Low cost, temperature stable, analog circuit RPM limiter

Details Low cost, temperature stable, analog circuit RPM limiter Low cost, temperature stable, analog circuit RPM limiter

1999-05-14

2001-02-27

Kwon, John (Department: 3747)

Internal-combustion engines

Engine speed regulator

Engine speed reduction by partial or complete omission of...

C123S406660

Reexamination Certificate

active

06192859

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to ignition systems, capacitive discharge, inductive, or other, for internal combustion engines for limiting maximum engine RPM (REV) as is required in racing and performance applications or other applications where REV limiting is required. In particular, this invention uses a simple analog circuit approaching the accuracy and temperature stability of the more complex hard wired digital base or microprocessor REV limiters.
BACKGROUND OF THE INVENTION AND PRIOR ART
Current ignitions typically use the more complex digitally processed or microprocessor REV limiting systems with two separate sets of code switches to allow user selectable REV limit and the number of engine cylinders, such as 4, 6 or 8 cylinders. Prior to the newer microprocessor based limiter, the more complex and expensive hard wired digital limiters replaced analog circuitry due to their significantly improved temperature stability. In contrast, the more recent microprocessor REV limiter replaces much of the hard wired digital circuitry, but additionally requires software to function.
A microprocessor REV limiter requires circuitry such as input trigger, output stages, and a stable clock oscillator for the processor chip to function and interface with the outside world. This forms the processor's basic hardware circuit which is internally controlled by software, i.e. a written set of programmed instructions that directs the entire operation of the processor. The software program is a set of detailed operating instruction on how the processor will function for sensing input ignition trigger signals, comparing that input trigger frequency to the REV limit selected by the user switches, accounting for the number of cylinders as selected by the cylinder switches, and how the output will occur when REV limit has been reached. Once the program has been written, debugged and proven reliable, it must then be download into each microprocessor integrated circuit (IC) chip for it to function as a REV limiter. This new approach accurately compares the elapsed time between ignition trigger pulses to that of a high frequency temperature stable crystal oscillator, also known as the microprocessor clock which steps the processor through each programmed instruction. The crystal oscillator frequency, typically in the mid megaHertz (mHz) range, is scaled down by division to an appropriate frequency as dictated by the particular application. Unfortunately, this very accurate approach to controlling RPM is complex and costly for such a simple task. Other techniques using analog circuitry are simpler but suffer from temperature instability. As engine ignition control electronics is normally subjected to wide temperature variations when operating in the proximity of the enclosed area of a hot engine, the thermal instability of the analog circuit limits its use for such applications.
However, by careful design consideration of the specific elements of analog circuitry that produces the thermal instability, the negative temperature coefficient of a semiconductor junction can effectively be canceled to produce a low cost, simple, thermally stable circuit that is well suited for ignition RPM control (REV limiting). Like its expensive microprocessor counter part, the analog RC timing circuit approach can easily incorporate all of the features for external user selectable RPM and the selection of the number of cylinders.
It is therefore an object of the present invention to limit RPM through an analog system, or substantially analog system with minor digital adjuncts, overcoming the disadvantages of the state-of-the-art digital and microprocessor based systems outlined above.
SUMMARY OF THE INVENTION
The REV limiter circuit of the present invention is applicable to any type of ignition system or any application requiring thermally stable and accurate timing control functions. In addition, the REV limiter can be easily interfaced with an existing ignition controller either internally or externally as a stand-alone system. Through the use of a temperature compensated RC timing circuit operating as a mono-stable multivibrator, input or output trigger signals from an ignition system can be inhibited at any preset RPM.
The timing circuit incorporates a simple topology comprised of a charging switching element to define and control the charging of a timing capacitor through a timing resistor (with an RC time constant) and a second switching element with a transistor base-emitter junction with a negative temperature coefficient (−2.5 mv/degree C) used as a voltage threshold switch to turn off the charging switching element to end the capacitor charging as well as to rapidly recharge the capacitor to its initial value. The temperature sensitivity of the transistor voltage threshold switch junction can be overcome by placing an additional semiconductor junction in the capacitor's initial condition current recharging path. The additional semiconductor junction should be configured as a diode junction from the same type transistor as the voltage threshold transistor switch forming a simple mono-stable circuit that overcomes its inherent temperature instability. In this way, circuit sensitivity to temperature variations can be effectively canceled producing a temperature stable timing circuit that can function as reliably as the more complex and expensive digital and micro-processor version for performance application.
While this analog REV limiter will not achieve the extreme accuracy of the digital of microprocessor based limiter, it can achieve 1% accuracy through the use of higher accuracy, off-the-shelf 1% resistors and 2% or better capacitors, which is an accuracy suitable for most applications while maintaining a simple low cost and reliable solution.
Other objects, features, and advantages of the invention will be apparent from the following detailed description of a preferred embodiment thereof including the accompanying drawing in the figures, in which:


REFERENCES:
patent: 5048486 (1991-09-01), Okuda et al.
patent: 5245965 (1993-09-01), Andersson
patent: 5383433 (1995-01-01), Fiorenza, II
patent: 5755199 (1998-05-01), Costello et al.

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