Last Modified: April 30, 2013
Contents: Basics; Coil Over Plug; Fuel Injectors; On The Horizon;
Without doubt, the most pervasive RFI (Radio Frequency Interference) noise sources in any modern vehicle (diesel or gas) are the ignition and injection systems. These systems contain a coil of wire in the form of a transformer and/or electromagnet. When the coil field collapses (after the supply current is removed), a CEMF (counter-electromotive force) pulse is generated. It is this pulse which generates the spark across the spark plug's gap, and generates the majority of the RFI. A goodly portion of the RFI is radiated not only by the secondary wiring, if any, but into the primary wiring as well. While steps are taken to minimize the level of induced RFI, it nonetheless plays havoc with amateur radio receivers.
We've come a long way since the early days of mobile radio, including the methods we use to quell ignition RFI, which the old Allen Bradley, QST advertisement at right exemplifies. Then, as now, we know the best place to cure RFI is at its source. Nowadays, we use both resistive ignition wires, and plug with resistors built into them. In an effort to explain why we use resistors, here are a few words from Mike Dale, a senior editor for Motor Magazine, and a well-known automotive engineer.
Basically the source of the noise is the spark gap itself. By definition, the spark gap is a broad spectrum noise generator. Early spark Gap transmitters worked on this principle.
There are a couple of basic strategies to suppress this noise. The concept is to make sure it cannot reflect back from the gap and into the coil or B+ wiring system. For those vehicles with spark plug wires it is about making those wires conduct while at the same time making them poor radiators.
The first step is the plug itself. There is a distributed capacitance of about 9 pf that is the result of how the plug is constructed. The idea is to put a resistor inside the plug to make the path to ground for the high frequency noise prefer the impedances of that capacitance as opposed to the resistance placed in series with the path back to the coil. Basically it is a high frequency shunt off to ground.
As we all know, automobile manufacturers are faced with ever increasing EPA demands to reduce exhaust emissions, and increase mileage figures. They have to weigh these governmental demands, against the demands of their customers for performance, reliability, value, and content; not an easy task by any account. As a result, very few new vehicles utilize high-tension ignition wiring, as most have switched over to Coil Over Plug (COP) technology in one form or another (General Motors' Gen III and IV small-block V6s and V8s are exceptions).
For the most part, COP ignition systems are much quieter than wired ones. If you're still plagued with ignition RFI, there are a few things you can do, and some you shouldn't do! Bonding the various bolted on pieces, particularly horizontal ones like the exhaust system, helps keep the RFI under the hood. Some forms of RFI, AFI, and EMI, may be caused or exacerbated by ground loops, so proper wiring practices are important too. If your vehicle still uses high tension ignition wiring, and replacement parts should be direct OEM replacements. Using non resistor wire and plugs will increase the level of RFI, as will shielding in any form. If nothing else is apparent, reducing ignition RFI is not a one-step, cure-all process. Just remember the age-old adage... If at first you don't succeed, try, try again.
Coil Over Plug
In the days gone by, every internal combustion gas engine was equipped with spark plugs, ignition wiring, a coil, and a distributor. Or perhaps a magneto combining some of the aforementioned parts. There were tried and true methods to eliminate or at least reduce the ignition hash we all endured. As mentioned above, after the first EPA rules went into effect, automobile manufactures have been scrambling to meet the ever-tightening emission and mileage standards. As a result the distributor was the first to go, plug wires are disappearing, and being replaced with Coil Over Plug (COP) assemblies.
As we learned above, this first line of defense is the spark plug itself. Mr. Dale: ...the voltage required to ionize the gap is between 8 KV and 20 Kv. The required voltage is very gap “shape” sensitive. Sharp edges are what is needed. An ideal gap would be two fine points pointing at each other. We don’t do too much of that because the shape is fragile relative to the combustion forces. In practical plugs what is wanted is a cylinder with sharp edges facing a ground leg with similarly sharp edges. Worn plugs are worn because over time and in combustion heat-pressure, these edges wear and round off. The idea of Platinum plugs is that they are resistant to this erosion and stay “sharp” for the life of the plug/engine.
These are my words, not Mr. Dale's, but excessive RFI from the ignition system is indicative to worn plugs and/or wires. It has always been prudent to replace them as a first step in noise abatement, especially if the mileage is over 25,000 or so.
The second step is actually built into the COP unit itself, a fact which some might find hard to believe. Inside the odd looking blocks of plastic, are coils, capacitors, inductors (in the form of a spring), and even a chunk of ferrite material. Here's Mr. Dale's take.
The second step is the spring and the ferrite rod. In this case the number of turns surrounding the rod are controlled by the way the spring is wound. The rod is “lossy” and the high frequency noise is dampened by eddy current losses in the ferrite. The combination of the turns and the rod make the suppression frequency sensitive with the goal of making the normal range of radio communications safe from the noise generated at the plug. Effectively the structure of the cylinder head and the way it surrounds the “boot” of the coil also absorbs and shields the spring/boot combination
The third step is actually on the B+ side of the coil [nominal 13.8 vdc]. There is on most vehicles a 0.22 mfd cap to ground whose job it is to shunt any noise that does come back to the coil and try's to transform back to the primary side of the coil. We want to get that noise off the B+ line to ground before it can radiate back into the rest of the vehicle’s electrical system.
