• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Need help please with car rev limiter - ECU fuel injection cut off.

Status
Not open for further replies.

artful

New Member
Can anyone help please with my basic idea for a car rev limiter that needs to high side switch the ignition feed to the fuel injectors normally driven by the ECU.

The ECU has a common ignition supply voltage at the top of each injector coil via an engine control relay and individually switches the ground to open the injectors in turn. I want to hook into this ignition supply and inhibit it above a preset rev limit.

Using a pic micro based shift light I can get a tacho signal which brings on an led above a set rev limit so I need an interface to modify this led o/p and drive an NPN high side switch to inhibit the injector supply. Please see the attached circuit for my initial idea using components available from UK supplier Maplin and help me calculate the resistor values including power ratings. Alternatively any ideas/ improvements comments are welcome.
 

Attachments

dr pepper

Well-Known Member
Most Helpful Member
A relay should do the trick, 10ms delay from the machanism shouldnt mess things up too much, if you want you could replace the shift led with the led section of an optocoupler, and use the phototranny part of the opto to drive a transistor whch switches the relay.

You say the system is sequential, most systems switch modes from fully sequential to 2 at once then all 4 at once (batch fire) as rpm increases towards max, though this shouldnt be a problem for your application.

Also later systems monitor the current drawn by the injectors, disconnecting them may put the ecu into error mode, switching to a load resistor might get around this if you switch it quick enough.

Oh yes, welcome to the forum.
 

artful

New Member
A relay should do the trick, 10ms delay from the machanism shouldnt mess things up too much, if you want you could replace the shift led with the led section of an optocoupler, and use the phototranny part of the opto to drive a transistor whch switches the relay.

You say the system is sequential, most systems switch modes from fully sequential to 2 at once then all 4 at once (batch fire) as rpm increases towards max, though this shouldn't be a problem for your application.

Also later systems monitor the current drawn by the injectors, disconnecting them may put the ecu into error mode, switching to a load resistor might get around this if you switch it quick enough.

Oh yes, welcome to the forum.
Thank you for your reply.

Firstly I have tried a relay and unfortunately it does introduce a noticeable delay with the revs dropping by about 1000 before switching back in.

Secondly if you look at the attachment of the circuit I uploaded (couldn't find a way to embed the .bmp file) I have replaced the led and 150R from the shift light, pin 5 of the pic to the i/p of an optocoupler and taken this to a transistor circuit which I need to switch the 6A load using high side switching and this is where I need specific help.

I am trying to use components I can buy from a local Maplin store and have placed what they have in stock where I think the circuit would work. It is a long time since I have tried to design any electronic circuit and am uncertain of calculating the resistors to enable full saturation and efficient switching in this configuration.

Lastly I am not worried about any error lights etc that the ecu may show up as this is to be used in a racing car. It is an old/early ecu system circa 1993 so not much in the way of error monitoring. The engine is a 6 cylinder and the injector switching has been confirmed as sequential on a scope trace I found on another car forum but it doesn't make any difference even if it were to change to batch firing as my initial relay circuit worked except for the enormous delay.

Thank you again for the welcome!
 

alec_t

Well-Known Member
Most Helpful Member
Your pin 5 signal is presently programmed to go low to turn on the opto. But when pin 5 goes high, current will still flow through the opto-diode via the inherent protection diodes of the pin 5 port. Can you rewire the opto-diode to power it from the MCU 5V supply (via the current-limiting resistor), rather than from the 14V supply? If not, you will need a level-shifter between pin 5 and the opto.

Edit: Can you source a suitable P-channel MOSFET instead of the BD912?
 
Last edited:

artful

New Member
Your pin 5 signal is presently programmed to go low to turn on the opto. But when pin 5 goes high, current will still flow through the opto-diode via the inherent protection diodes of the pin 5 port. Can you rewire the opto-diode to power it from the MCU 5V supply (via the current-limiting resistor), rather than from the 14V supply? If not, you will need a level-shifter between pin 5 and the opto.

Edit: Can you source a suitable P-channel MOSFET instead of the BD912?
Thank you alec_t I have made a mistake with my diagram and the opto-diode is currently powered by the 5V supplying the MCU which I originally had switching a relay but found it was far too slow.

I can source a P-channel Mosfet but the delivery is a few days so I was hoping to use components I can get locally from Maplin hence the BD912. They do not stock any P-channel Mosfets.

As alec_t said, You might try 1 or more, high amperage P channel MOSFETs. They make them with logic level input. I didn't have much luck with them but they might work for your application.
You might get insight here: http://www.electro-tech-online.com/threads/question-about-headlights-relay-in-car.131751/
Thank you ClydeCrashKop interestingly I had found a source for a P-channel Mosfet IRF4905 55V-74A mentioned in the above post on ebay but not available in a local shop ... http://www.ebay.co.uk/itm/130943960837

If I can't get the BD912 to work then the P-channel will be my next attempt.

I have corrected the mistake I had in my previous cct so the opto-diode is powered correctly from the 5V MCU supply.
 

Attachments

Last edited:

alec_t

Well-Known Member
Most Helpful Member
Looking good.
BTW, could you please post files as .gif or .png? (Much smaller to download.)
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top