Wheel Slippage, Coupler Failure, Slack Management and Bad Winter Drivers

Posted by PITHOCRATES - January 8th, 2014

Technology 101

Starting a Train to Move is like Starting a Car to Move on Snow and Ice

Starting and stopping a train takes great skill.  Because one of the greatest advantages of rail transport is also one of its greatest weakness.  Steel wheels and steel rails.  With very little friction between the two.  Allowing trains to travel very efficiently.  Rolling effortless over great distances.  Once they get moving, that is.  Which is where that skill comes in.

Starting a train to move is like starting a car to move on snow and ice.  If you stomp the accelerator the wheels will just spin on the snow and ice.  Just as steel wheels on steel rails will.  Because of the low amount of friction between the two.  The throttle on a North American locomotive has 8 ‘run’ positions.  And one ‘idle’ position.   The engineer starts the train moving by moving the throttle to position one.  As the train begins picking up speed the engineer advances the throttle through all the positions until reaching run eight.

As the engineer moves the throttle he (we will use the pronoun ‘he’ for simplicity in lieu of ‘he or she’) watches the amp meter and wheel slip indicator.  Which is why he advances the throttle through each position.  To slowly start the train moving.  If he ‘stomped the accelerator’ the wheels would slip and spin freely on the steel rail.  Damaging both wheels and rail.  Without moving the train.  In addition to preventing wheel slippage he is also trying to prevent one other thing.  Coupler failure.

Getting a Train Moving is Difficult but Keeping it Safely on the Track can be Harder

Driving a train is a study in slack management.  Each coupler on a train has slack in it.  They are not permanently affixed to the railcar or engine.  They can move forward and backward a little bit.  With a shock absorbing device that deals with the compression and tension forces between cars.  This slack exists at each coupler.  The longer the train the more couplers and the more slack.  When a train starts moving it takes very little effort to pick up the slack in a coupler.  But it takes a lot more effort to get the car moving once you do pick up the slack.  And if you apply that force too quickly you can snap the coupler right off of the car.

An engineer picks up this slack by moving slowly while in run one.  And he moves slowly by having the brakes partially set.  That is, he moves the throttle to run one and slowly releases some air in the train line.  As he does the brakes release.  A little bit.  Just enough to allow the train to move at a crawl.  Slowly picking up the slack without breaking a coupler.  Once he picks up all the slack he releases the brakes completely.   And slowly picks up speed.  Able to pull great weights of freight trailing behind as there is so little friction between steel wheels and steel rail.

Of course, that is also a problem.  For curves.  Where the engineer has to slow the train down so the centrifugal force doesn’t pull the train off the tracks.  Or on gradients.  Where the engineer has to slow the train on downhill portions to prevent a runaway.  Or add sand to the track on uphill runs (through automatic sand feeders in front of the drive wheels).  To prevent wheel slippage by adding friction between the wheel and track.  Getting a train moving is difficult.  But keeping it safely on the track can be harder.  Which requires the ability to slow a train in time for curves and downhill gradients.  Which takes time.  And a mile or so of track.

When it comes to Driving a Car in the Winter you have to approach it like Driving a Train

Driving a train is like driving a car on snow and ice.  There’s a lot of wheel slippage.  It’s difficult to slow down.  And you really have to slow down for curves.  For if you turn the steering wheel at speed your front wheels will just slide across the snow and ice and the car will keep going straight.  If you stomp on the brake pedal and lock the wheels your wheels will just slide across the snow and ice in the general direction you were traveling in.  Today, modern cars have systems to help people drive on snow and ice.  Like anti-lock brake systems.  And traction control systems.

An anti-lock brake system prevents the wheels from locking up during braking.  The system monitors wheel rotation.  If it senses a wheel that is no longer rotating it will begin pulsating the brakes.  Applying and releasing the brakes some 15 times a second.  So the wheel keeps rotating, giving the driver control.  A traction control system also monitors wheel rotation.  If it senses a wheel rotating faster than another (because it’s spinning in ice and snow) it will slow that wheel and/or apply more power to the non-slipping wheel.  Giving today’s drivers more control of their cars in the ice and snow.

