Flow through valves

1. Timing

Figure 6-12 shows the main geometric parameters of a poppet valve head and seat.

Figure 6-12 :

Figure 6-13 shows the proportions of typical inlet and exhaust valves and ports, relative to the valve inner seat diameter D. The inlet port is generally circular, or nearly so, and the cross-sectional area is no larger than is required to achieve the desired power output. For the exhaust port, the importance of good valve seat and guide cooling, with the shorten length of exposedvalve stem, leads to a different design. Although a circular cross section is still desirable, a rectangular or oval shape is often essential around the guide boss area. Typical valve head sizes for different shaped combustion chambers in terms of cylinder bore B are given in Table 6.1. Each of these chamber shapes allow higher maximum air flows for a given cylinder displacement.

Figure 6-13 :

Typical valve timing, valve-lift profiles, and valve open areas for a four-stroke cycle spark-ignition engine are shown in Fig. 6-14. There is no universally accepted criterion for defining valve timing points. Some are based upon a specific lift criterion. For example, SAE defines valve timing events based on reference valve-lift points :

1. Hydraulic lifters. Opening and closing positions are the 0.15mm (0.006 in) valve-lift points.

2. Mechanical lifters. Valve opening and closing positions are the 0.15mm (0.006 in) valve-lift points plus the specified lash.

Table 6-1 :

Alternatively, valve events can be defined based on angular criteria along the lift curve. What is important is when significant gas flow through the valve-open area either starts or ceases.

The instantaneous valve flow area depends on valve lift and the geometric details of the valve head, seat, and steam. There are three separate stages to the details of the flow area development as valve lift increases, as shown in Fig. 6-14b.

For low valve lifts, the minimum flow area corresponds to a frustrum of a right circular cone where the conical face between the valve and the seat, which is perpendicular to the seat, defines the flow area. For this stage :

Formula 6-7 :

where B is the valve seat angle, Lv is the vcalve lift, Dv is the valve head diameter and w is the seat width.

For the second stage, the minimum area is still the slant surface of a frustrum of a right circular cone, but this surface is no longer perpendicular to the valve seat. The base angle of the cone increases from (90 B)° toward that of a cylinder, 90°. For this stage :

Formula 6-8 :

where Dp is the port diameter, Ds is the valve stem diameter and Dm is the mean seat diameter (Dv w).

Figure 6-14 :

(a) Typical valve timing for high-speed 2.2-dm³ four-cylinder spark-ignition engine.
(b) Schematic showing three stages of valve lift.
(c) Valve-lift curve and corresponding minimum intake and exhaust valve open areas as a function of camshaft angle. Inlet and exhaust valve diameters are 3.6 and 3.1 cm, respectively.

Finally, when the valve lift is sufficiently large, the minimum flow area is no longer between the valve head and seat ; it is the port flow area minus the sectional area of the valve stem.

Thus for

Formula 6-9

Intake and exhaust valve open areas corresponding to a typical valve-lift profile are plotted versus camshaft angle in Fig. 6-14c. These three different flow regimes are indicated. The maximum valve lift is normally about 12 percent of the cylinder bore.

Inlet valve opening (IVO) typically occurs 10 to 25° BTDC. Engine performance is relativly insensitive to this timing point. It should occur sufficiently before TDC so that cylinder pressure does not dip early in the intake stroke.

Inlet valve closing (IVC) usually falls in the range 40 to 60° after BDC, to provide more time for cylinder filling under conditions where cylinder pressure is below the intake manifols pressure at BTC. IVC is one of the principal factors that determines high-speed volumetric efficiency (VE) ; it also affects low-speed VE due to backflow into the intake.

Exhaust Valve Opening (EVO) occurs 50 to 60° before BDC, well before the end of the expansion stroke, so that blowdown can assist in expelling the exhaust gases. The goal here is to reduce cylinder pressure to close to the exhaust manifold pressure as soon as possible after BDC over the full engine speed range. Note that the timing of EVO affects the cycle efficiency since it determines the effective expansion ratio.

Exhaust valve closing (EVC) ends the exhaust process and determines the duration of the valve overlap period. EVC typically falls in the range 8 to 20° after TDC. At idle and light load, in spark-ignition engines (which are throttled), it therefore regulates the quantity of exhaust gases that flow back into the combustion chamber through the exhaust valve under the influence of intake manifold vaccum. At high engine speeds and loads, it regulates how much of the cylinder burned gases are exhausted. EVC timing should occur sufficiently far after TDC so that the cylinder pressure does not rise near the end of the exhaust stroke. Late EVC favors high power at the expense of low-speed torque and idle combustion quality. Note from the timing diagram (Fig. 6-14a) that the points of maximum valve lift and maximum piston velocity do not coincide.

The effect of valve geometry and timing on air flow can be illustrated conceptually by dividing the rate of change of cylinder volume by the instantaneous minimum valve flow area to obtain a pseudo flow velocity for each valve :

Formula  6-10 :

where V is the cylinder volume, B is the cylinder bore,
s is the distance between the wrist pin and crank axis
and Am is the valve area given by Eqs (6.7), (6.8) or (6.9).

Instantaneous pseudo flow velocity profiles for the exhaust and intake strokes of a four-stroke four-cylinder engine are shown in Fig. 6-15.

Figure 6-15 :

Note the appearance of two peaks in the pseudo flow velocity for both the exhaust and the intake strokes. The broad peaks occuring at maximum piston velocity reflect the fact that valve flow area is constant at this point. The peaks close to TDC result from the exhaust valve closing and intake valve opening profiles. The peak at the end of the exhaust stroke is important since it indicates a gigh pressure drop across the valve at this point, which will result in higher trapped residual mass. The magnitude of this exhaust stroke pseudo velocity peak depends strongly on the timing of exhaust valve closing.

The pseudo velocity peak at the start of the intake stroke in much less important.

That the pseudo velocities early in the exhaust stroke and late in the intake stroke are low indicates that phenomena other than quasi-steady flow governe the flow rate. These are the periods when exhaust blowdown and ram and tuning effects in the intake are most important.