This invention relates generally to diesel-electric traction vehicles such as locomotives that have turbocharged diesel engines on board, and it relates more particularly to improved means for preventing undesirable smoke in the exhaust of a locomotive engine when the engine speed and/or load is increased.
Large self-propelled traction vehicles such as locomotives commonly use a thermal prime mover (typically a 16-cylinder turbocharged diesel engine) to drive an electrical transmission comprising generating means for supplying electric current to a plurality of direct current (d-c) traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator. These voltages are rectified and applied to the armature windings of the traction motors.
During the "motoring" or propulsion mode of operation, a locomotive diesel engine tends to deliver constant power, depending on throttle setting and ambient conditions, regardless of locomotive speed. For maximum performance, the electrical power output of the traction alternator must be suitably controlled so that the locomotive utilizes full engine power. For proper train handling, intermediate power output levels are provided to permit graduation from minimum to full output. But the load on the engine must not exceed whatever level of power the engine can develop. Overloads can cause premature wear, engine stalling or "bogging," excessive exhaust smoke, or other undesirable effects. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power, in discrete steps between zero and maximum, and so that the engine develops whatever level of power the traction and auxiliary loads demand.
Engine horsepower is proportional to the product of the angular velocity at which the crankshaft turns and the torque opposing such motion. For the purpose of varying and regulating the amount of available power, it is common practice to equip a locomotive engine with a speed regulating governor which adjusts the quantity of pressurized diesel fuel (i.e., fuel oil) injected into each of the engine cylinders so that the actual speed (RPM) of the crankshaft corresponds to a desired speed. The desired speed is set, within permissible limits, by a manually operated lever or handle of a throttle that can be selectively moved in eight steps or "notches" between a low power position (N1) and a maximum power position (N8). The throttle handle is part of the control console located in the operator's cab of the locomotive. (In addition to the eight conventional power notches, the handle has an "idle" position and a "shutdown" position).
The position of the throttle handle determines the engine speed setting of the associated governor. In a typical governor system, the output piston of an electro-hydraulic device is drivingly connected, via a mechanical linkage, to a pair of movable fuel pump racks which in turn are coupled to a plurality of fuel injection pumps that respectively meter the amounts of fuel supplied to the power cylinders of the engine. The governor compares the desired speed (as commanded by the throttle) with the actual speed of the engine, and its output piston moves the fuel racks as necessary to minimize any deviation therebetween.
For each of its eight different speed settings, the engine is capable of developing a corresponding constant amount of horsepower (assuming maximum output torque). When the throttle notch 8 is selected, maximum speed (e.g., 1,050 rpm) and maximum rated gross horsepower (e.g., 4,000) are realized. Under normal conditions the engine power at each notch equals the power demanded by the electric propulsion system which is supplied by the engine-driven main alternator plus power consumed by certain electrically and mechanically driven auxiliary equipments.
The output power (KVA) of the main alternator is proportional to the product of the rms magnitudes of generated voltage and load current. The voltage magnitude varies with the rotational speed of the engine, and it is also a function of the magnitude of excitation current in the alternator field windings. For the purpose of accurately controlling and regulating the amount of power supplied to the electric load circuit it is common practice to adjust the field strength of the traction alternator to compensate for load changes and minimize the error between actual and desired KVA. The desired power depends on the specific speed setting of the engine. Such excitation control will establish a balanced steady-state condition which results in a substantially constant, optimum electrical power output for each position of the throttle handle.
In practice the above-summarized system of controlling a diesel-electric locomotive also includes suitable means for overriding normal operation of the system and reducing engine load in response to certain temporary abnormal conditions, such as loss of wheel adhesion, low pressure in the lubricating oil system or the engine coolant system, or a load exceeding the power capability of the engine at whatever speed the throttle is commanding. This response, which is generally referred to as "deration," helps the locomotive recover from such conditions and/or prevents serious damage to the engine. In addition, the excitation control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits. Current limit is effective when the locomotive is accelerating from rest. At low locomotive speeds, the traction motor armatures are rotating slowly, so their back emf is low. A low alternator voltage can now produce maximum load current which in turn produces the high tractive effort required for acceleration. On the other hand, the alternator voltage magnitude must be held constant at its maximum level whenever locomotive speed is high. At high speeds the traction motor armatures are rotating rapidly and have a high back emf, and the alternator voltage must then be high to produce the required load current.
