DJA3032 CHAPTER 5

CHAPTER 5: FLUID MOTION IN COMBUSTION CHAMBER AND EXHAUST FLOW

byMOHD SAHRIL BIN MOHD FOUZI, Grad. IEM (G 27763)

DEPARTMENT OF MECHANICAL ENGINEERING

© MSF @ POLITEKNIK UNGKU OMAR

(DJA3032) INTERNAL COMBUSTION ENGINE

Introduction:

This chapter discusses air, fuel, and exhaust gas motion that occurs within the cylinders during the compression stroke, combustion, and power stroke of the cycle. It is important to have this motion to speed evaporation of the fuel, to enhance air-fuel mixing, and to increase combustion speed and efficiency. In addition to the normal desired turbulence, a rotational motion called swirl is generated on the air-fuel mixture during intake.

Near the end of the compression stroke, two additional mass motions are generated: squish and tumble. Squish is a radial motion towards the center-line of the cylinder, while tumble is a rotational motion around a circumferential axis. One additional flow motion will be discussed: that of crevice flow and blow-by. This is the flow into the very small crevices of the combustion chamber due to the very high pressures generated during compression and combustion,(Willard W. Pulkrabek ).





SWIRL:

Swirl greatly enhances the mixing of air and fuel to give a homogeneous mixture in the very short time available for this in modern high-speed engines. It is also a main mechanism for very rapid spreading of the flame front during the combustion process, (Willard W. Pulkrabek ).

Figure 5.1: (a) Swirl motion within engine cylinder. Methods to generate (b) air entering cylinder from tangential direction.



Figure 5.2: (c) countered intake runner, (d) countered valve

Swirl ratio is a dimensionless parameter used to quantify rotational motion within the cylinder. Figure 5.3 shows how swirl ratio changes through a cycle of the engine. During intake it is high, decreasing after BDC in the compression stroke due to viscous drag with the cylinder walls.

Combustion expands the gases and increases swirl to another maximum part way into the power stroke. Expansion of the gases and viscous drag quickly reduce this again before blow-down occurs. One-fourth to one-third of angular momentum will be lost during the compression stroke, (Willard W. Pulkrabek ).



Figure 5.3: Average cylinder swirl ratio as a function of crank angle for a typical SI engine. Swirl is high during the intake process, with a maximum near TDC. It then is reduced by viscous drag during the compression stroke. There is a second maximum near ~e end of compression when the radius of rotation is decreased near TDC and expansion from combustion occurs. Viscous drag with the cylinder walls during the expansion stroke quickly reduces this again before blow-down occurs, (Willard W. Pulkrabek ).



TURBULENCE:

Due to the high velocities involved, all flows into, out of, and within engine cylinders are turbulent flows. The exception to this are those flows in the corners and small crevices of the combustion chamber where the close proximity of the walls dampens out turbulence. As a result of turbulence, thermodynamic transfer rates within an engine are increased by an order of magnitude.

When flow is turbulent, particles experience random fluctuations in motion superimposed on their main bulk velocity. These fluctuations occur in all directions, perpendicular to the flow and in the flow direction. This makes it impossible to predict the exact flow conditions at any given time and position.

There are many levels of turbulence within an engine. Large-scale turbulence occurs with eddies on the order of the size of the flow passage (e.g., valve opening,diameter of intake runner, height of clearance volume, etc.),(Willard W. Pulkrabek ).



Turbulence in a cylinder is high during intake, but then decreases as the flow rate slows near BDC. It increases again during compression as swirl, squish, and tumble increase near TDC. Swirl makes turbulence more homogeneous throughout the cylinder.

The high turbulence near TDC when ignition occurs is very desirable for combustion. It breaks up and spreads the flame front many times faster than that of a laminar flame. The air-fuel is consumed in a very short time, and self-ignition and knock are avoided.

Local flame speed depends on the turbulence immediately in front of the flame. This turbulence is enhanced by the expansion of the cylinder gases during the combustion process. The shape of the combustion chamber is extremely important in generating maximum turbulence and increasing the desired rapid combustion, (Willard W. Pulkrabek ).



Figure 5.4: Turbulence level of gas flows in engines as a function of engine speed.

Turbulence intensity is a strong function of engine speed (Fig. 5.4). As speed is increased, turbulence increases, and this increases the rate of evaporation, mixing, and combustion. One result of this is that all engine speeds have about the same burn angle (i.e., the crank angle through which the engine turns as combustion takes place). The one phase of this process which is not totally changed by the increase in turbulence is ignition delay, (Willard W. Pulkrabek ).



SQUISH AND TUMBLE:

As the piston approaches TDC, the gas mixture occupying the volume at the outer radius of the cylinder is forced radially inward as this outer volume is reduced to near zero. This radial inward motion of the gas mixture is called squish. It adds to other mass motions within the cylinder to mix the air and fuel, and to quickly spread the flame front. Maximum squish velocity usually occurs at about 10° BTDC.

