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A design for an engine speed limiting device ("Rev Limiter").

Rev limiter circuit / How it works / More information

All material on this page is Copyright, all rights reserved. Unauthorised commercial use prohibited. (Personal use is OK).
All these ideas are unproven. Any design should be verified by a qualified, competent engineer.
If you don't understand 100% how these things work you should not be using them on your car!!


Rev Limiter Circuit:

In 1992 I designed & built a rev limiter that can be connected to any conventional Kettering ignition (but has only ever been tested on a points ignition).

It's based on an LM2917 and an SCR, which has the very nice side effect of being impossible to generate any mis-timed sparks, a highly desirable attribute for high output rotary engines. When the limit is reached by accelerating to the rev limit, the transition is smooth and causes no backfiring. (BUT it has the potential to damage the exhaust from a backfire if the engine is downshifted and consequently over-revs; due to unburned air/fuel mixture coming from the engine that may be ignited by combustion when the RPMs fall below the limit).


GIF Circuit Diagram
(800x600)

How it works:

The heart of the circuit is a National Semiconductor LM2917 Tachometer IC which detects the engine RPM and when it exceeds a given level the 'load' SCRs are turned on which reduces/eliminates the spark energy and hence the engine power.

The input signal comes via the 10k, 22k resistors and C1 (.02u). This forms a signal shaping network that protects the IC input and filters out any high frequency signals (eg coil ringing). This filtered input is compared against a 0.6volt reference formed by D1 and it's 10k resistor. This further serves to minimise false triggering.
Now there is a signal which pretty much represents a cleaned up version of what's happening at the points. This passes into a "Charge pump" with C2, C3 and the 100k resistor. This converts the frequency of the input signal (i.e. the engine RPM) into a voltage level.
Up until this point everything is pretty much how a standard tachometer works.
From the charge pump, this voltage level is compared with the preset voltage level (the 50k "Set rev limit" potentiometer - the 15k preset associated with this is optional, and sets the absolute maximum rev limit. This is handy if the rev limit is changed depending on the situation). When the tachometer voltage exceeds the limit voltage (i.e. the ENGINE RPM IS OVER THE LIMIT), the transistor in the tachometer IC turns on.
When this happens, the 2N2905 is turned on (the 9k, 220ohm and 18k resistors simply limit the current in the 2905), which in turn operates the C122E

When the C122E turns on, it effectively "shorts out" the points. The 4.7ohm resistor IS REQUIRED. Basically what happens is when the points open and the SCR is turned on the 4.7 ohm resistor is across the points. This causes the magnetic field inside the coil to collapse only marginally, which in turn generates a much weaker spark than usual. However, there is still enough signal for the IC to be able to sense the points opening/closing.

As a bonus because of the way SCRs work, when the SCR control signal is removed, the SCR will remain turned on until the points close again.
Because it can't turn off by itself, it cannot generate any mis-timed sparks. This is a very important advantage.
Furthermore the SCR can be turned on at any time because sparks are only made when the points open, not close.

Please note when using this circuit that it was only ever built as a prototype, which does work properly, however use at your own risk.
It did not seem to be able to repeatably restrict RPM (eg when used on different days) to an accuracy of better than 500rpm. I have not investigated exactly why this happens or any possible fixes - and as my electronics knowledge has slipped over the last few years do not intend to.
I think it may be due to the wildly varying voltages in the operation of the ignition system and the relative difficulty of making exact measurements (for the triggering of the timing circuit), there is probably also some thermal effects.


More Information:

 This design was an original creation based on application notes for the LM2917 and my own ideas.

Further reading and acknowledgements:
* The best reference for further reading is the Bosch automotive electrical handbook, which goes into detail about how ignition systems work.
* National Semiconductor LM2917 datasheet

Other relevant reading at Craig's Rotary Page (Please go via the INDEX page):
* 30 LED air-fuel meter

Other relevant sites on the Internet (Please go via the LINKS page):
* National Semiconductor/Bosch websites; Possibly Amazon for the Bosch book (You'll have to find these things; I have no links to these).


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This page last updated 17/3/2001
Update History:
24/3/2001 - Renamed page from PG01.HTM to PG13.HTM, all images renamed to PG13_XXY.Z
17/3/2001 - Changed more information section
16/3/2001 - Converted all text to new standard (Headings as Heading1, Some sub-headings (e.g. tables) as 14 point normal bold italic, Most text as Normal, Internal page links at top not all uppercase)
11/3/2001 - Minor content update (Spelling mistakes, tidy up). Removed Word document version of file to avoid virus risk. PG01_02B.GIF edited to change author to Craig's Rotary Page. Changed from Netscape to FrontPage. Background image changed to PG00_02B.JPG
27/11/1997 - Previous known update (May have been some before this)


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