Inside the science of F1 fuel

The current Formula 1 engine is a model of efficiency matched only, and surprisingly, by some huge maritime diesel engines. In terms of small petrol engines, the F1 V6 is significantly ahead of anything found on the road or racetrack. How this is achieved is both innovative and relevant as the future of the internal combustion engine extends beyond some projections of its imminent demise.

There has always been a truism in F1 engine design that the secret to performance was to get as much air as possible into the engine in a given time and then adding the appropriate amount of fuel, and burning it fully and efficiently.

Getting maximum air into the engine was generally achieved by good gas flow and tuning of the inlet system, increasing engine revs or turbocharging. In terms of combustion the ratio of air to fuel at which complete combustion takes place is known as the stoichiometric ratio, and for gasoline is around 14.7 times the mass of air to fuel. Maximum power was generally produced when the mixture was slightly rich, in other words the ratio was slightly lower.

The regulations introduced in 2014 turned this philosophy on its head by dictating that the fuel flow would be limited to 100kg/hour, around 60% of the flow of the previous 2.4-litre V8 engines. At the same time, turbocharging was reintroduced and direct fuel injection allowed. This led to a complete re-think of engine design. The game now would be to see how lean they could run an engine, in other words the opposite of previous thinking, and run a mixture that had excess air rather than fuel.

It may sound like a simple change, but the problem was that lean mixtures are difficult to ignite in a controlled fashion and burn very slowly. Ideally, when the spark plug fires, it initiates a controlled burning process (not an explosion as many think) which rapidly spreads out from the plug to the piston and cylinder walls.

A lean mixture is not only difficult to ignite when required but also, since the ensuing combustion is slower, combustion temperatures stay hotter for longer – reducing efficiency while increasing the propensity for pre-ignition and severe thermal damage.

F1 cars no longer need to refuel mid-race, and the composition of the fuel itself is very different since the turbo hybrid rules were introduced in 2014 too

Photo by: Sutton Images

The reintroduction of turbocharging meant it wasn’t a real problem to introduce excessive amounts of air into the engine, even at quite low engine speeds, but there remained the problem of igniting it. The solution lay in a modification to the spark plug that was termed passive pre-chamber ignition.

This design places a small chamber, with a few small holes, over the electrodes of the spark plug. This chamber will be around 2% of the total compression volume. When fuel injection is initiated, a small amount is pushed into the chamber leading to a locally rich mixture easily ignited by the spark.

Once ignited, the mixture is ejected through the same small holes as a highly reactive jet of radicals penetrating deep into the combustion chamber, creating a high-energy distributed ignition source to the main mixture. The high energy of ignition, coupled with multiple ignition sites, compensate for the slow burn-rate normally associated with lean combustion, resulting in optimised combustion phasing and a vast improvement in peak efficiency.

Ethanol, since it contains oxygen, improves the anti-knock value of the fuel but in this concentration would perhaps only take the octane rating from around 104 to 104.2

Another technique used in an F1 engine is to close the inlet valve before the piston reaches the bottom of its stroke in what is called the Miller cycle. By separating the thermodynamic compression ratio from the geometric compression ratio, the expansion ratio will exceed the compression ratio, allowing an increase in efficiency at the cost of reduced turbulence and slower combustion. Nevertheless, it provides an overall gain.

Of course, while the combustion system is designed to extract the maximum energy from the fuel, the fuel itself has a first-order importance. Before 2022 the regulations called for 5.75% of the fuel to comprise bio-components.

In general, fuel suppliers used a bio-derived iso-octane or iso-octene to comply with this regulation. Iso-octane has a high chemical energy content known as the lower heating value (LHV), a property of significant importance when fuel-mass flow is limited. Iso-octene, which has a double chemical bond, thereby containing less hydrogen, has a slightly lower LHV but better knock resistance.

In 2022 the regulations changed the non-specific 5.75% bio-content requirement to a specific 10% ethanol content. Ethanol, since it contains oxygen, improves the anti-knock value of the fuel but in this concentration would perhaps only take the octane rating from around 104 to 104.2 – however it has a more significant effect on the LHV, reducing it by between 2.5% and 3%, with a direct and proportional effect on the power of the engine.

The sustainable content of F1 fuel has steadily risen over recent years and shows no signs of slowing

Photo by: Shell Motorsport

In 2026 there will be a change to the engine and fuel. The engine may lose peak efficiency due to the removal of the MGU-H, the device which recovers energy from the exhaust which would otherwise have been lost as heat. In addition, the fuel must be sustainable and will see an increase in the concentration of oxygenates from 10% to around 20% volume (by ethanol equivalency). A maximum LHV of 41megajoules/kilogram is also mandated, around 5% lower than is achieved at present, which may lead to a reduction in efficiency.

PLUS: The new fuel war raging as F1's 2026 overhaul arrives

Since the regulated fuel-flow limit will change from mass flow to energy flow in 2026, LHV itself will no longer have a direct impact on engine performance. In fact we may find that fuels with a lower LHV have improved knock resistance and burn-rate in some engine concepts. However, LHV will still form an important consideration when a fuel is optimised for a given PU/chassis package.

One thing is very certain. Just as four-valve technology, fuel injection and overhead camshafts have moved from race engines to mainstream production engines, so too will the advanced combustion features of the current F1 power unit (as well as the advanced fuels) contribute to increased efficiency for transport systems relying on the internal combustion engine.

The technological advances used for fuels powering F1 cars will surely find their way into regular road usage

Photo by: Mark Sutton