DIESEL ENGINES

6 Jun

One of the most common prime movers is the diesel engine.  Before gaining an understanding of how the engine operates a basic understanding of the engine’s components must be gained.   This chapter reviews the major components of a generic diesel engine.

Introduction

Most DOE facilities require some type of prime mover to supply mechanical power for pumping, electrical power generation, operation of heavy equipment, and to act as a backup electrical generator for emergency use during the loss of the normal power source.  Although several types of prime movers are available (gasoline engines, steam and gas turbines), the diesel engine is the most commonly used.  Diesel engines provide a self-reliant energy source that is available in sizes from a few horsepower to 10,000 hp.  Figure 1 provides an illustration of a common skid-mounted,   diesel-driven   generator.  Relatively   speaking,   diesel   engines   are   small, inexpensive, powerful, fuel efficient, and extremely reliable if maintained properly.

Because of the widespread use of diesel engines at DOE facilities, a basic understanding of the operation of a diesel engine will help ensure they are operated and maintained properly.  Due to the large variety of sizes, brands, and types of engines in service, this module is intended to provide the fundamentals and theory of operation of a diesel engine.  Specific information on a particular engine should be obtained from the vendor’s manual.

Figure 1 Example of a Large Skid-Mounted, Diesel-Driven Generator

H istory

The modern diesel engine came about as the result of the internal combustion principles first proposed by Sadi Carnot in the early 19th century.   Dr. Rudolf Diesel applied Sadi Carnot’s principles into a patented cycle or method of combustion that has become known as the “diesel” cycle.  His patented engine operated when the heat generated during the compression of the air fuel charge caused ignition of the mixture, which then expanded at a constant pressure during the full power stroke of the engine.

Dr. Diesel’s first engine ran on coal dust and used a compression pressure of 1500 psi to increase its theoretical efficiency.  Also, his first engine did not have provisions for any type of cooling system.  Consequently, between the extreme pressure and the lack of cooling, the engine exploded and almost killed its inventor.  After recovering from his injuries, Diesel tried again using oil as the fuel, adding a cooling water jacket around the cylinder, and lowering the compression pressure to approximately 550 psi.  This combination eventually proved successful. Production rights to the engine were sold to Adolphus Bush, who built the first diesel engines for commercial use, installing them in his St. Louis brewery to drive various pumps.

D iesel E ngines

A diesel engine is similar to the gasoline engine used in most cars.  Both engines are internal combustion engines, meaning they burn the fuel-air mixture within the cylinders.

Both are reciprocating engines, being driven by pistons moving laterally in two directions.  The majority of their parts are similar.

Although a diesel engine and gasoline engine operate with similar components, a diesel engine, when compared to a gasoline engine of equal horsepower, is heavier due to stronger, heavier materials used to withstand the greater dynamic forces from the higher combustion pressures present in the diesel engine.

The greater combustion pressure is the result of the higher compression ratio used by diesel engines. The compression ratio is a measure of how much the engine compresses the gasses in the  engine’s  cylinder.        In  a  gasoline  engine  the  compression  ratio  (which  controls  the compression temperature) is limited by the air-fuel mixture entering the cylinders.  The lower ignition temperature of gasoline will cause it to ignite (burn) at a compression ratio of less than 10:1.   The average car has a 7:1 compression ratio.

In a diesel engine, compression ratios ranging from 14:1 to as high as 24:1 are commonly used.  The higher compression ratios are possible because only air is compressed, and then the fuel is injected.  This is one of the factors that allows the diesel engine to be so efficient.  Compression ratio will be discussed in greater detail later in this module.

