Taylor Made: CHI-GSC Part I - Big Machines
In January and again in March of this year, I took the Amtrak train California Zephyr from Chicago to Glenwood Springs, Colorado. The following two-part piece discusses my trip experience as well as the mechanical aspect of train travel.
Amtrak, which began operations in 1971, got its name by blending the words “America” and “track.” As of last year, Chicago’s Union Station was its fourth busiest station, behind those in New York City, Washington DC, and Philadelphia respectively.
Amtrak currently operates fifteen long-distance trains. Besides the California Zephyr, there’s the Capitol Limited, Cardinal, City of New Orleans, Coast Starlight, Crescent, Empire Builder, Lake Shore Limited, Palmetto, Silver Meteor, Silver Star, Southwest Chief, Sunset Limited, and Texas Eagle. Last but not least is the Auto Train, which carries cars from Virginia to Florida over the course of 17.5 hours. The long-distance routes range from 780 miles (Capitol Limited) to 2,728 miles (Texas Eagle) one way. The California Zephyr’s route is second longest at 2,438 miles one way—my ride to GSC covered more than 1,100 over the course of 25 hours. The final stop in Emeryville, California, is a 52-hour ride from the starting point in Chicago.
California Zephyr trains are typically driven by two P42DC locomotives. The P42DC is part of the General Electric Genesis locomotive series and weighs roughly 268,000 pounds, or 134 tons. It has an engine output of 4,250 horsepower and a maximum speed of 110 miles per hour, with a DC traction motor directly attached to each of the four pairs of wheels.
The four-stroke diesel engine on board the P42DC is used to generate mechanical energy, which is then converted to electrical energy by a generator. The four traction motors act as the transmission, receiving the electricity and thus delivering energy from the engine to the wheels. This system is used in place of a traditional transmission and clutch system to prevent parts from wearing out prematurely under the heavy load.
Diesel engines are characterized as compression-ignition (CI), meaning that the fuel ignites by compression alone. Most American cars use spark ignition (SI) engines, which achieve fuel ignition using a spark plug. Both types follow the same four-stroke process, with different resulting heat cycles according to fuel, timing, etc.
The first stroke of the cycle is called the intake stroke, when the piston moves to bottom dead center (i.e. the piston head goes as low as possible in the cylinder). The downward motion draws fresh air into the cylinder from the intake manifold through an open valve. Next, during the compression stroke, the valve closes so that the cylinder is sealed. The piston moves to top dead center, compressing the air inside. When the piston hits TDC, or slightly before or after TDC depending on how the engine is tuned, the spark plug sparks to ignite the compressed air and fuel. For CI engines, the fuel ignites when a certain pressure is reached. The explosion forces the piston back to BDC, called the power stroke. Then, in the exhaust stroke, the piston returns to TDC while a second valve is open, pushing the combustion product (exhaust) out of the cylinder and into the exhaust manifold.
Combustion by pressure means that diesel engine components need to withstand much higher pressure than those of an SI engine and are more robust. Diesel engines also present a unique phenomenon known as the soot-NOX trade-off. Soot consists mainly of black carbon, while NOX refers to nitrogen oxide pollutants (NO, NO2, etc.). Diesel contains hydrocarbon chains, which form COX products such as carbon dioxide and carbon monoxide when reacted with oxygen from air, and oxygen, which can react with nitrogen in the air to produce NOX.
Lowering the air:fuel ratio (i.e. making the mixture more fuel-rich) causes an excess of hydrocarbon chains in the cylinder chamber. Without sufficient oxygen, some of the chains do not react to form COX products and instead break down to become an exhaust product in the form of soot. Conversely, leaning out the mixture by increasing the amount of air causes more of the oxygen in the fuel to react with nitrogen from the outside air and produce NOX. Diesel engines can be tuned to favor one product over the other, or to reach the point at which both products have the same low value, as shown in the sample graph. A great challenge ensues in figuring out how to engineer a diesel engine with strong performance specs that also achieves relatively low emissions through a balance between soot and NOX.
Check back for CHI-GSC part II to read about riding the train and see photos from the trip.
Image, at top: A map of Amtrak train routes with the California Zephyr track marked in thick red.