SMALL ENGINES
CARBURETOR SYSTEMS VERSUS EFI SYSTEMS FUEL FILTER
CARBURETOR
FLOW FUEL PUMP
MANIFOLD FUEL TANK
CARBURETOR SYSTEM PRESSURE REGULATOR
FUEL PUMP
INJECTOR
IGNITION COIL
FILTER
FAST IDLE MECHANISM INTAKE TEMPERATURE SENSOR TO ENGINE ELECTRONIC CONTROL UNIT
ENGINE TEMPERATURE SENSOR CRANK ANGLE SENSOR
EFI SYSTEM
Figure 5‑38. Electronic fuel injection systems are installed in small engines, rather than carburetor systems, in order to improve engine efficiency and emissions.
Diesel EFI Systems Up until the 1980s, diesel engines operated by atomizing the fuel as it entered the cylinder using high fuel pressure and spray nozzles. In the 1980s however, EFI systems, which allowed injection pressures up to 22,000 psi, be‑ gan replacing the mechanical systems, which could only handle injection pressures of about 5000 psi. In a typical diesel engine fuel injection system, a low‑pressure supply pump delivers fuel from the fuel tank to the injection pump, which pressurizes, times, and meters the fuel charge and then delivers it through a tube to an injection nozzle, which sprays the pressurized fuel into the combustion chamber. The high fuel‑injection pressure is produced by an engine‑driven reciprocating piston pump either in the injection pump assembly or in a unit injector. 134
The two types of injection pumps used with diesel engines include an in‑line pump and a distributor pump. An in‑line pump has at least two pumping elements that pressurize the fuel and time and/or meter it. Each pumping element (a combination of a pump plunger and its barrel) serves one engine cylinder. A distributor pump uses one pumping element to pro‑ vide metered delivery to all cylinders. The plunger stroke delivers the fuel to a rotating distributing device, which then directs the fuel to the appropriate engine cylinder. The number of plunger strokes during one revolution of the of the injection pump driveshaft corresponds to the number of engine cylinders. EFI Advantages The main advantages of fuel injection can be broken down into three categories: improved fuel economy, reduced emissions, and improved performance. In gasoline engines, fuel injection systems are intelligent and have a control center, commonly referred to as the electronic control module (ECM). The ECM works with several sensors on the engine to monitor its operating conditions. All information from the sensors is then fed to the ECM so that it can make adjustments that will allow the engine to operate efficiently. See Figure 5‑39. The ECM contains base operation parameters as a starting point for various functions it controls. The overall basis for engine operation is often referred to as the calibration. From this starting point, the ECM is designed to modify the standard calibration based upon dynamic input from a variety of sensors. The dynamic adjustments to operation parameters occur for the pur‑ pose of continually trying to operate the engine most efficiently in the given conditions. Contrary to how engines with carburetors operate, the fuel is not exposed to the intake manifold for enhanced atomization where it can often become stuck to the walls and never even make it to a cylinder. With fuel injection systems, the fuel is atomized as it leaves the tip of a high‑pressure fuel injector. Liquid fuel doesn’t burn, but fuel vapor does. Therefore, more fuel broken into small droplets (vapor) as it’s placed into the moving air stream means more fuel carried into the cylinder. The fuel will then easily ignite and burn more efficiently. If the majority of the fuel is reaching each cylinder, then there will be little waste, and the engine will often yield more power from a given fuel quantity. Also, the ECM allows the fuel mixture delivered to each cylinder to be individually tailored (more fuel mixture delivered to one cylinder, less delivered to the other cylinder) to further optimize overall engine operation.