Technical Tour - Fuel Systems
Most of the engine tuning on a supercharged methanol powerplant is done with the fuel system. The heart of our system is the 21 gallon per minute Waterman "Red" fuel pump. This pump sucks fuel out of the tank through a 1.5 inch main fuel line. The volume is similar to the amount of water that might come out of a shower - we use 5 gallons of fuel during each run. The basic idea of the system is to use jets to bleed away any excess fuel so that the correct amount can be delivered to the engine. This requires an adjustment to the thought process that is common with carburetors. A bigger jet leans the engine by bleeding away more fuel, while a smaller jet richens it up. Because we run at such varied altitudes, reaching as high as 10,000 feet density altitude in Denver, the fuel pump has a pump loop with a jet and a check valve that bleeds fuel away before it can even reach the barrel valve. The fuel goes from the pump up to the barrel valve which is located on the injector "hat". The barrel valve is timed so that it opens and closes in direct proportion to the throttle pedal movement. At an idle, the barrel valve is set to let a small amount of fuel get by it to the engine. This setting is done by putting a leakdown meter on the barrel valve and setting it to leak between 65 % and 75%. The higher the leakage, the more fuel is let by and the richer the engine runs at idle. The main jet is located in the barrel valve. It controls the amount of fuel that is bled away from the system through the main bypass line. There are two main feeder lines going from the barrel valve - one takes fuel to the injector hat and the other takes fuel to the manifold port nozzles. The line to the hat goes to a distribution block which then feeds 4 lines that each have a nozzle on them. These nozzles control the amount of fuel that is introduced into the engine through the supercharger. The line to the port nozzles first goes through a check valve that is set to only open at a pre-determined fuel pressure. Generally this is set so that the fuel does not go to the port nozzles until somewhere between 3,500 and 5,500 rpm. Once this check valve opens, fuel is sent to an individual fuel line for each manifold port. Each of these has a nozzle that regulates the amount of fuel introduced to that cylinder. The cylinders do not burn evenly and there can be wide variations in the nozzle sizes from cylinder to cylinder.
The next part of the system is the bypasses that actually regulate the amount of fuel as the car goes down the track. The conventional wisdom says that an alcohol funny car must be kept fairly rich while it is under the most load, which is in low gear. As the load decreases, the engine can be leaned out. In 1998 we accomplished this by setting up a "high speed" bypass that was triggered by the air pressure that was used to shift the transmission. The car would have a certain amount of fuel going to the engine on the launch and through low gear, then when the shift to 2nd occurred, the air pressure opened a solenoid that allowed fuel to be bled away. A jet is made a part of this bypass so that the actual amount of fuel being bled away can be precisely controlled. For 1999, we anticipate having a 2 timer system. This will allow the car to launch "lean" for better 60 foot times, go rich about 0.3 seconds into the run to avoid the dreaded tire shake in low gear, then go lean again about 0.5 seconds after the shift to 2nd gear. This system will obviously be more of a challenge to tune, but has the potential to improve elapsed times by another tenth to 15 hundredths of a second.
Click here to go on to the next newsletter article - "Tentative 1999 Schedule".
Click here to return to the newsletter table of contents.