Venting

The fuel tank is required to be able to vent excess vapor pressure while the car is in the correct orientation, but not allow fuel leaking in the event of a vehicle rollover. A common solution is to purchase or create a custom vented fuel cap. Custom vented fuel caps may be subject to a water leak test during tech inspection.

Pump and Pressures

Fuel pressures can be divided into two categories for different applications. Most FSAE/FS teams run fuel pressures under 10 bar, which are classified as low pressure. Low pressure fuel is sufficient for single or multipoint manifold injection. Some teams chasing performance or academic goals may opt for high pressure fuel injection to facilitate direct injection.

Pumps are usually specified to pressures greater than that used by the injectors. A fuel pressure regulator is used in-line to achieve final fuel pressure. Fuel pressure regulators can be blocking (returnless or non-return-style) or bypass (return-style). Returnless fuel injectors have only one input and and one output port allowing for a simpler routing system and thus reducing points of failure. These returnless fuel injectors do need a bypass valve at the pump to relieve pressure. The design of these regulators allows for pressure creep, is more sensitive to debris, and are not able to consistently/accurately read pressure without the engine running. Bypass regulators can be more expensive and drive more complex routing, but yield more accurate fuel readings, have a longer life, and are easier on the fuel pump.

Low Pressure

Low pressure is often the default injection pressure for FSAE as most motorcycle or snowmobile engines run at pressures around 3-3.5 bar . Single point injection or throttle body injection (TBI) is when fuel is injected at the throttle, similar to a carburetor. Most modern fuel systems use multipoint injection or port injection (PI), placing the injectors after the plenum and as close as possible to the engine. This results in fuel being sprayed into the intake ports.

High Pressure

In order to run a direct injection setup, fuel pressures need to exceed 10 bar. These extreme pressures force higher safety requirements by rules. The major regulation in FSAE is that fuel lines must be stainless steel hard-line or "Aeroquip FC807 smooth bore PTFE hose with stainless steel reinforcement and visible Nomex tracer yarn". Teams can run something similar if the team gets approval before competition. Any fuel line before the boost pump is considered low pressure and is not subject to the fuel line restrictions.

The fuel rail and it's attachments must be able to withstand maximum force from the fuel line (not including cylinder pressure).

Injectors

Placement

The injector placement can be divided into two categories: manifold injection or direct injection. Manifold injection is easier to implement as the pressures are lower, rules are less strict, and it does not require modifications to most engines used in FSAE.

Throttle Body Injection

Throttle body injection (TBI) or single point manifold injection is the oldest electronically controlled fuel injection. It is analogous to a carburetor in concept but allows much more precise and tunable control. This is uncommon as it is older technology and has less benefits for efficiency than other approaches. Because the fuel is injected so high in the intake, a greater proportion of the fuel is lost to the walls of the manifold so control is less precise.

Port Fuel Injection

Fuel Injected into the ports or multipoint manifold injection just before entering the combustion chamber. The most common type of fuel injection in FSAE, port injection offers high levels of control at a lower cost than DI.

Injecting fuel into the port can be done when the intake valve is open or when the intake valve is closed. Injecting into the port when the valve is open is colloquially referred to as "poor man's direct injection". There are benefits and drawbacks to each.

Classification by Resistance

Low Resistance

Usually 0.5 - 5 Ohm. Also called "peak-and-hold-injectors". From this name you can directly infer the mode of operation. In the beginning the ECU has to give a high current to the valve to open it quickly. This is the peak. Then a lower current is sufficient to keep the valve open. A common ratio of the currents is 4:1.

However, for this the ECU must also have a controller that can supply the different currents. The advantage of this system is that due to the high current at the beginning the valve can be opened very quickly. However, this is no longer necessary nowadays, because even valves with high resistance can provide the necessary flow.

High Resistance

Ususally 8 - 15 Ohm. Also known as a "saturated drive injector". These are much easier to control, as only the circuit has to be closed and the resistance of the valve controls the current.

Critical Fasteners

All fasteners on the fuel system are critical. Nylon locking fasteners are not appropriate near the engine such as the fuel rail. The nylon will soften and will not retain the nut. Most technical inspectors will not catch this but it's a pain to change and better to just do it right the first time.

Fuel Strategy

How it's scored at comp

Points per L Fuel used in 2021 Michigan Competition

Points per L Fuel used in 2024 Michigan Competition

Since FSAE and FS considers how much fuel is used as well as how fast each car goes, the cost of fuel in terms of competition score must be considered. There are many ways to analyze the impact of the fuel usage on the competition score.

Unfortunately, there is no dial on the car that teams can turn to raise or lower their fuel consumption. Other factors that impact fuel use besides Efficiency Event scores are usually prioritized such as engine choice, torque, tuning, etc.

A simplistic, black box analysis using the 2021 Michigan Efficiency Event scores shows that gasoline is worth about 17 points per gallon used, and E85 is worth about 15 points per gallon used. The lap-time of the car has almost no correlation to the efficiency scores.

Fuel Choice

One of the biggest choices fueling system design engineers must make is which fuel to run. Teams with limited resources may find that cost and/or convenience may outweigh all other design considerations. Proper documentation and explanation of these restrictions will prevent this concession to real world conditions from being counted against a team in the design event.

The team must decide whether or not to pursue running E85 before many other fuel system decisions. The octane rating of E85 lies around 100, but specific aspects of the fuel mean it is not a direct replacement. The largest difference is the specific energy. E85 has about 75% of the energy per unit mass that gasoline has so the fuel tank will have to be bigger.

Efficiency

BSFC

Brake specific fuel consumption is another way to view efficiency and is used to compare engine efficiency despite size differences. It measures how much fuel is used (in lbs/hr) divided by power (hp). It is usually used to show a map of engine operating points to visualize engine efficiency across load (usually BMEP) and RPM.

Calculation
copying from old school notes so i'll have to add context later -simon

this is for finding fuel consumption (mpg or equivalent) w bsfc chart

Pme	Mean Effective Pressure
nmot	Motor speed
iG	Gear Ratio (for selected gear)
isec	FDR
rdyn	Dynamic Radius of Tire
Vd	Displacement Volume of Engine
i	Constant
Freq	Tractive force required

$P_{me}={\frac {(2\pi *T_{motor,required})}{(V_{d}*i)}}={\frac {(2\pi )}{(V_{d}*i)}}*{\frac {(F_{req}*r_{dyn})}{(i_{G}*i_{sec})}}$

$n_{motor}={\frac {Velocity*i_{G}*i_{sec}}{2\pi *r_{dyn}}}$

b_e (fuel consumption) usually experimentally determined

$B_{time}={\frac {b_{e}*P_{me}}{\rho _{fuel}}}={\frac {b_{e}*P_{me}*V_{d}*n_{motor}*i}{\rho _{fuel}}}$

$B_{distance}={\frac {B_{time}}{V}}$

i = 0.5 for 4-stroke engine