Ignition Timing and How it Works

[Ludester]

The spark is ignited in the figure shown at 30 degrees BTDC (-30 on the X-axis to the far left).

At about 10 degrees BTDC on the graph (-10 on the X-axis), the igniting of the air-fuel mix is finished and the air-fuel mix starts to increase cylinder pressure from the expanding force of the combustion event. Cylinder pressure rises, as the burning air-fuel mix expands and pushes down on the piston top.



The greatest force or highest cylinder pressure occurs at 20-25 degrees After TDC (ATDC) [+20-25 degrees on the x-axis]. This is the finish of the combustion event.

You want to "time" the start of the igniting process so that the combustion is finished (the highest peak cylinder pressure) when there is a good connecting-rod-to-crankshaft angle.


What do I mean by that?:


What is a "good" Connecting Rod to Crankshaft Angle for making more power? When should peak cylinder pressures happen?

Cartoon of a Piston ( [O] ) and Connecting Rod ( I ) On Side View:

1. ...........[O]............... 2. [O]
.................I..................... /

.........Rod @ TDC...........Rod @ 12-15 degrees ATDC


If the force of the explosion (or peak cylinder pressure) occurs at TDC, the connecting rod is straight up and down or "in line" vertically with the piston (1. in the above cartoon figure). A downward force or push by the explosion on top of the piston at this time does NOT help turn the crank. The force just travels down the rod vertically.

If the highest cylinder pressure occurs later ATDC (2. in the above cartoon figure) , the connecting rod to crankshaft angle is more ideal. The downward force or push of the explosion on the piston top helps turn the crankshaft due to the rod not being straight up and down or in line vertically with the piston. The rod is travelling clockwise and downward already at 12-15 degrees ATDC from inertia and an additional "push" from the peak cylinder pressure would accelerate it faster in this direction. As a result, the force turning the crank would be greater.

Most people say that the downward push should be happening no earlier than 12-15 degrees ATDC (+ 12-15 degrees on the X-axis) so that there is a mechancial advantage of turning the crank.


The time at which you START the ignition of the air-fuel mix influences when the combustion event finishes and where peak cylinder pressures occur.

You want the peak cylinder pressure or greatest downward push to occur at a time when the rod is at a good angle relative to the crankshaft.

This ensures that you get the most out of the downward force from the explosion to turn the crank.

Cont....

[Ludester]

How does this all relate to advancing you're ignition timing?


Why did we do all that math at the start of the post?:



If the slowest turbulent flame speed is 1.35 m/sec or 1,350 mm/sec. and the distance from the plug to the piston top at TDC is 0.89 mm:

1. How long do you have @8000 rpm WOT for the ignition process and combustion process to be completed in an Integra ?

Answer : approximately a 0.6 millisecond window to light the air fuel mix and finish the combustion event.

The Math (if you are interested):



0.89 mm / 1,350 mm/sec. = 0.0006 sec. = 0.6 milliseconds




2. How much is that in terms crankshaft degrees?


Answer: 29 crankshaft degrees.


The Math (if you are interested) :

if you have a 0.6 msec window from the start of igniting the air-fuel mix to the finish of the combustion event and 1 crankshaft degree takes 0.021 msec.


0.6 msec / 0.021 msec. / degree = 29 degrees

[Aside: If we do the same calculation for a flame speed of 2.2 m/sec, the window for the complete igniting of the air fuel mix and the expanding explosion is only 19 degrees. ]



3. What's happening when you advance the ignition timing? :


When we advance the ignition timing, we start the igniting process and combustion event earlier. The cylinder pressure builds up and finishes earlier. Peak cylinder pressure (highest downward push on the piston top) occurs earlier.



4. What's our goal or how do we make more power?


We want to minimize the time that the expanding combustion event occurs BTDC while the piston is coming up and squeezing the mix. If the air-fuel mix is expanding while you are still squeezing the mix with the piston BTDC on the compression stroke, you are making the piston work harder.

You also want the maximum cylinder pressure (i.e. the finish of the combustion or expanding event) to be located at least 12-25 degrees ATDC to get a good rod angle to help turn the crank. If you nail the timing for the highest downward force at the right rod to crank angle, you make more power.

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Back To Our Example of the Integra Type R @8000 rpm

A. 14 degrees BTDC Ignition Timing

We calculated that the time window from start to finish is 29 degrees. If we ignite the mix at 14 degrees BTDC (-14 on the X-axis of the graph above), the whole event should finish at 15 degrees ATDC (+15 on the X-axis). This is still within a good rod to crank angle to generate power.


B. 16 degrees BTDC Ignition Timing

When we advance the timing to 16 degrees BTDC (-16 on the X-axis), we move the peak cylinder pressure location earlier (i.e. the finish of the combustion event).

The start of the combustion event or expanding force is earlier. More combustion time is spent BTDC making the piston work harder by having it squeeze the mix as the force is expanding. A side effect of this is the peak cylinder pressures also increase to a much higher level.

In this example when, we advance to 16 degrees BTDC, the peak pressures will happen at 13 degrees ATDC (+13 on the X-axis) which is still at a good rod to crankshaft angle.

C. What if we keep on advancing the start of the ignition event to say 20 degrees BTDC?

Sooner or later when you advance too far, you will have most of the expanding force occur while the piston is still squeezing the mix BTDC and the peak cylinder pressure will occur too early at a nonadvantageous rod to crankshaft angle. The peak cylinder pressures now would occur at 9 degrees ATDC, if the timing was advanced to 20 degrees BTDC. Now, the rod to crank angle is too vertical and most of your combustion window occurs BTDC, when the piston is still on it's way up and squeezing the mix. You stop gaining power and have very, very high detonation-friendly cylinder pressures BTDC.

High cylinder pressures wear out components faster and can initiate auto-ignition (i.e. start combustion at the wrong time without a spark) which is also called " detonation" .

There's a trade-off or compromise:

On the one hand, you "advance" or move the start of the ignition process earlier so that the peak cylinder pressure is higher but still located earlier at a good rod to crankshaft angle to make more power. Higher cylinder pressures generate more downward force onto the piston top.

However, when you advance the start of the igniting process too far, the peak cylinder pressures are located too early at a rod to crankshaft angle that is almost vertical or inline with the piston which does not help turn the crank. The higher peak cylinder pressures create a condition that can generate detonation.

You are trying to locate the "sweet spot" (best rod to crank angle) for when the peak cylinder pressures should happen when you adjust ignition timing.

If you advance too far then you risk detonation from too high of a cylinder pressure or decrease the life of the engine over time.


Bottomline Take Home Message:

From this you should have surmised that the best way to go is to find the least advance you can get away with.

It puts less of a strain on the engine bottom end components, makes your pistons work less harder when they are squeezing the mix, and still places your peak cylinder pressures within a good rod to crank angle range from 12-25 degrees ATDC.

Good stuff....