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  #1  
Old 12-29-2020, 06:27 PM
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Why do OM615's have throttles?

Serious question. Saw a 615 and mistook it for a gas engine, embarrassed myself. What does the air side throttle do?


https://mbmanuals.com/pics/om615engine-parts-diagram.jpg

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  #2  
Old 12-29-2020, 06:33 PM
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I used to know but forgot. I'm not sure it was not discussed here years ago.
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  #3  
Old 12-29-2020, 06:48 PM
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Search our site for Pneumatic Governor Questions. Tried but was unable to post the link.
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  #4  
Old 12-29-2020, 06:49 PM
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I found a post that suggested that it was engaged to stop airflow if the engine started backwards. Seems like an elaborate solution to a rare problem, if that's the case. Ever hear of a Diesel turning backwards?
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Old 12-29-2020, 07:01 PM
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Diesel running backwards? Yes heard of it on cold starts and it's not good because the oil pump is then turning backward.
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  #6  
Old 12-29-2020, 10:54 PM
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Its not a tight fitting thing like a throttle plate.
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  #7  
Old 12-29-2020, 11:19 PM
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I thought it was to minimize "diesel run on" after shutdown. I had a 76 OM616 with a throttle plate, and vaguely recall that explanation.
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Old 12-29-2020, 11:35 PM
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Quote:
Originally Posted by Mxfrank View Post
I found a post that suggested that it was engaged to stop airflow if the engine started backwards. Seems like an elaborate solution to a rare problem, if that's the case. Ever hear of a Diesel turning backwards?
Quote:
Originally Posted by Sugar Bear View Post
Diesel running backwards? Yes heard of it on cold starts and it's not good because the oil pump is then turning backward.
A four-stroke engine will not actually "run" in reverse rotation. If combustion occurs too early on the compression stroke the piston may be forced down, rather than continuing to rise, and the crankshaft will briefly spin backwards ("kick-back").

Two-stroke engines, by contrast, can be, and are, arranged to run in either rotation by changing the ignition or injection point to suit the desired direction of rotation. This is the technique used with large marine two-stroke diesels. Want your ship to back up? Reverse the engine rotation.

In the case of the OM615 the "throttle" is the governor; air supply is reduced if the engine approaches an RPM limit value.
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  #9  
Old 12-30-2020, 09:22 AM
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And yet my research suggests that's exactly why the secondary flap is there. I had my doubts at first, but I see nothing preventing reverse rotation in a 4 stroke Diesel. Both the intake and exhaust manifolds are open pipes to the atmosphere. Fuel enters by an independent path, direct to either cylinder or prechamber. The only question is that injector timing is going to be early. Valve timing would be a bit screwy, but not a show stopper. So it very well might chug along backwards.

The primary flap appears to be there to increase flow around the venturi port at idle and light load, in order to provide a cleaner vacuum signal to the injection pump. The intake passage is never full obstructed. I would guess that intake pressure in this engine is slightly below atmospheric in most run conditions.
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  #10  
Old 12-30-2020, 01:27 PM
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Just the difference between a diaphragm and mechanical controlled throttle.
the diaphragm was earlier and simpler.

I make use of the slight vacuum the throttle plate creates in the intake to control other accessories.
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  #11  
Old 12-30-2020, 06:41 PM
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The throttle plate does indeed create a vacuum signal that's used to control fuel delivery from the IP. Inside the governor housing on the back of the IP there's a coil spring that pushes the diaphragm/fuel rack forward. Without vacuum (pedal to the metal) to pull the diaphragm back you get full throttle thanks to that coil spring. At idle you have max vacuum pulling the diaphragm back thereby limiting fuel output. And between these two positions there's a whole range of control depending on throttle position, engine speed, and load.

Problems can arise when the system leaks, reducing the amount of vacuum for fuel control. A leaky system will cause the injection system to overfuel because it can't overcome the governor's coil spring. Leaks can come from a faulty diaphragm, bad fit between the governor housing and the IP body, bad o-rings where a shaft goes through the housing, bad gasket between the two parts of the housing, and trouble with the plastic tube that runs from the throttle body to the IP.

This system is also used on OM616.916 engines that came in '74-up W115 and also in early W123 240Ds.
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Last edited by gmog220d; 12-30-2020 at 08:23 PM.
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  #12  
Old 01-01-2021, 06:17 AM
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Just to add, many many older diesels have the pneumatic governors on them, Japanese, British makers to note a couple
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  #13  
Old 01-04-2021, 11:50 PM
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Very common in early diesel days. It is a throttle and its job is to make manifold vacuum, same as a gas engine. But its not to control the air/fuel ratio, its simply to pull on a diaphragm attached to the fuel rack.

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