Another difference between a gasoline engine and a diesel engine is the manner in which engine speed is controlled.  In any engine, speed (or power) is a direct function of the amount of fuel burned in the cylinders.  Gasoline engines are self-speed-limiting, due to the method the engine uses to control the amount of air entering the engine.  Engine speed is indirectly controlled by the butterfly valve in the carburetor.  The butterfly valve in a carburetor limits the amount of air entering the engine.  In a carburetor, the rate of air flow dictates the amount of gasoline that will be mixed with the air.  Limiting the amount of air entering the engine limits the amount of fuel entering the engine, and, therefore, limits the speed of the engine.  By limiting the amount of air entering the engine, adding more fuel does not increase engine speed beyond the point where the fuel burns 100% of the available air (oxygen).

Diesel engines are not self-speed-limiting because the air (oxygen) entering the engine is always the maximum amount.

Therefore, the engine speed is limited solely by the amount of fuel injected into the engine cylinders. Therefore, the engine always has sufficient oxygen to burn and the engine will attempt to accelerate to meet the new fuel injection rate.   Because of this, a manual fuel control is not possible because these engines, in an unloaded condition, can accelerate at a rate of more than 2000 revolutions per second.  Diesel engines require a speed limiter, commonly called the governor, to control the amount of fuel being injected into the engine.

Unlike a gasoline engine, a diesel engine does not require an ignition system because in a diesel engine the fuel is injected into the cylinder as the piston comes to the top of its compression stroke.

When  fuel  is  injected,  it  vaporizes  and  ignites  due  to  the  heat  created  by  the compression of the air in the cylinder.

Major C om ponents of a D iesel E ngine

To understand how a diesel engine operates, an understanding of the major components and how they work together is necessary. Figure 2 is an example of a medium-sized, four-stroke, supercharged, diesel engine with inlet ports and exhaust valves.

Figure 3 provides a cross section of a similarly sized V-type diesel engine.

Figure 3  Cross Section of a V-type Four Stroke Diesel Engine

T he C ylinder Block

The cylinder block, as shown in Figure 4, is generally a single unit made from cast iron. In a liquid-cooled diesel, the block also provides the structure and rigid frame for the engine’s cylinders, water coolant and oil passages, and support for the crankshaft and camshaft bearings.

C rankcase and O il P an

The crankcase is usually located on the bottom of the cylinder block.  The crankcase is defined as the area around the crankshaft and crankshaft bearings.  This area encloses the rotating crankshaft and crankshaft counter weights and directs returning oil into the oil pan.   The oil pan is located at the bottom of the crankcase as shown in Figure 2 and Figure 3.  The oil pan collects and stores the engine’s supply of lubricating oil.  Large diesel engines may have the oil pan divided into several separate pans.

C ylinder Sleeve or Bore

Diesel engines use one of two types of cylinders.  In one type, each cylinder is simply machined or bored into the block casting, making the block and cylinders an integral part.  In the second type, a machined steel sleeve is pressed into the block casting to form the cylinder.  Figure 2 and Figure 3 provide examples of sleeved diesel engines.  With either  method,  the  cylinder  sleeve or  bore provides  the  engine  with  the  cylindrical structure needed to confine the combustion gasses and to act as a guide for the engine’s pistons.

In engines using sleeves, there are two types of sleeves, wet and dry.  A dry sleeve is surrounded by the metal of the block and does not come in direct contact               with    the   engine’s    coolant (water).  A wet sleeve comes in direct contact with the engine’s coolant. Figure 5 provides an example of a wet sleeve.   The volume enclosed by the sleeve or bore is called the combustion chamber and is the space where the fuel is burned.

In either type of cylinder, sleeved or bored, the diameter of the cylinder is called the bore of the engine and is stated  in  inches. For  example,  the bore of a 350 cubic inch Chevrolet gasoline engine is 4 inches.

Most diesel engines are multi-cylinder engines and typically have their cylinders arranged in one of two ways, an in-line or a “V”, although other combinations exits.  In an in-line engine, as the name indicates, all the cylinders are in a row.   In a “V” type engine the cylinders are arranged in two rows of cylinders set at an angle to each other that align to a common crankshaft.  Each group of cylinders making up one side of the “V” is referred to as a bank of cylinders.

