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Fuel Injection Absolute Basics

Blog Date - Monday, March 23, 2020

I am assuming you already have a basic grasp of how an engine works with this article. This also covers Port Injection which is different to Direct Injection.

A simply diagram showing the basics of a Fuel Injection system
The very basic FI system.

We'll start with the fuel pump. Traditionally fuel pumps just had to trickle fuel from the tank to the carb. An injection fuel pump needs to be powerful enough to ensure high pressure at the injector. 

From the pump to the injector is a high pressure fuel pipe.  

High pressure fuel pipe connector on a Kawasaki
High pressure pipes have high pressure connectors too.

The injector itself is a glorified electronic tap with a few tiny teeny little holes where the fuel comes out into the airflow in the throttle body. The tap is in the form of an electronic solenoid that opens and closes the fuel flow very rapidly and precisely. This opening/closing is controlled by the computer, the ECU, the brain. When the fuel passes through the tiny teeny holes under pressure it becomes a fine spray. This helps the fuel mix with the air before it enters the engine to be burnt.

A simple injector unit and the tiny holes that form the spray
A simple injector unit and the tiny teeny holes that spray the fuel into the throttle body.

And it really really is that simple. No? Oh, you want to know how the computer knows how much fuel to put into the engine? Why do you have to complicate EVERYTHING?!?!? Pffffft OK.

Sensors, that's how. Depending on the vehicle it could be 4 or 5 sensors through to 10 or 15. Happy? Pfffft.

You will definitely need a throttle position sensor. This tells the computer how much you have turned the throttle. Do you want to go FAST FAST FAST, a bit fast, slower or not at all? Generally speaking more throttle equals more fuel, but erm, it's more complex than that.

Throttle position sensor off a Fazer 600
A throttle position sensor. A glorified radio volume dial really.

Engine temperature sensor. This tells the computer how warm the motor is. If it's cold it might do a digital choke and throw in more fuel. When it's warm it'll back off on the rich mixture.

An engine temperature sensor
Most engine temperature sensors monitor the oil temperature.

Air Flow sensor(s). Before the throttle (Air Flow), after the throttle (Manifold Pressure (or vacuum)) and possibly more in the air box. If the computer can see how much air is going into the engine it can calculate how much fuel needs to be added. More air equals more fuel, but erm, it's more complex than that.

Air temperature sensor. Cold air is dense, warm air is thin. This affects how much fuel the engine needs. Knowing the temperature helps work out the right mixture.

A plastic housing that contains sensors and fixes to the throttle body
On the CBF the air flow, air temperature and throttle position sensors are combined in one unit.

Lambda Exhaust sensor. By monitoring the amount of oxygen left in the exhaust gasses the computer can work out if it is running rich or lean. It can then alter it's equations from all the other sensors to self correct. Ideally. Sort of. Lambda sensors are "slow" so the feedback is out of date. Think of them more as a vague guide rather than a precision response to engine conditions at all times.

Hidden under the engine the exhaust lambda sensor is protected
The Lambda sensor monitors the exhaust gasses.

Engine speed sensor. Normally the crank sensor used to organise the spark timing also provides engine speed data. More revs typically would require more fuel but once again... not always.

These are the most common sensors, others are available but beyond the scope of "basic". 

So your computer sees what you are doing with the throttle, sees how much air is flowing and how warm the air is, checks how warm the motor is, if it can be bothered it might ask the exhaust if it's running rich, does some amazing complex mathematics in a fraction of an engine revolution and then uses the result to calculate how often and/or how long to open the tap in the injector. 

See, I told you, SIMPLE!!

Oh wait, sorry, just one more thing. Tickover/Idle. 

You'll see on the diagram the light blue passageway that bypasses the main throttle body? The main throttle when closed is, erm, fully closed. To allow an accurate and small amount of air through for tickover there is this idle bypass tube. 

To really tightly control the tickover or idle there's a stepper motor that can alter the air flow through this already narrow passage. The computer now has a very accurate way to precisely control how much air gets in when the main throttle is closed. By altering the air gap and how much fuel is injected idle can be controlled to within a handful of revs. 

