science

Ignition timing

Ignition timing is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ignition timing rather than just read about it. In short: In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke. The need for advancing (or retarding) the timing of the spark is because fuel does not completely burn the instant the spark fires.

Key takeaways

Ignition timing belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ignition timing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ignition timing from memory before moving on to harder problems.

Reference excerpt

In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke. The need for advancing (or retarding) the timing of the spark is because fuel does not completely burn the instant the spark fires. The combustion gases take a period of time to expand and the angular or rotational speed of the engine can lengthen or shorten the time frame in which the burning and expansion should occur. In a vast majority of cases, the angle will be described as a certain angle advanced before top dead center (BTDC). Advancing the spark BTDC means that the spark is energized prior to the point where the combustion chamber reaches its minimum size, since the purpose of the power stroke in the engine is to force the combustion chamber to expand. Sparks occurring after top dead center (ATDC) are usually counter-productive (producing wasted spark, back-fire, engine knock, etc.) unless there is need for a supplemental or continuing spark prior to the exhaust stroke. Setting the correct ignition timing is crucial in the performance of an engine. Sparks occurring too soon or too late in the engine cycle are often responsible for excessive vibrations and even engine damage. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Many variables also affect what the "best" timing is. Modern engines that are controlled in real time by an engine control unit use a computer to control the timing throughout the engine's RPM and load range. Older engines that use mechanical distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM and load range. Early cars required the driver to adjust timing via controls according to driving conditions, but this is now automated. There are many factors that influence proper ignition timing for a given engine. These include the timing of the intake valve(s) or fuel injector(s), the type of ignition system used, the type and condition of the spark plugs, the contents and impurities of the fuel, fuel temperature and pressure, engine speed and load, air and engine temperature, turbo boost pressure or intake air pressure, the components used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.

Background The spark ignition system of mechanically controlled gasoline internal combustion engines consists of a mechanical device, known as a distributor, that triggers and distributes ignition spark to each cylinder relative to piston position—in crankshaft degrees relative to top dead centre (TDC). Spark timing, relative to piston position, is based on static (initial or base) timing without mechanical advance. The distributor's centrifugal timing advance mechanism makes the spark occur sooner as engine speed increases. Many of these engines will also use a vacuum advance that advances timing during light loads and deceleration, independent of the centrifugal advance. This typically applies to automotive use; marine gasoline engines generally use a similar system but without vacuum advance. In mid-1963, Ford offered transistorized ignition on their new 427 FE V8. This system only passed a very low current through the ignition points, using a PNP transistor to perform high-voltage switching of the ignition current, allowing for a higher voltage ignition spark, as well as reducing variations in ignition timing due to arc-wear of the breaker points. Engines so equipped carried special stickers on their valve covers reading “427-T.” AC Delco’s Delcotron Transistor Control Magnetic Pulse Ignition System became optional on a number of General Motors vehicles beginning in 1964. The Delco system eliminated the mechanical points completely, using magnetic flux variation for current switching, virtually eliminating point wear concerns. In 1967, Ferrari and Fiat Dinos came equipped with Magneti Marelli Dinoplex electronic ignition, and all Porsche 911s had electronic ignition beginning with the B-Series 1969 models. In 1972, Chrysler introduced a magnetically-triggered pointless electronic ignition system as standard equipment on some production cars, and included it as standard across the board by 1973. Electronic control of ignition timing was introduced a few years later in 1975-'76 with the introduction of Chrysler's computer-controlled "Lean-Burn" electronic spark advance system. By 1979 with the Bosch Motronic engine management system, technology had advanced to include simultaneous control of both the ignition timing and fuel delivery. These systems form the basis of modern engine management systems.

Setting the ignition timing

… excerpt ends here. Continue reading the full article.

Illustrations

Ignition timing: Distributor weights — Distributor weights

Worked examples

Example 1 — a first encounter with Ignition timing

Start with the simplest possible case. Write down what Ignition timing claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ignition timing before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ignition timing ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ignition timing

In research: Ignition timing appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ignition timing in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ignition timing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Ignition systems, Synchronization, so understanding it makes those chapters shorter.
In everyday life: Look for Ignition timing outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ignition timing in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ignition timing means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ignition timing out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ignition timing in simple terms?

Why does Ignition timing matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ignition timing?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ignition timing.