Engine Tech http://www.enginetech.maxa1.com Wed, 21 Mar 2012 05:11:07 +0000 en hourly 1 http://wordpress.org/?v=3.3.1 Ignition Spark Energy http://www.enginetech.maxa1.com/2010/04/15/hello-world/ http://www.enginetech.maxa1.com/2010/04/15/hello-world/#comments Thu, 15 Apr 2010 00:03:40 +0000 admin http://www.enginetech.maxa1.com/?p=1 Ignition fundamentals. we get into how the fire gets started . . . and different ways to initiate all the pyrotechnics

We’ve deliberately omitted discussion of the ignition process and the means by which it can be accomplished. Our reason was that it might be better to delay discussion on what happens both up and downstream of this part of an engine’s operation so that when we got to it— and now we’re there—it would be a more easily understood subject. Hopefully, this decision wasn’t a mistake.
Return with us now to inside an engine’s cylinder, after air and fuel have been compressed prior to combustion but just at the instant before ignition. Keep in mind also that what we have here is a chemical mix of air and hydrocarbons (fuel) which will undergo chemical and energy changes during the combustion process. Technically, this whole soggy mess is termed “oxidation” and might be called the combination of oxygen (air) and fuel.

Actually, at the risk of oversimplification, this is true even of fuels that are oxygen carriers (such as nitromethane).
For you students of chemistry, such compounds as carbon, hydrogen and oxygen are important “participants” in the combustion process, because they may be formed as so-called intermediate compounds during fuel oxidation. Of these, aldehydes are particularly interesting, since the “stink” from an engine’s exhaust is normally formed by the various aldehydes. Of course if you aren’t a student of chemistry (and we are not), you might think of an aldehyde as the outer skin of an aide. But this is a Series on ignition systems and processes. And we’ve digressed.

Back inside the cylinder, we suggested there was a compressed air/ fuel mixture ready for ignition. Assuming that a properly timed spark is delivered, three basic types of combustion can result. By way of review, these are (1) normal flame travel (propagation), (2) spontaneous combustion of unburned fuel (detonation), and (3) preignition or ignition of the air/fuel mixture by means other than controlled spark. Of these three, detonation is the most severe in terms of parts damage from sudden cylinder pressure rise. Preignition usually results in lost net cylinder pressure from early ignition and can sometimes lead to detonation. Most of this was discussed in the Shop Series on The Combustion Process.
So, on the assumption that we can avoid the problems of uncontrolled combustion, let’s examine first what happens when ignition is correct. Then we’ll discuss conventional ways of providing the proper amount of timed ignition spark, and maybe even a couple of unconventional methods. Keep in mind, at this point, that we’re examining the characteristics of spark ignition engines. The auto-ignition (diesel) stuff will follow.

A. Ignition voltage requirements are affected by the condition of air/fuel mixture ratios near the spark plug electrode at the time of ignition. An engine’s mechanical compression ratio also affects ignition voltage needed to initiate combustion, but air/fuel ratio typically requires a change in ignition voltage. As the chart indicates, leaner mixtures require higher ignition voltages (spark energy).
B. In a conventional spark-ignition system, a set of contact points is caused to open and close in accordance with the shape of a distributor points cam. The amount of time a set of points remains closed (current flowing through the primary side of the ignition coil) is called point dwell. The amount of time (distributor shaft rotation) the points remain open affects maximum point clearance (gap). For example, the earlier a point set is caused to open, the longer (and wider) will be the gap or unseated time. In effect, this is a form of advancing ignition timing (causing secondary voltage to be delivered to the spark plugs sooner than with a smaller point
gap)