To increase the maximum amount of useful power that a locomotive engine of given size can develop when running at a given speed, it is usual practice to equip the engine with a supercharger. For a 4-stroke diesel engine (which hereinafter is assumed), it is advantageous to use a free-wheeling centrifugal supercharger (commonly known as a turbocharger) the rotor assembly of which is driven by the engine exhaust gases. The turbocharger raises the air pressure in the intake manifold of the engine, whereby each cylinder is supplied with more fresh air during the intake stroke of its piston. This permits more fuel to be burned in the cylinder, and therefore the expanding products of combustion will exert more force on the piston during each power stroke.
When the throttle handle of the above-summarized locomotive is advanced from a relatively low notch to a higher power notch, the engine speed governor responds by immediately increasing the amount of fuel injected into the engine cylinders in an attempt to increase engine speed to the new speed as set by the throttle. At the same time, the throttle commands the excitation control system to strengthen the field of the traction alternator so that the traction load on the engine will increase to whatever magnitude is determined by the new throttle setting. However, the rate at which the load is actually applied needs to be controlled in order to prevent engine bogging and undesirable smoke.
A finite period of time is required for a large diesel engine to accelerate from a relatively low speed to a higher speed. During this period an appreciable portion of the power developed by the engine is being used to raise the angular velocity of the engine crankshaft and the rotating mass that it drives. During the same period the speed of the free-wheeling turbocharger proportionately lags behind the speed of the accelerating engine, particularly so while the traction load is relatively light. (The turbo speed, and hence the amount of combustion air supplied to the intake manifold, depends on the energy in the engine exhaust gases and is nearly a linear function of the engine's gross horsepower.) The relatively fast response of the engine speed governor and slower response of the turbocharger when the throttle is advanced make it possible to supply more fuel to the engine than can be burned efficiently with the air available from the turbocharger. As a result, there is a transient imbalance of fuel and air that leads to poor combustion and hence the undesirable emission of visible smoke (i.e., unburned fuel) from the engine exhaust stack.
For the foregoing reasons it is conventional practice to include a time delay or slow rise circuit in the alternator excitation control system so as to delay the application of traction load to the engine in response to any increase of the throttle setting. In one prior art locomotive, the maximum loading rate is limited to (1) a first relatively low, constant value so long as the actual KVA is less than a preselected first amount, (2) another, substantially higher value if the KVA were more than a preselected second amount which is higher than the aforesaid first amount, and (3) a predetermined intermediate value when the KVA is between the preselected first and second amounts. By thus controlling the rate of change of traction power demand following movement of the throttle from an idle or low setting to a higher setting, the engine speed is allowed to increase rapidly to its new setting without overloading the engine, while the engine horsepower (and hence the required amount of fuel) will gradually change, in concert with the increasing supply of air from the more slowly accelerating turbocharger, to the new power setting. Of course any delay in loading undesirably reduces the productivity of the locomotive, and accordingly the loading rate of this prior art locomotive is increased from its initially low value to the aforesaid intermediate value and then to an even higher value as the actual power (and hence the turbo acceleration) attains each of the two progressively higher preselected levels.
There are other known techniques that help to reduce or avoid excessively smoky exhaust during load and speed changes. One is to design the governor to limit the maximum available fuel as a function of manifold air pressure and to call for reduced engine load as the fuel-limit point is approached, thereby. approximating a proper air-to-fuel ratio for complete combustion. This technique is very useful but not perfect because it has a relatively slow response time and because it is difficult to adjust for optimum smoke control over the entire horsepower (i.e., throttle) range.
Another prior art technique for minimizing smoke in the exhaust of a locomotive engine when accelerating is disclosed and claimed in U.S. Pat. No. 3,878,400-McSparran. According to the McSparran patent, when the throttle is advanced to a higher notch the reference voltage that normally determines the power output of the traction alternator will increase to its new value at a predetermined limited rate, and during the transition the alternator excitation is controlled as a function of the turbocharger speed. Consequently the rate at which the engine load actually increases will track the increasing turbocharger speed. When properly applied, this "bootstrapping" method can satisfactorily control smoke during acceleration transients. However, some problems have been encountered in practice due either to variations in the characteristics of different turbochargers or to the variations in turbo performance that can result from ambient temperature or barometric pressure changes.