Friday, June 29, 2012

Ignition Timing and Vacuum Advance - Part 1

Do take note that you need to have an interest in automotive engines before proceeding to read as this article can get quite technical. 

This article was written for owners of automotive vehicles using mechanical and vacuum advance ignition timing in mind. Certain parts of the article may no longer be applicable for modern cars with its own computer controlled, distributorless Engine Management System.

Ignition Timing Advance

We all know spark plugs are used to ignite the air fuel mixture in the engine cylinder chamber. The combustion/explosion results in the sudden expansion of the volume of gases and in turn pushes the piston. The vertical forces of the piston is then translated to rotational forces by means of the crankshaft.

Step by step diagram of the 4 stroke cycle

So if we look at the rotations of the crankshaft, there is this critical point whereby the piston on its upwards motion reaches its maximum height before it starts to move downwards. That is know as the TOP DEAD CENTER (TDC) of the stroke. The expansion of the volume of air, resulted from the combustion of the air fuel mixture, should (theoretically?) start to push the piston downwards at the TDC, as it starts its downward motion, to achieve maximum power to the crankshaft.

TDC Explanation
Although it looks like TDC as the best maximum position of the explosion to happen, it is, however, not the best position to ignite the spark plugs.

This is because there is a lag time between the ignition point and the resulting complete expansion of the gases. And to ensure that the resulting expansion hits the pistons as close to the TDC as possible, the ignition has to happen a short moment before the TDC ( aka BTDC )

Instead of predicting the ignition point using the vertical motion of the pistons, it is much easier to use the rotational motion to predict when is the best moment to ignite the spark. And this is measured in degrees. So for most engines out there, at its initial idling state, the ignition point is set between 15 to 30 degrees of ignition timing advance. That means that the spark plugs are ignited at each cylinder at 15-30 degrees before even the piston reaches the Top Dead Center (TDC). This initial setting of the ignition timing is known as the "Base Advance".

But that does not mean that the timing advance remains fixed flat throughout the different running conditions of the engine ( ie part load, full load, idle, etc ).

The lag time between the sparking and the complete expansion of the Air Fuel (AF) mixture is also a varying factor based on the AF mixture (lean/rich) and the load of the engine ( measured by its vacuum pull into the intake manifold ).

A leaner AF mixture will take longer to burn and fully expand and on the other hand, a richer AF mixture will burn faster. And as the fuel management system is constantly adjusting the AF mixture between lean and rich based on the throttle position ( and hence affecting the intake manifold vacuum ), there will be varying lag times of complete combustion (and expansion of resulting gases) due to the varying AF mixtures.

There has to be another way of increasing the timing advance to cater for the leaner AF ratios (which burn and expand slower) as the rich AF ratio condition is already taken care by the "base advance".

The missing plug in was carefully thought of and implemented as the "Vacuum Advance"

Kindly await the next installment for elaboration of the Vacuum Advance and the importance of vacuum in older cars that still uses the spark plug distributor with Vacuum Advance.

Last but not least, the optimum angle at which the expanding gases should exert force on the piston is actually not at TDC. 

It is actually 20 degrees After Top Dead Center (ATDC), where the downwards expanding gases will have the most effective power transfer to the crankshaft.