I don't know what the minimum RPM is, and it may well depend upon the engine. Your point 1 and 2 are good observations. Your last paragraph however is incorrect. But I'm not really interested in pissing contest.
dj
Runaway diesel
- Thread starter Captain Larry-DH
- Start date
dj
I agree with you there as I had previously mentioned somewhere back in this (soon to become) endless book of words, if my piston rings are well soaked in oil, the engine will start in 1/2 a rotation or less. And that's even when the engine is hovering around 0°C or 32°F. If the rings are relatively dry (even after cranking with the stop pulled out) it's grunt, grind, and groan until those rings are soaked in oil. As a matter of fact, somewhere in my horder's collection of 2GM20F operator and service manuals, I seem to remember a statement of using extreme caution when rotating the engine by hand in that the decompression levers are open to avoid accidental starting.
Well, I would but I've already had my morning leak and I don't have the capacity to get into a match now, other than to say that if you can roll over that compression stroke quickly and your rings are wet, it's off to the races NOW.
Next time you have absolutely nothing better to do, check your compression by hand (with a slack V belt) after it hasn't run for a couple of weeks. It's slow but you can get it past the compression stoke. Now fire up your engine for about 5 seconds and shut it down. Leave teh engine to cool down for a half hour to ensure it's back to ambient temperature and try the poor man's compression test again. You will have real problems getting it over the compression stroke with the rings still wet. If it's still relatively easy, you've likely got leaky valves.
Food for thought.
For your reference, the minimum cranking speed for a diesel engine to start is between 150 and 250 RPM - too many refences to give for this range...
an excellent source is https://www.nrc.gov/docs/ml1122/ml11229a127.pdf
This document states (bold put in by me):
The single most important element in attaining the compression pressure, and therefore the air charge temperature required for ignition of the fuel, is the cranking or rotational speed of the engine during the starting sequence. Figure 7-1 depicts a typical relationship between the cranking speed (RPM) and the compression pressure (PSIG) achieved for a high speed diesel engine. Several factors contribute to this relationship:
• The more rapid the rate of compression is, the higher the pressure and temperature of the air charge.
• Rapid compression allows less time for pressure to be lost past the piston rings and valves.
• At higher cranking speeds, less time is available for the heat of compression to be lost from the engine cylinders.
At colder temperatures you may need high rotational speed. Also note that all engines we have been talking about are classified as high speed diesel engines.
dj
an excellent source is https://www.nrc.gov/docs/ml1122/ml11229a127.pdf
This document states (bold put in by me):
The single most important element in attaining the compression pressure, and therefore the air charge temperature required for ignition of the fuel, is the cranking or rotational speed of the engine during the starting sequence. Figure 7-1 depicts a typical relationship between the cranking speed (RPM) and the compression pressure (PSIG) achieved for a high speed diesel engine. Several factors contribute to this relationship:
• The more rapid the rate of compression is, the higher the pressure and temperature of the air charge.
• Rapid compression allows less time for pressure to be lost past the piston rings and valves.
• At higher cranking speeds, less time is available for the heat of compression to be lost from the engine cylinders.
At colder temperatures you may need high rotational speed. Also note that all engines we have been talking about are classified as high speed diesel engines.
dj
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Cranking speed is a rate of change, not a velocity, as explained in the text above. The engine doesn't have to get to a 200rpm velocity to start - it only has to achieve a 200rpm rate of change over a small distance. This can be a single compression stroke on a cylinder. This is why a low battery fails to start an engine - it cannot crank through the compression stroke fast enough.
I can hear the number of crankshaft turnovers on our engine as it starts (and see them if I'm looking at the engine at the time), and it starts on one or two turns of the crankshaft. Additionally, we have a tach sensor measuring the flywheel rotation, and the tachs never show any velocity until after the engine starts. If I have the stop lever pulled, the tach does show velocity as the engine fruitlessly spins.
Mark
I can hear the number of crankshaft turnovers on our engine as it starts (and see them if I'm looking at the engine at the time), and it starts on one or two turns of the crankshaft. Additionally, we have a tach sensor measuring the flywheel rotation, and the tachs never show any velocity until after the engine starts. If I have the stop lever pulled, the tach does show velocity as the engine fruitlessly spins.
Mark
I had a runaway engine with a Yanmar 1gm10. Motoring at 3000 rpm. Martec prop spun off shaft. Engine immediately pegged tach at 4000rpm/(3600 is max). Put it in nuetraL AND PULLED ENGINE STOP. nothing. Engine screaming and white smoke. Stuffed towel in the air cleaner and engine stopped. Sailed it back to he dock. No damage o the engine after mechanic did compression check. Boat pulled and new prop fitted. All good. I wonder how long it would have run before the engine would have hand grenaded.
I'm not sure what you are pointing out with the above.
When one wants to hand crank a diesel engine, you use the decompression levers so the pistons, fly wheel, that entire structure, can be spun up fast enough so that when you drop the decompression lever the engine can start. While you are spinning up, you are likely going to go through the compression stroke several times while getting everything up to speed. Obviously, with the decompression lever on, you don't get compression - it's purpose.
