Thread: how td5 Injection actually works

Is there any interest in me running down how the injection setup works in the Td5 (and similar engines)

If theres interest I'll edit this post

And so here comes the first edit.

So youve got an engine and yovue been filling it with diesel and when you goto check the plugs you see these monsterously huge metal thingys with pipes on em where the spark plugs ought to be and theres these little pen like things with a wire on em near those. The pipes run back to a large metal body attached to the engine and back to the fuel tank as well. Congratulations youve just found your injectors, glow plugs and injector pump as well as the injector lines and the return lines.

So whats it all about... simple really the injectors preform the same function as the spark plug in an petrol engine they make the bang happen that pushes the piston down and the downwards motion of the piston with the help of lots of other metal bits makes the round rubber bits go round and that makes the vehicle move. No really thats what they do they dont do all that by themselves just like the spark plugs dont make the spark themselves the injectors are just one part of a seriosly well thought out bunch of wierdly shaped bits of metal, rubber glass and in some cases plastic bits that make up "The Diesel Fuel System"

ok so the key thing that seperates a Diesel engine from a petrol engine is that a diesel engine inhales clean fresh air then squeezes it lots till it gets really really hot and then adds the fuel where a petrol engine inhales a mix of air and atomised fuel and then puts a bit of a squeeze on it then lights it up with a spark. (and yes you technological historian anoraks I know that there are still some diesel engines in existance that use a vaporisor and squeezes the whole fuel air mix till it goes bang but I havent ever seen one in a comercially available car so they dont count here hmmmkay)

So in keeping with older school engines for a while (carbied petrols and Cav/VE/inline diesels) we know that the carby in a petrol engine meters the fuel into the engine to get the required power and the points tell the coil when to spark and the distributor points the spark to the right pot and if we get it all right then we get nice smooth power from the engine. For your basic diesle engine if you think of the fuel injection pump being a combination of all the previous components your not far from the right track. To make it easy lets take to our hypothetical 4 pot engines with our magical angle grinders and turn them into single cylinder engines. now that weve done that lets forget about the petrol engine and lets focus on the diesel as other than the vague similarity of function they dont have too much more in common when it comes to the fuel system.

so you know where this is all heading lets start with an outline of the whole system. Whats in it and whats it do for a diesel engine. Theres 7 primary components to most early diesel fuel systems although the layout will vary from one car to another.

1. The fuel tank(s) -they hold the fuel and connect to
2. Fuel lines and valves- these get the fuel from the various components to other componants and if you have more than one tank pick which tank your going to eventually empty
3. sedimentors, seperators and filters - these make sure the fuel is clean and not contaminated with water, grit, dirt, small children and hoefully keep diesel snot out of the rest of the system by cloging up and starving the engine of fuel.
4. the lift pump or priming pump - this gets the fuel flowing around the system its a relativly low pressure high volume pump
5. the injection pump - this is the bit that does the bulk of the work nutting out how much fuel to squirt in and when to squirt it in
6. high pressure lines - these get the fuel from the injection pump to the injectors
7. injectors - these squirt the fuel into the combustion chamber to make the engine run

now Im not going to go into things like tanks filters and lift pumps here those things are all common to carbied petrol engines.

the concept of how diesel injection works is no more complicated than your average cheapy Kmart water pistol your finger represents the effort of the engine on the pump the triggery bit represents the fuel injection pump the small hose from the triggery bit is a high ressure injector line and the nozzle is your injector. Yes Im being serious. When you strip off all the govenor bits max fuel setting bits idle adjust bits and all the other trickery bits of the injector pump that control how much fuel its going to squirt your left with a plunger in a barrel that works on exactly the same principal as your average run of the mill water pistol.

Ok so we chuck in some controls that measure the fuel and at this point all we need to worry about is that they are mechanical in nature and work by allowing a small opening in the plunger to dump the fuel out of the collar back into the injector pump then on its merry way back to the tank via the return lines. so now we have some fuel at very high pressure (8000psi +) that gets directed via some valving to a high pressure injector line to wind up at the fuel injector.

The fuel injector is pretty much a glorified checkvalve and spray nozzle and providing that the injectors in good nick when the high pressure fuel turns up at one end of it then all the bits inside do their thing and open up the galleries to the nozzle that then takes the high pressure fuel and turns it into a mist that then pretty much spontaneously combusts in the heat of the combustion chamber. a simple equivilent is putting your thumb over the end of your garden hose turn it on just a little bit and as the pressure builds up little streams will start to spray out from between your thumb and the hose and then when the pressure drops off it stops and your thumb stops any air from getting back into the hose. swap out fuel for water a set of pin***** holes for the gap between your thumb and the hose and combustion gasses for air and thats what an injector does.


now unlike a carbied engine a diesel engine uses the fuel to do 2 very important things 1. it keeps the injectors cool and 2 it keeps them lubricated at the pressures and temperatures your injectors are working at they wouldnt last long without some kind of cooling system and lubrication. The diesel also lubricates the injection pump. and with all the fancy wierd shaped bits in the diesel fuel injection system there are tolerances that are sometimes meausred in the hundredths of mm. Easy to see why diesel engine have much better filtration systems than carbied petrol engines.

now that thats out there digest it for a bit while I get ready to type up the way the td5 does business.

So now that we have all the elements of a TD5 injector developed lets add some lectronicory to it and make it out of some exotically hard metals with super fine tolerances...

as you might have guessed from the lead up the TD5 injector is A unit type injector being the injector pump and the injector all rolled into one and driven by the camshaft. Where it differs is in the fact that its electronically controled.

In place of a mechanical spill port a spill valve is controled by pilot pressure which is controled by the electrical solenoid that sits on top of the injector (its the bits with the numbers on it that you can see when the rocker cover is off)

and thats essentialy the heart of it all, all the electronical geewhizadry exist for one reason to turn on and off at the right time one of the 5 solenoids that control the amount of fuel that goes into each cylinder.

how it decides all of that in the scope of this post is best left to 2 words...

"whiteman Magic"

but now hopefully you should have a basic idea of whats going on where.