Downforce (not drag) and its effects on fuel consumption

[raymondu999]

My point was - ultimate race-distance fuel is indeed fixed at 100kg, or whatever it is.

However I'm quite sure Car A and Car B, two cars identical in everything except for downforce, you would not complete a lap with the same fuel consumption rate. Remember that the two cars are identical even in engine maps and modes. So first question is - which would use more fuel? This initial question is posted, regardless of 2014 regulations.

Now for the 2014 fuel regs...

Depending on the answer to part a above, one car would have to run slightly leaner/richer than the other to fully comply with the 100kg/race limits. Ultimately both will use equal fuel of 100kg over the race distance (assuming neither are lapped by the other) but I am putting forth the hypothesis that given the different df levels, they will run different engine modes to achieve said fuel consumption target over the race distance.

[olefud]

I see that you are still missing the main point. Next year all cars will be having roughly the same fuel consumption due to the way the formula is written. Hence the original question is faulty. If you have two cars with the same drive train and one has higher aero efficiency it simply means the performance will be higher. That would be the correct answer for the question if it were written the right way.

Just what performance will be higher? Look at it this way. Say the lower downforce car will have to slow more for a given corner such an amount that it exits the corner one gear down from the high downforce car. Exiting the corner both cars are producing the same power (same rate of fuel consumption), but the LDF is going slower and using a portion of the fuel to build speed, i.e. accelerate and invest energy in regaining kinetic energy that the HDF car preserved by cornering faster. At the next corner, same story –the LDF car dumps the more kinetic energy into heat entropy as it has to slow more than the HDF car.
Thus, when the HDF car finishes the race, the cars will have used the same amount of fuel. But the LDF car, having lower performance, still has to race some finite distance to finish the race. It is the fuel it consumes after the higher performance, HDG car has finished and shut down that results in the LDF car having poorer fuel efficiency in terms of fuel used to complete the same race distance as the HDF car.
Higher performance in this instance results in both higher speed and higher fuel efficiency.

[hollus]

The little fallacy in this gedankenexperiment is that car A will be faster and hence have to pay a fuel penalty for the extra speed. To prove that wrong I will assume that car A has higher downforce and equal drag than car B at the same speed (or at any and all speeds).
A clever driver in car A knows that he has equal or superior performance to car B at all times. He can choose to drive at all times at the exact same speed as car B, hence having at all times the same drag and using the same fuel in the same time and distance. Or he can do that for most of the lap, coast when car B starts to brake, and brake where it will allow him to match car B's speed at the end of the braking area. Now car A uses the exact same amount of fuel than car B, all all positions in the track, yet is faster for a part of the lap and equal everywhere else.
Faster does not need to carry a fuel penalty.
Once the fallacy is established, we can look into the subtleties (like braking less and coasting the extra cornering speed, or trading high speed / high drag performance for low speed / low drag performance), and rolling resistance definitively is one of them.

[WhiteBlue]

Very good Hollus. we are getting somewhere. You can make any point you want and arrive nowhere if you do not define a meaningful experiment. The most simple situation as you say is still the assumption that the car with the higher downforce has equal drag due to higher aerodynamic efficiency. This makes it a very simple solution. Straight line speed will be equal because power and drag are equal. But fast cornering will be done with higher performance by the car with higher downforce. This is very simple and indisputable IMO. And it shows that more downforce does not necessitate higher fuel consumption. Exactly that strategy is used by Newey all the time - maximizing aero efficiency. And he is usually able to beat cars with higher power. It shows how valuable aero efficiency really is. It is winning championships for several years. And the beauty is that nothing will change fundamentally if the engines come out with similar efficiency, which the narrow spec makes very likely.

[olefud]

Once the fallacy is established, we can look into the subtleties (like braking less and coasting the extra cornering speed, or trading high speed / high drag performance for low speed / low drag performance), and rolling resistance definitively is one of them.

The thread premise comprises a number of stipulations that do not correlate to reality and a few ambiguities. I lumped rolling resistance into “drag” though I also inferred that equal drag would be equal drag vs. speed curves. While there could be exceptions, i.e. a track at which both cars ran wide open for the whole distance, with the usual fairly intense braking, the high down force car would be more efficient for the same reason that driving at low drag, low speed urban conditions in a road car yields poorer fuel mileage than driving at higher drag, higher speeds on the open road.