If you are having noise problems from the ignition, it is probably that one of these defense mechanisms is not working. Some models did not include the ferrite rod in the spring. Installing the wrong plug, without the internal resistor, can cause a lot of trouble in terms of RF noise. Sometimes the 0.22 mfd shunt to ground gets lost or disconnected or whatever. The first step is to make sure these defenses are in place and functioning.
In the following paragraph Mr. Dale refers to the shielding heretofore mentioned in this article. For those who haven't seen the shielding, here is an example at left. The reason this is no longer recommended, will become evident.
The downside to the shield you show is that it increases secondary capacitance. In inductive storage coils, this results in a reduced output. Probably not horrible but less than design intent. A second disadvantage is that we designed the coil to be at least 6mm away from any metal or ground planes. This is all about hanging on to 35Kv in a small package. We did the best we could with the insulation, but you really need that spacing to help. A third thing to note is that your shielding comes very close to the boot to housing interface. We relied on the compliant seal between the boot and the housing to help seal the high voltage that is present at the Christmas tree. The [Christmas tree] is that little piece of brass that the spring contacts at the output of the coil. It has a serrated edge that grabs the turns of the spring.
My suggestion to you is that the best way to suppress RFI is just as you say; at the source. In this case the source is the spark gap at the plug. The defenses that are there are pretty effective if they are all in place and hooked up. You could add the spring and ferrite from a junkyard coil if your particular model didn't come with that. You could also check to see that you have resistor plugs and that the resistor inside the plug has not failed. Generally they are around 5K ohms if memory serves. You could even add to the 0.22 mfd another bypass cap in the 0.01 range at 400V
The latter suggestions from Mr. Dale are for those who understand the ramifications if you don't do the job correctly. If you don't, then don't!
It is interesting to note, that Ford has a new heavy-duty truck engine (2011). The 6.2 L V8 is SOHC, with variable valve timing. These facts don't make it unique, but the two spark plug design does! The two plugs are fired using a COP, but the bottom plug is fed HV by an interconnecting plug wire (see photo). Whether these short jumpers will cause additional ignition noise, remains to be seen.
Every modern engine uses some form of fuel injection, yet few people know how they actually work, or even what they look like. The ones shown at left are made by Delphi, one of the largest manufacturers of injectors. One of the best explanations of how they work can be found on Wikipedia. It is important to remember that fuel injectors utilize an electromagnet (solenoid), and when their field collapses (power is shut off to them), there is an electrical pulse generated. Although this pulse is RFI suppressed in most cases, it can still be heard in our receivers. It sounds similar to an ignition pulse, but is slightly narrower in bandwidth. Examples of these pulses are in my Noise ID article.
In the last few years, there has been great strides in the design and applications of fuel injectors for both gasoline and diesel engines. Heretofore, most gasoline injectors sprayed fuel into the intake manifold, but the latest iterations spray the fuel directly into the combustion chamber like a diesel does. Note the difference in the design of the direct injection unit at right, and those above left.
Fuel rail pressures have increased too, and in some current systems the pressure is nearly 3,000 psi (200 bar). And think about this; one future design calls for a fuel rail pressure exceeding 21,000 psi (1,500 bar)! Imagine what would happen if the fuel line failed?
As a result of this increase in pressure, solenoid power requirements have increased dramatically, and the level of RFI from them has also increased, factory suppression techniques notwithstanding. If you can read French, you can order a document here which describes the current EU requirements for RFI suppression from fuel injectors. It is far from ideal.
Unfortunately, there isn't much one can do to further suppress the RFI generated by fuel injectors. On most V style engines, the injectors are mounted below the intake manifold and are inaccessible. On some designs, they're under the valve covers! About all that can be done is to shield their control wires with copper tape (at least the parts we can get to).
Just in case you are interested, any RFI generated by transportation vehicles, are exempt from Part 15 of the FCC's Rules and Regulations. To wit:
Section 15.103 Exempted devices.
The following devices are subject only to the general conditions of operation in Sections 15.5 and 15.29 and are exempt from the specific technical standards and other requirements contained in this Part. The operator of the exempted device shall be required to stop operating the device upon a finding by the Commission or its representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected. Although not mandatory, it is strongly recommended that the manufacturer of an exempted device endeavor to have the device meet the specific technical standards in this Part.
(a) A digital device utilized exclusively in any transportation vehicle including motor vehicles and aircraft.
On The Horizon
In the near future, laser diodes will replace the centenarian-old spark plug as the source of ignition in vehicle engines, so says Takunori Taira, of Japan's National Institutes of Natural Sciences, and his colleagues. The driving force is lower emissions, and better fuel economy obviously. In order to meet those demands, the fuel mixture will have to become very lean indeed. The leaner the mixture, the more difficult it is to ignite, and we're almost at the limit of what spark plug can do in this regard. Not only that, the spark plug's placement in multiple valved engines isn't always ideal, especially in engines sporting direct cylinder fuel injection. Laser diodes can fix that problem too.
The best part is, other than the small digital signal when the laser is fired, they're virtually RFI quiet! The reason is, there is no high voltage spark gap rich in harmonics, to wreak havoc with out receivers. Let's all hope it happens quickly!
I should point out that like LED headlights, the lasers used in ignition systems are pulse width modulated (PWM). Whether these PWM devices themselves (whatever they are) cause RFI is unknown. We'll soon know, as at least ten 2013 models sport LED headlights as standard equipment, with about double this number as an option.