Diesel genset torque

[azuredragon]

When we run our diesel powered 250VAC alternator the boat feels like it heels a bit which I assume is caused by the torque of the engine being transferred to the hull through the engine mounts. Does this sound logical? Has anyone else observed such a phenomenon? It’s not a serious problem; just interesting.

[tanglewood]

I don’t think so. The torque to generator electricity is between the generator end and the generator engine, and is taken by the coupler housing. None should be transmitted to the hull, other than perhaps during transients.

[tkeithlu]

Whoa. The torque has to go somewhere, or the genset would be spinning rather than the generator. Gotta think it goes coupling/engine/motor mounts. It's going to end up in the water.

[tanglewood]

Originally Posted by tkeithlu

Whoa. The torque has to go somewhere, or the genset would be spinning rather than the generator. Gotta think it goes coupling/engine/motor mounts. It's going to end up in the water.

But if that were the case, wouldn't a portable generator tumble across the floor when you apply a load?

[tkeithlu]

If you were able to put bathroom scales under all four corners, you would find different "weights," AKA forces, on the two sides. All of it ends up as heat. Little ones just don't generate enough torque for you to notice. You might notice that a portable genset is sitting on springs.

[tkeithlu]

This is really the same situation as your propeller in water. Torque in your drive shaft is converted to force of your propeller working against the water. Notice that when you do this your boat tips slightly to port or starboard, depending on your propeller's direction. If the propeller was locked in ice, in theory your boat would spin rather than your propeller, just in the opposite direction. Even your car is an example. Accelerate hard, and the rear end drops while the front suspension extends. Drag racers have little wheels behind the big wheels to keep the car from flipping over backwards, and a very long "arm" for the front wheels.

[a64pilot]

There is torque, but it’s slight, not anywhere near enough to heel even a small boat.
It’s actually more of a gyroscope, yet it won’t keep your boat from moving in a seaway, the forces are just way too small.

[a64pilot]

Originally Posted by tkeithlu

This is really the same situation as your propeller in water. Torque in your drive shaft is converted to force of your propeller working against the water. Notice that when you do this your boat tips slightly to port or starboard, depending on your propeller's direction. If the propeller was locked in ice, in theory your boat would spin rather than your propeller, just in the opposite direction. Even your car is an example. Accelerate hard, and the rear end drops while the front suspension extends. Drag racers have little wheels behind the big wheels to keep the car from flipping over backwards, and a very long "arm" for the front wheels.

Torque on a car that has its crankshaft run fore and aft, which almost all drag cars do, will roll the car, not lift the front end or squat the rear, that’s acceleration, specifically the thrust being below the centerline of the car, the rear wheels literally are being forced under the car, so the front end lifts.
A car with a big motor will actually roll a little just from goosing the engine, and of course if your drag racing you almost certainly have one side of the engine chained down, cause if you don’t your going to bust engine mounts from the torque, if memory serves on an American car it was the left side that you had to chain so I guess the motor rolls to the right.
It will be opposite crankshaft rotation of course.

[Jim Cate]

A64, I think you are forgetting that the gears in the differential change the rotation from fore and aft to transverse. The tendency for dragsters to do wheelies is mostly due to the engine torque driving the rear axles and trying to climb up over the axis of the drive axles. If you nailed the tires to the ground, thus eliminating fore and aft inertial weight transfer from the equation the front wheels would still lift. The engine torque does also load one side, thus locked or limited slip differentials are used to keep both wheels driving more or less equally. A narrow tread in the drive wheels helps to keep the different tire loading from steering the car too much. (Thinking about all this is like time travel, back to the pre-sailing days).

Now, in the case of the engine directly coupled to a generator, there is no external torque delivered to the boat, other than when you accelerate the mass of the crank and rotor. The reaction to that acceleration will try to rotate the whole mass of the genset in the opposite direction to the acceleration. When at steady state, no external torque forces exist.

Jim

[Wotname]

Originally Posted by a64pilot

Torque on a car that has its crankshaft run fore and aft, which almost all drag cars do, will roll the car, not lift the front end or squat the rear, that’s acceleration, specifically the thrust being below the centerline of the car, the rear wheels literally are being forced under the car, so the front end lifts.........

Jim has correctly addressed this comment; I only wish to add the following observation - my little 16 hp 50 year tractor wants to lift the front wheels when driving off in low first and it ain't from the acceleration I promise you

[a64pilot]

Acceleration or change in inertia whichever, but the lift is I believe from the thrust centerline being below the vehicle’s centerline, just as say a Lake Amphibian airplane’s nose is pushed down whenever you add power because it’s engine is above the centerline of the aircraft.

