Auto Electrics
PART SEVEN: Petrol injection – explaining mixture calculation and emissions control. Jack Moore is our guide.
Part 7: Petrol injection.
Mixing and igniting air and fuel is the basis on which engines operate, with the ratio of air to fuel directly influencing engine performance, fuel efficiency and exhaust emissions. In a modern fuel injected engine, it is the job of the engine control unit (ECU) to monitor and control this mixture. For complete combustion to take place in a petrol engine, the theoretical ideal ratio is about 14.7:1 by weight; that means you need 14.7 grammes of air to completely burn 1 gramme of petrol. If the mixture is lower – 12.5:1 for example – this is considered a rich mixture; if it’s higher – say 18:1 for example – it’s a lean mixture. For exhaust emissions and fuel efficiency purposes, most modern petrol engines work somewhere close to the 14.7:1 ratio, usually fluctuating above and below it. In order to control and monitor the air/fuel mixture, the ECU receives information from various sensors. Two of the key sensors are the oxygen sensor, sometimes called a Lambda sensor, and the mass air flow or MAF sensor.
Mass air flow meters are probably the most accurate way to measure the air intake. They can compensate for changes in air density and reverse air flow. They operate by passing an electrical current through a ‘hot film’ sensor element, heating it to 200°C above ambient temperature. The incoming air passes over the film and cools it. Electronics within the meter increase the current passing through the film to maintain it at 200°C above ambient, with the increase in current proportionate to the quantity of air passing through the meter. In a digital MAF sensor this value is converted to a frequency signal and sent to the ECU via the engine wiring harness. Symptoms of a faulty MAF could include lack of power, poor acceleration, excessive smoke, excessive emissions and the presence of specific fault codes.
In order to monitor the actual air/fuel ratio, the ECU receives a voltage generated by the oxygen sensor. Oxygen sensors can be narrow-band or wide-band type. The sensor is mounted directly in the exhaust pipe and generates a voltage based on the amount of oxygen remaining in the exhaust gases after combustion. Narrow-band sensors do not require a supply voltage and are typically used downstream of the catalytic convertor to measure its efficiency. Wide-band sensors are used upstream of the cat as they can provide more accurate feedback on the air/fuel ratio. Sensors are fitted with heating elements in order to quickly reach operating temperature; these heaters are often the cause of sensor failure.
The ECU is programmed with sophisticated software to enable it to deliver the optimum air/ fuel mixture for a range of operating conditions. In addition to air mass and oxygen, it monitors several other sensors, including coolant temperature, intake manifold pressure, engine speed, driver demand, throttle position, etc. The MAF sensor signal is used to calculate engine load and adjust the quantity of injected fuel accordingly. Feedback from the oxygen sensor enables closed-loop control of the mixture.
Solenoid-type injectors have an internal copper winding that, when energised, creates a magnetic field. This magnetic field lifts an internal needle off its seat, allowing pressurised fuel to flow through the nozzle, which has numerous tiny holes that atomise the fuel into a fine mist as it leaves the injector. This enables the fuel to mix with the incoming air for better combustion. The period of time that the injector is energised determines how much fuel will flow. A typical opening time for an engine running at idle is about 2-3 milliseconds. Varying the opening time is the primary means by which the ECU adjusts the air/fuel ratio.
The ECU performs complex calculations and precision adjustments to achieve a balance between power output, exhaust emissions and fuel economy. A richer mixture can provide more power, but carbon emissions will be higher and fuel economy will suffer. Too lean a mixture will reduce carbon emissions and improve fuel economy, but power output will reduce and harmful NOX gases can be created.