ACTUAL CYCLES AND THEIR ANALYSIS
4.5 EXHAUST BLOWDOWN
0 4 8 12 16
20 84
80
76
72
68
64 0.04 0.08 0.12
CO
O2
Fuel-air ratio by weight
Stoichiometric mixture
CO2 N2
Per cent volume of CO , CO, O 22 Per cent volume of N2
Fig. 4.7 Composition of exhaust gases for various fuel-air ratios
Fuel-air cycle Real cycle Heat loss 12%
Time loss 6%
Exhaust loss 2%
0 2 4 6 8 V /Vcyl c
60 50 40 30 20 10 0
Pressure (bar)
70
Fig. 4.8 Time loss, heat loss and exhaust loss in petrol engines
the early part of the exhaust stroke. If the exhaust valve is opened too early, a part of the expansion stroke is lost. The best compromise is to open the exhaust valve 40◦ to 70◦ beforeBDC thereby reducing the cylinder pressure to halfway (say 3.5 bar) before the exhaust stroke begins. This is shown in Fig.4.9 by the roundness at the end of the diagram.
Exhaust valve opening at bottom dead centre Optimum exhaust valve opening Early exhaust valve opening
Expansion stroke
Volume
Pressure (bar)
3.5 7.0
Ideal diagram
Fig. 4.9 Effect of exhaust valve opening time on blowdown
4.5.1 Loss Due to Gas Exchange Processes
The difference of work done in expelling the exhaust gases and the work done by the fresh charge during the suction stroke is called the pumping work. In
other words loss due to the gas exchange process (pumping loss) is due to pumping gas from lower inlet pressurepito higher exhaust pressure pe. The pumping loss increases at part throttle because throttling reduces the suction pressure.Pumping loss also increases with speed. The gas exchange processes affect the volumetric efficiency of the engine. The performance of the engine, to a great deal, depends on the volumetric efficiency. Hence, it is worthwhile to discuss this parameter in greater detail here.
4.5.2 Volumetric Efficiency
As already stated in section 1.8.4, volumetric efficiency is an indication of the breathing ability of the engine and is defined as the ratio of the volume of air actually inducted at ambient condition to swept volume. However, it may also be defined on mass basis as the ratio of the actual mass of air drawn into the engine during a given period of time to the theoretical mass which should have been drawn in during that same period of time, based upon the total piston displacement of the engine, and the temperature and pressure of the surrounding atmosphere.
The above definition is applicable only to the naturally aspirated engine.
In the case of the supercharged engine, however, the theoretical mass of air should be calculated at the conditions of pressure and temperature prevailing in the intake manifold. The volumetric efficiency is affected by many variables, some of the important ones are:
(i) The density of the fresh charge : As the fresh charge arrives in the hot cylinder, heat is transferred to it from the hot chamber walls and the hot residual exhaust gases, raising its temperature. This results in a decrease in the mass of fresh charge admitted and a reduction in volumetric efficiency. The volumetric efficiency is increased by low temperatures (provided there are no heat transfer effects) and high pressure of the fresh charge, since density is thereby increased, and more mass of charge can be inducted into a given volume.
(ii) The exhaust gas in the clearance volume : As the piston moves from T DCtoBDC on the intake stroke, these products tend to expand and occupy a portion of the piston displacement greater than the clearance volume, thus reducing the space available to the incoming charge. In ad- dition, these exhaust products tend to raise the temperature of the fresh charge, thereby decreasing its density and further reducing volumetric efficiency.
(iii) The design of the intake and exhaust manifolds : The exhaust mani- fold should be so designed as to enable the exhaust products to escape readily, while the intake manifold should be designed so as to bring in the maximum possible fresh charge. This implies minimum restriction is offered to the fresh charge flowing into the cylinder, as well as to the exhaust products being forced out.
(iv) The timing of the intake and exhaust valves : Valve timing is the reg- ulation of the points in the cycle at which the valves are set to open
and close. Since, the valves require a finite period of time to open or close for smooth operation, a slight “lead” time is necessary for proper opening and closing. The design of the valve operating cam provides for the smooth transition from one position to the other, while the cam setting determines the timing of the valve.
The effect of theintake valvetiming on the engine air capacity is indicated by its effect on the air inducted per cylinder per cycle, i.e., the mass of air taken into one cylinder during one suction stroke. Figure 4.10 shows representative intake valve timing for both a low speed and high speed SI engine. In order to understand the effect of the intake valve timing on the charge inducted per cylinder per cycle, it is desirable to follow through the intake process, referring to the Fig.4.10.
