Signal to Noise - CD Ignition

- How Things Work / DIY Section -

Mercedes-Benz | Porsche | Motorcycles & Miscellanea | How Things Work (DIY) Section

Inductive ignition systems use the coil to store the energy that's eventually utilized to power the spark plugs. If the coil generates the maximum amount of energy it can hold before the spark is needed, any excess energy is usually converted to heat. If the coil hasn't generated the maximum amount of energy it can hold before the ignition signal is given, the coil will send what energy it does have to the spark plug. That reduced energy level results in a weaker spark. Weak spark at high RPM can result in misfire. That's because low voltage sparks typically don't have enough strength to ignite all of the fuel.

That's one of the major flaws of the inductive ignition system - the amount of time it takes to build up voltage. As engine speed increases, the amount of time available for the coil to build up power decreases. At high RPMs, there isn't enough time for the coil to generate full power. Some manufacturers partially get around this by using multiple coils.

Capacitive Discharge ignitions get around this by storing the energy in a capacitor. The capacitor takes less time store the maximum amount of energy it can hold. In fact, the capacitive discharge ignition boxes that I examined were all capable of generating full voltage throughout the rev range. Having full voltage available at high RPMs allows for more complete combustion. More complete combustion translates into less pollutants and more power. As an added bonus, capacitive discharge ignition systems generate more powerful sparks, even at lower RPMs.

There is a downside to capacitive discharge ignitions. All sparks are not created equal. Capacitive discharge ignition systems produce a much hotter spark, but the spark itself doesn't last as long as a spark from an inductive ignition system. At high RPMs, this doesn't matter. The engine is moving so fast, that a short duration spark is exactly what you want. At lower speeds, a long duration spark burns the fuel much more efficiently. To correct this problem, many capacitive discharge ignition systems also have a feature called MSD.

MSD stands for Multiple Spark Discharge. This works because a capacitive discharge ignition can store energy so quickly that it can produce multiple sparks where an inductive ignition could only produce one. Igniting the fuel with several short sparks actually works better than one long spark. With one long spark, there are times when the flame can actually burn itself out, only to reignite a few microseconds later. That results in lost power. This doesn't occur with MSD because the fuel is being repeatedly ignited. The multiple sparks burn the fuel more efficiently than a single spark. That results in a smoother idle, less pollution, and better throttle response. In most capacitive discharge ignitions, the multiple spark feature only works until 3000 RPM. At higher engine speeds, there isn't enough time to generate multiple sparks. At that point the system reverts back to the conventional single spark mode. This gives us more complete ignition throughout the entire rev range.

Because you already understand how an inductive ignition system works, understanding a capacitive discharge system is easy. A capacitive discharge ignition stores energy in a capacitor. When it receives the ignition signal from an ECU or distributor, the capacitive discharge ignition box then dumps that energy into a coil. Rather than building and storing energy in a magnetic field, the coil simply ramps the voltage up even higher as it passes the energy out of the secondary circuit and through the high-tension terminal. The rest is exactly the same as an inductive ignition, except it's much more powerful.

To some people, this may sound too good to be true. Those people may not realize that the inductive ignition system has been around since 1908. What we're seeing are the advancements in ignition technology that have occurred since then. Others may be wondering, "If this is so great why aren't the auto manufacturers using capacitive discharge ignitions?" Mercedes Benz and Porsche have been using capacitive discharge ignitions since the 1960's. Cost has probably been a big reason why other manufacturers haven't joined in. Today, an original equipment Bosch capacitive discharge ignition box for a 1974 Porsche 911 is about $1800. Trust me, those kinds of expenses are not likely to be overlooked by the bean counters at many manufacturers.

As was the case with VCRs, cell phones, and PCs, things have grown more affordable over time. I've seen capacitive discharge ignition systems from the major manufacturers for under $100. The average prices are in the $150 range. What can you honestly expect from one of these things? You car will start easier, you'll get better gas mileage, the engine will run better at full throttle, and you'll go through spark plugs at a faster rate. You won't start devouring plugs, but they won't last as long as they used to. That is, unless you used to foul plugs. In that case, they will last longer, but you really should fix whatever is causing you to foul plugs.

Rather than just suggesting you buy one, let me explain the most confusing part about capacitive discharge ignitions - the specifications.

Capacitive discharge ignitions are commonly called CD ignitions.
You may see a spec called Operating Voltage. It may say something like 7 to 16 volts. That just means it will operate anywhere in that range. The CD box will probably not function with less than the minimum voltage, and it may break if given more than the maximum. The people who really pay attention to this number are drag racers. They like to run their cars on just the battery, no alternator. They start the day with a battery that may show 14+ volts, and if they forget to charge the battery between rounds, the voltage may drop down below 10.
The RPM Range spec is only important if the number is lower than your car's redline. If it is, find another CD box.
Primary Output Voltage. This number is how much voltage the CD box will hand over to the coil - provided the coil can handle it. If the CD box is rated at 500 volts, but the coil can only hold 400, then 400 is all the coil will get. Within reason, the larger number is better.
Maximum Energy Output. People who post their dyno numbers love this number. It says how much power the CD box is capable of. Like the Primary Output Voltage, this number is dependant on the coil. The Maximum Energy Output is rated in millijoules per spark. It is often abbreviated as "mJ". If you have a turbo or a supercharger and you have problems getting spark high in the RPM band, a big number here will get the spark you're looking for. That also applies to you guys with oil consumption issues who regularly foul plugs. Although rather than a CD box, you may want to save your money for the valve job that's fast approaching.
The last spec you may come across will be whether it's an Analog or a Digital CD box. The higher-powered CD boxes will almost always be digital. Digital CD boxes run cooler and can generate higher power levels at higher RPMs.

Final Note:

The CD ignition systems I investigated had CARB numbers. The sole exception was any CD ignition system with a rev limiter. The reason for the rev limiter restriction was because ignition rev limiters stop spark. This allows raw gasoline to run right out the exhaust ports. That drives the HC numbers through the roof.

Home | Mercedes-Benz | Porsche | Motorcycles & Miscellanea | How Things Work (DIY) Section