The only way to adjust the air/fuel mixture on electric fuel injection is to remap the ECU.
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The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline fuel, the stoichiometric air–fuel mixture is about 14.7:1 i.e. for every one gram of fuel, 14.7 grams of air are required.
Normally fuel injectors need about 46 to 65 psi of fuel pressure to make them operate. The GDI fuel injectors will require upwards of 2000 psi. They require more pressure because they have to over come the high pressures of the combustion chamber.
The theoretical air–fuel ratio mixture for a gasoline fueled engine, for complete gasoline fuel combustion, the stoichiometric air–fuel ratio is about 14.7:1. In order to completely burn 1 kg of gasoline fuel, the combustion process needs 14.7 kg of air.
This method involves looking at the base of the spark plug insulator (white part of the plug) for a slight coloring on the insulator just above where the insulator comes through the steel case. If the mixture is too lean, it will leave no color, while a rich mixture will cause the fuel ring to become more prominent.
On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture. Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.
It`s important that all idle mixture screws be adjusted the same way so that the output from the carburetor idle circuit is balanced across both idle mixture outlets. One to 1½ turns out is a typical place to start.
Direct injection pressure is measured with sensors, and the signals are used to determine pump speed and/or volume. So, you`ll need a scan tool to look at the pressures. Most direct-injection systems use piezoresistive pressure sensors on the low side of the system.
For optimum fuel economy 16-17:1 is usually best, leaner than that and the car will begin to misfire. Maximum power is usually found between 12-14:1, but this may be too lean for safety on many engines. For maximum reliability at full power, air fuel ratios from 10.5-12.5:1 are considered best, depending on the engine.
The 14.7:1 ratio is perfect for idling and light throttle cruising conditions as it`s the most efficient mixture possible, meaning the best fuel economy and lowest emissions.
The key difference between lean and rich fuel mixture is that we use a lean mixture for maximum efficiency while we use a rich mixture for maximum power in an engine.
Lean Air/Fuel Mixture
Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn`t have enough gasoline can cause a backfire, too. A “lean” mixture is one that doesn`t have enough fuel, and too much air.
Adding in extra fuel prevents detonation and just generally makes the engine happier. So, yes, if you lean out your engine, going from 12:1 to around 14:1, your engine will run a bit hotter.
A lean fuel mixture occurs when there is too much air or not enough fuel in your engine. This can result in your car`s engine not operating optimally. Since there isn`t enough fuel being burnt, your car may stall, operate roughly or struggle to accelerate.
Tip: The factory position for most air fuel mixture screws is usually between 1.5 and 2.5 turns out from being screwed all the way in. If you ever want to start fresh, turn the screw clockwise until it is lightly seated, then back it out about 2 turns. Then you can make adjustments from this position.
For peak power, most engines like an air/fuel ratio of 12.8 to 13.2. If you can tune a carb to deliver a consistent A/F within this range, the engine will usually run at its full potential from off-idle to wide open throttle. The air/fuel ratio will change with ambient temperature, barometric pressure and humidity.
If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without “blipping” the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.
The air screw is on the side of the carburetor and meters the amount of air that makes it to the pilot-jet nozzle. A fuel screw is located underneath the float bowl and meters the amount of fuel that makes it to the carb`s main body.
Direct-injection systems can have rail voltages >100 V (as opposed to around 50 V for standard injection systems). Thus higher voltage MOSFETs need to be specified, along with voltage boost power supply circuits. Piezoelectric injectors also require high voltages to be activated, typically 200 V or above.
In order to increase fuel delivery in a shorter amount of time to produce more power, the fuel pressure is increased. The fuel rail pressure on a GDI engine typically ranges from 300 PSI at idle to 2200 PSI at full load.
The static flow rate of an injector is determined by holding the injector fully open (no pulsing) and measuring the total volume dispensed over a given time. The static flow rate is dependent on fuel pressure, the higher the pressure the higher the flow.
Too much fuel, too rich a mixture, produces too much cooling and excess unburned fuel may accumulate and foul spark plugs, valve guides and other important parts. Running the engine too cool or allowing fouling to occur reduces power output.
As air density decreases with increasing altitude, the mixture control is used to reduce the amount of fuel entering the system to maintain the correct fuel/air mixture.
Generally, engines with compression ratios of 9.3 : 1 or less will safely operate with unleaded 87 octane fuel. Engines with higher compression ratios usually require higher octane fuels. Many owners who operate vehicles designed to operate on 87 octane fuel experience ping and knock.