Now, once you've got everything spinning at a high enough RPM, you flip the decompression lever. When that happens, you don't really know where in the cycle that piston is at, so all this stored energy has to be able to carry that RPM through an entire cycle in order to have a complete compression cycle. Now, if you think you can flip that decompression lever at exactly the bottom of the compression stroke, or if you think you can get everything moving at the required RPM from the bottom of that stroke to the top while hand cranking, well, you are a better than I am.
But maybe there is something I'm not understanding. I'm all ears.
dj
so what actually caused the crank oil to generate the run-away condition?
I always hate when Ralph posts pictures of his engine compartment because I have the same engine and even after I cleaned/repainted it and varnished surrounding wood it looks like crap next to his. Oh, And it's warmer in Chicago than San Diego in February WTF ????
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I'm confused by this sentence. I've always thought RPM was rotational speed, velocity is synonym for speed, that I understand. However, I've never heard of RPM being a measure of rate of change. Rate of change is a measure of acceleration (or deceleration) but not a velocity or speed.
To a lay person who has a love-hate relationship with the internal combustion engine (gas or diesel), it seems rotational speed makes a difference in starting because heat generated on compression will be dissipated with a slow rotational speed which would inhibit firing, whereas with a faster rotational speed the heat will have less opportunity to dissipate before the next compression cycle.
Am I missing something in my plain language explanation?
You kind of changed the focus from "the engine has to be at 200rpm to start" to something about hand cranking. I was pointing out that the 200rpm has to be a rate of change, and not a velocity. Sort of like when you tap your knee reflex spot, your leg moves at a higher speed for a short distance than you could ever run.
A compression stroke is not a compression stroke without compression. So that part is neither here nor there when the decompression levers are open.
There isn't anyone here who can hand crank an engine at 200rpm regardless of decompression levers open. That is 3.3 revolutions per second. Put a winch handle in an unloaded winch and try to turn it 3 times each second. 60-80rpm there will have you huffing.
Rate of change is velocity over time - a vector quantity, while speed is the magnitude of this rate - a scalar quantity. So one could achieve a 200rpm rate of change when starting the engine by quickly enough moving the handle just the 100mm (4") distance of the piston's stroke if that piston was at the bottom of its compression stroke.
If one piston isn't at the bottom of its compression stroke, one of the other pistons will be - assuming it isn't a single cylinder engine. So even if you needed to move the crank handle more than 4", it will likely be less than 12".
So to start a diesel, the engine must achieve a 200rpm rate of change for a very short distance. The engine itself does not have to achieve a speed of 200rpm. The high torque of starters make this rate of change easily achievable in such a short distance without having to get the engine up to 200rpm speed. A weak battery does not allow the starter to turn hard enough to get this rate of change.
Going back to hand starting, if there is enough momentum at the slow cranking speed achieved by us mortals, then when the decompression levers are closed, the instantaneous rate of change of a piston stroke during a compression stroke will be higher than the engine's speed when the levers were closed.
Mark
My terminology may not be precise or even correct, and may be confusing. Go back to the section of the paper quoted about starting diesels. They talk about rpm being important, but then put that in terms of rate of change of a piston in a compression stroke, and not the speed of the engine itself.
It is a bit sloppy, but what they are describing is the piston moving through its 3-4" stroke at a speed that would be equivalent to the engine operating at 200rpm. So the engine doesn't have to be at 200rpm to start - just that one of the pistons has to move that fast for 3-4" during its compression stroke. This can be achieved by a starter or a human far easier than getting the engine itself up to 200rpm. It can actually be achieved at less than 1rpm on the engine.
So rpm is a scalar value for when the engine is running, but it does not have a vector direction. The paper talks about the vector quantity - the speed of moving the piston up a certain distance. It would have been more clear if they had discussed it in terms of the piston needing to move at a certain speed for a fixed distance, but that wouldn't give most people a familiar frame of reference to quantify it.
Mark
Hey, psssst, buddy, want to keep your engine looking like brand new ?
Cover the gear clamp meat hooks with these neat little covers and no one will know the engine is actually 1997 vintage.
My big advantage here is that I've had the boat and engine from brand new and was able to keep up the appearance.
And truthfully, the meat hook covers have kept the blood and guts off the engine from each time I worked on it. The SS strap ends were a magnet to my hands.
Looks like the price has multiplied since I bought mine a couple of years ago so best to shop around if interested.
This makes more sense, because a piston doesn't rotate, it moves vertically in a straight line, whereas the crank shaft does rotate. So the faster the crankshaft rotates, the faster the piston moves vertically and compression is developed quickly and more heat is retained in the cylinder facilitating ignition.
When hand cranking the momentum built up by rapidly rotating the flywheel helps over come the resistance from compression.
I.m not sure. When the prop fell off it immediately over revved. Mechanic mentioned that engine was eating it's own oil. Throttle didn't slow it down nor engine stop which meant it wasn't running on diesel at that point. Clouds of white smoke.