P iston and P iston R ings

The piston transforms the energy of the expanding gasses into mechanical energy. The piston rides in the cylinder liner or sleeve as shown in Figure 2 and Figure 3. Pistons are commonly made of aluminum or cast iron alloys.

To prevent the combustion gasses from bypassing the piston and to keep friction to a minimum, each piston has several metal rings around it, as illustrated by Figure 6.

These rings function as the seal between the piston and the cylinder wall and also act to reduce friction by minimizing the contact area between the piston and the cylinder wall. The rings are usually made of cast iron and coated with chrome or molybdenum.  Most diesel engine pistons have several rings, usually 2 to 5, with each ring performing a distinct function.  The top ring(s) acts primarily as the pressure seal.  The intermediate ring(s) acts as a wiper ring to remove and control the amount of oil film on the cylinder walls.  The bottom ring(s) is an oiler ring and ensures that a supply of lubricating oil is evenly deposited on the cylinder walls.

C onnecting R od

The connecting rod connects the piston to the crankshaft.  See Figure 2 and Figure 3 for the location of the connecting rods in an engine.  The rods are made from drop-forged, heat-treated steel to provide the required strength.  Each end of the rod is bored, with the smaller top bore connecting to the piston pin (wrist pin) in the piston as  shown in Figure 6.  The large bore end of the rod is split in half and bolted to allow the rod to be attached to the crankshaft.  Some diesel engine connecting rods are drilled down the center to allow oil to travel up from the crankshaft and into the piston pin and piston for lubrication.

A variation found in V-type engines that affects the connecting rods is to position the cylinders in the left and right banks directly opposite each other instead of staggered (most common configuration). This arrangement requires that the connecting rods of two opposing cylinders share the same main journal bearing on the crankshaft.  To allow this configuration, one of the connecting rods must be split or forked around the other.

C ranksha ft

The crankshaft transforms the linear motion of the pistons into a rotational motion that is transmited to the load.  Crankshafts are made of forged steel.  The forged crankshaft is machined to produce the crankshaft bearing and connecting rod bearing surfaces.  The rod bearings are eccentric, or offset, from the center of the crankshaft as illustrated in Figure 7.  This offset converts the reciprocating (up and down) motion of the piston into the rotary motion of the crankshaft.  The amount of offset determines the stroke (distance the piston travels) of the engine (discussed later).

The crankshaft does not ride directly on the cast iron block crankshaft supports, but rides on  special bearing  material as  shown in  Figure 7.

The  connecting rods  also  have bearings inserted between the crankshaft and the connecting rods.  The bearing material is a soft alloy of metals that provides a replaceable wear surface and prevents galling between two similar metals (i.e., crankshaft and connecting rod).  Each bearing is split into halves to allow assembly of the engine.  The crankshaft is drilled with oil passages that allow the engine to feed oil to each of the crankshaft bearings and connection rod bearings and up into the connecting rod itself.

The crankshaft has large weights, called counter weights, that balance the weight of the connecting rods.

These weights ensure an even (balance) force during the rotation of the moving parts.

F lywheel

The flywheel is located on one end of the crankshaft and serves three purposes. First, through its  inertia, it reduces  vibration by smoothing out the power stroke as  each cylinder fires.  Second, it is the mounting surface used to bolt the engine up to its load. Third, on some diesels, the flywheel has gear teeth around its perimeter that allow the starting motors to engage and crank the diesel.

C ylinder H eads and V alves

A diesel engine’s cylinder heads perform several functions.  First, they provide the top seal for the cylinder bore or sleeve.  Second, they provide the structure holding exhaust valves (and intake valves where applicable), the fuel injector, and necessary linkages.  A diesel engine’s  heads  are  manufactured  in one  of  two ways.                                                                         In  one  method,  each cylinder has its own head casting, which is bolted to the block.   This method is used primarily on the larger diesel engines.  In the second method, which is used on smaller engines, the engine’s head is cast as one piece (multi-cylinder head).

