Valve Timing. Air expenditure with variable IC

The valve timing is always a compromise since the engine operates within wide ranges of speed and load. Because of the factors described in Section 10.6.1, one cannot simultaneously optimize the charge cycle for maximum torque and maximum rated horsepower without additional features such as the camshaft adjustment system, the control cam system, or a multistage manifold. The offsetting of the valve timing is related to these factors. The terms “early” and “late” indicate a relative position to the basic control times that are indicated as the degree of crankshaft angle relative to the closer dead center.

- Exhaust opens (EO)

The exhaust usually opens in spark-ignition engines at a 50°-30° crankshaft angle from В DC shortly before the end of the expansion cycle. This control time represents a compromise between a gain in expansion work and greater exhaust work.

If the EO is moved in the late direction (i.e., the EO occurs closer to the BDC), the working gas expands longer and exerts work on the piston, the thermal efficiency increases, and consumption falls. A longer expansion lowers the hydrocarbon emissions and the exhaust temperature. At greater speeds and loads, the exhaust work substantially increases at the start of the expulsion cycle, which, in turn, increases consumption. A late EO is primarily relevant for partial loads, and its influence on full loads is slight (Fig. 10-44).

Fig. 10-44. Increase in expansion work by shifting the EO in the "late" direction

When the EO shifts in the early direction, the opposite occurs: Expansion work is lost, the thermal efficiency drops, and the fuel consumption increases. The hydrocarbon emissions and exhaust temperature rise. However, less exhaust work is required since the cylinder pressure is always at a higher level, and the exhaust leaves the cylinder more quickly. An important factor is that the consumption increases at a partial load. Another fact is that the thermal load on the exhaust valve rises with an early EO and, hence, increases the material wear.

The pressure loss during expulsion also depends on the lifting curve of the exhaust valves. When the valve stroke rises strongly during opening, it is easiest for the exhaust to leave the cylinder. For this reason, the required compromises with two exhaust valves are less critical than with only one exhaust valve: When there are two exhaust valves, there is a more effective opening area available for expulsion at a faster rate. The exhaust can, therefore, leave the cylinder at the beginning of the expulsion cycle since it is at a higher pressure. There is, therefore, less exhaust work for the piston.

- Exhaust closes (EC)

A common approach to EC is an 8°-20° crankshaft angle after TDC, which indicates the end of the valve overlap phase. In addition to IO (inlet opens), EC is the control time that can be used to control the length of the overlap. At low speeds and load levels, the EC controls the amount of exhaust drawn back by the exhaust system, and at higher load levels and speeds, it controls the residual gas that can be expelled.

Under a full load, the cylinder can be thoroughly purged by a late EC, which increases the volumetric efficiency. This is used for engines with a higher rated horsepower such as sports engines. An increasingly greater portion of the fresh charge flows through the cylinder without participating in combustion (scavenging loss from short-circuit flow), which increases consumption and the hydrocarbon emissions.

Under a partial load, an increasingly greater portion of the exhaust is drawn back (internal exhaust recirculation) by the suction of the piston. This can yield substantial advantages for consumption and emissions. The last part of the exhaust is always relatively rich in uncombusted hydrocarbons since the combustion is incomplete of the cylinder charge zones close to the wall. This component is expelled relatively late. If this component in the exhaust is “recombusted,” consumption is reduced, and there are fewer hydrocarbon emissions. Because of the diluted charge, the combustion temperature is lower, which reduces nitrogen emissions.

Another consideration is that the fresh mixture becomes homogenized because of the hot residual gases and, hence, produces a better mixture. There is less intake work with a later EC. This occurs for two reasons: First, the drawn back exhaust component expands in the cylinder and supports expansion. Second, when there is more residual exhaust gas in the cylinder charge, less throttling for load control is required to compensate for this quantity while retaining the load. This further reduces consumption. The restriction on internal exhaust gas recirculation is determined by the residual gas compatibility during combustion.

With an early EC, the combustion gas cannot leave the cylinder at the right time (exhaust lockup) so that the residual exhaust gas in the cylinder rises. This causes the volumetric efficiency and the rated horsepower to drop. The scavenging loss is lower, which slightly lowers consumption. In this case as well, the last component of the exhaust is recombusted, which can have advantages for consumption and emissions under a partial load (the nitrogen oxide emissions are reduced because of the low combustion temperature).

The exhaust remaining in the cylinder continues to flow (partially guided by the piston) very strongly into the induction pipe, which improves the mixture preparation. Since there is a continually smaller area for expelling the exhaust after a certain piston position, the exhaust work is increased. At the end of the expulsion cycle, the residual gas can be compressed by an early EC, which slightly increases consumption. An early EC is limited by the increased exhaust work, a fresh charge diluted with exhaust, and an inhomogeneous mixture from a strong inflow of exhaust into the induction pipe.

When dynamic effects in the exhaust system are optimized, the efficiency of the expulsion can be improved if a vacuum wave reduces the static pressure in the exhaust port shortly before EC and thereby sucks the exhaust out of the cylinder.

- Inlet opens (10)

The control time 10 is commonly set at 20°-5° crankshaft angle before TDC for spark-ignition engines. As the beginning of the valve overlap phase, it is also important like the EC for regulating the residual amount of gas in the fresh charge under partial loads and for scavenging the residual gas under full loads. As such, it has a substantial influence on idling quality.