Of course none of these systems will help if the driver is irresponsible behind the wheel.  And lazy.  If you don’t shovel your driveway after it snows.  Or if you do but push that snow into the street in your driveway approach.  For a car needs to have the rubber in contact with the pavement for traction.  If not you get wheel slippage.  And we all probably have a neighbor who thinks the best thing to do when this happens is to step down on the accelerator.  To spin those wheels faster.  And does.  Digging a hole in the snow.  And then begins swearing because the stupid car got stuck in the snow.

When it comes to driving a car in the winter you have to approach it like driving a train.  You need to start slowly and monitor your wheel slippage.  Sometimes it’s best to just let the engine idle in gear to slowly get the car moving.  Then once the car is moving on top of the snow and ice you can slowly increase the speed.  But never so much to cause wheel slippage which will just dig a hole in the snow and ice that you may not be able to drive out of.  And you have to start slowing down long before you have to stop.  Always being careful not to lock your wheels.  Simple stuff.  Something every driver can do.  For these are things every engineer does.  And driving a locomotive is a lot more difficult than driving a car.



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Generator, Current, Voltage, Diesel Electric Locomotive, Traction Motors, Head-End Power, Jet, Refined Petroleum and Plug-in Hybrid

Posted by PITHOCRATES - June 6th, 2012

Technology 101

When the Engineer advances the Throttle to ‘Run 1’ there is a Surge of Current into the Traction Motors

Once when my father suffered a power outage at his home I helped him hook up his backup generator.  This was the first time he used it.  He had sized it to be large enough to run the air conditioner as Mom had health issues and didn’t breathe well in hot and humid weather.  This outage was in the middle of a hot, sweltering summer.  So they were eager to get the air conditioner running again.  Only one problem.  Although the generator was large enough to run the air conditioner, it was not large enough to start it.  The starting in-rush of current was too much for the generator.  The current surged and the voltage dropped as the generator was pushed beyond its operating limit.  Suffice it to say Mom suffered during that power outage.

Getting a diesel-electric locomotive moving is very similar.  The massive diesel engine turns a generator.  When the engineer advances the throttle to ‘Run 1’ (the first notch) there is a surge of current into the traction motors.  And a drop in voltage.  As the current moves through the rotor windings in the traction motors it creates an electrical field that fights with the stator electrical field.  Creating a tremendous amount of torque.  Which slowly begins to turn the wheels.  As the wheels begin to rotate less torque is required and the current decreases and voltage increases.  Then the engineer advances the throttle to ‘Run 2’ and the current to the traction motors increases again.  And the voltage falls again.  Until the train picks up more speed.  Then the current falls and the voltage rises.  And so on until the engineer advances the throttle all the way to ‘Run 8’ and the train is running at speed. 

The actual speed is controlled by the RPMs of the diesel engine and fuel flow to the cylinders. Which is what the engineer is doing by advancing the throttle.  In a passenger train there are additional power needs for the passenger cars.  Heating, cooling, lights, etc.  The locomotive typically provides this Head-End Power (HEP).  The General Electric Genesis Series I locomotive (the aerodynamic locomotive engines on the majority of Amtrak’s trains), for example, has a maximum of 800 kilowatts of HEP available.  But there is a tradeoff in traction power that moves the train towards its destination.  With a full HEP load a 4,250 horsepower rated engine can only produce 2,525 horsepower of traction power.  Or a decrease of about 41% in traction horsepower due to the heating, cooling, lighting, etc., requirements of the passenger cars.  But because passenger cars are so light they can still pull many of them with one engine.  Unlike their freight counterparts.  Where it can take a lashup of three engines or more to move a heavy freight train to its destination.  Without any HEP sapping traction horsepower.

There is so much Energy available in Refined Petroleum that we can carry Small Amounts that take us Great Distances

The largest cost of flying a passenger jet is jet fuel.  That’s why they make planes out of aluminum.  To make them light.  Airbus and Boeing are using ever more composite materials in their latest planes to reduce the weight further still.  New engine designs improve fuel economy.  Advances in engine design allow bigger and more powerful engines.  So 2 engines can do the work it took 4 engines to do a decade or more ago.  Fewer engines mean less weight.  And less fuel.  Making the plane lighter and more fuel efficient.  They measure all cargo and count people to determine the total weight of plane, cargo, passengers and fuel.  So the pilot can calculate the minimum amount of fuel to carry.  For the less fuel they carry the lighter the plane and the more fuel efficient it is.   During times of high fuel costs airlines charge extra for every extra pound you bring aboard.  To either dissuade you from bringing a lot of extra dead weight aboard.  Or to help pay the fuel cost for the extra weight when they can’t dissuade you.

