Fuel Injection

Fuel injection, this is the norm these days, back int he 80's it made people say 'oooooohhh' and they thought it was something special. It made them think fast cars and power.

Fuel injection has been around for a long, long time, but only in the 90's has every car had it and only now in the new century do most cars have multipoint injection.

So what actually is it? Its basically a more efficient way to introduce fuel into the engine.

What benefit does that give? It improves power, torque and fuel economy thus makes the cars driveability better.

So its perfect then? No, not quite. Many think that fuel injection gives the exact amount of perfectly atomised fuel for the given temp, load and speed. Easy way to disprove this is the fast cold idle speed and the fact theres cold start enrichment.

Theres basically 2 types of injection mechanical fuel injection (MFI) and electrial fuel injection (EFI). EFI is much better as it uses sensors for input whilst MFI uses a fuel distributor and flaps in the ar ducting to regulate fuel flow.

The fuel injection that affects our Renaults is EFI though, its been used since the mid 80's. The engines use both multi point injection (MPI) and single point injection (SPI).

Our cars use batch fire injection, this means all 4 injectors or single injector fires for every crank revolution. This is not the most efficent way to do things, Sequential fuel injection as used most commonly known on Vauxhall 2litre 16V lumps (with red SFI on them) fires each injector only when its needed, this brings about better driveability and fuel economy, but it restorts back to batch fire at increased revs.

The difference between SPI and MPI is in the position and number of fuel injectors used. On SPI systems there is a single fuel injector mounted centrally in the inlet manifold to feed all the cylinders. On multipoint injection there is a fuel injector for each cylinder, this is mounted close to the inlet valves themsevles and the injector usually sprays at the back of them to improve atomisation.

This should tell you straight away that MPI is more efficient than SPI. SPI engines suffer charge robbing (a cylinder that doesn't need that fuel takes it and leaves the one that does lean) and poor atomisation. Also the problem of the fuel dropping out of suspension and condensing ont he manifold walls. This generally means SPI's have to run a tad richer (more fuel injected in) to compensate for these problems. The problems are real but they are not really as dramatic as they sound.

MPI is the only way to make a 'decent' engine, by decent I mean driveable, powerful but not a fuel guzzler - to an extent anyway.

The way the injection knows what to do is by taking readings from various sensors. To run a fuel injected engine it needs to know 2 things, speed and throttle position. To run with any sort of success it needs to know load as well. To run nicely and without any snatch, fear of stalling etc it needs a few more inputs from secondary inputs.

Engine speed, this is taken care by the crank postion sensor, this is mounted on the gearbox bellhousing, its an electro magnetic sensor which reads a toothed wheel fitted to the flywheel. On coil pack engines there is 1 tooth thats different to the rest and on other engines theres 2 teeth, these signify TDC and in the 2 teeth flywheel it signifies TDC for cylinders 2 and 3.

Throttle postion is taken care of by the throttle position sensor (shock horror!), this is much like a volume control on a HI-FI. Its mounted on the throttle body and activated off of the throttle spindle itself. It varies its resistance through out its travel so the ECU can acalculate what position the throttle is in.

Load is determined by the MAP (Manifold Absolute Pressure) sensor. This device is linked to the inlet manifold after the throttle plate by a pipe. The sensor itself has a diagphram inside it with a swinging arm attached which again varies resistance. It calculates load by the vacuum created in the inlet manifold. If the throttle is wide open, the engine speed is low and the vacuum is low then it must mean the engine is being heavily loaded, like pulling up a hill so it needs richening to provide more power, if the engine speed is high, the vacuum high, the throttle barely open it means the car is cruising at high speed so it can be leaned off to improve fuel economy.

The coolant temperature sender tells the ECU what the temperature of the coolant is at, if its cold then it needs enrichment to heat the engine up to a more suitable working temperature so more fuel is injected. If its hot then the opposite.

Air temperature sender, this is mounted on the air intake or the manifold and it measures the temperature of incoming air. Hot air means there is a lower concentration of oxygen molecules for a given amount of air whilst cold air there is more oxygen content for the same amount, its denser. Hot air means less fuel and cold air means more.

Knock sensor, this is mounted in the cyinder head. Its basically a microphone that listens for engine knock, if it detects any the ignition timing is pulled back until it stops. The sensor does not compensate for increased fuel RON, the ecu will run the maps it was programmed or, however cheap fuels even throguh they are meant to be a certain RON don't act as if they are so pinking occurs, in these cases an increased RON fuel will bring about a power increase back to STANDARD, nothing else.

Lambda sensor is mounted in the exhaust system, its mouted either at the collector in the manifold or in the downpipe just before the CAT. This sensor determines the mixture richness, this device switches, if its slow to switch or doesn't switch all sorts of running problems arise.

Load sensors, these are used in power steering pipework etc, they are there to raise the idle speed when something is used to prevent the engine stalling.

Idle speed is controlled by regulating the flow of air into the engine. This is done by idle control valves and idle control solenoids. On SPI engines a solenoid acts directly on the throttle spindle opening and closing the throttle as required.

On MPI engines its controlled by a rotary valve and a plunger port valve. These systems work by allowing certain amounts of air to bypass the throttle plate thus regulate the idle speed.

So thats the componenets and what they do. Due to the amount of sensors etc the fueling and ignition control can be very accurate as long as the inputs from the sensors are accurate. So if theres a problem with any of the sensors the car will not run as well as it could. The trouble is these days cars run quite well when somethings wrong, so it can be a while until it worsens before you realise anythign is worng.

What is open and closed loop?

Closed loop is when the ecu takes information from all the sensors and adjusts the fueling and ignition to suit. This gives economical running and low emmisions. Closed loop running is normally happening when the cars at idle and constant engine speeds.

Open loop, this is when the cars accelerating hard, deceleration and during warm up. During warm up the engine and CAT need to get up to temp asap, to provide, economical and emmision friendly running. To do this it runs rich, the lambda knows this and tells the ecu the ecu ignores it as the coolant temp sender is saying the coolants still cold. When the engine heats up to temp it reverts back to closed loop. When accelerating hard the mixture has to richen from the stoich mix of 14.7:1 to 13/12:1 to give best power, the lambda again would tell the ecu this is a bit rich needs leaned off a tad. The ECU is told by the MAP sensor the engines loaded and the throttle position sensor is saying the throttles full open so full power is needed. When the cars slowing down and your foot is off the throttle then over run fuel cut off kicks in, basically fuel stops being injected, this of course sends a lean signal to the ECU via the lambda but once again its ignored.

Well now you should know most of the in's and outs of fuel injection, another article will discuss independant throttle bodies and another article which will no doubt be quite long will discuss the faults and rememdies to fuel injected proeblems.