The test cell is arranged to test the engines under ideal conditions relating to the diesel fuel delivery system. The test cell fuel tanks are stationary and located six (6) to ten (10) feet above the engine. This setup provides two things that do not exist in the real world. One, positive fuel pressure to the injection pump. Two, diesel fuel free from fuel vapor and entrained air. This is directly related to the stationary fuel tank. With no agitation there is virtually no entrained air in the fuel and with the fuel being at a positive pressure to the pump, no fuel vapor is being produced!!
Real World Conditions
In the real world, the fuel tanks are usually mounted at least two (2) feet below and ten (10) feet away from the fuel pump, This configuration creates multiple problems associated with the fuel delivery to the engine. First, the injection pump has to draw the fuel from the tank by creating a vacuum. When diesel fuel is drawn by a vacuum fuel vapor is produced. Second, while the vehicle is underway the fuel is sloshing around in the fuel tanks creating entrained air. This circumstance has created a situation far from the test cell conditions which results in air and vapor being mixed in with the fuel when it is delivered to the injector. By having entrained air and air vapor in the fuel the engine does not perform at the engine manufacturer's specification. Air and fuel vapor occupy the space that should have pure fuel, thus delaying the injection of fuel into the combustion chamber. This phenomenon is better known as "RETARDED TIMING." The air/vapor present in fuel is very inconsistent making it virtually impossible to adapt the varying degrees of changes in injection timing. The retarded injections cause a multitude of problems in the combustion chamber!