During combustion, the expansion stroke begins and the volume of the combustion chamber increases. As the piston moves away from TDC, the burning gases are propelled radially outward to fill the now-increasing outer volume along the cylinder. walls. This reverse squish helps to spread the flame front during the latter part of combustion.

As the piston nears TDC, squish motion generates a secondary rotational flowcalled tumble. This rotation occurs about a circumferential axis near the outer edgeof the piston bowl as shown in Fig. 5.5, (Willard W. Pulkrabek ).



Figure 5.5: Tumble action caused by squish as piston approaches TDC. Tumble is a rotational motion about a circumferential axis near the edge of the clearance volume in the piston bowl or in the cylinder head.



BLOWDOWN:

Exhaust blowdown occurs when the exhaust valve starts to open towards the end of the power stroke, somewhere around 60° to 40° bBDC. At this time, pressure in the cylinder is still at about 4-5 atmospheres and the temperature is upwards of 1000 K. Pressure in the exhaust system is about one atmosphere, and when the valve is opened the resulting pressure differential causes a rapid flow of exhaust gases from the cylinder, through the valve, into the exhaust system (i.e., exhaust blowdown),

In an ideal air-standard Otto cycle or Diesel cycle, the exhaust valve opens at BDC and blowdown occurs instantaneously at constant volume (process 4-5 in Fig. 5.6). This does not happen in a real engine, where blowdown takes a finite length of time. So that the pressure in the cylinder has been fully reduced by BDC when the exhaust stroke starts, the exhaust valve starts to open somewhere around 60° to 40° bBDC. When this happens, the pressure is quickly reduced, and what would have been additional useful work is lost during the last part of the expansion stroke, (Willard W. Pulkrabek ).



Figure 5.6: Air-standard Otto cycle in P-V coordinates showing exhaust gas after blowdown at hypothetical state 7.



EXHAUST MANIFOLD:

After leaving the cylinders by passing out of the exhaust valves, exhaust gases pass through the exhaust manifold, a piping system that directs the flow into one or more exhaust pipes. Exhaust manifolds are usually made of cast iron and are sometimes designed to have close thermal contact with the intake manifold. This is to provide heating and vaporization in the intake manifold.

From the exhaust manifold, the gases flow through an exhaust pipe to the emission control system of the engine, which may consist of thermal and/or catalytic converters. One argument says these should be as close to the engine as space allows to minimize heat losses. On the other hand, this creates high temperature problems in the engine compartment. These converters promote reduction of emissions in the exhaust gases by additional chemical reaction. They are discussed in the next chapter, (Willard W. Pulkrabek ).



CATALYTIC CONVERTERS:

The most effective after treatment system for reducing engine emissions is the catalytic converter found on most automobiles and other modern engines of medium or large size. HC and CO can be oxidized to Hz 0 and COz in exhaust systems and thermal converters if the temperature is held at 600°-700°C.

If certain catalysts are present, the temperature needed to sustain these oxidation processes is reduced to 250°-300°C, making for a much more attractive system. A catalyst is a substance that accelerates a chemical reaction by lowering the energy needed for it to proceed. The catalyst is not consumed in the reaction and so functions indefinitely unless degraded by heat, age, contaminants, or other factors.

Catalytic converters are chambers mounted in the flow system through which the exhaust gases flow. These chambers contain catalytic material, which promotes the oxidation of the emissions contained in the exhaust flow.

Generally, catalytic converters are called three-way converters because they promote the reduction of CO, HC, and NOx. Most consist of a stainless steel container mounted somewhere along the exhaust pipe of the engine. In most converters, the ceramic is a single honeycomb structure with many flow passages, (Willard W. Pulkrabek ).



The surface of the ceramic passages contains small embedded particles of catalytic material that promote the oxidation reactions in the exhaust gas as it passes. Aluminum oxide (alumina) is the base ceramic material used for most catalytic converters. Alumina can withstand the high temperatures, it remains chemically neutral, it has very low thermal expansion, and it does not thermally degrade with age. The catalyst materials most commonly used are platinum, palladium, and rhodium.



TAILPIPE AND MUFFLER:

After exiting the catalytic converter, exhaust gases flow through a tailpipe that ducts the flow away from the passenger compartment of the vehicle and vents it to the surroundings. This is usually under and out the back (or side) of an automobile and often upward behind the cab of large trucks.

Somewhere in the tailpipe section there is usually a larger flow chamber called the mumer. This is a sound chamber designed to reduce the operating noise of the engine, most of which is carried out with the exhaust flow. Mufflers use two general methods of sound reduction. One method absorbs the energy of sound pulses by flow through a porous medium. Other mufflers reduce sound by the cancellation of waves. Instead of fully dampening all engine noise, some mufflers are designed to give a louder, sporty sound.

DJA3032 CHAPTER 5

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