]]> http://www.enginetech.maxa1.com/2010/04/15/hello-world/feed/ 0 Spark Timing http://www.enginetech.maxa1.com/2010/04/15/spark-timing/ http://www.enginetech.maxa1.com/2010/04/15/spark-timing/#comments Thu, 15 Apr 2010 01:51:28 +0000 admin http://www.enginetech.maxa1.com/2010/04/15/spark-timing/ Now, in a single-spark ignition engine (one per cylinder), the combustion flame begun at the spark plug travels comparatively slowly away from the plug. As the flame moves throughout the combustion space, heat released by the burning fuel causes burned gas expansion. This tends to increase the velocity of fame travel. Spark timing, therefore, has much to do with this increase in cylinder pressure and rate of combustion flame. For example, starting it too early (too much initial timing) can lead to problems of detonation, while starting it too late can cause cylinder pressure loss and reduced power. And believe it or not, probably the most influential factor affecting flame rate (aside from actual air/ fuel ratios) is turbulence within the combustion chamber . But if you’re into building race engines, you probably already know about this. If you don’t…. But back to our cylinder.
Actual spark voltage requirements vary with the amount of spark plug electrode gap and pressure conditions

within the cylinder. Even variations in air/fuel mixture ratios can affect the amount of required spark. For example, if mixture conditions near the plug are such that a lean air/fuel ratio exists, higher spark voltages may be required to initiate combustion. This is especially true if the engine is a so-called “lean-burn” version or if the mixing of air and fuel is not uniform at the time of combustion. Cold engines also require more spark voltage, since the atomization of cold fuel is more difficult than when it’s warm. The point of all this is fairly simple: Spark is required to start the combustion process. Timing of the spark is also important, as is the amount of available spark energy. So the methods of providing ignition spark and the controls used to properly time it now become the focus of our attention.
Most cars have a wet-cell battery. Some of the voltage such a battery supplies is fed into an ignition coil consisting of
two sets of windings.

As voltage is passed through one set of “coiled wires” (windings), a magnetic field much like you’ve probably observed from science class study of electromagnets is established. That is, assuming your lab partner wasn’t one of the school’s cheer leaders. Lucky for you, ours wasn’t. In fact, we didn’t have any cheer leaders. Just labs.
Now let’s assume that this flow of primary winding voltage (see illustration of basic point-type ignition system) is interrupted. Collapse of the magnetic field “induces” a voltage in the other (secondary) windings of the ignition coil, resulting in the delivery of a much higher voltage to the engine’s spark plugs. Such a boost in secondary voltage is the result of many more turns (or coils) of wire on the secondary side of the coil. This increased voltage is then delivered to the engine’s ignition distributor, a mechanical means of providing spark to each cylinder.

C. This is a schematic of a so-called induction voltage coil. But it really isn’t all that complicated in terms of what actually happens. Around a soft iron core, two sets of wires (windings) are coiled. A magnetic field (much like the sheet of paper, iron filings and permanent magnet experiment you saw back in the seventh grade) is established. Several turns of wire (primary windings) create this field when primary voltage is passed through them. A second set of windings (many more turns than in the primary set) also experience the magnetic field, such that when the primary voltage is interrupted, an “induced” secondary voltage results from collapse of the magnetic field. Because of the greater number of secondary windings, a much higher voltage results on the secondary side of the coil. And this becomes spark plug voltage. Nothing to it, right? D. A set of ignition points is not the only method of interrupting primary voltage. Various forms of magnetic “pickup” devices are now being used in certain types of ignition distributors. Spark dwell and “point open” time are governed by the width of the distributor rotors and resulting air-gap between them. Of particular interest, of late, is the so-called Wiegand trigger distributors in which a rotating “vane” (such as the one shown here) passes between a “saturating magnet” and a Wiegand module. The result (at the risk of oversimplicity) creates a pulse which causes secondary voltage output to the spark plugs. You’ll see more of this concept in the near future.