Diesel engines have two methods of admitting and exhausting gasses from the cylinder. They can use either ports or valves or a combination of both.          

Ports are slots in the cylinder walls located in the lower 1/3 of the bore.

See Figure 2 and Figure 3 for examples of intake ports, and note their relative location with respect to the rest of the engine.

When the piston travels below the level of the ports, the ports are “opened” and fresh air or exhaust gasses are able to enter or leave, depending on the type of port.

The ports are then “closed” when the piston travels back above the level of the ports.      

Valves (refer to figure 8) are mechanically opened and closed to admit or exhaust the gasses as needed.

The valves are located in the head casting of the engine.  The point at which the valve seals against the head is called the valve seat.

Most medium-sized diesels have either intake ports or exhaust valves or both intake and exhaust valves.

T iming  G ears,  C amshaft,  and

V alve Mechanism

In order for a diesel engine to operate, all of its components must perform their functions at very precise intervals in relation to the motion of the piston. To accomplish this, a component called a camshaft is used. Figure 9 illustrates a camshaft and camshaft drive gear. Figure 2 and Figure 3 illustrate the location of a camshaft in a large overhead cam diesel engine.

A camshaft is a long bar with egg-shaped eccentric lobes, one lobe for each valve and fuel injector (discussed later). Each lobe has a follower as shown on Figure 10. As the camshaft is rotated, the follower is forced up and down as it follows the profile of the cam lobe. The followers are connected to the engine’s valves and fuel injector s thr ough var ious types of linkages called pushrods and rocker arms.

The pushrods and rocker arms transfer the reciprocating motion generated by the camshaft lobes to the valves and injectors, opening and closing them as needed.  The valves are maintained closed by springs.

As the valve is opened by the camshaft, it compresses the valve spring. The energy stored in the valve spring is then used to close the valve as the camshaft lobe rotates out from  under  the  follower. Because  an  engine  experiences  fairly  large  changes  in temperature  (e.g.,  ambient  to  a  normal  running  temperature  of  about  190 F),  its components must be designed to allow for thermal expansion.   Therefore, the valves, valve pushrods, and rocker arms must have some method of allowing for the expansion. This is accomplished by the use of valve lash.   Valve lash is the term given to the “slop” or “give” in the valve train before the cam actually starts to open the valve.

The camshaft is driven by the engine’s crankshaft through a series of gears called idler gears and timing gears. The gears allow the rotation of the camshaft to correspond or be in time with, the rotation of the crankshaft and thereby allows the valve opening, valve closing, and injection of fuel to be timed to occur at precise intervals in the piston’s travel. To increase the flexibility in timing the valve opening, valve closing, and injection of fuel, and to increase power or to reduce cost, an engine may have one or more camshafts. Typically, in a medium to large V-type engine, each bank will have one or more camshafts per head. In the larger engines, the intake valves, exhaust valves, and fuel injectors may share a common camshaft or have independent camshafts.

Depending on the type and make of the engine, the location of the camshaft or shafts varies. The camshaft(s) in an in-line engine is usually found either in the head of the engine or in the top of the block running down one side of the cylinder bank. Figure 10 provides an example of an engine with the camshaft located on the side of the engine. Figure 3 provides an example of an overhead cam arrangement as on a V-type engine. On small or mid-sized V-type engines, the camshaft is usually located in the block at the center of the “V” between the two banks of cylinders. In larger or multi-camshafted V- type engines, the camshafts are usually located in the heads.

Blow er

The diesel engine’s blower is part of the air intake system and serves to compress the incoming fresh air for delivery to the cylinders for combustion.

The location of the blower is  shown on Figure 2.

The blower can be part of either a turbocharged or supercharged air intake system.  Additional information on these two types of blowers is provided later in this module.

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