The duration of the valve overlap phase is shortened with a late 10. Under a partial load, this produces a charge that is less diluted with exhaust, which increases the speed of combustion. Under such conditions, the rpm can be lowered during idling, which reduces consumption. Given the lower residual exhaust gas and the fast combustion, the combustion temperature increases, and emissions of nitrogen oxide increases. The hydrocarbon emissions can be lowered under the following conditions: Since the intake valve opens later, the flow in the cylinder is faster at a specific piston position, which increases the flow within the cylinder.

This, in turn, improves the mixture preparation, and combustion is more thorough, which shortens the combustion or ignition delay as well as the length of combustion. When the 10 is late, the intake work increases since a vacuum is generated in the cylinder in the first phase of intake. This increases consumption. Under a full load, the mean effective pressure is less since the air expenditure is lower.

With an early IO, the valve overlap phase is lengthened, and a particularly large amount of exhaust returns into the induction pipe under a partial load. This has a negative influence on combustion since the mixture becomes inhomogeneous and bums more slowly. However, this effect can also be put to positive use for induction pipe injection with throttle-free load control (variable valve actuation). Since there is no induction pipe vacuum with throttle-free load control, there is frequently insufficient mixture preparation, which causes the combustion to last longer and be incomplete. Fuel deposits can also form close to the valve.

These deposits can be vaporized by the returning hot exhaust and be sucked back inside, which heats the induction pipe wall and improves the mixture. Investigations have shown [Gobel, MTZ] that this method can positively influence mixture preparation despite the reaction-inhibiting higher residual exhaust gas, which in the final analysis enhances the reactivity of the mixture.

- Inlet closes (IC)

The valve timing element that is primarily responsible for the torque and power characteristic is the IC. It usually lies at a 40°-60° crankshaft angle after BDC, and it influences the charging of an engine much more than the other control times. The characteristic quantities such as torque and output are primarily determined by the IC.

Offsetting the IC in the late direction to a time optimized for the maximum torque yields greater air expenditure and volumetric efficiency at higher speeds. A higher rated horsepower is correspondingly attained with a late IC. As illustrated in Section 10.6.2.1, the gas dynamic effects at higher speeds play the most important role (especially the recharging effects). When the IC is offset, the most important task is to exploit these effects by capturing the overpressure wave in the cylinder. At lower speeds and under a full load, a long opening time has a negative influence on the torque. Since the intake valve is closed later, a greater amount of charge is pushed back into the induction pipe by the piston.

This is countered by a lower pulse because of the lower gas speed, which, in turn, lowers the volumetric efficiency. The influence of the IC control time on air expenditure under a full load is shown in Fig. 10-45. By offsetting the inlet camshaft by a 20° crankshaft angle toward late, the air expenditure is clearly reduced at low speeds. At the nominal rpm, the air expenditure is contrastingly increased by approximately 8% in an eight-cylinder spark-ignition engine with four valves per cylinder.

Fig. 10-45. Air expenditure with variable IC

Under a partial load, a late IC lowers the intake work since the charge is aspirated with less throttling. This lowers the consumption. The thermal efficiency of the process is lower since the effective compression becomes increasingly low. The combustion temperature is reduced by the low peak pressure, which, in turn, reduces the nitrogen oxide emissions.

With variable valve actuation and a late IC, the engine can be operated without throttling. The goal is either to achieve a higher rated horsepower or to reduce the consumption under a partial load. Under a partial load, the excess charge is returned by the piston into the induction pipe during the compression cycle. Because of the throttle-free load control, less intake work is required. As described above, this reduces consumption, thermal efficiency, the consumption temperature, and nitrogen oxide emissions. The limit for a late IC is the drop in thermal efficiency and the worse mixture preparation in the intake port because of the lack of a vacuum (lower gas speed).

Given an early IC and conventional valve actuation, the intake phase becomes shorter, which reduces air expenditure. Under a full load and at higher speeds, this reduces the volumetric efficiency and yields a low rated horsepower. However, since less charge is returned into the induction pipe at low speeds, the volumetric efficiency and torque increase. Under a partial load, the required load can be attained with last throttling because of the shorter intake phase, which reduces the amount of intake work. This has a positive effect on consumption.

With variable valve actuation and an early IC, the load no longer has to be controlled by throttling; rather it can be regulated by the selected IC valve timing. The goal can be either to increase the torque under a full load or to reduce the consumption under a partial load. As soon as the amount of charge is in the cylinder that is required for the load, the intake valve is closed. In this phase, the piston is still moving toward BDC, and a vacuum is generated in the cylinder. Since the load control is throttle-free, the amount of intake work is much lower than when throttling is used to control the load, and this reduces consumption.

The difference in pressure between the intake and exhaust systems is low, and only a slight amount of its exhaust is sucked back by the outlet. Assuming that the 10 timing is at a conventional position and that the overlap phase is not long, an early IC produces stable combustion under low loads at slow speeds. The limits on a low IC is the mixture formation. Since the intake ends earlier than the BDC, there is frequently negligible charging movement in the cylinder during ignition, which can make combustion longer and incomplete after a long combustion delay.

This can produce greater hydrocarbon emissions and increase consumption despite the low amount of work involved in the charge cycle. Furthermore, there is the danger of fuel condensation in the cylinder from the charge cooling because of the generated vacuum. As mentioned under the section “Inlet opens,” the mixture is insufficiently prepared in the inlet port because of the absence of a vacuum, which makes the aspirated mixture inhomogeneous. Fuel deposits can form close to the valve.