Mechanical Versus Electrical System Control

By Sean Kelly

The computer-controlled injection and timing control systems implemented in cars of the OBD age run at speeds so far surpassing the mechanical systems they control, that the implementation of the control algorithms and actions can be analyzed as stop-motion system control. A reasonable engine speed is 4500 RPM. To further influence this point, we will analyze the engine at twice that, 9000 RPM.

In 1963 computers operated at 50,000 cycles per second, or 50 kilohertz. In 1980, the average speed was 60 kHz. Modern personal computers operate in the 800 Megahertz to 3 Gigahertz range. The processor in modern cars runs as 16 MHz. To compare how much faster this is compared to the mechanical engine system, we look at the granularity per degree:

Mechanical System

{150}\frac{cycles}{second}*{360}\frac{degrees}{cycle}={54000}*{10}^{9}\frac{degrees}{second}

Electrical System

{16}*{10}^{6}\frac{cycles}{second}*{360}\frac{degrees}{cycle} = 5.76 * {10}^{9}\frac{degrees}{second}}

Ratio of computer speed versus mechanical speed

\frac{{5.76}*{10}^{9}}{{16}*{10}^{6}}=106,000

This means that for every degree of rotation, even at 9000RPM, the computer is looking at the engine over 100,000 times. A fuel injector goes from completely closed to completely open in a little over half a millisecond. This equates to a cycle time of 1+ millisecond, or N>1000 Hz. Even this electromechanical system is over 15,000 times slower than the processor controlling it. The open time for an injector is in the range of 3-8 ms, so on comparison the opening time is actually substantial, but the processor can easily overcome any inefficiency due to mechanical lag simply by advancing the injection point a fraction of a millisecond forward in the injection cycle. The ignition system response time is on the order of microseconds, and delay of ignition to recuperation of coil primary voltage is 1-1.5 ms. This ensures that the ignition coil could be capable of firing again well before the ignition point of the next cycle occurs.

Figures courtesy of IEEE