It’s similar with cars.  To meet strict CAFE standards manufacturers have been aggressively trying to reduce the weight of their vehicles.  Using front-wheel drive on cars saved the excess weight of a drive shaft.  Unibody construction removed the heavy frame.  Aerodynamic designs reduced wind resistance.  Use of composite materials instead of metal reduced weight.  Shrinking the size of cars made them lighter.  Controlling the engine by a computer increased engine efficiencies and improved fuel economy.  Everywhere manufacturers can they have reduced the weight of cars and improved the efficiencies of the engine.  While still providing the creature comforts we enjoy in a car.  In particular heating and air conditioning.  All the while driving great distances on a weekend getaway to an amusement park.  Or a drive across the country on a summer vacation.  Or on a winter ski trip.

This is something trains, planes and automobiles share.  The ability to take you great distances in comfort.  And what makes this all possible?  One thing.  Refined petroleum.  There is so much energy available in refined petroleum that we can carry small amounts of it in our trains, planes and automobiles that will take us great distances.  Planes can fly halfway across the planet on one fill-up.  Trains can travel across numerous states on one fill-up.  A car can drive up to 6 hours or more doing 70 MPH on the interstate on one fill-up.  And keep you warm while doing it in the winter.  And cool in the summer.  For the engine cooling system transfers the wasted heat of the internal combustion engine to a heating core inside the passenger compartment to heat the car.  And another belt slung around an engine pulley drives an air conditioner compressor under the hood to cool the passenger compartment.  Thanks to that abundant energy in refined petroleum creating all the power under the hood we need.

The Opportunity Cost of the Plug-in Hybrid is giving up what the Car Originally gave us – Freedom 

And then there’s the plug-in hybrid car.  That shares some things in common with the train, plane and (gasoline-powered) automobile.  Only it doesn’t do anything as well.  Primarily because of the limited range of the battery.  Electric traction motors draw a lot of current.  But a battery’s storage capacity is limited.  Some batteries offer only about 20-30 miles of driving distance on a charge.  Which is great if you use a car for very, very short commutes.  But as few do manufacturers add a backup gasoline engine so the car can go almost as far as a gasoline-powered car.  It probably could go as far if it wasn’t for that heavy battery and generator it was dragging around with it.

This is but one of many tradeoffs required in a plug-in hybrid car.  Most of these cars are tiny to make them as light as possible.  For the lighter the car is the less current it takes to get it moving.  But adding a backup gasoline engine and generator only makes the car heavier.  Thus reducing its electric range.  Making it more like a conventional car for a trip longer than 20-30 miles.  Only one that gets a poorer fuel economy.  Because of the extra weight of the battery and generator.  Manufacturers have even addressed this problem by reducing the range of the car.  If people don’t drive more than 10 miles on a typical trip they don’t need such a large battery.  Which is ideal if you use your car to go no further than you normally walk.  A smaller battery means less weight due to the lesser storage capacity required to travel that lesser range.  Another tradeoff is the heating and cooling of the car.  Without a gasoline engine on all of the time these cars have to use electric heat.  And an electric motor to drive the air conditioner compressor.  (Some heating and cooling systems will operate when the car is plugged in to conserve battery charge for the initial climate adjustment).  So in the heat of summer and the cold of winter you can scratch off another 20% of your electric range (bringing that 20 miles down to 16 miles).  Not as bad as on a passenger locomotive.  But with its large tanks of diesel fuel that train can still take you across the country.

The opportunity cost of the plug-in hybrid is giving up what the car originally gave us.  Freedom.  To get out on the open road just to see where it would take us.  For if you drive a long commute or like to take long trips your hybrid is just going to be using the backup gasoline engine for most of that driving.  While dragging around a lot of excess weight.  To make up for some lost fuel economy some manufacturers use a gasoline engine with high compression.  Unfortunately, high compression engines require the more expensive premium (higher octane) gasoline.  Which costs more at the pump.  There eventually comes the point we should ask ourselves why bother?  Wouldn’t life and driving be so much simpler with a gasoline-powered car?  Get fuel economy with a range of over 300 miles?  Guess it all depends on what’s more important.  Being sensible.  Or showing others that you’re saving the planet.



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