FUNDAMENTALS OF THE DIESEL CYCLE

6 Jun

Diesel engines operate under the principle of the internal combustion engine. There are two basic types of diesel engines, two-cycle and four-cycle. An understanding of how each cycle operates is required to understand how to correctly operate and maintain a diesel engine.

The Basic Diesel Cycles

A diesel engine is a type of heat engine that uses the internal combustion process to convert the energy stored in the chemical bonds of the fuel into useful mechanical energy.  This occurs in two steps.   First, the fuel reacts chemically (burns) and releases energy in the form of heat. Second the heat causes the gasses trapped in the cylinder to expand, and the expanding gases, being confined by the cylinder, must move the piston to expand.  The reciprocating motion of the piston is then converted into rotational motion by the crankshaft.

To convert the chemical energy of the fuel into useful mechanical energy all internal combustion engines must go through four events: intake, compression, power, and exhaust.                                  How these events are timed and how they occur differentiates the various types of engines.

All diesel engines fall into one of two categories, two-stroke or four-stroke cycle engines.  The word cycle refers to any operation or series of events that repeats itself.  In the case of a four- stroke cycle engine, the engine requires four strokes of the piston (intake, compression, power, and exhaust) to complete one full cycle.  Therefore, it requires two rotations of the crankshaft, or 720  of crankshaft rotation (360   x 2) to complete one cycle.  In a two-stroke cycle engine the events (intake, compression, power, and exhaust) occur in only one rotation of the crankshaft, or 360 .

Timing

In the following discussion of the diesel cycle it is important to keep in mind the time frame in which each of the actions is required to occur. Time is required to move exhaust gas out of the cylinder and fresh air in to the cylinders, to compress the air, to inject fuel, and to burn the fuel.

If  a four-stroke diesel engine is  running at a constant 2100 revolutions per minute (rpm), the crankshaft would be rotating at 35 revolutions, or 12,600 degrees, per second.  One stroke is completed in about 0.01429 seconds.

Intake

As the piston moves upward and approaches 28 before top dead center (BTDC), as measured by crankshaft rotation, the camshaft lobe starts to lift the cam follower. This causes the pushrod to move upward and pivots the rocker arm on the rocker arm shaft. As the valve lash is taken up, the rocker arm pushes the intake valve downward and the valve starts to open. The intake stroke now starts while the exhaust valve is still open. The flow of the exhaust gasses will have created a low pressure condition within the cylinder and will help pull in the fresh air charge as shown in Figure 16.

The piston continues its upward travel through top dead center (TDC) while fresh air enters  and  exhaust  gasses  leave.

At  about 12   after  top  dead center  (ATDC),  the camshaft exhaust lobe rotates so that the exhaust valve will start to close.  The valve is fully closed at 23   ATDC.  This is accomplished through the valve spring, which was compressed when the valve was opened, forcing the rocker arm and cam follower back against the cam lobe as it rotates.   The time frame during which both the intake and exhaust valves are open is called valve overlap (51   of overlap in this example) and is necessary to allow the fresh air to help scavenge (remove) the spent exhaust gasses and cool the cylinder.  In most engines, 30 to 50 times cylinder volume is scavenged through the cylinder during overlap.   This excess cool air also provides the necessary cooling effect on the engine parts.

As the piston passes TDC and begins to travel down the cylinder bore, the movement of the piston creates a suction and continues to draw fresh air into the cylinder.

Compression
At 35 after bottom dead center (ABDC), the intake valve starts to close. At 43 ABDC (or 137 BTDC), the intake valve is on its seat and is fully closed. At this point the air charge is at normal pressure (14.7 psia) and ambient air temperature (~80 F), as illustrated in Figure 17. At about 70 BTDC, the piston has traveled about 2.125 inches, or about half of its stroke, thus reducing the volume in the cylinder by half. The temperature has now doubled to ~160 F and pressure is ~34 psia. At about 43 BTDC the piston has traveled upward 3.062 inches of its stroke and the volume is once again halved. Consequently, the temperature again doubles to about 320 F and pressure is ~85 psia. When the piston has traveled to 3.530 inches of its stroke the volume is again

halved and temperature reaches ~640 F and pressure 277 psia.   When the piston has traveled to 3.757 inches of its stroke, or the volume is again halved, the temperature climbs to 1280 F and pressure reaches 742 psia.   With a piston area of 9.616 in2  the pressure in the cylinder is exerting a force of approximately 7135 lb. or 3-1/2 tons of force.

The above numbers are ideal and provide a good example of what is occurring in an engine during compression.   In an actual engine, pressures reach only about 690 psia. This is due primarily to the heat loss to the surrounding engine parts.

Fuel Injection

Fuel in a liquid state is injected into the cylinder at a precise time and rate to ensure that the combustion pressure is forced on the piston neither too early nor too late, as shown in Figure 18. The fuel enters the cylinder where the heated compressed air is present; however, it will only burn when it is in a vaporized state (attained through the addition of heat to cause vaporization) and intimately mixed with a supply of oxygen.

The first minute droplets of fuel enter the combustion chamber and are quickly vaporized. The vaporization of the fuel causes the air surrounding the fuel to cool and it requires time for the air to reheat sufficiently to ignite the vaporized fuel. But once ignition has started, the additional heat from combustion helps to further vaporize the new fuel entering the chamber, as long as oxygen is present. Fuel injection starts at 28 BTDC and ends at 3 ATDC; therefore, fuel is injected for a duration of 31 .

Power
Both valves are closed, and the fresh air charge has been compressed. The fuel has been injected and is starting to burn. After the piston passes TDC, heat is rapidly released by the ignition of the fuel, causing a rise in cylinder pressure. Combustion temperatures are around 2336 F. This rise in pressure forces the piston downward and increases the force on the crankshaft for the power stroke as illustrated in Figure 19.