While the intake valve should open, theoretically, atT DC, almost all SI engines employ an intake valve opening of a few degrees beforeT DCon the exhaust stroke. This is to ensure that the valve will be fully open and the fresh charge starts to flow into the cylinder as soon as the piston reachesT DC. In Fig.4.10, the intake valve starts to open 10◦ before T DC. It may be noted from Fig.4.10 that for a low speed engine, the intake valve closes 10◦ after BDC, and for a high speed engine, 60◦ after BDC.
Compression IVO
EVO
Intake opens Exhaust closes
TDC
Valve overlap
BDC
Intake closes Exhaust opens
10o
10o 25o 5o
Compression IVO
EVO
60o 55o
Intake opens Exhaust closes
TDC
10o 20o
BDC
Intake closes Exhaust opens
Power Power
Slow speed High speed
Fig. 4.10 Valve timing diagram of four-stroke engines
As the piston descends on the intake stroke, the fresh charge is drawn in through the intake port and valve. When the piston reachesBDC and starts to ascend on the compression stroke, the inertia of the incoming fresh charge tends to cause it to continue to move into the cylinder. At low engine speeds, the charge is moving into the cylinder relatively slowly, and its inertia is relatively low. If the intake valve were to remain open much beyondBDC, the up-moving piston on the compression stroke would tend to force some of the charge, already in the cylinder back into the intake manifold, with consequent reduction in volumetric efficiency. Hence, the intake valve is closed relatively
early afterBDCfor a slow speed engine. High speed engines, however, bring the charge in through the intake manifold at greater speeds, and the charge has greater inertia. As the piston moves up on the compression stroke, there is a “ram” effect produced by the incoming mixture which tends to pack more charge into the cylinder. In the high speed engine, therefore, the intake valve closing is delayed for a greater period of time after BDC in order to take advantage of this “ram” and induct the maximum quantity of charge.
For either a low speed or a high speed engine operating in its range of speeds, there is some point at which the charge per cylinder per cycle becomes a maximum, for a particular valve setting. If the revolutions of the low speed engine are increased beyond this point, the intake valve in effect close too soon, and the charge per cylinder per cycle is reduced. If the revolutions of the high speed engine are increased beyond this maximum, the flow may be chocked due to fluid friction. These losses can become greater than the benefit of theram, and the charge per cylinder per cycle falls off.
The chosen intake valve setting for an engine operating over a range of speeds must necessarily be a compromise between the best setting for the low speed end of the range and the best setting for the high speed end.
The timing of theexhaust valvealso affects the volumetric efficiency. The exhaust valve usually opens before the piston reachesBDCon the expansion stroke. This reduces the work done by the expanding gases during the power stroke, but decreases the work necessary to expel the burned products during the exhaust stroke, and results in an overall gain in output.
During the exhaust stroke, the piston forces the burned gases out at high velocity. If the closing of the exhaust valve is delayed beyondT DC, the inertia of the exhaust gases tends to scavenge the cylinder better by carrying out a greater mass of the gas left in the clearance volume, and results in increased volumetric efficiency. Consequently, the exhaust valve is often set to close a few degrees afterT DC on the exhaust stroke, as indicated in Fig.4.10. It should be noted that it is quite possible for both the intake and exhaust valves to remain open, or partially open, at the same time. This is termed thevalve overlap. This overlap, of course, must not be excessive enough to allow the burned gases to be sucked into the intake manifold, or the fresh charge to escape through the exhaust valve.
The reasons for the necessity of valve overlap and valve timings other than atT DC orBDC, has been explained above, taking into consideration only the dynamic effects of gas flow. One must realize, however, that the presence of a mechanical problem in actuating the valves has an influence in the timing of the valves.
The valve cannot be lifted instantaneously to a desired height, but must be opened gradually due to the problem of acceleration involved. If the sudden change in acceleration from positive to negative values are encountered in design of a cam. The cam follower may lose the contact with the cam and then be forced back to close contact by the valve spring, resulting in a blow against the cam. This type of action must be avoided and, hence, cam contours are so designed as to produce gradual and smooth changes in directional acceleration.
As a result, the opening of the valve must commence ahead of the time at
which it is fully opened. The same reasoning applies for the closing time.
It can be seen, therefore, that the timing of valves depends on dynamic and mechanical considerations.
Both the intake and exhaust valves are usually timed to give the most sat- isfactory results for the average operating conditions of the particular engine, and the settings are determined on the prototype of the actual engine.