Diesel Engine

DIESEL BASIC
At first glance, a diesel engine looks like a heavy-duty gasoline engine, minus spark plugs and ignition wiring (Fig. 2-1). Some manufacturers build compression ignition (CI) and spark ignition (SI) versions of the same engine
Caterpillar G3500 and G3600 SI natural-gas fueled engines are built on diesel frames and use the same blocks, crankshafts, heads, liners, and connecting rods. But there are important differences between CI and SI engines that cut deeper than the mode of igniting the fuel

When air is compressed, collisions between molecules produce heat that ignites the diesel fuel. The compression ratio (c/r) is the measure of how much the air is compressed (Fig. 2-2).
Compression ratio swept volume clearance volume swept volume Swept volume the volume of the cylinder traversed by the piston in its travel from top dead center (tdc) to bottom dead center (bdc) Clearance volume combustion chamber volume Figure 2-3 graphs the relationship between c/r’s and thermal efficiency, which reaffirms what every mechanic knows: high c/r’s are a precondition for power and fuel economy. At the very minimum, a diesel engine needs a c/r of about 16:1 for cold starting. Friction, which increases more rapidly than the power liberated by increases in compression, sets the upper limit at about 24:1.
Other inhibiting factors are the energy required for cranking and the stresses produced by high power outputs. Diesels with c/r’s of 16 or 17:1 sometimes benefit from a point or two of higher compression. Starting becomes easier and less exhaust smoke is produced. An example is the Caterpillar 3208 that has a tendency to smoke and “wet stack,” that is, to saturate its exhaust system with unburned fuel. These problems can be alleviated with longer connecting rods that raise the compression ratio from 16.5:1 to 18.2:1. It should be noted that a compressor, in the form of a turbocharger or supercharger, raises the effective c/r. Consequently, these engines have c/r’s of 16 or 17:1, which are just adequate for starting. Once the engine is running, the compressor provides additional compression. Gasoline engines have lower c/r’s—half or less—than CI engines. This is because the fuel detonates when exposed to the heat and pressure associated with higher c/r’s. Detonation is a kind of maverick combustion that occurs after normal ignition.
The unburned fraction of the charge spontaneously explodes. This sudden rise in pressure can be heard as a rattle or, depending upon the natural frequency of the connecting rods, as a series of distinct pings. Uncontrolled detonation destroys crankshaft bearings and melts piston crowns. Induction Modern SI engines mix air and fuel in the intake manifold by way of one or more low-pressure (50-psi or so) injectors. A throttle valve regulates the amount of air admitted, which is only slightly in excess of the air needed for combustion. As the throttle opens, the injectors remain open longer to increase fuel delivery. For a gasoline engine, the optimum mixture is roughly 15 parts air to 1 part fuel. The air-fuel mixture then passes into the cylinder for compression and ignition. In a CI engine, air undergoes compression before fuel is admitted. Injectors open late during the compression stroke as the piston approaches tdc. Compressing air, rather than a mix of air and fuel, improves the thermal efficiency of diesel engines.
To understand why would require a course in thermodynamics; suffice to say that air contains more latent heat than does a mixture of air and vaporized fuel. Forcing fuel into a column of highly compressed air requires high injection pressures. These pressures range from about 6000 psi for utility engines to as much as 30,000 psi for state-of-the-art examples. CI engines dispense with the throttle plate—the same amount of air enters the cylinders at all engine speeds. Typically, idle-speed air consumption averages about 100 lb of air per pound of fuel; at high speed or under heavy load, the additional fuel supplied drops the ratio to about 20:1. Without a throttle plate, diesels breathe easily at low speeds, which explains why truck drivers can idle their rigs for long periods without consuming appreciable fuel. (An SI engine requires a fuel-rich mixture at idle to generate power to overcome the throttle restriction.) Since diesel air flow remains constant, the power output depends upon the amount of fuel delivered. As power requirements increase, the injectors deliver more fuel than can be burned with available oxygen. The exhaust turns black with partially oxidized fuel.
How much smoke can be tolerated depends upon the regulatory climate, but the smoke limit always puts a ceiling on power output. To get around this restriction, many diesels incorporate an air pump in the form of an exhaust-driven turbocharger or a mechanical supercharger. Forced induction can double power outputs without violating the smoke limit. And, as far as turbochargers are concerned, the supercharge effect is free. That is, the energy that drives the turbo would otherwise be wasted out the exhaust pipe as heat and exhaust-gas velocity. The absence of an air restriction and an ignition system that operates as a function of engine architecture can wrest control of the engine from the operator. All that’s needed is for significant amounts of crankcase oil to find its way into the combustion chambers. Oil might be drawn into the chambers past worn piston rings or from a failed turbocharger seal. Some industrial engines have an air trip on the intake manifold for this contingency, but many do not. A runaway engine generally accelerates itself to perdition because few operators have the presence of mind to engage the air trip or stuff a rag into the intake. Ignition and combustion SI engines are fired by an electrical spark timed to occur just before the piston reaches the top of the compression stroke. Because the full charge of fuel and air is present, combustion proceeds rapidly in the form of a controlled explosion. The rise in cylinder pressure occurs during the span of a few crankshaft degrees. Thus, the cylinder volume above the piston undergoes little change between ignition and peak pressure. Engineers, exaggerating a bit, describe SI engines as “constant volume” engines (Fig. 2-4). Compared to SI, the onset of diesel ignition is a leisurely process
Some time is required for the fuel spray to vaporize and more time is required for the spray to reach ignition temperature. Fuel continues to be injected during the delay period. Once ignited, the accumulated fuel burns rapidly with correspondingly rapid increases in cylinder temperature and pressure. The injector continues to deliver fuel through the period of rapid combustion and into the period of controlled combustion that follows. When injection ceases, combustion enters what is known as the afterburn period. The delay between the onset of fuel delivery and ignition (A–B in Fig. 2-5) should be as brief as possible to minimize the amount of unburnt fuel accumulated in the cylinder. The greater the ignition lag, the more violent the combustion and resulting noise, vibration, and harshness (NVH). Ignition lag is always worst upon starting cold, when engine metal acts as a heat sink.
Mechanics sometimes describe the clatter, white exhaust smoke, and rough combustion that accompany cold starts as “diesel detonation,” a term that is misleading because diesels do not detonate in the manner of SI engines. Combustion should smooth out after the engine warms and ignition lag diminishes. Heating the incoming air makes cold starts easier and less intrusive. In normal operation, with ignition delay under control, cylinder pressures and temperatures rise more slowly (but to higher levels) than for SI engines. In his proposal of 1893, Rudolf Diesel went one step further and visualized constant pressure expansion: fuel input and combustion pressure would remain constant during the expansion, or power, stroke.
He was able to approach that goal in experimental engines, but only if rotational speeds were held low. His colleagues eventually abandoned the idea and controlled fuel input pragmatically, on the basis of power output. Even so, the pressure rise is relatively smooth and diesel engines are sometimes called “constant pressure” devices to distinguish them from “constant volume” SI engines McGraw-Hill eBooks Copyright © 2008, 1995, 1991, 1975 by Paul Dempsey