Is there any reason why one should not start a diesel routinely in the decompression mode (other than convenience)? It would certainly be easier on the battery. In case it matters, I have a Yanmar lGM.
diesel starting
- Thread starter gmeyerga
- Start date
- Status
well...
It won't start in decompression mode. When you drop the lever your puting a shock load on the starter. Remember an electric motor develops peak torque at 0 rpm. Pat McCartin Inland Marine Diesel Buford, GA imd_ga@hotmail.com
It won't start in decompression mode. When you drop the lever your puting a shock load on the starter. Remember an electric motor develops peak torque at 0 rpm. Pat McCartin Inland Marine Diesel Buford, GA imd_ga@hotmail.com
Very good point, Pat
I would also think that the immediate spin (and lack of resistence) on the starter may cause premature wear on the starter. Therefore, one should save the decompression starting for an emergency?
I would also think that the immediate spin (and lack of resistence) on the starter may cause premature wear on the starter. Therefore, one should save the decompression starting for an emergency?
Wrong - It is a lot LESS strain on the starter
to spin up the engine without compression. Then if memory serves me correctly, because the angular momentum of the engine is conserved (simple physics), the starter has FAR less work to do when the compression lever is dropped. If it is not easier on the engine and starter, why is this the preferred method when ones batteries are low? That logic alone tells me that it is much easier on the starting system. If you add to that the fact that when the engine spins in decompression mode, it begins to pump oil to the vital engine bearing surfaces. That reduces friction and is better for all of the engine bearing surfaces than just cranking with compression. The usual starting method is harder on the starter and harder on the engine. As I understand it, most engine wear occurs during the first few seconds or minutes after starting due to lack of immediate bearing lubrication.
to spin up the engine without compression. Then if memory serves me correctly, because the angular momentum of the engine is conserved (simple physics), the starter has FAR less work to do when the compression lever is dropped. If it is not easier on the engine and starter, why is this the preferred method when ones batteries are low? That logic alone tells me that it is much easier on the starting system. If you add to that the fact that when the engine spins in decompression mode, it begins to pump oil to the vital engine bearing surfaces. That reduces friction and is better for all of the engine bearing surfaces than just cranking with compression. The usual starting method is harder on the starter and harder on the engine. As I understand it, most engine wear occurs during the first few seconds or minutes after starting due to lack of immediate bearing lubrication.
My oppinion....
I am not a physist or an enginer. I've built many diesel and gas engines over the years. I understand how an engine is desinged and assembled. Patrick, You are partialy correct. The majority of engine wear does come at start up. BUT all bearing surfaces retain a small film of oil after shut down to protect them at startup. The oil filter has a check valve to achieve oil pressure quicker. I disagree that the loads are easier on the starter. I'd like an enginer to analize this. If you get the starter spining with no compression and shock load it with compression in the range of 250# the amount of current draw increase has to be detrimental to the starter. The reason this is the prefered method when your batteries are low is simple. The batteries can't produce enough power to overcome the compression and turn the engine a full rotation let alone multiple rotations. Once you get the 40# mass of the flywheel spinning inertia helps the weak starter. This is also the reason it's posible to hand start a small diesel. The above is my oppinion. Flames will be ignored. Pat McCartin Inland Marine Diesel Buford, Ga imd_ga@hotmail.com
I am not a physist or an enginer. I've built many diesel and gas engines over the years. I understand how an engine is desinged and assembled. Patrick, You are partialy correct. The majority of engine wear does come at start up. BUT all bearing surfaces retain a small film of oil after shut down to protect them at startup. The oil filter has a check valve to achieve oil pressure quicker. I disagree that the loads are easier on the starter. I'd like an enginer to analize this. If you get the starter spining with no compression and shock load it with compression in the range of 250# the amount of current draw increase has to be detrimental to the starter. The reason this is the prefered method when your batteries are low is simple. The batteries can't produce enough power to overcome the compression and turn the engine a full rotation let alone multiple rotations. Once you get the 40# mass of the flywheel spinning inertia helps the weak starter. This is also the reason it's posible to hand start a small diesel. The above is my oppinion. Flames will be ignored. Pat McCartin Inland Marine Diesel Buford, Ga imd_ga@hotmail.com
Pat, it is a simple issue
All due respect, it takes a certain given amount of energy to start a given engine. Think of it as amp-hours. Spinning up the engine puts some energy into the system over a longer time period with less strain (fewer amps for more time) to the starter. Just look at the amps when you do it either way. The current required to start under compression is far more proportionally (far more amps for shorter time) than getting the flywheel spinning. It is logical that if you can start an engine one way and not the other with weak batteries (less amp-hours available), that way is taking less energy. Less energy in this case equals less strain and less wear on all of the parts of the system including batteries. Lubrication is just an added benefit. I have rarely seen flames when starting an engine but it was a very exciting event which I could hardly ignore. Did you read the fire extinguisher thread? I am sure someone with your expertise could add a story or two on that subject.
All due respect, it takes a certain given amount of energy to start a given engine. Think of it as amp-hours. Spinning up the engine puts some energy into the system over a longer time period with less strain (fewer amps for more time) to the starter. Just look at the amps when you do it either way. The current required to start under compression is far more proportionally (far more amps for shorter time) than getting the flywheel spinning. It is logical that if you can start an engine one way and not the other with weak batteries (less amp-hours available), that way is taking less energy. Less energy in this case equals less strain and less wear on all of the parts of the system including batteries. Lubrication is just an added benefit. I have rarely seen flames when starting an engine but it was a very exciting event which I could hardly ignore. Did you read the fire extinguisher thread? I am sure someone with your expertise could add a story or two on that subject.
In the navy, I used to start Lister diesel gen....
generators by hand crank all the time, that was the only way we started them. Cranked them up as fast as I could with the decompression on, and then flipped them off while still cranking. I dont think I could have ever started them without the decompression on.
generators by hand crank all the time, that was the only way we started them. Cranked them up as fast as I could with the decompression on, and then flipped them off while still cranking. I dont think I could have ever started them without the decompression on.
Well I guess...
we'll have to agree to disagree. I stand by my oppinion and posts untill proven wrong.
we'll have to agree to disagree. I stand by my oppinion and posts untill proven wrong.
starter shock load
I take it for granted that starting in the decompression mode is easier for the battery, but the question of shock load on the starter when kicking in the compression is unresolved. Even in a regular start the engine spins before firing so there is a shock load on the starter. It would seem to me that the shock is porportional to the speed difference before and after firing. In that case a faster spinning engine before firing would see a lower shock load than an engine turning against compression - unless the engine spins fast enough for the starter to experience a shock in the reverse direction or which it is not designed. Could that happen? There must be a reason why engine designs make the starter work so hard to get the engine going rather than mimic a handstart.
I take it for granted that starting in the decompression mode is easier for the battery, but the question of shock load on the starter when kicking in the compression is unresolved. Even in a regular start the engine spins before firing so there is a shock load on the starter. It would seem to me that the shock is porportional to the speed difference before and after firing. In that case a faster spinning engine before firing would see a lower shock load than an engine turning against compression - unless the engine spins fast enough for the starter to experience a shock in the reverse direction or which it is not designed. Could that happen? There must be a reason why engine designs make the starter work so hard to get the engine going rather than mimic a handstart.
- Status