The idea of direct injection, for gasoline engines, has been around for quite a while. Many newer vehicles offer it and marketing states it is the wave of the future. Direct injection offers greatly increased performance, but not without a cost.
Direct and port injection
Direct injection or DI, and the more standard port injection, are similar in many ways. Both use electronic controls to open magnetic solenoids. Fuel flows each time the solenoid opens and the computer controls the opening, for a precise amount of time. Longer injector open-time means more fuel will pass into the engine. The biggest difference is where they apply the fuel.
Port injection sprays fuel just above the intake valve, into the air flow. When the intake valve opens, gasoline and air are drawn in and burn to produce power. With DI, the design injects the fuel directly into the cylinder, using the intake valve only for air control. This allows far more precise metering, increased power and huge variability for engineers.
Fuel timing
Early engines use variable ignition timing to control the combustion event. Firing the fuel-air mixture sooner in the compression stroke increases power. Variable cam timing, on later engines, adds increased control of fuel and air in the process. With port injection, they control fuel-air mixture by the time the injector stays open and it relies on the intake valve to enter the combustion chamber.
With DI, fuel timing does not rely on the intake valve. This allows far better control of the combustion process. It also means the injectors have less time to fill the cylinder with fuel. Under high loads, the port injector may stay open. This means fuel flows for 720 degrees of crankshaft revolution (firing every other stroke.) DI must wait, until the exhaust valve closes, to avoid spraying fuel out of the exhaust. The cylinder must fill before the spark plug fires, normally about 310 degrees.
To accomplish this task, DI requires extremely high fuel pressure. The fuel rail has about 2200 PSI pressure compared with 60-65 PSI with port injection. Fuel pumps, injector pumps, lines and rails must be much heavier to handle the load.
With increased fuel control, bringing a catalytic converter to operating temperature becomes easier. Simply apply a small amount of fuel into the exhaust and the converter is hot. Spinning a turbo charger is also easier. By allowing fuel to burn, as it enters the exhaust, they increase pressure on the turbo. More pressure means the turbo spins faster and produces more power.
Potential drawbacks to DI
With engineering, nothing is free. The increases in power, come at the expense of production costs. High pressure fuel pumps and injectors cost much more than port injection components. The computer is also much more complex and expensive. Increased complexity adds an opportunity for problems. Expensive components and more potential for problems means increased down-road cost.
Direct injection has also exposes the injector to combustion heat and carbon. This will make durability and hygiene a much larger problem. The intake valve provides a shield for the port injector. With the port system, heat and carbon, in the combustion chamber, does not directly reach the injector. Replacing a set of direct-injectors may cost several thousand dollars.
Intake valve cleanliness is another potential problem. Detergent in the gasoline sprays onto the intake valve, with the port injection system. With DI, fuel does not flow in the intake. This may cause deposits to form on intake valves and require expensive cleaning regularly.
Is direct injection the wave of the future? My bet is, they will solve many potential problems in time. For me, that is a bet, not a buy at this time.
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