The energy generated by the combustion process is not all harnessed. In a two stroke diesel engine, only about 38% of the generated power is harnessed to do work, about 30% is wasted in the form of heat rejected to the cooling system, and about 32% in the form of heat is rejected out the exhaust. In comparison, the four-stroke diesel engine has a thermal distribution of 42% converted to useful work, 28% heat rejected to the cooling system, and 30% heat rejected out the exhaust.

Exhaust


As the piston approaches 48 BBDC, the cam of the exhaust lobe starts to force the follower upward, causing the exhaust valve to lift off its seat. As shown in Figure 20, the exhaust gasses start to flow out the exhaust valve due to cylinder pressure and into the exhaust manifold. After passing BDC, the piston moves upward and accelerates to its maximum speed at 63 BTDC. From this point on the piston is decelerating. As the piston speed slows down, the velocity of the gasses flowing out of the cylinder creates a pressure slightly lower than atmospheric pressure. At 28 BTDC, the intake valve opens and the cycle starts again.

The  Two-Stroke  Cycle

Like the four-stroke engine, the two-stroke engine must go through the same four events: intake, compression, power, and exhaust. But a two-stroke engine requires only two strokes of the piston to complete one full cycle. Therefore, it requires only one rotation of the crankshaft to complete a cycle. This means several events must occur during each stroke for all four events to be completed in two strokes, as opposed to the four-stroke engine where each stroke basically contains one event.

In  a two-stroke  engine the camshaft is  geared so  that  it rotates  at  the same speed as  the crankshaft (1:1).  The following section will describe a two-stroke, supercharged, diesel engine having intake ports and exhaust valves with a 3.5-inch bore and 4-inch stroke with a 16:1 compression ratio, as it passes through one complete cycle.  We will start on the exhaust stroke. All the timing marks given are generic and will vary from engine to engine.

E xhaust and Intake

At 82  ATDC, with the piston near the end of its power stroke, the exhaust cam begins to lift the exhaust valves follower.  The valve lash is taken up, and 9  later (91  ATDC), the rocker arm forces the exhaust valve off its seat.  The exhaust gasses start to escape into the exhaust manifold, as shown in Figure 21.  Cylinder pressure starts to decrease.

After the piston travels three-quarters of its (down) stroke, or 132  ATDC of crankshaft rotation, the piston starts to uncover the inlet ports.  As the exhaust valve is still open, the uncovering of the inlet ports lets the compressed fresh air enter the cylinder and helps cool the cylinder and scavenge the cylinder of the remaining exhaust gasses (Figure 22). Commonly, intake and exhaust occur over approximately 96  of crankshaft rotation.

At 43   ABDC, the camshaft starts to close the exhaust valve.    At 53   ABDC (117

BTDC), the camshaft has rotated sufficiently to allow the spring pressure to close the exhaust valve. Also, as the piston travels past 48 ABDC (5  after the exhaust valve starts closing), the intake ports are closed off by the piston.

C om pression

After the exhaust valve is on its seat (53  ATDC), the temperature and pressure begin to rise in nearly the same fashion as in the four-stroke engine. Figure 23 illustrates the compression in a 2-stroke engine.

At 23   BTDC the injector cam begins to lift the injector follower and pushrod.  Fuel injection continues until 6  BTDC (17 total degrees of injection), as illustrated in Figure 24.

Power
The power stroke starts after the piston passes TDC. Figure 25 illustrates the power stroke which continues until the piston reaches 91 ATDC, at which point the exhaust valves start to open and a new cycle begins.

Summary

The important information in this chapter is summarized below.

F und a mentals of the Diesel Cycle Summary

Ignition occurs in a diesel by injecting fuel into the air charge which has been heated by compression to a temperature greater than the ignition point of the fuel.

A diesel engine converts the energy stored in the fuel’s chemical bonds into mechanical energy by burning the fuel.  The chemical reaction of burning the fuel liberates heat, which causes the gasses to expand, forcing the piston to rotate the crankshaft.

A four-stroke engine requires two rotations of the crankshaft to complete one cycle.  The event occur as follows:

Intake – the piston passes TDC, the intake valve(s) open and the fresh air is admitted into the cylinder, the exhaust valve is still open for a few degrees to allow scavenging to occur.

Compression – after the piston passes BDC the intake valve closes and the piston travels up to TDC (completion of the first crankshaft rotation).

Fuel injection – As the piston nears TDC on the compression stroke, the fuel is injected by the injectors and the fuel starts to burn, further heating the gasses in the cylinder.

Power – the piston passes TDC and the expanding gasses force the piston down, rotating the crankshaft.

Exhaust – as the piston passes BDC the exhaust valves open and the exhaust gasses start to flow out of the cylinder. This continues as the piston travels up to TDC, pumping the spent gasses out of the cylinder.  At TDC the second crankshaft rotation is complete.

F und a mentals of the D iesel C y cle S um m a ry ( Cont.)

A two-stroke engine requires one rotation of the crankshaft to complete one cycle.  The events occur as follows:

Intake – the piston is near BDC and exhaust is in progress.  The intake valve or ports open and the fresh air is forced in.  The exhaust valves or ports are closed and intake continues.

Compression – after both the  exhaust and intake valves or ports are closed, the piston travels up towards TDC.  The fresh air is heated by the compression.

Fuel injection – near TDC the fuel is injected by the injectors and the fuel starts to burn, further heating the gasses in the cylinder.

Power – the piston passes TDC and the expanding gasses force the piston down, rotating the crankshaft.

Exhaust – as the piston approaches BDC the exhaust valves or ports open and the exhaust gasses start to flow out of the cylinder.

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