As emission laws got tighter it was neccesary for manufacturers to achieve better results from their engines. They did this in many ways and the battle for better economy and lower emissions wont stop!
There have been lots of improvements, but I will stick to the ones that matter the most for fiting an LPG system.
Both of the systems mentioned on the previous page are multipoint injection, as they have one injector per cylinder, so it would be possible to use a multipoint injection LPG kit. However the benefit of this type of conversion over that of using an induction or 'mixer' system is limited. This is because these engines are full group injection (all injectors fire at once) and half group injection ( four injectors fire, then the other four.)
There are a couple of changes that made multipoint injection LPG kits essential. The most obvious is the use of plastic inlet manifolds, many manufacturers started using these as they are cheaper to make and a lot lighter than a cast alloy manifold.
The problem is that an LPG induction kit works by fitting a mixer to the air intake of the engine, this in turn fills the inlet manifold with a perfect gas and air mix to go BANG in the cylinder, which is fine until it goes bang a bit early! This is known as a back fire and can be caused by many things. The most common cause is either a weak mixture (not enough gas and to much air) or, bad ignition. This can be due to worn spark plugs, old HT leads or distributor cap/rotor arm. Basicly any part of the ignition circuit. This can be a problem on any vehicle, but if it happens to one with a plastic inlet manifold it can be a serious problem! A split inlet manifold can cost a lot to replace and can be split open again in a quick bang.
The other advance in technology that made multipoint LPG neccesary was the increased accuracy and control of the petrol fuel mixture. Any vehicle built after 1993/4 had a catalitic converter fitted into the exhaust, this filters the exhaust gasses to reduce the harmful fumes created by burning petrol. The lambda probe, or oxgen sensor is fitted before the cat so that the engine control unit (ECU) can adjust the mixture of petrol and air. Later a second lamda probe was fitted after the cat so that the ECU could check that the cat was doing its job and cleaning the exhaust fumes.
This system demanded greater control from the ECU and computer technology was there to help. This type of control is called OBD, quite simply, on board diagnostics. All modern engines use this and in fact OBD2 is now very common.
So the closed loop injection system was now even better and its works like this:-
The ECU monitors many things, the volume and temperature of the air entering the engine, the throttle position, cam position, crank position and engine timing which it increases to achieve the best possible burn. Knock sensors fitted to the cam shaft tell the ECU when it is too far advanced. And, most importantly to us, sequential petrol injection timing.
Petrol injectors have two wires, one is an ignition fed live which is common to all of the injectors, the other is a negative which runs from the ECU to the injector, each injector has its own negative, so by tapping these wires down to earth the ECU can control how long each individual injector is held open for.
Petrol is added to the cylinders via the injectors, the ECU then reads the signal from he lambda probe to see if the mixture is rich or lean, if it is lean it increases the injection timing, which opens the injectors longer thereby adding more fuel. When it sees that the mixture is rich it decreases the injection timing and reduces the fuel. This fuel timing is very important to our LPG system.
The sequential Multipoint LPG kit reads this petrol timing by interrupting the signal from the cars ECU to the petrol injectors, this is done quite simply by cutting the negative of each petrol injector and connecting a loom to the two cut ends of the wire.
So, the signal to the petrol injector is sent by the ECU, but before it gets there it goes into the new gas ecu which reads the timing. If the system is set to run on petrol then the signal simply travels back down the other wire to the petrol injector as normal. However if the system is set to run on LPG the signal is stolen by the gas ecu which converts it and fires the relevant LPG injector (for more detail on how the LPG system works see Sequential Systems) . A greatly reduced signal is sent to the petrol injector which is not strong enough to open the injector, but is enough for the petrol ECU to not see a fault. The car always thinks it is running on petrol. This means that by reading this one signal or injection timing, the LPG ECU gets all the benefits of the very clever petrol ECU which is monitoring all the different sensors on the engine to control the mixture.
The petrol injection timing is constantly adjusting to keep the mixture correct, but it does have limits. These limits are called fuel trim.
Example
Let us say that the petrol injection timing at idle is 3.00 milli seconds
The Petrol ECU will be able to increase or decrease this timing by 25%
If the mixture is lean the ECU will start to increase the timing - 3.02 - 3.10 - 3.45 and so on until it reads a rich signal, then it will decrease the milli seconds.
If the ECU adjusts the injection timing but does not see a rich signal it will keep adjusting until it reaches the maximum of its fuel trim, in this case 25%.
If it reaches this point and the mixture still doesn't change it will store an error in its memory and switch on the engine check light. If an induction LPG Kit is fitted to a vehicle with this advanced OBD it will not be able adjust quickly or precisely enough to keep up with the petrol system so check lights and fault codes can be a common occurence.
Also if the rear lambda probe (the one behind the cat) senses to much unburnt fuel in the exhaust gasses, due to a rich mixture, it will store a code and turn on the engine check light.
To read these codes a OBD reader is required. There are lots of these available on the market, but some are more advanced than others. A cheap code reader will do just that, read the code and will probably be capable of clearing the fault. However more advanced and of course expensive, devices will also allow you to see the all important fuel trim. By monitoring this whilst driving it is possible to see at which point the fuel trim is reaching its limits and make any necessary adjustment.
They will also have a greater in built memory of what each code stands for and be able to tell the user lots of information about when the fault was stored.
Example :
Fault code - PO071
Explanation - System to lean-Bank 1
By reading the freeze frame data that was created when this fault was stored, you will be able to see what was happening at the time, including engine revs, air temperature, throttle position and so on. Not all vehicles will be capable of telling you all things, but generally the newer they are the more data they will show.
For a list of common fault codes see the fault code page.