The lowdown on how fuel injection works
Question: I was having a conversation about cars with my 14-year-old son, and the topic of fuel injection came up. He wanted to know how it works, and I began to tell him but then realized I was talking out of my a-and managed to change the subject. Could you help me out? I hope to revisit this with him after getting my facts straight. D.L.
Answer: Fuel injection has evolved through the years from mechanical systems to a variety of electronically operated (EFI) systems.
EFI systems employ an electric fuel pump, typically within the fuel tank, to move fuel to the engine at about 40-50 PSI. Fuel pressure is accurately managed by either a mechanical regulator or by altering the electrical commands to the pump. More recent direct-fuel-injection systems boost fuel pressure significantly further at the engine with a mechanical pump to pressures as high as 3,000 PSI.
Fuel injectors are an electrically operated valve — a shower head of sorts — and are accurately pulsed for perhaps two to 50 milliseconds each time that cylinder’s intake valve opens, or the piston reaches a certain position. This broad range of pulse width allows accurate fuel delivery under a wide range of conditions.
As a rule, the greater the engine load (power desired), the longer the squirt. As the engine is cranked to start or operated cold, increased pulse width is also required. Older/traditional EFI systems injected fuel into the intake manifold ports (multiport EFI) while more recent systems inject fuel directly into the combustion chamber (direct injection) at very high pressure and in a variety of pulse strategies. Direct injection increases performance and fuel economy by perhaps 20 per cent. Diesel engines have used direct injection and its predecessor, indirect injection, for many moons.
An assortment of sensors are used to measure crankshaft and camshaft position, engine and air temperature, engine airflow and/or manifold pressure, throttle position and exhaust composition, among others, so just the right air-fuel mixture and ignition timing can be computed and commanded. Monitoring the exhaust oxygen level allows trimming (adjusting) fuel delivery in compensation for factors the upstream sensors couldn’t see such as engine wear, a dirty air filter or less-than-par fuel pressure, to name a few. The powertrain control module (PCM) manages all this, striving to deliver the best balance of power, fuel economy, low emissions, and durability. The PCM also manages diagnostic monitoring of components, circuits and performance, and it lists helpful diagnostic trouble codes along with driver notification (the check engine light) as potential problems arise.
Fuel injection is only one of the many jobs performed by the PCM. Other systems it manages are the ignition system, emission control system, variable valve system, turbocharger, electronic throttle, variable intake system, transmission, air conditioning compressor, charging system, among others. It’s the seamless integration of all these functions that delivers the amazing performance we have come to know. Back in the carburetor days, it required five or six litres of engine to deliver 350 horsepower, whereas today a two-litre engine can do the same without belching emissions, wasting fuel and running poorly when cold.