[WhiteBlue]

The thread premise comprises a number of stipulations that do not correlate to reality and a few ambiguities. I lumped rolling resistance into “drag” though I also inferred that equal drag would be equal drag vs. speed curves...

Naturally you try to build a model that focusses on the variables that have the dominant impact on the question on hand. You try to eliminate the non essential to show the fundamental drivers of the situation. I have not been very impressed with the way the original post was structured in that regard and I have made proposals to improve on its original structure. If that looks like unfair criticism to some people I can assure you that it was only done to have a more stringent discussion and not to antagonize anybody.

[Forza]

Bosch-motorsport.de uploaded the study done by Chris van Rutten in Bosch LapSim (published in RCE) concerning the new 2014 ACO regulations. It explains the basics so you can get some points for F1 related discussion of this topics.

You can read the full study on this link: bosch-motorsport.de/TECHNOLOGY - LMP 2014 REGS

Here are the final results

[dragosmp]

If both cars are absolutely identical in weight, tyres, engine, etc. and the only difference being the downforce, then for me it's clear that the higher downforce car will use more fuel.

For an exact analysis you could create a histogram that shows how much time is spent for different engine power output regions. The more time is spent under 100% throttle and at high power numbers the higher the potential fuel consumption.

And higher downforce will allow a driver to earlier push the throttle down and brake later, which means that more time is spent under 100% throttle.

I'm with this line of reasoning too. A higher DF car should consume more per mile, but not necesarily more per hour because:
*more DF, can accelerate earlier
*more DF so lower top speed; since the car accelerates earlier it will arrive faster at its lower top speed, but if the gearbox is properly set the motor would be at top revs, so top consumption
*more DF so braking comes later, so again more time at full throttle
...for these reasons the consumption per mile should be higher, ever so slightly. But the car will finish the lap faster (supposedly), so the fuel per lap consumed with high DF may be lower due to the faster lap time.

About drag vs DF, I guess drag = DF + losses. For a given amount of drag you'd want as much as possible to be useful (DF) in order to minimise loss.

my 2cents

[WhiteBlue]

bosch-motorsport.de/TECHNOLOGY - LMP 2014 REGS

This table shows in great simplicity the point that I'm trying to make for some time. You need to compare the base case in line two to the case of 5% more downforce. The assumption would be that the 5% higher downforce is caused by higher aerodynamic efficiency. Fuel consumption remains the same but performance goes up. This is exactly the ideal case that designers will try to achieve in F1 as well.

It is almost impossible to make valid assumptions how the absolute level of downforce impacts. It is so much easier to analyse the case of higher aero efficiency and constant fuel use which would be practically relevant anyway.

[hollus]

If the car consumes more fuel per mile (I don't think that's necessarily the case), it will consume more fuel to finish the race. The number of miles is the same for everyone.

Again I'll repeat a hidden subtlety here. Just because the car can be faster, doesn't mean that is has to be. The driver can decide if he uses all the speed advantage or only part of it, in in that case he decides when and how.

[olefud]

Whilst a higher average speed results in a higher average power, it also results in a lower time spent at that average power. I would expect that there is a point where increasing downforce increases fuel economy as well as where increasing downforce decreases fuel economy.

A higher average speed doesn’t necessarily result in higher average engine power. The engine produces pretty much the same power independent of speed, particularly with a good number of close gear ratios. What differs is how the resulting energy is invested. Relatively, a high downforce car will use more of the energy overcoming drag at higher speeds. The low down force car will invest more energy in kinetic energy accelerating from corners at lower speeds with the energy lost again at the next such corner, though at any given speed both cars will consume the same energy overcoming drag.

On an open course with no braking, both cars will use the same amount of fuel. On a normal course with substantial braking, the low down force car will use more energy regaining and braking away kinetic energy while the high downforce car will have a higher duty cycle in the high drag area of the speed aero drag curve, though the high downforce car will spend less overall time consuming fuel during the race. Which car is more fuel efficient will depend on unknowns such as the actual relative downforce of the car, the nature of the course, tire drag at different speeds etc.

[riff_raff]

While the overall balance between aero downforce levels and driving conditions (speed) is very important, there are also other factors to consider such as the contribution in mechanical grip provided by the suspension and tires in low-speed corners. Being able to go thru a corner faster means there is less use of the braking system. Using the brakes essentially dumps large amounts of energy produced from combustion of the fuel.