Now chain the tractor to a stump and of course the front end will lift, but if it’s not chained to anything, surely the lift has to come from inertia tending to hold the vehicle still as the force is applied just as the chain would.
I may not be using inertia or acceleration correctly as terms, but it’s the rapid application of force to move the vehicle that lifts the front, and not engine torque.

I’m also not sure that in steady state there isn’t torque either, I know a helicopter needs a tail rotor to counter torque, in a steady state.

[Jim Cate]

I may not be using inertia or acceleration correctly as terms, but it’s the rapid application of force to move the vehicle that lifts the front, and not engine torque.

Nope, I must disagree. No new arguments, but in this case I think you are just wrong.

Having the CoG up high does add weight transfer to the rear wheels under acceleration (I was a pioneer in use of this idea in drag classes where the engine could only be moved back 25% of the wheelbase), and it certainly helps), but this is a minor effect compared to the drive torque trying to lift the front end.

Jim

[Captain Bill]

I would argue that any time you have rotary motion you have torque. The effects depend on the moments of inertia of the two items in question. In this case the engine and generator head. If neither were attached to anything i.e floating in space the parts would rotate in opposite directions proportionally to the ration of their moments of inertia. Of course since there is no reference plane in space one could decide arbitrarily that one or the other was sitting still and the other was rotating. Here on earth the ground or water gives us a fixed reference plane and since the engine is coupled to it either by the boat or and engine mount to the ground the engine has a a huge moment of inertia and the generator head has a very small moment. Running in a steady state the engine only has to provide enough torque to replace the energy being extracted in the form of electricity and of course bearing and cooling friction. If the engine stopped applying this torque the generator head would come to a halt rather quickly as the energy stored in the rotating mass of the armature would be quickly extracted in the form of electricity. Remembering that for every action there is an equal and opposite reaction for every ft-lb or newton of torque applied the the generator head an equal and opposite force is applied to the engine. Only the large moment of inertia made by coupling (even if only by gravity and a frame of some sort) to the large mass of the earth prevents the engine from spinning. If this coupling is rather flexible such as on a floating boat there may in fact be detectable movement between the boat and the earth. In a monohull not in motion the keel might be heeled until the righting force (which is also a torque) is equal to the torque being applied to the armature by the engine.



In AH64s description of helicopters the torque applied to the rotor blades in a steady state is simply to replace the energy being extracted from the blades to keep the helicopter in the air. The tail rotor is used to counteract this torque and prevent the helicopter from rotating the the opposite direction of the blades. The long tail boom increases the moment arm of the anti-torque rotor allowing it to be much smaller than main rotors. Most helicopters moving forward also are designed to get some of the anti-torque effect from aerodynamic forces. Even in the event of a tail rotor failure at low speed the fuselage of the helicopter will rotate in the opposite direction of the main rotor much slower than the rotors because it has a much higher inertia than the light weight rotors. If a helicopter has to lift a larger load more energy is required to lift the load and more torque must be applied to the main rotors and more counter torque is required from the tail rotor.



On my boat the generator is mounted so that the torque is applied to the boat along the fore-aft axis and since the boat has a huge moment of inertia along that axis any movement is essentially undetectable. If I had a rather narrow monohull and mounter the genset so that the torque was applied along the roll axis I would not be surprised at all if there was a bit of detectable heel, especially if there was not a heavy keel and one needed to depend on buoyancy to counter the torque.

[tanglewood]

Originally Posted by tkeithlu

This is really the same situation as your propeller in water. Torque in your drive shaft is converted to force of your propeller working against the water. Notice that when you do this your boat tips slightly to port or starboard, depending on your propeller's direction. If the propeller was locked in ice, in theory your boat would spin rather than your propeller, just in the opposite direction. Even your car is an example. Accelerate hard, and the rear end drops while the front suspension extends. Drag racers have little wheels behind the big wheels to keep the car from flipping over backwards, and a very long "arm" for the front wheels.

You see, I think it's quite different from a propulsion engine. In a gen set the driving torque comes from the engine, and the restive counter torque comes from the generator end. The two are bolted together, allowing for transfer of the torque through the frames of the parts.


For a propulsion motor the driving torque once again comes from the engine. But the counter torque comes from the prop pushing against the water. So the engine is pushing against the water. The engine and water are coupled via the engine mounts, and the weight of the boat, or more precisely it's resistance to heeling, and resistance to moving forward. But it all travels through the engine mounts in one form or another. If you unbolted the engine mounts, the engine would definitely tumble across the engine room/compartment.



The rubber mounts on a generator are for vibration isolation. Now there sill be some torque in the system where there are transition in the moment of inertia of the moving parts, but that's pretty small.

[tkeithlu]

Enough of basic physics class for one day.