Ah, no, I did not change the focus. The original post that started this was me replying about using the nut on the alternator to spin his engine over rather than using the hand crank which is located in a hard to access location.
Looking at that, I brought up the point he should be sure to take into consideration a minimum RPM needed for starting. The gear ratios are backwards on that method. Hand cranks have a gear ratio where when you are spinning the hand crank, it spins the engine faster than your crank speed. However the alternator is geared in the opposite manner, whatever RPM you are achieving at the alternator nut, the RPM of the engine will be less.
I would imagine from this response you have not had a hand crank diesel engine. I absolutely would have to get my engine spinning somewhere in that 150 to 250 RPM range for it to start. That is achieved due to the gear ratios between crank and fly wheel. In my first post, I did not remember what RPM was needed, hence the suggestion to look at idle RPMs - which was way off as has been both pointed out and i have subsequently clarified.
dj
I was scratching my head on your first post on this. This second post clarifies - you are mis-reading what that document said.
It mentioned the rate of compression, not the rate of change of the piston. The graph shows the relationship between RPM and compression. The rate of compression changes due to the change in RPM.
It does not show the change of compression vs a rate change in RPM. The graph is discrete compression numbers vs discrete RPM's. It would be a very complicated graph that would try to represent the change in compression vs the rate of change in RPM. In fact you can't do it. You would have to either create a 3 dimensional graph, or you would have to create multiple graphs showing varying RPM change rates vs compression. That would be very complicated to both represent and to actually study.
dj
Actually, I have owned an engine that could be hand cranked, although I never tried to start it that way. The crank for that engine had no gearing to it - the handle connected to a piece on the crankshaft that would kick it out when the engine started.
It remains that I can watch my engine start while the starter cranks it, and no way it achieves 200rpm before starting. It usually achieves less than 1 rpm before it starts. I'm sure it achieves the equivalent of 200rpm during the 4" the piston needs to move to get compression/ignition.
Does your engine spin up to 200rpm before it will start?
BTW, that post about using the nut to spin the engine was mine, and I was talking about using the nut on the crankshaft, not the alternator. I suspect most engines trying to be started by turning the alternator would just have belt slippage.
Mark
You may be confusing scalar and vector quantities. RPM is scalar - it has no direction. When it is applied to a piston, then it becomes a vector quantity describing a rate of change of the piston, or the rate of change of compression if looking at what the piston is doing at the time. It is easy to graph this in two dimensions, and the paper did just that.
So the vector rate of change of the piston or compression can certainly be higher than the engine scalar value of rpm. It will be equivalent to rpm, but only for a short distance. For example, if you had a timing light that could count 1/10 revolutions and measure the engine speed for 1/2 of a revolution, it would indicate 200rpm, although the engine never did a single full revolution. Meanwhile, that piston would have experienced a rate of change over 4" equivalent to 200rpm.
Mark
Was the engine that you hand cranked, or could, gasoline or diesel? The only direct, no gearing, crank start engines I've seen are gasoline engines.
Yes, but then it was said he couldn't reach his crank shaft and was looking to using his alternator nut.
Any diesel engine with an electric starter will have the torque to spin the engine up to the required starting RPM even though it may not need to spin the engine multiple times.
I was once working on a diesel engine we couldn't get to start. The batteries were full, everything was perfect, but the engine would just sit there and crank over and not start. One of the guys there came over and was listening and said, the starter isn't spinning the motor fast enough. We changed out the starter and the engine fired up in less than one rotation.
dj
I'm very aware of the difference between scalar and vector quantities.
The relationship between them dealing with a specific engine is directly related to details of the specific engine and the size of the goose neck of the crank shaft, etc. The relationship between RPM and linear velocity of the piston is simply a sine wave. If you add in acceleration of RPM - the sine wave gets distorted - if trying to incorporate varying accelerations, it gets hard to graph. But all this said, all of these are variations on the sine wave and the linear velocity of the piston at top dead center is exactly the same in every single internal combustion engine using pistons - it' s 0. So linear velocity of the piston versus rotational speed of the crankshaft is a useless number when designing a starting system - we already know the linear velocity of the piston is 0 at top dead center.
Nothing in that paper nor the graph, is related to the change of linear velocity with respect to position of the piston. One of the limitations of piston driven internal combustion engines is the inefficiency related to the constant loss of energy by the fact the piston goes to 0 linear velocity twice in each 360 degree rotation of the crank - once at the top of the stroke and once at the bottom of the stroke. All of the energy needed for both acceleration and deceleration is simply lost energy to the drive shaft. That's one of the reasons the Wankle engine was originally thought it would be a major break-through in internal combustion engines was the fact this energy loss did not exist.
There was no relationship between rotational velocity and linear velocity expressed in that paper- if there was, you would have seen a sine curve. But I'm not really sure what this discussion does for someone trying to figure out how to hand crank a diesel engine. It's irrelevant if one wishes to talk linear velocity of the piston - quite difficult to measure on your own boat - or the rotational velocity of the crankshaft - a much easier measurement on your own boat - the point is: you need to achieve not only the torque, but the speed to achieve ignition in a diesel engine.
dj
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