]]> http://www.enginetech.maxa1.com/2010/04/15/spark-timing/feed/ 0 Ignition Voltage http://www.enginetech.maxa1.com/2010/04/15/ignition-voltage/ http://www.enginetech.maxa1.com/2010/04/15/ignition-voltage/#comments Thu, 15 Apr 2010 02:01:20 +0000 admin http://www.enginetech.maxa1.com/2010/04/15/ignition-voltage/ And in accordance with the engine’s particular firing order, we’re now back to the combustion process. But for purposes of a clearer understanding of how this combustion spark was developed, let’s return to the engine’s battery and see what takes place from wet cell to “fire.”
In spark ignition engines, there are two basic methods by which ignition voltage is produced. One requires battery voltage input, the other uses permanent magnets and the disruption of magnetic fields to produce secondary voltage output. This second form of ignition voltage is called a magneto (so named because of its use of magnets for the production of ignition voltages). However, since battery ignition systems are the more common, let’s first examine how they develop ignition voltage.

We mentioned that battery voltage flows into the primary side of the ignition coil. Interruption of this voltage flow is required to produce secondary (plug) voltage. This is accomplished as follows: Voltage passes from the battery through a set of contact points inside the ignition distributor (see schematic illustration). While the points are closed, this flow of voltage continues through the primary side of the ignition coil until the points are caused to open. A “distributor cam” (see illustration) opens and closes the ignition points which act as a switch in the delivery of ignition voltage to the spark plugs. When the points are opened, the primary/ secondary magnetic field collapses, resulting in an “induced” voltage to the spark plugs. Timing of this voltage is governed by a voltage-conducting rotor mounted on the same shaft as the distributor cam. A metal tip is located on the outer end of the rotor so that ignition voltage is passed from the rotor to each of the engine’s spark plug wires connected to the distributor cap (see illustration).

On the chance that all this good theory is not coming into focus, let’s simplify it a little. First, compare the movement of electrical current to water. What we’re trying to do is get electrical energy into an engine’s combustion chamber at the proper time and with the required amount of force. As cylinder pressure increases (a result of high compression ratios, lean air/fuel mixtures, or early spark ignition timing), more “water pressure” or voltage strength is going to be required. This means that more ignition voltage will be needed to initiate combustion. And if you haven’t already figured out where all this is leading, an obvious solution is the use of an HEI (high-energy ignition) system.

Such systems provide the necessary voltage to initiate combustion during conditions of lean air/fuel mixtures or otherwise high precombustion cylinder pressure. As a natural result of such conditions, multiple-spark ignition systems have evolved for both passenger car and race engines. The object here is the reduction of misfire (lost power) during lean air/fuel mixture conditions, contaminated air/ fuel mixtures, or high-rpm engine operation. Such systems provide additional ignition spark as air and fuel (mixed to some degree of combustibility) move past an engine’s spark plug. Elements (and their functions) of a conventional battery ignition system include the ignition coil with a primary-to-secondary coil ratio typically of 1:100. This means, for example, that for every single coil of primary wire there will be 100 secondary coils. More exactly, an ignition coil of 200 turns of primary wire will have 20,000 comparable turns of secondary wire, usually wound in layers and insulated from adjacent layers by some form of coated paper. In order to increase the intensity of the magnetic field developed as a result of voltage through the primary windings, a soft iron core is frequently used. And to further aid the dissipation of heat within the coil unit and help prevent insulation failures among all the coils of wire, oil is often used to fill the inside of the coil assembly case.

E. Spark plug heat range is related to ignition timing and net engine output. What you can boil it all down to is the length of the heat path from electrode to cylinder head (or cooling) area and the actual surface area of plug insulator (usually porcelain) exposed to combustion heat. As a rule of thumb, the shorter the cooling path, the colder the plug temperature. The longer the path, the hotter the plug will operate. For engines of high-combustion temperatures, colder plugs are required to prevent preignition and lost power. Lower combustion heat engines require hotter spark plugs to prevent the buildup of deposits that prevent proper plug operation. F. Here you can see the relationship among components of a conventional battery-spark ignition system. Functionally, voltage passes through the primary side of the ignition coil and ignition points (closed). When the points open, the magnetic field generated by the primary voltage collapses, inducing a much higher secondary voltage into the windings of this side of the coil. Properly timed with the distributor, this voltage passes from distributor rotor to one of the engine’s spark plugs, and the process repeats according to the firing order.

