24/07/2021
Injectors.
Increasingly these days the market is awash with dodgy fakes being sold, not only on platforms like Amazon and eBay but are also sold by some supposedly bona fide companies who you’d expect to care about their reputation. It appears that they don’t think they’ll get caught out selling fake parts because “they’re just the same”. The situation is getting so bad that it’s actually becoming difficult to buy genuine parts. The fakes are much more profitable and, because greed comes with a degree of blindness, suppliers tend to stock rip-offs instead of genuine parts.
Injectors are an absolutely critical part of the smooth running and performance of your engine, of that there is no doubt, so why cheap out with a critical part on your expensive build?
“What’s the difference anyway?” you may ask.
Poor delivered fuel quantity control leads to uneven mixture between individual cylinders and that means that each cylinder won’t perform as well as it should, even if the overall mixture coming out the back is ‘perfect’. Half of the cylinders running rich and half running lean by the same degree will average out between them and may appear to be ‘perfect’ overall, but all cylinders will be far from perfect, won’t perform well, nor will the engine run as smoothly as it otherwise would.
Solenoid injectors (which account for more than 99% of them) are fed with current to open and they are closed by a spring and fuel rail pressure combined. Because the valve is a physical thing that has to move they don’t open or close instantly. The forces act on the mass of the moving parts of the injector and the mass accelerates one way or the other depending which force is the greater. Injectors SHOULD close more quickly than they open, and the difference between opening and closing time is usually called ‘dead time’ or ‘latency’. Because opening should always be slower than closing, dead time should always be positive and time can be added by the ECU to the linear part of the injector signal to account for it.
Technically speaking, dead time is the difference in the areas of the displacement curves but ‘time’ will do for the purposes of this explanation. The dead time is thus an OFFSET added to the injector signal to make the delivered fuel quantity analogous to the injector duty cycle calculated from measured air mass, VE etc.
So, if you want an injector which has a dead time of 1 ms at the prevailing voltage to deliver 5 ms worth of fuel you send current to it for 6 ms (5+1). Likewise, if you want to double the delivered fuel quantity then you would open it for 11 ms (5+5+1).
One of the biggest problems with Chinese rip off injectors is that the closing springs usually have very variable installed strength, so the dead time of them can often be negative when measured (by halving the duty and doubling the frequency, and vice versa). No ECU that I have ever come across has the facility to deal with negative dead times. This, along with nozzle hole size inaccuracy, account for the large delivered quantity variations between individual injectors, and the poor running that goes with it.
Poor spray pattern and too large a fuel droplet size both contribute to excessive unburnt fuel, which brings a loss of combustion efficiency. Power is reduced whilst fuel consumption and Hydrocarbon and Particulate emissions increase. Large droplets have a lower ‘surface area to volume ratio’, so there’s less of the fuel molecules adjacent to oxygen molecules that are required for it to burn. Likewise, but in a different way, if the spray pattern is bad and is spraying on the port wall, floor or roof, the surface area to volume ratio becomes very low because there’s only one side of the fuel film adjacent to the air that is supposed to be carrying it into the cylinder and it basically just dribbles into the cylinder over the valve seat and doesn’t get too involved in the combustion because it’s just lazy and lying around.
OK, so that’s the theory bit talked about, so how do we tell if we’ve been ripped off (or foolish enough to think we really can get new, genuine parts for half price)?
Very often I can just tell from the ‘feel’ of the motor after a while. I’ve seen it so many times that I’ve learned to know when to ask the question “Where did you get your injectors from?” Unfortunately, a few hours of dyno time have usually been wasted by that point.
Rough running, higher than expected hydrocarbon emissions, spitting back through the throttles even though the overall mixture is OK, the throttles balanced and the ignition timing are where they need to be, etc etc. Sometimes the symptoms are caused by something else, tight valve clearances for example, but more often than not it’s crappy injectors.
Me ‘having a feeling’ isn’t much use to you though is it, so how do you tell?
Testing for dead time differences isn’t easy without equipment but an easy tell is the visual quality of them. Genuine injectors tend to have very ‘cleanly’ moulded plastic parts, especially when you compare the finer areas and the edges of holes. Rip offs tend to be made using inferior plastics, in inferior moulds, so the edges have flash and rough bits, sometimes the symbols and characters on the injectors are misaligned, not square or twisted. The ‘+’ symbol denoting the positive side of the injector is often off-centre in the recess, and/or twisted from the position that it would be in on a genuine part.
If you have a microscope, or at least much better eyes than I do, you can tell for certain by looking at the delivery nozzles. The genuine ones have sharp-edged holes that promote the formation of fine droplets, often the centre is slightly raised to create a ‘cone’ pattern, or has a clean line down the centre for a ‘split’ spray pattern, which the copies don’t have. The ring of weld around the edge looks so nice that you’d expect it to be impossible to be made like that. The copies have much rougher-looking welds and sometimes have ‘spatter’.
Here are some pictures that I took earlier of a genuine injector and a copy. The difference is obvious.
