Over rev limit

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bthood
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Over rev limit

Post by bthood »

Hey guys I’ve got a question. Is there a way to control the rev limits on an IDI Engine? The more I read about accidentally over revving a turbo engine the more it concerns me. I know the obvious is to keep your foot on the pedal but sometimes you forget things and bad things happened at that point. Is there such thing as some sort of a rev limiter that you could wire into the IP pump that would turn the fuel off momentarily if you over rev ?
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JoeIsuzu
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Re: Over rev limit

Post by JoeIsuzu »

The pump has a mechanical (centrifugal) rev limiter built in. However, it doesn't respond fast enough to prevent damage. Keep in mind that although there's a "redline", there's nothing magic about that number. Even if a rev limiter had 100% accuracy with instantaneous response when you hit 4,250 RPM (for example), it would not prevent you from driving all day at 4,249 RPM, and stressing the engine just as much.

Jack
bthood
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Re: Over rev limit

Post by bthood »

OK then would it be possible to wire in some sort of a switch that would turn the fuel pump off if it hits 4200 RPMs? There must be some sort of a way to prevent an engine from going that high?
bthood
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Re: Over rev limit

Post by bthood »

Would it hurt anything as far as hurting the injection pop or anything internally with the engine if the injection pump was to be electrically disconnected while the engine was running?
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JoeIsuzu
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Re: Over rev limit

Post by JoeIsuzu »

The only electrical connection on the injection pump (on a Federal-spec model) is the fuel cutoff solenoid.
No, it would not hurt.
Yes, the engine would stop. ;)

Jack
bthood
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Re: Over rev limit

Post by bthood »

OK then so is there such a thing as a rev limit switch that you could wire up that would cut power to something? I don’t wanna reinvent the wheel for something already on the market that’s why I’m asking all these questions. If there was some kind of a switch that would turn power off to the electrical part of the IP pump when the engine reaches a certain RPMs (whatever that may be) that should protect Engine from over rev right?
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JoeIsuzu
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Re: Over rev limit

Post by JoeIsuzu »

Theoretically, I suppose so. And I assume you want it to re-engage once the RPMs drop below the magic number. Let's say it does this perfectly and instantaneously. Whenever you hit the magic RPM limit and don't apply some pedal discipline, it will simply "flap". Is that REALLY what you want? That sounds destructive to me.

Jack
bthood
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Re: Over rev limit

Post by bthood »

I’m not sure that’s what I want because your scaring me with the “destructive” word. What do you mean? I’m asking because I’m not sure I understand. If it’s going g to tear up something then no that’s what I’m trying to avoid in the first place. If it’s just something where the engine will cut out till the rpms drop then yes that’s what I want.
Brian
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JoeIsuzu
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Re: Over rev limit

Post by JoeIsuzu »

Let's give it some time for others to weigh in. I'm not a diesel mechanic. My opinion comes from an IT perspective (my life during the day). We (in IT) often find that users are surprised and disappointed when they get exactly what they asked for -- because they didn't think through the possible scenarios that will happen in real life.

What you're proposing might actually work just fine. In fact, it may be the same solution that JLEMOND proposed to Isuzu engineers when Isuzu was having to replace so many turbo diesel engines. They liked it, but the diesel was dropped before the solution could be added to production. I'll have to search some old posts.

Jack
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Paul
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Re: Over rev limit

Post by Paul »

Actually, the rev limiter in the injection pump works reasonably well. It works by gradually reducing the fuel as the speed increases above a set speed. The pumps are calibrated to run full bore to about 4000rpm and then to start cutting back until the fuel is near zero at around 5000rpm. This means that the engine could run at approaching 5000rpm if it is totally unloaded. Not too likely, I think.

If this is too high for you, you can adjust the rev limiter by backing out the max throttle stop on the pump. This is the screw that stops the full throttle swing of the throttle lever on the passenger side of the pump. I can't tell you exactly how much to back the screw out but I suggest one full turn for about 500rpm, maybe. I can measure this on the next pump we rebuild. This will not affect the fuel delivery at lower speeds or throttle settings.

All that said, I think that most over revving damage to these engines probably occurs due to down shifting at too high a speed. There is nothing a governor can do to prevent damage form this.

More of all that said, I think that the best control to prevent damage is a tachometer and an alert driver. I seldom find any need to rev my engine over 3000 rpm, which with my extra tall 5th gear gives me around 80 mph. I rarely drive that fast.

Shutting the fuel completely off and then back on at high speed likely will cause a very uncomfortable jerking engine. Also, if the fuel cut solenoid on this pump is switched off at high pump speed it usually will not turn back on until the engine speed is reduced to around 1500rpm or so.

Paul
'84 P'UP 2 wd diesel, 5 spd with 0.78 fifth gear and differential back to 3.73.
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Paul
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Re: Over rev limit

Post by Paul »

I see that I made an error in my prior post.

To adjust the rev limiter to a lower maximum speed the screw I mentioned needs to be screwed in (not out) so as to further limit the travel of the throttle lever.

Paul
'84 P'UP 2 wd diesel, 5 spd with 0.78 fifth gear and differential back to 3.73.
paulthepilot5
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Re: Over rev limit

Post by paulthepilot5 »

bthood wrote:
Wed Jul 28, 2021 12:41 pm
Hey guys I’ve got a question. Is there a way to control the rev limits on an IDI Engine? The more I read about accidentally over revving a turbo engine the more it concerns me. I know the obvious is to keep your foot on the pedal but sometimes you forget things and bad things happened at that point. Is there such thing as some sort of a rev limiter that you could wire into the IP pump that would turn the fuel off momentarily if you over rev ?
The injection pump does control RPM, particularly under load. While it isn’t a hard cut at a particular rpm, it just gradually reduces fuelling above around 3600 through to around 2100. (Depending on pump spec).
You would be surprised what they will do without damage, I’ve seen over 5k on the tach when the input shaft snapped in 4th under full load during a 1/4mile run, still running that engine at over 300% factory output over the last few years.
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