]]> http://www.enginetech.maxa1.com/2010/04/15/ignition-voltage/feed/ 1 Distributor http://www.enginetech.maxa1.com/2010/04/15/distributor/ http://www.enginetech.maxa1.com/2010/04/15/distributor/#comments Thu, 15 Apr 2010 02:11:21 +0000 admin http://www.enginetech.maxa1.com/2010/04/15/distributor/ Next comes the ignition distributor, the ignition points it houses, a capacitor and some sort of timing advance mechanism (one mechanical and one, usually, governed by intake manifold vacuum). We’ve already mentioned that a distributor’s ignition points serve as an electrical “switch” for the interruption of current (voltage) flow through the ignition points. Such points are opened and closed Dy the rotation of an eccentric cam fixed to the distributor shaft which is driven by the engine (and in speed proportion to engine rpm, normally at one-half engine speed). Also attached to the distributor shaft is a rotor which is in electrical contact with a terminal in the distributor cap and secondary voltage wires leading directly from the ignition coil. As the rotor turns (sounds like some sort of soap opera), its tip comes into alignment with terminals located around the inner circumference of the distributor cap, each of which leads to one of the engine’s spark plugs by way of a plug wire. This provides plug voltage “timed” with the engine’s piston position, since the distributor shaft (and, consequently, its rotor) is related to crankshaft position, which is related to piston position—which lives in the house that Jack built!
Also inside the distributor, or at least an electrical part of its operation, is a condenser (or capacitor).

This device, sometimes called an “electrical shock absorber,” is responsible for the prevention of excessive sparking at the ignition points (when they open) and the speeding up of the magnetic field collapse (thus affecting the rate of induced voltage in the secondary windings of the ignition coil). Since the capacitor provides an alternate path for current (voltage) passage through the points at the time of point opening, collapse of the magnetic field is rapid and ignition point “pitting” is reduced.
In addition to these distributor components, there is also the capability of providing variable amounts of timing based on engine conditions. One of these methods relies on distributor shaft rotation speed and is called centrifugal advance, since a set of weights is caused to move radially outward as a function of distributor shaft speed. Working against spring tension, these weights cause the ignition points to “move ahead” on the distributor point cam, resulting in earlier point opening and ignition timing as engine rpm increase.
The other basic type of timing advance mechanism uses intake manifold vacuum. Especially during part-throttle engine operation, manifold vacuum is high. As a result, smaller amounts of air and fuel enter the engine’s cylinders.
Such mixtures are less compressed, causing slower combustion rates (burning times) and requiring earlier ignition timing for increased fuel economy. So don’t disconnect your engine’s vacuum advance if mileage is the object.
The reasons underlying an ignition’s ability to vary timing according to engine conditions can be discussed as follows: Part-throttle operation does not require the production of maximum power. Mixture compression pressures are reduced, resulting in the need for earlier ignition timing to compensate for slower combustion rates. If it burns slower, we need to get the fire started sooner to make certain a high percentage of mix is consumed.
Under conditions of wide-open throttle and low engine speed, large quantities of air/fuel mixtures are passing into the engine, resulting in greater mixture compression, faster burning rates and the need for less ignition timing (in order to prevent detonation). At such times, intake manifold vacuum drops, vacuum advance is reduced, and the engine relies more on centrifugal advance and initial timing for best combustion efficiency.

G. In a transistor ignition system, beneficial because of much reduced current passage across the ignition points (and subsequent increased point life), a conventional positive-negative-positive (P-N-P) transistor is used, consisting of a collector (C), base (B) and emitter (E). You students of electronics know this is pretty basic, so be patient with those of us who still think of current as how fast the creek flows. Part (a) shows current flow from the ground side of the transistor through the primary side of the ignition coil and back through the battery to ground. Simultaneously, a small amount of current passes from ground through the ignition points to the transistor base (B). When the points open (by action of the distributor cam), the coil’s primary field collapses, causing induced secondary voltage to pass along the spark plug(s). If it were any more simple, we’d understand it, too.

