Faux Poppet for Asymmetric Exhaust Timing
/projects/homemade-simulation-tools
This report discusses the process of creating and implementing asymmetric exhaust-port timing by designing the geometry of the cylinder port and a rotary exhaust valve in EngMod2T. In this model, the cylinder features standard exhaust and scavenging ports; however, a rotary exhaust valve is added downstream of the port. When closed, flow is restricted just after the port entrance.
At the time of writing, EngMod2T does not offer an option for REV implementation, but it does offer an exhaust poppet valve setting. This can effectively be used for the same purpose. Although the flow and pressure differences between a REV and a poppet valve are drastically different, it is possible to account for these differences. Also, since EngMod2T is not CFD software, the nuances of the two valves are modeled through equivalent effectiveness in the 0D/1D calculations.
How a REV is Designed
In this case, a simple REV is used that has no complex port geometry beyond a length and width. Although the current REV designer program has the capability to define more complex profiles, the basic foundation is the same as discussed here.
With that being said, below are the basic geometries used for the following calculations.
| Geometry | Description |
|---|---|
| Rotor Diameter | Physical diameter of the rotor. |
| Rotor Width | How wide the rotor is. This is usually less important because the port will often be as wide as the cylinder port. |
| Port Width | How wide the open port is. Commonly the same as the cylinder port. |
| Port Height | How tall the port is. Commonly much taller than the cylinder port so the port can stay open longer. |
| Phase Offset | Changes when the port opens relative to the crankshaft and piston position. |
| Drive Ratio | Changes the gear ratio between the crankshaft drive gear and the REV gear. |
It is also important to note that the widths and heights of the port on the rotor are measured and calculated in arc lengths.
Base REV Timing
For flow to travel through the exhaust system, the cylinder port must be open, meaning the piston is below the port, and the REV must also be open. If either of the two restrictions is closed, then no exhaust flow will occur. Because of this, there are two effective open-area curves. Consequently, this setup is capable of asymmetric timing by overlapping the two curves in a precise way to create a desirable outcome. The phase offset and drive ratio are easy ways to tune how these two base curves overlap. In this explanation, it is assumed that the REV is fully open at EPO and closes slightly after BDC.
The timing-curve overlap product is the equivalent area curve, Aq(θ), which is used to define the actual open area between the two systems as follows:
Valve Lift Area Overview
By using the above curve, it is possible to calculate the lift curve for the faux camshaft. The leading question is: what poppet lift produces the same area as the equivalent area curve? This is discovered through experimental testing and literature review and can be described as follows.
The open flow area changes geometric form as valve lift increases. At ordinary low lift, the flow surface is represented by a conical curtain originating at the inner seat diameter. When a larger valve back-cut is present, the back-cut edge may instead control the opening over part of the lift range. At sufficiently high lift, the flow surface spans the inner and outer seat edges and is evaluated from its average circumference and slant length. The area is therefore evaluated using three geometric expressions.
This model was popularized primarily by John B. Heywood in his textbook, Internal Combustion Engine Fundamentals.
In this, let ds represent the mean diameter of the valve seat and ws represent the radial width between the inner and outer seat edges.
And the transition lifts can be written as:
These two equations represent the geometric thresholds that state at what valve lift the shape controlling the open flow area changes. With this, there are three possible area equations for calculating open area.
- High-lift seat-spanning area, when the flow path spans the inner and outer seat edges.
- Back-cut-controlled area, when the valve back-cut controls the opening.
- Ordinary low-lift seat area.
Selection Rules
These equations are selected through the following set:
| aspan(l), | l > l1 |
| aback(l), | l ≤ l1, db > di, l ≤ l2 |
| alow(l), | l ≤ l1, and either db ≤ di or l > l2 |
By being able to use these base equations, it is then possible to effectively design poppet-valve and lift geometries to get the required timing and flow characteristics as the REV designed above.
Finding the Required Lift Curve
At this point in the operation, both the REV equivalent area curve Aq(θ) and the poppet area equation Av(l) are known. The model must then solve a central geometry-matching operation as follows:
Which simply means: find the smallest nonnegative lift that produces at least the required target area. In normal operation, this is simply:
This is solved at every crank angle.
Pressure Ratio and Flow Direction
A big finding in this study is that open area is not the whole equation; actual flow is the most important thing to match. This is difficult since there is no CFD study on REV discharge coefficients, so they are not currently used in the poppet design.
During blowdown, the two most important pressures are the cylinder pressure and the exhaust pressure. These two combine to create pressure ratios that are important for describing inflow and outflow of the cylinder. These ratios must be above 1.
This is used because if reverse flow happens, a different calculation series is applied. By using both the pressure ratio and normalized lift coefficients, the workflow uses bilinear interpolation to take a weighted average of four surrounding table values.
The final effective restriction equation is:
The above inputs can then create a poppet-valve setup that mimics the rotary exhaust valve designed. The above workflow is integrated into the main REV designer explained here.