Imagine screaming down the front straight at your favorite track, when suddenly your engine begins to bog and sputter. You look over at the gauges and discover a loss of fuel pressure, and then the engine dies. You pull off track, and try to refire it but it won’t cooperate.
After being towed back in, just for curiosity’s sake, you turn hit the starter button and it fires right up, like nothing happened. What the … ?
On carbureted cars, you could have boiled the fuel, causing a condition known as vapor lock. On fuel-injected cars, it’s more like you are experiencing a condition known as cavitation. Their symptoms are similar. We sat down for a telephone interview Rob Scharfenberg, chief engineer at Fuelab, to find out the causes and cures for these phenomena. For NASA racers with carbureted cars, let’s explore vapor lock first.
“Traditional vapor lock, as in older carbureted systems, the fuel is boiling because the fuel line is running near the engine and it prevents fuel from flowing because you’re producing pressure from the vapor itself and that’s preventing fuel flow, and that will stop an engine,” Scharfenberg said.
Fuel boils at anywhere from 100 to 400 degrees, according to EngineeringToolbox.com. To correct the problem, it could be as simple as insulating fuel lines that supply fuel to the carburetor. It also could mean rerouting them away from heat sources. You want to reduce a fuel line’s exposure to heat sources anyway, Scharfenberg said, because hot fuel decreases power output. Here’s why.
“Just as cold-air induction increases horsepower, the same can be said for the fuel going in there,” he said. “The colder it is, the more mass of reactants per volume that the engine is pumping is higher. So if you look at an engine that’s only capable of so much volumetric flow rate, if you want more power you can increase the mass that it’s getting. The colder the fuel the more mass per unit of volume.”
Cavitation exhibits similar symptoms, but can result in damage severe enough to warrant replacing the fuel pump. When conditions are right, fuel will vaporize within the fuel pump itself. The higher the temperature of the fuel, the more likely it can happen. Likewise the more restriction on the fuel pump inlet the more likely it can happen. Also, the higher the pressure the pump operates at, the more likely cavitation can occur.
According to Wikipedia, information Scharfenberg verified as correct, cavitation is “the formation of vapor cavities in a liquid, small liquid-free bubbles or voids that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shockwave.”
That shockwave is what does the damage. It happens on boat propellers, too. If you see a propeller with bunch of pits on the pressure (back) side, that’s damage from cavitation.
“For high-flow systems, a lot of times you see sumps instead of pickup tubes,” Scharfenberg said. “A pickup tube is more restrictive and makes a system more prone to cavitation. Sumps are used not only to capture the fuel, but their primary purpose is to make it easier to feed the pump.”
When devising a fuel system on your racecar, any fine filters, check valves or cross-drill fittings should come after the pump. You also want to reduce restriction in the inlet side as much as possible.
Fuelab minimizes the conditions for cavitation with its variable-speed Prodigy fuel pumps, which accomplishes a few things. First, it minimizes fuel return due to oversupply. Recirculating fuel transfers heat into it, which increases the chances of cavitation. Second, variable speeds mean the pump itself reduces turbulence when necessary, but also means less current draw and less heat generated through electrical resistance. Further, the slower the pump turns, the longer it takes fuel to return to the tank, which gives it more time to cool off. The Fuelab pump can be controlled through aftermarket ECUs or with the company’s proprietary regulator, which sends signals to the microprocessor in the pump itself.
“Our fuel pump, that capability is automatically built in because we have a DC brushless system. Basically, that means we have electronics built into it that control the motor itself,” Scharfenberg said. “We also have a patented regulator that actually senses engine demand by looking at return line flow rate that comes through the regulator. A pressure transducer monitors the return rate. Below a certain value it increases supply. Above a certain value, it decreases supply.”
That’s probably more information than you need on fuel supply and demand, unless you’re that guy in the car that quit on the front straight. Then it might be just enough.