Optimizing for lap time

The idea of simulating potential fuel saving approaches can be expanded from a basic example of just one straight to simulating an entire lap, but doing so brings additional complexity. Distributing where the driver lifts and coasts throughout the lap increases the number of simulations by several orders of magnitude. Putting aside engine modes for this example, say that at a (fictional) street course with five corners, a push lap burns 0.742 gallons and the fuel target is 0.668 gallons (or 10% reduction in consumption). Now, the driver wants to know where the best places to lift are and how much should they be lifting in each corner. Is it faster to only do one big, long lift on one straight to save the entire 0.074 gallons in one go? Or should the driver do smaller, shorter lifts throughout the lap adding up to 0.074 gallons saved in total? Once again, lap time simulation is an engineer’s best friend. These simulations can run hundreds of thousands of laps at the click of a button, and then data analysis software can immediately point to the best solutions. Below is a typical sweep of various lift and coast distances for each corner of the track.

Move your cursor over the graphs to reveal additional details

After using lap time simulation to create this data, finding the combination of lift and coast distances in each corner that can stay under the fuel target while also going the fastest is now a straightforward data analysis problem. Code can be written to create theoretical laps for every combination of lift and coast distance in every corner (that’s 16,807 laps in this example!), and the fastest approach that hits the fuel target will become apparent. In this sweep, the engine mode has been fixed and only one fuel target has been given. But in reality, both of those parameters are also variable during the race and so the amount of pre-event simulation done by the teams and manufacturers in the weeks building up to a race weekend can become enormous, as they have to be prepared for all possible scenarios.

The simulations say that when fuel consumption is reduced by 10%, the lap time loss can be anywhere from 0.121s when done optimally to 1.678s if done inefficiently. Since fuel saving simulations like this are so important and are used week-in and week-out, teams have typically developed software and code to automate these tasks to immediately solve for an ideal approach.

Also from these results, something else that has already been touched on (but can now be seen in the data) is that the higher the entry speed, the better suited that corner is for lift and coast. From this, it follows that the fuel saving characteristics of each track are different from one another. Tracks with long straights and big braking zones (like Nashville) tend to be more suited for lift and coast, whereas tracks with shorter straights and flowing corners (like Barber) tend to prefer using engine modes.

Traffic and track position also plays a big factor in how teams save fuel throughout the race. Thus far, fuel saving has only been discussed through the lens of optimizing lap time. But for a track like Nashville, the end of the two long straights are also great overtaking spots. If a driver needs to save fuel but also defend their track position, a big lift and coast at the end of each straight leaves them vulnerable to being passed. In these situations, drivers will do their fuel saving in parts of the track where overtaking is impossible, like the back section of Nashville from T4 to T7. While this certainly isn’t ideal for lap time, it does allow the driver to hit the fuel target while giving them the best chance of maintaining position. The optimum fuel save approach will always depend on how much fuel needs to be saved, but the characteristics of the track and traffic are just more variables for drivers and engineers to consider as they hone in on a best solution.

The driver

A deeper look at these overlays can also begin to show why resorting to lift and coast requires a change in driving style, and why some drivers are better at it than others. Adding a lift at the end of the straight obviously lowers the entry speed compared to pushing at 100%, so drivers can actually brake later when doing lift and coast and still achieve the same apex speed. This means that braking is done over a much shorter distance when saving fuel, which changes the required peak brake pressure and the bleed-off technique that should be used. Because there is only a finite amount of tire grip to be allocated for both braking and turning, the knock-on effect of changing braking technique is that the steering technique to rotate the car on entry changes as well.

The most effective drivers are able to stretch their fuel with virtually no penalty to their lap time. Joe Skibinski/Penske Entertainment

In fact, the driving style through the entire entry phase to the corner changes when doing lift and coast: switching from a late braking, aggressive style towards being more ‘efficient’ in the way drivers simultaneously slow down and rotate the car in order to scrub off as little speed as possible. There is no fuel saving done whatsoever from apex to exit, so despite having to change the way they approach the entries, drivers need to be sure that minimizing time loss on entry doesn’t compromise the exit phase. Depending on a driver’s ability to adapt their style, some teams are better off using engine modes for the same fuel target that other teams might choose to lift and coast. Ultimately, each team is going to do whatever is fastest for them.

As fuel targets often change throughout the race, lift and coast distances (and thus entry speeds) will also change. Drivers that can adjust their style the quickest are at a huge advantage in these situations. If strategy dictates going directly from pushing flat-out to a massive amount of fuel saving, a driver that takes a couple of laps to settle in to such a big shift may lose heaps of time to drivers that can adapt at a moment’s notice.

Driver consistency can also make a big difference. Once a lift and coast distance is determined, a new braking point is found, and the driving style has adapted accordingly, being able to repeat this every time gives a competitive advantage over drivers who may struggle to drive the car the same way each lap. Consistency is obviously beneficial in many aspects of driving, but when it comes to minimizing losses from fuel saving, it becomes even more critical.

Finally, a driver being good at fuel saving isn’t dictated purely by their driving technique. Before each race, engineers and drivers will review the best combination of engine mode and lift and coast distances in each corner for a series of possible fuel targets. Once a fuel target is called out on the radio, the driver should know what engine mode and where/how much lift and coast to do. Still, when adjusting to a new fuel target there is always a period of feeling it out. In the heat of the moment, drivers with the ability to recall the most efficient approach to save fuel for a given fuel target can gain time on drivers that might need a couple of laps to arrive at the most efficient approach through experimenting, or drivers that may require some coaching on the radio.

All this to say, drivers can have a huge effect on race pace when fuel saving. The best drivers can save fuel and minimize their lap time loss to the point that they are almost as fast  as they are when pushing. It is truly impressive to see how drivers can adapt their braking points and driving style on a moment’s notice and then consistently run lap after lap without fault.

Running it dry

There’s an old adage in race car design that because of the compromise between speed and reliability, the perfect racecar takes the checkered flag and immediately falls apart.

Ultimately, any fuel left in the car at the end of a stint is a performance loss. That fuel is weight that was carried around the track for a whole stint to eventually serve no purpose. When this happens, a higher consumption could have been used to complete the same number of laps. Every car in the paddock has an engineer dedicated to fuel strategy during the race: they are typically calculating and recalculating fuel targets in live time, updating pit windows as the race goes on, all in order to run the car as close to empty as they dare at every opportunity. While it may sound obvious, the best way to go fast while saving fuel is to save less fuel! Still, no sensor is perfect and so engineers will always want to err on the conservative side if they can’t be exact. Fuel left over in the tank is certainly better than running out on track.

The perfect stint means the car gets to pit lane, or finishes the race, practically dry. These engineers are playing the world’s scariest game of the The Price is Right: get as close to the perfect amount without ever going over. Run out of fuel and they’ll be out of the race, leave too much fuel in the tank and they are leaving performance on the table. While it’s not a particularly popular move with mechanics, when it comes to threading that needle, finishing the race – but not the cooldown lap – is an engineering gold star!