Fuel Pickups
Denny Hamlin must wonder whether there are gremlins at Bristol. In 2007, he had fuel pump problems and lost the race after leading 177 laps. In 2008, he lost the lead when his car just didn't go on a restart and Denny ended up with a 15th place finish instead of what should have been at least a top five.
When a car that had been running just fine all of a sudden–often on a restart–sputters like it's out of gas, but then recovers and continues, it's likely that the problem is fuel not getting to the engine. If the car were entirely out of fuel, it would continue to slow down. The diagram at left shows the path the fuel takes from the fuel cell to the engine and then through the exhaust.
Let's start where the fuel starts, which is in the fuel cell. The picture below is from the ATL catalog. Fuelsafe and ATL are two companies that supply NASCAR-legal fuel cells. Both companies have catalogs with a lot of helpful information.
A fuel cell schematic from ATL
The fuel cell is a rectangular box filled with foam to prevent the fuel from sloshing around too much. There are one or two fuel pickups (also called "collectors") inside the box that are sometimes called "duck feet" because some company's versions are shaped like, well, duck feet, as you can see in the picture at right. There are other designs that serve the same function, but look a little different. Fuel pickups have one-way doors that allow fuel to enter and then holds it in the pickup.
The collectors (teams are allowed to have more than one) usually are located on the right-hand side of the fuel cell on oval tracks. When the car turns, the fuel sloshes toward the right side of the car. (This happens because of the centripetal force that turns the car.) The doors on the collectors prevent the fuel from coming out again on the straightaways.
This strategy can be problematic on steeply banked tracks. Liquids always seek their own level. The drawing below shows what the fuel would look like if the car were sitting still on a banked track. (Amusingly enough, this is a picture I was asked to draw during my doctoral comps. The request was to show what happens to water in a glass that is tilted, but the picture is the same regardless of whether it is a glass of water or a fuel cell. You never know when you're going to use something you learned.)
The fuel level in a fuel cell where the car is sitting still on a banked track.
When there is a lot of fuel in the tank, the banking doesn't cause a problem, as the top picture shows; however, when the fuel level gets low (as in the bottom picture), even the turning action of the car may not get enough fuel into the fuel pickup, causing the engine to sputter. The problem is more likely to happen on a restart because the force with which the gas enters the pickup changes with the square of the car's speed. Under caution, the cars are going more slowly; however, the force of gravity is the same regardless of the car's speed. It becomes a tug of war between the turning force and gravity. If gravity wins, the driver often loses.
A second problem, which was pointed out to me by Dr. Andy Randolph, is when there is something in the fuel line that isn't liquid and/or isn't fuel. Fuel vapor, air bubbles or water (remember second Atlanta last year?) all cause problems if they get in the fuel line. One of the more common culprits for sputtering on a restart is vapor lock, which occurs when liquid fuel becomes vaporized before reaching the cylinder. Under caution, fuel is in the fuel lines for a longer time because it is not being used as quickly. The inlet lines that bring fuel to the engine can get hot if they are close to the exhaust. If the fuel is moving to the engine quickly, there isn't enough time for the heat to affect it significantly; however, some of the hydrocarbons in the fuel vaporize at relatively low temperatures. As one of my engine experts put it, "Vapor is difficult to pump!" (I need to check–I think the unleaded fuel, which has higher concentrations of toluene, is more likely to vaporize at lower temperatures than the old, leaded fuel did.)
Fuel vapor makes it difficult for the fuel pump to properly regulate fuel pressure, which again leads to sputtering on the restart. The problem is compounded when air gets into the fuel lines because, in addition to messing up the pressure regulation, combustion requires gasoline as well as air (in a pretty picky ratio).
Some teams mount their fuel pumps on the fuel cell as opposed to on the engine. The idea is that doing so decreases the likelihood of vapor lock; however, any type of pressure regulating device near the engine that is expecting liquid and gets vapor will cause problems with fuel supply.
The big question, of course, is why this would affect some teams and not others. How much fuel is in the fuel tank, the type of fuel pump (some are mounted back by the tank and others are mounted by the engine), and even the way components are mounted relative to the effective net force (i.e. the vector sum of centripetal force and gravity) can impact whether or not the driver suffers from a fuel intake problem on the restarts.
Related Information
- An ESPN article mentions some of the same issues.