A small stepper motor and a plastic plunger that alter the air flow IACV
By moving a plastic slider in and out the IACV can control the air flow, thence the idle.

If you'd like to share your own explanation of something related to motorcycling - click here.

Reader's Comments

Ian Soady said :-
Very good Ren, I think you're getting the idea of this web thingy. For newcomers to FI (or I suppose EFI is what you've described as some of us may remember the old Lucas mechanical system used on Triumph 2.5 PI cars which had a distressing habit of bursting into flames) it should be very helpful. I often read about people using the throttle cable to try to adjust the idle speed which is of course totally wrong.

You might want to include A/F maps and maybe a chapter 2 about how the computer controls the ignition timing.....
24/03/2020 04:11:06 UTC
CrazyFrog said :-
Excellent, thanks Ren. Nowhere near as complicated as I thought!
24/03/2020 05:43:34 UTC
Ren - The Ed¹ said :-
I have also seen people suggesting altering the throttle stop on a fuel injection system - NO! Maybe, maybe maybe some of the really early examples were done like this but I am unaware of any modern systems where the throttle stops could or should be altered.

To be honest Ian - I know next to nothing about actual mappings. If you'd like to provide a SIMPLE starter guide I'd be happy to publish it.

Ignition timing I could probably manage, that may come at a future date.

The principal of fuel injection operation is quite simple CrazyFrog. What makes it complicated are the magic pixies that keep the smoke in the wires. A broken float bowl looks, erm, broken. A Mass Air Flow sensor never looks any different so you need new skills and more importantly data to check them. Resistances, voltages, blah blah blah. To be honest like computing there are very very few people capable of actually diagnosing and fixing problems, most like myself replace things till it works again.
24/03/2020 09:22:25 UTC
nab301 said :-
There's a Haynes book number 3514 "Fuel systems Tech book" by the late great John Robinson which I have a copy of , covers carbs and fuel injection . I've found it an interesting read and reference book.

24/03/2020 10:24:27 UTC
Bill said :-
Hi Ren I thought your basic introduction to EFI was good, you and the readers may find this article interesting.
25/03/2020 12:20:41 UTC
Ren - The Ed¹ said :-
Bill - can you email me that link because it's taking me to a generic Honda page, maybe my code is messing it up?

nab301 - and I thought I was a nerd!
25/03/2020 07:28:47 UTC
Ren - The Ed¹ said :-
I've fixed Bill's link. Fascinating stuff and it makes me want a CX500 Turbo now...
26/03/2020 07:51:05 UTC
Mark Noel said :-

Yes, we all knew that EFI was coming our way, courtesy of the Eurocrats in Brussels who, having eaten their sprouts, decided to make things more complicated for us in the name of cleaning up the environment (a noble cause) and binding us to the techno-motor mechanics who understand this stuff and charge large fees accordingly.

My only experience of an EFI propelled motorcycle was brief ownership of a Moto Guzzi V7 Special. Like all things designed in Italy it was svelte and gorgeous, rather like Sophia Loren, but it surged at low speed, unlike Sophia Loren. When trickling through traffic at modest RPM it seemed that the computer could not decide exactly how to solve the multiparametric polynomial equation that determined how much distilled petroleum to squirt through the thingy that resembles the hair strainer on my bath plug. Perhaps Sophia's flaxen locks had got in there during our warm up?

If fuel injection was introduced to improve emissions then it does not make any sense to mandate its use on engines under 250cc which are already doing an eco-kind 80+ mpg. Yet that is what we see fitted on today's small bikes. All that technology requires additional materials to manufacture, with associated environmental costs. The same applies to the new breed of cars where the engine stops after a few seconds halted at the lights, then are re-started, causing additional wear on the started motor and battery which then have to be replaced sooner, thus needing more metal to be dug out of the ground.

Until there is a real threat of being sent to Guantanamo for riding my 125cc motorcycle without fuel dejection then I will stick to machines that have carburetors, even of I do need to wear a tweed jacket, brogues and a Stadium cork helmet to look the part. Sophia was cool in that too.

Phew. Now I can climb down off my soap box made, of course, from organic, fair traded material!

02/04/2020 11:42:42 UTC

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