Fuel thermal efficiency
Thermal efficiency is a way to measure efficiency of an internal combustion engine. Even an advanced modern F1 engine is very inefficient when it comes to converting the power available from the fuel/air mixture into power at the rear wheels. For an F1 engine this value is typically around 30%. This mean that if a typical F1 engine produces slightly under 650 KW (approx. 850 bhp) on the dyno, something like 1500 KW (or potentially 2000 bhp) of the energy is lost, mainly trough the heat.
This picture and energy path is for modern gasoline road cars. F1 car is about 5-15% more effective! (green part of diagram)
For example, oil heat dissipates around 120 KW of energy, water cooling system around 160 KW and hydraulics around 30 KW.
34% of remainder lost energy is lost trough exhaust and heat, while up to 15% of the available energy can be accounted for unburnt fuel. A small percentage is turned into the distinctive sound of an F1 car.
To dissipate this heat in surrounding air is real challenge for designers. While the heat exchangers on a racing car are extremely efficient, their ability to cool the engine is a function of the 'air-side capacity'. Essentially, how big a mass of air you can make flow through the radiator for a given area in given moment. This depends on generating high air velocities in the radiator intake ducts. However, typically, air velocity in the radiator ducts (sidepods of F1 car) will only be 10-15% of the car's velocity. So even if the car is traveling at 300 kph, the air in the ducts is probably only at 30-40 kph. This data is more or less the same for all racing cars without additional fan. For family car speed of air is even slower, but helped with cooling fan.
If designer make cooling duct intakes openings to big, that will improve cooling, but will ad to drag. If they are too small, overheating will be a problem. They must find the correct balance between cooling and aero performance because the more air they channel through the radiators, the less efficient the overall aerodynamics become. More air they channel through the radiators, less air remain for underflor, diffuser and rear wings to play with.
They can't make internal aerodynamics so clean and efficient like external one. In fact, changing between minimum and maximum cooling can reduce downforce by as much as 5%, which translates to a lap-time deficit of around 0.4s on an average circuit. Because air inlet is defined mostly during early stages of designing of an F1 car and can't be changed easily during the season (air inlet is very often designed like part of side impact area), airflow passing trough sidepods is controlled by different configurations of radiator outlet, and the F1 car has a lot of different possible configurations to cope with all kind of conditions. The configuration used at a particular circuit is defined according to the ambient temperatures, 'circuit factors' such as how much full throttle is used, and the temperature limits they can run the engine.
Typically, oil temperature is around and over 100 C and water is pressurized at 3.75 bar (limited by FIA) to allow boiling point to be pushed to around 120 C. Running these higher water temperatures means that they require less airflow through the radiators, and in this way they can improve aerodynamic performance.
This choice carry a penalty: each extra 5°C of water temperature they run, allowing the radiator outlets to be smaller, robs the engine of over 1 bhp. However, the importance of aerodynamics in modern F1 means they continue devoting significant resources and wind tunnel time to cooling and internal aerodynamics. This is better illustrated by the fact that the penalty in terms of aero efficiency they must accept for a 10°C drop in car temperatures is 80% smaller than it was just four years ago. This is proving that internal aerodynamics of an F1 car is as much important as external aerodynamics. Only we can't see that.
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