valve overlap

valve overlap

valve overlapclick for a larger image
The portion of the operating cycle of a reciprocating engine in which, when the piston is passing TDC (top dead center) on the exhaust stroke, both the intake and exhaust valves are open. Valve overlap increases the efficiency of the engine operation by allowing the low pressure caused by the exhaust gases leaving the cylinder to help the fresh charge of fuel-air mixture in the induction system to start moving into the engine.
References in periodicals archive ?
The significant reduction of intake pressure along with the changing valve overlap during the tip-out results in a large increase of internal residual.
Valve phasing was set to maximize internal residuals within the combustion stability limit of 3% CoV IMEP, with the valve overlap centered at TDC.
This enables variable valve timing, valve dwell (how long the valve remains open) and ability to vary the exhaust valve and inlet valve overlap. In this example, each valve is operated by two independent cams, one to open and another to close the valve.
Especially the combination of scavenging due to positive valve overlap and a rich in-cylinder mixture leads to a significant rise of thermocouple temperature at turbine inlet of up to 80 K.
Contract award for finisher discharge valve overlap compatible with use in flushing box brand deca.
He hypothesized that injecting a small amount of fuel during negative valve overlap would allow unburned hydrocarbons in the internal residual to react, which could facilitate compression ignition during the following cycle.
This two-stage system varies intake valve timing and lift between high- and low-speed settings, thus optimizing valve overlap. The previous system, on the other hand, deactivated half of the intake valves to create swirl in the combustion chamber to increase torque output below 3,400 rpm.
Intake valve timing and clearance settings switch to match for low- and high-speed driving and optimized valve overlap in the low-, mid-, and high speed ranges enhances intake and exhaust efficiency for power output of 88 kW(120PS) and superior fuel economy of 19.6 km per liter[16].
In some ways it can get easier, because if you add technologies such as variable cam timing, you get more power by changing valve overlap, but it also gives you improvements in economy and emissions.
Internal EGR can be achieved as a result of valve overlap between the intake and exhaust valve where the piston draws some of the exhaust present in the exhaust port back into the cylinder.
Further, the exhaust valve closes before the engine valve opens, which eliminates valve overlap.
This allows unrestricted control of valve overlap, such that trapped residuals can be readily managed.