]]> http://www.enginetech.maxa1.com/2010/04/15/distributor/feed/ 2 High Engine RPM http://www.enginetech.maxa1.com/2010/04/15/high-engine-rpm/ http://www.enginetech.maxa1.com/2010/04/15/high-engine-rpm/#comments Thu, 15 Apr 2010 02:18:07 +0000 admin http://www.enginetech.maxa1.com/2010/04/15/high-engine-rpm/ As engine speed increases to a maximum, less combustion time is available, volumetric efficiency (cylinder filling efficiency) is reduced, so less fuel and air are available for combustion, flame travel is slower, and ignition timing needs to be advanced. And if you’ve heard of devices that retard timing at high engine rpm, then there was probably too much “intermediate rpm” advance in getting from low to high engine speed. High-gear ignition-retard units that improve race track speeds do work, but not for your point A to point B fuel miser.
Probably not worthy of expanded discussion, but related to ignition systems, are engines operated by “compression ignition.” You know them as diesei engines that initiate combustion spontaneously as a result of extremely high (by comparison to spark ignition engines) compression ratios. Such ratios are often on the order of 12 to 21:1, so the precombustion temperature of air near the end of the compression stroke is very high. Just prior to the piston reaching top dead center, a measured quantity of fuel is injected into an engine’s cylinder and, in combination with the now-heated air, ignites and combusts just about as fast as it can be injected into the cylinder. But this is the diesei principle, which may become the subject of a future Shop’Series … if there is sufficient

reader interest. All you have to do is let us know. Which brings us around to wrap-up time.
Keep in mind that spark timing and intensity (joule level vs. time) are the ingredients of successful ignition. What happens from ignition forward depends on many conditions not related to the ignition system. The conventional system uses points, a set of coils (primary and secondary windings) placed around an iron core, and a spark distributor that times secondary voltage to each of the engine’s spark plugs. Substitutions (as in the case of types of electronic components) and additions (such as multiple spark systems) can be made to this basic method of spark timing and frequency. But the object of it ail is initiation of the combustion process. What happens from then depends on other non-ignition conditions.
It’s like the old saying “You can lead a horse to water, but he’ll only eat the okra.” Well, maybe you heard a different version. We’re probably misteafcen . . . again.

REVIEW QUESTIONS: True or False
1. The combination of air and fuel, in the presence of heat and pressure, can be called sublimation.
2. Any form of uncontrolled combustion is a form of detonation.
3. In a typical combustion chamber, ignition of the air/fuel charge takes place directly opposite the spark plug and travels uniformly toward the exhaust valve.
4. Other than the conditions and amount of air/fuel mixture in a given engine’s cylinders, turbulence is the most important factor in flame travel.
5. Lean air/fuel mixtures require less ignition voltage to begin combustion than do mixtures that are richer.
6. In a conventional battery ignition coil, the secondary windings are outnumbered by the primary windings, resulting in higher secondary voltages necessary for spark plug operation.
7. An ignition system’s points, operated by a distributor cam, are used to interrupt primary voltage flow.
8. Condensers act as “electrical shock absorbers,” resulting in rapid collapse of the magnetic field and increased point life.
9. Vacuum advance mechanisms provide increases in the amount of ignition timing as intake manifold vacuum decreases.
10. During part-throttle engine operation, the compression of air/fuel mixtures is quite high, resulting in rapid combustion flame speed and less ignition timing.
11. Diesel engines incorporate air/fuel mixture combustion as a result of heat and pressure, with fuel being injected after the compression and heating of air within the cylinder.
12. Multiple-spark ignition systems are excellent ways of improving part-throttle combustion efficiency when cylinder pressures are high and combustion rates are rapid.
13. So-called high-energy ignition (HEI) systems have become commonplace with the advent of lean mixture (lean-burn) engines designed to meet exhaust emissions standards through the reduction of misfire.
14. Reasons our school had no cheer leaders were mentioned in a previous Shop Series on rear.. . uh, differentials.

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